CHAPTER 2: SATELLITE ORBIT AND INSTRUMENTATION

Chapter Objectives

After reading this chapter, you should be able to:-

Understand different types of orbits and the orbits that are useful for meteorology. Understand different payloads onboard current satellites and their utilities.

Understand various geophysical parameters that are derived from satellite data.

Structure

1. Introduction

2. Orbits and Kepler’s Laws

3. Different Kinds of Satellite Orbits

4. Basic Orbital Mechanics

5. Tracking System and Earth Station

6. Meteorological Satellite Instrumentation

7. Instrumentation in Operational Geostationary Satellite

8. Instrumentation in Operational Polar Orbiting Satellite

9. Meteorological Satellite from other countries Satellite

10. Tropical Rainfall Measuring Mission

11. The Earth Observing system satellite

12. Conclusion

Introduction

1. Today the meteorological services are a part and parcel of each country and serve a variety of people, viz., agriculturists, aviators, mariners, tourists and planners. The weather systems, particularly over the tropics, viz. cyclones, monsoon, etc have their origin in oceans, which are large reservoirs of energy for meteorological systems. The knowledge that upper air observations and weather of distant regions (global) are equally important for understanding and predicting the local weather beyond one or two days has resulted in a network of stations all over the globe for measuring basic meteorological parameters. Satellite meteorology is the logical extension to develop more versatile, coordinated and uniform measurement system over the globe. 01 Apr 60 heralded a new era in meteorological observations with the launching of TIROS-1 satellite. A new platform for synoptic and repetitive coverage over vast oceanic areas and inaccessible land became available for the forecaster. The satellite orbits are "grouped" into general categories because a major characteristic of a particular orbit in the "group" produces a highly desired ground track or an aspect of the orbit which is needed to accomplish the main purpose of the satellite. In general, a satellite orbit gives rise to particular desirable ground track. For example, a communications satellite needs to stay where it can always be seen from the ground, a weather satellite needs to view the earth with the sun in the same relative position every time the satellite passes over a country. Thus, a satellite is placed in an orbit which capitalizes on an aspect of the orbit which helps the satellite meet its mission, be that scientific, military, or commercial. This chapter also deals with the current satellites of India and other countries of the world.

Orbits and Kepler’s Laws

2. Satellites are those moving around the gravitational force of a central mass. The path followed by the satellite is called orbit. The satellite moves as per Kepler’s laws. Kepler’s first law states “Each planet moves around the sun in an ellipse, with the sun at one focus”(Fig.1). An ellipse is not just an oval, but is a very specific and precise curve more mathematically; it is the locus of all the points the sum of whose distances from two fixed points (the foci) is a constant.

Fig.1: Explanation of Kepler’s First Law

3. Kepler’s second law is “ The radius vector from the sun to the planet sweeps out equal area in equal area of time”. The planets do not go around the sun at a uniform speed, but move faster when they are nearer the sun and more slowly when they are farther from the sun, in precisely this way: suppose an planet is observed at any two successive times, let us say a week apart, and that the radius vector is drawn to the planet during the week, and the two radius vectors , bound a certain plane area , at a part of the orbit farther from the sun( where the planets moves more slowly), the similarly bounded area is exactly the same as in the first case. So, in accordance with the second law, the orbital speed of each planet is such that the radius “sweep out” equal areas (A1=A2) in equal times (Δt) (Fig.2).

Fig.2: Explanation of Kepler’s Second Law

4. Finally, a third law was discovered by Kepler much later; this law is of a different category from the other two, because it deals not with only a single planet, but relates one planet to another. This law says that when the orbital period and orbit size of any two planets are compared, the periods are proportional to the 3/2 power of the orbit size. In this statement the period is the time interval it takes a planet to go completely around its orbit, technically known as the time required to go around the circle would be proportional to the 3/2 power of the diameter (or radius) . Thus Kepler’s third law is “The squares of the periods of any two planets are proportional to the cubes of the semi major axes of their respective orbits”.

5. Period (T) of an elliptical orbit. It is defined as the time to complete one revolution. T is proportional to the semi-major axis raised to the power 3/2 power.

T= the orbital period, µ = the Earth’s gravitational parameter

= 3.986x105 Km3s/Sec2

The condition for satellite to orbit is that the Gravitational force should be equal to centrifugal force of the body.

Rmν

RmGM 2

2e

Where, G - Universal gravitational constant, R = re + h, Me - Mass of Earth

n - Orbital Velocity

mass satellite oft independanhr

GM

e

e2

n

It is found that n is independent of the satellite mass for for h=0, v = 7.9 km/sec.

T)(2 hre

n

seconds)in TKm,in h and (r )(10)(GM2T e

2322

3

e

hrhr ee

6. Sub-Satellite Point. It is the point on the earth below the satellite (Fig.3).

Fig.3: Sub-satellite point

The Sub-satellite velocity is given by the equation

h

re

Sub-satellite point

hr

rVV

e

esg

7. Ground Track. The ground track of the satellite is the line connecting all the sub-satellite points. The Rotation of earth causes westward shift of ground track. The ground track of IRS -1A is given below in Fig.4:-

Fig.4: The ground track of IRS-1A

Different Kinds of Satellite Orbits

8. Low Earth Orbit (LEO). Any orbit, in which the satellite completes one full orbit around the earth (the "period") in less than 225 minutes, is called a "low earth orbit." In some documents these orbits are called "near earth" orbits. The reason for the 225 minute definition is the factors which affect the satellite in orbit. Satellites so low that they orbit in fewer than 225 minutes are far more susceptible to the earth's atmosphere and earth gravitational anomalies than any other source of disturbance. Satellites with a period greater than 225 minutes are more likely to be affected by the gravitation of the sun, moon and planets, and the earth's natural radiation belts. Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO")(Fig.5). This orbit is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.

Lat it

ude

LongitudeDescending ground traces of IRS-1A/1B for one day. The satellite crosses equator every 103.192 minutes. During this time Earth rotates a distance of 2871.8km at equator causing a westward drift for the ground track (1, 2, 3, ….). In 24hrs satellite makes 13.9545 revolutions around the earth. The orbit on the second day (15th orbit) is shifted westward from orbit No.1 by about 130 km. The ground traces repeat after every 307 orbits in 22 days.

15 Orbit Number

12345678910

11

12

13

14

Fig.5: Low Earth Orbit

9. Advantages and Disadvantages of LEO. Low Earth Orbit is used for things that we want to visit often with the Space Shuttle, like the Hubble Space Telescope and the International Space Station. This is convenient for installing new instruments, fixing things that are broken, and inspecting damage. It is also about the only way we can have people go up, do experiments, and return in a relatively short time. There are two disadvantages to having things so close, however. The first is that there is still some atmospheric drag. Even though the amount of atmosphere is far too little to breath, there is enough to place a small amount of drag on the satellite or other object. As a result, over time these objects slow down and their orbits slowly decay. Simply put, the satellite or spacecraft slows down and this allows the influence of gravity to pull the object towards the Earth. The second disadvantage has to do with how quickly a satellite in LEO goes around the Earth. As you can imagine, a satellite traveling 18,000 miles per hour or faster does not spend very long over any one part of the Earth at a given time. So what happens if we want a satellite to spend all of its time over just one part of the Earth? For instance, a weather satellite wouldn't be very effective for us in North America if it didn't have a long dwell time over us. (Dwell time = the time a satellite sits over one part of the globe.) Also, a communications satellite wouldn't work very well for us in North American if it spent most of its time over Africa or Asia. There are two ways to accomplish this. One solution is to put a satellite in a highly elliptical orbit and the other is to place the satellite in a geosynchronous orbit.

10. Low-Inclination Orbits. This has to be one of the poorest choices of terms in the satellite industry. A "low-inclination" orbit is whatever the term means to the user. The inclination of a satellite, defined as the angle between the orbital plane of the satellite and the equatorial plane of the earth, manifests itself in the highest north or south geographic latitude the satellite reaches in its orbit as viewed from the ground. The inclination for a particular satellite is a particular number having a clear meaning and mathematical significance. When referring to a "low-inclination orbit", there simply is no established definition or mathematical significance. There is no military or civilian definition of a "low-inclination orbit". One finds a low-inclination orbit can be somewhat arbitrarily defined as an inclination less than 45 degrees but no accepted source of authority, including the USA Space Command (USSPACECOM), has defined the term. Some of the general orbit types are shown below in Fig.6.

Fig.6: General Orbit Types

11. Polar Orbits. Strictly defined, a "polar orbit" is when the inclination is exactly 90 degrees. Some latitude (no pun intended) is allowed so that any orbit within a few degrees of 90 is considered a polar orbit.

12. Sun-synchronous Orbits. The sun-synchronous orbit is one of the special categories alluded to in the opening paragraph of this section. All satellites, at any inclination other than exactly 90 degrees, are affected gravitationally by the fact that the earth is not a perfect sphere. This "mass asymmetry" of the earth causes the orbit of the satellite to change. The greatest effect is on the argument of the perigee, and the right ascension of the ascending node. In simple terms, not only does the satellite go around the earth on its orbit, but the orbit itself rotates, or "regresses", around the earth. This is "nodal regression" and is greatly dependent on the satellite's orbital altitude and inclination. At 185 km (100 nm) altitude, 40 degrees inclination, the nodal regression is about 6.8 degrees per day westward. For 555 km (300 nm) altitude, 130 degrees inclination, the nodal regression is 4.7 degrees per day eastward. The sun synchronous orbit is shown in Fig.7.

Fig.7 : Sun Synchronous Orbit

One can take advantage of nodal regression and launch a satellite into an orbit where the nodal regression nearly exactly cancels out the daily change in the position of the sun over any point on earth, caused by the earth's orbit around the sun. This means that every day, when the satellite passes over a point on earth, the position of the sun in relation to the satellite and the earth would be the same. This is a very useful thing to do for a weather or surveillance satellite. The satellite always "sees" the point on the earth, when the sun is shining on the earth from the same angle - the same "sun time". The orbit which has this unique characteristic is called "sun-synchronous" and is an orbit where the

combination of orbit altitude and inclination causes a nodal regression of 0.98 degrees per day eastward. This turns out to be, depending on the altitude of the satellite, about 95 to 100 degrees inclination.

13. What is an n-th resonant orbit?. A resonant orbit is one that completes an integral number of complete revolutions in one day. For example, a 15th-order resonant orbit has exactly 15 revolutions in 24 hours. Resonant orbits can be used in geophysical studies because the satellite passes over the same locations every day, and minor gravitational irregularities accumulate and become measurable. The repeated passing over the same location characteristic is also taken advantage of by classified "spy" satellites (which also can use "two day" resonances, such as 31 revolutions in 2 days). Lastly, the Space Shuttle sometimes uses a 16th order resonant orbit because it permits same-day synchronization of work and sleep schedules.

14. Geostationary Orbit. A geostationary orbit is a special case of a geosynchronous orbit. Put the satellite in a very nearly circular orbit (no eccentricity) and give it zero inclination and the satellite will stay over the same point of the earth's equator – in other words, appear to be stationary in the sky. This is the ideal condition for a communications satellite. One would simply point their ground antenna to the spot in the sky where the satellite appears. Unfortunately, orbits are easily perturbed through natural causes, and a geostationary satellite soon drifts from the position and must be forced back to position by firing thrusters. This uses up the satellite's fuel, and soon, with all fuel exhausted, the satellite drifts permanently away from its geostationary point. Communication satellite owners get around this problem by allowing the satellite to have a small inclination and a very small eccentricity. Ground antennae "see" a large enough area of the sky that if the satellite stays within that area, communications is retained. Thus, all "geostationary" satellites are really allowed to be geosynchronous. They make tiny Fig. eights in the sky instead of staying in exactly one place. It takes less fuel to let the satellite wander around a little, and the lifetime supply of fuel on the satellite is extended. The communications industry somewhat whimsically refers to geostationary communications satellites as "wobbliest." All geostationary satellites must be located along the celestial equator as viewed from the earth. An international commission "assigns" who gets to put a satellite on a particular longitudinal subpoint. Interestingly, a perturbation, caused by the Earth's oblateness, causes a longitudinal acceleration of a satellite in geostationary orbit. The acceleration is zero at 75 degrees east longitude (over the Indian Ocean) and 255 degrees east longitude (over the eastern Pacific Ocean). The lucky owners of these slots get to put their satellites where they are least likely to need fuel to maintain position! All other satellites must use fuel to retain their positions, or they will drift toward these two stable longitudinal points!

15. Geosynchronous Orbit. A geosynchronous orbit is achieved when the satellite completes one orbit around the earth in one sidereal day. (A

sidereal day is the time it takes the earth to rotate once with respect to the stars (not the sun). A sidereal day is 23 hours 59 minutes, 4.091 seconds, compared to a mean solar day which is 24 hours.) This gives the satellite an altitude of about 35,786 km (19,300 nm or 22,236 statute miles). A satellite in a geosynchronous orbit, pretty closely matches the earth's rotation and "appears" from the ground to stay overhead at all times. It is only "pretty close" because, though the satellite has a sidereal period, nothing has been said about the inclination or eccentricity. Give the satellite a nearly circular orbit, but some inclination, say 10 degrees, and the satellite will, over the course of an entire day, appear to inscribe a line in the sky - 10 degrees above the celestial equator to 10 degrees below it. Change the eccentricity a little and the apparent path of the satellite can be changed to some rather odd shapes, from lopsided Fig. eights to a circle. Communications and surveillance satellites use geosynchronous orbits. The geosynchronous satellite coverage is shown in Fig.8.

Fig.8: The geosynchronous satellite coverage

16. Molniya Orbit. In addition to nodal regression discussed above under sun-synchronous orbits, the earth causes the perigee of the satellite orbit to change its position with respect to the stars. The perigee of the orbit (satellite's lowest altitude) literally moves along the plane of the orbit at a rate dependent on the inclination. The condition is known as "rotation of the apsides." A satellite launched such that the perigee is in the southern hemisphere will soon find its perigee in the northern hemisphere. It turns out that at two special inclinations, the apsidal rotation rate is zero. The inclinations are 63.4 and 116.6 degrees. If a satellite is at 63.4 degrees inclination, and the perigee is in the southern hemisphere, the perigee stays in the southern hemisphere. One can take advantage of this. Say you live at northern latitude where a geostationary satellite is too low in your sky to be any use, yet you want a communications satellite to be high in your sky as long as possible. If you launch a satellite in a highly eccentric orbit, at 63.4 (or 116.6) degrees inclination, putting the perigee in the southern hemisphere, then the satellite path during the time it is at or near apogee will spend most of its time in the northern hemisphere. If you further give this satellite a useful period, say, 12 hours (2 revolutions per day), then the satellite spends a majority of its time in the northern hemisphere over your country. Russia uses this technique for many of its communications satellites, and these

orbits have become known as Molniya orbits. Molniya orbits typically have large eccentricities (0.73 for example) and perigee altitudes of 200 to 1000 kilometers, keeping the satellite in view of the high northern latitudes for around 11 hours of every 12.

17. Mid-Earth Orbit (MEO). Mid-earth orbit is also known as Semi-synchronous (Fig.9). Satellites said to be semi-synchronous have a period of 1/2 a sidereal day. Thus, they orbit the earth two times per day. Geo-positioning and navigation satellites, such as GPS and GLONASS, use this type orbit. If we want continuous coverage over the entire planet at all times, such as the Department of Defense's Global Positioning System (GPS), then we must have a constellation of satellites with orbits that are both different in location and time.

Fig.9: Mid-earth orbit

18. What is a "transfer orbit"? What does GTO mean? When it is desired to change the altitude of an orbit, to raise the perigee of a nearly circular orbit to a higher perigee for example, it is accomplished through a "transfer orbit." Orbital maneuvers can take place anywhere in an orbit, but you can accomplish the change using the least amount of propellant, if the maneuver is performed at certain points on the orbit. To raise the satellite to a higher orbit, you fire a thruster to increase its speed, wait for it to arrive at its new apogee, then fire a thruster to adjust the satellite's speed to the new orbit. The satellite literally was put in a new orbit until the thruster was fired again to achieve the desired end orbit. That temporary new orbit was the transfer orbit and was an elliptical orbit which intersected the old orbit and the new orbit. A special case of a transfer orbit is the Geosynchronous Transfer Orbit or GTO. The GTO is the elliptical orbit needed from earth launch to geosynchronous altitude. At geosynchronous altitude, either the payload or the final stage of the launch vehicle conducts a velocity change burn to correct the speed of the payload to that needed for the geosynchronous orbit.

19. High-Earth Orbit (HEO). A high-earth orbit is any orbit greater than geosynchronous. Thus, if the period is greater than a sidereal day, it is a high-earth orbit, also known as super-synchronous. These orbits are often highly inclined and highly elliptical to get the satellite out of the earth's natural magnetosphere (Fig.10 and 11). Many satellites designed for astronomical work are placed in HEO. Remember Kepler's second law: an object in orbit about Earth moves much faster when it is close to Earth

than when it is farther away. Perigee is the closest point and apogee is the farthest (for Earth - for the Sun we say aphelion and perihelion). If the orbit is very elliptical, the satellite will spend most of its time near apogee (the furthest point in its orbit) where it moves very slowly. Thus it can be above home base most of the time, taking a break once each orbit to speed around the other side.

Fig.10: High-earth orbit

With the highly elliptical orbit described above, the satellite has long dwell time over one area, but at certain times when the satellite is on the high speed portion of the orbit, there is no coverage over the desired area. To solve this problem we could have two satellites on similar orbits, but timed to be on opposite sides of the orbit at any given time. In this way, there will always be one satellite over the desired coverage area at all times.

Fig.11: HEO for two satellites

20. Solar Orbit. The earth is in solar orbit. Any satellite given enough energy to leave earth orbit, but not enough energy to leave the solar system, will enter solar orbit. Many science satellites are placed in a solar orbit - Ulysses and Galileo for example. The two USA Pioneer spacecraft of 1972 and 1973 are in a highly elliptical solar orbit. One special case is a "Halo" orbit. A Halo orbit relies on a gravitationally stable point between the earth and the sun, one of the "Lagrange Points." A satellite placed at the Lagrange point between the earth and sun, approximately 1.6 million kilometers from earth, will execute a three dimensional elliptical orbit about the Lagrange point as the Earth, moon,

and satellite system orbit the sun. The satellite ISSE 3 was put in this orbit to detect solar wind products and thus provide an early warning to observers on the ground when solar flare protons were heading toward the earth.

21. Beyond Solar Orbit. Given enough energy, a satellite can be in orbit about nothing, literally free flying through space until it encounters an object with enough gravity to change the energy of the satellite. The new path may be a highly elliptical orbit about the gravitational source, or the satellite may have been given more energy and a new direction of travel. The two USA Voyager spacecraft are currently leaving our solar system. They have enough energy to be free of the sun's gravity and used the gravity of Jupiter to gain energy.

Basic Orbital Mechanics

22. The 3 Kepler’s laws of planetary motion are as follows: –

(a) A planet moves around the sun in an elliptical orbit, with the sun at one focus.

(b) The line joining the Sun’s centre to the planet sweeps out equal areas in equal time.

(c) The square of the period of revolution of the planet is proportional to the cube of its semi-major axis.

23. Newton could `derive’ these laws (and also apply similarly to the Moon around the Earth, besides explaining projectiles, pendulums etc. on Earth) from the following 4 laws: –

(a) Everybody tends to stay at rest or in uniform linear motion unless an external force acts on it (`inertia’ property).

(b) The rate of change of momentum (Mass x velocity) of a body equals the external force applied.

(c) Every action is met by an equal and opposite reaction(These 3 are called Laws of Motion)

(d) Every pair of bodies exert attractive force on one another proportional to the product of their masses, and inversely proportional to the square of the distance between them (and implicitly: the constant of proportionality/involved herein is a `universal gravitational’ constant – it is denoted as capital G). (This is called the Law of Gravitation).

24. A simple demonstration of Kepler’s 3rd law being consistent with the above Newton’s laws, for a special case of a circular orbit, is feasible and is not covered in this write up. At any point in the orbit the small orbiting body (say, satellite around the Earth) is trying to go tangentially as per Newton’s 1st law but is deflected towards the larger body by the gravitational force (4th & 2nd laws).25. We have to equate GMm/R2 = m x v2 /R . This says that the force = mass x acceleration. Now, why is acceleration written here as v2/R? If you draw tangents indicating velocities at 2 close by points on the circular orbit, showing the same length (magnitude or speed) but slightly differing directions, then the vector-difference of these velocities (divided by time-difference) represents acceleration, and it is perpendicular to both the original velocity vectors (towards the central large body, as it should, since the driving force is gravitational attraction towards it by Newtons 4th hypothesis). The velocity-difference vector’s magnitude is (speed) v x small angle between the two velocity-vectors (expressed in radians of course). By similarity of triangles, switch over your attention to the `perpendicular’ triangle formed by the two position (radius) vectors drawn from the Earth to the 2 satellite positions. Then the opposite side (which should be v x time-difference i.e. distance flown by the satellite) divided by orbital radius, should equal the same angle as in the previous triangle!. So, acceleration must be v x (v x time-diff.)/[R x time-diff.] = v2

/R! That’s why we wrote acceleration for a circular motion as v2 /R. (Please don’t get misled into believing that there is a `centrifugal force’!). Going back to the original equation of Newton’s 2nd law, we can now solve it in terms of the period of revolution by noting that v=2(pi)R/T i.e. circular circumference divided by period. Hence GM m/R2 = mv2 /R becomes, after cancelling m (so, satellites’ mass doesn’t matter as long as it is small – otherwise it can even disturb the Earth itself! For example, Earth and Moon rotate together around their common center of mass which is 5000 km below the earth’s surface, on a line joining to moon!!) :

GM/R2 = (4.pi-square. R2/T2)/RWhich = 4. Pi-square.R/T2

Hence T2 = 4. pi-square. R3 /GM

26. This not only demonstrates (rather, `recovers’) Keplers’ third-law but also finds the constant of proportionality in terms of the central body’s mass! A heavier central body forces the orbiting body to circle faster as it should; imagine that the circling object is `falling in or down’ faster.

27. Kepler’s 2nd law is a consequence of the conservation of angular momentum – in an elliptic orbit when you are nearer the central body you have to move faster to preserve `mrv’, (In fact, for Kepler's 2nd law to be valid one does not need the force to be an inverse square law. It can be any function of R! The only condition is that the force between the two bodies must act in the directions of R) or if you like, you can invoke Kepler’s 3rd law and think that the period is `variable’ within the orbit

because the distance R itself is variable such that locally at each point of the orbit, T is proportional to R raised to power 1.5 i.e. movement is faster/slower at nearer/farther point on the orbit! However, any 'central' force (towards a point like earth) yields 2nd law. 28. Kepler’s 1st law is beyond the scope of this Lecture as we shall not enter full geometry. It may be remarked that an actual satellite experiences more complex forces for example, oblateness of the earth (mentioned under `Sun-synchronous’ orbits earlier) causes a torque; the force is of the order 0.1% of the basic gravity for a low-earth satellite; and higher order non-sphericity-of-earth forces, Sun/Moon attractions, Air-drag, Solar radiation etc. ( all these are like 1 part in a million to 10 in a billion). This reason, besides inclination being not precisely 0, makes geosynch. Sat. make 8-shaped excursion, so we can't call it geostationary (fixed).

29. Finally, one common term which in popular Science literature you may have come across, deserves some discussion – viz. `escape velocity’. This is not really applicable to an orbiting satellite since it is very much `confined’ or `bound’ to the earth’s vicinity. The term `escape velocity’ is that minimum speed, given to an object outwards (`up’ from the earth’s surface) which will permit it to go completely out of the earth’s clutches (mathematically: `to infinity’), never to return again. This is obtained by letting its gravitational potential energy + kinetic energy = 0 (considering infinite distance as a reference, gravitational energy at earth’s surface will be taken as negative). The work = force x distance, hence potential = integral of force = integral of GMm/R2 = -GMm/R. So we place mv2/2-GMm/R=0 and get v=11km/sec.

30. Orbital Elements. Although you may never have to actually carry out orbital path calculations of satellites (you may use readymade software), at least the definition, terminology and geometry by which orbital position is described, may be of use to you, as these `orbital elements’ often come in the annotation of Metsat’s data stream. These are:

(a) Semi-major-axis (a). Half the longer axis of the elliptical orbit.

(b) Eccentricity (e). Distance from centre of ellipse to focus divided by semi-major-axes.

(c) Inclination (i) . Angle between equatorial plane and plane of orbit.

(d) Right ascension of the ascending node (greek capital omega) . Angle at center of earth, measured in the equatorial plane, between the vernal equinox and the ascending node ( satellite crossing equator from S to N).

(e) Argument of perigee (Greek small omega). Angle at center of earth, measured in the orbital plane, between the ascending node

and perigee (closest approach of satellite to earth)– defined for circular orbit.

(f) True anomaly (Greek small new): Angle at center of earth, measured in the orbital plane, between the perigee and the satellite position at a given time – this is the fastest changing `dynamic’ variable compared to above 5 – but even some of those 5 do undergo slow movement e.g. in Sun-synchronous (e.g. NOAA) orbit, item 4 changes by 1 degree per day; the item 5 can change slowly + or – as per inclination being less or more than 63.4 degrees. Soviet Molinya (communication satellites) are kept at this `critical’ inclination of 63.4 degrees with perigee in Southern Hemisphere and apogee in Northern Hemisphere to get `quasi-fixed’ communication satellites at about 40000 km at northern apex from where nearly half the earth is visible (over Asia).

(g) Mean anomaly(M): This is sometimes used as an alternate to item (f) i.e. true anomaly; this is not physically an angle but a measure of “time expressed in angle” as 360 x (t – t of perigee)/period. For diagrams explaining orbital elements, please see Text (Kidder & von der Haar), Section 2.2. Only `true anomaly, is not drawn in this Text. For this, in Fig.12 (a&b) imagine a satellite a little beyond the perigee, and take the angle between the perigee and satellite as the true anomaly.

Fig.12(a&b) : Schematics showing the co-ordinate system to locate a satellite in space.

Tracking System of Earth Station

Earth

X

Perigee

Equitorial plane

Line of nodes

Vertical Equinox

i

Orbital plane

Apogee

Satellite

Y

Z

Perigee

Apogee

b

b

a

Satellite

31. Once an Earth Station is installed, it is necessary to determine Azimuth and Elevation angles for it for a given target for manually positioning to any desired direction or incorporate certain methods so that the Antenna follows the target all by itself i.e. auto tracking. Fig.13 shows three most common dual axis mounts. Auto tracking for satellite tracking Antenna is achieved using systems giving out errors if Boresight axis is not aligned with target and then feeding to a closed loop servo control system to nullify this error. The tracking system is described in two parts: (a) error derivation methods and (b) Servo control system for driving the Antenna. Various methods for generating the error signals are classified as sequential lobbing, conical scan and mono pulse.

Fig. 13 .Earth Station Antenna Mounts (a),(b)Elevation over azimuth;(c),(d) Earth over elevation;(e) Polar(HA-DEC)in

northern hemisphere

(a) In first method obtaining the direction and the magnitude of the angular error is by alternately switching the antenna beam between two positions. The difference in amplitude between the voltages obtained in the two-switched positions is a measure of the angular displacement of the target from the switching axis with the direction of the target. When the voltages in two switched positions are equal the target is on axis. This is given in Fig. 14.

Fig. 14: Lobe switching antenna pattern and error signal(a)Polar representation of switched antenna patterns (b )Rectangular representation (c) Error signal

(b) The second method involves rotation of feed (in cases of focal feeds) or of sub-reflector (in case of cassegrain feed system). This can be considered as extension of lobe switching technique to rotate continuously an offset antenna beam rather than discontinuously step the beam between four discrete positions. This

is conical scan tracking. However, this system has several draw backs for using this for fast moving satellite tracking applications viz. poorer signal to noise ratio, susceptibility to noise etc. which can effect the accuracy of tracking.

(c) Third method for deriving errors uses simultaneous lobbing (monopulse). Commonly used method is Amplitude Comparison monopulse (or more simply monopulse). In this technique the RF signals received from two offset Antenna beams are combined so that both sum and difference signals are derived simultaneously which are multiplied in a phase sensitive detector to obtain both magnitude and direction of error signal.

32. Monopulse Tracking System. Monopulse tracking (y systems are the most commonly used technique for tracking fast moving satellite target due to simplicity of mechanical fabrication, ease of maintenance and accuracy. The system consists of four horns, placed about central focus of the paraboloid reflector (In case of cassegrain feed, main feed horn can consist of four segments (Fig.15 a&b). Target illuminates all the four horns simultaneously. A duplexer using hybrids is used to obtain three signals; the sum of all horn outputs (A+B+C+D), the vertical difference (A+C) (B+D) and the horizontal difference (A+B) - (C+D). No difference is recorded if the target is in the axial direction of Antenna, but once the position of the target has been acquired, any deviation from the central position will result in the generation of three signals i.e. horizontal & vertical differences and a sum signal. The receiver for such a system has three separate input channels, one for each signal, consisting of three complete receive chains. The output of sum channel forms the data, while each of the difference signals feeds a closed loop servo system controlling the antenna, so as to keep it pointed exactly at the target. The drawback of having three separate channels for processing can be overcome by an arrangement called signal channel monopulse tracking system. This consists of phase commutation unit in which the Azimuth and Elevation difference signals are amplitude modulated and sequentially coupled to the reference signal. The flexibility and cost effectiveness of this technique has resulted in it being the predominant choice for telemetry tracking applications for last two decades.

Fig. 15(a): Feed Aperture Configuration(Horn)

Fig. 15(b) : Feed Aperture Configuration(Horn)

33. Single channels monopulse system for Remote Sensing Satellite Earth Station shall be described here in some detail. The Antenna, feed, monopulse comparator and polarisation together give three signals to phase communication unit when the components are arranged as in Fig. 15(a). These three signals are obtained in monopulse comparators as follows [letters refer to horn signals in Fig. 15(b):

e = A+B+C+DD EL = (A+B) - (C+D)D AZ = (A+D) - (B+D)

34. These signal operations are performed using magic tees and hybrids for horn type feed incase of short slot hybrids for dipole type feeds as used for S-band focal feed. Sum signal (å) is the total power received by the feed and the difference signals represent the differences between the signals received by the two Antenna elements in the same plane. Sum signal is fairly constant over the usable beam width of the antenna while the difference signals have sharp null along Boresight axis of the antenna. Deeper null signifies higher sensitivity and precision of summary of location of feed elements or horns about the axis and thus determine the accuracy of the tracking.

35. The phase commutation unit amplitude modulates the sum signal with difference signals such that the three signals are reduced to one. This is accomplished without data degradation by providing all switching in the difference channels. In the tracking converter, Azimuth and Elevation demodulators convert the tracking video into dc error signals that are connected to the servo system. The scan code generator provides switching signals to the phase commutation unit and also to tracking demodulator. The tracking video and demodulator reference signals are synchronised and hence only AM tracking video produces dc error signal. The demodulator outputs are fed to servo control system which cause Antenna pedestal to drive the antenna axis towards the target.

36. Antenna Control System. The tracking of satellite is performed by varying the position of two axes of the tracking antenna, which are orthogonal. The drive for each axis uses DC servo motors with very low inertia which are coupled to antenna axis through separate gear boxes and is energised by fully reversible solid state SCR power amplifiers. Inherent backlash in the gearboxes is eliminated using two motors per axis and employing counter torque arrangement. The speed of the motors can be continuously varied with an accuracy of better than 1% of rated speed employing velocity feedback. The system is designed with appropriate compensation circuits to have desired frequency response and stability.

37. Description of Servo System. The servo system is a selectable single or double integration (Type-I or type-II) system. For each axis this broadly comprises of the following sub systems:-

(a) Motor drive with SCR amplifier

(b) Velocity loop with torque bias

(c) Position loop

(d) Angular display and positioning system.

Apart from this necessary protection interlocks, limit and related control circuit is also provided.

38. Drive and Scramplifiers. The servo drive for each axis utilises two permanent magnet DC servomotors driving a common bull gear through separate gearboxes. Torque capabilities of the motors selected comply with the maximum torque required under maximum allowable wind condition of 80kmfhour. Motors are capable to operate up to 0.2% of the rated speeds and can be over loaded two and half times for a duration of two minutes. Each motor is armature controlled with fully

reversible SCR bridge amplifiers. The SCR power amplifier is single phase, full wave controlled bridge rectifier, employing eight SCRs per bridge, for reversible operation. All SCRs are triggered using pulse train generated as per the amplitude and direction of errors. Chokes are connected in series with the SCR bridge to keep circulating currents within safe limits.

39. Velocity Loop. Velocity loop is closed with tacho feed back and with an integration to provide adequate stiffness to the system. The velocity loop achieves speed stability, acceleration of proper magnitude and smoothness of desired response. The rate loop encloses both the current loops and the input signal, which is fed from the position loop compensation networks in all modes except in rate (slew) mode. The function of current loop is to produce and control currents in the motors as per load demand and clamp at the desired values. Two types of current clamps are provided one continuous acting type for peak current and other for steady state current. Torque biasing is done for two motors of each axis such that in cases of zero input, both motors are opposing each other, thus making backlash zero. At higher speeds both motors aid each other. The torque bias circuitry also performs velocity and torque synchronization to take care of instantaneous difference of speeds of two drive trains and in case of over loads when torque coupling may slip. 40. Position Loop. Position loop encloses the velocity loop and consists of compensation networks, rate loop and antenna structure. The compensation networks include integrator, type I and type 11 servo networks. The position loop receives error signals from tracking controller (Demodulator), synchro demodulator or programme track unit when the system is operated in auto track, manual and programme track mode respectively. For AZ axis a secant correction is incorporated in the position loop to compensate Azimuth gain variation that results from changing Elevation angle of antenna dish in the auto track mode. In the auto track mode the antenna follows the position of the satellite nullifying the error signals generated by the tracking receiver. In the manual mode the position of Antenna can be controlled by rotation of hand wheels causing rotation of synchro transmitter in the manual position, thus resulting in misalignment of rotor position of this with that of synchro control transformer coupled to antenna. Error voltages thus produced are demodulated and fed to input of position loop.

41. Indication Loop. The indication system provides indication of angular position, for both axes. Digital indications of positions are C, directly obtained by synchro to digital converters and display circuits. Analog indications of positions are also provided. Real time monitoring, of critical parameters of servo control systems is also provided wherein values of auto errors, tacho voltages, and angular positions of system are stored in a PC for analysing any malfunction later. Other back-up modes are also provided for the system to be more reliable and easy to operate. Here, if target is lost after acquisition, system is put to Position memory mode in which antenna is driven to the last position where target was

seen. If target is lost after some time of tracking,, the system shall be transferred to rate memory mode, wherein both the axes shall continue to move with speeds which were existing prior to loss of target. If target appears, within 30 seconds then system is again placed to auto track mode or else it is put back to ready mode or manual mode for operators intervention.

Meteorological Satellite Instrumentation

Evolution of Meteorological Satellites

42. The first successful photographs of cloud systems of the earth from space can be traced to those obtained in 1947 using a World War II converted V2 rocket from an altitude of about 140 km. The first "Meteorological Instrument" was the first Earth Radiation Experiment launched on 13 Oct 58 by Verne Suomi of University of Wisconsin. The era of satellite meteorology truly began with the launch of TIROS-1 on 01 Apr 60. There has been a continuous record of improvement in the satellite sensing systems since that time. Television pictures taken by the spinning TIROS-1 in a near-earth (500 km height) non sub-synchronous orbit for the first time showed the organization of weather systems with a clarity never before seen.

43. In the ensuing years many new types of instruments have been developed with improved camera systems, wide field radiometers, scanning radiometers, quantitative atmospheric sounders, data collection platforms and data relay systems. Many instruments contained multispectral bands over a broad range of frequencies, including the near and far infrared and microwave parts of the electromagnetic spectrum. Two basic types of orbits emerged; polar orbits (sun synchronous, near earth) and geostationary; and now a third the low inclination (to observe the world Tropics). The Russians had the highly eccentric orbits, which essentially provide a geosynchronous type of coverage at high latitudes.

44. In contrast to Remote Sensing satellites, the meteorological satellites have poorer horizontal resolution (km compared to meters), but have much higher swath and better repetivity. Radiometrically accurate imaging capabilities have allowed the applications to spread from old imaging device to atmospheric sounders and active instruments. Spinscan, 3-axis stabilised, conical scanning, push-broom charged coupled device (CCD), along track, limb scanning etc have been some of the ways of using the instruments for collecting the data over a larger area and with specific utility of atmospheric corrections. In fact it can be said that satellites have given a great fillip to the numerical weather prediction (NWP) activity by providing quantitative data all over the globe at frequent intervals. It was again Verne Suomi (who can be called the father of Satellite Meteorology) who suggested that a spinning geostationary satellite would offer an ideal system for taking cloud pictures form, which

wind speed could be estimated. Even in a short notice he provided a spin scan radiometer on ATS-1 flown in 07 Dec 66.

45. Because of the image animation possibilities, the applications of qualitative weather analysis have shifted from the polar orbiting to geostationary satellites. The former provides global coverage in 12 hours with nearly 200-km swath, while the latter provides continuous data over a location (coverage about 70 deg. around the sub-satellite point).

46. All weather observing capability has been achieved with the development of microwave sensors. The passive radiance observations are now being supplemented by active sensors, which measure the returned signal from clouds, land, and sea surface or through rain.

47. USA, USSR, Japan, ESA, China and India have been the countries to have launched meteorological satellites. The US operational satellites have included TIROS, ESSA, NOAA and GOES series of satellites. Besides they had the highly successful Nimbus experimental Metsat series, Defence Metsat Programme and Seasat, UARS, ERB type of individual satellites. The USSR has launched a number of meteorological instruments in the COSMOS programme and Meteor series of operational meteorological satellites. The ESA, Japan and India have provided basically geostationary satellites, which together with US GOES are providing world-wide tropical coverage for winds at two levels of the atmosphere.

48. The operational sun-synchronous twin - NOAA satellite system are providing worldwide cover four times a day. They give imagery at a very high resolution - 1 km - in five spectral bands in visible, near, mid and thermal infrared. They also provide atmospheric temperature and humidity sounding using a 20 channel (-20 km resolution) High Resolution Infrared Sounder (HIRS), 4 channel (-100 km resolution) Microwave Sounding Unit (MSU) and 3 channel pressure Stratospheric Sounding Unit (SSU). The five geostationary satellites and the NOAA polar orbiting satellites are today the workhorse for the operational meteorological services the world over. The Man-computer Interactive Data Access System (McIDAS), another brain-child of Suomi, has provided a new analysis tool for handling and display of a large amount of data from a variety of sources.

Instrumentation in Operational Geostationary Satellites

49. Indian Met Satellites in Operation. The current operational satellites of India and their pay loads are given in the following paragraphs.

50. Kalpana-1. Kalpana-1(Fig.16), an exclusive meteorological, geostationary satellite was launched by an upgraded, four-stage PSLV-C4 rocket from Sriharikota, at 10:24 UTC on September 12, 2002. The triaxially-stabilized, 1,050 kg (including 560 kg of propellant), 550 W satellite carries a VHRR (Very High Resolution Radiometer) scanning radiometer for three-band images: one in the visible, the second in the thermal infrared and the third in the water vapor infrared bands, each at a spatial resolution of 2-km x 2-km resolution, to obtain atmospheric cloud cover, water vapor and temperature. It carries also a Data Relay Transponder (DRT) to provide data from fixed/mobile ground level weather platforms. It was maneuvered from the transfer orbit to a geostationary at 37 degrees E longitude on 16 September, and then to the final parking at 74 degrees E longitude on 24 September. Initially known as METSAT-1, the satellite was renamed as KALPANA-1 on February 5, 2003, in honour of the late Indian-born American astronaut, Kalpana Chawla, who died in the Columbia Space shuttle accident.

51. INSAT-3A. INSAT-3A (Fig.17) is a multipurpose satellite for providing telecommunications, television broadcasting, meteorological and search & rescue services. It carries twenty four transponders _ twelve operating in the normal C-band frequency, six in Extended C-band and six in Ku-band. Nine of the twelve normal C-band transponders provide expanded coverage and the remaining three have India coverage beam. All the extended C-band as well as the Ku-band transponders have India coverage beams. INSAT-3A also carries a Ku-band beacon. INSAT-3A also carries another transponder for Satellite Aided Search and Rescue (SAS & R) as part of India's contribution to the international Satellite Aided Search and Rescue programme. INSAT-3A is launched by European Ariane-5

Fig.16. Kalpana1

Launch Vehicle into a Geosynchronous Transfer Orbit (GTO) with a perigee of 200 km and an apogee of 35,980 km. The satellite is manoeuvered to its final orbit by firing the satellite's apogee motor. Subsequently, the deployment of solar array, antennae and the solar sail is carried out and the satellite is commissioned after in-orbit checkout. INSAT-3A is the third satellite in the INSAT-3 series. INSAT-3B and INSAT-3C were launched in March 2000 and January 2002 respectively by the Ariane launch vehicle and both the satellites are now providing regular service. INSAT-3A is to be located at 93.5 deg east longitude in the geostationary orbit

.

52. INSAT-3A carries the following Meteorological Instruments:-

(a) A Very High Resolution Radiometer (VHRR) with imaging capability in the Visible (0.55-0.75 microns), Thermal Infrared (10.5-12.5 microns) and Water Vapour (5.7-7.1 microns) channels, providing 2x2 km, 8x8 km and 8x8 km ground resolution respectively.

(b) A Charge Coupled Device (CCD) camera in the Visible (0.63-0.69 micron), Near Infrared (0.77-0.86 micron) and Shortwave Infrared (1.55-1.70 micron) bands providing 1x1 km ground resolution.

53 INSAT-3D. India has launched an advanced satellite, INSAT-3D (Fig.18) dedicated to weather forecasting on 26 Jul 13. The satellite has a six channel imager and 19 channel sounder. “This satellite is almost similar to GOES satellites of US. The INSAT-3D data provides necessary quantitative products in addition to earth imagery in six channels need for making quality weather forecast. The new advanced satellite provides vertical profiles of temperature and humidity, atmospheric motion vectors, outgoing longwave radiation, quantitative

Fig.17. INSAT – 3A

precipitation estimates, sea surface temperature, Himalayan snow cover, snow depth, data on fire, smoke and aerosol. It will also monitor upper troposphere humidity, ozone layer, fog, vegetation index, flash flood. All these data are being used in weather forecasting by conventional methods and most of it is assimilated in numerical weather prediction models to achieve greater accuracy.

54. The channel description and purpose of the 6 Channel Imager and 19 channel sounders are listed in Table 1 and Table 2 respectively. The geophysical parameters derived from INSAT – 3D are given in Table 3.

Table 1. Characteristics of 6 channel Imager& Applications of INSAT-3D

Payload Spectral Bands (μm) Resolution (km) Application

6 Channel Imager

Visible : 0.55-0.75Short Wave IR : 1.55 - 1.70Mid Wave IR : 3.70 - 3.95Water Vapour : 6:50 - 7.10 Thermal IR - 1 : 10.30 - 11.30Thermal IR - 2 : 11.30- 12.50

1 km1 km4 km8 km4 km8 km

Cloud characterization

Meso-scale processes

Fig.18. INSAT-3D

Table.2. Description and purpose of the 19 channel sounders of INSAT-3D

Table.3. Geophysical Parameters to be derived from INSAT –3D

INSAT-3D Sounder Channels Characteristics

Detector Ch. No.

c

(m)c

(cm-1)

NET@300K

Principal absorbing

gasPurpose

Long wave

1 14.67 682 0.17 CO2

Stratosphere temperature

2 14.32 699 0.16 CO2

Tropopause temperature

3 14.04 712 0.15 CO2

Upper-level temperature

4 13.64 733 0.12 CO2

Mid-level temperature

5 13.32 751 0.12 CO2

Low-level temperature

6 12.62 793 0.07water vapor

Total precipitable water

7 11.99 834 0.05water vapor

Surface temp., moisture

8 11.04 906 0.05 window Surface temperature

9 9.72 1029 0.10 ozone Total ozone

10 7.44 1344 0.05water vapor

Low-level moisture

11 7.03 1422 0.05water vapor

Mid-level moisture

12 6.53 1531 0.10water vapor

Upper-level moisture

13 4.58 2184 0.05 N2O Low-level temperature

14 4.53 2209 0.05 N2O Mid-level temperature

15 4.46 2241 0.05 CO2

Upper-level temperature

16 4.13 2420 0.05 CO2

Boundary-level temp.

17 3.98 2510 0.05 window Surface temperature

18 3.76 2658 0.05 window Surface temp., moisture

19 0.695 14367 - visible Cloud

55. MTSAT. The Multi-functional Transport Satellite (MTSAT) series (Fig.19) fulfills a meteorological function for the Japan Meteorological Agency and an aviation control function for the Civil Aviation Bureau of the Ministry of Land, Infrastructure and Transport. The MTSAT series succeeds the Geostationary Meteorological Satellite (GMS) series as the next generation of satellites covering East Asia and the Western Pacific. To improve meteorological services over a wide field of activity (such as weather forecasts, natural-disaster countermeasures and securing safe transportation), the MTSAT series replaced the GMS series that had been in operation since 1977. It has taken over the role of the GMS series, covering East Asia and the Western Pacific region from 140 degrees east above the equator. The Operation plan of MTSAT-1R (also known as Himawar-8) and MTSAT-2 (Himawar-9) is shown in Fig 20.

Fig.19. MTSAT-1R

       

Fig 20.The operation plan of the MTSAT-1R and MTSAT-2

56. Observation by the MTSAT. MTSAT provides information to 27 countries and territories in the region, including 1) imagery for monitoring the distribution/motion of clouds, 2) sea surface temperatures and 3) distribution of water vapor. The MTSAT series provides imagery for the Northern Hemisphere every 30 minutes in contrast to the previous hourly rate, enabling JMA to more closely monitor typhoon and cloud movement. The MTSAT series carries a new imager with a new infrared channel (IR4) in addition to the four channels (VIS, IR1, IR2 and IR3) of the GMS-5. MTSAT imagery is more effective than GMS-5 imagery in detecting low-level cloud/fog and estimating sea surface temperatures at night and has enhanced brightness levels, enabling a whole new level of image imagery. By further computation of cloud imagery, data obtained by MTSAT's observations can be used to 1) in calculate wind data for numerical weather prediction, 2) make nephanalysis charts and 3) analyze the distribution of cloud amounts according to area.

57. Major characteristics of MTSAT series is given in Table 4.

Table 4: Characteristics of the MTSAT series and meteorological payload

Position Geostationary orbit 35,800 km above the equator at 135 degrees east longitude, 140 degrees east longitude (operational position for meteorological function) or 145 degrees east longitude

Attitude control

Three-axis stabilization(system to control roll, pitch and yaw using thrusters or momentum wheels)

Designed lifetime

5 years for the meteorological function, 10 years for the aviation function

Channel andwavelength (micrometers)

VIS0.55 - 0.90

IR110.3 - 11.3

IR211.5 - 12.5

IR36.5 - 7.0

IR43.5 - 4.0

Resolution 1 km (VIS) and 4 km (IR) at the sub-satellite pointBrightness level 10 bits for both VIS and IR channels (1,024 gradations)

FrequencyS-band (reception: 2026 - 2035 MHz, transmission: 1677 - 1695 MHz)UHF (reception: 402 MHz, transmission: 468 MHz)

58. FY( Even No. Series) Satellite.Commission of Science Technology and Industry for National Defense held a handover ceremony on July 14 for China's first Earth Observation geostationary operational meteorological satellite successfully developed and launched by China Aerospace Science and Technology Corporation. After trial operation for six months, the satellite is officially delivered to China Meteorological Administration (CMA) for full operation. Speaking at the ceremony, Sun Laiyan, deputy director of Commission of Science Technology and Industry for National Defense and director general of the China National Space Administration (CNSA), said that the successful delivery marks a great leap forward of Chinese aerospace industry on its way to service-oriented practice from application and experiment based practice. Zhang Qingwei, managing director of China Aerospace Science and Technology Corporation, noted that FY - 2C is the third geostationary meteorological satellite China has ever launched. It employs the self-spin stabilization method and has a designed service life of three years. The remote sensing instruments onboard the satellite could perform the 36,000-km high-altitude observation over the earth, which is of high time resolution and has an advantage in dynamically monitoring the disastrous weather caused by meso- and micro-scale weather system. Meanwhile, FY - 2C is also good at obtaining and transmitting data, enabling continuous meteorological monitoring over the earth. Qin Dahe, director of CMA, note that China has so far established polar orbit and stationary operation series of meteorological satellites, among which the FY meteorological satellites have all been included into the World Meteorological Organisation Satellite Observation Network, with China's meteorological satellites to be the key members.

59. Four-band HgCdTe infrared (IR) detector is developed for the first operational Chinese geostationary meteorological satellite FY-2C launched on October 19, 2004. As the Visible Infrared Spin-Scan Radiometer (VISSR) is the primary payload on FY-2C, the IR detector is one of the most important modules for such an imaging instrument. Compared with its predecessors FY-2A and FY-2B (experimental models, launched in June 1997 anJune 2000 respectd ively), the detector used in FY-2C is quite different in band selection and detector package. The four band IR detector for FY-2C application consists of four photoconductive (PC) detector chips made of Hg1-xCdxTe with different compositions x, corresponding to the wavelengths of 3.5 to 4.0μm, 6.3 to 7.6μm, 10.3 to 11.3μm and 11.5 to 12.5μm respectively. Four cooled IR filters are included in one detector package, which enables us to simplify the system without any IR beam splitters and IR filters outside the detector for

defining separate bands. The IR detector operates at radiative cooler temperature ranging from 92 to 102K.

60. According to the plan for the Chinese meteorological satellite development approved by the State Council, currently FY – 2C, FY-2D, FY-2E and FY-2F are operational and FY-4 series satellites are to be launched during 205-2019. The research and development for FY-3, the second generation of Chinese polar-orbiting meteorological satellite, is on the way. The application system for FY-3 will be designed and constructed to meet the requirement of FY-3 satellite, in order to receive, process and distribute data of this new satellite. Characteristics of FY-2C satellite is given in Table.5.

Table.5: Characteristics of FY-2C satellite

 CharacteristicsFY-2 C, D, E

Vis IR1 IR2 IR3 WV

Wavelength (mm) 0.5-0.75 10.3-11.3 11.5-12.5 3.5-4.0 6.3-7.6

Spatial resolution (km) 1.25 5 5 5 5

Temperature resolution

S/N 1.5 (0.5%) 0.4-0.2K 0.4-0.2K 0.5-0.3K 0.6-0.5K

Number of detectors 4 + 4

1(main) +

1(alternate)

1(main) +

1(alternate)

1(main) +

1(alternate)

1(main) + 1(alternate

)

Quantizing level 64 1024 1024 1024 256

61. Meteosat Second Generation (MSG) Satellites. The Meteosat Second Generation satellites ( MSG-7 to MSG-10) (Fig.21) have been designed to take advantage of new technology and to improve on the already successful and proven design of the original Meteosat satellites. The SEVIRI radiometer on-board the MSG satellite has a total of 12 channels that generate images by scanning the Earth every 15 minutes. The High Resolution Visible channel provides data at 1km sampling, the other channels sample at 3km. In addition to the main SEVIRI payload the satellite carries an instrument for the measurement of terrestrial radiation (GERB), telecommunications equipment for the dissemination of

processed imagery and products, as well as components for the reception and relay of distress messages for search and rescue (GEOSAR).

Fig.21. MSG satellite

62. The main and Technical Characteristics of MSG satellites are in table 6.

Table.6: Technical Characteristics of MSG satellites

Main CharactersticsLaunch Date MSG-1: 28 August 2002

MSG-2 : 21 December 2005Spacecraft Mass 2000 KgActive Lifetime 7 YearsLauncher Ariane 4, Ariane 5Developer EUMETSATOrbit Type Polar OrbitingTechnical Characterstics:Imaging Cycle 15 minChannels

VisibleHRVVIS 0.6VIS 0.8IR 1.6

Water Vapour WV 6.2, WV 17.3

IR WindowIR 3.9IR 8.7IR 10.8IR 12.0

Air Mass Analysis IR 9.7 + WVIR 13.4

Sampling Distance 1 km (HRV)3 km (others)

Number of Detectors 42

Telescope Diameter 500 mmScanning Principle Scan mirrorData Transmission Rates :Raw Data Transmission Rate 3.2 MbsDisseminated Image 1 Mbs (HRIT)

128 Kbs (LRIT)

63. Meteor-3 and GOMS. The meteorological equipments onboard Meteor-3 and Geostationary Operational Meteorological Satellite (GOMS) – Russia are shown in the Table 7 and 8 respectively. In the framework of national weather satellite systems modernization the efforts are focused on the development and manufacturing the next generation of polar-orbiting

Table.7. Meteorological equipments onboard Meteor-3

INSTRUMENT

SPECTRAL

BAND

(mm)

GROUND RESOLUTION

(km)

SWATH WIDTH

(km)

OPERATING SCHEDULE

Scanning TV-sensor with on-board data recording system for global coverage mode

0.5-0.8 0.7 x 1.4 3100Recording, direct transmission

Scanning TV-sensor for automatic data transmission mode

0.5-0.8 1 x 2 2600 Direct transmission

IR-radiometer for global coverage and direct data transmission modes

10.5-12.5 3 x 3 3100Recording, direct transmission

Scanning 10-channel IR-radiometer

9.65-18.7 35 x 35 400Recording, direct transmission

Table.8. Meteorological equipments onboard GOMS

CHANNEL VISIBLE INFRARED-1INFRARED-II

(from GOMS-2)

Wavelength (mm) 0.46-0.7 10.5-12.5 6.0-7.0

Resolution at sub-

satellite point (km)1.25 6.25 6.25

64. GOES. GOES is a major component of the NOAA/National Weather Service modernization program and represents a significant advance in geostationary remote sensing that has been under development for the last decade. All major components of the GOES-12 to 15 are new and greatly improved. The channel specification of GOES satellites is given in Table. 9.

Table.9. Channel specifications of GOES satellites

Channel No.

Satellite (GOES)

Wave length (mm) Objective

Resolution (km)

1 I,J,K,L,M 0.55-0.75 Day cloud cover 1

2 I,J,K,L.M 3.8-4.0 Night time cloud 4

3I,J,K,L,M

6.5-7.0

13.0-13.7

Water vapour

Cloud cover

8

4

4 I,J,K,L,M 10.2-11.2 SST/Cloud cover 4

5 I,J,K,L,M11.5-12.5

5.8-7.3

SST/Cloud cover

Water vapour

4

8

*19 channel sounder, IGFOV 10 km

65. GOES Sounder. The GOES-sounder measures emitted radiation in 19 thermal infrared bands that are sensitive to temperature, moisture, and ozone, and reflected solar radiation in one visible band. The "footprint" or spatial resolution of the Earth sampling is 8 km, with 13 bit data transmitted to the GOES receiving facilities. The first soundings from GOES-8 on 6 June 1994 were of excellent quality. The full time availability of the GOES-8/M sounder enables operational sounding products for the first time. Shortwave bands ( near 4 um) enhance lower troposphere vertical resolution, an accurate "split window" provides atmosphere -corrected temperature measurements of the earth surface, and three moisture-sensing bands improve the vertical resolution for moisture sounding. The specification of GOES channels is given in Table 10.

Table.10: GOES-Sounder Channels

Long Wave IR Medium Wave IR Short Wave IR Visible 19

Channel Number

s

Wave Length (µm)

Channel Number

s

Wave Length (µm)

Channel Number

s

Wave Length (µm)

Channel Number

s

Wave Length (µm)

1234567

14.7114.3714.0613.6413.3712.6612.02

89101112

11.039.717.437.026.51

131415161718

4.574.524.454.133.983.74

19 0.70

66. GOES Imager . Presently GOES-12 to GOES-15 are in orbit. The GOES imager had a five band multispectral capability with 10 bit precision and high spatial resolution (with a sixth band available on GOES-12(M), replacing band #5):

(a) 0.52 - 0.72 um (visible) at 1 km, useful for cloud, pollution, and haze detection and severe storm identification;

(b) 3.78 - 4.03 um (shortwave infrared window) at 4 km, useful for identification of fog at night, discriminating between water clouds

and snow or ice clouds during the daytime, detecting fires and volcanoes, and nighttime determination of sea surface temperature;

(c) 6.47 - 7.02 um (upper level water vapor) at "4" km, useful for estimating regions of mid-level moisture content and advection, and tracking mid-level atmospheric motions;

(d) 10.2 - 11.2 um (long wave infrared window) at 4 km, familiar to most users for cloud-drift winds, severe storm identification, and location of heavy rainfall; and,

(e) 11.5 - 12.5 um (infrared window more sensitive to water vapor) at 4 km, useful for identification of low-level moisture, determination of sea surface temperature, and detection of airborne dust and volcanic ash. 12.9 - 13.8 um (lower level (CO2) temperature) at 8 km, useful for determination of cloud characteristics (such as cloud top pressure).

Instrumentation in Operational Polar Orbiting Satellites

67. The current operational satellites of India and other countries and their pay loads are given in the following paragraphs.68. Megha-Tropiques. Megha-Tropiques ('Megha' in Sanskrit language means clouds and 'Tropiques' is the  French word for tropics) is the name given to a 500 kg satellite to be launched from India under a CNES (French Space Agency) and ISRO collaboration project by 2009. This mission is an experimental mission without plan for operational follow-up. In order to be able to study time scales from the scale of the large convective events to inter-annual variations, the duration of the mission is expected to be 3 years. It will be the first dedicated satellite devoted to atmospheric and climate research in the tropics. The Satellite is planned to be launched using ISRO's Polar Satellite Launch Vehicle (PSLV) from Sriharikota range.  The satellite would carry multi frequency microwave and optical sensors to provide information on convective and  cloud physical processes, water vapour and rainfall characteristics over ocean and land surfaces, monsoon radiation budget etc. The satellite will have a unique 20 degree inclined orbit of 867 km so as to provide maximum coverage of the tropical region of interest. . Changes in energy and water budget of the land-ocean-atmosphere systems in the tropics influence the global climate to a great extent and Megha-Tropiques could provide the answer. The information will supplement and complement the data received from other Indian and global geo-stationary and polar orbiting satellites. The satellite data will be utilised by well identified scientists and

their groups in both the countries. Megha Tropiques is expected to carry a multi-frequency microwave scanning radiometer (MADRAS), a multi-channel microwave instrument (SAPHIR) providing humidity profile of the atmosphere and a multi-channel radiation instrument, ScaRaB, providing data on the earth's radiation budget. The key of this mission is the repetitivity of the measurement in the Tropics. The orbit of the platform must be in a low inclination on the equatorial plane. The altitude of the orbit has to be high enough to allow a wide swath of the instruments.

69. The Objectives of launching Megha-Tropiques are as follows:-

(a) To improve the knowledge of the water cycle in the intertropical region, to evaluate its consequences on the energy budget,

(b) To study the life cycle of tropical convective systems over ocean and continents, the environmental conditions for their appearance and evolution, their water budget, and the associated transports of water vapor.

(c) To provide data about the processes leading to dramatic weather events affecting the Tropical countries, as hurricanes, systems producing heavy rainfalls, processes governing monsoons variability or droughts.

70. The mission would help retrieve Water vapor (integrated and vertical distribution), cloud condensed water content, ice/water, and precipitation, solar reflected and terrestrial emitted fluxes at the top of the atmosphere.

71. The main payload instruments are as follows;-:

(a) MADRAS. It is a microwave imager, with conical scanning (incidence angle 56°), close from the SSM/I and TMI concepts. The main aim of the mission being the study of cloud systems, a frequency has been added (150 Ghz) in order to study the high level ice clouds associated with the convective systems, and to serve as a window channel relative to the sounding instrument at 183 GHz. The main characteristics of the MADRAS channels are given in Table 11.

Table.11. Characteristics of MADRAS Channels

Frequencies Polarization

Pixel size Main use

18.7 Ghz ± 100 Mhz H + V 40 km Ocean rain and surface wind

23.8 Ghz ± 200 Mhz V 40 km Integrated water vapor

36.5 Ghz ± 500 Mhz H + V 40 km Cloud liquid water

89 Ghz ± 1350 Mhz H + V 10 km Convective rain areas

157 Ghz ± 1350 Mhz H + V 6 km Cloud top ice

72. The main uses given here are only indicative, most of the products being extracted from algorithms combining the different channels information. The resolutions are those expected in the different channels, accounting for the specification of 10 km given for the 89 Ghz channel.

(b) SAPHIR. It is a sounding instrument with 6 channels near the absorption band of water vapor at 183 Ghz. These channels provide relatively narrow weighting functions from the surface to about 10 km, allowing retrieving water vapor profiles in the cloud free troposphere. The scanning is cross-track, up to an incidence angle of 50°. The resolution at nadir is of 10 km.

(c) ScaRaB. ScaRaB is a scanning radiative budget instrument, which has already been launched twice on Russian satellites. The basic measurements of ScaRaB are the radiances in two wide channels, a solar channel (0.2 - 4 µm), and a total channel (0.2 - 200 µm)(Table.12), allowing to derive long wave radiances. The resolution at nadir will be 40 km from an orbit at 870 km. The procedures of calibration and processing of the data in order to derive fluxes from the original radiances have been set up and tested by CNES and LMD.

Table.12. he ScaRaB Channels. Channel No. Channel Name Wavelengths

1 Visible 0.5 to 0.7 µm2 Solar (Short Waves) 0.2 to 4 µm3 Total 0.2 to 200µm4 Infra-Red 10.5 to 12.5 µm

73. As the key of this mission is the repetitivity of the measurement in the Tropics. One has to combine the choice of the inclination of the orbit, the scanning capability of the instruments and the height of the orbit. The limitation of the swath is determined mainly by the microwave imager, which has a conical swath. Simulations have shown that it was possible to obtain a repetitivity of more than 3.5 visibilities per day of each point of the zone situated between 22°S and 22°N for an orbit inclination of 20° at 870-km height. The repetitivity reaches more than 5 per day around 13°N and 13°S. The Megha-tropiques orbit for a 1-day period and general configuration of the swath of the three instruments are shown in Fig 22 and 23 respectively

Fig. 22. Megha-Tropiques orbit for a 1-day period.

 MADRAS

Fig.23. General configuration of the swath of the three instruments of Megha-Tropiques. Size of the footprints has been enhanced in order to show their geometric behaviour.

74. Oceansat- II. OCEANSAT- II (Fig 24) is envisaged to continue the services of OCEANSAT-1. It will carry two payloads for ocean related studies, namely, Ocean Colour Monitor (OCM) and SCAT a Ku-band Pencil

Beam Scatterometer. An additional piggy-back payload called ROSA (Radio Occultation Sounder for Atmospheric studies) developed by the Italian Space Agency (ASI) is also proposed to be included. The major applications of data from OCEANSAT- II are identification of potential fishing zones, sea state forecasting, coastal zone studies and inputs for weather forecasting and climatic studies. OCEANSAT- II is planned for launch by PSLV in 2009. It will be launched into a near polar sun-synchronous orbit of 720 km with equatorial crossing time of 12 noon. The repetitivity achievable will be two days.

Fig.24. Ocean Sat- II

(a) OCM is a 8-band multi-spectral camera operating in the Visible – Near IR spectral range ( 400-900 nm). This camera provides an instantaneous geometric field of view of 360 meter and a swath of 1420 km. OCM can be tilted up to + 20 degree along track.

(b) SCAT (Scanning Scatterometer), an active microwave device designed and developed at ISRO/SAC, Ahmedabad. The objective is to monitor ocean surface wind speed and directions. The instrument is a pencil beam wind scatterometer operating at Ku-band of 13.515 GHz. SCAT is being utilized for the estimation of the radar backscattered power and subsequent local and global wind vector (velocity magnitude and direction) retrieval over the ocean, from the normalized radar cross-section (o), for cell resolution grids of 25 km x 25 km over a swath of 1400 km. The aim is to provide global ocean coverage and wind vector retrieval with a revisit time of 2 days. The specifications of the scatterometer are shown in Fig 25.

(c) ROSA (Radio Occultation Sounder for Atmospheric Studies), a new GPS occultation receiver provided by ASI (Italian Space Agency). The objective is to characterize the lower atmosphere and the ionosphere, opening the possibilities for the development of several scientific activities exploiting these new radio occultation data sets.

Meteorological Satellites from Other Countries Satellites

NOAA Satellites

75. The United States has the NOAA series of polar orbiting meteorological satellites, presently NOAA 17, NOAA 18 and NOAA 19 as primary spacecraft, NOAA 15 and NOAA 16 as secondary spacecraft, NOAA 14 in standby, and NOAA 12. NOAA-18 is a weather forecasting satellite run by NOAA. It was launched on 20 May 2005, and is currently operational, in a sun-synchronous orbit, 854km above the Earth, orbiting every 102 minutes. It hosts the AMSU-A, MHS, AVHRR and High Resolution Infrared Radiation Sounder (HIRS) instruments. It is the first NOAA POES satellite to use MHS in place of AMSU-B.

76. AVHRR Instrument Characteristics. The NOAA-POES series are regarded as the backbone of the US meteorological program. Image data are being recorded by the "Advanced Very High Resolution Radiometer (AVHRR)", a sensor operating onboard of the NOAA - POES series (Polar-Orbiting Operational Environmental Satellites;(Fig.26).  The current POES series satellites are named simply NOAA-9 through NOAA-19 in order of launch date. The program has evolved over several years starting in 1960

Fig.25. Scat on board Oceansat- II

with TIROS. The philosophy of NOAA is to maintain at least two operational satellites in complementary orbit.

Fig.26. The NOAA satellite with AVHRR on board

77. AVHRR is a five channel scanning radiometer in visible, near infra-red and infra-red wavelengths for analysis of hydrological, oceanographic and meteorological parameters such as vegetation index (ie greenness), clouds, snow and ice cover and sea surface temperatures. Data are obtained by all the five channels with a resolution of 1 km. The digitised AVHRR data is transmitted from the satellite in real-time (High Resolution Picture Transmission or HRPT) as well as selectively recorded on board the satellite for subsequent playback when the satellite is in communication range of the ground control station. This high resolution data is called Local Area Coverage (LAC).AVHRR data is also sampled on real time to produce lower resolution Global Area Coverage (GAC) data. The effective resolution of GAC data is about 4 Km. The spectral characteristics and imaging application of AVHRR are given below in Table. 13.

Table.13. AVHRR Spectral Characteristics

Channel

Spectrum interval(μ

m)Resoluti

on Application

1 0.50 - 0.68 1.1 Cloud Mapping

2 1.58-1.64 1.1 Surface water Boundaries

3 3.55-3.93 1.1 Thermal mapping,cloud distribution fire detection

4 10.3-11.3 1.1 Cloud Distribution, SST, WV Correction

5 11.5-12.5 1.1 ----------------------do---------------------

78. All NOAA series satellites starting from NOAA15 onwards carry dedicated MW sounders and improved AVHRR with additional band at 1.6 mm. The major payloads are :-

(a) Advanced Very High Resolution Radiometer (AVHRR/3)(b) High Resolution IR radiation Sounder (HIRS/2)(c) Advanced Microwave Sounding Unit- A1 (AMSU-A1)(d) Advanced Microwave Sounding Unit - A2 (AMSU-A2)(e) Advanced Microwave Sounding Unit –(AMSU-B)(f) Solar Backscatter UV Radiometer (SBUV)

79. The Advanced Microwave Sounding Unit-A (AMSU-A) system is implemented in two separate modules: the AMSU-A1 and AMSU-A2. The AMSU-A is a multi-channel microwave radiometer that will be used for measuring global atmospheric temperature profiles and will provide information on atmospheric water in all of its forms (with the exception of small ice particles, which are transparent at microwave frequencies) from the NOAA KLM spacecraft.

80. AMSU-A is a cross-track, line-scanned instrument designed to measure scene radiances in 15 discrete frequency channels which permit the calculation of the vertical temperature profile from about 3 millibars (45 km) pressure height to the Earth's surface. At each channel frequency, the antenna beam-width is a constant 3.3 degrees (at the half power point). Thirty contiguous scene resolution cells are sampled in a stepped-scan fashion every eight seconds, each scan covering 50 degrees on each side of the sub satellite path. These scan patterns and geometric resolution translate to a 50 km diameter cell at nadir and a 2,343 km swath width from the 833 km nominal orbital altitude.

81. The Advanced Microwave Sounding Unit-B (AMSU-B) is a 5 channel microwave radiometer. The purpose of the instrument is to receive and measure radiation from a number of different layers of the atmosphere in order to obtain global data on humidity profiles. It works in conjunction with the AMSU-A instruments to provide a 20 channel microwave radiometer. AMSU-B covers channels 16 through 20. The highest channels: 18, 19 and 20, span the strongly opaque water vapor absorption line at 183 GHz and provide data on the atmosphere's humidity level. Channels 16 and 17, at 89 GHz and 150 GHz, respectively, enable deeper penetration through the atmosphere to the Earth's surface.

Tropical Rainfall Measuring Mission

82. The Tropical Rainfall Measuring Mission (TRMM) is a joint space mission between NASA and the Japan Aerospace Exploration Agency (JAXA) designed to monitor and study tropical rainfall. The term refers to both the mission itself and the satellite that the mission uses to collect data. TRMM is part of NASA's Mission to Planet Earth, a long-term, coordinated research effort to study the Earth as a global system. The satellite was launched on November 27, 1997 from the Tanegashima Space Center in Tanegashima, Japan. It is a low earth orbit satellite. The Scan Patterns of all the instruments of TRMM is shown in Fig. 27.

83. The instruments aboard the TRMM are:-

(a) Precipitation Radar (PR)

(b) TRMM Microwave Imager (TMI)

(c) Visible and Infrared Scanner (VIRS)

(d) Clouds and the Earth's Radiant Energy System (CERES)

(e) Lightning Imaging Sensor (LIS)

Fig.27. Scan Patterns of the instruments of TRMM

84. Precipitation Radar (PR). The Precipitation Radar was the first spaceborne instrument designed to provide three-dimensional maps of storm structure. These measurements yield invaluable information on the intensity and distribution of the rain, on the rain type, on the storm depth and on the height at which the snow melts into rain. The estimates of the heat released into the atmosphere at different heights based on these measurements can be used to improve models of the global atmospheric

circulation. The Precipitation Radar has a horizontal resolution at the ground of about 3.1 miles (five kilometers) and a swath width of 154 miles (247 kilometers). One of its most important features is its ability to provide vertical profiles of the rain and snow from the surface up to a height of about 12 miles (20 kilometers). The Precipitation Radar is able to detect fairly light rain rates down to about .027 inches (0.7 millimeters) per hour. At intense rain rates, where the attenuation effects can be strong, new methods of data processing have been developed that help correct for this effect. The Precipitation Radar is able to separate out rain echoes for vertical sample sizes of about 820 feet (250 meters) when looking straight down. It carries out all these measurements while using only 224 watts of electric power?the power of just a few household light bulbs. The Precipitation Radar was built by the National Space Development Agency (JAXA) of Japan as part of its contribution to the joint US/Japan Tropical Rainfall Measuring Mission (TRMM).

85. TRMM Microwave Imager (TMI). The Tropical Rainfall Measuring Mission?s (TRMM) Microwave Imager (TMI) is a passive microwave sensor designed to provide quantitative rainfall information over a wide swath under the TRMM satellite. By carefully measuring the minute amounts of microwave energy emitted by the Earth and its atmosphere, TMI is able to quantify the water vapor, the cloud water, and the rainfall intensity in the atmosphere. It is a relatively small instrument that consumes little power. This, combined with the wide swath and the good, quantitative information regarding rainfall make TMI the "workhorse" of the rain-measuring package on Tropical Rainfall Measuring Mission. TMI is not a new instrument. It is based on the design of the highly successful Special Sensor Microwave/Imager (SSM/I) which has been flying continuously on Defense Meteorological Satellites since 1987. The TMI measures the intensity of radiation at five separate frequencies: 10.7, 19.4, 21.3, 37, 85.5 GHz. These frequencies are similar to those of the SSM/I, except that TMI has the additional 10.7 GHz channel designed to provide a more-linear response for the high rainfall rates common in tropical rainfall. The other main improvement that is expected from TMI is due to the improved ground resolution. This improvement, however, is not the result of any instrument improvements, but rather a function of the lower altitude of TRMM 250 miles (402 kilometers) compared to 537 miles (860 kilometers) of SSM/I). TMI has a 547 mile (878-kilometer) wide swath on the surface. The higher resolution of TMI on TRMM, as well as the additional 10.7 GHz frequency, makes TMI a better instrument than its predecessors. The additional information supplied by the Precipitation Radar further helps to improve algorithms. The improved rainfall products over a wide swath will serve both TRMM as well as the continuing measurements being made by the SSM/I and radiometers flying on the NASAs EOS-PM and the Japanese ADEOS-II satellites.

86. Visible and Infrared Scanner (VIRS). The Visible and Infrared Scanner VIRS) is one of the primary instruments aboard the Tropical Rainfall Measuring Mission (TRMM) observatory. VIRS is one of the three instruments in the rain-measuring package and serves as a very indirect indicator of rainfall. It also ties in TRMM measurements with other measurements that are made routinely using the meteorological Polar Orbiting Environmental Satellites POES) and those that are made using the Geostationary Operational Environmental Satellites (GOES) operated by the United States. VIRS, as its name implies, senses radiation coming up from the Earth in five spectral regions, ranging from visible to infrared, or 0.63 to 12 micrometers. VIRS is included in the primary instrument package for two reasons. First is its ability to deliniate rainfall. The second, and even more important reason, is to serve as a transfer standard to other measurements that are made routinely using POES and GOES satellites. The intensity of the radiation in the various spectral regions (or bands) can be used to determine the brightness (visible and NIR) or temperature (infrared) of the source. If the sky is clear, the temperature will correspond to that of the surface of the Earth, and if there are clouds, the temperature will tend to be that of the cloud tops. Colder temperatures will produce greater intensities in the shorter wavelength bands, and warmer temperatures will produce greater intensities in the longer wavelength bands. Since colder clouds occur at higher altitudes the measured temperatures are useful as indicators of cloud heights, and the highest clouds can be associated with the presence of rain. A variety of techniques use the Infrared (IR) images to estimate precipitation. Higher cloud tops are positively correlated with precipitation for convective clouds (generally thunderstorms) which dominate tropical (and therefore global) precipitation accumulations. One notable exception to this rule of thumb are the high cirrus clouds that generally flow out of thunderstorms. These cirrus clouds are high and therefore "cold" in the infrared observations but they do not rain. To differentiate these cirrus clouds from water clouds (cumulonimbus), a technique which involves comparing the two infrared channels at 10.8 and 12.0 micrometers can be employed. Nonetheless, IR techniques usually have significant errors for instantaneous rainfall estimates. The strength of the IR observations lies in the ability to monitor the clouds continuously from geostationary altitude. By comparing the visible and infrared observations on the Tropical Rainfall Measuring Mission with the rainfall estimates of the TRMM Microwave Imager and Precipitation Radar, it is hoped that much more can be learned about the relationship of the cloud tops as seen from geostationary orbit. VIRS uses a rotating mirror to scan across the track of the TRMM

observatory, thus sweeping out a region 833 kilometers wide as the observatory proceeds along its orbit. Looking straight down (nadir), VIRS can pick out individual cloud features as small as 2.4 kilometers.

87. Clouds and the Earth's Radiant Energy System (CERES). CERES measures the energy at the top of the atmosphere, as well

as estimates energy levels within the atmosphere and at the Earth's surface. Using information from very high resolution cloud imaging instruments on the same spacecraft, CERES also will determine cloud properties, including cloud-amount, altitude, thickness, and the size of the cloud particles. All of these measurements are critical for understanding the Earth's total climate system and improving climate prediction models.

88. Lightning Imaging Sensor (LIS). The Lightning Imaging Sensor is a small, highly sophisticated instrument that detects and locates lightning over the tropical region of the globe. Looking down from a vantage point aboard the Tropical Rainfall Measuring Mission (TRMM) observatory, 250 miles (402 kilometers) above the Earth, the sensor provides information that could lead to future advanced lightning sensors capable of significantly improving weather "nowcasting." Using a vantage point in space, the Lightning Imaging Sensor promises to expand scientists' capabilities for surveying lightning and thunderstorm activity on a global scale. It will help pave the way for future geostationary lightning mappers. From their stationary position in orbit, these future lightning sensors would provide continuous coverage of the continental United States, nearby oceans and parts of Central America. Researchers hope that future sensors will deliver day and night lightning information to a forecaster's work-station within 30 seconds of occurrence providing an invaluable tool for storm "nowcasting" as well as for issuing severe storm warnings. The lightning detector is a compact combination of optical and electronic elements including a staring imager capable of locating and detecting lightning within individual storms. The imager's field of view allows the sensor to observe a point on the Earth or a cloud for 80 seconds, a sufficient time to estimate the flashing rate, which tells researchers whether a storm is growing or decaying. The sensor provides information on cloud characteristics, storm dynamics, and seasonal as well as yearly variability of thunderstorms.

GPM(Global Precipitation Measurement) Core Observatory

89. It is a joint mission between JAXA of Japan and NASA of USA. The satellite’s launch date was on 17 Feb 14. This will unify and advance precipitation measurements from space to provide next-generation global precipitation products within a consistent framework. It is an international satellite mission that will unify and advance precipitation

measurements from a constellation of microwave sensors for scientific research and societal applications. The Key advances include:-

(a) More accurate instantaneous precipitation information, especially light rain & solid precipitation

(b) Better space-time coverage through international partnership

(c) High spatial resolution (DPR & GMI on Core Observatory)

(d) Next-generation global precipitation products building on inter-calibrated constellation radiometric measurements and unified physical retrievals using a common observation-constrained hydrometeor database

The Earth Observing System Satellites

90. NASA planned a series of satellites known as the Earth Observing System (EOS) as the nucleus of a "Mission to Planet Earth." From high above Earth, EOS satellites would monitor land, sea and atmosphere for changes in the environment. In 1991, NASA upgraded the project to a more comprehensive program studying Earth as an environmental system. The space agency refers to it as the Earth Science Enterprise (ESE). By using satellites to examine Earth thoroughly, ESE expands human understanding of how natural processes affect human beings, and how human beings affect natural processes. Research scours such diverse resources as land surfaces; the waters, oceans, glaciers, and polar ice sheets; atmospheric chemistry and processes, and clouds; energy cycles; and ecosystem processes. Such research improves many things, including weather forecasts, predictions of climate changes, agricultural and forest management, and planning by diverse users such as fishermen and local governments. ESE has three components:

(a) a series of Earth-observing satellites,

(b) a data storage and retrieval system,

(c) teams of scientists to interpret the data.

91. Today, EOS is the centerpiece of NASA's Earth Science Enterprise. To help us understand Earth as an integrated system, EOS maintains a series of polar-orbiting and low inclination science satellites for long-term global observations of the land surface, biosphere, atmosphere, and

oceans. EOS is the first observing system to integrate measurements of Earth's processes.

(a) Terra, the first Earth Observing System satellite, was launched aboard an Atlas-Centaur IIAS expendable rocket from Vandenberg Air Force Base, California, at 1:57 p.m. EST on December 18, 1999.

(b) Aqua (formerly EOS PM-1), the second EOS satellite, was launched aboard a Delta II expendable rocket from Vandenberg Air Force Base, California, at 5:55 a.m. EDT on May 4, 2002. Aqua is studying Earth’s water cycle with an imaging spectrometer, two radiometers and infrared and microwave sounders.

(c) Aura (formerly EOS Chem), the third EOS satellite, was launched aboard a Delta II expendable rocket from Vandenberg Air Force Base, California, at 6:01 a.m. EDT on July 15, 2004. It s sounders measure air quality, stratospheric ozone and climate change.

(d) PARASOL (Polarization & Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) was launched aboard an Ariane 5 at 1626 GMT on December 18, 2004, from the European Space Agency's (ESA) launch complex ELA3 at Guiana Space Center at Kourou, French Guiana. PARASOL is a project of the French space agency Centre National d'Etudes Spatiales (CNES). Its radiometer studies the atmosphere by measuring the direction and polarization of light reflected by clouds and aerosols.

(e) CloudSat and CALIPSO were launched together aboard a Delta II expendable rocket from Vandenberg Air Force Base, California, in July 2005. CloudSat and CALIPSO complement each other in providing a 3-D view of clouds and aerosols showing how they form, evolve, and affect weather and climate. CloudSat is a U.S.-Canadian project using radar to measure thick ice and water in clouds. CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), a U.S.-French mission, has a lidar and infrared and visible imagers to measure thin clouds and aerosols over the globe.

92. The A-Train. A-Train (Fig.28) is a flotilla composed of the Aqua, Aura, CloudSat, PARASOL and CALIPSO satellites flying in formation in low polar orbits 438 miles (705 km) above Earth at an inclination of 98 degrees. Together, their overlapping science instruments give a comprehensive picture of Earth weather and climate. The satellites are referred to as the A-Train because the caravan has been said to resemble

a train of satellites flying around Earth. The satellites do not follow each other in single file. Rather, they fly independently and cross over the equator a few minutes apart starting just after 1:30 p.m. local time. The A in A-Train also stands for "afternoon" because the satellites cross the equator shortly after noon. Aqua leads the train. It is the largest satellite in the group and the first to cross the equator each day (about 1:40 p.m. on ascending passes) and night (at 1:40 a.m. on descending passes). The spacecraft travels around the planet at more than 15,000 mph. Ground controllers maintain their orbits within 15 minutes of the leading and trailing satellites. CloudSat and CALIPSO fly within 15 seconds of each other, so they can measure the same clouds at the same time.

93. NASA's Orbiting Carbon Observatory joined the A-Train in 2008. OCO will be placed 15 minutes ahead of Aqua and will measure the concentration of carbon dioxide in the atmosphere. The EOS Project Science Office (EOSPSO) makes its scientific information and resources available to scientists and the general public. NASA has combined its Earth science and space science programs into an integrated Science Mission Directorate. The directorate is involved in NASA's Vision for Space Exploration through its support of scientific exploration activities.

Fig.28. A-Train

94. MODIS (or Moderate Resolution Imaging Spectroradiometer). It is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days, acquiring data in 36 spectral bands, or groups of wavelengths (see MODIS Technical Specifications). These data will improve our understanding of global dynamics and processes occurring on the land, in the oceans, and in the lower atmosphere. MODIS is playing a vital role in the development of validated, global, interactive Earth system models able to predict global

change accurately enough to assist policy makers in making sound decisions concerning the protection of our environment. The instrument specifications and primary uses are given in Table 14 and 15 respectively.

Table 14.Specifications of MODIS (onboard Terra and Aqua)

Orbit:705 km, 10:30 a.m. descending node (Terra) or 1:30 p.m. ascending node (Aqua), sun-synchronous, near-polar, circular

Scan Rate: 20.3 rpm, cross track

Swath Dimensions: 2330 km (cross track) by 10 km (along track at nadir)

Telescope: 17.78 cm diam. off-axis, afocal (collimated), with intermediate field stop

Size: 1.0 x 1.6 x 1.0 m

Weight: 228.7 kg

Power: 162.5 W (single orbit average)

Data Rate: 10.6 Mbps (peak daytime); 6.1 Mbps (orbital average)

Quantization: 12 bits

Spatial Resolution:

250 m (bands 1-2)500 m (bands 3-7)1000 m (bands 8-36)

Design Life: 6 years

Table.15. Primary Uses and Band details of MODIS (onboard Terra and Aqua)

Primary Use Band Bandwidth1SpectralRadiance2

RequiredNE[delta]T(K)4

Land/Cloud/AerosolsBoundaries

1 620 - 670 21.8 128

2 841 - 876 24.7 201

Land/Cloud/AerosolsProperties

3 459 - 479 35.3 243

4 545 - 565 29.0 228

5 1230 - 1250 5.4 74

6 1628 - 1652 7.3 275

7 2105 - 2155 1.0 110

Ocean Color/Phytoplankton/Biogeochemistry

8 405 - 420 44.9 880

9 438 - 448 41.9 838

10 483 - 493 32.1 802

11 526 - 536 27.9 754

12 546 - 556 21.0 750

13 662 - 672 9.5 910

14 673 - 683 8.7 1087

15 743 - 753 10.2 586

16 862 - 877 6.2 516

AtmosphericWater Vapor

17 890 - 920 10.0 167

18 931 - 941 3.6 57

19 915 - 965 15.0 250

Surface/CloudTemperature 20 3.660 - 3.840 0.45(300

K) 0.05

21 3.929 - 3.989 2.38(335K) 2.00

22 3.929 - 3.989 0.67(300K)

0.07

23 4.020 - 4.080 0.79(300K) 0.07

AtmosphericTemperature

24 4.433 - 4.498 0.17(250K) 0.25

25 4.482 - 4.549 0.59(275K) 0.25

Cirrus CloudsWater Vapor

26 1.360 - 1.390 6.00 150(SNR)

27 6.535 - 6.895 1.16(240K) 0.25

28 7.175 - 7.475 2.18(250K) 0.25

Cloud Properties 29 8.400 - 8.700 9.58(300K) 0.05

Ozone 30 9.580 - 9.880 3.69(250K) 0.25

Surface/CloudTemperature

31 10.780 - 11.280

9.55(300K) 0.05

32 11.770 - 12.270

8.94(300K) 0.05

Cloud TopAltitude

33 13.185 - 13.485

4.52(260K) 0.25

34 13.485 - 13.785

3.76(250K) 0.25

35 13.785 - 14.085

3.11(240K) 0.25

36 14.085 - 14.385

2.08(220K) 0.35

1 Bands 1 to 19 are in nm; Bands 20 to 36 are in µm2 Spectral Radiance values are (W/m2 -µm-sr)

3 SNR = Signal-to-noise ratio4 NE(delta)T = Noise-equivalent temperature difference

Note: Performance goal is 30-40% better than required

Conclusion

95. Remarkable progress has been made in satellite meteorology over the five decades. Weather services in the tropics and mid-latitude regions will become increasingly dependent on the data there from, and their role in support of environmental warning services, particularly those relating to the rapid onset of severe weather, will undoubtedly expand. Existing sensors and techniques will continue to undergo progressive development, bringing corresponding improvements in data quality and accurate retrieval techniques. Satellite systems are unsurpassed as means of making global observations of the atmosphere and oceans. With their further development in the coming years they will not simply increase the rate at which we advance our basic knowledge of the planet on which we live, though this is important. They will, above all, make a growing and indispensable contribution to improvements in weather forecasts and all the other services which meteorologists, oceanographers and hydrologists together strive to provide for the ultimate benefit of the whole of mankind.

Check Assimilation

State true or false / Fill in the blanks

1. The ground track of the satellite is __________________________________ .

2. Inclination (i) is the angle between _____________________________ and _________________________.

3. The era of satellite meteorology truly began with the launch of _____________ satellite on _______________.

4. INSAT-3A carries _____________________and ___________________________ for meteorological purposes.

5. Out of the 19 channels in the sounder of INSAT-3D, _________________ numbers of channels are operating in window region.

Check Assimilation: The Key

1. The ground track of the satellite is the line connecting all the sub-satellite points.

2. Inclination (i) is the angle between equatorial plane and plane of orbit.

3. The era of satellite meteorology truly began with the launch of TIROS-1 on 01 Apr 60.

4. INSAT-3A carries a Very High Resolution Radiometer and Charge Coupled Device (CCD) camera for meteorological purposes.

5. Out of the 19 channels in the sounder of INSAT-3D, four channels are operating in window region.

Bibliography

References

1. Lecture Notes for Post Graduate Course on “Satellite Meteorology and Global Climate” Volume I to III by MOG at SAC Ahmadabad.2. http:// www.isro.org

3.

http://winds.jpl.nasa.gov/missions/quikscat/index.efm

4.

http://www.nasa.gov/missions/current/index.html

5. http://www.esa.int/esatmi/msg

6. http://www.jma.go.jp/jma-avg/satellite