Satellite Communication. 17.2Satellite Networks Orbits Three Categories of Satellites GEO Satellites...

Satellite Communication

17.217.2 Satellite NetworksSatellite Networks

Orbits

Three Categories of Satellites

GEO Satellites

MEO Satellites

LEO Satellites

Figure 17.13 Satellite orbits

Example 1Example 1

What is the period of the moon according to Kepler’s law?

SolutionSolution

The moon is located approximately 384,000 km above the earth. The radius of the earth is 6378 km. Applying the formula, we getPeriod = (1/100) (384,000 + 6378)1.5 = 2,439,090 s = 1 month

Example 2Example 2

According to Kepler’s law, what is the period of a satellite that is located at an orbit approximately 35,786 km above the earth?

SolutionSolution

Applying the formula, we get

Period = (1/100) (35,786 + 6378)1.5 = 86,579 s = 24 h

A satellite like this is said to be stationary to the earth. The orbit, as we will see, is called a geosynchronous orbit.

Figure 17.14 Satellite categories

Figure 17.15 Satellite orbit altitudes

Table 17.1 Satellite frequency bandTable 17.1 Satellite frequency band

Band Downlink,

GHzUplink, GHz

Bandwidth, MHz

L 1.5 1.6 15

S 1.9 2.2 70

C 4 6 500

Ku 11 14 500

Ka 20 30 3500

Figure 17.16 Satellites in geosynchronous orbit

Figure 17.17 Triangulation

Figure 17.18 GPS

Figure 17.19 LEO satellite system

Figure 17.20 Iridium constellation

The Iridium system has 66 satellites in six LEO orbits, each at an

altitude of 750 km.

NoteNote::

Iridium is designed to provide direct worldwide voice and data

communication using handheld terminals, a service similar to cellular

telephony but on a global scale.

NoteNote::

Figure 17.21 Teledesic

Teledesic has 288 satellites in 12 LEO orbits, each at an altitude of 1350 km.

NoteNote::

Satellite Components

• Satellite Subsystems– Telemetry, Tracking, and Control– Electrical Power and Thermal Control– Attitude Control– Communications Subsystem

Satellite Orbits

• Equatorial

• Inclined

• Polar

Orbital Mechanics

Without Force

Gravity

Effect of Gravity

Here’s the Math…

• Gravity depends on the mass of the earth, the mass of the satellite, and the distance between the center of the earth and the satellite

• For a satellite traveling in a circle, the speed of the satellite and the radius of the circle determine the force (of gravity) needed to maintain the orbit

But…

• The radius of the orbit is also the distance from the center of the earth.

• For each orbit the amount of gravity available is therefore fixed

• That in turn means that the speed at which the satellite travels is determined by the orbit

Let’s look in a Physics Book…

• From what we have deduced so far, there has to be an equation that relates the orbit and the speed of the satellite:

T 2r3

4 1014

T is the time for one full revolution around the orbit, in seconds

r is the radius of the orbit, in meters, including the radius of the earth (6.38x106m).

The Most Common Example

• “Height” of the orbit = 22,300 mile

• That is 36,000km = 3.6x107m

• The radius of the orbit is3.6x107m + 6.38x106m = 4.2x107m

• Put that into the formula and …

The Geosynchronous Orbit

• The answer is T = 86,000 sec (rounded)

• 86,000 sec = 1,433 min = 24hours (rounded)

• The satellite needs 1 day to complete an orbit

• Since the earth turns once per day, the satellite moves with the surface of the earth.

Assignment

• How long does a Low Earth Orbit Satellite need for one orbit at a height of 200miles = 322km = 3.22x105m

• Do this:– Add the radius of the earth, 6.38x106m– Compute T from the formula– Change T to minutes or hours

T 2r3

4 1014

GEO Coverage

• Altitude is about 6 times the earth’s radius• Three satellite can cover the surface of the earth

Orbit Examples

• Geostationary– Equatorial and Geosynchronous

• Inclined Geosynchonous– Satellite moves north/south relative to the

earth station

• Polar LEO– Satellite group covers the entire earth

LEOS Coverage

• Altitude is 1/6 of the earth’s radius

Communication Frequencies

• Uplink (Earth to Satellite)– C Band: around 6 GHz

– Ku Band: around 14 GHz

– Ka Band: around 30 GHz

• Downlink (Satellite to Earth)– C Band: around 4 GHz

– Ku Band: around 12 GHz

– Ka Band: around 20 GHz

Sputnik I

Sputnik I -- 60 cm (about 2 ft.) diam. sphere with straight-wire antennas

Explorer I

Explorer I -- 1 m. long and 20 cm in diam., spin stabilized (like a gyroscope), with flexible antennas

A generic military/meteorological/communications satellite

1-3 m. on each side, stabilized with internal gyroscopes or external thrusters

Dual-spin stabilized satellite

1-3 m. in diameter, up to several meters tall; lower section spins to provide gyroscopic stability, upper section does not spin

LIONSATLocal IONospheric Measurements

SATellite

•will measure ion distrib. in ram and wake of satellite in low orbit

•student-run project

(funded by Air Force, NASA and AIAA)

•www.psu.edu/dept/aerospace/lionsat

Hubble Space Telescope

http://www.stsci.edu/hst/proposing/documents/cp_cy12/primer_cyc12.pdf

Propulsion

• Provides force needed to change satellite’s orbit.• Includes thrusters and propellant.

Spacecraft Propulsion Subsystem

• Uses of onboard propulsion systems– Orbit Transfer

• (Low Earth Orbit) LEO to (Geosynchronous Earth Orbit) GEO

• LEO to Solar Orbit

– Drag Makeup– Attitude Control– Orbit Maintenance

Types of Propulsion

– Chemical Propulsion• Performance is energy limited• Propellant Selection

– Electric Propulsion• Electrostatic—Ion Engine• Electrothermal—ArcJet• Electomagnetic—Rail gun

Types of Propulsion

– Solar Sails• Would use large (1 sq. km.) reflective sail (made of

thin plastic) • Light pushes on the sail to provide necessary force

to change orbit.• Still on the drawing board, but technologically

possible!

– Nuclear Thermal

• Provides, stores, distributes, and controls electrical power.

• Need power for (basically everything) communications, computers, scientific instruments, environ. control and life support, thermal control, and even for propulsion (to start the rocket engine)

Power

Power

• Solar array: sunlight electrical power– max. efficiency = 17% (231 W/m2 of array)– degrade due to radiation damage 0.5%/year– best for missions less than Mars’ dist. from Sun

• Radioisotope Thermoelectric Generator (RTG): nuclear decay heat electrical power– max. efficiency = 8% (lots of waste heat!)– best for missions to outer planets– political problems (protests about launching 238PuO2)

• Batteries – good for a few hours, then recharge

Power

• Dynamic Power Sources– Like power plants on Earth.

• Fuel Cells– Think of these as refillable batteries.– The Space Shuttle uses hydrogen-oxygen fuel

cells.

Power

• The design is highly dependent on:– Space Environment (thermal, radiation)– Shadowing– Mission Life

Thermal

• Thermal Control System– Purpose—to maintain all the items of a

spacecraft within their allowed temperature limits during all mission phases using minimum spacecraft resources.

Thermal

• Passive– Coatings (control amt of heat absorbed & emitted)

• can include louvers

– Multi-layer insulation (MLI) blankets– Heat pipes (phase transition)

Thermal

• Active (use power)– Refrigerant loops– Heater coils

Communications

• Transmits data to ground or to relay satellite (e.g. TDRS)

• Receives commands from ground or relay satellite

Communications

• Radios (several for redundancy) – voice communications if humans onboard – data sent back to Earth from scientific

instruments – instructions sent to s/c from Earth

• Video (pictures of Earth, stars, other planets, etc.)

• various antennas: dish, dipole, helix

Attitude Sensing and Control

• Senses and controls the orientation of the spacecraft.

Attitude Sensing

• star sensor – – The light from stars and compares it to a star

catalog.

Attitude Sensing

• sun sensor measures angle between "sun line"

Attitude Sensing

• gyroscopes -- spinning disk maintains its orientation with respect to the fixed stars -- onboard computer determines how the s/c is oriented with respect to the spinning disk.

Attitude Control

• Thrusters -- fire thrusters (small rockets) in pairs to start rotation, then fire opposite pair to stop the rotation. 

Attitude

• gyroscopes -- use electric motor in s/c

wheel

motor

satellite

Attitude Determination and Control

• Sensors– Earth sensor (0.1o to 1o)

– Sun sensor (0.005o to 3o)

– star sensors (0.0003o to 0.01o)

– magnetometers (0.5o to 3o)

– Inertial measurement unit (gyros)

• Active control (< 0.001o)– thrusters (pairs)

– gyroscopic devices

• reaction & momentum wheels

– magnetic torquers (interact with Earth’s magnetic field)

• Passive control (1o to 5o)– Spin stabilization (spin entire sat.)

– Gravity gradient effect

x

y

Earth sensor

photocells

wheel

motor

satellite

• Motor applies torque to wheel (red)

• Reaction torque on motor (green) causes satellite to rotate

rotation

field of view

Command and Data Handling

• Principal Function– Processes and distributes commands;

processes, stores, and formats data

• Other Names– Spacecraft Computer System– Spacecraft Processor

Command and Data Handling

• Commands– Validates – Routes uplinked commands to subsystems

• Data– Stores temporarily (as needed)– Formats for transmission to ground– Routes to other subsystems (as needed)

• Example: thermal data routed to thermal controller, copy downlinked to ground for monitoring

Command and Data Handling

• provide automatic capability for s/c, reducing dependence on expensive ground control

• must include backups or redundant computers if humans onboard

• need to be protected from high-energy radiation • cosmic rays can alter computer program (bit flip)

without human ground controllers realizing it.

Structure

• Not just a coat-rack!• Unifies subsystems• Supports them during launch

– (accel. and vibrational loads)

• Protects them from space debris, dust, etc.

Launch Vehicle

• Boosts satellite from Earth’s surface to space• May have upper stage to transfer satellite to

higher orbit• Provides power and active thermal control

before launch and until satellite deployment

Creates high levels of accel. and vibrational loading

Launch System

• System selection process– Analyze capable systems

– Maximum accelerations– Vibration frequencies and amplitudes– Acoustic frequencies and amplitudes– Temperature extremes– LV/satellite interface– Kick motor needed?

Delta II Rocket

Image:http://www.boeing.com/companyoffices/gallery/images/space/delta_ii/delta2_contour_08.htm

Titan IV Rocket

Image: www.spaceline.org/galleries/cpx-40-41/blowup41.jpg.html

Ground Control• MOCC (Mission Operations Control Center)

– Oversees all stages of the mission (changes in orbits, deployment of subsatellites, etc.)

• SOCC (Spacecraft Operations Control Center)– Monitors housekeeping (engineering) data from sat.– Uplinks commands for vehicle operations

• POCC (Payload Operations Control Center)– Processes (and stores) data from payload (telescope

instruments, Earth resource sensors, etc.)– Routes data to users– Prepares commands for uplink to payload

• Ground station – receives downlink and transmits uplink

Payload Operations Control Center

NASA Marshall Space Flight Center, Huntsville Alabama

Mission Control Center

NASA Johnson Spaceflight Center, Houston Texas