Dr Mark Cresswell

Anatomy of a satelliteTypes of satellite orbitThe radiometerLaunch vehiclesOperational demandsDecommissioning and “space junk”

A satellite is any object that orbits or revolves around another object. For example, the Moon is a satellite of Earth, and Earth is a satellite of the Sun.

Man-made satellites are designed to orbit the Earth and the Sun - for scientific, military or commercial purposes

A satellite is a complex machine. All satellites are made up of several sub-systems that work together as one large system to help the satellite achieve its mission.

There are two key elements to any satellite: the PAYLOAD and the BUS

The payload is all the equipment a satellite needs to do its job. This can include antennas, cameras, radar, and electronics. The payload is different for every satellite.

For example, the payload for a weather satellite includes cameras to take pictures of cloud formations, while the payload for a communications satellite includes large antennas to transmit TV or telephone signals to Earth.

The bus is the part of the satellite that carries the payload and all its equipment into space. It holds all the satellite's parts together and provides electrical power, computers and propulsion to the spacecraft.

The bus also contains equipment that allows the satellite to communicate with Earth.

Geosynchronous Equatorial Orbit: A satellite in geosynchronous equatorial orbit (GEO) is located directly above the equator, 36,000 km out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together.

So, a satellite in GEO always stays directly over the same spot on Earth. (A geosynchronous orbit can also be called a GeoSTATIONARY Orbit.)

Geosynchronous orbit FOOTPRINT: Because they're so far away, GEO satellites have a very broad view of Earth. For instance, the footprint of Meteosat covers All of Africa, Europe, part of the Middle-East and part of South America

Since they stay over the same spot on Earth, we always know where GEO satellites are. If our antenna points in the right direction, we will always have direct contact with the satellite.

Polar Orbit: A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction.

Why use a Polar Orbit?: Polar orbits are useful for viewing the planet's surface. As a satellite orbits in a north-south direction, Earth spins beneath it in an east-west direction. As a result, a satellite in polar orbit can eventually scan the the entire surface.

For this reason, satellites that monitor the global environment, like remote sensing satellites and certain weather satellites, are almost always in polar orbit. No other orbit gives such thorough coverage of Earth.

Satellites on constant motion are said to have an INCLINATION. A polar orbiter is inclined at 90 degrees (North-South) whereas a geostationary craft has an inclination of zero degrees (East-West)

Elliptical Orbit: A satellite in elliptical orbit follows an oval-shaped path. One part of the orbit is closest to the center of Earth (perigee) and the other part is farthest away (apogee). A satellite in this orbit takes about 12 hours to circle the planet. Like polar orbits, elliptical orbits move in a north-south direction.

Problem of Polar Coverage with Geostationary Satellites: While most communications satellites are in Geosynchronous orbit, the footprints of GEO satellites do not cover the polar regions of Earth. So communications satellites in elliptical orbits cover the areas in the high northern and southern hemispheres that are not covered by GEO satellites.

Not all artificial satellites orbit the Earth. Research craft orbit the Sun. SOHO (SOlar and Heliospheric Observation) - SOHO is an international project managed by Europe and the United States. Its very sophisticated instruments can measure activity inside the Sun, look at its atmosphere or corona, and study its surface.

The most important instrument on board any remote sensing satellite is the radiometer. This instrument is sensitive to specific wavelengths of energy and detects and stores the magnitude of this energy

The magnitude of energy is encoded digitally at either 8 or 16-bit digital numbers (DN)

MeteosatRadiometer

For Geosynchronous satellites such as Meteosat, the radiometer “scans” the earth a line at a time as the satellite rotates. A stepper motor moves the instrument by small degree steps until it has observed the entire Earth disc

A polar orbiter usually faces the surface and scans the orbital strip

Radiometric calibration is usually performed on Earth prior to launch

Operational on-board calibration may be performed on a regular basis to maintain sensor efficiency and safeguard against technical problems

Meteosat-6 was affected by trapped water vapour - required software correction

One of the most expensive and technically challenging aspects of satellite remote sensing is getting the satellite into orbit (or “inserted” into orbit)

Europeans have had success with the Ariane 5 rocket - launches usually occur near the equator. The Indian space programme has also seen great success

Once a satellite is in orbit it requires some maintenance

Hydrazine propellant is used to constantly correct and readjust the orbital parameters

Calibration data must be maintained and monitored

Data must be downloaded to ground segment stations for pre-processing and quality checks before archiving and retransmission

Once a satellite nears the end of its operational life the orbit can begin to decay (polar orbiters)

Analysis of NOAA-AVHRR has shown that data collected at the start and end of its life can be quite different and careful pre-processing must be performed prior to use

A satellite may be “retired” either by placing the craft in a burnout orbit

Or..it becomes part of the ever increasing collection of space junk!

The United States Space Command keeps track of the number of satellites in orbit. This is a graphic display of the objects in low earth orbit. According to the USSC, there are more than 8,000 objects larger than a softball now circling the globe.