TYPES OF ORBIT
Most satellites are launched into one of four main orbits. A nearly circular low-Earth orbit is up to about 250 km above Earth. Polar orbits are often 800 km high. A highly elliptical orbit has a much lower altitude at its closest approach to Earth (its perigee) than when it is most distant (its apogee). A geostationary orbit is 36,000 km above the equator.
TELEMETRY, TRACKING AND COMMAND
Telemetry – literally, measuring from far away – allows people on the ground to receive measurements from satellites in orbit. The measurements, sent as radio signals, might include information that allows operators to pinpoint the satellite’s position. This allows people to track the satellite, and to send command signals that can change its position. Telemetry also includes data that allow ground controllers to check that the satellite is operating correctly.
STABILIZING SATELLITES
If a satellite is not stable – if it swings about in an unpredictable way – it cannot do its job. For example, the dish of a communications satellite must always point towards its receiving station or towards the right country if it is transmitting television signals. Two techniques commonly used to maintain stability are spin and three axis stabilization.
SPIN STABILIZATION
Things that spin are naturally stable. A spinning op remains stable if it is spun fast enough, and the turning of its wheels helps to keep bicycle upright. In the early days of satellites, designers decided to exploit this principle. The result is spin-stabilized satellites. These are often cylindrical in shape, and make about one revolution every second. The antenna dish must always point to Earth, so it does not spin. Designers must take care that the dish does not destabilize the satellite.
THREE-AXIS STABILIZATION
Three-axis stabilized satellites contain small spinning wheels that rotate in such a way that they always keep the satellite in the same orientation to the Earth and Sun. if the satellite’s sensors detect a deviation on any of the three axes of the cube, a signal is sent to the wheels to spin faster or slower. These changes restore the satellite to the correct orientation.
Most satellites are launched into one of four main orbits. A nearly circular low-Earth orbit is up to about 250 km above Earth. Polar orbits are often 800 km high. A highly elliptical orbit has a much lower altitude at its closest approach to Earth (its perigee) than when it is most distant (its apogee). A geostationary orbit is 36,000 km above the equator.
TELEMETRY, TRACKING AND COMMAND
Telemetry – literally, measuring from far away – allows people on the ground to receive measurements from satellites in orbit. The measurements, sent as radio signals, might include information that allows operators to pinpoint the satellite’s position. This allows people to track the satellite, and to send command signals that can change its position. Telemetry also includes data that allow ground controllers to check that the satellite is operating correctly.
STABILIZING SATELLITES
If a satellite is not stable – if it swings about in an unpredictable way – it cannot do its job. For example, the dish of a communications satellite must always point towards its receiving station or towards the right country if it is transmitting television signals. Two techniques commonly used to maintain stability are spin and three axis stabilization.
SPIN STABILIZATION
Things that spin are naturally stable. A spinning op remains stable if it is spun fast enough, and the turning of its wheels helps to keep bicycle upright. In the early days of satellites, designers decided to exploit this principle. The result is spin-stabilized satellites. These are often cylindrical in shape, and make about one revolution every second. The antenna dish must always point to Earth, so it does not spin. Designers must take care that the dish does not destabilize the satellite.
THREE-AXIS STABILIZATION
Three-axis stabilized satellites contain small spinning wheels that rotate in such a way that they always keep the satellite in the same orientation to the Earth and Sun. if the satellite’s sensors detect a deviation on any of the three axes of the cube, a signal is sent to the wheels to spin faster or slower. These changes restore the satellite to the correct orientation.
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