![]() The laser retro-reflector allows the measurement of the satellite's altitude to within a few centimetres by reflecting a laser beam transmitted by a ground station. The Sun sensors are visible light detectors which measure angles between their mounting base and incident sunlight. ![]() The infrared Earth sensors do this by detecting the contrast between the cold of deep space and the heat of the Earth's atmosphere. The infrared Earth sensors and the Sun sensors both help to keep the spacecraft pointing at the Earth. The S-band antennas also receive, process and transmit ranging signals that measure the satellite's altitude to within a few metres. They transmit housekeeping data about the payload and spacecraft to ground control and, in turn, receive commands to control the spacecraft and operate the payload. Two S-band antennas are part of the telemetry, tracking and command subsystem. The integrity information is incorporated into the navigation signal for transmission to users. This includes data to synchronise the on-board clocks with a ground-based reference clock and integrity data which contains information about how well each satellite is functioning. ![]() The C-band antenna receives signals containing mission data from Galileo Uplink Stations. The SAR (Search and Rescue) antenna picks up distress signals from beacons on Earth and transmits them to a ground station for forwarding to local rescue services. The L-band antenna transmits the navigation signals in the 1200-1600 MHz frequency range. ![]()
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