The star tracker unit is designed to improve image location accuracy by providing precise data on the satellite's attitude. This unit is composed of a new star tracker, a structure to interface with the satellite, and central software functions for determining attitude. These functions filter star tracker measurements and calculate the estimated attitude that is then inserted in the auxiliary data sent with image telemetry.
The satellite's onboard attitude and orbit control system does not use the measurements from the star tracking unit. However, these measurements are used to determine the satellite's attitude very precisely for ground processing of images acquired by the HRS, HRG and VEGETATION instruments, thus significantly increasing their geometric location accuracy-from 350 metres for SPOT 4 to 50 metres for SPOT 5.
The star tracker uses the measured position of stars to estimate the satellite's attitude accurate to better than 0.01°. It operates autonomously and requires no information from ground control, other than the Julian date.
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SPOT 5 star tracker on the space-facing side of the satellite between the two HRG instruments. |
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Star tracker on SPOT5 |
