Page:Project Longshot - Advanced Design Program Project Report.pdf/53

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3.6 Guidance
A system of star trackers will be used for both navigation and attitude determination. This system has been chosen for its high accuracy (4 arc sec) and adaptability, and for its low weight (7 kg) and power requirements (18 watts). Star scanners were not chosen since the spacecraft is not rotating, and they are less accurate. The trackers will be coupled to a computer system which will have a star catalogue of 200-300 stars' locations. "Adaptability" refers to this catalogue, because a "best guess" of star locations that the probe will "see" on the trip and in orbit in the Centauri system can be programmed into the computer before launch, thus increasing the accuracy of position/attitude determination. This best guess could even be updated enroute or in Centauri-orbit by the astrometry calculations that the computer will make.

Initially, during the transit between Earth and the point where the probe-head separates from the propulsion system, trackers located on the last fuel tanks will be used. These trackers will be oriented in different directions in order to gain a nearly complete field of view. 18 trackers will be used, 3 in each of the 6 axis directions to get the greatest field of view and triple redundancy. The power for these trackers will come from generators drawing energy from the propulsion system waste heat. In the final mission phase, 9 trackers