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Space Shuttle Navigation: Difference between revisions
→The star tracker
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Usually the star tracker should require minimal intervention and perform its function fine on its own. | Usually the star tracker should require minimal intervention and perform its function fine on its own. | ||
=== COAS === | |||
==== Hardware ==== | |||
The COAS system consists of a mountable frame which can be applied to the forward (+X) or upward (-Z) windows. Alternatively the HUD can be used. The frame contains an aiming pattern. | |||
The idea is to maneuver the Shuttle such that a known star is in the center of the pattern and then to mark an attitude reference (i.e. the current attitude of the IMUs is stored). By taking two such attitude references and comparing to the stored star database, the inertial attitude correction for the propagaed state vector can be computed. | |||
==== Usage in FG ==== | |||
The COAS procedure is fully supported, albeit only for a limited number of stars. As of March 2016, no COAS frame is modeled, so the Shuttle has to be aimed at a star using the HUD (and thus COAS marks can only be taken in the +X direction). | |||
Like the star tracker, COAS operations are controlled from SPEC 22. | |||
[[File:Shuttle avionics spec22.jpg|600px|STRK/COAS display of the Space Shuttle]] | |||
To use the COAS, wait for the night portion of the orbit till the stars come out. You need some knowledge of the night sky to identify constellations and find the star you need. Pick one of the stars from the supported table and enter its index as item 21. | |||
{| | |||
| index || star | |||
|- | |||
| 11 || Shedir | |||
|- | |||
| 12 || Mirphak | |||
|- | |||
| 13 || Mizar | |||
|- | |||
| 14 || Arcturus | |||
|- | |||
| 15 || Betelgeuse | |||
|- | |||
| 16 || Procyon | |||
|- | |||
| 17 || Spica | |||
|} | |||
Make sure the COAS position forward (item 26) is selected. Point the Shuttle at the chosen star. When it is centered, press {{Key press|Ctrl|A}} to take an attitude reference. If you feel it wasn't good, you can take more, the last will overwrite all previous ones. Once you think the reference is good, accept it by executing item 23. Enter the next star ID via item 21 (ideally make sure they're about 90 degrees separate). Take another attitude reference, and once you're happy, accept with item 23. | |||
You'll now get to see ΔBIAS values which tell you how much your just determined attitude deviates from the state vector. If you pointed at the wrong star, expect the biases to be very large. If you're just outside the star tracker operating range, a bias of 40 deg is probably not real. | |||
If you are convinced that the COAS is okay, execute item 28 to update the state vector. If you want to start from scratch, execute item 20 which clears the star table and all COAS references. Usually, the star tracker should be back in working condition after a COAS procedure. | |||