2.1.1 Guidance and Navigation
Digital computers were first placed on launch vehicles such as Atlas, Centaur, Minuteman and Titan to perform guidance and navigation (G&N) calculations. To accomplish this C&N task, the computer first processes data from various sources (e.g., inertial measurement units, star trackers, and horizon sensors) to estimate the present attitude, position, and velocity of the space vehicle. Using stored or computed trajectory data, it then determines any velocity corrections which are necessary and generates the appropriate steering commands, including thrust commencement and/or termination signals. Normally, the computation requirements for guidance and navigation do not by themselves dictate an excessively large or fast machine by present day standards. However, extreme accuracy and reliability demands can place stringent requirements on the spaceborne computer.
Guidance and navigation of a spacecraft in orbit involve calculations for such functions as 1) orbit parameters for use with experiment data, 2) steering commands for orbit maintenance, generation of ground tracks and display of latitude, longitude, and altitude, and 3) rendezvous guidance commands. Some missions require the capability of orbit changes in addition to orbit maintenance. In the Apollo guidance computer (AGC), the performance of all those G&N tasks requires some 24,000 words of memory and 26,000 operations per second (peak), out of the totals available for all calculations of 38,000 words and about 40,000 operations per second.
The Apollo spacecraft uses a stable platform for the primary inertial navigation system, whereas future vehicles may use strapdown systems which require additional calculations to update the inertial reference (e.g., see refs. 3 to 6). The memory requirement remains about the same as for a stable platform system, but the speed requirement depends heavily on the resolution desired and the rate at which the vehicle attitude is changing. For example, it is estimated that a system comprising three single-degree-of-freedom strapdown gyros and accelerometers in a vehicle rotating at 0.35 rad (20 deg)/sec would require approximately 120,000 operations per second to establish a resolution of 0.15 mrad (±30 sec of arc) (ref. 7).
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