Excerpt from Professional Computing, Oct/Nov 1984:

Six months before the first shuttle flight, Terry Hart was asked to find the best calculator for the astronauts. He looked at the TI-59 and the HP 41, the most powerful units available, and decided that the 41’s alphanumeric display capability made it the clear winner.

Once he decided on the 41, Hart realized he had more than just the handheld scientific calculator NASA had wanted. He started to look for more complex jobs to use it for. The deorbit program is one example. Computation of a deorbit opportunity would have been easy for the on-board computers, but the software for it was never developed.

Since the 41s would already be on board as general-purpose calculators, Hart began to develop additional programs for them. …

CG, the first 41 shuttle program, computes the center of gravity of the orbiter as fuel from the tanks is consumed.

Another program, Landtrack, computes the ground track of the shuttle, identifying points of interest on the earth’s surface for observation (such as the Great Wall of China? — rrd). These two programs were on board the first two shuttle flights.

The most widely used 41 shuttle program is Deorbit/Alarm/AOS…

Acquisition of Signal (AOS), which runs continuously throughout the mission, is important because there is direct communication with ground controllers only during passes over one of 13 earth stations. These passes last about 10 minutes, less if the shuttle does not pass directly over the earth station.

AOS beeps at the start of a pass over an earth station and displays the time remaining to loss of signal (LOS).