Orb-rate explanation

The ORDEAL (Orbital Rate Display - Earth And Lunar) drives the FDAI (Flight Director Attitude Indicator, or "8-ball") to make it display the spacecraft's attitude relative to the ground below.

If a spacecraft is in orbit with a fixed attitude relative to the celestial sphere (i.e. the spacecraft keeps pointing to the same stars no matter where it is in its orbit), then its attitude relative to the body it's orbiting (Earth, Moon or whatever) is constantly changing. For example, at one point in its orbit, the front of a spacecraft can be pointing directly at the planet below. Half an orbit later, it will be pointing directly away. This is known as "stellar inertial."

Conversely, if the spacecraft is to be flown in 'orb-rate,' keeping the same face towards the surface (to point cameras for example), it must rotate around one of its axes at a rate which matches the orbital period. Normally, the FDAI displays the spacecraft's attitude relative to the celestial sphere (i.e. it normally shows the inertial attitude) but the function of the ORDEAL is to provide the correct drive signal to rotate the FDAI at a rate which also matches the orbital period. With the ORDEAL, the FDAI will display attitudes relative to the surface below.

[edit] Apollo 15

During Apollo 15's TLI, the crew are going to use the ORDEAL to drive the FDAI at a rate which matches the pitch rate of the S-IVB during its powered flight. This way, the crew can monitor the progress of the TLI as far as the vehicle's attitude is concerned, and they can take over manual attitude control if required during the burn. They must start the ORDEAL working at a precise time if the FDAI is to show zero attitude errors, otherwise the spacecraft's further motion around the Earth will cause it to be offset one way or the other. (ap15fj)