We're presently about 25 minutes away from the separation burn which will be performed by Mike Collins in the Command Module to give the LM and the CSM a separation distance at the Descent Orbit Insertion maneuver of about 2 miles. When next we hear from them, the Lunar Module should be undocked from the Command and Service Module. We're now less than 2 minutes from reacquiring the spacecraft on the thirteenth revolution. This is Apollo Control at 100 hours, 14 minutes. Since the craft is in freefall, they should indicate zero acceleration and any reading from them would indicate their inherent bias. At the same time, the accelerometers in the AGS are also calibrated. This should yield a measure of their drift which is stored and used in subsequent computations. The idea is that the spacecraft holds a steady attitude using the primary system and the amount of drift in the AGS gyros is measured over a 302-second time frame. They are inherently more prone to drift and an attempt is made during their calibration to compensate for this drift. It gets its sense of orientation from a set of strapped-down gyroscopes, units that are affixed to the spacecraft body and which derive rate of rotation rather than absolute rotation by measuring the forces that any rotation will place on their mountings. It has no inertially stabilised platform for accurate attitude determination and its computer is much simpler than the LGC machine that will control the landing. It's only intention is to get the LM to a safe orbit should the primary system fail. The Abort Guidance System (AGS) is less sophisticated than the primary system.

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