From: tholen@newton.ifa.hawaii.edu (Dave Tholen)
Subject: Re: Command Loss Timer (Re: Galileo Update - 04/22/93)
Organization: Institute for Astronomy, Hawaii
Lines: 23

Alan Carter writes:

>> 3.  On April 19, a NO-OP command was sent to reset the command loss timer to
>> 264 hours, its planned value during this mission phase.

> This activity is regularly reported in Ron's interesting posts. Could
> someone explain what the Command Loss Timer is?

The name is rather descriptive.  It's a command to the spacecraft that tells
it "If you don't hear from Earth after 264 hours, assume something is wrong
with your (the spacecraft) attitude, and go into a preprogrammed search mode
in an attempt to reacquire the signal from Earth."

The spacecraft and Earth are not in constant communication with each other.
Earth monitors the telemetry from the spacecraft, and if everything is fine,
there's no reason to send it any new information.  But from the spacecraft's
point of view, no information from Earth could mean either everything is
fine, or that the spacecraft has lost signal acquisition.  Just how long
should the spacecraft wait before it decides that something is wrong and
begins to take corrective action?  That "how long" is the command loss timer.
During relatively inactive cruise phases, the command loss timer can be set
to rather long values.  In this case, Earth is telling Galileo "expect to
hear back from us sometime within the next 264 hours".
