NASA SOHO High Gain Antenna Status 21 Jun 2003

This morning we continued with the troubleshooting of the HGA motor. There were several attempts in high speed mode and one attempt in low speed mode to move the antenna by 30 steps, just enough to get another encoder pulse. None was successful.

The lead we are investigating is the puzzling fact that whenever the antenna was not used for more than 24 hours, i.e., when there were only minimal thermal gradients inside the motor, it moved by about 30 steps (for approx. 5 min in low speed mode) before sticking. The hypothesis is that when the motor is uniformly cool enough, a single set of windings may be enough to drive the gears.

It was decided to increase the temperature of the antenna slightly to about 10C. Antenna motor commanding tests will resume Monday, when all thermal gradients inside the motor should have settled.

Other explanations for the behavior of the antenna motor include debris and/or lubricant thinning, though these have been discounted by the manufacturer.