Hi guys,

I saw the Australian movie, "The Dish" over the holidays. It's about the
Parkes Observatory and the large dish antenna used to relay video, audio,
and telemetry feeds from Apollo 11 in 1969 back to NASA.

At one point in the movie, they lose track of where Apollo 11 is and have to
scramble to manually point the dish to get a signal again. However, once
they DO manage to get a signal, they flip a switch and the dish continues to
automatically track Apollo 11.

I'm curious... how is this down? 3 or 4 slightly offset (from the dish's
central feedpoint) receivers, the outputs of which are compared to determine
which way the transmitting source is 'drifting' (then feed back to the
motion control system to move the dish that way)? Or is there a simpler
means?

Thanks,
---Joel Kolstad