QUESTION:
How does the HGA unavaliability affect your uplink capacity? We have 
heard a lot about the low downlink capacity, but don't that affect the 
uplink as well?

ANSWER from Jim FH Taylor on April 2, 1997:
The short answer is "not at all".  The essential purpose of the uplink
is to command the spacecraft.  Without an assured command capability,
the mission is over.  During Galileo's design phase in the late 1970s,
the end-to-end uplink system was designed to ensure reliable command
capability under adverse conditions.  These adverse conditions were
defined as:

1. Use of the low gain antenna only;
2. Spacecraft sun-pointed (not earth-pointed);
3. Maximum Jupiter-earth distance.

These criteria established the command bit rate of 32 bits/second and
the use of the high power transmitter (100 kW) at the 70-meter ground
stations.

It was this good design, not luck, that resulted in the uplink
capability not being an issue when the high gain antenna failed to
open.  

If the high gain antenna had opened, our primary downlink would have
been at X-band and could have used the 34-meter ground stations.  The
uplink would still have been 32 bits/second on S-band, and would have
used the low-power (20 kW) transmitter during the 34-m tracking passes.
  
We command the spacecraft up to several times a week, using the 100 kW
transmitter during regular tracking passes.  Command periods range
from a few seconds for a single command to several hours for a major
sequence.  

Sun-point is the worst-case attitude that may exist after some fault
conditions that cause execution on the on-board Spacecraft Safing
routine. At Jupiter distance, the maximum angle between sun-point and
earth-point is about 12 degrees.  At that angle, the low gain antenna
has 0.5 dB less uplink capability than at its boresight (point of
maximum gain).

Galileo has a single command data rate, and the uplink is not coded.
Tracking time is dominated by downlink requirements.  The uplink can
afford to be inefficient compared to the downlink.  Both Galileo links
are S-band over the low gain antenna.  The uplink enjoys a raw RF
power advantage over the downlink of 100,000 watts (uplink) to 14
watts (downlink).