QUESTION:
Why is there still a short period of LOS for the shuttle with the TDRSS and what are the other TRDS besides East & West? Does the South Atlantic Anomaly Zone play into it at all?

ANSWER from Steve Sides on October 3, 1997:
The reason for the loss of signal (LOS) is because the ground station that receives the downlink from the tracking and data relay satellites (TDRS) is located in southern New Mexico near Las Cruses. Since the satellites must have a line of sight view to the ground site they can only be so far apart. The primary TDRS satellites used by the shuttle are located at -174 degrees West and -41 degrees West. These are about as far apart as they can be and still see the ground site. The shuttle must have a line of sight view of the TDRS as well. There is a small area over the world where the shuttle can't see the satellites. That is the LOS or zone of exclusion (ZOE).

There are several other TDRS satellites in orbit. Some of them are very old as far as satellites go and have lost some of their capabilities. The shuttle has 4 satellites available to it including the two that it primarily uses. For the shuttle to use these other satellites a command must be sent to the shuttle to tell it which two satellites to use. We do this to support the deorbit burns and any very critical time frames like the MIR dock and undock times on STS 86.

The South Atlantic Anomaly (SAA) does not have any affect on the TDRS links to the shuttle or the ground. The main impact of the SAA is on the laptop computers and the TV cameras on the shuttle. If you look real close during a night time downlink of the payload bay cameras you will see a streak of colored light. That is a proton hit on some of the camera image elements. Over time the pixels (image elements) can be damaged by the SAA. You can see that as spots on the picture. The laptops that fly on the shuttle are just like the kind that you can buy in the store. They can be affected by the same thing that affects the cameras. The impact is that the computers will stop working because the proton has hit the memory of the computer. All the crew has to do is power off then on to fix the computer.