Answers Your Questions
| From: Jeff Kupke, of Dover,
To: Kelly Beck, flight director
Question: I have noticed that on the screen which shows the shuttle's (or station's) current orientation and attitude, you can see the Tracking and Data Relay Satellites, and that you can see each satellite's approximate coverage area. My question is, why does each one have a small chunk missing from it's coverage area, rather than a perfect circle? Is it due to antenna orientation on the satellite?
Answer: The coverage of each TDRS satellite, when projected onto a globe of the Earth, does appear as a circle for lower-Earth orbiting vehicles, such as the shuttle orbiter and the space station. The circle represents the boundary at which the orbiter/ISS and the TDRS gain or lose line of sight contact with each other due to being blocked by the Earth. When this coverage area is projected onto a flat map, however, the area within the resulting "circle" (or rather the flattened oval extending off the top and bottom of the map) represents where this line of sight no longer is possible. It's the area outside this circle where contact is possible.
The two primary TDRS satellites are not located directly opposite each other with respect to the Earth. Instead, each is positioned such that it has a direct line of sight to the ground station located at White Sands, N.M. This positioning causes a gap where neither satellite has contact with the orbiter/ISS. The gap is located on the opposite side of the Earth from White Sands and is referred to as the Zone of Exclusion (ZOE). This is most likely the "missing chunk" you refer to. It's actually where the two "circles" (the flattened ovals) showing where there is no coverage overlap.
Data can still be obtained in the ZOE either by ground station at Diego Garcia if the orbiter or ISS is passing over it or via a third TDRS that must relay its data through an extra path to reach controllers on the ground.
Ascent/Entry/Orbit 1 Mechanical Systems