From: Bob Baggs, of Ottawa, Ontario
To: Tony Ceccacci, flight director for the orbit 2 team

Question: How come at certain times of the orbit there is a communications blackout or LOS?

Answer: Bob,

Great question. The space shuttle is designed to work with the Tracking and Data Relay Satellite (TDRS) System. There are only five TDRS satellites that can be used by the shuttle. Since these satellites are used by other spacecraft like the International Space Station we must share. This is why we have loss of signal (LOS) sometimes. Generally during EVAs, we have total coverage and the only time we do not have communications with the shuttle is when we are switching between TDRS. Thanks for your interest in the space program.

Steve Sides (Orbit 2 Instrumentation and Communications Officer)