The current TDRS (Tracking and Data Relay Satellites) configuration is made up of several geosynchronous satellites which have been distributed in such a way that they are able to provide nearly continuous information relay service to missions. Missions such as other satellites, on-going missions, balloons & aircraft, Hubble and the ISS all utilize this. While at present 11 such satellites have been launched, at any given time there are only seven in use.
Let me make it simpler; do you remember the iconic movie “Independence Day”? In one scene Jeff Goldblum was explaining how the aliens were using our own satellites against us? Well, he was speaking of a satellite system such as that of the TDRS.
The TDRS satellites were built with the understanding that they would have to support several mission at one time. For this reason each satellite has several bands on which to communicate. In addition, the newer models (after the seventh unit) are capable of supporting more communications.
GROUND TERMINALS: There are two ground terminals for the TDRS system. Both are located at NASA’s White Sands Complex. Originally only one ground system was built, but thanks to the increased user demand NASA ordered the design/construction of the second terminal system 3.1 miles away.
Odds are that although NASA has successfully completed Laser communications bi-directionally not only in the testing of the LLCD (onboard the LADEE) but also in the download of data from the LADEE when its mission was complete. (The data took a total of 5 minutes to capture whereas had it been communicated through “normal” channels it would have required several days.) The TDRS system will still be active as laser communications require line of sight and also the satellites to date only have capability for the earlier kind. Down the line most likely we will see satellites and rovers launched that are capable of both as neither system is free from various impediments but together they are able to offer greater communications.
In addition to the TDRS system for communications there are two other NASA run communications systems. One is the Near Earth Network that similar to TDRS monitors satellites and spacecraft in low-Earth orbit; however it does this using ground based antennae’s.
Also there is the Deep Space Network which uses ultra-sensitive ground based radio telescopes. It is through this that NASA is able to monitor far off spacecraft such as those that are currently leaving the solar system – Voyager 1 & 2.
From the beautiful images sent back looking Earth bound or into the Galaxy, plus looking at weather tends, supporting the ISS or supporting the 100 or so missions on any given day, basically seen going to or in space is enabled by the TDRS system. It isn’t responsible for all the communications for crewed vehicles, but it does have an awesome reach.