The United States has been launching communications satellites into Earth orbit for 60 years. But the satellites of the Space Network are the only satellites to provide continuous communications coverage with Earth from anywhere in orbit. Thirty-five years ago, on April 4, 1983, NASA launched TDRS-1, the first of the Tracking and Data Relay Satellites (TDRS), which make up a global communications network known as the Space Network, or SN.
The launch of TDRS-1 was the culmination of decades of research and hard work. On May 25, 1961, President John F. Kennedy addressed a joint session of Congress. During that address, he famously committed the United States to “landing a man on the Moon and returning him safely to the Earth" before the end of the decade. However, this was only the first of a number of goals the president set for the nation in regards to space. One little-remembered goal he also set in that speech was to accelerate “the use of space satellites for worldwide communications.” This spurred forward work on space communications technology that would eventually become the SN.
Before the existence of the SN, in order to communicate with the ground from orbit, satellites needed to have an unobstructed view of a station on the ground with antennas capable of receiving the satellite’s signal. While NASA built a system of ground stations all over the world for this purpose, the cost to build and maintain these stations meant that building enough to provide 100 percent coverage of Earth was nearly impossible. There were always spots in a spacecraft’s orbit that would not be in view of a ground station, known as zones of exclusion.
The SN solves this problem by placing TDRS spacecraft in geosynchronous orbit. This orbit is such that, as the satellite travels around Earth, it always stays above the same relative point on the ground as the planet rotates. Geosynchronous orbit is also a high-altitude orbit of about 22,000 miles. This means TDRS have a wider view of Earth than the lower orbit user spacecraft do. Spacecraft such as the International Space Station or the Hubble Space Telescope send their signals to a TDRS, then the TDRS relays the signal back down to the ground stations in White Sands, New Mexico. The use of a TDRS satellite and the Guam ground station, made operational in 1998, provide communications to parts of the Indian Ocean that was the SN zone of exclusion.
“A lot goes into the development and operations of space hardware,” said Marco Toral, TDRS’s telecommunications system manager. “The Space Network is a national asset that provides vital communication services to a wide range of missions from Earth science to space science missions, human spaceflight missions, and service missions. The system operates 24/7, and provides approximately 500 hours of user service a day.”
Before TDRS, the United States created a number of communications satellites. Some of the first ones, the Echo satellites, were quite simple, technologically. These were literally giant balloons with a 100 feet diameter that were made of reflective Mylar. The Echo satellites functioned as enormous mirrors in orbit. The light they reflected could be seen from the ground with the naked eye. Ground stations could send radio signals up to one of the satellites, and the signals would literally bounce off of them down to another ground station in another part of the world. Later, NASA created the more advanced Syncom satellites, which stands for “synchronous communications” satellites. These were the first geosynchronous communications satellites. They were barrel-shaped spacecraft covered in solar cells to use the power of the sun to keep them running. They also spun like tops to remain stable in orbit.
Throughout the 1960’s and 70’s, technology continued to improve. Satellites soon used gyroscopes to keep stable. Gyroscopes are essentially wheels inside the spacecraft that spin to counteract force that pushes on it, so that the spacecraft itself does not spin. This allows communication satellites to have larger and more powerful antennas, as well as larger solar arrays to accommodate the power the antennas need. Advances in technology also allowed for more powerful radio frequency signals used to transmit data. This meant that ground stations could use smaller and less expensive antennas to receive the data. Eventually, these advancements made TDRS and the Space Network possible.
Over the decades, the SN has developed three generations of TDRS to increase the capacity of the network, and keep pace with the advance of technology. Collectively, the TDRS fleet has over 200 years of on-orbit operations.
With TDRS-13 joining the Space Network in 2018, there are currently a record number of 10 active TDRS. TDRS-2 was lost in the Challenger accident in 1986. TDRS 1 and 4 were retired in 2010 and 2012, respectively.
By Matthew D. Peters
NASA’s Goddard Space Flight Center, Greenbelt, Md.