Space Network (SN)
The Space Network (SN) empowers missions with reliable and secure relay communications and tracking services. The SN's global coverage enables near-continuous, bi-directional communications, a unique capability well-suited for launch vehicles and flagship missions like the International Space Station and the Hubble Space Telescope.
The SN's architecture is comprised of two segments. The space segment consists of a constellation of Tracking and Data Relay Satellites (TDRS) in geosynchronous orbit. A series of ground-based antennas make up the network's ground segment. All TDRS are within sight of at least one ground station at all times.
Before the existence of the SN, in order to communicate with the ground from orbit, satellites needed an unobstructed view of a station on the ground with antennas capable of receiving the satellite’s signal. This provided communications coverage for only about 15% of any orbit. TDRS’s relay capabilities removes the need for direct line of sight with the ground to provide communications coverage for nearly 100% of a spacecraft’s orbit.
The SN provides a variety of services to its customers. Perhaps its most important service is ensuring mission data reaches its destination. The SN’s customers gather data that help with predicting weather patterns, take pictures of the stars, and much more.
Another service is a related set of communication capabilities called Tracking, Telemetry, and Command (TT&C). Tracking helps mission operators know where their satellite is. Telemetry gathers data on the satellite such as its state of operational health. Command allows mission operators to control their spacecraft.
The Space Network is going through large-scale upgrades to its ground segment through the Space Network Ground Segment Sustainment (SGSS) project. These upgrades will modernize SN ground stations to increase data rates and volumes, improve data quality and user coverage, reduce maintenance requirements, and extend the system’s longevity. An upgrade of this magnitude while simultaneously maintaining operational viability has never before been performed.
Currently, NASA uses radio frequency (RF) signals for communication. RF has been used in space based communications for decades due to the fact that it is not affected by interference in Earth’s atmosphere such as cloud cover. A new technology, optical communications, will be used alongside RF on future relay satellites. Optical communications transmits data using infrared lasers. This technology is smaller, lighter, requires less power and can transmit 10 to 100 times more data per second than RF. What this means for future missions is that scientists can collect much more detailed data, and more room and power will be available for additional instruments on spacecraft. What’s more, the narrower beam of light optical uses to communicate makes the data harder for outsiders to access, thus making communications more secure.
SN FLEET OPERATIONS
The TDRS fleet consists of three generations of spacecraft spanning over 35 years. Each successive generation improved upon the last with additional RF band support and increased automation. There have been 12 TDRS to reach orbit. The first TDRS launched April 4, 1983, and was designed for a mission life of 10 years. The robust satellite lasted 26 years before it was decommissioned in 2009. While they were being built, and prior to launch, TDRS were referred to by letters. After launch, they are referred to by numbers. For example, the first TDRS was called TDRS-A prior to launch, and TDRS-1 once it became operational. The last in the third generation of TDRS, TDRS-M (now called TDRS-13) was launched August 18, 2017, replenishing the fleet for many more years to come.
While a space-based communications network is useful for all sorts of missions, one of the largest early drivers for the SN’s development was human space flight. When the SN was first being conceived, the Space Shuttle’s development was also on the horizon. A craft that could launch, return to Earth, and launch again later meant crewed missions could become much more frequent. The needs of living people in space to complete their tasks are much greater than the needs of a satellite. One of these needs is increased connectivity with Earth. Once the International Space Station was completed, and humans began to occupy space on a continuous basis, it became necessary to provide two way communications 24/7 for human health and safety. Now, NASA’s upcoming Artemis missions, sending the first woman and next man to the moon with an eye towards Mars, connecting our astronauts to Earth is more important than ever.
What makes the SN’s continuous communications possible is TDRS’s geosynchronous orbit. Geosynchronous orbit is a high-altitude orbit of about 22,000 miles. With this orbit, TDRS remains above the same relative point on the ground as the planet rotates. This means TDRS have a wider view of Earth and near Earth space where most spacecraft operate 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 a ground station.
The TDRS are grouped in three strategic locations: over the Indian Ocean, the Atlantic Ocean and the Pacific Ocean. This allows them to relay signals around what is known as "zones of exclusion".
Before TDRS, the United States created a number of communications satellites. Some of the first, the Echo satellites, were quite simple, technologically. These were literally giant balloons with a 100-foot 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.
As 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.
While a space-based communications network is useful for all sorts of missions, one of the largest early drivers for the SN’s development was human space flight. When the SN was first being conceived, the Space Shuttle’s development was also on the horizon. A craft that could launch, return to Earth, and launch again later meant crewed missions could become much more frequent. The needs of living people in space to complete their tasks are much greater than the needs of a satellite. One of these needs is increased connectivity with Earth.
For over 35 years, the SN has been a trusted provider of reliable and secure communications services.