Calling All Spacecraft! Tracking and Unveiling the Solar System through the Deep Space Network (DSN)
When it comes to making a long-distance call, it's hard to top NASA's Deep Space Network.
The Deep Space Network, or DSN, is an international network of communication facilities that supports interplanetary spacecraft missions, as well as radio and radar astronomy observations for the exploration of the solar system and the universe. It is best known for its large dish radio antennas. The network also supports selected Earth-orbiting missions. The DSN is part of NASA's Jet Propulsion Laboratory (JPL).
The forerunner of the DSN was established in January, 1958, when JPL, then under contract to the U.S. Army, deployed portable radio tracking stations in Nigeria, Singapore, and California to receive telemetry and plot the orbit of the Army-launched Explorer 1, the first successful U.S. satellite. NASA was officially established on October 1, 1958, to consolidate the separately developing space-exploration programs of the Army, Navy, and Air Force into one civilian organization.
On December 3, 1958, JPL was transferred from the Army to NASA and given responsibility for the design and execution of lunar and planetary exploration programs using remote controlled spacecraft. Shortly after the transfer of JPL to NASA, NASA [the space agency] established the concept of the DSN as a separately managed and operated communications system that would accommodate all deep space missions, thereby avoiding the need for each flight project to acquire and operate its own specialized space communications network. The DSN was given responsibility for its own research, development, and operation in support of all of its users. Under this concept, it has become a world leader in the development of low-noise receivers; large parabolic-dish antennas; tracking, telemetry, and command systems; digital signal processing; and deep space navigation.
The largest antennas of the DSN are often called on during spacecraft emergencies. Almost all spacecraft are designed so normal operation can be conducted on the smaller (and more economical) antennas of the DSN, but during an emergency the use of the largest antennas is crucial. This is because a troubled spacecraft may be forced to use less than its normal transmitter power, attitude control problems may preclude the use of high-gain antennas, and recovering every bit of telemetry is critical to assessing the health of the spacecraft and planning the recovery. The most famous example is the Apollo 13 mission, where limited battery power and inability to use the spacecraft's high gain antennas reduced signal levels below the capability of the Manned Space Flight Network, and the use of the biggest DSN antennas (and the Australian Parkes Observatory radio telescope) was critical to saving the lives of the astronauts. Although in this case Apollo was also a USA/NASA mission, DSN also provides this same emergency service to other space agencies as well, in a spirit of inter-agency and international cooperation. For example, the recovery of the Solar and Heliospheric Observatory (SOHO) mission of the European Space Agency (ESA) would not have been possible without the use of the largest DSN facilities.
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The challenge facing the DSN is an impressive one. Imagine setting your car radio to a station, and then driving out of town. If it's a good signal, you may still pick it up after a couple hundred miles or so. In space, however, that only gets you into low Earth orbit. Some of the signals the DSN has to receive are coming from beyond the edge of our solar system. And, the transmitters on spacecraft do not have nearly the power available to them that a radio station tower does. So, how can DSN continue to receive good information from tens of millions of times farther away than your car radio can?
In short, the DSN is the largest and most sensitive scientific telecommunications system in the world. The DSN is a collection of three communications complexes that support interplanetary spacecraft missions. Each complex has several antennas with diameters ranging in size from 26 meters (85 feet) to 70 meters (230 feet). In addition, the powerful receivers also conduct radio and radar astronomy observations of our solar system and beyond, and support selected spacecraft in Earth orbit. One of the complexes is at Goldstone in California's Mojave Desert; another is near Madrid, Spain; and the third is near Canberra, Australia. This placement puts the three facilities about 120 degrees around the world from each other, allowing constant contact with spacecraft as the Earth rotates.
The antennas can be steered toward a particular direction with very high accuracy. The two-way communications system between the ground and the spacecraft makes it possible to receive telemetry data from spacecraft and determine their position and velocity and to transmit commands back to the spacecraft. The DSN is operated by NASA's Jet Propulsion Laboratory (JPL), which also operates many of the agency's interplanetary spacecraft missions.
Although the DSN wasn't formally established until 1963, its roots date back even further. In 1958, the year that NASA was established, the first antennas were built at the Goldstone site, while additional facilities were being developed in Woomera, Australia, and Johannesburg, South Africa. In 1963, the director of JPL created the DSN, combining the various facilities into a global network. Since that time, the two overseas sites have been relocated, the antennas' receiving systems have been upgraded, and antennas have been added. While the DSN originally used 26-meter (85-foot) antennas operating at low frequency L band (1.7 gigahertz [GHz]), today it makes use of 34-meter (112-foot) and 70-meter (230-foot) antennas operating at very high frequencies such as X-band (8.4 GHz) and Ka Band (32 GHz). At its beginning, the DSN usually supported only one spacecraft at a time. Today, it supports more than three dozen in a year. Some of those are so far out that getting a message to the spacecraft and receiving its response at the speed of light takes more than a day. Originally, the network could only receive 8 bits of data per second. It now regularly receives multi-megabit telemetry signals. Today, the DSN currently tracks more than 24 missions as they continue their journeys exploring the many mysteries of our Solar system, and even beyond.
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Among the spacecraft the DSN is currently communicating with are two that it has been talking to the longest. The Voyager 1 and 2 spacecraft have been communicating through the DSN since they were launched in 1977. Voyager 1, farther away than any other spacecraft, is now at the edge of our solar system, and is searching for the region where our Sun's influence ends and interstellar space begins. To compare, when the DSN sends a signal to Mars, it takes 15 minutes to reach the Red Planet. Voyager 1 is so far out that the signal takes about 13 hours just to reach the spacecraft.
While its powerful antennae make up the core of the DSN, parts of today's communication system are out of this world. NASA is now operating a communications network at Mars. Rovers on the surface have two ways of communicating with Earth. They can send signals directly back to Earth through the DSN, or they can send signals to spacecraft in orbit around Mars, which then relay the signals to Earth through the DSN. In fact, most of the scientific data from the rovers is relayed back to Earth by the Odyssey and Mars Global Surveyor spacecraft. Later this decade, the first step will be taken in expanding the DSN into space with the launch of a spacecraft to Mars with the sole purpose of being a relay communications satellite like those in orbit around the Earth.
As NASA's exploration of the solar system has spread through the solar system and beyond, the DSN has grown to provide the support they need. Perhaps, someday, people will look back on the early days when all of the DSN's antennas were still just on Earth.
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