The majority of NASA's scientific investigations of the solar system are accomplished through the use of robot spacecraft. The Deep Space Network (DSN) provides the two-way communications link that guides and controls these spacecraft and brings back the spectacular planetary images and other important scientific data they collect.
The DSN consists of telecommunications complexes strategically placed on three continents—providing almost continuous contact with scientific spacecraft traveling in deep space as Earth rotates on its axis. The Deep Space Network is the largest and most sensitive scientific telecommunications system in the world. It also performs radio and radar astronomy observations in support of NASA's mission to explore the solar system and the universe. The Jet Propulsion Laboratory (JPL) in Pasadena, California, manages and operates the Deep Space Network for NASA.
The Jet Propulsion Laboratory established the predecessor to the DSN. Under a contract with the U.S. Army in January 1958, the laboratory deployed portable radio tracking stations in Nigeria (Africa), Singapore (Southeast Asia), and California to receive signals from and plot the orbit of Explorer 1—the first U.S. satellite to orbit Earth successfully. Later that year (on December 3, 1958), as part of the emergence of the new federal civilian space agency, JPL was transferred from U.S. Army jurisdiction to that of NASA. At the very onset of the U.S. civilian space program, NASA assigned JPL responsibility for the design and execution of lunar and planetary exploration programs by robot spacecraft. Shortly afterward, NASA embraced the concept of the DSN as a separately managed and operated telecommunications facility that would accommodate all deep space missions. This management decision avoided the need for each space flight project to acquire and operate its own specialized telecommunications network.
Today, the DSN features three deep space communications complexes placed approximately 120 degrees apart around the world: at Goldstone in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. This global configuration ensures that, as Earth rotates, an antenna is always within sight of a given spacecraft, day and night. Each complex contains up to 10 deep space communication stations equipped with large parabolic reflector antennas.
Every deep space communications complex within the DSN has a 230-foot- (70-m-) diameter antenna. These antennas, the largest and most sensitive in the DSN network, are capable of tracking spacecraft that are more than 10 billion miles (16 billion km) away from Earth. The 41,450-square-feet (3,850-square-meter) surface of the 230-foot- (70-m-) diameter reflector must remain accurate within a fraction of the signal wavelength, meaning that the dimensional precision across the surface is
A view of the 230-foot-(70-m-) diameter antenna of the Canberra Deep Space Communications Complex located outside Canberra, Australia. This facility is one of the three complexes that comprise NASA's deep space network (DSN). The other complexes are located in Goldstone, California, and Madrid, Spain. The national flags representing the three DSN sites appear in the foreground of this image. (NASA)
maintained to within 0.4 inch (1 cm). The dish and its mount have a mass of nearly 15.8 million pounds (7.2 X 106 kg).
There is also a 112-foot- (34-m-) diameter high-efficiency antenna at each complex, which incorporates advances in radio frequency antenna design and mechanics. The reflector surface of the 112-foot- (34-m-) diameter antenna is precision-shaped for maximum signal-gathering capability.
The most recent additions to the DSN are several 112-foot (34-m) beam waveguide antennas. On earlier DSN antennas, sensitive electronics were centrally mounted on the hard-to-reach reflector structure, making upgrades and repairs difficult. On beam waveguide antennas, the sensitive electronics are now located in a below-ground pedestal room. Telecommunications engineers bring an incident radio signal from the reflector to this room through a series of precision-machined radio frequency reflective mirrors. Not only does this architecture provide the advantage of easier access for maintenance and electronic equipment enhancements, but the new configuration also accommodates better thermal control of critical electronic components. Furthermore, engineers can place more electronics in the antenna to support operation at multiple frequencies. Three of these new 112-foot (34-m) beam waveguide antennas have been constructed at the Goldstone, California, complex, along with one each at the Canberra and Madrid complexes.
There is also one 85-foot- (26-m-) diameter antenna at each complex for tracking Earth-orbiting satellites, which travel primarily in orbits of 100 miles (160 km) to 620 miles (1,000 km) above Earth. The two-axis astronomical mount allows these antennas to point low on the horizon to acquire (pick up) fast-moving satellites as soon as they come into view. The agile 85-foot- (26-m-) diameter antennas can track (slew) at up to three degrees per second. Finally, each complex also has one 36-foot- (11-m-) diameter antenna to support a series of international Earth-orbiting missions involving very long baseline interferometry.
All of the antennas in the DSN network communicate directly with the Deep Space Operations Center (DSOC) at JPL in Pasadena, California. The DSOC staff directs and monitors operations, transmits commands, and oversees the quality of spacecraft telemetry and navigation data delivered to network users. In addition to the DSN complexes and the operations center, a ground communications facility provides communications that link the three complexes to the operations center at JPL, to spaceflight control centers in the United States and overseas, and to scientists around the world. Voice and data communications traffic between various locations is sent via landlines, submarine cable, microwave links, and communications satellites.
The Deep Space Network's radio link to scientific robot spacecraft is basically the same as other point-to-point microwave communications systems, except for the very long distances involved and the very low radio frequency signal strength received from the robot spacecraft. The total signal power arriving at a network antenna from a typical robot spacecraft encounter among the outer planets can be 20 billion times weaker than the power level in a modern digital wristwatch battery.
The extreme weakness of this radio frequency signals results from restrictions placed on the size, mass, and power supply of a particular spacecraft by the payload volume and mass-lifting limitations of its launch vehicle. Consequently, the design of the radio link is the result of engineering trade-offs between spacecraft transmitter power and antenna diameter and the signal sensitivity that engineers can build into the ground receiving system.
Typically, a spacecraft signal is limited to 20 watts, or about the same amount of power required to light the bulb in a refrigerator. When the spacecraft's transmitted radio signal arrives at Earth—from, for example, the neighborhood of Saturn—it has spread over an area with a diameter equal to about 1,000 Earth diameters. (Earth has an equatorial diameter of 7,928 miles [12,756 km]). As a result, the ground antenna is able to receive only a very small part of the signal power, which is also degraded by background radio noise, or static.
Radio noise is radiated naturally from nearly all objects in the universe, including Earth and the Sun. Noise is also inherently generated in all electronic systems including the DSN's own detectors. Since noise will always be amplified along with the signal, the ability of the ground receiving system to separate noise from the signal is critical. The DSN uses state-of-the-art, low-noise receivers and telemetry coding techniques to create unequaled sensitivity and efficiency.
Telemetry is basically the process of making measurements at one point and transmitting the data to a distant location for evaluation and use. A robot spacecraft sends telemetry to Earth by modulating data onto its communications downlink. Telemetry includes state-of-health data about the spacecraft's subsystems and science data from its instruments. A typical scientific spacecraft transmits its data in binary code, using only the symbols 1 and 0. The spacecraft's data-handling subsystem (telemetry system) organizes and encodes these data for efficient transmission to ground stations back on Earth. The ground stations have radio antennas and specialized electronic equipment to detect the individual bits, decode the data stream, and format the information for subsequent transmission to the data user (usually a team of scientists).
Data transmission from a robot spacecraft can be disturbed by noise from various sources that interferes with the decoding process. If there is a high signal-to-noise ratio, the number of decoding errors will be low. But if the signal-to-noise ratio is low, then an excessive number of bit errors can occur. When a particular transmission encounters a large number of bit errors, mission controllers will often command the spacecraft's telemetry system to reduce the data transmission rate (measured in bits per second) to give the decoder (at the ground station) more time to determine the value of each bit.
To help solve the noise problem, a spacecraft's telemetry system might feed additional or redundant data into the data stream, which additional data are then used to detect and correct bit errors after transmission. The information theory equations used by telemetry analysts in data evaluation are sufficiently detailed to allow the detection and correction of individual and multiple bit errors. After correction, the redundant digits are eliminated from the data, leaving a valuable sequence of information for delivery to the data user.
Error detecting and encoding techniques can increase the data rate many times over transmissions that are not coded for error detection. DSN coding techniques have the capability of reducing transmission errors in spacecraft science information to less than one in a million.
Telemetry is a two-way process, having a downlink as well as an uplink. Robot spacecraft use the downlink to send scientific data back to Earth, while mission controllers on Earth use the uplink to send commands, computer software, and other crucial data to the spacecraft. The uplink portion of the telecommunications process allows human beings to guide spacecraft on their planned missions, as well as to enhance mission objectives through such important activities as upgrading a spacecraft's onboard software while the robot explorer is traveling through interplanetary space. When large distances are involved, human supervision and guidance is limited to non-real time interactions with the robot spacecraft. That is why deep space robots must possess high levels of machine intelligence and autonomy.
Data collected by the DSN is also very important in precisely determining a spacecraft's location and trajectory. Teams of human beings (called the mission navigators) use these tracking data to plan all the maneuvers necessary to ensure that a particular scientific spacecraft is properly configured and is at the right place (in space) to collect its important scientific data. Tracking data produced by the DSN let mission controllers know the location of a spacecraft that is billions of miles (kilometers) away from Earth to an accuracy of just a few feet (meters).
NASA's Deep Space Network is also a multifaceted science instrument that scientists can use to improve their knowledge of the solar system and the universe. For example, scientists use the large antennas and sensitive electronic instruments of the DSN to perform experiments in radio astronomy, radar astronomy, and radio science. The DSN antennas collect information from radio signals emitted or reflected by natural celestial sources. Such DSN-acquired radio frequency data are compiled and analyzed by scientists in a variety of disciplines, including astrophysics, radio astronomy, planetary astronomy, radar astronomy, Earth science, gravitational physics, and relativity physics.
In its role as a science instrument, the DSN provides the information that is needed to: select landing sites for space missions; determine the composition of the atmospheres and/or the surfaces of the planets and their moons; search for biogenic elements in the interstellar space; study the process of star formation; image asteroids; investigate comets, especially their nuclei and comas; search the permanently shadowed regions of the Moon and Mercury for the presence of water ice; and confirm Albert Einstein's theory of general relativity.
The DSN radio science system performs experiments that allow scientists to characterize the atmospheres and ionospheres of planets, determine the compositions of planetary surfaces and rings, look through the solar corona, and determine the mass of planets, moons, and asteroids. It accomplishes this by precisely measuring the small changes that take place in a spacecraft's telemetry signal as the radio waves are scattered, refracted, or absorbed by particles and gases near celestial objects within the solar system. The DSN makes its facilities available to qualified scientists as long as the research activities do not interfere with spacecraft mission support.
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