Tracking of Spacecraft

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Tracking of Spacecraft

For tracking purposes, there are two types of space objects—cooperating and noncooperating. Normal civilian satellites and some military satellites use beacons and radar reflectors to assist ground stations in keeping track of satellite locations. Beacons are transmitters that broadcast a simple radio signal that, in essence, tells the Earth-based tracking radar, "Here I am. Here I am. Here I am." Radar reflectors are simply the normal parts of satellites that effectively reflect the radar signal. Examples of radar reflectors are solar panels and heat radiators.

How Objects Are Tracked

The U.S. Space Command has primary responsibility for keeping track of everything in orbit. Information from radars, optical systems, and space-based sensors are integrated by the Space Control Center, which is located deep underground at Cheyenne Mountain, Colorado. As of the spring of 2001, the Space Command was keeping an eye on more than 8,300 objects in space. Of these, only about 250 were active satellites.

The primary tool for this task is the computerized Space Objects Catalogue, maintained by the Space Command, which tells the system where the thousands of objects being tracked should be at any given moment. The sensors then make observations to make sure these predictions are correct—in the jargon of the operators, to see that they have not "jumped the fence." If any object has gone outside of its scheduled flight path, more sensors are alerted to see what the object is doing and to recalculate its new orbit.

Objects in orbit change orbital paths fairly often. This happens when ground control sends a command to a satellite to fire its maneuvering thrusters, for example, when a reconnaissance satellite needs to take a look at an unexpected event on Earth or when a communications satellite has its orbit adjusted to serve new customers. When these changes are detected, the Space Command must adjust its calculations to take these changes into account.

It is even more difficult to estimate the effects of Earth's upper atmosphere and the impact of radiation storms from the Sun on the behavior of space objects. In low Earth orbit , there are enough fragments of hydrogen and helium in the ionosphere (the part of the atmosphere located above about 50 kilometers [30 miles]) to exert a constant drag on all orbiting objects. This may cause objects to change orbits unexpectedly.

Coronal mass ejections send huge, unpredictable masses of energy in the form of radiation and tiny particles, toward Earth. This causes the phenomenon called the northern lights, also known as the aurora borealis. It also sometimes causes spacecraft to behave erratically, again requiring adjustments to the calculations in the Space Objects Catalogue. The Living with a Star program, launched by the National Aeronautics and Space Administration (NASA), will help scientists better understand these events.

Components of the U.S. Network

The space objects tracking network of the United States includes systems originally built to give early warning of missile attacks. The two phased-array PAVE PAWS radars, located at Otis Air Force Base on Cape Cod, Massachusetts, and at Beale Air Force Base northeast of Sacramento, California, were designed to detect submarine-launched missiles. But because a phased-array radar signal can be shaped and controlled by electronically shifting the radar's signal using the hundreds of different elements of which it is composed, the high power of this radar and its ability to electronically aim its beams in whatever direction required make it an ideal part of the space surveillance network.

In addition to the PAVE PAWS radars, the U.S. network uses the radars of the Ballistic Missile Early Warning System based at Clear in Alaska, at Thule in Greenland, and at the well-known U.S./U.K. facility at Fyling-dales in Yorkshire, England. Designed to detect Soviet intercontinental ballistic missiles, these radars provide excellent radar coverage of Earth's northern hemisphere.

The main U.S. radar specifically designed to track space objects is located at Eglin Air Force Base in Florida. Other radars are reportedly located at Incirlik, Turkey; at Kaena Point, Hawaii; on Ascension Island in the Atlantic; and on Diego Garcia in the Indian Ocean.

The Air Force Space Command's 21st Space Wing at Peterson Air Force Base in Colorado also controls the Ground Based Electro-Optical Deep Space Surveillance System (GEODSS). Its three bases are at Socorro, New Mexico; on Maui, Hawaii; and on Diego Garcia. Each base is equipped with two 1-meter (40-inch) aperture telescopes and a 36-centimeter (14-inch) aperture auxiliary telescope, in addition to low-light TV cameras and computer systems.

GEODSS operates by taking pictures of the sky, combining them, and removing known stars, a process that results in the images of space objects showing up as streaks on the resulting computer-generated image. Analysis of these streaks gives the operators of the GEODSS system information on how big the object is, how fast it is moving, and in what orbit. It is claimed that the system is sensitive enough to detect basketball-sized objects in geosynchronous orbit , 35,786 kilometers (22,300 miles) above Earth.

Air Force Space Command took over control of the Midcourse Space Experiment (MSX) satellite, which was launched in 1996 by the Ballistic Missile Defense Organization. Originally designed to test sensors for the Missile Defense Program and to collect astronomical data, MSX is equipped with an imaging infrared telescope, other infrared sensors, an ultraviolet imager, and visible light imaging systems. After its designed life was over, control was transferred to Space Command, and the satellite began serving as the space-based element of the space tracking network.

The U.S. space object tracking system is controlled and almost entirely paid for by the U.S. Department of Defense. Its capabilities are used both by NASA and, through NASA, by other international space agencies. When other nations or international agencies need information about their satellites—for example, after a malfunction—NASA serves as a civilian intermediary between them and the Defense Department.

Other Nations' Tracking Networks

The former Soviet Union had a complex space tracking network of its own based, in part, on large phased-array radars. These served the needs of the Soviet Space Tracking Network, as well as those of the country's early warning and missile defense systems. By the early twenty-first century, with Russia (the successor to the Soviet Union) struggling to remain in the forefront of space exploration and development, it had become an open question whether that nation's tracking network was a real alternative to the American system.

The Europeans are working hard on the problem of tracking space debris. Their efforts are coordinated by the European Space Agency. As of mid-2002, they have not built a worldwide space object tracking system comparable to those of the United States or Russia. The Japanese have their own tracking systems but, like the Europeans, they have limited themselves to their own region.

see also Ground Infrastructure (volume 1); Guidance and Control Systems (volume 3).

Taylor Dinerman