TDRSS satellites can take advantage of satellite-to-satellite ranging to obtain exceptionally precise orbit determination.
Classical orbit determination has always relied on passive sensors such as telescopes. As technology has advanced, new ways of tracking objects in space have arisen, but passive sensors still play a role.
For example, the Space Surveillance Network of the Air Force Space Command has a system known as GEODSS (ground-based electro-optical deep-space surveillance sensors) that uses telescopes and television cameras. The telescopes scan an area of space at the same speed that distant stars appear to move, effectively keeping the stellar background static. The cameras take rapid snapshots of the area, and these are superimposed on the telescopic images to reveal any moving objects. Accuracy is excellent, but operation is restricted to night and fair weather conditions. The system offers other advantages. For example, GEODSS operators do not need to interact with any other country or program, which makes the system particularly useful for tracking objects of foreign or unknown origin. Technology of the sort used for GEODSS continues to advance. Scientists at Phillips Laboratory in New Mexico are developing a telescope that uses charge-coupled devices and advanced computer processing; Aerospace researchers are analyzing the orbit-determination accuracies possible with this new technology.
The Space Surveillance Network also uses radar to track satellites. In contrast to telescopes, which are essentially passive receivers, radar systems are considered active because they emit a microwave pulse toward a space object and measure the reflected energy.
All sensors used by the Space Surveillance Network, both passive and active, are considered noncooperative because they require no action on the part of the object being tracked. This feature permits tracking of objects that are not under U.S. control; it also enables tracking of space debris. In fact, the Space Surveillance Network is attempting to calculate the trajectories of every Earth-orbiting object bigger than a grapefruit.
Cooperative sensor systems require action by both the spacecraft and the ground station. One common example is the Space-Ground Link Subsystem, or SGLS, used by the Air Force Satellite Control Network. In this case, a pseudorandom numeric code can be imposed on an S-band carrier signal and uplinked to a spacecraft, which returns the signal after applying a frequency shift. The ground system correlates the received signal with a replica of the transmitted signal to generate a time-delay measurement. This measurement, when multiplied by the speed of light, provides an approximation of the round-trip distance. Accuracy can range from a few kilometers to a few meters, depending on the level of resources employed.
A somewhat more accurate and much more expensive technique is satellite laser ranging. In this case, a laser transmitter on the ground, combined with a telescopic/photometric receiver, bounces a precise laser pulse off a reflector on a spacecraft and computes the round-trip distance.
The ultimate cooperative technique is known as intersatellite crosslink ranging. In this technique, two satellites exploit special characteristics of a communication channel to extract ranging information. This has proved quite beneficial for both Milstar, a military communications system, and the Tracking and Data Relay Satellite System (TDRSS), primarily associated with NASA Earth-orbiting experiments. Both satellite systems are in geosynchronous orbits.
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TDRSS satellites can take advantage of satellite-to-satellite ranging to obtain exceptionally precise orbit determination. |
In the Milstar system, the crosslink ranging includes a mechanism for time transfer as well as relative distance measurements. Each satellite clock is autonomously referenced to a master clock, which is in turn synchronized to a reference cesium clock using time-offset measurements. These measurements also contain range information for estimating the orbits of each satellite. The resulting ephemerides and system time are accurate enough to permit communications and autonomy.
The orbits of the TDRSS satellites are determined from the Bilateral Ranging Transponder System (BRTS), which is similar to SGLS. In this case, however, operators can take advantage of satellite-to-satellite ranging and knowledge of the TDRSS orbits to perform their orbit estimations. For example, in processing data for Topex/Poseidon (a radar altimetry satellite), scientists found that the TDRSS ephemeris accuracy could be greatly enhanced by using TDRSS tracking from a satellite in a low Earth orbit. The Topex/Poseidon orbit was determined using satellite laser ranging and radiometric techniques. The orbits of the TDRSS satellites were then determined using one- and two-way TDRSS-Topex/Poseidon ranging. This technique reduced the TDRSS total position ephemeris error from 30 meters to less than 3 meters. Subsequently, the reduced ephemeris error improved the orbit estimations of satellites that used ranging to the TDRSS satellites in their calculations. These techniques demonstrated the benefits of satellite-to-satellite ranging for precise orbit determination.
Orbit determination for the GPS satellites is a curious mix of active and passive techniques. The GPS satellites actively radiate a radiometric signal similar to SGLS, while ground-based GPS receivers passively collect this information without providing direct feedback to the GPS satellites.
Orbit determination of other satellites using GPS moves this composite one step further. The client satellite passively collects the GPS signal, just as a ground-based receiver would; but the space-to-space measurements enjoy the geometric benefits associated with crosslinks. This added dimension allows the user satellite to apply the superior orbit determination underlying the GPS system to obtain highly accurate orbit determination for itself.