Worldwide Tracking via Satellite
Small, microprocessor-controlled "smart" beacons allow the location of objects such as aircraft, ships, and cargo vehicles to be easily monitored
by M. LaVarre Bushman and M. Pike Castles
We can locate things by seeing them in reflected light or other radiation: a ship on the horizon, a soldier on a hill, a submarine by active sonar, an aircraft by radar. We can also locate things by observing the light or other radiation they emit: a distant car at night by its headlights, an earthquake by seismic waves, a downed pilot by his radio beacon, a star by its light. But frequently, we locate things through some form of communication: looking up a name in the telephone book, finding a city on a map, or receiving a radioed position report from an aircraft in flight or a ship at sea.
Of course, in order for location by communication to work, the "target" has to be able to communicate and to provide information in terms that are comprehensible to the receiver. Fifty years ago, a ship at sea determined its location by a combination of celestial observation, time-keeping, and dead reckoning, and then radioed its position to shore. Today, that same dual capability is possessed by a small box (approximately the size of a VHS cassette) assembled in the Surveillance Engineering Department at Southwest Research Institute (SwRI) with the help of modern space and communications technology.
The Institute has been involved in tracking and locating since the 1960s. Customers include various government agencies and the military services, as well as foreign clients.
Early SwRI work in tracking and location relied on small beacon transmitters and companion direction finding (DF) sets. The DF sets were configured for portable operation or for operation from vehicles such as helicopters, fixed-wing aircraft, boats, or land vehicles. A number of innovative techniques were developed by Institute engineers at the time to enhance the performance of tracking systems, including a technique in which the signal transmitted from the beacon used unique modulation codes that could be correlation-processed and recovered from receiver noise. These developments led to worldwide tracking via satellites.
Global Positioning Satellite System
One of the outstanding technological accomplishments of recent years is the $12 billion Global Positioning Satellite (GPS) system developed for and controlled by the U.S. Department of Defense. The completed GPS constellation consists of 24 satellites, in 12-hour orbits 11,000 miles above the earth, so that several are always in view from any position. Each GPS satellite transmits orbital position and time (based on atomic clocks) with an accuracy equivalent to one second in 160,000 years. By precisely measuring the time of received signals from several GPS satellites, a GPS receiver can determine its position (longitude, latitude, and altitude) with an accuracy of better than 30 meters, and can also calculate its speed and heading. Using the SwRI-developed tracking devices, GPS information is processed and formatted by an onboard microprocessor and is then transmitted to the user via a prearranged communication link.
Although the GPS is a military system, many non-military applications are evident. Candidate communication links include VHF/UHF line of sight, cellular telephone, and a variety of satellite systems. Manufacturers have recognized these opportunities, and today GPS receivers are being fabricated that are small and economical enough to be used by hikers, sailors, and surveyors, as well as by the soldiers for whom the system was originally designed.
The GPS receiver is the first component in SwRI's tracking system -- as long as the system can receive the signals transmitted by at least three GPS satellites, it knows precisely where it is.
Communicating the Information
For remote tracking, the position calculated by the GPS receiver must be communicated before it is of use. The tracking device digital data, representing longitude and latitude, can drive a display for local use, or a whole sequence of position data, together with the precise time, can be stored for later retrieval to permit an accurate reconstruction of where the receiver has been. For use in remote tracking, however, provisions must be made for transmitting the data to the interested observer.
The straightforward solution is to use a radio transmitter that transmits position data to a distant receiver. If the receiver is within the line of sight -- for example, in a high-flying surveillance aircraft -- the small, low-powered transmitter can send a continuous stream of accurate position information to the data receiver. The entire "smart" beacon package, comprising the GPS receiver, control computer, and transmitter, is small and battery-driven. However, coverage is restricted to only that part of the earth visible from the data receiver -- 100 miles at most.
If the data receiver is a geosynchronous spacecraft, coverage can include an entire hemisphere of the earth, or the entire earth's surface when several such satellites are provided. Geosynchronous orbit is at a considerable distance, so substantial transmitter (hence electrical) power and/or transmit antenna size is needed to establish the link.
Low Earth Orbit Concept
In 1989, the Institute began an internally funded project to find and exploit a middle ground: a communication system that used satellites in low earth orbit (LEO) rather than synchronous orbit, thereby reducing the transmit power/antenna requirement, while maintaining the capability of whole-earth coverage. Such a system, when fully implemented, would permit the use of small, low-powered, self-contained smart beacons that could be carried in a pocket or attached to vehicles or cargo, and that would reliably report their positions from anywhere in the world to a central data collection site.
It was recognized at the outset of this development work that suitable polar-orbit LEO satellites would not always be in view, and thus internal storage for a series of position measurements was provided in the beacon's microcomputer, together with appropriate schedules that would permit the computer to command the beacon to transmit data when a LEO satellite was in view. The data received onboard the LEO satellite would then be stored until within range of the central data receiver, at which time the satellite would be commanded to download the data.
Both of these steps involve a delay, which means the real-time aspect of the tracking operations is lost -- the location data may be a few minutes to a few hours old when received. However, for many applications such short delays are not a problem, and the concept has been carried through several testing and verification phases.
Dissemination and Display
The SwRI smart beacon, or RS-5, is equipped to communicate with LEO satellites and has been used in several recent tests of the concept. In one test, in which the beacon was carried by a U.S. Navy Vice Admiral on a round-the-world tour of military installations, details of his journey were accurately reported. In another test, the progress of the movement of the U.S. Army 25th Infantry Division from its home base in Hawaii to maneuvers in Arkansas was followed. In a recent project for a European customer, the location of a vehicle containing the beacon was successfully followed around rural Europe.
In all these cases, an important part of the program is to devise effective and efficient means of communicating the position track, as received by the LEO satellites, to possible users, and to provide map displays that make the information immediately useful.
For this purpose, Institute engineers have invested considerable time developing mapping tools that use existing CD ROM-based digital map databases to plot the progress of the smart beacon on high quality maps. The satellite-transmitted information provides a series of times with associated longitude and latitude. Other high-resolution displays are produced using optical imaging programs.
Many of the world's map resources have already been reduced to digital form, and GPS-based surveying is replacing traditional surveying methods for many applications. For example, all U.S. city maps, as well as the Ordinance Survey of Great Britain, are now available in purely digital form, and the location of a vehicle used in beacon tests can be plotted in precise detail on these maps.
The work carried out thus far by SwRI has involved the use of polar orbit LEO and geosynchronous spacecraft, and it was funded by various government agencies as well as the SwRI internal research program. Although there is no standard constellation of LEO satellites yet available to provide sufficiently frequent service, several commercial LEO satellite constellations are being designed and tested. As these constellations become available, it will be possible to program the smart beacons to use them. Furthermore, as this type of location-by-communication becomes standardized, provisions could be made for the smart beacon to receive instructions directly from the LEO regarding when to transmit data, at what interval, and with what power, thereby increasing the efficiency of the process. Institute engineers are further miniaturizing the RS-5 smart beacon, and arranging its configuration for ease of automated manufacture of an RS-6 model. The RS-6 may be the first of many generations of smart beacons used in the 21st century.
It is clear that for many purposes, the time delay inherent in the LEO concept is not acceptable, and work is continuing to refine the use of synchronous or near-synchronous satellites. Additional power required for geosynchronous communications is appropriate when using the beacons on vehicles where the power drain would be a negligible problem.
It is also clear that for purposes such as tracing valuable or hazardous cargo, power is available, and the geosynchronous satellite option may prevail because of its simpler system configuration. Work is under way on a system that is optimized for use with a constellation of geosynchronous satellite data receivers. This could develop into a widespread and standard way of keeping track of a variety of vehicles in the near future.
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