The use of GPS in the test and evaluation of weapons

Abstract

The Global Positioning System (GPS) has played a major role in weapons test and evaluation (T&E) since the early 1980s. Such use of GPS is becoming even more widespread on the T&E ranges in the 1990s. The reason for this continued use of GPS is that it has many advantages over other available range tracking techniques. ADVANTAGES OF GPS FOR RANGE TRACKING Since GPS is a satellite-based tracking system, and the distance measurement from satellites in space decides the position of the players, it requires no ground reference. The position accuracy is determined only by the accuracy to which the position of the satellites is known and our ability to measure the distance from the satellites to the player. It has nothing to do with the location of the player anyplace in the world. GPS precision code accuracy is about 50 feet for nondifferential operation and in the vicinity of 6feet when differential techniques are used, depending upon the relative position of the satellites used for position determination. The position of the player can be found to 50 feet or better despite his location. Other tracking techniques such as radar, trilateration, phototheodolites, and lasers have excellent position accuracy under certain conditions. However, these accuracies depend upon the player's position in relation to the tracking device. At extended ranges and very low tracking angles, for example, the accuracy of these tracking devices deteriorates drastically. This ability to determine accurate player position in any location is the significant advantage of GPS over alternate tracking systems. Distance from the base or tracking site has nothing to do with the accuracy of player determination. The other significant advantage of GPS for range tracking applications is that it is capable of providing position location to an unlimited number of players simultaneously. These two advantages of GPS are the main reason that GPS is the preferred method for time-space-position information (TSPI) determination in weapons test and evaluation. These advantages can be realized to their maximum extent in operational test and evaluation where operations involving many players take place over extended range areas. IMPLEMENTATION OF GPS In using GPS for target tracking, there are two principal methods in which it may be applied. First, a GPS receiver may be installed aboard the vehicle where its exact location is determined. Thus each vehicle knc .lls its precise position. This position information mdy then be recorded on the vehicle or sent by data link to a ground station. The other method of implementation is to receive the satellite signals on board the vehicle and retransmit them to the ground where the vehicle position is determined. Each of these techniques has its own advantages. A GPS receiver-data link system is usually larger than a GPS frequency translator. A translator system is therefore selected for very small vehicles such as missiles. Where GPS receiver acquisition time may be a few seconds, a GPS frequency translator processing system working with a GPS reference receiver can provide precise player position within a second of signal acquisition. This rapid acquisition time is mandatory when using GPS tracking for missile range safety. A GPS receiver-data link system, however, has the advantage of deciding position aboard the player and is the preferred system for aircraft tracking. Between these two GPS system implementations, an optimum solution can be selected for almost any range tracking problem. PACKAGING OF AIRBORNE GPS SYSTEMS FOR TEST APPLICATIONS For most test applications, the GPS hardware is temporarily added to the vehicle under test. For fighter aircraft the GPS hardware is generally installed in a pod mounted at a wing weapon pylon. These pods are the same weight and dimension as an AIM-9 missile that is mounted on the pylon. The addition of the test instrumentation thus does not cause any difference in performance of the test vehicle. For larger aircraft the GPS instrumentation is located on a plate that is internally installed. These plate installations have also been used in helicopters. For special installations on ships, these systems have been installed in small drums attached to the mast. All these applications have been for the temporary installation of test instrumentation. When GPS navigation systems are installed aboard all vehicles, a separate GPS instrumentation system will not be needed. When these GPS navigation systems are normally installed on the test vehicles and are coupled to a data bus and precise position data is available throughout the test vehicle, installing special instrumentation GPS will not be necessary. We estimate that this data will be available aboard all vehicles in about 10 years. APPLICATION OF GPS FOR TEST AND EVALUATION GPS was first used for test and evaluation on the U.S. Navy's fleet ballistic missile (FBM) program in the early 1990s. The Trident, a long-range missile, was to be tested farther at sea, and more than one could be in the air at once. The existing range tracking systems were not adequate for range safety and metric tracking. Interstate Electronics Corporation then developed a GPS tracking system for the Trident program. This was a GPS frequency translator system with a translator located on the missile. This translator received the L-band signals from all satellites in view, shifted the frequency to S-band, and retransmitted them to a ground-based translator processing system. The translator processing system, with fast signal acquisition capability, produced a state vector of the missile trajectory used by the range safety officer. In addition precise tracking information is used for guidance system evaluation. Because of the success of GPS on the Trident program, other range tracking requirements were considered for GPS application. In the mid-1980s the U.S. Department of Defense directed a study of the tracking requirements on all U.S. ranges to see if GPS offered a practical solution. The results of this study showed that 95 percent of the range tacking requirements could be satisfied with GPS and that GPS was more cost effective than alternate tracking techniques. The Air Force was then directed to develop a family of GPS instrumentation hardware for the test and training community. lnterstate Electronics was selected to perform the development of these GPS instrumentation systems. A whole family of GPS hardware and associated systems were developed. A system consisted of an instrumentation pod containing a P-code GPS receiver, an inertial reference unit, a data link transceiver, GPS and data link antennas, and an encryption device (see figure 1). This pod-mounted equipment was also furnished on a plate for internal aircraft and helicopter mounting (see figure 2). A complete data link receiving station and GPS reference station were also developed to support the airborne equipment (see figure 3). A new, smaller GPS frequency translator was then developed for reentry vehicle application along with a new ground-based translator processing system (see figures 4 and 5). These systems were applied to a variety of applications. The pod-mounted systems have been used for fighter aircraft test and evaluation such as the F-16 upgrade program at Edwards Air Force Base. The plates have been used on helicopters, transport aircraft, and small commercial aircraft for test and evaluation. In a special application one of our pod-mounted GPS receivers and an inertial reference unit was used to guide a 2,000-pound bomb to a preselected target from an altitude of approximately 30,000 feet and 16 miles downrange from the target with amazingly successful results. The GPS frequency translators were used to test the Strategic Defense Initiative (SDI) program's ERlS (exoatmospheric reentry-vehicle interception subsystem. In this application an ERlS interceptor was fired from the Kwajalein Atoll to intercept a reentry-vehicle fired on a Minuteman missile installed in both the interceptor and the reentry vehicle. From these various types of applications, the versatilityof GPS as a precise TSPl instrumentation device for weapon system test and evaluation is exhibited. THE FUTURE OF GPS IN TEST AND EVALUATION Because of its relatively high accuracy and its ability to track many vehicles simultaneously over an extended range area, GPS is becoming the desired TSPl source for the test and evaluation of weapon systems. As we look to the T&E requirements of the future, with more exotic weapons and platforms, the full advantage of GPS will be appreciated. These advantages will become more and more apparent as much larger range areas are required for the testing of long-range, high-velocity weapons and the number of weapons and targets to be simultaneously tracked is increased. GPS has earned its place in the T&E community and will be in even more demand in the future. Figure 1. Instrumentation Pod Containing a P-Code GPS Receiver, an Inertial Reference Unit Data Link Transceiver, Antennas, and an Encryption Device Figure 2. Plate Used for Internal Aircraft and Helicopter Mounting Figure 3. Complete Data Link Receiving Station and GPS Reference Station Figure 4. Small GPS Frequency Translator for Reentry Vehicle Application Figure 5. Ground-Based Frequency Translator System