Abstract:
Processing GPS signals received at moving targets and at a fixed location near instrumentation associated with timing and recording an event allows the determination of instantaneous target position to be made without a requirement for additional active tracking. An example is the recording and display of a test event involving a missile intercepting a target. The missile, target, and an instrumentation site all receive GPS signals. The signals are time tagged and the data contained thereon verified by a source and forwarded from each platform to a ground station. The data is correlated with other test data to provide a real time record and display of the missile intercepting the target. Both Time Space Position Information (TSPI) and Miss Distance Indication (MDI) are derived to a very high degree of accuracy using double difference error correction techniques. Both absolute and relative position information can be derived.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 09/488,594 filed Jan 24, 2000, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a tracking system for tracking the position of a target and a missile pursuing the target. More particularly, the present invention relates to a tracking system means for determining the dynamic positions of a target and a missile tracking the target passively by correlating multiple GPS signals and event-related data. 
     2. Description of the Prior Art 
     There is a continued pressure on the military to deliver high performance weapon systems with quantifiable performance characteristics. It is expected that these weapons systems can be procured at a cost which is comparable to presently available weapon systems. Testing, in particular flight test on a full-scale range, is a major contributor to the cost of procuring a high performance system. Any opportunity to reduce test costs and thereby save defense funds for other purchases such as weapons systems is welcomed by the military. 
     Conventional range testing is heavily reliant on radar and optical instrumentation to provide Time, Space, Position Information (TSPI) and Miss Distance Indication (MDI) or Vectoring Scoring (VS) data. These conventional fixed systems are expensive to procure, operate, and maintain and represent a “sunk cost” independent of usage of a range testing facility. Further, the manpower needed to operate these complex range testing facilities must be available at odd hours of the day in order to meet mission requirements, necessitating the payment of overtime to specially trained operators in many cases. Also, the mobility, positioning and coverage of range testing systems is dependent on terrain. Communications among the many operators also calls for an elaborate secure communications network to be procured, maintained and operated. Finally, the ability to conduct tests in a secure mode is also severely constrained when using a specific active emitter, such as a radar, for a specified duration and readily identifiable location. A preferred embodiment of the present invention addresses each of the above deficiencies in a cost effective, reliable, and efficient package. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes some of the disadvantages of the past including those mentioned above in that it comprises a relatively in design yet highly efficient and effective tracking system for providing position information relative to a missile in flight tracking a target by correlating multiple satellite generated GPS signals and event-related data. Raw position data, generated from GPS receivers onboard a moving target and a missile tracking the target, is supplied to a ground station in a IRIG (Interrange Instrumentation Group) encrypted compatible data string. To properly correlate the data and eliminate any errors in the data, a ground station GPS receiver also collects raw position data as well as emphmeris data of all the satellites in view at the ground station. The data is processed using double difference error correction techniques for post mission real time processing and kinematic processing. 
     A preferred embodiment of the present invention integrates a “semi-passive tracking” subsystem into test range instrumentation and test vehicles to provide very accurate position information. Position data is obtained by data merged from multiple Global Positioning System (GPS) receivers with instrumentation data telemetered to the ground station. 
     The term “semi-passive tracking” is used to denote that although “active” signals from GPS satellites are used, their use is not traceable to a specific test or event since anyone can use these signals at any time. Unlike radars that emit signals and track objects for a specific purpose, and thus provide a fully active signal capable of being monitored, the GPS signals are always there. 
     The ground station or analysis site is normally a fixed location, but can be mobile such as a vehicle, given the unique capabilities that the present invention provides. The unique integration of receivers for intercepting GPS signals with existing systems allows development of an accurate reproduction of the test event. Further, it can be accomplished inexpensively, and in a very secure mode at remote locations, if necessary, to address unique mission requirements. 
     Each test article such as a missile or target and its instrumentation system location is precisely known at any given moment in time, by transmitting the raw GPS data and IMU measurements in “real time” to an analysis site or ground station where the GPS data is differentially corrected to merge the IMU data. At the site, which also has its own GPS receiver for generating its own position data, the GPS position information is then merged with other test data thus fixing the dynamic position of all elements engaged in the test. By dynamic position is meant the location and attitude of an article correlated to a given moment in time. Even the location and attitude of one test article relative to other test articles can be obtained by post-processing of the data. 
     Each test articles is fitted with a GPS receiver capable of intercepting GPS signals from a satellite. These receivers are built to withstand the test environment and have unique message formatting capabilities for transmitting raw GPS measurement data to a ground station. A pre-amplifier is added to each receiver to amplify the GPS signal. The GPS signal is then input to an onboard encoder where a data validation bit is added together with a timing bit and the entire data stream is telemetered as an encrypted signal to an analysis site where data from other test articles and range instrumentation is merged. 
     Test data is provided to the ground station, in near real time, for input to formal evaluations of a weapon system, such as a missile, as it flies an actual test mission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts the flow of data from a missile and a target to a ground station for a test setup using a preferred embodiment of the present invention; 
     FIG. 2 is a view of modifications made to a telemetry unit on board a missile in order to install a portion of a preferred embodiment of the present invention; 
     FIG. 3 is an electrical block diagram of a portion of a preferred embodiment of the present invention as installed in a missile; 
     FIG. 4 depicts the Data Word format for transmitting position data to a ground station; 
     FIG. 5 is an electrical block diagram incorporating a portion of a preferred embodiment of the present invention in a ground station associated with receiving an processing data from a missile; 
     FIG. 6 depicts a layout of a portion of a preferred embodiment of the present invention as installed in a drone or target; 
     FIG. 7 is an electrical block diagram of a portion of a preferred embodiment of the present invention as installed in a drone or target; 
     FIG. 8 is an electrical block diagram incorporating a portion of a preferred embodiment of the present invention in a ground station associated with receiving an processing data from a target; and 
     FIG. 9 is a block diagram depicting the interface of missile and target ground station data to a computer for further processing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, there is shown a schematic diagram which depicts the RF signal flow path  106  of position data from a missile  101  and a target  102  to a receiving antenna  114  located at a ground or tracking station  100  for processing by the ground station  100 . Ground station  100  also has a GPS receiver  107  and its associated antenna  112  which allows the ground station to receive and process GPS position data transmitted from GPS satellites  103  to antenna  112  via GPS signal flow paths  105 . Satellites  103  also transmit GPS data to the missile  101  and the target  102 . The missile  101  may be a Sidewinder missile and the target  102  may be an MQM-107 drone. 
     Referring to FIGS. 1,  2  and  3  missile  101  has on board an AN/DKT-80 missile telemetry system  200  that is modified to include an inertial measurement unit (IMU)  205 . Receiver  303  is interfaced with the missile telemetry system  200 . In addition to the standard parameters measured by missile telemetry system  200 , added telemetered signals include Pitch, Yaw, and the GPS position data supplied to the GPS receiver  303 . The modified missile telemetry system  200  for use with GPS receiver  303  includes: (1) a GPS interface circuit added to a Frequency Select Card  202 ; (2) the GPS receiver (identified by the reference numeral  201  in FIG. 2) added to the mother board in place of the original Frequency Select Card; (3) Pitch and Yaw rate sensors (not shown); and (4) a BOA power distribution board (not shown) added to power the additional sensors. 
     The additional rate sensors provide a six degree of freedom (DOF) Inertial Measurement Unit  205  so that post-processing of position data will permit improved position update rates with the use of a Kalman filter since the GPS receiver provides position updates at 20 Hz. The missile telemetry system  200  also includes a pair of signal conditioners  203 , an encoder  204 , and a frequency select board  202  that is used to store the transmitted frequency in non-volatile random access memory (NVRAM). 
     Referring to FIGS. 1 and 3, missile  101  is modified to incorporate a GPS receiver  303 . The receiver  303  used in missile  101  is an ASHTECH G12 High Dynamic Missile Application (HDMA) GPS receiver board  303  available from Thales Navigation (formally Magellan Corporation) of Santa Clara, Calif. Receiver  303  is a twelve channel receiver which allows for a selectable rate of 20 Hz for real time guidance, tracking position and raw data. Receiver  303  also allows for all-in-view tracking of up to twelve satellites with a loss of lock re-acquisition time of less than two seconds. 
     A GPS signal from satellite  103  is received at a GPS antenna  301 , and then supplied a filter  309  which supplies its output to a pre-amplifier  302 . Filter  309  prevents saturation of pre-amplifier  302  allowing the pre-amplifier to amplify the GPS signal. Pre-amplifier  302  has an amplification/gain of about 25 dB which is the gain required by GPS receiver  303 . The Antenna  301  is a dual-band wrap around antenna which has an L 1  element which receives GPS signals and S band antenna which functions as the transmitting antenna  308  for missile  101 . The dual band wrap around antenna which is mounted upon and used by missile  101  is manufactured by HAIGH-FARR of Salem, N.H. 
     In order to accurately measure position of missile  101  for 20 Hz updates, the phase center of the antenna  301  is determined. The phase center of antenna  101  is measured at about 1.5 inches aft of the GPS antenna connector and on the surface of the missile  101 . All GPS measurements are made from the phase centers of the antennas which requires that phase centers must be accurately measured. 
     The GPS receiver  303  provides GPS position data in an RS-232 MACM (Missile Application Condensed Measurements Record) format and supplies the MACM formatted data to a microcontroller  305  via an RS-232 data line  304 . Microcontroller  305  has a UART (universal asynchronous receiver/transmitter) which receives the MACM formatted data serially and than converts the MACM formatted data to a nine bit parallel format (illustrated in FIG. 4) prior to supplying the position data to encoder  306 . Microcontroller  305  may being any commercially available computer which would fit within the limited space available in missile  101 . 
     The ASHTECH G12 receiver has two RS-232 ports. Port A is used for telemetry data. Port B is brought out to a service connector (not separately shown) located under a hatch in the skin of the missile  101  at a location (not separately shown) of the onboard telemetry (TM) system  200 . This connector is used to configure the ASHTECH G12 receiver. A holding battery (not separately shown) saves the configuration. 
     Modifications were made to the format of the telemetered data to accommodate two added rate sensors and GPS data. The bit rate was increased from 1 MBs to 1.25 MBs and the common word size is a 12-bit word which is illustrated in FIG.  4 . The receiver  303  normally uses 8-bit words at a 115.2K-baud rate. To process data at this rate, the GPS data word is sampled at 20,000 samples per second (20 KS/s). 
     As shown in FIG. 4, the first eight bits of data D 0 -D 7  of each twelve bit byte contain the GPS MACM formatted data from receiver  303 . The ninth bit D 8 , is a status bit which verifies that the 8-bit GPS data word is a valid new word. When the ninth bit D 8  is a logic one, the 8-bit GPS data word is a valid word. Similarly, when the ninth bit D 8  is a logic zero, the 8-bit GPS data word is a non-valid word which results in the word not being processed at the ground station  100 . 
     Receiver  303  provides a timing bit D 11  which is supplied to encoder  306  via a data line  310 . Bit D 11  is a timing bit which is sampled about every 60 microseconds to determine its logic state. The rising edge of the D 11  bit(logic zero to one transition) indicates when GPS time has been incremented by one second. Bit D 11  allows all data to be correlated with GPS time. For example, Bit D 11  enables the time correlation of the GPS data with the IMU  205  data in post processing. Ten MACM messages are generated by receiver  303  between the time Bit D 11  first transitions to the logic one state and the time bit D 11  again transitions to the logic one state. Bits D 9  and D 10  are unused or spare bits. 
     Encoder  306  receives the nine bit parallel MACM messages from microcontroller  305  encryptes the messages and provides the messages in a pulse code modulated (PCM) format to a transmitter  307 . Transmitter  307  transmits the GPS position data to ground station  100 , using S-band antenna  308 . 
     Referring now to FIGS. 1 and 5, FIG. 5 is an illustration of the electrical components located at the ground station  100  for receiving and processing GPS position data from missile  101 . The GPS position data from missile  101  and drone  102  is received at a TM antenna  501  located at ground station  100  and forwarded to a receiver  502 . The GPS position data is supplied to a recorder  503  and then recorded on recorder  503  along with an IRIG timing signal  504  and a range radio signal  505 . The GPS position data is also supplied to a decommutator  507 . Decommutator  507  which receives the GPS position data in a PCM data stream decommutates or breaks out the PCM data stream into its individual words. Decommutator  507  includes a parallel port which is connected to a UART  509 . Decommutator  507  is also connected to at least one strip chart  511 . 
     Decommutator  507  supplies the GPS position data which is in a parallel format to UART  509  which converts the data to an RS-232 data stream prior to supplying the GPS position data to a personal computer  108 . Current state of the art decommutators can output GPS data directly to an RS-232 Ethernet port which eliminates the UART shown in FIG.  5 . 
     At this time it should be noted that decommutator  507  is a Loral ADS-550 decommutator manufactured by Loral Space and Communications of New York, N.Y. which was used to demonstrate operability of a preferred embodiment of the present invention. 
     There is also a GPS antenna  112  and a GPS receiver  107  located at the ground at station  100 . The GPS receiver  107  at the ground station  100  operates as a reference receiver providing satellite generated GPS position data without errors to computer  108 . Since GPS receiver  107  is non-moving, computer  108  has fixed reference GPS position data provided which computer  108  processes to insure position accuracy within approximately 3-10 centimeters. The position data output by computer  108  is supplied to a range display  111  which then displays the location of missile  101 . Computer  108  also includes a display (illustrated in FIG. 1) which displays Time, Space, and Position Information for missile  101 . 
     The telemetered GPS data is converted back into an RS-232 data stream/signal at a rate of 115.2K-baud, reversing the conversion process performed by the telemetry system  200  of missile  101 . There is also a check to see if the valid bit D 8  has been set. If bit D 8  has been set, then the bit is sent through the UART  509  where it is converted into the RS-232 format originally output from receiver  303  (FIG.  3 ). Personal Computer  108 , receives the GPS signal in the original format output by receiver  303  and thereby responds as if missile  101  is communicating its position directly with computer  108 . The GPS receiver  107  also in communicates directly with personal computer  108 , enabling computer  108  to calculate a Double Difference GPS correction which insures that position information relative to missile  101  is accurate within approximately 3-10 centimeters. 
     Referring to FIGS. 1 and 6, FIG. 6 depicts the location of an ASHTECH G12 receiver, packaged with its associated telemetry, within the target/drone  102  as installed in the altimeter&#39;s housing  601  of target  102 . The telemetry system sends only the data output by the G12 receiver. The location of the dual-band antenna  602  is in the radome (forward part of the nose of target  102 ). The dual band antenna  602  is a wrap around antenna having a 5-inch diameter, which incorporates an L-band antenna for receiving the GPS data from satellites  103  and an S-band antenna which operates as the telemetry transmitter antenna. 
     Referring now to FIGS. 1,  6  and  7 , as shown in FIG. 7, the L-band antenna is antenna  701  and the S-band antenna is antenna  714 . L-band antenna  701  is connected to a filter  702  which is connected to a pre-amplifier  704 , also located in the nose. The pre-amplifier  704  amplifies the GPS signal for use by receiver  706  and telemetry package located in the altimeter housing  601 . Filter  702  prevents saturation of pre-amplifier  704  allowing the pre-amplifier to amplify the GPS signal. Pre-amplifier  704  has an amplification/gain of about 25 dB which is the gain required by GPS receiver  706 . The GPS receiver  706  provides GPS position data in the RS-232 MACM (Missile Application Condensed Measurements Record) format and supplies the MACM formatted data to an RS-232 to TTL converter/randomizer  710 . Randomizer  710  converts the GPS data which is ±12 VDC to TTL logic levels of zero to five volts DC. Randomizer  710  also randomizes the GPS position data which is burst data. Randomizer  710  insures a state transition (e.g. 0 VDC to 1 VDC) within the GPS data stream since burst data includes lengthy streams of zeros between MACM messages. Transmitter  712  is adapted to receive data at TTL logic levels prior to antenna to transmission of the GPS position data to ground station  100 . Filter  711  is a 3-pole Bessel pre-mod filter set to 0.7 times the bit rate. Filter  711  deviates the transmitter  712  with a peak-to-peak (p2p) deviation of 0.7 times the bit rate and is part of transmitter  712 . 
     The L-band antenna  701  is connected to the telemetry package within housing  601  via a GPS data line  609  and bulkhead connector  605 , while the S-band antenna  714  is connected to the telemetry package within housing  601  via a data line  607  and bulkhead connector  603 . The phase center of antenna  602  was measured and found to be about 2 inches aft of the antenna on the surface of the 5-inch diameter mounting tube of the antenna. The port B of the receiver  706  (not shown in FIG. 6) is also brought out to the radome, providing access to the receiver  706  for configuration and checkout prior to flight. A switch on the target&#39;s control panel (not shown) for activating the altimeter now activates the receiver and associated telemetry using existing cable  604 . 
     Referring to FIGS. 1,  5  and  8 , in addition to the missile support equipment (illustrated in FIG. 5) at the ground station  100 , the target  102  requires supporting equipment at the ground station  100 . FIG. 8 is a block diagram of the target&#39;s GPS ground station support equipment. The telemetered GPS position data/signal from target  102  is received by the S-band antenna  801 , forwarded to the receiver  802  and from there is sent to a bit sync circuit  804  and a tape recorder  803 . The GPS position data/signal is recorded at recorder  803  as a video signal which is output from the receiver  802 . The signal is also sent to bit sync circuit  804  where it is de-randomized, creating an NRZL (non-return to zero logic) data stream and a clock signal which is a synchronized clock signal. The NRZL data and clock are supplied to a decoder  805  for conversion from NRZL to RS-232 data and subsequent processing by a personal computer  109  that also receives reference GPS data from the ground station GPS receiver  107  and its associated GPS antenna  112 . The reference GPS data is used by personal computer  109  to eliminate position errors due the atmosphere or other conditions which may result in position data which is not accurate. The position data output by computer  109  is supplied to a range display  111  which then displays the location of target  102 . Computer includes a display (illustrated in FIG. 1) for displaying Time, Space and Position Information for target  102 . 
     Referring to FIGS. 1,  5 ,  8  and  9 , FIG. 9 is a block diagram of the electrical components within the ground station  100  for the miss distance indicator (MDI) processing, display, and archiving. This diagram is a continuation of FIGS. 5 and 8. From the UART  509  located at ground station  100  (FIG. 7) and the NRZL-to-RS-232 decoder  805  located at ground station  100  (FIG.  8 ), two RS-232 data streams are provided to personal computer  110  where one of the two RS-232 data streams is chosen as the reference. A double difference error correction is made on these two streams in the same manner as processed for the missile and drone Time, Space, and Position Information. Personal Computer  110  then displays via a display which is illustrated in FIG.  1  and also archives the MDI data. 
     The MACM message provided by receiver  303  on board missile  101  and receiver  706  on board target  102  has the following response format. 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 [MACM:4][COUNT:2][RCVTIME:4][NAVT:4] 
                 Header: 14 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [PRN:1][WRN:1][POL:1][CNO:1][PHASE:8][RANGE:4][DPL:4][LCK_TIME:4] 
                 Prn Data: 24 
               
               
                 [CHECKSUM:1] 
                 Checksum: 1 
               
               
                   
               
             
          
         
       
     
     The total message length for eight SV measurements is 207 bytes of data. 
     Bytes  1 - 5  of the MACM message [MACM: 4 ] is the name of the message which includes a sync_word (ASCII “MACM”). Bytes  5  and  6  of the MACM message [COUNT: 2 ] identifies the number of structures (PRN record) to be sent for the current epoch Bytes  7 - 10  of the MACM message [RCVTIME: 4 ] comprises a signal received in milliseconds of week GPS system time and is the lag time for all measurements and position data. Bytes  11 - 14  of the MACM message [NAVT: 4 ] comprises a receiver clock offset in meters. Beginning with byte  15  of the MACM message and continuing to the end of the MACM message, which is variable in length, the following data is provided: 
     1. PRN is a one byte message which identifies a Satellite PRN number. 
     2. WRN is a one byte message which is a Warning Flay where setting bit  1  and/or bit  2  results in the following: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Bit 1 
                 Bit 2 
                   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 Same as 22 in goodbad flag (see next field) 
               
               
                 1 
                 0 
                 Same as 23 in goodbad flag 
               
               
                 0 
                 1 
                 Same as 24 in goodbad flag 
               
               
                   
               
             
          
         
       
     
     When bit  3  is set carrier phase is questionable; when bit  4  is set code phase is questionable; when bit  5  is set code phase integration is questionable; bit  6 ,is not used; when bit  7  is set there is a possible loss of lock; and when bit  8  is set there is a loss of lock counter reset. 
     3. POL is a one message comprising a number which is either 0 or 5. The number zero means the satellite is just locked and the number 5 means the beginning of the first frame has been found. CNO is a one byte message which signal-to-noise data of satellite observation. PHASE is an eight byte message of full carrier phase measurements in cycles. RANGE is a four byte message which includes raw_range G-8 ITA record having a pseudo range in seconds. DPL is a doppler MCA record which is 10-4 Hz. LCK_TIME is a continuous count since the satellite is locked. This number is incremented about 500 times per second. Checksum is a checksum an exclusive OR of all bytes from the count just after the header to the byte before the checksum. 
     Raw GPS position data is contained the messages PHASE, RANGE, DPL and LCK_TIME. 
     Software packages allow comparison of GPS position data from missile  101  and drone  102  to that received at the ground station  100 . From this comparison, a real time TSPI solution is created. Software also enables creation of miss distance indication information for missile  101  engagement of drone  102 . 
     Software from several packages is used to process various portions of the data. Software for real time processing of data for real time differential correction and post mission scoring of missile performance is provided by WAYPOINT CONSULTING, INC. of Calgary, Canada. Two packages are used for post mission processing: MULTI-SENSOR OPTIMAL SMOOTHER ESTIMATING SOFTWARE (MOSES) and WIDE AREA DIFFERENTIAL GPS (WADGPS). MOSES merges the GPS position data and IMU data with a Kalman filter and performs kinematic processing, providing much higher update rates for missile performance estimates. 
     Real time differential processing of missile  101  and drone  102  performance is performed using raw GPS measurements (e.g., pseudorange, carrier phase, and Doppler) in the Missile Application Condensed Message (MACM) format downlinked at 20 Hz from missile  101  and drone  102 . Personal computer  108  receives data in real time from the missile  101  and the ground station reference GPS receiver  107  simultaneously. A Double Difference Pseudorange navigation solution is formed and the position of missile  101  relative to the test range boundaries is displayed on the computer  108 . Personal computer  108  also logs the missile  101  and ground GPS reference receiver  107  MACM messages to files. An ephemeris message is also logged from the ground station GPS reference receiver  107 . This sequence is carried out simultaneously, though separately, for drone  102  on personal computer  109 . 
     On personal computer  110  a Double Difference solution, yielding relative positions of drone  102  and missile  101 , is composed. Supplying the time-variant position data of missile  101  relative to drone  102 , enables formation of an miss distance indicator database for a flight test. Choosing drone  102  as the reference GPS receiver, a solution can be produced for an miss distance indicator based score for missile  101  relative to drone  102  in a moving East-North-up coordinate system with origin at the phase center of the drone&#39;s GPS antenna. This is referred to as a moving baseline and is performed by post processing the logged missile and drone data. 
     As an alternative, the data is converted to a correct data protocol and sent to the range control center to provide a real time Independent Tracking Aid. This is very useful for insuring range safety. 
     Real time differential results based upon a double difference error correction pseudorange navigation solution are estimated to yield accuracy of ±3 meters. Post mission processing of recorded data in the MACM messages uses pseudorange and carrier phase measurements to estimate integer ambiguities with kinematic processing and Kalman filtering of the IMU data. This will permit accuracy yields of 1 foot or less and measure missile altitude to less than ±0.5 degrees. 
     From the foregoing, it may readily be seen that the present invention comprises a new, unique and exceedingly useful tracking system for providing position information which constitutes a considerable improvement over the known prior art. Many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.