Abstract:
The invention generally relates to a system for generating and transmitting a telemetry formatted message containing raw Global Positioning System (GPS) information, processed Inertial Measurement Unit (IMU) information corresponding to the position and attitude of a high speed vehicle in motion. This telemetry formatted message is received on the ground and used to improve Kalman filter operation. In particular, the telemetry formatted message is used as an input to a ground based Kalman filter that is set to track and predict the trajectory of the high speed vehicle. The telemetry formatted message content improves the overall operation of the Kalman filter by preventing Kalman filter resets that occur when a bit error is encountered in the IMU data and improves the time correlation of high data rate IMU information and low data rate GPS information, both necessary for accurate tracking of the high speed vehicle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a system for generating and transmitting a telemetry formatted message containing raw Global Positioning System (GPS) positional information, processed Inertial Measurement Unit (IMU) attitude information corresponding to a vehicle experiencing high dynamic motion. In addition to GPS and IMU information, events tagged with GPS time are formatted for telemetry output to a ground receiving station. The GPS and IMU telemetry formatted message is received by the ground receiving station and used to improve Kalman filter operation. In particular, the GPS and IMU telemetry formatted message is used as an input to a ground based Kalman filter that is set to track and predict the trajectory of the vehicle, necessary for fast tracking acquisition. The GPS and IMU telemetry formatted message content improves the overall operation of the Kalman filter by preventing Kalman filter resets that occur when a bit error is encountered in IMU data and improves the time correlation of high data rate IMU information and low data rate GPS information, both necessary for accurate tracking of the vehicle experiencing high dynamic motion. 
     2. Description of the Prior Art 
     Historically, time, space, position information (TSPI) information was obtained from radar tracking data, laser tracking data and optical tracking data. Global Positioning System (GPS) instrumentation systems have been developed and are widely used to derive TSPI data from GPS information transmitted from a vehicle and received by a ground station. TSPI data is derived by processing the GPS information and feeding vehicle positional updates into a Kalman filter which derives a track and a trajectory prediction for the vehicle. 
     Another method to obtain an even higher degree of accuracy in reported positional information requires the high speed vehicle to transmit raw GPS data and raw IMU data to a ground receiving station where ground based signal processing can be applied to time correlate the GPS and IMU data. Once time-correlated the GPS and IMU data may be used as an input to a Kalman filter for tracking and trajectory prediction. One disadvantage to correlating the IMU and GPS data and then feeding them concurrently into a Kalman filter is that an IMU bit error will reset the Kalman filter. Multiple IMU bit errors inducing multiple wait states in the Kalman filter degrades tracking accuracy. Another disadvantage of combining GPS and IMU data is the inherent difficulty in time correlating the high rate IMU data with the low rate GPS data at the receiving station. 
     This invention presents a solution to the problems of degrading tracking accuracy in the presence of IMU bit errors overcomes the inherent difficulty in time correlating GPS and IMU data by implementing an onboard vehicle message formatting system to improve ground station tracking accuracy derived by a Kalman filter. This improved tracking accuracy provides faster tracking acquisition time for a vehicle in high dynamic motion. 
     SUMMARY OF THE INVENTION 
     The preferred embodiment is a message formatting system that is compact, lightweight, consumes a low amount of power and can be configured to operate in a small high speed vehicle such as a missile. The message formatting system combines raw Global Positioning System (GPS) information, processed Inertial Measurement Unit (IMU) information, system status information and event marker type information corresponding to the position and attitude of a high speed vehicle. System status is a report of the health and proper operation of the subsystems that are required for the formatting system to operate properly. An event marker is a GPS time tagged indication that a specific action has occurred or has failed to occur. In the preferred embodiment, a high speed vehicle transmits a radio frequency (RF) telemetry (TM) stream to an existing receiving station where each individual message in the RF telemetry stream contains the raw GPS measurements, the processed IMU data and the status word combined into a message called time space position information unit message structure (TUMS). The information in the transmitted TUMS message represents the current position and attitude of the high speed vehicle. The TUMS message is received by an existing TM ground station and is used as an input to a standard Kalman filter that is set to perform a tracking operation and also set to predict the trajectory of the high speed vehicle. 
     The TUMS message content improves the overall operation of the Kalman filter by preventing repeated Kalman filter resets that occur when a bit error is encountered in the high rate IMU data telemetered to the ground receiving station. Increases in the stability of the tracking data and increases in the accuracy of the trajectory predictions generated at the ground station are generally attributed to the uninterrupted operation of the Kalman filter. A key feature of the TUMS message content is the improved time correlation of the IMU and GPS data necessary for accurate tracking of the high speed vehicle. This improved time correlation is due to a unique algorithm that integrates the attitude measurements for the IMU and time correlates them to the GPS positional information received by the vehicle when in motion. Over time the algorithm provides a series of stable and correlated relative position reports that serve as the input to the standard Kalman filter. The algorithm also compresses the correlated positional information sent to the existing receiving station and reduces the overall TM data rate, increasing the overall tracking system efficiency at the existing receiving station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a General Tracking System that includes the Message Formatter comprising the invention. 
         FIG. 2  is a Block Diagram of the Message Formatter with the input and output signals identified. 
         FIG. 3  is a Data Formatter Internal Block Diagram that depicts the interface and signals a Programmable Logic Device uses to build a TUMS. 
         FIG. 4  is a flowchart that describes the application software processing that is used to build the TUMS. 
         FIG. 5  is a diagram describing the timing relationship between the GPS data, IMU data and the TUMS message output. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A TUMS message follows Inter-Range Instrumentation Group, IRIG-106 Part II specification for packet telemetry. This allows the TUMS message to be a packet of data formatted for a packet telemetry system or embedded in a standard PCM data stream as an asynchronous data stream that can be de-commutated to create the TUMS packet on the ground. The preferred embodiment is a TUMS message formatted for a packet telemetry system. 
     Referring to  FIGS. 1 and 2 , a general tracking system  50  is composed of Global Positioning System (GPS) signals  53  received by a GPS antenna  55  which serves as the input  57  to a Filter Limiter Amplifier (FLA)  58 . The FLA  58  contains a filter and an amplifier with a gain of 25 decibels in a single unit. The filter portion of the FLA  58  prevents the amplifier portion of the FLA  58  from being driven into saturation when confronted with an input GPS signal  57  of high strength and filters out interfering telemetry transmitted RF signals. An inertial measurement unit (IMU)  60  ( FIG. 2 ) provides measurements that represent the angular rates and acceleration of the vehicle to which the IMU  60  is attached. Angular rate and linear acceleration data is commonly known as attitude information. The IMU data  63  is fed into a Data Formatter  81  for further processing. The message formatter  61  creates the time space position information unit message structure (TUMS) that is the core of this invention. 
     Referring to  FIG. 1  TUMS  54  is an input to the telemetry (TM) processor  64  that is coupled to a TM antenna  67  to facilitate radiation of the TUMS  54  and other TM data via RF signals  70  to the receiving station&#39;s TM antenna  73 . The output of the receiving antenna  73  is coupled to an existing track processing ground station  76  which is connected to and feeding  65  a standard Kalman filter  62 . The Kalman filter  62  is the device that hosts the algorithm that calculates the present position and predicted trajectory of the moving vehicle. The present position and predicted trajectory calculated by the Kalman filter are fed  42  to user range displays  51 . 
     Referring to  FIGS. 1 and 2 , the block diagram of the message formatter  61  (illustrated in  FIG. 1 ) depicts the essential control signals  91  for handshaking and program reconfiguration. The core of the message formatter  61  is the data formatter  81 . The data formatter  81  accepts input and output signals  84  from a GPS Sensor Unit (GSU)  87 . The GSU  87  is the primary unit for processing the filtered and amplified GPS signal  59 , where GPS signal  59  of  FIG. 1  is identical to GPS signal  59  in  FIG. 2 . The data formatter  81  also accepts an output signal  63  from the IMU  60  containing high data rate information that corresponds to angular rates and acceleration of the vehicle. The Data Formatter  81  outputs the TUMS messages in a data set  54  that the Message Formatter  61  feeds the TM processor  64  ( FIG. 1 ). The TM processor  64  combines the TUMS message data set  54  with the event marker and other vehicle information  44  ( FIG. 1 ) for transmission via RF signals  70  to the ground receiving station  76 . The event markers are incoming events  46  ( FIG. 2 ) time tagged with GPS time and are related to any internal vehicle state change, any external input sensed by the vehicle or any other information that is available and transmitted to the TM ground station  76  ( FIG. 1 ) for monitoring. Referring to  FIG. 2 , the incoming events  46  serve as an input to a differential buffer  48 . The buffered event signal  44  is connected to the TM processor  64  for transmission to the track processing ground station  76 , as shown in  FIG. 1 . 
     Referring to  FIG. 2 , an input power waveform  94  that is nominally positive 28 Volts is connected directly to a Power Conditioner  99  that is integral to the Data Formatter  81 . The input power  94  is converted by power conditioner  99  to other voltages that are used to supply power  98  via power lines to the GSU  87 , to the IMU  60  and to the standard electrical components and electronic circuits integral to Data Formatter  81  as illustrated in  FIG. 3 . 
     The power conditioner  99  of  FIG. 3 , which is identical to the power conditioner  99  in  FIG. 2 , supplies a number of output voltages  310  routed to various systems and components. The output voltages  310  are positive and negative 5 Volts Direct Current (VDC) for the standard electronic circuits, a positive 3.3 VDC for developing logic level signals, a positive 1.8 VDC for the standard electronic circuits, a positive 21 VDC to power both the IMU  60  and the GSU  87  ( FIG. 1 ). The input voltage  94  must conform to the input voltage specified by the manufacturer of power conditioner  99 . The output voltages  310  available from the power conditioner  99  must be compatible with the components and electronic circuits selected for use in constructing the Message Formatter  61 . 
     An internal block diagram of the Data Formatter  81  is shown in  FIG. 3 . The preferred embodiment uses a system on a programmable chip, also known as a programmable logic device (PLD)  305 , made by the Altera Corporation of San Jose, Calif. and drawn from their Stratix family of devices. The Stratix PLD  305  is the preferred device to use as a data formatting processor due to the inherent programming flexibility, the robustness of an internal soft-core processor, the size and speed of internal random access memory (RAM), the size and speed of internal read only memory (ROM), a number of available interrupt request (IRQ) ports and the variety and quantity of external interface circuits offered within PLD  305 . PLD  305  serves as the central processor for receiving, buffering and assembling the TUMS message  54  built from the data available from the IMU signals  63  and GSU serial data  84  and also provides the Data Formatter  81  operational control. There are other PLDs classified as systems on a chip capable of performing similar functions and are available for substitution in place of the Stratix PLD  305 . 
     Critical interface circuits available as part of, and internal to, the PLD  305  are the asynchronous receiver transmitters (UARTs), not shown in  FIG. 3 . The preferred PLD  305  is internally configured with at least two UARTs. 
     Critical interface circuits available as part of and internal to the PLD  305  are the Interrupt Request (IRQ) ports, not shown in  FIG. 3 . The preferred PLD  305  is internally configured with a plurality of IRQ ports. These IRQ ports are necessary for controlling software execution as depicted in the Applications Software Flowchart that is  FIG. 4 . 
     Referring to  FIG. 3 , in order to maintain proper overall system operation the data formatter  81  requires a stable clock  385  that provides a clock signal  375  that is external to PLD  305 . A stable clock signal must provide timing resolution that can synchronize a Flash Memory Controller  368  to a Flash Memory  365  and to any other component that operates in a time critical manner. 
     Referring to  FIG. 3 , external to PLD  305  are a Flash Memory Controller  368  and a Flash Memory  365 . The configuration file for PLD  305  and the software ( FIG. 4 ) that runs on the soft-core processor internal to PLD  305  reside in Flash Memory  365 . Upon powering up the Data Formatter  81 , the Flash Memory Controller  368  loads the configuration files to configure the UARTs, the IRQ ports and downloads the application software ( FIG. 4 ) from Flash Memory  365  into PLD  305  RAM. Flash Memory  365  is directly connected to both the Flash Control device  368  and PLD  305 , as illustrated by connection  380  in  FIG. 3 . This power up sequence is depicted as block  505  ( FIG. 4 ) and is written by the user in the language specified by the Altera Corporation&#39;s manual describing the operational requirements for the Stratix device. 
     In general, an RS-232 serial converter  320  ( FIG. 3 ) which has two input ports and two output ports, is used to convert serial data into 3.3 Volt logic level electrical signals compatible with the interface circuits internal to PLD  305 . 
     A first input port of the RS-232 serial converter  320  ( FIG. 3 ) receives a configuration signal  315  for conversion to a configuration output signal  330  that has a logic level suitable for further processing by a first UART internal to PLD  305 . The configuration output signal  330  results in the setting of an IRQ, block  520  ( FIG. 4 ) to halt normal application software operation and to direct the PLD  305  to execute a bootload program residing in Read-Only-Memory, block  525  ( FIG. 4 ) to accept a new version of the application software for loading into flash memory  365  ( FIG. 3 ). The first UART is internally configured as a configuration port. 
     A second input port of the RS-232 serial converter  320  receives GSU serial data  84  for conversion to a serial data output signal  325  that has a logic level suitable for further processing by a second UART internal to PLD  305 . The processing step is triggered by an IRQ, block  530  ( FIG. 4 ) set by the second UART indicating that serial data  325  is ready for further processing. Further signal processing includes the steps of receiving and storing serial data  325  into PLD  305  RAM. The steps of receiving and storing serial data  325  are also represented by block  535  ( FIG. 4 ). The second UART is to be configured as a serial port. 
     Referring to  FIG. 3 , external to the PLD  305  is an electronic device  345  that conditions and samples the raw high data rate analog IMU signals  63  for subsequent conversion to a sixteen bit digital word  392  compatible with a parallel input port interface that is internal to PLD  305 . 
     Electronic device  345  has an input to accept six parallel IMU signals  63  and is connects them to a signal conditioning circuit  348 . The output of the signal conditioning circuit  352  is applied as an input to a sample and hold circuit  350  having six input lines. The output of the sample and hold circuit  354  are six stable analog signals suitable for input to multiplexer  355 . Multiplexer  355  combines the six stable analog signals  354  into a single analog signal  356  suitable for input to an analog to digital converter (ADC)  357 . ADC  357  performs a conversion of analog signal  356  to a sixteen bit digital signal  358 . Digital signal  358  is connected to the input of a first-in-first-out (FIFO) buffer  360  for further processing by the internal soft-core processor of PLD  305 . Further signal processing is composed of setting an IRQ, block  550  in  FIG. 4  to indicate that the Analog-to-Digital (A/D) conversion is complete and to begin storing the high rate IMU messages, block  555  ( FIG. 4 ) into PLD  305  RAM. 
     The preferred electronic device  345  provides a plurality of signal conditioning circuits  348 , a plurality of sample and hold circuits  350 , a multiplexer  355 , an ADC  357 , and at least one FIFO buffer  360  all contained in a single integrated circuit. Electronic device  345  is manufactured by Burr Brown, a Texas Instrument company, a leading integrated circuit manufacturer. 
     Referring to  FIG. 3 , the variable frequency signal (VFREQ)  335 , occurs every GPS epoch and is correlated to GPS time. Using VFREQ  335  to time tag IMU signals  392  ensures that the IMU signals  392  are always in sync with the GSU serial data  84  received from GSU  87  ( FIG. 1 ). GSU  87  is configured to output VFREQ  335  ensuring that VFREQ  335  is correlated to current GPS time. The preferred embodiment will have a VFREQ  335  frequency in the range of 1000 Hertz to 2000 Hertz. 
     Time correlation of the GSU and IMU data is crucial to the usefulness of TUMS TM message  54  transmitted to the ground receiving station  76 . The timing between the IMU signals  63 , the IMU processed messages  392 , the GSU serial data  84  and the TUMS TM message  54  is represented in  FIG. 5 . The high data rate IMU signals are represented by the arrow heads labeled as  392  in  FIG. 5 , where  392  is the same  392  of  FIG. 3 . An integration of incoming IMU signals  392  is performed resulting in a medium rate of IMU processed messages  399  for every nth set of IMU signals. The integration of the high data rate IMU signals  392  is performed via a software algorithm running on the soft-core processor which is internal to PLD  305 . The algorithm begins by first retrieving the high rate IMU data from internal PLD  305  RAM, block  590  ( FIG. 4 ) and subjecting the retrieved IMU data to a successive mathematical integration for every integer number of high data rate IMU messages, block  615  in  FIG. 4 . At the completion of an integration cycle the result is an error free medium rate IMU message, illustrated by the arrowheads numbered  399  in  FIG. 5 . These error free medium rate IMU messages  399  are then stored in PLD  305  RAM, step  620  ( FIG. 4 ). An integer number of high data rate IMU messages  392  are required for each GPS cycle, between epochs, where an epoch is the instant in time a GPS measurement is made. The integration accumulation performed in block  615  eliminates the IMU signals that cause a Kalman filter to reset. Elimination of Kalman filter resets is a key objective of this invention. 
     The IMU medium rate processed messages  399  are now available for combining with the GSU serial data  84  that arrives at the triggering of an epoch. The TUMS message  54  is assembled by combining the IMU medium rate processed messages  399  with the GSU serial data  84  and a correlation time tag determined by the algorithm running on the soft-core processor internal to PLD  305 , reference  FIG. 4  software flowchart. Without this proper time correlation time tag the track processing ground station  76  ( FIG. 1 ) cannot be provided bit error free IMU data which is tightly correlated with GPS data  65  ( FIG. 1 ) for use in updating the Kalman filter  62 . 
     Referring to  FIG. 3 , a timing synchronization signal  327  that occurs every epoch (1PPE) is received from the GSU  87  ( FIG. 2 ) and is received by PLD  305  to set an IRQ, block  580  ( FIG. 4 ) to trigger processing in block  585 , which builds and formats the TUMS message. An epoch is defined as a GPS carrier phase measurement made for a given period, about every 30 seconds. The processing steps required to build the TUMS message (block  585 ) are retrieve from PLD  305  RAM the GSU data, the IMU medium rate data and the correlation time tag VFREQ  335 . Once the GSU, IMU and time tag are available the TUMS software combines GSU and IMU data into the TUMS message and stores the TUMS, block  587   FIG. 4  into PLD  305  RAM. 
     In the preferred embodiment it is necessary for the PLD  305  to ascertain proper operation of the GSU  87  ( FIG. 1 ), the IMU  60  ( FIG. 2 ), the power conditioner  99  and other critical components by monitoring a status signal  389  ( FIG. 3 ) made available to a parallel input output port, resident within PLD  305 . The status signal  389  is the final piece of data that is necessary to complete the TUMS message. 
     A complete TUMS message contains a packet header used to identify the message type and length, status words, GPS messages and IMU message and a checksum. When PLD  305  RAM contains the completely assembled TUMS message the TM IRQ, block  589 , flag will be set. The Application Software detects the setting of the TM IRQ, block  586   FIG. 4 , directing the retrieval of the TUMS message from internal PLD  305  RAM and sends the TUMS message, block  595 , to the TM Processor  64  ( FIG. 1 ) for telemetry transmit to the Track Processing Ground Station  76 . 
     Referring to  FIG. 1 , the Track Processing Ground Station  76  extracts the data  65  to feed Kalman filter  62  resulting in an improved vehicle track and range displays  51  that accurately reflect the dynamic motion of the vehicle.