Abstract:
A navigation system for tracking the position of an object includes a GPS receiver responsive to GPS signals for periodically providing navigation state measurement updates to a navigation processor. The system also includes a dead-reckoning sensor responsive to movement of the object for providing movement measurements to the navigation processor. The navigation processor determines object navigation states using the navigation state measurement updates and propagates the object navigation states between measurement updates using the movement measurements.

Description:
RELATED APPLICATION  
       [0001]    This application is a continuation of application Ser. No. 09/662,559 filed Sep. 15, 2000 and claims the benefit of provisional application Ser. No. 60/154,003 filed Sep. 15, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to navigation systems and more particularly to a system and method of calibrating sensors in a navigation system utilizing dead-reckoning mechanisms in conjunction with GPS devices to determining the position of vehicles while the vehicle is traversing through areas of Global Positioning Satellite (GPS) signal outage.  
           [0004]    2. Description of Related Art  
           [0005]    It is often desirable to maintain location information on objects such as vehicles traveling through areas of partial or total GPS signal blockage or areas that present multi-path signals, or areas where external interferences cause GPS signal outage. Such areas may include urban canyons having natural and/or artificial structures causing partial or complete GPS signal blockage. For example, an airport location provides a challenge as there are generally numerous partial and total blockages due to overhead roadways, underground tunnels and densely populated artificial structures comprising the airport.  
           [0006]    Typical navigation systems use dead-reckoning sensors to navigate through areas of partial or total GPS signal blockage. Dead-reckoning navigation systems utilize inertial measurements mechanisms, such as a flywheel, to provide navigation during partial signal outages. Sensors used in GPS-Dead-Reckoning (GPS-DR) systems include rate gyroscopes, magnetic heading sensors, accelerometers, odometers and differential odometers or wheel tick sensors. Automotive GPS applications suffer performance degradation when signals from the GPS satellites are blocked or reflected by local terrain, buildings, tunnels, and the vehicle itself. In certain applications, an automotive navigation system may still be required to provide an output even when the satellites are not visible. Use of a dead-reckoning system in these intervals is common practice. In particular, a GPS-DR system which uses a gyroscope to maintain heading and vehicle odometer input pulses to determine distance traveled is well known.  
           [0007]    GPS and inertial sensor solutions provide a synergistic relationship when used together in hybrid navigation systems. The integration of these two types of solutions not only overcomes performance issues found in each individual solution, but produces a system whose performance exceeds that of the individual solutions. GPS provides bounded accuracy, while inertial system accuracy degrades with time.  
           [0008]    In navigation systems, GPS receiver performance issues include susceptibility to interference from external sources, time to first fix, i.e., first position solution, interruption of the satellite signal due to blockage, integrity, and signal reacquisition capability. The performance issues related to inertial sensors are their associated quality and cost.  
           [0009]    A primary concern with using GPS as a stand alone source for navigation is signal interruption. Signal interruption can be caused by shading of the GPS antenna by terrain or manmade structures, e.g., buildings, vehicle structure, and tunnels, or by interference from an external source. Generally, when only three usable satellite signals are available, most GPS receivers revert to a two-dimensional navigation mode by utilizing either the last known height or a height obtained from an external source. However, if the number of usable satellites is less than three, some receivers have the option of not producing a solution or extrapolating the last position and velocity solution forward in what is called “dead-reckoning” (DR) navigation. Position aiding from the inertial system can be used to help the GPS receiver reacquire the satellite signal. By sending vehicle position to the receiver, the receiver can accurately estimate the range from the given position to the satellites and thus initialize its internal code loops.  
           [0010]    Generally, inertial sensors are of two types—gyroscopes and accelerometers. The output of a gyroscope is a signal proportional to angular movement about its input axis and the output of an accelerometer is a signal proportional to the change in velocity sensed along its input axis. A three-axis inertial measurement unit (MU) would then require three gyroscopes and three accelerometers to inertially determine its position and velocity in free space.  
           [0011]    One of the significant factors related to the quality of an inertial system is the drift of the gyroscopes, measured in degrees/hour. The drift of a gyro is a false output signal caused by deficiencies during the manufacturing of the sensor. In inertial sensors, these are caused by mass unbalances in a gyroscope&#39;s rotating mass and by non-linearities in the pickoff circuits as is seen in fiber-optic gyroscopes. This false signal is in effect telling the navigation system that the vehicle is moving when it is actually stationary. The manufacturing cost of gyros with low drift is approximately $1,000. An inertial unit with a drift from 1 to 100 deg/hr are currently priced from approximately $1,000 to $10,000. Inertial units providing accuracies of less than 1 deg/hr are available at prices significantly higher, ranging from ten to one hundred times the cost associated with lesser accurate units.  
           [0012]    As can be seen, the quality of the inertial sensors has a large role in the cost effectiveness of a navigation system. If 0.0001-deg/hr gyroscopes were relatively inexpensive, GPS may not be needed today. However, in actuality, inertial sensors are expensive, and a significant result of the integration of GPS with inertial sensors is the ability to use lower performing, more cost-effective sensors. As mentioned above, during the operation of a navigation system when both GPS and inertial components are operational, the inertial navigation errors are bounded by the accuracy of the GPS solution. Thus, one significant contribution the GPS receiver makes to the operation of the inertial subsystem is the calibration of the inertial sensors. Inertial instruments are specified to meet a turn-on to turn-off drift requirement (each time a gyro is powered up, its initial drift rate differs.)  
           [0013]    The major errors associated with inertial sensors used in conjunction with GPS systems (GPSI systems) are the gyro bias and accelerometerbias. The gyro bias and accelerometerbias typically occupy six of the states within an inertial or GPSI Kalman filter. During the operation of a GPSI system, the Kalman filter produces an estimate of these biases as derived from the velocity data received from the GPS receiver.  
           [0014]    Integrated GPS systems, wherein GPS sensors are used in conjunction with inertial sensors (GPSI systems) have typically been accomplished by utilizing a single Kalman filter to estimate the navigation state and sensor errors. Kalman filtering is a statistical technique that combines a knowledge of the statistical nature of system errors with a knowledge of system dynamics, as represented as a state space model, to arrive at an estimate of the state of a system. In a navigation system, we are usually concerned with position and velocity, at a minimum, but its not unusual to see filters for system models with state vector dimensions ranging from six to sixty. The state estimate utilizes a weighting function, called the Kalman gain, which is optimized to produce a minimum error variance.  
           [0015]    These designs have proven effective with high-quality inertial sensors but involve substantial costs. Unfortunately, increasing the Kalman filter state vector size involves substantial design time, tuning complexity, risk of numerical difficulties, and processing power costs. This often forces navigation designers into a harsh trade between estimating significant instrument errors and increasing state vector size.  
           [0016]    Existing GPS-DR hybrid systems have evolved into several classes, switched and filtered. The switched and filtered GPS-DR hybrid systems are feed-forward designs which calibrate the instruments when GPS is available but do not use the dead-reckoning instruments to improve the operation of the GPS receiver.  
           [0017]    Switched GPS-DR systems are simple and commonly available today. These systems are effective against a blockage, but do not offer increased resistance to multi-path or improved signal reacquisition. Switched GPS-DR systems typically use NMEA output from the GPS receiver and are therefore GPS vendor independent. The system switches between two states depending on the quality of the GPS solution. The two states include providing unaided GPS output or providing the dead-reckoning state propagated with the gyro and odometer. When the GPS solution quality is satisfactory the GPS velocity vector is used to update estimates of the odometer scale factor, heading, and gyro bias.  
           [0018]    Filtered GPS-DR systems provide two independent navigation solutions, one based upon GPS and the other based upon DR sensors and/or map-matching information. The solutions are combined to produce a best solution for output or display. As in the switched system, the combined solution is not fed back to the GPS receiver to aid in rejecting multi-path or signal reacquisition. There are several map-matched implementations of this class of systems available today. In general map matching provides a path constraint which can be used to calibrate the DR sensors directly or to filter the GPS output before using the combined state to calibrate the sensors.  
           [0019]    Hence, those concerned with improving efficient application of navigation systems while minimizing costs have recognized a need for a cost effective alternative to the single Kalman filter navigation system design. Furthermore, a need exists for an alternative to the switched aid filtered GPS-DR design that provides a feed back of DR measurements to improve the solution of the GPS-DR solution. This invention fulfills those needs and others.  
         SUMMARY OF THE INVENTION  
         [0020]    Briefly, and in general terms, the present invention relates to an integrated GPS-DR navigation system that uses dead-reckoning (DR) measurements to propagate the navigation states in the GPS receiver.  
           [0021]    In a first aspect, the invention relates to a navigation system for tracking the position of an object. The navigation system includes a GPS receiver responsive to GPS signals for periodically providing navigation state measurement updates to a navigation processor. The system also includes a dead-reckoning sensor responsive to movement of the object for providing movement measurements to the navigation processor. The navigation processor determines object navigation states using the navigation state measurement updates and propagates the object navigation states between measurement updates using the movement measurements.  
           [0022]    By utilizing the dead-reckoning sensors to propagate the GPS receiver&#39;s navigation state between measurement updates, the present invention reduces the uncertainty, or process noise, associated with advancing the state from one measurement epoch to the next. The reduced uncertainties allow for a less noisy state estimation, a tighter constraint on measurements for rejection of multi-path, and improved reacquisition because of the availability of the dead-reckoned state to provide superior pre-positioning data.  
           [0023]    In a detailed aspect, the navigation processor includes a navigation update unit that receives as input an updated dead-reckoning measurement and the navigation state measurement updates and provides as output navigation measurements and a modified dead-reckoning position measurement. The navigation update unit further includes a sensor update unit that receives as input the navigation measurements and the movement measurements and provides as output position changes. Also included in the navigation update unit is a navigation propagation unit that receives as input the position changes and the modified dead-reckoning measurement and provides as output the updated dead-reckoning measurement.  
           [0024]    In a second aspect, the invention relates to a method of tracking the position of an object. The method includes periodically obtaining navigation state measurement updates and processing the navigation state measurement updates to determine object navigation states. The method also includes obtaining movement measurements related to object movement and propagating the object navigation states between measurement updates using the movement measurements.  
           [0025]    In a third aspect, the invention relates to a navigation system for providing navigation information related to the movement of an object. The navigation system includes a global positioning satellite (GPS) receiver mounted to the object for receiving GPS signals and providing GPS measurements. The system further includes a navigation update unit that receives as input an updated dead-reckoning measurement and the GPS measurements and provides as output navigation measurements and a modified dead-reckoning position measurement. The system also includes at least one inertial sensor mounted to the object for sampling movement measurements and providing as output the movement measurements. Also included in the system in a sensor update unit that receives as input the navigation measurements and the movement measurements and provides as output position changes. The system also includes a navigation propagation unit that receives as input the position change and the modified dead-reckoning measurement and provides as output the updated dead-reckoning measurement.  
           [0026]    In a detailed aspect of the invention, the navigation measurements include measurement changes in at least one of heading, gyro bias, gyro scale factor, speed bias, and speed scale factor. In another detailed facet of the invention, the sensor update unit includes a first processor for processing the changes in heading, gyro bias, and gyro scale factor along with the gyro measurement to produce a heading measurement and a second processor for processing the changes in speed bias and speed scale factor along with the speed measurement to produce changes in direction measurements. In another detailed aspect of the invention, the navigation update unit includes a first filter receiving as input the updated dead-reckoning measurement and the GPS measurements and providing as output an estimated velocity value, a second filter for estimating changes in heading, gyro bias, and gyro scale factor from first a measurement derived from the estimated velocity value and a third filter for estimating change in speed bias and speed scale factor from a second measurement derived from the estimated velocity value.  
           [0027]    In its most basic form, the navigation system closely mimics the traditional single Kalman filter design by integrating separate GPS-navigation (first), heading (second) and speed (third) Kalman filters. This configuration makes up the federated filter architecture of the current invention. The federated filter architecture very nearly matches the single Kalman filter architecture thereby achieving the benefits of the single Kalman filter architecture at a substantially reduced throughput cost.  
           [0028]    In a fourth aspect, the invention relates to a method of providing navigation information related to the movement of an object. The method includes receiving GPS signals by a global positioning satellite (GPS) receiver mounted to the object and providing as output GPS measurements. Also included is calculating navigation measurements and a modified dead-reckoning position measurement from an updated dead-reckoning measurement and the GPS measurements. Further included is the sampling of movement measurements by at least one inertial sensor mounted to the object, the processing of position changes from the navigation measurements and the movement measurements and the propagation of the updated dead-reckoning measurement calculated by the position change and the modified dead-reckoning measurement.  
           [0029]    The navigation system is capable of calibrating the rate gyro bias, gyro scale factor, and odometer pulse scale factor and provides the advantage of continuous calibration of the sensor input data to provide accurate position solutions. One advantage of the system and method of the invention is the capability of using the dead-reckoning sensors in a feed back design to propagate the navigation state as they are calibrated. This continuous feed back propagation thereby reduces the process noise model in the primary navigation filter to take advantage of the reduced uncertainty in vehicle dynamics. The system further provides for adapting the measurement editing algorithm to take advantage of the reduced dynamic uncertainty allowing for tighter edit criteria to eliminate some multi-path corrupted measurements.  
           [0030]    In a detailed aspect, the invention utilizes the last stored position, stored heading, the gyro and the odometer to generate a dead-reckoning navigation solution prior to the acquisition of GPS thereby providing a more efficient reacquisition solution. In another detailed aspect, the invention utilizes the zero turn-rate measurement to calibrate the gyro bias at start-up and allows for starting the heading sub-filter before the primary GPS filter starts to take advantage of the availability of the zero turn-rate measurement prior to the acquisition of GPS signals.  
           [0031]    In a detailed facet of the invention, each individually integrated Kalman filter (KF), i.e., the primary navigation KF, heading KF, and speed KF, performs different estimation functions. The primary navigation KF is used as a position, velocity and clock state estimator. Although this filter is not substantially altered for adaptation in the present invention, there are modifications to the tuning parameters, changes to the process noise model, and changes in the moding logic and mechanisms to navigate with degraded constellations based on the availability of a dead-reckoning state. In another detailed aspect of the invention, the navigation system resets any of the three filters without resetting the other two thereby providing the advantage of continuous solution maintenance.  
           [0032]    Furthermore, the system allows for estimating road grade from altitude changes when GPS is available, decaying the estimated road grade to zero after entering an outage with no GPS-based visibility of altitude, using the estimated road-grade to divide the odometer measured distance traveled into horizontal and vertical components for use in propagating the state-vector, and performing independent checks of the heading, speed, and primary Kalman filters against floors, ceilings, and correlation coefficient range limits.  
           [0033]    These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate byway of example, the features of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a block diagram depicting a navigation system incorporating aspects of the present invention;  
         [0035]    [0035]FIG. 2 is a block diagram of the navigation update unit utilizing a federated filter architecture;  
         [0036]    [0036]FIG. 3 is a block diagram of the sensor update unit of FIG. 1 depicting the outputs of heading and speed filters utilized to update the instrument error states of the inertial sensors employed in the present invention;  
         [0037]    [0037]FIG. 4 is a block diagram of the navigation propagation unit depicting the propagation of calibrated dead-reckoning measurements;  
         [0038]    [0038]FIG. 5 is a map depicting the ground track over which a navigation system of the present invention was tested;  
         [0039]    [0039]FIG. 6 is an enlarged detail taken from the map of FIG. 5, depicting performance of the present invention under complete blockage of GPS signals;  
         [0040]    [0040]FIG. 7 is another enlarged detail taken from the map of FIG. 5, depicting performance of the present invention under maximum partial GPS signal blockage; and  
         [0041]    [0041]FIG. 8 is yet another enlarged detail taken from the map of FIG. 5, depicting performance of the present invention where the system is subject to severe multi-path signals.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    Referring now to the drawings, wherein like reference numerals denote like or corresponding parts throughout the drawing figures, and particularly to FIG. 1, there is shown a navigation system incorporating aspects of the present invention. The system itself is typically employed within a moving object, such as an automobile or other vehicle. The system includes a navigation update unit  29 , a sensor update unit  61  and a navigation propagation unit  110 . The navigation update unit  29  receives as inputs GPS measurements  162  from a GPS receiver  28  and dead-reckoning measurements R DR(in) . Using these measurements, the navigation update unit  29  calculates changes in navigation measurements  160 . The navigation measurements  160  includes changes in heading ΔH, gyro bias ΔG B , gyro scale factor ΔG S , speed bias ΔS B  and speed scale factor ΔS S . The navigation measurements  160  are fed into the sensor update unit  61 . Additionally, the navigation update unit  29  provides as output a dead-reckoning measurement R DR(out)  for input into the navigation propagation unit  110 .  
         [0043]    The sensor update unit  61  receives movement measurements  164  provided by inertial sensors  98  and the navigation measurements  160  from the navigation update unit  29 . Using these inputs, the sensor update unit  61  produces position-change measurements  165 , including changes in north/south heading ΔN, east/west heading ΔE and time change measurement, time ΔT. The position-change measurements  165  of the sensor update unit  61  and the dead-reckoning measurement R DR(out)  of the navigation update unit  29  are utilized by the navigation propagation unit  110  to calculate a new or modified dead-reckoning measurement R DR(in) . This new dead-reckoning measurement R DR(in)  is fed back to the navigation update unit  29  where it is once again processed. This feedback processing is continuously repeated.  
         [0044]    Referring to FIG. 2, the navigation update unit  29  includes a primary filter  30 , heading filter  60 , and speed filter  70 . In a preferred embodiment of the invention the filters are Kalman filters (KF). The primary filter  30  accepts as input GPS measurements  162  from the GPS receiver  28 , dead-reckoning position R DR(in)  measurements, velocity measurements V DR  and a process noise matrix Q provided from a process noise model  33 . From these various inputs, the primary navigation filter  30  calculates an estimated GPS velocity measurement V E . The primary filter  30  is connected to the heading filter  60 , speed filter  70  and a dead-reckoning position set unit  42 .  
         [0045]    The heading filter  60  has three states, heading error, gyro bias error, and gyro scale factor error. The heading filter  60  uses two measurement types. The first is the heading estimate  37  derived from the GPS velocity estimate V E , which is updated by the primary filter  30 . The second measurement type is a zero turn-rate measurement  47 , which is provided when the vehicle odometer is reporting  48  that the vehicle is stopped. This zero turn-rate measurement  47  is often available at turn-on, before the vehicle starts to move, and prior to the acquisition of GPS signals. This measurement is used to initialize the heading filter  60  and to begin dead-reckoning navigation using saved initial position and heading measurement values prior to the acquisition of GPS signals. The capability of the heading KF  60  to initialize and run prior to the primary navigation KF  30  is yet another advantage of the present invention.  
         [0046]    The mathematics associated with the heading filter  60  are as follows, the state transition matrix for the heading filter  60  is:  
         X t+Δt =ΦX t   (Eq. 1)  
         [0047]    where the heading error, bias error, and scale factor error form the state as:  
             X   =     [           δ                 H               δ                 B               δ                 S           ]             (     Eq   .              2     )                               
 
         [0048]    and the state transition matrix using as ω the mean rate measured on the propagation interval is:  
             Φ   =     [         1         Δ                 t             ω   ·   Δ                   t             0       1       0           0       0       1         ]             (     Eq   .              3     )                               
 
         [0049]    the covariance propagation equation for the heading filter  60  is of the standard form:  
           P   t+Δt   =Φ·P   t ·Φ T   +Q   (Eq. 4)  
         [0050]    where Q is the process noise matrix provided by the process noise model  33 . Q is based on the assumption that there is random walk in each of the error terms and is of diagonal form.  
         [0051]    The covariance matrix P is checked against upper and lower limits on each propagation and the update cycle to ensure that the variances remain in the allowed range and the correlation coefficients are in [−1, 1].  
         [0052]    The zero turn-rate measurement matrix is of the form:  
         h 1 =[ 0   1   0 ]  (Eq. 5)  
         [0053]    The measurement residual is the reported gyro rate. The measurement update for the zero turn-rate measurement  47  is of the usual form, with gain  
             K   =       P   ·     h   1   T             h   1     ·   P   ·     h   1   T       +   R               (     Eq   .              6     )                               
 
         [0054]    where the measurement noise value R is chosen as a constant to represent the smallest possible turn-rate of the vehicle assuming the odometer pulses have a “dead-zone” near zero velocity and considering that the car must be accelerating but may not have triggered the first pulse. The state and covariance update with the gain are of the usual form.  
         [0055]    As described above, the heading measurement  37  to the heading filter  60  is derived from the velocity estimate V E  of the primary filter  30  as:  
         H GPS =tan −1 (V E /V N )  (Eq. 7)  
         [0056]    The heading measurement partial derivative matrix is:  
         h 2 =[1 0 0 ]  (Eq. 8)  
         [0057]    The gain and update equations are the same as for the zero turn-rate measurement  48 .  
         [0058]    Referring still to FIG. 2, the speed filter  70  has two states, a speed bias state and an odometer scale factor state. Where applicable, the speed bias state may be designed to estimate velocity disturbances generated by selective availability (SA), which is a purposeful degradation of GPS signals that raises the value of the user range error. The speed bias prevents the SA velocity disturbances from corrupting the odometer scale factor estimation. The speed bias state is a fictitious state in that there is no physical mechanism for a bias in a wheel-tick sensor. However, the speed bias state has proven effective in GPS operation.  
         [0059]    As the automotive industry moves away from a drive wheel rotation sensor towards individual wheel tick sensors, the definition of the speed pulses will become more varied. Composite speed pulses may be generated to support the speedometer and cruise control functions, or these may be replaced with messaging on a data bus. It is noted that composite speed signals generated by the manufacturer&#39;s control unit may have a speed bias, particularly if they use inductive wheel tick sensors and attempt to compensate for loss of data at very low speeds. The present invention has been applied to and tested with one composite speed signal generated by the manufacturer&#39;s control unit using wheel tick sensors from all four wheels with good results. Therefore, the present invention may be utilized in conjunction with future automotive innovations.  
         [0060]    The navigation update unit  29  provides map-matching position updates to the dead-reckoned state vector R DR(out) . This is accomplished by utilizing the estimated velocity V E  provided by the primary filter  30  as an input into the dead-reckoning position set unit  42 . The set unit  42  receives the V E  after a switch  45  is triggered. Once triggered, the set unit  42  combines a map-matched position measurement  43  with a position measurement calculated from V E  and produces as output a dead-reckoning position measurement R DR(out)  that is updated by the map-matched position measurement. Generally, this usage of map-matching is common to navigation systems and is essential to automotive applications.  
         [0061]    As described above, the primary filter  30  may also take as input a process noise matrix Q generated from the process noise model  33 . This occurs when the dead-reckoning data is good. Dead-reckoning date is good when gyro and odometer data are available, the odometer scale factor and heading have been initialized, and the heading filter  60  has been updated twice to estimate the gyro bias G B  and gyro scale factor G S . The Q matrix produced by the process noise model  33  is used by the primary filter  30  to more accurately calculate the velocity estimation V E  to be supplied to the heading filter  60  and speed filter  70 .  
         [0062]    Referring to FIG. 3, the heading filter  60  and speed filter  70  (FIG. 2) produce updates to the instrument error states which may be utilized by the sensor update unit  61  when calculating changes in position. The heading filter  60  of the navigation update unit  29  produces as output, a change in heading ΔH, a change in gyro bias ΔG B , and a change in gyro scale factor ΔG S . The speed filter  70  of the navigation update unit  29  produces as output, a change in speed bias ΔS B  and a change in speed scale factor ΔS S .  
         [0063]    The sensor update unit  61  includes a first processor, a gyro bias  62 , gyro scale  64 , integrator  68  and mixers  63 ,  66  and a second processor, comprising an odometer scale  74 , speed bias  72 , mixer  76  and trigonometric function units  84 ,  86 . The change in gyro bias ΔG B  provided by the heading filter  60  of the navigation update unit  29  is used by a gyro bias unit  62  to generate a gyro bias value G B . The gyro bias value G B  and a gyro measurement G derived from sampling a gyro  100  at various time intervals serve as inputs to a mixer  63 . The mixer  63  generates a gyro scale value G S  that is inputted into a multiplier  64 . The multiplier  64  multiplies the G S  value with the change in gyro scale factor ΔG S  provided by the heading filter  60  to generate an updated gyro scale value for input into a mixer  66 . Mixer  66  combines the updated gyro scale value, the change in heading measurement ΔH provided by the heading filter  60 , and an integrated feedback heading value H I  to generate a current heading value H. The integrated feedback heading value H I  is calculated by an integration unit  68  utilizing the current heading value H provided by mixer  66 . The feedback integration unit  68  is connected to mixer  66  in a loop configuration. The current heading value H may be used in a variety of ways, such as, to update a user display unit.  
         [0064]    The change in speed scale factor ΔS S  provided by the speed filter  70  of the navigation update unit  29  and a speed measurement S derived from sampling an odometer pulse dump  102  serve as inputs to a multiplier  74 . The multiplier  74  multiplies the two measurements and provides a speed scale value S S  for input into a mixer  76 . Mixer  76  generates an updated speed scale value S SU  by mixing the speed scale value S S  and a speed bias value S B  provided by a speed bias unit  72 . The speed bias unit calculates the speed bias value S B  from the change in speed bias measurement ΔS B  provided by the speed filter  70 .  
         [0065]    The updated speed scale value S SU  generated by mixer  76  and the heading value H generated by mixer  66  serve as inputs to a cosine function unit  84 . The cosine function unit  84  calculates from these input values changes in the north/south heading represented by ΔN. The values of ΔN are summed up by a summation unit  92  to provide a single ΔN output value. Additionally, the values S SU  and H are used by a sine function unit  86  to calculate changes in the east/west heading represented by ΔE. The values of ΔE are collected and summed up by a summation unit  94  to provide a single ΔE output value.  
         [0066]    Finally, the sensor update unit  61  receives a time measurement T by sampling time values from a receiver clock  104 . The time measurement T are sampled concurrently with the sampling of the gyro  100  and the odometer  102  as controlled by a new data interrupt signal  106 . The time measurement T is mixed with its integrated value T I , as calculated by an integration unit  88 , to provide a changes in time value ΔT. The values of ΔT are collected and summed up by a summation unit  96  to provide a single ΔT output value.  
         [0067]    Referring to FIG. 4, the dead-reckoning position measurement R DR(out)  provided by the navigation update unit  29  serve as one input to mixer  114 . The other input to mixer  114  consists of the changes in direction and time (ΔN, ΔE, and ΔT) provided by the sensor update unit  61 . The values of ΔN, ΔE, and ΔT are converted by conversion unit  112  into the earth-centered earth-fixed coordinate system. Mixer  114  combines the two measurements and provides an updated dead-reckoning position R DR(in)  measurement. This R DR(in)  value is propagated back to the navigation update unit  29 . The propagation of the R DR(m)  back to the navigation update unit provides reduced uncertainty in the vehicle dynamics thereby reducing the process noise model in the primary filter  30  of the navigation update unit  29 . Furthermore, the reduced uncertainty achieved allows for tighter edit criteria thereby allowing the system to reject GPS measurements falling outside the range of measurements provided by the tighter edit criteria. Therefore, the system is capable of eliminating some multi-path corrupted measurements that may be mistaken by the GPS receiver  28  as good measurements.  
         [0068]    Referring to FIG. 5, the present invention was tested at Los Angeles International Airport (LAX). The LAX test course has several features that tests the present inventions capability. The Sepulvada tunnel  130  provides an area of complete blockage of GPS signals for approximately 23 seconds, the LAX lower deck  135  provides severe blockage of GPS signals for a period of approximately 90 seconds and includes a 180 degree turn at the west end of the airport, and the northeast corner  140  of the course provides a rich source of multi-path signals.  
         [0069]    Referring to FIG. 6, an enlarged map is provided to illustrate the performance of the present invention while traversing the Sepulveda tunnel  130 . The system experienced complete blockage for approximately 23 seconds. The results of the present invention, depicted by a dotted line  131  is clearly satisfactory as it closely parallels the mapped road/tunnel as depicted by line  134 . The offset between the results obtained from the system  131  and the actual mapped road/tunnel  134  is due to selective availability (SA). Point  133  is the location of the last GPS position fix prior to entering the tunnel  130 . Point  132  is the location of GPS reacquisition upon exiting the tunnel  130 . As can be seen by the results obtained, there is no discernable horizontal position error growth during the signal outage. Because the tunnel is straight it can be inferred that the gyro bias was correctly estimated prior to entering the tunnel  130 .  
         [0070]    Referring to FIG. 7, an enlarged map is provided to illustrate the performance of the present invention while traversing the LAX lower deck  135 . This area of the test course maximizes GPS signal blockage. While in this area, a system may occasionally pick up one or two low elevation satellites signals or multi-path signals, but the system is primarily blocked from any usable GPS signals. This type of blockage will generally force an unaided receiver into “no navigation” mode as can be seen from path line  137 . However, as can be seen from the path line  136 , generated by the present invention, the system of the present invention successfully dead-reckoned through the turn and reacquired GPS signals without any ground track discontinuity. The estimated growth in horizontal error from the dead-reckoning interval of 90 seconds is less than 30 meters (m). It can be inferred from the successful results of the present invention within this area of maximum blockage that the gyro bias and gyro scale factor were being successfully estimated by the heading filter.  
         [0071]    Referring to FIG. 8, an enlarged map is provided to illustrate the performance of the present invention while traversing through areas subject to multi-path signals. The northeast corner  140  of the course provides such an area because situated therein is a large office building  141  having metallic surfaces that provides a rich source of multi-path signals. As can be seen from a plot result  143  provided by the navigation system of the present invention, substantially better performance and accuracy is achieved than is possible with conventional navigation systems, as seen from plot result  144 . The dead-reckoning measurements of the present invention reduces the uncertainty in the time propagation of the position states and allow tighter screening of measurements for multi-path effects.  
         [0072]    It will be apparent from the foregoing that, while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.