Abstract:
A passive position fix system for a vehicle in which a gyro first provides a first gyro position of the vehicle, a map storage means which receives the first gyro position and in response thereto provides map-sets containing a set of gravity gradients and a gravity anomaly associated with geographic positions in the vacinity of the first gyro position, a gravity sensor for providing a GSS-set comprising a sensed set of gravity gradients values and a gravity anomaly at the position of the vehicle, a comparing means for sequentially receiving the map-sets so as to determine when there is a match between the GSS-set and a map-set and providing a displacement vector, and a processor for placing a second gyro position in the gyro based on the first gyro position and the displacement vector.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system for providing an accurate geographic position of a vehicle. 
     BACKGROUND OF THE INVENTION 
     In the past, gyros and accelerometers have been used in a combination, in order to provide inputs to a gyro navigation system. These inputs have been used by the gyro navigation system in order to provide a geographic position of a vehicle on which the system is placed. 
     However, with the passage of time, sensor axes of the gyros becomes less accurate, due to drift of spin axes of the rotors of the gyros. The provided geographic position of the vehicle becomes less accurate with time, due to such drift of the spin axes of the rotors of the gyros. 
     The passive position fix system of the present invention uses gyros and accelerometers of a gyro navigation system on a vehicle to find a gyro position of a vehicle on which the passive position fix system has been placed. The passive position fix system compares stored a gravity gradient and gravity anomaly map-set, for each of several geographic positions in a region around the gyro position of the vehicle, with measurements from gravity gradient sensors and a gravity anomaly sensor. The passive position fix system finds a best estimate of the position difference between the gyro position and the geographic position of the vehicle. The gyro navigation system is updated with this best estimate of position difference. The passive position fix system thus provides an updated gyro position of the vehicle. 
     The gyro navigation system, gravity gradient and gravity anomaly map-sets, and a gravity gradient and gravity anomaly sensor system are central parts of the passive position fix system. 
     The gravity gradient and gravity anomaly sensor system and the gravity gradient and gravity anomaly map-sets are used in a map matching technique in the passive position fix system. The gravity gradient and gravity anomaly sensor system and the gravity gradient and gravity anomaly map-sets can be used to periodically update the gyro position provided by the gyro navigation system. 
     The gravity gradient and gravity anomaly sensor system and gravity gradient and gravity anomaly map-sets could be used independently to provide geographic locations of a vehicle. 
     The passive position fix system has a gravity gradient and gravity anomaly sensor system. The gravity gradient and gravity anomaly sensor system firstly measures six gravity related parameters. The six measured parameters are five gravity gradients and one gravity anomaly. 
     These six measured parameters are then successively compared to each of several stored map-sets for geographic positions in the region of the gyro position as provided by the gyro navigation system. A stored map-set is a set of stored map parameters for a corresponding geographic position. A set of stored map parameters consists of five predetermined gravity gradients and one predetermined gravity anomaly of a corresponding geographic position. 
     Information derived from comparisons of the map-sets with the six measured gravity related parameters is used by a real-time filtering algorithm to compute a displacement vector between the gyro position as provided by the gyro navigation system and the geographic position whose map-set provides a match. The displacement vector is used to update the gyro position of the gyro navigation system of the passive position fix system. 
     An objective of the passive position fix system (PPFS) is to allow an operator on a vehicle to accurately determine the geographic position of the vehicle, in real-time, without exposure or radiation of energy, in a completely passive manner. 
     The objectives of the PPFS are: 
     a) passivity 
     b) accuracy 
     c) manual/automatic fix site selection and course guidance from site to site 
     SUMMARY OF THE INVENTION 
     A passive position fix system on a vehicle, the vehicle having a geographic position over the earth comprising gyro means for providing a first gyro position of the vehicle, map storage means for receiving the first gyro position and in response providing map-sets associated with geographic positions in the vicinity of the first gyro position provided by the gyro means, each map-set comprising a stored set of gravity gradients and a gravity anomaly at an associated geographic position on the earth, gravity sensor means for providing a GSS-set, the GSS-set comprising a sensed set of gravity gradients values and a gravity anomaly value at the geographic position of the vehicle, comparator means for receiving the GSS-set and for sequencially receiving the map-sets and for determining when there is a match between the GSS-set and a map-set and for providing a displacement vector based on which map-set matches the GSS-set, and processor means for placing a second gyro position in the gyro means based on the first gyro position and on the displacement vector. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of the passive position fix system. 
     FIG. 2 is a diagramic view of a vehicle over a point of land having a geographic position P 2 . 
     FIG. 3 is a block diagram of data processing sections used in a passive position fix determination. 
     FIG. 4 is a block diagram of map comparison means used in the passive position fix system. 
     FIG. 5 shows a strawman example of a passive position fix scenario. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The passive position fix system (PPFS)  10  of FIG. 1 is used on a vehicle. The vehicle has an actual geograpic position P 2 , as shown in FIG.  2 . 
     The passive position fix system  10  has an electrostatic gyro navigation system  12 . The navigator  12  provides a gyro position value p 1 . Navigation system  12  is also known as an ESGN. 
     The PPFS  10  has a map storage and retrieval means  14 . The map storage and retrival means  14  is connected to electrostatic gyro navigation system  12 . The gyro navigation system  12  sends a gyro position value p 1  to the map storage and retrieval means  14 . This gyro position value p 1  indicates, although incorrectly, that the vehicle is over a point of land having geographic position P 1 . 
     The map storage and retrival means  14  contains map-sets. Each map-set has five gravity gradient values for gravity gradients that exist at a certain height over a point of land, plus one gravity anomaly value for a gravity force that exists at the same height over that point of land. Each map-set has five gravity gradient values and one gravity anomaly value. 
     The map storage and retrieval means  14  retrieves a map MP 1  of FIG. 2, map MP 1  having a number of map-sets for geographic positions around geographic position P 1 , in response to means  14  receiving a gyro position value p 1  from system  12 . Means  14  places the map-sets of map MP 1  in a preprocessor  15 . In the map matching technique, several map-sets for geographic positions around geographic position P 1 , are thus nearly simultaneously retrieved from within means  14 , by means  14 , and stored in preprocessor  15 . 
     The PPFS  10  has a gravity gradient and gravity anomaly sensor system  16 . The system  16  is also known as a GSS. The system  16  consists of one gravity gradiometer and two gravimeters. The gravity gradiometer of sensor system  16  measures five gravity gradient values at the actual geographic position P 2  of the vehicle. One of the two gravimeters of system  16  measures one gravity value, also known as a gravity anomaly value, at the actual geographic position P 2  of the vehicle. The measured gravity gradient values and gravity anamoly value are set to preprocessor  15 . 
     The five measured gravity gradient values and one gravity anomaly value form a GSS-set. The GSS-set of the true gravity gradients and the true gravity anomaly is detected by using a continuous carrousel gradiometer and one functioning gravimeter. 
     The gradiometer has three wheels that are mutually at right angles to one another. Four accelerometers are on each wheel. Each wheel produces one in line, that is planar, gravity gradient value and one cross gravity gradient value. The six gravity gravity values, from the three wheels, are also referred to as gravity tensors. One of the gravity gradient values is not of viable use, since that gravity gradient value has a trivial value, that is a zero value, or a redundant value to that of another gravity gradient value. 
     The one functioning gravimeter measures the local force of gravity due to earth, beginning at land that is below the vehicle. From this measured value of gravity is subtracted a mean value of gravity for earth, over a wide ring of land over which the vehicle is operating. The difference value is the gravity anomaly value is sensed by the GSS  16 . 
     The PPFS  10  has the preprocessor  15  for storing and conditioning the GSS-set, and the map-data sets of map MP 1 . Again map MP 1  is provided to preprocessor  15  by means  14  based on the gyro position value p 1  as provided by gyro navigation system  12 . The map storage and retrival means  14 , and sensor system  16  are each connected to preprocessor  15 . 
     The PPFS  10  has a comparator  18 , also known as a passive position fix filter. The preprocessor  15  is connected to the comparator  18 . Computer algorithms are located within comparator  18  to successively compare each of the map-sets, with the GSS-set, all of which come from preprocessor  15 . Comparator  18  then determines a displacement vector, delta P, from the geographic position that corresponds to the gyro position value provided by the gyro navigation system  12 , to a geographic position, whose map-set matches the GSS-set. For example comparator  18  determines a displacement vector, delta P 2 , from the geographic position P 1  to geographic position P 2 , whose map-set matches the GSS-set. 
     The PPFS  10  also has a processor  19 . The processor  19  is connected to comparator  18 . Computer software in processor  19  is used for selecting/deploying nearby fix sites and for providing manual/automatic course guidance to the next site. Processor  19  uses the displacement vector, delta P 2 . This displacement vector, delta P 2 , is used in processor  19 , along with the first gyro position value p 1  from gyro navigator system  12 , to calculate a second gyro position value p 2 . The first gyro position value p 1  is replaced by the second gyro position value p 2  in gyro navigation system  12 . 
     Again, a cumulative movement of a vehicle (not shown) actually places the vehicle, having the PPFS  10  on board, over a geographic position P 2 . However, the ESGN  12  of the vehicle provides inaccurate gyro position value p 1 . 
     In an example of the process of the map matching technique, at time T 1  a gyro position value p 1  is read out of ESGN  12 . This position value is inaccurate since the submarine is at geographic position P 2 . Gyro position value p 1  is read into map storage and retrival means  14 . 
     Map storage and retrival means  14  reads out map-sets for geographic positions on map MP 1 . The map-data sets for the geographic positions on the map MP 1  are at and around geographic position P 1 . These map-sets are read into preprocessor  15 . Map storage storage and retrival means  14  contains map set S 1  for geographic position P 1 , map-set S 2  for geographic position P 2  . . . and map set S 9  for geographicposition P 9 . Geographic positions P 2 , P 3  . . . P 9  form a circle around geographic position P 1 . Geographic position P 1  is the center of the map that is read out of means  14 , since gyro position value p 1  indicates, although inacurately, that the geographic position of the vehicle is P 1 . 
     Map-set S 1  has five gravity gradient values and one gravity anomaly value for geographic position P 1 . Map-sets S 2 , S 3 , S 4  . . . S 9 , respectively for neighboring geographic positions P 2 , P 3 , P 4 , . . . P 9 , each have five gravity gradient values and one gravity anomaly value. Positions P 2 , P 3 , P 4  . . . P 9  forms a rectangle that encloses geographic position P 1 . The map-data sets S 1 , S 2 , S 3 , . . . S 9 , respectively, for geographic positions P 1 , P 2 , P 3  . . . P 9  on the map MP 1 , are serially read out of map storage and retrival means  14 . 
     A map-data set S for each of the nine geographic positions of the map MP 1  has five map gravity gradient values g xxmapP , g xymapP , g yymapP , g yzmapP , and g zzmapP  and one gravity anomaly value G mapP . Again, means  14  contains map-data sets, each map-data set having five gravity gradient values and one gravity anomaly value. 
     At time TO, the map-sets S 1 , S 2 , S 3 , S 4  . . . S 9 , are read into preprocessor  15  from map-data storage and retrival means  14 , under control of clock  22 . 
     At time TO, five gravity gradients and one gravity anomaly are sensed by GSS  16  and read into preprocessor  15 , while the vehicle is at geographic position P 2 . GSS  16  is under control of clock  22 . The gravity gradients values from GSS  16  are part of a GSS-set, G, and are g xxP2 , g xyPx , g yyP2 , g yzP2 , and g zzP2 . The gravity anomaly value from GSS  16 , also in GSS-set, G, is G P2 . These gravity gradient values and the gravity anomaly value correspond to the gravity gradients and gravity anomaly for geographic position P 2 , since the vehicle is actually over geographic position P 2 . 
     At time T 1 , the first map-set S 1 , of the map of geographic positions having map-sets S 1 , S 2 , S 3 , S 4  . . . S 9 , is read from preprocessor  15  into filter  18 , under control of clock  22 . 
     At time T 1 , the GSS-set, G, of five gravity gradient values and one gravity anomaly value, is read from preprocessor  15  into processor  18 . Preprocessor  15  is under control of clock  22 . The gravity gradient values in GSS data-set G are g xxP2 , g xyP2 , g yyP2 , g yzP2 , and g zzP2 . The gravity anomaly value in GSS-data set G is GP 2 . These gravity gradient values and the gravity anomaly value are valid and correspond to the gravity gradients and gravity anomaly for geographic position P 2 , since the submarine is actually over geographic position P 2 . At time T 1  a poor match occurs in filter  18  between map-set S 1  and GSS-set G. 
     At time T 2 , the GSS-set, G, is again read from preprocessor  15  into comparator  18 . At time T 2 , map-set, S 2 , is read from preprocessor  15  into comparator  18 . At this time, an excellent match occurs in filter  18  between the map-set S 2  and the GSS-set G. 
     Map-data set S 2  corresponds to geographic position P 2 . Map-data set S 2  has map gravity gradient values g xxmapP2 , g xymapP2 , g ypmapP2 , g yzmapP2  and g zzmapP2  and map gravity anomaly value G mapP2 . 
     At time T 2 , the comparator  18  compares the map gravity gradient value g xxmapP2  and sensed gravity gradient value g xxP2 , g xymapP2  and g xyP2 , g yymapP2  and g yyP2 , g yzmapP2  and g yzP2 , g zzmapP2  and g zzP2  that come from preprocessor  15 . The comparator  18  also compares the map gravity anomaly value G mapP2  and sensed gravity anomaly value G P2  that come from preprocessor  15 . An excellent match is found by comparator  18 . 
     Even though an excellent match has been found, the map matching process is repeated at times T 3 , T 4 , . . . T 9 , until the remaining map-sets S 3 , S 4  . . . S 9  for the map are compared with GSS-set G in comparator  18 . The comparisons of the nine map-sets S 1 , S 2 , S 3  . . . S 9  with GSS-set G reveals that the best match occurs between the map-set S 2  and the GSS-set G. The comparator  18  calculates displacement vector delta P 2 , since the vector distance from the center position P 1  of the map MP 1 , to the geographic position P 2 , is delta P 2 . P 2  is also referred to as the position difference vector. 
     Again, the map-sets S 1 , S 2 , S 3  . . . S 9  are successively read out of preprocessor  15  and the GSS-set G is repeatedly read out of preprocessor  15 . A map-set and the GSS-set are compared in comparator  18 . The comparison continues until all map-sets for the map MP 1  are compared with the GSS-set in comparator  18 . 
     A match between map-set S 2  and GSS-set G in filter  18  causes a displacement vector, delta P 2 , to be sent from filter  18  to processor  19 . The processor  19  calculates a second gyro position value p 2  by adding gyro position value p 1  and displacement vector delta P 2 . The calculated second gyro position value p 2  is transmitted to ESGN  12  from processor  19 . ESGN  12  is updated to contain second gyro position value p 2  instead of first gyro position value p 1 . ESGN  12  thus now provides correct information that the vehicle is at geographic position P 2 . 
     Components of the passive position fix system  10 , as described above, are shown in FIG.  3 . Data processing sections of the gravity sensor system  16  are shown. The first of these sections is a gravity data gathering section  16   a  for processing gravity inputs to system  16 . A preliminary data editing section  16   b  of the gravity sensor system  16  edits the gravity data, that is, places the data in proper memory locations. Then in section  16   c  of system  16 , north-east-down transformations, that is NED transformations, are calculated from the gravity data. In section  16   d  of system  16 , a smooth filter is used to refine the north-east-down gradients that have been calculated in section  16   c.    
     In the preprocessor  15  of the passive position fix system  10 , the GSS measurement is linearized. 
     In the filter  18  of passive position fix system  10 , the length of the displacement vector delta P 2  and its direction are determined. The length of the vector and its direction give an estimated position error between the true geographic position P 2  and the geographic position P 1  that corresponds to the gyro position value p 1 . The diplacement vector delta P 2  lies between the geographic position P 1 , that corresponds to the gyro position value p 1 , and the true geographic position P 2  of the vehicle. 
     In processor  19  of FIG. 3, the geographic position value p 1  and the displacement vector delta P 2  are processed by an algorithm to provide an adjusted gyro position value p 2 . The adjusted gyro position value p 2  is sent to the gyro navigator system  12 . 
     In FIG. 3, it is shown that fix-site reformatted maps are sent from map storage and retrieval means  14  to preprocessor  15 . Specific maps are sent as a result of and in agreement with the specific position value p provided by the gyro navigation system  12 . 
     In FIG. 4 it is shown that the passive position fix system  10  has major components including the gyro navigation system  12 , the map storage and retrieval means  14 , the gravity gradient and gravity anomaly sensor system  16 , preprocessor components  15   a  and  15   b , and the comparator  18 . The preprocessor  15  of FIG. 1 is shown as divided into two components  15   a  and  15   b  in FIG.  4 . Component  15   a  stores the gravity gradient portions of map-sets, in the area around a geographic position P, corresponding to gyro position value p from gyro navigation system  12 , and stores the gravity gradient portion of the GSS-set from GSS  16 . Component  15   b  stores the gravity anomaly portions of map-sets, in the area around a geographic position P, corresponding to gyro position value p from gyro navigation system  12 , and stores the gravity anomaly portion of the GSS-set from GSS  16 . 
     Again, the preprocessor  15  of FIG. 1 is divided into two components in FIG.  4 . Component  15   a  stores the gravity gradient map portions and the measured gravity gradient portion. The gravity gradient map portions come from the map storage and retrieval means  14  and the measured gradient portion comes from the GSS  16 . Component  15   b  of the preprocessor  15  stores the map gravity anomaly and the measured gravity anomaly. 
     Gradient map component  14   a  of the map storage and retrieval system  14  applies the map gravity gradients to the preprocessor component  15   a.    
     Anomaly map component  14   b  of-the map storage and retrieval system  14  applies the map gravity anomalies to the preprocessor component  15   b.    
     As shown in FIG. 4, position values are emitted from comparator  18  at periodic intervals to system  12  along the track of the the vehicle. The process described above is repeated at intervals so that a position of the vehicle is determined at various points along its track. In this way the exact position of the vehicle is arrived at over time. The corresponding exact position value is supplied to the gyro navigation system  12 . The gyro navigation system  12  is updated over time so as to hold an exact position value. 
     FIG. 5 shows a passive position fix scenario. In the scenario, the present position of the vehicle is shown as a triangle. Then the vehicle passed over a fix-site area of land. Gravity gradient and gravity anomaly maps for the fix-site area are stored in map storage and retrieval system  14 . As the vehicle passes over the fix-site number  2 , the gyro navigation system  12  is updated at periodic points along its track, as described above. After the vehicle exits the area of fix-site number  2 , the gyro navigation system  12  is completely updated and holds an exact position. 
     While the present invention has been disclosed in connection with the preferred embodiment thereof, it should be understood that there may be other embodiments which fall within the spirit and scope of the invention as defined by the following claims.