Abstract:
A precise positioning method and system is disclosed for allowing an array of ground transmitters to self-survey their relative location at centimeter-level accuracy by transmitting, amongst the ground transmitters in the array, bi-directional ranging signals associated with a wideband code modulation with a chipping rate faster than 30 MHz.

Description:
TECHNICAL FIELD 
     The present invention is directed to precise local positioning systems, and more particularly to a method and system for self-surveying of ground transmitters. 
     BACKGROUND 
     Local positioning systems that employ a set of ground transmitters (pseudolites) require pre-survey of the location of each of the ground transmitters at a centimeter-level of accuracy for initializing the precise positioning capabilities of each ground transmitter. The precise relative or absolute positioning of ground transmitters often requires surveying techniques that employ GPS Real Time Kinematic (RTK) solutions. However, a GPS RTK solution may not be possible when ground transmitters are placed at locations with poor or no access to GPS satellites. For example, a GPS RTK solution may not be achievable in a mining pit. 
     According to another approach, the location of a given ground transmitter can be determined using information on the geometric motion of an array of ground transmitters. However, such an approach requires the ground transmitters to move relative to each other for self-surveying of their relative locations. 
     According to another approach, ground transmitters can self-survey their locations by tracking each other&#39;s multi-frequency carrier signals for resolving integer biases in a cascaded manner with little or no geometric motion information. However, the ground transmitters are required to transmit and track multi-frequency carrier signals, which adds to undesired complexity. 
     Thus, in view of the above problems, there is a need for a method and system to allow ground transmitters to self-survey their location at a centimeter-level of accuracy without resorting to requiring geometric motion information on the ground transmitters and without requiring the ground transmitters to transmit and track multi-frequency carrier signals. 
     SUMMARY OF THE INVENTION 
     According to one aspect of certain non-limiting embodiments, a position solution can be determined using an array of ground transmitters. Each ground transmitter of the array is capable of self-surveying its own location relative to the location of other ground transmitters in the array by generating ranging signals associated with a wideband code modulation with a chipping rate of faster than 30 MHz, and by tracking other ranging signals generated by the other ground transmitters in the array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a high-level block diagram that illustrates an array of ground transmitters, according to certain non-limiting embodiments. 
         FIG. 2  is a flowchart that illustrates some high-level steps for implementing a self-survey of an array of ground transmitters, according to certain non-limiting embodiments. 
         FIG. 3  is a block diagram that illustrates an array of self-surveyed ground transmitters providing a positioning solution to a rover by broadcasting the self-surveyed locations of the ground transmitters and the relative clock bias estimates to the rover, according to certain non-limiting embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a high-level block diagram that illustrates an array of ground transmitters, according to certain embodiments.  FIG. 1  shows an array  100  of ground transmitters  102 ,  104 ,  106 , and  108 . Each ground transmitter includes a transceiver antenna, such as transceiver antennas  102   a ,  104   a ,  106   a , and  108   a . The transceiver antennas are capable of both receiving and transmitting ranging signals. Further, each ground transmitter comprises: 1) an optional wireless data modem, such as modems  102   b ,  104   b ,  106   b , and  108   b , 2) a transmitter section, such as transmitter sections  102   c ,  104   c ,  106   c , and  108   c , 3) a receiver section, such as receiver sections  102   d ,  104   d ,  106   d , and  108   d , and 4) a single common oscillator clock, such as clocks  102   e ,  104   e ,  106   e , and  108   e.    
     Each ground transmitter in array  100  can transmit and receive wideband signals  110 . According to certain embodiments, the receiver section and the transmitter section of each ground transmitter of array  100 , are clocked by the single common oscillator clock, such as clocks  102   e ,  104   e ,  106   e , and  108   e.    
     In an array of two ground transmitters, each ground transmitter is capable of determining their relative distance to the other ground transmitter in the array. In an array of three ground transmitters, each ground transmitter is capable of determining its 2-D location. If an array of at least four ground transmitters is deployed over a local area of interest, the ground transmitters are capable of determining their 3-D relative locations, and together, the ground transmitters in the array can provide a 3-D positioning solution to a given rover. 
     In the case of an array with more than four ground transmitters, it is not necessary for all the ground transmitters to have line of sight (LOS) to all the other ground transmitters in the array. If a particular ground transmitter in the array has LOS to at least three other ground transmitters in the array, then the particular ground transmitter can be self-surveyed relative to the ground transmitters that are in the line of sight. 
       FIG. 2  is a flowchart that illustrates some high-level steps for implementing a self-survey of an array of ground transmitters, according to certain non-limiting embodiments. 
     At block  202  of  FIG. 2 , an array of ground transmitters is deployed over a local area of interest. Each ground transmitter in the array is capable of transmitting and receiving wideband ranging signals with a chipping rate that is faster than 30 MHz. At block  204 , each ground transmitter in the array transmits a wideband signal. At block  206 , each ground transmitter in the array receives wideband signals that are transmitted by other ground transmitters in the array, and which are in the line of sight of the corresponding receiving ground transmitter. 
     At block  208 , each ground transmitter that has received wideband ranging signals measures the code phases of the received wideband ranging signals. At block  210 , the code phase measurements measured by the ground transmitters are collected at a central unit. The central unit may reside in one of the ground transmitters in the array, according to certain embodiments. In other embodiments, the central unit may reside elsewhere. 
     At block  212 , the collected phase measurements are used in formulating a matrix equation. The formulation of such a matrix equation is described in greater detail herein. At block  214 , the matrix equation of block  212  is solved for cross ranges and/or relative clock biases. The solution of such a matrix equation is described in greater detail herein. At block  216 , the relative location of each ground transmitter can be determined based on the resolved cross ranges. 
     Thus, the array of ground transmitters as described with reference to  FIG. 1  and  FIG. 2  allows each ground transmitter in the array to perform a self-survey of its location relative to the location of other ground transmitters. 
     The code phase measurements that are measured by the ground transmitters in the array include information on cross ranges and clock biases. For example, in an array comprising four ground transmitters, such as in  FIG. 1 , the code phase measurements of the four ground transmitters can be mathematically modeled as illustrated below. 
     Code phase measurements at ground transmitter, GT 1 , can be modeled mathematically as:
 
Φ 1   2   =R   1-2   +T   2   −T   1 +ε 1   2 
 
Φ 1   3   =R   1-3   +T   3   −T   1 +ε 1   3 
 
Φ 1   4   =R   1-4   +T   4   −T   1 +ε 1   4 
 
     where, 
     Φ 1   2 : Code phase of signal transmitted by GT 2  and measured at GT 1   
     Φ 1   3 : Code phase of signal transmitted by GT 3  and measured at GT 1   
     Φ 1   4 : Code phase of signal transmitted by GT 4  and measured at GT 1   
     R 1-2 : Cross range between GT 1  and GT 2   
     R 1-3 : Cross range between GT 1  and GT 3   
     R 1-4 : Cross range between GT 1  and GT 4   
     T 1 : Clock bias of GT 1   
     T 2 : Clock bias of GT 2   
     T 3 : Clock bias of GT 3   
     T 4 : Clock bias of GT 4   
     ε 1   2 : Noise of signal transmitted by GT 2  and measured at GT 1   
     ε 1   3 : Noise of signal transmitted by GT 3  and measured at GT 1   
     ε 1   4 : Noise of signal transmitted by GT 4  and measured at GT 1   
     The noise (ε) includes line biases that may be pre-calibrated. 
     Code phase measurements at ground transmitter, GT 2 , can be modeled mathematically as:
 
Φ 2   1   =R   1-2   +T   1   −T   2 +ε 2   1 
 
Φ 2   3   =R   2-3   +T   3   −T   2 +ε 2   3 
 
Φ 2   4   =R   2-4   +T   4   −T   2 +ε 2   4 
 
     where, 
     Φ 2   1 : Code phase of signal transmitted by GT 1  and measured at GT 2   
     Φ 2   3 : Code phase of signal transmitted by GT 3  and measured at GT 2   
     Φ 2   4 : Code phase of signal transmitted by GT 4  and measured at GT 2   
     R 1-2 : Cross range between GT 1  and GT 2   
     R 2-3 : Cross range between GT 2  and GT 3   
     R 2-4 : Cross range between GT 2  and GT 4   
     T 1 : Clock bias of GT 1   
     T 2 : Clock bias of GT 2   
     T 3 : Clock bias of GT 3   
     T 4 : Clock bias of GT 4   
     ε 2   1 : Noise of signal transmitted by GT 1  and measured at GT 2   
     ε 2   3 : Noise of signal transmitted by GT 3  and measured at GT 2   
     ε 2   4 : Noise of signal transmitted by GT 4  and measured at GT 2   
     Code phase measurements at ground transmitter, GT 3 , can be modeled mathematically as:
 
Φ 3   1   =R   1-3   +T   1   −T   3 +ε 3   1 
 
Φ 3   2   =R   2-3   +T   2   −T   3 +ε 3   2 
 
Φ 3   4   =R   3-4   +T   4   −T   3 +ε 3   4 
 
     where, 
     Φ 3   1 : Code phase of signal transmitted by GT 1  and measured at GT 3   
     Φ 3   2 : Code phase of signal transmitted by GT 2  and measured at GT 3   
     Φ 3   4 : Code phase of signal transmitted by GT 4  and measured at GT 3   
     R 1-3 : Cross range between GT 1  and GT 3   
     R 2-3 : Cross range between GT 2  and GT 3   
     R 3-4 : Cross range between GT 3  and GT 4   
     T 1 : Clock bias of GT 1   
     T 2 : Clock bias of GT 2   
     T 3 : Clock bias of GT 3   
     T 4 : Clock bias of GT 4   
     ε 3   1 : Noise of signal transmitted by GT 1  and measured at GT 3   
     ε 3   2 : Noise of signal transmitted by GT 2  and measured at GT 3   
     ε 3   4 : Noise of signal transmitted by GT 4  and measured at GT 3   
     Code phase measurements at ground transmitter, GT 4 , can be modeled mathematically as:
 
Φ 4   1   =R   1-4   +T   1   −T   4 +ε 4   1 
 
Φ 4   2   =R   2-4   +T   2   −T   4 +ε 4   2 
 
Φ 4   3   =R   3-4   +T   3   −T   4 +ε 4   3 
 
     where, 
     Φ 4   1 : Code phase of signal transmitted by GT 1  and measured at GT 4   
     Φ 4   2 : Code phase of signal transmitted by GT 2  and measured at GT 4   
     Φ 4   3 : Code phase of signal transmitted by GT 3  and measured at GT 4   
     R 1-4 : Cross range between GT 1  and GT 4   
     R 2-4 : Cross range between GT 2  and GT 4   
     R 3-4 : Cross range between GT 3  and GT 4   
     T 1 : Clock bias of GT 1   
     T 2 : Clock bias of GT 2   
     T 3 : Clock bias of GT 3   
     T 4 : Clock bias of GT 4   
     ε 4   1 : Noise of signal transmitted by GT 1  and measured at GT 4   
     ε 4   2 : Noise of signal transmitted by GT 2  and measured at GT 4   
     ε 4   3 : Noise of signal transmitted by GT 3  and measured at GT 4   
     The network-based approach allows the formulation of all the code phase measurements from the array of ground transmitters into a single matrix as follows: 
     
       
         
           
             
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     The above matrix equation can be solved for cross ranges and relative clock biases as follows: 
     
       
         
           
             
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     The code-based cross ranges and relative clock biases are observable at each measurement epoch. The use of wideband signals can provide the accuracy of code-based cross ranges at about 15 cm in typical multi-path environments. Averaging may help achieve better accuracy. 
     Once all the cross ranges are resolved, the relative locations of the ground transmitters can be determined in local array fixed coordinates. For example, local Cartesian coordinates are defined as the first ground transmitter at the origin, the second ground transmitter on the X-axis, the third ground transmitter on the X-Y plane with positive Y. If needed, the local coordinates can be mapped to other coordinates (e.g., GPS WGS84) when at least three of the ground transmitters are determined in the desirable coordinates. 
     If the ground transmitters need to be synchronized, the estimate of the relative clocks (T i −T j ) can be used as feedback to steer the relative clock biases to zero. 
     The relative clock bias estimate can be broadcast to rovers for transmitter clock bias corrections, as described herein with reference to  FIG. 3 . In other words, there is no requirement for a separate reference station in the rover&#39;s proximity for conventional differential correction. 
       FIG. 3  is a block diagram that illustrates an array of self-surveyed ground transmitters that provide a positioning solution to a rover by broadcasting the self-surveyed locations of the ground transmitters and the relative clock bias estimates to the rover, according to certain non-limiting embodiments. Various techniques may be used to broadcast such information to a rover. One such technique is described in U.S. patent application entitled, “PRECISE LOCAL POSITIONING SYSTEMS USING GROUND-BASED TRANSMITTERS” filed by Masayoshi Matsuoka, Kurt R. Zimmerman, Paul Y. Montgomery, and David G. Lawrence on Feb. 22, 2006, the contents of which are incorporated herein by reference. 
       FIG. 3  shows an array  300  of ground transmitters  302 ,  304 ,  306 ,  308  and a rover  312 . Rover  312  comprises a receiver antenna  312   a , a data modem  312   b , a receiver section  312   c  and a clock  312   d . According to certain embodiments, rover  312  can transmit wideband ranging signals. In such a case, the rover would include a transmitter and a transceiver antenna. 
     Each ground transmitter includes a transceiver antenna, such as transceiver antennas  302   a ,  304   a ,  306   a , and  308   a . The transceiver antennas are capable of both receiving and transmitting ranging signals. Further, each ground transmitter comprises: 1) an optional wireless data modem, such as modems  302   b ,  304   b ,  306   b , and  308   b , 2) a transmitter section, such as transmitter sections  302   c ,  304   c ,  306   c , and  308   c , 3) a receiver section, such as receiver sections  302   d ,  304   d ,  306   d , and  308   d , and 4) a clock, such as  302   e ,  304   e ,  306   e , and  308   e . Each ground transmitter broadcasts clock bias estimates  316  to rover  312  as correction data. Further, each ground transmitter in array  300  can transmit and receive wideband ranging signals  310 . According to certain embodiments, the receiver section and the transmitter section of each ground transmitter of array  300 , are clocked by a single common oscillator clock, such as clocks  302   e ,  304   e ,  306   e , and  308   e.    
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The invention is intended to be as broad as the appended claims, including all equivalents thereto.