Calibration for wireless location system

Abstract

Methods and apparatus for calibrating a Wireless Location System to enable the system to make highly accurate TDOA and FDOA measurements are disclosed. An external calibration method in accordance with the present invention comprises the steps of transmitting a first reference signal from a reference transmitter; receiving the first reference signal at first and second receiver systems; determining a first error value by comparing a measured TDOA (or FDOA) value with a theoretical TDOA (or FDOA) value associated with the known locations of the receiver systems and the known location of the reference transmitter; and utilizing the first error value to correct subsequent TDOA measurements associated with a mobile transmitter to be located. An internal calibration method in accordance with the present invention comprises the steps of injecting a comb signal into the first receiver system; utilizing the comb signal to obtain an estimate of the manner in which the transfer function varies across the bandwidth of the first receiver system; and utilizing the estimate to mitigate the effects of the variation of the first transfer function on the time measurements made by the first receiver system.

What is claimed is:  
     
         1 . An external calibration method for a TDOA wireless location system (WLS), comprising the steps of: transmitting a reference signal, receiving the reference signal at first and second receivers, determining an error value, and using the error value to correct subsequent TDOA measurements.  
     
     
         2 . A method as recited in  claim 1 , further comprising transmitting a second reference signal, receiving the second reference signal, determining a second error value, and combining the first and second reference signals for use in making corrections to subsequent TDOA measurements.  
     
     
         3 . A method as recited in  claim 3 , wherein the first and second error values are combined in a weighted average.  
     
     
         4 . A method as recited in  claim 3 , wherein the error values are combined using a time series weighted averaging method.  
     
     
         5 . A method as recited in  claim 4 , wherein the time series weighted average comprises a Kalman filter.  
     
     
         6 . A method as recited in  claim 1 , wherein the system monitors a rate of change of the error value and changes a rate of the calibration.  
     
     
         7 . A method as recited in  claim 6 , wherein the rate of calibration is changed by automatically paging reference transmitters.  
     
     
         8 . A method as recited in  claim 1 , wherein the first and second receiver systems have wideband receivers, and the first and second reference signals are narrowband at different carriers.  
     
     
         9 . A method as recited in  claim 1 , wherein the error values are stored for each baseline.  
     
     
         10 . A method as recited in  claim 1 , wherein the error values are weighted by a quality factor prior to correcting subsequent TDOA measurements.  
     
     
         11 . A method as recited in  claim 10 , wherein the quality factor is based on an output of a cross correlation function of reference signals received by first and second receivers.  
     
     
         12 . A method as recited in  claim 10 , wherein the error values are used only if the quality factor exceeds a prescribed threshold value.  
     
     
         13 . An internal calibration method for a receiver system characterized by a time- and frequency-varying transfer function, wherein the receiver system is used in a wireless location system, comprising the steps of: injecting a stable, known wideband signal into the receiver system; using the wideband signal to estimate the transfer function across a prescribed bandwidth of the receiver system; and using the estimate to mitigate the variation of the transfer function on subsequent time measurements.  
     
     
         14 . A method as recited in  claim 13 , wherein the internally generated wideband signal is a comb signal.  
     
     
         15 . A method as recited in  claim 14 , wherein the comb signal is characterized by multiple discrete frequencies and consistent amplitude and spacing between the discrete frequencies.  
     
     
         16 . A method as recited in  claim 13 , wherein the estimate of the transfer function is weighted by a quality factor prior to mitigation of the effects of the transfer function.  
     
     
         17 . A method as recited in  claim 16 , wherein the estimate of the transfer function is used only if the quality factor exceeds a predetermined threshold value.  
     
     
         18 . A method as recited in  claim 17 , wherein the quality factor is based on a cross correlation function before and after the transfer function.  
     
     
         19 . A method as recited in  claim 13 , wherein an antenna of the receiver system is first isolated from the receiver prior to injection of the known wideband signal.  
     
     
         20 . A method as recited in  claim 19 , wherein an electronically controlled RF relay is used for isolation.  
     
     
         21 . A method as recited in  claim 20 , wherein the known wideband signal is routed to the receiver system using the electronic RF relay.  
     
     
         22 . An external calibration method for use with a wireless location system having first and second receivers, wherein the location system estimates velocity using frequency difference of arrival (FDOA), comprising: transmitting a first reference signal, receiving the reference at the first and second receivers, determining an error value, and utilizing the error value to correct subsequent FDOA measurements.  
     
     
         23 . An external calibration method for use with a wireless location system including a receiver system characterized by a time- and frequency-varying transfer function, comprising the steps of: transmitting a stable, known wideband calibration signal from an external transmitter; using the wideband calibration signal to estimate the transfer function across a prescribed bandwidth; and using the estimate to mitigate effects of variation of the transfer function on subsequent time measurements.  
     
     
         24 . A method as recited in  claim 23 , wherein the external transmission is of short duration and low power so as not to cause interference to a wireless system hosting the location system.  
     
     
         25 . A method as recited in  claim 23 , wherein the receiver system is synchronized with the external transmitter so that the receiver system is programmed to receive and process the entire wideband of the calibration signal only at the time that the calibration signal is being sent.  
     
     
         26 . A method as recited in  claim 25 , wherein the receiver system will not perform calibration processing at any other time except when in synchronization with the external calibration transmissions.  
     
     
         27 . A method as recited in  claim 25 , wherein the receiver system and external calibration transmitter are synchronized using GPS timing units.  
     
     
         28 . A method as recited in  claim 27 , wherein a wireless communications link is used between the receiver system and the external calibration transmitter to exchange commands and responses.  
     
     
         29 . A method as recited in  claim 23 , wherein the external transmitter uses a directional antenna to direct the wideband signal only at antennae of the receiver system.  
     
     
         30 . A method as recited in  claim 29 , wherein the directional antenna is a Yagi antenna.  
     
     
         31 . A method as recited in  claim 29 , wherein the method includes making the external transmission only when the directional antenna is aimed at the receiver system antennae and the risk of multipath reflections is reduced.  
     
     
         32 . A method as recited in  claim 23 , wherein the transfer function includes antennae, filters, amplifiers, and cabling associated with the receiver system.  
     
     
         33 . An external synchronization system for a receiver system used in a wireless location system (WLS), comprising a GPS receiver, an oven controlled crystal oscillator, and a phase-locked loop that generates various signals required for use within the receiver system, each signal being synchronized in time and frequency to a signal output by the GPS receiver, and each generated signal having a low phase noise.  
     
     
         34 . A system as recited in  claim 33 , wherein the generated signals are characterized by phase noise that is less than 0.7 degrees RMS.  
     
     
         35 . A system as recited in  claim 33 , wherein the phase-locked loop is designed for low phase noise.  
     
     
         36 . A system as recited in  claim 33 , wherein the GPS receiver comprises a receiver capable of receiving time signals from GPS satellites and additional circuitry and processing capability that removes the time varying effect of a selective availability function in the GPS satellites.  
     
     
         37 . A method for calibrating and correcting for station biases in a receiver system employed in a wireless location system (WLS), the receiver system comprising an antennae array, cabling, a filter, preamplifier, and a wideband receiver, and a station bias defined as the finite delay between when an RF signal from a mobile transmitter reaches the antennae and when that same signal reaches the wideband receiver, comprising the steps of: measuring the length of cable from the antennae to the filter and determining the corresponding delay associated with the cable length; injecting a known signal into the filter and measuring the delay from the filter input to the wideband receiver; combining the delays and using the combined delays to correct subsequent location measurements.  
     
     
         38 . A method as recited in  claim 37 , wherein, when used with a GPS based calibration scheme, the method further comprises correcting for GPS cable lengths.  
     
     
         39 . A method as recited in  claim 37 , wherein an externally generated reference signal is used to monitor changes in station bias that may arise due to aging and weather.  
     
     
         40 . A method as recited in  claim 37 , wherein the station bias for each receiver system in the WLS is stored in tabular form for use in subsequent location processing.  
     
     
         41 . An external calibration method for calibrating a location system having a first receiver system and a second receiver system, wherein the location system is operative to estimate the location of a mobile transmitter by, in part, determining a difference in time of arrival of a signal transmitted by said mobile transmitter and received by said first and second receiver systems, and wherein the accuracy of the location estimate is dependent, in part, upon the accuracy of time difference of arrival (TDOA) measurements made by the location system, the method comprising the steps of: 
 (A) transmitting a first reference signal from a reference transmitter;  
 (B) receiving said first reference signal at said first and second receiver systems;  
 (C) determining a first error value by comparing a measured TDOA value with a theoretical TDOA value associated with the known locations of the receiver systems and the known location of the reference transmitter; and  
 (D) utilizing said first error value to correct subsequent TDOA measurements associated with a mobile transmitter to be located.  
 
     
     
         42 . A method as recited in  claim 41 , further comprising the steps of transmitting a second reference signal from a second reference transmitter; receiving said second reference signal at said first and second receiver systems; determining a second error value by comparing a second measured TDOA value with a second theoretical TDOA value associated with the known locations of the receiver systems and the known location of the second reference transmitter; and utilizing said second error value in combination with said first error value to correct subsequent TDOA measurements associated with said mobile transmitter to be located.  
     
     
         43 . A method as recited in  claim 42 , wherein said first and second error values are combined in a weighted average.  
     
     
         44 . A method as recited in  claim 42 , wherein said first and second receiver systems include wideband receivers and said first and second reference signals are narrowband signals with different carrier frequencies.  
     
     
         45 . A method as recited in  claim 41 , wherein the error values arc stored in tabular form for each baseline in the location system.  
     
     
         46 . A method as recited in  claim 42 , wherein the error values are stored in tabular form for each baseline in the location system.  
     
     
         47 . A method as recited in  claim 43 , wherein the error values are combined in a time series weighted averaging method prior to being used to correct subsequent TDOA measurements associated with a mobile transmitter to be located.  
     
     
         48 . A method as recited in  claim 47 , wherein the time series weighted averaging method is based on a Kalman filter.  
     
     
         49 . A method as recited in  claim 41 , wherein the error values are weighted by a quality factor prior to being used to correct subsequent TDOA measurements associated with a mobile transmitter to be located.  
     
     
         50 . A method as recited in  claim 42 , wherein the error values are weighted by a quality factor prior to being used to correct subsequent TDOA measurements associated with a mobile transmitter to be located.  
     
     
         51 . A method as recited in  claim 49 , wherein the quality factor is based upon the output of a cross-correlation function of a reference signal received by the first and second receivers.  
     
     
         52 . A method as recited in  claim 49 , wherein the error values are used by the location system only if the quality factor exceeds a prescribed threshold value.  
     
     
         53 . A method as recited in  claim 47 , wherein the location system monitors the rate of change of the error values and changes the rate of calibration, or time interval between calibrations, to exceed the rate of change of the error values.  
     
     
         54 . A method as recited in  claim 53 , wherein the rate of calibration is controlled by automatically paging the reference transmitters.  
     
     
         55 . An internal calibration method for calibrating a first receiver system, wherein said first receiver system is characterized by a time- and frequency-varying transfer function, said transfer function defining how the amplitude and phase of a received signal will be altered by said first receiver system, and wherein the first receiver system is utilized in a location system that is operative to determine the location of a mobile transmitter by, in part, determining a difference in time of arrival of a signal transmitted by a mobile transmitter and received by said first receiver system and another receiver system, and wherein the accuracy of the location determination is dependent, in part, upon the accuracy of time measurements made by said receiver systems, the method comprising the steps of: 
 (A) injecting an internally generated stable, known wideband signal into said first receiver system;    (B) utilizing the generated wideband signal to obtain an estimate of the manner in which said transfer function varies across the bandwidth of said first receiver system; and    (C) utilizing said estimate to mitigate the effects of said variation of the first transfer function on the time measurements made by the first receiver system.    
     
     
         56 . A method as recited in  claim 55 , wherein the internally generated stable, known wideband signal is a comb signal having multiple discrete frequency elements of consistent amplitude and spacing.  
     
     
         57 . A method as recited in  claim 55 , wherein the estimate of the manner in which the transfer function varies across the bandwidth of the first receiver system is weighted by a quality factor prior to being used to mitigate the effects of the transfer function.  
     
     
         58 . A method as recited in  claim 57 , wherein the estimate of the transfer function is used by the location system to mitigate the effect of the transfer function only if the quality factor exceeds a prescribed threshold value.  
     
     
         59 . A method as recited in  claim 57 , wherein the quality factor is based upon the output of a cross-correlation function of the internally generated stable, known wideband signal and the same signal after it has passed through the transfer function.  
     
     
         60 . A method as recited in  claim 55 , wherein the antenna is first isolated from the receiver system prior to the injection of the internally generated stable, known wideband signal.  
     
     
         61 . A method as recited in claim  60 , wherein an electronically controlled RF relay is used to automatically isolate the antenna from the receiver system.  
     
     
         62 . A method as recited in  claim 55 , wherein the internally generated stable, known wideband signal is routed to the receiver system using an electronically controlled RF relay.  
     
     
         63 . An external calibration method for calibrating a location system having a first receiver system and a second receiver system, wherein the location system is operative to estimate the location and velocity of a mobile transmitter by, in part, determining a difference in frequency of arrival of a signal transmitted by said mobile transmitter and received by said first and second receiver systems, and wherein the accuracy of the location and velocity estimate is dependent, in part, upon the accuracy of frequency difference of arrival (FDOA) measurements made by the location system, the method comprising the steps of: 
 (A) transmitting a first reference signal from a reference transmitter;    (B) receiving said first reference signal at said first and second receiver systems;    (C) determining a first error value by comparing a measured FDOA value with a theoretical FDOA value associated with the known locations of the receiver systems and the known location of the reference transmitter; and    (D) utilizing said first error value to correct subsequent FDOA measurements associated with a mobile transmitter to be located.