Abstract:
A method and apparatus for estimating bias errors in a time-difference-of-arrival/frequency-difference-of-arrival (TDOA/FDOA) geolocation system using a reference signal transmitter in which position and/or motion information of the reference signal transmitter is encoded into the reference signal. The motion information may include the velocity and/or acceleration of the reference signal transmitter. The reference signal is received by multiple collection platforms operating in conjunction with a geolocation system and a reference correction processing system. The reference correction processing system receives, via the multiple collection platforms, the position and/or motion information, which is immediately and unambiguously associated with specific reference signal transmissions. The geolocation system estimates the position and/or velocity of the reference signal transmitter using conventional TDOA/FDOA techniques. The estimated position and/or velocity of the reference signal transmitter is compared to the information contained in the reference signal to estimate bias errors.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to time-difference-of-arrival (TDOA) and frequency-difference-of-arrival (FDOA) measurement techniques and systems.  
       BACKGROUND INFORMATION  
       [0002]     Errors in time-difference-of-arrival (TDOA) and frequency-difference-of-arrival (FDOA) measurements used to perform geolocation of non-cooperative emitters (i.e., signals of unknown format or content, transmitted from an unknown location) can be reduced by use of a reference beacon that transmits a signal from a known position. By performing a reverse geolocation using signals transmitted from a reference beacon with a known position and velocity, it is possible to estimate bias errors in the TDOA/FDOA measurements taken by the collection system. The estimated bias errors can then be subtracted from measurements taken of non-cooperative emitter signals to reduce the bias error in those measurements. Such correction is typically performed by a reference correction processing system that is part of or operates in conjunction with the geolocation system.  
         [0003]     In order to correctly compute the bias errors using signal data collected from a reference beacon transmitter, it is necessary to know the position (and/or velocity) of the transmitter at the time of transmission.  
         [0004]     A reference beacon signal need not contain any information in order to provide useable bias corrections. A random or pseudorandom waveform with good correlation properties can be used as a reference beacon signal. In fact, it is generally simpler to generate a signal containing a pseudorandom waveform, than one which contains information.  
         [0005]     Currently deployed reference beacon systems typically use stationary reference beacon transmitters whose locations are constant and known to geolocation processing systems, which compute the TDOA/FDOA bias error corrections based on the reference beacon signals and the locations of the transmitters. The beacon signals transmitted by these transmitters typically consist of pseudorandom waveforms, which contain no data.  
         [0006]     The Boeing Company is currently developing a reference beacon system that can be placed on a moving platform (e.g., an aircraft). In such an arrangement, the position of the reference beacon transmitter is provided to the geolocation processing system over a network connection. This approach, however, requires the geolocation processing system to associate sets of position data received over the network from the reference beacon transmitter with sets of RF signal data independently received from one or more collection platforms. This association is accomplished by having the geolocation processing system command the reference beacon to transmit at a known time. In the aforementioned system, the reference beacon transmits its position data over the network at the time it starts transmission of the reference beacon signal. As with other conventional approaches, the reference beacon signals consist of pseudorandom waveforms.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to a method and apparatus for estimating bias errors in a TDOA/FDOA system using a reference beacon signal in which position and/or motion information relating to the reference beacon transmitter is encoded. The motion information may include the velocity and/or acceleration of the reference beacon signal transmitter. The reference signal is received by one or more collection platforms operating in conjunction with a reference correction processing system. The reference correction processing system receives, via the one or more collection platforms, the position and/or motion information, which is immediately and unambiguously associated with specific reference signal transmissions.  
         [0008]     Moreover, any separate data path from the reference signal transmitter to the reference correction processing system can be eliminated.  
         [0009]     The aforementioned and additional features and advantages of the present invention are further described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a block diagram of an exemplary embodiment of a reference beacon parameter information arrangement in accordance with the present invention.  
         [0011]      FIG. 2  is a flow-chart illustrating an exemplary embodiment of a method in accordance with the present invention.  
         [0012]      FIG. 3  is an exemplary format of information encoded into a reference beacon signal in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows a block diagram of an exemplary embodiment of a bias error estimation arrangement for TDOA/FDOA, in accordance with the present invention. The arrangement of  FIG. 1  includes a reference beacon transmitter  110  which transmits a reference beacon signal  115 . It is contemplated that the position and/or velocity of the reference beacon transmitter  110  can vary with time (e.g., the transmitter  110  is carried in a moving aircraft), although the present invention will also operate with stationary transmitters. In the exemplary embodiment shown, position, velocity and acceleration information relating to the reference beacon transmitter  110  are provided to the transmitter  110  in known ways from elements  111 ,  112 , and  113 , respectively. The operation and implementation of the elements  111 - 113  are conventional and may include, for example, sensors, transducers, accelerometers, or the like. Moreover, as can be appreciated, even though illustrated as three separate blocks, elements  111 - 113  may be implemented as one or more units. As will be clear from the description below, the methods and systems of the present invention can be implemented to use only position information, position and velocity information, or position, velocity and acceleration information.  
         [0014]     In accordance with the present invention, the reference beacon transmitter  110  periodically constructs a message containing information relating to the position of the reference beacon transmitter  110  at a specific time. The message is contained, as described in greater detail below, in the reference beacon signal  115  that is transmitted. Messages can be generated and transmitted periodically at a fixed time interval, for example, or in accordance with changes in the position of the transmitter  110 . A particular message may also be transmitted more than once.  
         [0015]     In an exemplary embodiment, the information contained in the message includes an estimate of the position of the reference beacon transmitter  110  at the time of transmission of the message. The message may include additional information, such as the velocity of the reference beacon transmitter  110  at the time of message transmission. In alternative embodiments, the message may include the position and/or velocity of the reference beacon transmitter  110  at a particular time, which time is also included in the message. Error correction coding and other information, described more fully below, may also be included.  
         [0016]     The reference beacon signal  115  is received by a plurality of collection platforms  121 - 123  which are in communication with and operate in conjunction with a geolocation processing system  130 . The collection platforms  121 - 123  can be conventional. Although three collection platforms  121 - 123  are shown in  FIG. 1 , as can be appreciated by one of ordinary skill in the art, two or more collection platforms can be used for geolocation depending on the particular implementation.  
         [0017]     The geolocation processing system  130  includes a reference correction processing sub-system or function  135  (also referred to as reference correction processor  135 ). The reference correction processor  135  may be implemented as part of the geolocation processing system  130  or as a separate element.  
         [0018]     In conjunction with the collection platforms  121 - 123 , the geolocation processing system  130  operates in accordance with conventional TDOA/FDOA techniques to estimate the location and/or velocity of various emitters, including non-cooperative emitters as well as reference beacons. As described below, the reference correction processor  135  uses the information contained in the signals received from the reference beacon transmitter  110  to estimate TDOA/FDOA bias errors that can be used to improve the geolocation processing system&#39;s  130  estimates of emitter location and/or velocity.  
         [0019]     It should be noted that the TDOA/FDOA bias errors can be estimated in terms of position and motion or in terms of time and frequency. In other words, the reference correction processor  135  can preferably generate at least one of two types of correction. The first is a position/motion error correction: e.g., “all computed 2D geolocations should be corrected by −100 m east and +300 m north.” The second is a correction to the TDOA (and/or FDOA) measurements that are used to perform the geolocation: e.g., “all TDOA and FDOA measurements between collection platforms A &amp; B should be corrected by +25.4 ms and +0.0043 Hz.” Such a correction can be determined by computing the “true” TDOA/FDOA measurements that would be expected based on the known transmitter and receiver positions (and velocities) and comparing to the “measured” TDOA/FDOA values.  
         [0020]     After reception, the information contained in the reference beacon signal  115  may be extracted by the collection platforms  121 - 123  and communicated to the geolocation processing system  130  and/or reference correction processor  135 . Alternatively, the received signal  115  may be conveyed by the collection platforms  121 - 123  to the geolocation processing system  130  and/or reference correction processor  135  which then extract(s) the relevant contents. In either case, the reference correction processor  135  obtains, by way of the collection platforms  121 - 123 , the information in the messages transmitted by the reference beacon transmitter  110 . As such, no other link between the reference correction processor  135  and the reference beacon transmitter  110  is required.  
         [0021]     The position and/or velocity estimates generated by the geolocation processing system  130  and the position and/or motion information contained in the received reference beacon signal  115  are used by the reference correction processor  135  to estimate a measurement bias error. The estimated measurement bias error can then be used to improve the estimated position and/or velocity of other emitters.  
         [0022]     As mentioned, in an exemplary embodiment of the present invention, the position and/or velocity of the reference beacon transmitter  110  at some future time is predicted and transmitted in the reference beacon signal. Previous values of position and motion (velocity and/or acceleration) can be used to predict the position and/or velocity of the transmitter  110  at a future time of interest. The future time of interest can be the time of transmission of the message containing the predicted information.  
         [0023]     In an alternative exemplary embodiment, the reference beacon transmitter  110  transmits a message containing its last known position, velocity, and, if available, acceleration, and the time at which those parameters were determined (i.e., “time of fix”). The reference correction processor  135  could then use the received information to estimate the position and/or velocity of the reference beacon transmitter  110  at the time of transmission of the message containing said information.  
         [0024]     To allow for the possibility that demodulation and decoding errors may occur while demodulating the signal, the reference beacon transmitter  110  may add additional data to the position message before modulation and transmission. Such additional data may include error correction codes, for example.  
         [0025]     Additional information, such as the identity of the reference beacon transmitter  110  could also be added to a message.  
         [0026]      FIG. 2  shows a flow-chart summarizing the steps in an exemplary embodiment of a method in accordance with the present invention. At step  201 , the information to be contained in the reference beacon signal is assembled, and may include, as discussed above, the transmitter position (P), velocity (V), and acceleration (A), time of fix (T) of the position and/or motion information, transmitter identification (ID), and error correction (EC) information, for example. At step  202 , the beacon signal containing said information is transmitted and received by the geolocation system at step  203 . Time of arrival and/or frequency of arrival at the collection platforms of the geolocation system is measured at step  204 . The information embedded in the received reference beacon signal is then extracted at step  205 . Based on the extracted information, the geolocation system, at step  206 , determines the TDOA/FDOA measurements that would be expected at the receiving collection platforms. The bias error is then determined at step  207  based on the expected TDOA/FDOA measurements and the actual measurements made at step  204 . The bias error may include a timing bias error and/or a frequency bias error, as described above.  
         [0027]      FIG. 3  shows an exemplary arrangement of information in a reference beacon signal generated in accordance with the present invention. Information elements  321 . 1 - 321 .N and  322  each contain position (P), velocity (V), acceleration (A), time of fix (T) and identification (ID) information for the reference beacon transmitter from which they are transmitted. Error correction (EC) information is also included in the aforementioned information elements. Error correction information can be included in each information element or for a group of information elements.  
         [0028]     To allow for the possibility that a portion of the transmitted waveform may not be properly received, or may be corrupted, the reference beacon transmitter position and/or motion information may be transmitted multiple times in the beacon signal. Referring to  FIG. 3 , the information elements  321 . 1 - 321 .N may all contain the same information. The duplicate information elements may be contiguous (e.g.,  321 . 1 - 321 .N) or may be separated by periods of pseudo-random waveforms. Thus, for example, the information element  322  may contain the same information as information elements  321 . 1 - 321 .N but is separated therefrom by a pseudo-random waveform period  312 . An information element  321 ,  322  may be transmitted one or more times between intervening pseudo-random waveform periods  311 - 313 .  
         [0029]     Any of a variety of encoding or modulation schemes can be used to embed the reference beacon information in the reference beacon signal including, for example, pulse amplitude modulation (PAM), frequency shift keying (FSK), or phase shift keying (PSK), among others.  
         [0030]     Once received, the reference beacon signal can be demodulated, decoded and otherwise processed by the collection platforms  121 - 123  and/or the geolocation processing system  130 . Conventional TDOA/FDOA signal collection systems, however, do not typically demodulate or decode the RF signals that they capture. Rather, conventional collection platforms will generate RF signal data, typically raw, digitized sample data of the captured RF signals. The RF signal data may be digitally filtered, resampled, or re-tuned, but there is typically no detection or demodulation processing performed on the data samples by the collection platforms  121 - 123 . Instead, the digitized RF data is sent directly to the geolocation processor where the RF waveforms are correlated to produce TDOA/FDOA measurements. This allows the reference beacon signal to be demodulated by the geolocation processing system, rather than the signal collectors.  
         [0031]     It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.