Abstract:
A cooperative position location device (CPLD) that integrates a broadcast digital transmission (BDT) receiver, a data link transceiver, and a displacement sensor; a computer program product tangibly stored in computer-readable media; and associated methods for receiving and processing special codes embedded in BDT signals from a plurality of transmitters to produce time of arrival (TOA) measurements thereof; for sending and receiving special messages between cooperative position location devices (CPLDs) to produce time difference of arrival (TDOA) of common events of BDT at and time of flight (TOF) measurements between the CPLDs; and for integrating differential and relative ranges between CPLDs to a plurality of BDT transmitters and displacement measurements to yield a joint position solution of the CPLDs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to radio geolocation and particularly to cooperative receivers for position location using periodic codes in such broadcast digital transmissions as broadcast digital television (DTV) and wireless local area network (WLAN) signals. 
     2. Description of the Prior Art 
     Among different radio geolocation and navigation systems, there are two important systems in wide use today. One is the 100 kHz Long Range Navigation-C (LORAN-C) system which evolved to the present form in the mid-1950s. It uses terrestrial radio transmitters to provide navigation, location, and timing services for suitably equipped air, land and marine users, civil and military alike. A LORAN-C receiver measures the difference in times of arrival of pulses transmitted by a chain of three to six synchronized transmitter stations separated by hundreds of kilometers. There are many LORAN chains of stations around the globe. Modernization effort is underway to enhance the accuracy, integrity, availability, and continuity of the LORAN system, known as Enhanced LORAN or eLORAN for short. 
     The other is the increasingly popular satellite-based Global Positioning System (GPS). Fully operational since 1994, the GPS relies upon a nominal constellation of twenty-four satellites in six different orbit plans around the Earth for position location, navigation, survey, and time transfer. Each satellite carries a set of ultra precise atomic clocks and transmits pseudo-noise (PN) code modulated signals at several frequencies. By tracking four or more satellites, a user can solve for the variables of longitude, latitude, altitude and time to precisely determine the user&#39;s location and calibrate its clock. More details are provided in the books entitled,  Global Positioning System: Theory and Applications  (Vols. I and II), edited by B. W. Parkinson and J. J. Spilker Jr., AIAA, 1996 ; Understanding GPS: Principles and Applications , edited by E. D. Kaplan, Artech House Publishers, 1996 ; Fundamentals of Global Positioning System Receivers—A Software Approach , by J. B. Y. Tsui, John Wiley &amp; Sons, Inc., 2000; and  Global Positioning System, Signals, Measurements, and Performance , by P. Misra and P. Enge, Ganga-Jamuna Press, 2001. 
     Despite of its increased popularity, GPS cannot function well when the line-of-sight (LOS) view between a receiver and a GPS satellite is obstructed due to foliage, mountains, buildings, or other structures. To satisfy the requirements of location-based mobile e-commerce and emergency call location (E911), there have been ongoing efforts so as to improve GPS receiver sensitivity to operate on GPS signals of very low power level. One example is the assisted GPS (AGPS). The AGPS approach relies upon a wireless data link to distribute, in real time, such information as time, frequency, navigation data bits, satellite ephemeredes, and approximate position as well as differential corrections to special GPS receivers equipped with a network modem so as to reduce the uncertainty search space, to help lock onto signals, and to assist navigation solution. This approach, however, comes with a price associated with installing and maintaining the wireless aiding infrastructure and services. 
     GPS cannot function well either when GPS signal is heavily jammed or overwhelmed by unintentional interference. GPS signals may be turned off altogether from newer GPS satellites with flexible power and flexible signal capabilities when it orbits over certain region. In such circumstances, no GPS solution is available. 
     Amid the process of replacement of National Television System Committee (NTSC) analog television signals by an Advanced Television Systems Committee (ATSC) digital television (DTV) signal, there has been a considerable amount of efforts devoted to the use of DTV signals for position location, thus serving as a complement to and/or a substitute for GPS. This is exemplified by the U.S. Pat. No. 6,861,984, entitled, Position Location Using Broadcast Digital Television Signals, by M. Rabinowitz and J. J. Spilker Jr., issued Mar. 1, 2005. 
     Designed primarily for indoor reception, DTV signals exhibit several advantages. It is much higher in power (40 dB over GPS) and at lower and more diverse frequencies (nearly half of the spectrum between 30 MHz and 1 GHz). The geometry offered by a network of terrestrial DTV transmitters is superior to what a satellite constellation can provide. As such, it has better propagation characteristics with greater diffraction, larger horizon, and stronger penetration through buildings and automobiles. DTV signals have a bandwidth of 6 to 8 MHz, which is much wider than the primary lobe of GPS C/A-code (2 MHz), thereby minimizing the effects of multipath and permitting higher accuracy tracking. 
     With DTV transmitters fixed on the ground, their lines of sight to a user changes very slowly, only adding a small amount of Doppler shift to a DTV signal frequency. This allows the signal to be integrated over a long period of time, thus easing the task of acquisition and tracking of a weak signal considerably. As a further benefit, the component of a DTV signal that can be used for timing is of low duty factor (e.g., 1 of 313) in contract to GPS wherein the ranging code is repeatedly transmitted and has to be tracked continuously. 
     However, one inherent technical difficulty faced by position location using broadcast digital transmissions (BDT) such as DTV signals is the clock bias and drift of the signal timing source at a transmitter, which are unknown to a user. Although it may be possible to have all DTV stations use ultra-precise atomic clocks or GPS-disciplined clocks, the synchronization of all signal transmissions across a large region is a daily operational challenge. It may also be possible to time-tag all transmissions and embed the clock offset information in the broadcast signals for all stations in a given region. However, these approaches require coordinated involvement of local DTV operators who are in broadcasting and not time transfer business. 
     Many inventions exemplified by the U.S. Pat. No. 6,861,984 by M. Rabinowitz and J. J. Spilker Jr. mentioned earlier make use of base stations, location servers, and monitor units to calibrate the DTV transmitter timing biases and to provide the calibration data to mobile users via dedicated data links. The position location mechanism in such inventions is referred to as “reference-aiding,” wherein the signal source timing errors are estimated explicitly at the reference station and sent to users (a parametric approach) or the measurement difference is employed to remove the timing errors common to the reference station and users (a non-parametric approach). There is a significant cost associated with installing and maintaining the infrastructure of base stations, location servers, and monitor units on a large scale. A user has to subscribe to a service coverage in addition to special equipment for the service signals. 
     Clearly, a user is subject to the potential risk of service discontinuity when moving from one region (or a country for the matter) to another without a global service network in place or a valid global subscription. These prior-art approaches further prevent broadcast digital transmission (BDT) signals from being used for military applications as signals of opportunity (SOOP) because of lack of pre-surveyed reference/monitor units. In the U.S. Pat. No. 7,388,541, entitled “Self-Calibrating Position Location Using Periodic Codes in Broadcast Digital Transmissions,” issued Jun. 17, 2008 to the present inventor, two position location mechanisms, referred to as “self-referencing” and “self-calibrating,” respectively, are disclosed by which position location systems can make use of broadcast digital transmissions such as DTV and WLAN signals without requiring the service from external base stations, location servers, and monitor units. However, these self-aiding methods, in contrast to the above-mentioned reference-aiding methods, are useful for one user at a time and may require long time to complete the self-referencing and/or self-calibrating process. 
     Applications arise wherein a team of cooperative mobile users need to know not only their own location but also those of their teammates without relying upon GPS signals. By cooperative, we mean the teammates have a means to communicate to one another via a wireless data link to coordinate their activities, exchange data, and perform mutual aiding in the form of cooperative referencing and calibration. This need is met by the present invention as described and claimed below. 
     SUMMARY OF THE INVENTION 
     The present invention is (1) a cooperative position location device (CPLD) that integrates a broadcast digital transmission (BDT) receiver, a data link transceiver, and a displacement sensor, (2) a computer program product tangibly stored in computer-readable media, and (3) associated methods for (i) receiving and processing of special codes embedded in BDT signals from a plurality of transmitters to produce time of arrival (TOA) measurements thereof, (ii) for sending and receiving special messages between cooperative position location devices (CPLDs) to produce time difference of arrival (TDOA) measurements of common events of BDT at and time of flight (TOF) measurements between the CPLDs, and (iii) for integrating the differential ranges to a plurality of BDT transmitters, relative ranges between CPLDs, and displacement measurements to yield a joint position solution of the CPLDs. 
     A broadcast digital transmission (BDT) receiver includes an antenna and a radio-frequency (RF) front-end to intercept the incoming RF signal and to convert it to an appropriate intermediate frequency (IF) for digitization. A baseband signal processor is organized into functionally identical channels, each dynamically assigned to a different BDT transmitter. Special periodic codes of BDT transmissions such as those for synchronization are typically of low duty factor; as such, search thereof is conducted in the baseband signal processor over small overlapping data windows covering the entire code repetition interval. Once acquired, the BDT signal is tracked via a closed loop wherein update is windowed in sync with the low duty cycle so as to save precious resources. In both acquisition and tracking, the baseband signal processor performs a correlation between the incoming signal samples and a reference code over a number of code lags and for a number of Doppler frequency bins. This results in a two-dimensional delay-Doppler map of complex correlations from which the baseband signal processor further extracts the code delay and carrier phase and frequency parameters to close a joint code and carrier tracking loop and measures time of arrival (TOA) of the special codes relative to a local clock time. 
     In a particular embodiment, the broadcast digital transmission (BDT) is a broadcast digital television (DTV) signal and the broadcast digital television signal is an Advanced Television Systems Committee (ATSC) digital television signal. The special periodic code is a field synchronization segment with an ATSC/DTV data frame, a segment synchronization sequence within a data segment within an ATSC/DTV data frame, or a combination thereof. In addition, an ATSC DTV signal may also contain a pseudorandom sequence as a “RF watermark” that is uniquely assigned to each DTV transmitter for transmitter identification (TxID) in system monitoring and measurement, which can also serve as the special periodic code. However, different from GPS signals, general broadcast digital transmissions do not contain timing information directly. In addition, as signals of opportunity, BDT signals are typically not synchronized (in contrast to GPS wherein all satellites operate on the well-maintained GPS time). Besides, the timing source at BDT transmitters is subject to different clock bias and drift (whereas atomic clocks onboard GPS satellites are constantly calibrated by ground stations). These are inherent technical difficulties in using signals of opportunity by a standalone device to derive position location information. In the present invention, times of arrival (TOA) of the special codes measured at individual CPLDs are communicated to one another via data link transceivers to form a time difference of arrival (TDOA) so as to eliminate the clock errors of BDT transmitters. 
     A data link transceiver contains a data receive channel and a data transmit channel. The data receive channel includes an antenna and a RF front-end to intercept the incoming RF signal and to down-convert it to an appropriate IF for digitization. The data transmit channel includes a data modulator and a RF front-end to up-convert the modulated signal from the baseband to an appropriate RF frequency and power-amplify it prior to a transmit antenna. A data link transceiver data processor contains a receive signal processor and transmit signal processor. The receive signal processor and the transmit signal processor of two correspondent CPLDs exchange ranging messages according to a ranging protocol via request send, request receive, reply send, and reply receive to obtain relative range and time offset thereof. Furthermore, the receive signal processor and transmit signal processor communicate, by modulating and demodulating such data as times of arrival (TOA) of the special codes measured at individual CPLDs onto and from the data link, for cooperation and coordination. 
     However, in some practical situations, the number of independent BDT transmitters may be fewer than necessary to solve for position location unknowns or the geometrical distribution of BDT transmitters (e.g., co-located in the same transmission tower) is too poor for an accurate solution. The use of a displacement sensor as disclosed in the present invention alleviates this difficulty. A rudimentary displacement sensor can be made of a magnetic compass and a tape measure. The magnetic compass determines the direction of travel relative to the magnetic north, which can be related to a common coordinate frame in which the location of BDT transmitters are known, while the tape measure indicates the distance travelled, thus providing a displacement vector. For a wheeled ground vehicle, the average wheel speed provides an estimate of the speed, which is integrated over time to provide the distance traveled. The scaled difference between the left and right wheel speeds provides an estimate of the turning rate, which is integrated into the heading change (yaw). Together, the two measurements provide an estimate of relative velocity vector, which is integrated into a displacement vector. Inertial sensors such as accelerometers and gyros can also be mechanized to provide displacements. 
     An onboard/online database is used to supply a location of a plurality of transmitters. Auxiliary sensors are also available to provide other pertinent information upon demand such as weather conditions and local topographic data. A cooperative position location processor, coupled to the BDT receiver data processor, the data link receiver data processor, the displacement sensor, the onboard/online database, and a local clock, operates on time of arrival measurements of a plurality of transmitters, time of flight measurements between cooperative devices, and displacement vector measurements, to produce a joint position solution for the cooperative devices by a least-squares or Kalman fixed-point smoother in a batch or sequential processing manner. 
     When a BDT receiver is implemented as a software receiver, any change in broadcast signal characteristics can be easily accommodated by simply reprogramming the receiver when moving from one region or one country to another. Similarly, when a data link transceiver is implemented as a software receiver, any change in ranging protocols and signaling waveforms can be easily accommodated by simply reprogramming the transceiver. When a different displacement sensor (or other auxiliary sensors for the matter) is used, it suffices to modify sensor error models in the joint calibration and positioning solution by reprogramming. In an urban environment, the direct signal from a transmitter may be attenuated whereas the multipath signals reflected from surroundings, although arriving later in time, may appear stronger. When moving from an environment to another, the cooperative position location device can be programmed to use a channel impulse response or a generalized frequency-domain correlator (GFDC) instead of a conventional delay-lock loop (DLL). As a preferred method, the use of a channel impulse response or a GFDC in the present invention produces a major portion of the correlation function (or the entire function if desired) from which the direct signal and multipath fingerprint can be isolated. And more importantly, the impulse response or the generalized correlation peak is much sharper, thus being less susceptible to multipath and leading to better timing and ultimately to better position location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the following description of an exemplary embodiment thereof, considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a schematic drawing illustrating an exemplary embodiment of the present invention including a pair of cooperative position location systems that receive broadcast digital transmissions (BDT) from a plurality of transmitters and communicate with each other via a data link; 
         FIG. 2  is a schematic illustrating the architecture of an exemplary embodiment of a broadcast digital transmission (BDT) receiver in accordance with the present invention; 
         FIG. 3  is a drawing graphically illustrating the fundamental equations relating measured times of arrival and a time offset to a total time of flight and a differential time of flight, with which a known transmitter at a known location is tied to unknown cooperative user locations to be determined; 
         FIG. 4  is a schematic illustrating the architecture of an exemplary embodiment of a data link transceiver in accordance with the present invention; 
         FIG. 5  is a drawing graphically illustrating the fundamental equations relating time stamps of ranging messages (request send, request receive, reply send, reply receive) to a relative range and a clock offset between two cooperative transceivers; 
         FIG. 6  is a table illustrating the solvability of position location in terms of the number of displacements to make by cooperative receivers as a function of the number of independent transmitters; 
         FIG. 7  is a drawing graphically illustrating the procedure of making displacement to a different location and accumulating differential and relative range measurements at the location in accordance with the present invention; 
         FIG. 8  is a schematic illustrating an exemplary embodiment of method steps of initialization using measured times of arrival from a plurality of transmitters, times of flight between cooperative receivers, and a sequence of displacement vectors in accordance with the present invention; and 
         FIG. 9  is a schematic illustrating an exemplary embodiment of method steps to jointly determine a time offset and a trajectory of position locations using measured times of arrival from a plurality of transmitters, times of flight between cooperative receivers, and a sequence of displacement vectors in accordance with the present invention. 
     
    
    
     The leading digit of each reference numeral used in this specification indicates the number of the drawing in which the reference numeral first appears. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention can be used for position location with broadcast digital transmissions (BDT) from a plurality of transmitters at known locations that employ repetitive codes for synchronization and it is particularly well suited for digital television (DTV) and wireless local area network (WLAN) signals. Accordingly, an exemplary embodiment of the present invention will be described in an application to the Advanced Television Systems Committee (ATSC) DTV signals in the United States for cooperative position location in guidance, navigation, and control. The techniques disclosed herein are applicable to other television and radio broadcasts and communication signals. 
     Referring to  FIG. 1 , an exemplar embodiment  100  of the present invention is sketched wherein two cooperative position location systems i  102  and j  124 , respectively, are present. Broadcast digital transmissions  140 ,  144 , and  148  from a plurality of transmitters are received by broadcast digital transmission (BDT) receivers  104  and  126  via antennas  106  and  128 , respectively. This may include surface transmitters  142   a  and  142   b  through  142   n  at known locations on the ground for instances. It may also include airborne transmitters  146   a  and  146   b  through  146   n  with their locations either embedded in digital transmissions or made available to cooperative position location systems  102  and  124  by other means. Satellite transmitters  150   a  and  150   b  through  150   n  can also be used in a similar manner. In a sense, GPS satellites fall into this category because a GPS satellite broadcasts its orbit and clock information within navigation messages modulated on the radio signal. More generally, surface transmitters do not need to be fixed on the ground for use in the present invention so long as their locations at time of broadcasting are precisely known to cooperative systems  102  and  124 . 
     Still referring to  FIG. 1 , cooperative position location systems  102  and  124  are connected to each other for cooperation and coordination by means of a radio channel  122  using data link transceivers  108  and  130  via antennas  110  and  132 , respectively. Also included in a cooperative position location system  102  or  124  are a local clock (an oscillator)  116  or  138  that drives BDT receiver  104  or  126 , data link transceiver  108  or  130 , and cooperative position location processor  120  or  142  and a digital database  114  or  136  that contains such information as locations of ground transmitters and their spectrum band allocation (also referred to as a channel) as well as digital road maps and local terrain elevation/topographic information. 
     Referring to  FIG. 1  again, a cooperative position location system  102  or  124  further includes a displacement sensor  112  or  134 , which, as well as other components, will be further described in this specification later on. A user interface  144  or  146  is used to display, preferably graphically, results to and receive commands and/or data from users. Optionally, it may further include auxiliary sensors  118  or  140 . This may include such popular devices as wireless Internet adapter and/or a cell phone for online access to road maps and local weather information for instance. An altimeter or a barometer may be used to provide an indication of (differential) altitude. 
     Referring to  FIG. 2 , an exemplary embodiment  200  of a broadcast digital transmission (BDT) receiver  104  or  126  is depicted with a radio frequency (RF) front-end  202  coupled to a BDT receiver data processor  230  with software running either on a digital signal processor (DSP)/a microprocessor (μP) or hardwired on a field programmable gate array (FPGA)/an application-specific integrated circuit (ASIC) chip in accordance with the present invention. More specifically, an antenna  204  intercepts RF signals that come from a plurality of transmitters on the ground  140 , in the air  144 , and/or from the space  148 . Captured signals are low-noise amplified (LNA) and bandpass-filtered (BPF)  206  and coupled to a RF to IF converter  208  with mixers. There may be several RF to IF down-conversion stages, each followed by an amplifier (AMP)/a bandpass filter (BPF)  210  in order to reach the desired gain and bandwidth while minimizing unwanted nonlinearity and interference effects. To drive the RF front-end  202 , various frequency components  218   d  are produced by a frequency synthesizer  212  driven in turn by, say, a temperature-compensated crystal oscillator (TCXO)  214 , which is referred to as local clock  116  or  138 . The frequency synthesizer  212  also produces a sampling clock  218   a  to an analog to digital converter (ADC)  216  that samples and quantizes the IF signal and then passes the samples over for digital processing. The sampling rate should be sufficiently high to obtain an accurate representation of the BDT signal, as would be apparent to one skilled in the art. The frequency synthesizer  212  may also provide a clock  218   b  to drive digital signal processing that follows. The frequency synthesizer  212  may be tuned via an instruction line  218   c  to a desired center frequency of a particular broadcast digital transmission by a channels selector  220 . 
     Still referring to  FIG. 2 , the incoming signal samples provided by an analog to digital converter  216  are first processed in a baseband signal processor  222  wherein the signals from transmitters chosen by the channels selector  220  are detected, acquired, and tracked. Features or events of broadcast signals that are common to cooperative receivers  102  and  124  are extracted and time-tagged according to the local clock  214  to produce a time of arrival (TOA) measurement  230 . The TOA measurements  230  are then passed on to a cooperative position location processor  120  or  142  via a path  224  and to a data link transceiver  108  or  130  via a path  226 . In return, a cooperative position location processor  120  or  142  sets up a channels selector  220  and a baseband signal processor  222  via a control path  228 . Due to low duty factor, a baseband signal processor  222  operates on windowed data, thus easing throughput demand and saving power consumption. Details of the operations and interactions of a baseband signal processor  222  with others will be presented when describing subsequent figures. 
     The techniques for design and construction of antennas, RF front-end, data link modems, user interfaces, digital databases, and auxiliary sensors that possess the characteristics relevant to the present invention are well known to those of ordinary skill in the art. 
     As stated earlier, although the present invention is applicable to many broadcast digital transmissions, a preferred embodiment is described in this specification for the ATSC DTV signal. The current ATSC DTV signal is described in  ATSC Digital Television Standard  ( A/ 53),  Revision E, with Amendment No.  1, Dec. 27, 2005, with  Amendment No.  1 dated Apr. 18, 2006. The ATSC DTV signal uses the 8-ary Vestigial Sideband (8VSB) modulation and is organized into frames. Each frame has two fields, each field has 313 segments, and each segment has 832 symbols. The symbol rate is 10.762237 Mega-samples per second (Msps) and a symbol duration is 92.92 nanoseconds (ns), which is derived from a 27.000000 MHz clock. The segment rate is 12.935482 kilo-segments per second (ksps) and a segment duration is 77.307348 microseconds (μs). The field rate is 41.327096 fields per second and a field duration is 24.197200 milliseconds (ms). The frame rate is 20.663548 frames per second and the frame duration is 48.394400 (ms). There are a total of 260,416 symbols in a field and 520,832 symbols in a frame. 
     There are two types of segments, namely, field synchronization segment and data segment. Each segment starts with 4 symbols that are used for segment synchronization purpose (thus known as the segment sync code). There are two field synchronization segments in each frame, one for each field. Following each field synchronization segment are 312 data segments. The two field synchronization segments in a frame differ only to the extent that the middle set of 63 symbols are inverted in the second field synchronization segment. 
     The first 4 symbols of a data segment are 1, −1, −1, 1, which are known as segment sync code and used for segment synchronization. The other 828 symbols in a data segment are information-carrying that are randomized to be different from the segment sync code. Since the modulation is 8VSB, each symbol carries 3 bits of coded data using a rate ⅔ coding scheme. The 8VSB symbol values are −7 (000), −5 (001), −3 (010), − 1  (011), 1 (100), 3 (101), 5 (110), 7 (111) before pilot insertion. A pilot is a carrier signal, which has −11.5 dB less in power than the data signal, and is used to aid coherent demodulation of the ATSC DTV signal. The symbol pulse has a raised-cosine waveform, which is constructed by filtering, as described in the book entitled,  Digital Communications  (3 rd  Ed.), by J. G. Proakis, McGraw-Hill, 1995. 
     The code sequence used as a feature or an event to be acquired and time-tagged by all cooperative receives can be any known digital sequence in the received signal. However, it is preferred to be repetitive with a reasonable periodicity. In a preferred embodiment with ATSC DTV signals, such DTV signal components as pilot, symbol clock, or even carrier could be used for timing and ranging purposes. However, the use of such signal components would produce inherent ambiguities due to their high repetition rate (or equivalently short wavelength). There are well-known techniques in the art to resolve such ambiguities but their use would entail additional complexity. A preferred code is therefore a repetitive synchronization code in an ATSC frame such as a field synchronization segment within an ASTC DTV frame or a segment synchronization symbol sequence within a data segment within an ATSC DTV frame or a combination of both. Pseudorandom DTV transmitter identification (TxID) watermark signals can also be used to serve the same purpose. However, the use of multiple wavelengths may be desired with the timing of field, segment, symbol, and pilot/carrier in the order from the coarsest wavelength to finest wavelength. 
     One of the most important tasks of a baseband signal processor  222  is to search and detect a code sequence embedded in a BDT signal sample stream, which is disclosed in the U.S. Pat. No. 7,388,541, entitled, Self-Calibrating Position Location Using Periodic Codes in Broadcast Digital Transmissions, by the present inventor, and is hereby incorporated into this specification by reference. 
     As part of its operations, the baseband signal processor  222  performs correlation between signal samples over data windows and a desired code sequence (called a code replica) for match. A preferred method for such correlation is disclosed in U.S. Pat. No. 7,471,241, entitled, Global Navigation Satellite System (GNSS) Receivers Based on Satellite Signal Channel Impulse Response, by the present inventor, issued Dec. 30, 2008. Another preferred method is the generalized frequency-domain correlation (GFDC) disclosed by the present inventor in the paper entitled “Symmetric Phase-Only Matched Filter (SPOMF) for Frequency-Domain Software GPS Receivers,” in  ION Journal: Navigation , Vol. 54, No. 1, Spring 2007, pp 31-42, which is incorporated into this specification by reference. 
     Referring to  FIG. 3 , the range equations  320  and  322  between an event of interest at transmission  308  and the same event at reception  310  and  312  are illustrated on the k-th transmitter&#39;s time base  302 , the i-th receiver&#39;s time base  304 , and the j-th transmitter&#39;s time base  306 , respectively, in accordance with the present invention. In an embodiment, the leading edge of a field sync segment within an ATSC DTV frame is taken as an event of interest. The timing relationship between the event of interest at transmission  308  and the same event at reception  310  (and  312 ) is characterized by a time of flight (TOF) Δt i   k    314  (and Δ j   k    316 ), which is calculated from the time of transmit (TOT) t k    330  and a time of arrival (TOA) t i   k    326  (and t j   k    328 ) relative to the i-th receiver&#39;s time base  304  (and to the j-th receiver&#39;s time base  306 ). The times of flight, Δ i   k    314  and Δ j   k    316 , are then scaled into ranges, r i   k    320  and r j   k    322 , by the speed of light, c. As a result, the times of flight, Δ i   k  and Δ j   k , are related to the k-th transmitter&#39;s location (ξ k , η k ) (which is known) and the i-th receiver&#39;s locations (x i , y i ) and the j-th receiver&#39;s location (x j , y j ), respectively (which both are unknown and are to be estimated). 
     However, the time of transmit t k    330  is not known. Besides, the transmitter&#39;s clock is subject to bias and drift due to frequency instability. Prior art techniques attempt to estimate the transmitter&#39;s clock drift by various means. In the present invention, the time of transmit t k    330  (and the transmitter&#39;s clock error) is removed by the technique of measurement differentiation. Referring back to  FIG. 3 , a time difference of arrival Δt ij   k    318  is formed between the i-th and j-th receivers from their respective times of arrival t i   k    326  and t j   k    328 . However, a time offset ε ij    332  may exist between time bases of cooperative receivers. This is translated into an error term in differential range r ij   k    324 . Means to estimate time offset between cooperative receiver time bases and to communicate times of arrival of common events to each other will be described in connection with subsequent figures. 
     Referring to  FIG. 4 , an exemplary embodiment  400  of a data link transceiver  108  or  130  is depicted with a data receive channel  402 , a data transmit channel  420 , and a data link transceiver data processor  440  in accordance with the present invention. The data receive channel  402  comprises an antenna  404 , a low-noise amplifier (LNA) and a bandpass-filter (BPF)  406 , several RF to IF down-conversion stages  408 , each followed by an amplifier (AMP)/a bandpass filter (BPF)  410 , and an analog to digital converter (ADC)  412 . On the other hand, the data transmit channel  420  consists of a data modulator  430 , several IF to RF up-conversion stages  426 , each preceded by an amplifier (AMP)/a bandpass filter (BPF)  428 , a power amplifier  424 , and an antenna  422 . To drive the data receive channel  402  and data transmit channel  422 , various frequency components  418   a  and  418   b  are produced by a frequency synthesizer  416  driven in turn by a crystal oscillator  414  (from  116  or  138 ). The frequency synthesizer  416  also produces a sampling clock  418   c  and a clock  418   d  to drive digital signal processing that follows. The frequency synthesizer  416  may be tuned via an instruction line  432  to a desired center frequency for the data receive and transmit channels  402  and  420 , respectively. The techniques for design and construction of data transceivers that possess the characteristics relevant to the present invention are well known to those of ordinary skill in the art. 
     Still referring to  FIG. 4 , a data link transceiver data processor  440  consists of a receive signal processor  442  and a transmit signal processor  444 . The incoming signal samples provided by an analog to digital converter  412  are processed in a receive signal processor  442  wherein the signals from cooperative transceivers are detected, acquired, and tracked. Data bits and symbols of embedded messages are demodulated from the signals with their arrival times tagged in the local time. The demodulated messages include times of arrival  448  of common features of broadcast digital transmissions from common known transmitters at cooperative receivers. In a preferred protocol, it may also include transmit times of these very messages as they leaving cooperative receivers. As explained in  FIG. 5 , the demodulated transmit time of a message from a cooperative receiver and its arrival time in the local time tag are used by a cooperative time offset calibration and relative ranging block  446  to produce relative range Δt ij  (or r ij )  450   a  and time offset ε ij    450   b , which are then used by a cooperative position location processor  120  or  142 . 
     Still referring to  FIG. 4 , the time of arrival of a common feature  452  from a BDT receiver  104  or  126  is passed on to a transmit signal processor  444 . It formulates request and reply messages and converts times of arrival and time tags into proper formats according to a communications protocol before sending them to a data modulator  430  for transmission. 
     Referring to  FIG. 5 , an exemplary embodiment  500  of a clock offset calibration and relative ranging block  446  between two cooperative transceivers via a data link  122  (in addition to data exchanges) is illustrated in accordance with the present invention. This not only allows the cooperative transceivers to choose the most appropriate positioning mechanism but also provides them with additional measurements so as to eliminate the undesired dependency on a large number of broadcast digital transmissions that may not be available in a less populated area. The cooperative clock offset calibration and relative ranging is accomplished via the exchange of ranging request message  502  and ranging reply message  504 . As shown, the i-th transceiver sends out a ranging request message  502 . The leading edge of the message is time-tagged and the time-tag t 1    506  is immediately embedded into the same message (request send t 1    506 ). The message may also include time tags of selected features of broadcast digital transmissions it has captured. The j-th transceiver, which is ready to cooperate, will reply. But it first time-tags the request message when it arrives (request receive τ 1    508 ). After a certain turn-around time  518 , it will send a reply message (reply send τ 2    510 ). The reply message will include not only the time tags of request receive τ 1    508  and reply send τ 2    510  (and request send t 1    506  optionally) but also the time tags of selected features of broadcast digital transmissions it has captured. The reply message is again time-tagged when it reaches back the initiating transceiver (reply receive t 2  ( 512 ). 
     The time tags of selected features of broadcast digital transmissions are used to form measurements of time difference of arrival (scaled into differential range measurements by the speed of light plus certain corrections) to common sources as illustrated in  FIG. 3 . At the same time, the time tags of request send, request receive, reply send, and reply receive are used to estimate the clock offset between the two cooperative receivers. The various events are time-tagged relative to local time bases t  522  and τ  524 , respectively, which differ by a time offset ε ij    520 . 
     Still referring to  FIG. 5 , the ranging request message is transmitted at t 1    506  and received at τ 1    508 . Their difference yields the apparent time of flight Δt 1    526 , which consists of the true time of flight Δt  514 , the clock offset ε ij    520 , and measurement noise n 1    528 . Similarly, the ranging reply message is transmitted at τ 2    510  and received at t 2    512 . Their difference yields the apparent time of flight Δt 2    530 , which consists of the true time of flight Δt  516 , the clock offset ε ij    520 , and measurement noise n 2    532 . 
     Referring to  FIG. 5  again, the relative range between the two cooperative transceivers is calculated as the round-trip average Δ  t   534 , which will not only eliminate the clock bias ε ij    520  but also reduce the random noise by average. That is, the average noise sum  n   538  has a smaller variance. Similarly, the clock offset between the two time bases is calculated as the average difference  ε   ij    536 , which will also reduce the random noise by average. That is, the average noise difference ñ  540  has a smaller variance. 
     To implement the clock offset calibration and relative ranging mechanism as depicted in  FIG. 5  for a plurality of cooperative users, it requires the wireless data links to be capable of multiple access using either time division, code division, frequency division or a combination thereof. In addition, protocols are required to specify actions taken at cooperative transceivers so as to transform time of arrival (TOA) information into time of flight (TOF) and range estimate for ultimate positioning. One example of such protocols is the IEEE 802.15.4a Wireless Personal Area Network (WPAN) standard that is intended for the creation of a physical layer for short-range and low-data rate communications and for precise localization with ultra wideband (UWB) radios. Instead of embedding transmit time tags into ranging request/reply messages (as in GPS navigation messages), separate messages can be sent that carry transmit and receive time tags together with turn-around times and internal propagation delay and other protocol-related delay errors. In this case, each cooperative device acts like a transponder echoing back a ranging request/reply message immediately. Other time-based ranging protocols exist to implement the relative ranging and clock offset calibration as illustrated in  FIG. 5 . Examples include two-way time of arrival based ranging protocol (TW-TOA), differential two-way time of arrival ranging protocol (DTW-TOA), and symmetric double-sided ranging protocol (SDS) to name a few. This and other techniques are described in the book entitled  Ultra - Wideband Positioning Systems: Theoretical Limits, Ranging Algorithms, and Protocols , by Zafer Sahinoglu, Sinan Gezici, and Ismail Guvenc, Cambridge University Press 2008 (ISBN 978-0-521-87309-3), which is incorporated into the present specification by reference. 
     A host of techniques have been set forth recently for wireless network-based positioning. It typically relies on anchor nodes at known locations. A mobile node measures its ranges to several anchor nodes via times of arrival (TOA) and/or time differences of arrival (TDOA) through various communication protocols and finally determines its location by multilateration. The present invention differs from such wireless network-based positioning techniques significantly. First, all cooperative nodes in the present invention are at unknown locations (i.e., no anchor nodes), which are what to be estimated. Of course, the problem is greatly simplified if any of the nodes are known. Second, those sources at known location (i.e., broadcast digital transmitters) in the present invention are not “correspondent” in the sense that ranges to these known transmitters have to be estimated using a method other than two-way communications. 
     For the two-dimensional case considered in  FIG. 3 , there are four unknowns for two cooperative transceivers (i.e., (x i , y i ) and (x j , y j )) assuming that the clock offset between the two receivers (i.e., ε ij ) can be satisfactorily calibrated. Then, the differential range r ij   k    324  and the relative range r ij    534  only provide two equations, which are insufficient to solve for the four unknowns. Each additional BDT source will bring an extra range equation. At least three independent BDT sources are therefore required to provide an initial position location, as shown in the second column  602  of the table  600  depicted in  FIG. 6 . 
     However, there may not be enough number of independent BDT transmitters with a good geometrical distribution in practical situations, particularly in less populated areas. The difficulty is circumvented for mobile users if their displacement can be measured. Referring to  FIG. 6 , the third column  604  indicates that only two independent BDT sources are needed to solve for the unknowns if at least one of the cooperative users makes a displacement that can be accurately measured. Similarly, the fifth column  606  of  FIG. 6  provides a general formula  608  relating the number of displacements (m) needed for one or two of the cooperative users (l) to make as a function of the number of independent BDT sources (n). 
     Referring to  FIG. 7 , cooperative position location via wireless data link using broadcast digital transmissions is illustrated for a two-dimensional case in accordance with the present invention. A BDT source k  702  is at a known location (ξ k , η k ), referenced to a common coordinate frame ξ−η  700 . At time t 1 , the two cooperative receivers i and j are at unknown locations (x i (t 1 ), y i (t 1 ))  704  and (x j (t 1 ), y j (t 1 ))  706 , which we want to determine. At this time, the differential range r ij   k (t 1 )  712  calculated from r i   k (t 1 )  708   a  and  r   j   k (t 1 )  708   b  according to the method  324  and the relative range r ij (t 1 )  710  according to the method  534  are available but they are insufficient to solve the problem. 
     Still referring to  FIG. 7 , receiver j  706  is assumed to be stationary whereas receiver i  704  is to move. Consider a general case wherein receiver i  704  can only measure its displacement (r 1 , θ 1 )  718  relative to a local coordinate frame x−y  714 . Further, receiver i can track the change of its local frame relative to the common coordinate frame ξ−η  700  except for its initial orientation θ 0    716 , which is however unknown. This leads to a set of five unknowns (x i (t 1 ), y i (t 1 ), x j (t 1 ), y j (t 1 ), θ 0 )  720  to solve for. 
     Still referring to  FIG. 7 , at time t 2 , after the first displacement (r 1 , θ 1 )  718 , receiver i is at (x i (t 2 ), y i (t 2 ))  722 . The differential range r ij   k (t 2 )  728  is calculated from r i   k (t 2 )  724   a  and  r   j   k (t 2 )  724   b  according to the method  324  and the relative range r ij (t 2 )  726  according to the method  534  are available. Since the unknown location at time t 2  (x i (t 2 ), y i (t 2 ))  722  can be related to the initial unknown location at time t 1  (x i (t 1 ), y i (t 1 ))  704  via equation  730  where the displacement vector is calculated via equation  732 . There are four measurements but they are still insufficient to solve the problem. 
     Referring to  FIG. 7  again, after another displacement (r 2 , θ 2 )  736 , receiver i reaches at (x i (t 3 ), y i (t 3 ))  734 . At time t 3 , two more measurements are available and they are the differential range r ij   k (t 3 )  738  and the relative range r ij (t 3 )  740 . Since the unknown location at time t 3  (x i (t 3 ), y i (t 3 ))  734  can be related to the initial unknown location at time t 1  (x i (t 1 ), y i (t 1 ))  704 , then after two displacements and taking measurements at three separated locations, there are six measurements and they are sufficient now to solve for the five unknowns (x i (t 1 ), y i (t 1 ), x j (t 1 ), y j (t 1 ), θ 0 )  720 . 
     Measuring displacements (i.e., distance travelled and orientation turned) in one&#39;s own frame is a form of dead-reckoning. For example, an inertial measurement unit (IMU) has a three-axis accelerometer assembly to measure accelerations and a three-axis gyro assembly to measure rotation rates. The IMU outputs are integrated over time in an inertial navigation system (INS) to produce an inertial solution (position and attitude). To curb its ever-growing errors, an INS solution needs to be frequently updated by a navigational aid (navaid) such as the Global Positioning System (GPS). However, the integration of differential and relative ranges with displacements in the present invention significantly differs from conventional integrations such as GPS/INS in two major aspects. First, conventional dead-reckoning adds up displacements forward from a given initial condition, which is however unknown and is to be estimated in the present invention. Second, the integration of all measurements in the present invention is done at the initial point, leading to a fixed-point smoother whereas a Kalman filter is typically used for conventional GPS/INS integration. As a matter of fact, the present invention offers an alternative approach that can be used to integrate an INS with other navaids such as GPS without the need for precise initialization otherwise required for conventional inertial solution. 
     Once the initial point is determined, the displacements are integrated forward from the initial point to yield a solution trajectory. This processing procedure is somewhat similar to satellite orbit determination. However, in orbit determination, the displacements are numerically integrated from precise mathematical models of the geopotential and other perturbation force fields. 
     Referring to  FIG. 8 , an exemplary embodiment  800  of an initialization step  802  of a cooperative position location processor  120  or  142  is illustrated in accordance with the present invention. A data link transceiver  108  or  130  provides a time of flight between two cooperative receivers  804  (or  534 ), a time offset estimate  810  (or  536 ), and a time tag of a common event of BDT at another cooperative receiver  808  (or  328 ). At the same time, a BDT receiver  104  or  126  of this receiver provides a time tag of the same event at this receiver  806  (or  326 ). The time tag of a common event of BDT at this receiver  806  and the time tag of the same event at the other cooperative receiver  808  are used to calculate a time difference of arrival between the two cooperative receivers  814 . A time offset estimate  810  is used to apply time offset correction  816 . A next step is to apply tropospheric/topographic correction  818 . This is because the speed of a radio signal changes when propagating through media as a function of weather conditions, e.g., air temperature, atmospheric pressure, and humidity. The weather information in the vicinity of a BDT receiver can be obtained via an online/onboard database  114  or  136  from the Internet or other sources such as National Oceanic and Atmospheric Administration (NOAA), from which the actual propagation velocity can be determined. Furthermore, radio geolocation with terrestrial transmitters is better suited for two-dimensional latitude/longitude position location because the vertical dimension (i.e., height above the ground) of transmitters is rather small compared to horizontal dimensions. In terrains with hills and valleys relative to a transmitter antenna&#39;s phase center, a user may not lie on the circle of constant range around a transmitter if the user has a different altitude even though its line of sight distance to the transmitter is the same as the circle radius. In this case, a terrain topographic map can be used to compensate for the effect of user altitude on the surface of the earth. Optionally, an altimeter or a barometer can be used to obtain an estimate of (differential) altitude. For the same reason, a next step is to apply topographic, protocol related, and internal circuitry propagation errors correction  820  to a time of flight  804 . These time tags are converted into differential and relative range measurements  822 , which are collected at a location  824  and then cumulated at multiple locations  830 . 
     Still referring to  FIG. 8 , a simultaneous step is to obtain displacement vectors  826  from a displacement sensor  812  ( 112  or  134 ). A next step is to relate displacement  812  from different locations to an initial location  828 . A final step of initialization  802  is to apply a nonlinear least squares method to the collected measurements  830  and their relationships to the initial location  828  to solve for initial location  832  to afford an initial solution  834 . Alternatively, an initial solution estimate  840  can be obtained from a user interface  836  ( 144  or  146 ) via a selection switch  838 . A direct search can be used to solve the nonlinear least squares formulation to obtain an initial estimate. A preferred Nelder-Mead simplex method, disclosed in the paper entitled “A Simplex Method for Function Minimization,” by J. A. Nelder and R. Mead in  Computer Journal,  7 (1965), 308-313, is incorporated into this specification by reference. 
     A magnetic compass and a tape measure can serve as a rudimentary displacement sensor. The magnetic compass determines the direction of travel relative to the magnetic north while the tape measure indicates the distance travelled, thus providing a displacement vector. So long as the displacement is large in magnitude as compared to errors in differential and relative range measurements, it can help solving the positioning equations provided that the line of sight (LOS) vectors are sufficiently moved, thus affording a good geometry. For a four-wheeled ground vehicle when not skidding, the average of all wheel speeds from its antilock break system (ABS) provides an estimate of the speed, which is integrated over time to provide the distance traveled. On the other hand, the scaled difference between the pairs of left and right wheel speeds provides an estimate of the turning rate, which is integrated into the heading change (yaw). Together the two measurements provide an estimate of relative velocity vector, which is integrated into a displacement vector. Inertial sensors such as accelerometers and gyros can also be mechanized to provide displacements as disclosed in the previously mentioned U.S. Pat. No. 7,388,541 by the present inventor. 
     Referring to  FIG. 9 , an exemplary embodiment  900  of a cooperative position location processor is illustrated for continuous processing after initialization in accordance with the present invention. Similar to the initialization step  802  in  FIG. 8 , a data link transceiver  108  or  130  provides a time of flight between two cooperative receivers  904  (or  534 ), a time offset estimate  910  (or  536 ), and a time tag of a common event of BDT at another cooperative receiver  908  (or  328 ). A time tag of the same event at this receiver  906  (or  326 ) is provided by a BDT receiver  104  or  126 . A first step of measurement collection  902  is to calculate time difference of arrival between two cooperative receivers  912  from the time tag of a common event of BDT at this receiver  906  and the time tag of the same event at another cooperative receiver  908 . The time offset estimate  910  is used to apply time offset correction  914  initially. A next step is to apply tropospheric/topographic/protocol delay correction  916  to both time of flight  904  and time difference of arrival after time offset correction  914 . These time tags are converted into differential and relative range measurements  918 , which are collected per location  920 . 
     Still referring to  FIG. 9 , a simultaneous step is to obtain displacement vectors  926  from a displacement sensor  112  or  134 . A next step is to compensate for bias and scale factor  928  of displacement measurements  924 . Given an initial solution  940  ( 840 ) and compensated displacements from  928 , a next step is to obtain predicted locations  930 , from which to calculate a geometry scaling matrix  934  on the one hand and to obtain differential and relative range measurement prediction errors  932  on the other hand. A next step is to obtain a joint location and sensor calibration solution  936  from measurement prediction errors  932  and a geometry scaling matrix  934 . Due to nonlinear nature, a next step is to make several in-step iterations or to calculate next-step prediction  938 . The joint estimates of location errors and sensor calibrations  942  are used to correct previous estimates to obtain improved locations for next step  940 , to compensate for displacement sensor errors  928 , and to apply time offset correction  914  in a refined manner. The exemplary scheme  922  for joint position location and sensor calibration in  FIG. 9  can be understood as a least squares fixed-point smoother wherein all displacement vectors are brought back to the initial point and a batch processing is applied. An alternative joint position location and calibration scheme is to use sequential processing. 
     Referring to  FIG. 9  again, the compensated displacement measurements  928  and the location solution  942  are finally used to construct a solution trajectory  944  to yield a location trajectory over time  946 . The techniques for design and construction of a least-squares fixed-point smoother or a sequential processing Kalman filter that possesses the characteristics relevant to the present invention are well known to those of ordinary skill in the art. In particular, the ridge regression technique may be used to alleviate rank-deficiency in the geometry scaling matrix if occurred. 
     Although the description above contains much specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, the present invention provides a generic signal processing architecture of versatility where many processing steps can be tailored to achieve a desired combination of functionalities. As a result, some steps may be made optional and others are omitted. Flexibility is also provided for design parameters tradeoff to best suit a particular application. As a preferred method, a least-squares smoother detailed in this specification may be substituted with such nonlinear estimators as an unscented Kalman smoother, a particle smoother, and a variant thereof for the same purposes. These filters and smoothers may be replaced by numerical methods of direct search such as the Nelder-Mead simplex method mentioned earlier. A constant bias and scale factor are modeled in the error state vector for time offset and displacement sensor errors in the exemplary joint estimation formulation of this specification. It is possible to account for higher order effects by including such terms as drift in the error state. 
     Reference has been made mostly to digital television signals but the present invention is equally applicable to broadcast digital radio/audio signals, wireless local area network (WLAN) and wireless personal area network (WPAN), other broadcast digital transmissions in general, and even partially available GPS signals. One example is the high power, low frequency radio signal broadcast by the National Institute of Standards and Technology (NIST) WWVB station near Ft. Collins, Colo., that has been used by millions of people throughout North America to synchronize consumer electronic products like wall clocks, clock radios, and wristwatches. 
     As a preferred embodiment, the use of range measurements is described in detail in the present specification. However, other measurements such as angles of arrival (AOA) can be used instead. Similarly, visual odometers (via optical flow for instance) can be used to construct a displacement sensor in a personal dead-reckoning system. Although the drawings as presented in this specification are two-dimensional for the sake of simple and clear presentation, it can be easily generalized to three-dimensional cases. Furthermore, the cooperative mechanism is described in the present invention for position location. It can however be used for joint sensor location and target tracking as well as for cooperative simultaneous location and mapping (CSLAM). 
     It is understood that the various figures described above illustrated only the preferred embodiments of the present invention system and method. A person skilled in the art can therefore make numerous alterations and modifications to the described embodiments utilizing functionally equivalent components and method steps to those shown and described. All such modifications are intended to be included within the scope of the present invention as defined by the appended claims.