Patent Publication Number: US-7719467-B2

Title: Digital camera with GNSS picture location determination

Description:
FIELD OF THE INVENTION 
     The invention relates to digital cameras and more particularly to a digital camera for providing a short burst of global navigation satellite system (GNSS) signal samples in a picture data file medium for reading by a computer apparatus an arbitrary time later for determining the geographical location and an accurate time of a picture. The invention also relates to GNSS position determination of an event an arbitrary time after the event from a short burst of GNSS signal samples without having an accurate GNSS signal reception time. 
     DESCRIPTION OF THE BACKGROUND ART 
     Global Navigation Satellite Systems (GNSS)&#39;s now come in several forms. The United States government maintains the global positioning system (GPS) of earth orbiting GPS positioning satellites that broadcast GPS signals in several formats. The European Space Agency is deploying a Galileo constellation of positioning satellites. And, Russia has been developing its global navigation satellite system (GLONASS) of positioning satellites for many years. The GNSS satellites provide signals having location-determination information and code timing that can be received, measured and decoded by a GNSS receiver for determining a geographical location of the receiver and an accurate GNSS-based clock time. 
     The acquisition process for finding signal power in a GNSS signal involves correlating a pseudorandom noise (PRN) code carried on the incoming GNSS signal broadcast by the GNSS satellite against a locally generated PRN code replica. The local code replica is correlated with the incoming code at successive code phase offsets until a code phase offset is found that shows signal power. This process is known as a code search. 
     When signal power is found, the GNSS receiver uses inversions of the code for determining data bit timing. The data bit timing is used for monitoring the GNSS data bits of the incoming GNSS signal until a GNSS clock time for a time-of-transmission is decoded from the data bits. The time-of-transmission is used with orbital ephemeris parameters for a GNSS satellite for calculating the satellite&#39;s location-in-space. The locations-in-space for several GNSS satellites are used with the code phase offsets and the data bit timings for providing pseudoranges between the GNSS receiver and the satellites. These ranges are termed “pseudo” because they depend upon the local replica clocking offset. The GNSS receiver performs arithmetic operations on the locations-in-space and the pseudoranges for resolving the replica clocking offset and the location of the receiver. The resolution of the clocking offset and times-of-transmission are used for determining the GNSS clock time. 
     Conventional GNSS receivers use four GNSS satellites for resolving the four GNSS unknowns for the three dimensions of the geographical location of the GNSS receiver and the fourth dimension for the clocking offset. It is also conventional for GNSS receivers to use fewer than four satellites when other positioning information, such as altitude, inertial motion or map matching, is available to substitute for the positioning information of a pseudorange; and to use more than four satellites for overdetermining the four unknowns. 
     The GPS C/A code signal data bits have frames having time periods of thirty seconds. The frames are segmented into five subframes of six seconds each. The time-of-transmission for the GPS signal is encoded in the GPS data bits in a Z-count at six second intervals near the beginnings of the subframes. Unfortunately, this means that about six seconds of GPS data bits must be observed in order to ensure receiving the Z-count. Further, in order to ensure that random data is not mistaken for the Z-count, two subframes or about twelve seconds, are sometimes observed. However, there are certain GPS positioning systems where it is undesirable or impractical to receive or use a GPS signal burst that is twelve or even six seconds long. 
     GPS processing techniques have been developed using a relatively accurate, for example 100 milliseconds, knowledge of time as an assumed time for eliminating the requirement for receiving the Z-count. Unfortunately, there are circumstances where these techniques cannot be used because time with this accuracy is not available. For example, a stand-alone event capture device such as a digital camera might have a real time clock that accumulates an error of one second per day or might be incorrectly set by a user for a time error of several hours or even days. 
     SUMMARY 
     It is therefore desirable to provide apparatus and methods for providing a short data burst of GNSS signal samples for later determination of a GNSS location where the GNSS signal reception time may have a receiver time error of several hours or even days. 
     The disclosure discusses a way to use measurements from an extra GNSS satellite and velocity estimates for GNSS satellites for overcoming the effect of a large receiver time error. An event location system is disclosed that involves the mutual resolution of the conventional four unknowns of a GNSS system of the three dimensions of a location and the fourth dimension of a replica clocking offset and also a fifth dimension of an unknown for the receiver time error. This resolution requires correlation measurements on GNSS signals from one more than a conventional number of GNSS satellites. 
     Briefly, a system is disclosed that includes a digital camera, or other event capture device, and a computer apparatus. The event capture device receives GNSS signals at an event; samples the GNSS signals; and then writes GNSS signal samples into an event data file medium with an approximate signal reception time tag that differs from GPS-based clock time by a receiver time error. The computer apparatus includes a GNSS sample processor. The GNSS sample processor measures correlations and performs arithmetic operations on the GNSS signal samples in the event data file medium at a later time for determining the location of the event and the GNSS-based time of the event. 
     In an embodiment a digital camera comprises: a picture device for taking a picture and converting the picture into picture data; a global navigation satellite system (GNSS) signal sampler for sampling the GNSS signals when the picture is taken; a real time clock for generating a time tag for the GNSS signal samples, the time tag having a time uncertainty window greater than a time length of a frame of data bits for the GNSS signals; and a picture file formatter for writing the picture data, the GNSS signal samples and the time tag into a computer-readable picture data file medium configured for processing by a computer apparatus for determining a GNSS picture position based on the GNSS signal samples and providing the picture with the GNSS picture position to a user. 
     In another embodiment a method for providing a global navigation satellite system (GNSS)-based position for a picture comprises: converting a scene into digital picture data; sampling GNSS signals from GNSS satellites; generating a time tag for the GNSS signal samples, the time tag having a time uncertainty window greater than a time length of a frame of data bits for the GNSS signals; and writing the picture data in association with the GNSS signal samples and the time tag into a computer-readable picture data file medium in a configuration for processing by a computer apparatus for determining a GNSS picture position based on the GNSS signal samples and providing the picture with the GNSS picture position to a user. 
     An advantage that may be obtained is that a GNSS-based location of a picture taken by a digital camera is resolved in a separate computer apparatus, thereby reducing the cost of the camera by minimizing the GNSS hardware and software in the camera. 
     An advantage that may be obtained is that a GNSS-based location where a picture was taken can be determined at a different location at an arbitrary later time, using GNSS signal samples collected at the time and location the picture was taken. 
     An advantage that may be obtained is that a GNSS-based location is determined without decoding of encoded GNSS data bits for a time-of-transmission, thereby enabling the determination of the GNSS-based location from a burst of GNSS signals shorter than the time between the time-of-transmission data bits in the GNSS signals. 
     An advantage that may be obtained is that a GNSS-based location and an accurate time that a picture was taken can be resolved where the receiver time error for receiving and time tagging the GNSS signals is several hours, or even days. 
     An advantage that may be obtained is that the idea of the digital camera may be generalized to an event capture device for recording an event where a GNSS-base location and time of the event may be determined at an arbitrarily later time from GNSS signal samples in an event data file medium without having an accurate time when GNSS signals are received by the event capture device. 
     These and other embodiments and advantages will no doubt become obvious to those of ordinary skill in the art after reading the following detailed descriptions and viewing the various drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram for an event capture device at an event for writing GPS signals samples into an event data file medium.  FIG. 1B  is a block diagram for a computer apparatus for using the data on the event data file medium to determine the location and time of the event; 
         FIG. 2  is a block diagram of a digital camera for the event capture device of  FIG. 1A ; 
         FIG. 3  is a block diagram of the computer apparatus of  FIG. 1B  having a GPS sample processor having compensation for a large receiver time error; 
         FIG. 4  is a signal chart for GPS signal samples for the system of  FIGS. 1A-B ,  2  and  3 ; 
         FIG. 5  is a block diagram of a GPS locator for the GPS sample processor of  FIG. 3 ; 
         FIG. 6  is a flow chart of a GPS positioning method; 
         FIG. 7  is a timing chart showing a replica clocking offset and a receiver time error for the system of  FIGS. 1A-B ,  6  and  8 ; and 
         FIG. 8  is a flow chart of a method for using an extra GPS satellite and velocity estimates of GPS satellites for resolving the location and time of an event when the GPS signals were received with a large receiver time error. 
     
    
    
     DETAILED DESCRIPTION 
     The details of several embodiments for carrying out the ideas of the invention will now be presented. It should be understood that it is not necessary to employ all of the details of the embodiments in order to carry out the idea of the invention. Consequently, these details are not to be read as limitations of the below-listed claims of the invention. 
     The embodiments are described in terms of the global positioning system (GPS) having GPS signals modulated with a coarse/acquisition (C/A) code. However, the idea of the invention may be carried out with other GPS signal formats. Further, the invention may be carried out with a global navigation satellite system (GNSS) where the global positioning system (GPS), the global navigation satellite system (GLONASS), the Galileo system or similar satellite positioning systems, or a combination of these systems provides GNSS positioning signals. It should also be noted that the GNSS signals sources for broadcasting GNSS positioning signals may sources other than satellites, such as pseudolites, or a combination of satellites and sources other than satellites. 
       FIGS. 1A and 1B  show sections  10 A and  10 B of a block diagram of a GPS positioning system. The system section  10 A includes an event capture device  12  and a digital event data file medium  14 . The system section  10 B includes the event data file medium  14  and a computer apparatus  16 . The computer apparatus  16  is referenced to and discussed below as the computer  16 . The event capture device  12  may be a still camera, video camera, a tape recorder, telephone, cellular telephone, Wi-Fi base station or the like. 
     The event capture device  12  converts an event, such as a picture or photograph, a video or series of photographs, a burst of sound, data taken over a time period, data for attributes of a machine operation, or the like into digital form. The device  12  also receives and takes samples  18  of a short burst of GPS signals  19 . A digital representation of the event and the GPS signal samples  18  are stored on the medium  14 . The medium  14  is a flash card, floppy disc, compact disc (CD), digital video disc (DVD), portable hard disc (HD) or some other type of portable memory. 
     At some time later the data file medium  14  is read by the computer  16 . The later time may be a few seconds to a few years later. There are no restrictions for the location of the computer  16  with respect to the location of the event capture device  12 . The data file medium  14  may be physically carried to the computer  16 . Alternatively, the data in the data file medium  14  may be transmitted over a communication link to the computer  16 . The computer  16  processes the data on the medium  14  for providing a display, print out, or re-formatted data for the event with characters in the form of subtext or appended data for the GPS-based location and/or time of the event. 
     The system  10  uses GPS signals  19  from one more GPS satellite than the numbers of GPS satellites that a GPS receiver conventionally uses (with or without other positioning information and with or without overdetermination) for determining location and time. The system  10  uses the GPS signals  19  from the extra GPS satellite and estimates of satellite velocities for overcoming and compensating for large receiver time errors, up to plus and minus six hours, and sometimes longer, without knowing or decoding GPS clock time data bits (Z-counts in a C/A code) of the GPS signals  19 . The receiver time uncertainty window is permitted to be up to twelve hours for the system  10  to determine a GPS-based position. 
     In a typical case for three dimensional location (position) determination with no other source of positioning information, the conventional number of GPS satellites without overdetermination is four in order to resolve the four conventional GPS unknowns of the three dimensions of position and a fourth dimension for the local replica clock offset. For this case, the system  10  uses five GPS satellites and satellite velocity estimates for determining the four conventional GPS unknowns and a fifth unknown for receiver time error. 
     For position determination when altitude is separately available, the conventional number of GPS satellites without overdetermination is three in order to resolve the three unknowns of two dimensions of horizontal position and the third dimension of the local replica clock offset. For this case the system  10  uses four GPS satellites and satellite velocity estimates for determining the three conventional GPS unknowns and a fourth unknown for receiver time error. 
     Additional GPS satellites are conventional for overdetermination of the four GPS unknowns. For example for first order overdetermination, a conventional number of GPS satellites is five in order to resolve the four conventional GPS unknowns. For this case the system  10  uses a sixth GPS satellite for a first order overdetermination of the five unknowns of the location, replica clocking error and receiver time error. 
     For position determination when altitude is separately available, the conventional number of GPS satellites for first order of overdetermination is four in order to resolve the three unknowns of two dimensions of horizontal position and the third dimension of the local replica clock offset. For this case the system  10  uses a fifth GPS satellite for overdetermining the three conventional GPS unknowns and the fourth unknown for receiver time error. 
       FIG. 2  is a block diagram for the event capture device  12  as a digital camera device  12 A and the medium  14  as a picture data file medium  14 A. The digital camera  12 A includes a picture device  22 , a real time clock  23 , a GPS antenna  24 , and GPS hardware and software for a GPS signal sampler  26 . The GPS signal sampler  26  includes a GPS frequency downconverter  27  and a sampler  28 . 
     The GPS antenna  24  receives GPS signals  19  from GPS satellites and passes the GPS signals  19  in a conducted form to the GPS frequency downconverter  27 . The GPS frequency downconverter  27  converts the radio frequency (RF) of the GPS signals  19  to a lower frequency and passes the lower frequency GPS signals to the sampler  28 . The sampler  28  samples and digitizes the lower frequency GPS signals for providing the GPS signal samples  18 . 
     The sampler  28  digitizes the downconverted GPS signals for providing digital GPS signal samples  18  for I (in-phase) and Q (quadrature phase) components (see  FIG. 4 ). The samples  18  at this stage represent, at a lower frequency, the superimposed GPS signals  19  simultaneously received by the GPS antenna  24  from several GPS satellites. These signal samples  18  are not the GPS data bits that carry GPS data of Z-count, satellite health, almanac and ephemeris parameters, and the like but are instead discrete digital representations of the incoming RF signals from which the GPS data bits may be recovered by GPS-specific correlation and timing measurements using GPS replica signals. 
     It is worth noting at this point that the device  12 , 12 A has a low cost because it does not include hardware and software for performing these correlation or timing measurements on the GPS signal samples  18 , does not perform the arithmetic operations for processing the measurements for determining location or time, and does not need the extra memory for storing ephemeris or almanac orbital parameters for the GPS satellites. This shifts bill of material (BOM) costs for performing these functions away from the device  12 , 12 A to the computer  16 . This is important because the event capture device  12  and especially the digital camera device  12 A may have such a low cost that even a few dollars added to the BOM cost can result in a significant percentage increase to the total cost. The construction of the device  12 , 12 A and the computer apparatus  16  shifts much more BOM cost from the device  12 , 12 A to the computer apparatus  16  than previous attempts to provide device location at a computer. 
     The picture device  22  typically includes a lens, charge coupled devices, displays, user operated keys and other hardware and software for taking a picture for capturing an image or likeness of a scene and converting the picture into digital data. The picture is an event. The real time clock  23  provides an approximate time and a date having an accuracy that depends on the accuracy to which it was set by a user and its time drift since the time it was set. This time error with respect to GPS time is an unknown receiver time error that is local to the device  12 , 12 A. When a picture is taken, the clock time from the real time clock  23  provides an event time tag for the time in the device  12 , 12 A that the GPS signals  19  were received where this time tag has an error of the receiver time error. 
     The event capture device  12  or digital camera device  12 A includes a data file formatter  31  for configuring the data for GNSS signals samples  18 , the time tags and the data for the event or picture into the event or the picture data file medium  14  or  14 A. 
     The device  12  or camera  12 A optionally includes an altimeter  33  for providing an altitude Z. The altitude Z at the time of the event or picture is formatted by the formatter  31  into the data for the medium  14 , 14 A. Manual entry of the altitude Z may be provided. 
     The burst of GPS signal samples  18  written in the data file medium  14 , 14 A has a range of about twenty milliseconds to about one second. In exemplary cases the burst is less than about 30 milliseconds, 50 milliseconds, 100 milliseconds, 500 milliseconds, or one second. The length of the burst may be increased when the GPS signals  19  have a low signal strength at the device  12 , 12 A and decreased when the GPS signals  19  are strong at the device  12 , 12 A. 
       FIG. 3  is a block diagram of the computer  16 . The computer  16  includes a GPS sample processor  40  and an event display  42 ; and hardware and software for a microprocessor computing system typically including monitor, user controlled keys, mouse, hard disc drive, drives for removable memory, electrical interface connections, and the like that are conventional. The computer apparatus  16  may include a printer. One or more of the drives for removable memory or electrical interface connections is constructed as an event data file reader  44  or picture data file reader  44 A for receiving the data in the event data file medium  14  or picture data file medium  14 A at any time after the event or picture and passing the data to the GNSS sample processor  40 . The time lag between the event or picture and the reading of the medium  14 , 14 A may be hours, days, weeks, months or even years. 
     The hardware and software in the microprocessor system are configured for processing the event and/or picture data and displaying the features of the event and/or photograph as the event display  42  with alphanumerical characters as subtext for the GPS-based location (position) and time of the photograph. The terms “location” and “position” are used interchangeably in this application. The characters may express the location in geographic form such as latitude, longitude and altitude and/or may use reverse geocoding for providing a name or address or geographical information system (GIS) attribute that is proximate to the geographical location. It should be noted that the computer  16  may be a personal computer with the addition of the GPS sample processor  40  where the monitor of the personal computer shows the event display  42  in a form that is viewable by a human user. 
     The location of the event or picture is determined in the form of geographical coordinates such as latitude, longitude and altitude or some other two or three dimensional geographical coordinate system. The computer  16  optionally includes a reverse geocoder  46 . The reverse geocoder  46  optionally maps the geographical coordinates into names or addresses such as countries, provinces or states, cities, postal codes, street addresses or provides GIS attributes or features of the location of the event. 
     The GPS sample processor  40  includes a correlation machine  48  and a GPS locator  50 . The correlation machine  48  includes carrier correlators  52 , code correlators  54 , PRN replica generators  56  and a code clock  58  for making correlation and timing measurements on the GPS signal samples  18 . The carrier correlators  52  include hardware and software for generating local carrier frequencies that are representative of GPS signal carrier frequencies for performing carrier correlations for each GPS satellite that is under consideration. Generally a GPS satellite may be put into consideration if it is healthy and above a selected elevation mask above the horizon. 
     When more than a required number of satellites meet these qualifications, certain satellites may be selected for providing a constellation having the best expected dilution of precision (DOP) or the highest signal strengths; or the excess satellites may be used for overdetermination of the GNSS unknown variables. In a conventional GPS receiver GPS signal sample time tags are used with ephemeris parameters for estimating satellite velocities. The satellite velocities are used for estimating Doppler frequency shifts. The estimated Doppler frequency shifts are then used for predicting the correct GPS signal carrier frequencies in order to find GPS signal power more quickly by reducing the number of frequencies that need to be searched to find the GPS carrier frequency. However, the satellite velocities in the system  10  are used, after signal power has already been found, for an entirely different function of resolving a position when there is a large receiver time error for the GPS signal reception time. 
     The code correlators  54  include hardware and software for using pseudorandom noise (PRN) code replicas for each GPS satellite under consideration for performing code correlations. In order to recover the incoming GPS signal carrier frequencies in the GPS signal samples  18 , the carrier correlators  52  multiply the GPS signal samples  18  by local signals at the local carrier frequencies to mix the carrier frequencies in the incoming GPS signal samples  18  to baseband or a predetermined intermediate frequency. The local carrier frequencies that are used are typically much lower than the RF carrier frequencies of the GPS signals  19  originally received by the GPS antenna  24 . Several local carrier frequencies for a particular GPS satellite may be tried in series or parallel in what is known as a frequency search. 
     The PRN replica generators  56  use a replica clocking signal from the code clock  58  for generating phase shifted versions of the PRN replica codes corresponding to the GPS satellites under consideration. For each local carrier frequency that is tried, the code correlators  54  correlate the PRN replica codes from the generators  56  against the PRN codes in the incoming GPS signal samples  18 . The replica clocking signal has an unknown replica clocking offset with respect to ticks of GPS time. For example, when the replica clocking signal has a one millisecond time period, the replica clock offset is defined with respect to one millisecond ticks of GPS clock time. The GPS clock time of the ticks is not known until all the unknown variables for GPS location and time are resolved. 
     The incoming signal PRN codes represent the GPS satellites from which GPS signals  19  were received and sampled in the event capture device  12  and/or digital camera  12 A. In general, incoming signal PRN codes will be present and superimposed in the GPS signal samples  18  for all the GPS satellites that are healthy and above the horizon as long as the line of sight to the satellite is not blocked. The correlation machine  48  correlates the PRN code replicas at successive code phase offsets with the incoming signal PRN codes until correlation levels are found that show signal power for a GPS satellite. This process is commonly known as a code search. 
     The code search may test several code phase offsets in parallel. Fast Fourier Transform (FFT) techniques may be used for the code search or for a combination of the code and carrier searches. The code phase offsets for several GPS satellites are determined in like manner in parallel or in series. The code phase offsets that correspond to the signal powers for the GPS satellites are then used in arithmetic operations for the determination of the GPS-based position and time of the event capture device  12  and/or digital camera  12 A. It should be remembered at this point that the replica clocking offset adds an unknown time or phase offset that is common for the code phase offsets for all GPS satellites under consideration. 
     The carrier correlators  52  recover and wipe out the GPS signal carrier variations from the IQ digitized data and pass carrier-less (meaning the variability of the carrier has been eliminated) digital I and Q components to the code correlators  54 . The code correlators  54  operate on the carrier-less digital I and Q components for providing the code phase offsets, with respect to the replica clocking offset, for the code correlations between the pseudorandom noise (PRN) codes carried in the carrier-less digital I and Q signals and the I and Q components of the internally generated PRN code replicas. 
     After signal power has been found the code correlators  52  use inversions of the code phase correlations for determining data bit timing (DBT). In the case of the GPS C/A code, the data bit timing is used for determining a code integer for the number of code epochs between a DBT edge and the start of the code epoch for which the code phase offset was determined. The code integer, also known as a data bit edge (DBE) integer, corresponds to the number of code epoch time periods between the DBT edge and the start of the code epoch for which the code phase offset was determined. The DBE integer has a range of the number of code epochs in a data bit. For example, for the GPS C/A code, the range is twenty, meaning there are twenty possible DBE integers from zero to nineteen, and the integer increments represent one millisecond. The correlation machine  48  provides the code phase offsets and DBE integers to the GPS locator  50 . 
     The GPS locator  50  includes a GPS matrix resolver  60 , a satellite velocity calculator  62  and a GPS time calculator  64 . The GPS locator  50  uses the GPS signal samples  18  from an extra GPS satellite and the satellite velocities estimated by the velocity calculator  62  for compensating for large receiver time errors in the time tag reception time of the GPS signals  19  by the device  12 , 12 A. 
     The GPS matrix resolver  60  uses the code phase offsets together with the DBE integers for a conventional number of GPS satellites plus one extra GPS satellite for determining the location of the event corresponding to the GPS signal samples  18 ; the local (at the device  12 , 12 A) receiver time error when the GPS signals  19  at the event were received; and the local (at the computer  16 ) replica clocking offset. For an exemplary case the receiver time error is less than 6 hours and it may be greater than 100 milliseconds, 200 milliseconds, 500 milliseconds, 1 second, 2 seconds, 5 seconds, 10 seconds, 20 seconds, 50 seconds, 100 seconds, 200 seconds, 500 seconds, 10 minutes, 20 minutes, 1 hour, 2 hours or 5 hours. In general, greater receiver time errors will require more resolution iterations and a longer time to converge on the resolution of the unknown variables for the location of the event. 
     Overdetermination is a feature for using more than the required number of GPS satellites for providing better accuracy or more confidence in the position result for the number of unknowns. For example, the system  10  requires five GPS satellites (or four satellites plus altitude) for determining the five unknowns for the position, replica clocking offset and receiver time error. The GPS sample processor  40  may use a sixth GPS satellite (or a fifth satellite plus altitude) for overdetermining the five unknowns. 
       FIG. 4  is a time chart of the I (in-phase) and Q (quadrature phase) GPS signal samples  18  for an exemplary downconverted GPS signal. The GPS signal samples  18  have I and Q components in a lower frequency representation of the GPS signals  19  received by the GPS antenna  24 . Conceptually, the I and Q GPS signal is captured by a sampling signal at periodic sample times. In an embodiment, the I and Q GPS signal is integrated for time periods centered at the sample times. The levels of the I and Q GPS signal at the sample times are Iss and Qss, respectively. 
     The Iss and Qss levels are compared to a threshold. The I GPS signal sample Iss takes a “1”, or the equivalent, when the Iss level is greater than the threshold and takes a “0”, or the equivalent, when the Iss level is less than the threshold. The same for the Q GPS signal sample Qss. These signal samples  18  are not the GPS data bits that carry GPS data of Z-count, satellite health, almanac and ephemeris parameters, and the like but are instead sampled versions the GPS signals  19  that were received by the GPS antenna  24 . 
     The GPS signal can be sampled in several ways. Some GPS receivers use only one bit to represent the I samples and one bit to represent the Q samples (the so-called “sign bits”). Other GPS receivers use 2 bits each to represent the I and Q samples respectively (using both a “sign bit” and a “magnitude bit”). For two bit sampling, three thresholds are used and the I and Q GPS signal samples Iss and Qss take the values “11”, “10”, “01 and “00”. 
     While two bit I and Q samples can be stored in many ways, for non-real-time software-based processing the samples would preferably be arranged in blocks of I sign samples, I magnitude samples, Q sign samples and Q magnitude samples interleaving each other, with the block size adapted to the processors used throughout the entire processing system. Thus, for modern day 32 bit architectures the samples would be interleaved such that there would be 32 consecutive I sign samples, followed by 32 consecutive I magnitude samples belonging to the same time interval, in turn followed by similarly arranged 32 samples of the Q sign and Q magnitude. These blocks would in turn be aligned with the (block size divided by 8) byte-address boundaries for convenient access by the processors involved, thereby avoiding unnecessary bit or byte shifting operations in the subsequent processing steps. 
     In the case of one bit sampling, there would be only sign blocks in the data; the magnitude blocks would not be present. For processing chains with weaker 16 bit processors the block size can be scaled back to be 16 or even 8 bit, as appropriate. Similarly, 64 bit blocks, 128 bit and so on blocks could be realized as appropriate on very modern or future processors. 
     The data file formatter  31  ( FIG. 2 ) for the digital camera device  12 A can store the GPS data within a picture image file for formats and applications that allow for such storage of meta-data. In such cases the image file format would typically allow for the inclusion of arbitrary binary data for this purpose in a section of the file, and the I and Q data would be injected into this section in accordance with the file format specification. The internal arrangement of the I&amp;Q data could be as described previously. 
     In cases where it is for any reason not convenient to store the I and Q samples inside the picture file itself, it can be stored in a separate file. In this case there can be an explicit or implicit association between the image and the I&amp;Q sample files. This can be done by filename convention (e.g. the image file img001.jpg being implicitly associated with the I&amp;Q sample file gps001.iq in the same directory or a suitable subdirectory) or by including a reference inside the image file to the I&amp;Q sample file where the file format and application allow for such storage of meta-data. 
       FIG. 5  is a block diagram of the GPS locator  50 . The GPS locator  50  includes the GPS matrix resolver  60 , the satellite velocity calculator  62  and the GPS time calculator  64 ; and also includes a hot start memory  66 , a satellite location calculator  68 , a unit vector calculator  72 , a range calculator  74 , and an event time calibrator  76 . The hot start memory  66  retains ephemeris parameters EP for the GPS satellites, an assumed location vector E* for an estimate of the position of the device  12 , 12 A when the GPS signals  19  were received, and an estimate of the receiver time error (RTE*). The asterisk mark “*” is used to represent an estimate that is later refined and improved by arithmetic processing in the GPS locator  50 . 
     The assumed location E* and the RTE* may be based on the resolution of location and RTE of a previous event or picture. When there is a sequence of events or pictures in the file  14 , 14 A the assumed location E* and the RTE* of an event or picture may be estimated by extrapolation either forward in time or backward in time or by interpolation from the resolved locations and RTE*s of the other events or pictures. 
     The event time calibrator  76  receives the GNSS signal event time tags from the data file medium  14 , 14 A for providing time tags tt*. As an option, the calibrator  76  uses the receiver time error resolved by the GPS matrix resolver  60  for a previous (or a later) event or picture stored in the memory  66  for calibrating a new GNSS signal event time tag for offsetting the time tag tt*. The time error calibrator  76  calibration by the last receiver time error may be used to speed the resolution convergence for the GPS matrix resolver  60  and/or to offset the time uncertainty window of the receiver time error. The GPS matrix resolver  60  expects the time tag tt* to be within ±6 hours of GPS clock time when the event occurred when no estimate of position is available and ±12 hours when position is estimated with ±90° of longitude. Even greater receiver time errors are allowable when better estimates of position are available. 
     The satellite location calculator  68  uses the time tag tt* with the ephemeris parameters EP for providing estimated location-in-space vectors L* for several GPS satellites. The satellite velocity calculator  62  uses the time tag tt* with the ephemeris parameters EP for providing estimated velocity vectors V* for the same GPS satellites. In some cases the less accurate GPS satellite almanac parameters may be used in place of the ephemeris parameters for calculating location-in-space vectors L* and/or satellite velocity vectors V*. 
     The unit vector calculator  72  uses the estimated location-in-space vectors L* and the assumed event location vector E* for providing estimated unit vectors U* (also called directional cosines) for the directions between the device  12 , 12 A and the same GPS satellites. The range calculator  74  uses the estimated location-in-space vectors L* and the assumed event location vector E* for providing estimated ranges R* between the device  12 , 12 A and same GPS satellites. It should be noted that several of the elements of the GPS locator  50  may be implemented in software stored on a medium that is read by the microprocessor in the computer  16 . 
     The GPS matrix resolver  60  uses the code phase offsets and DBE integers for five GPS satellites, denoted SV 1  through SV 5 , the estimated ranges R*, and the speed of light C for providing linearized pseudoranges between the device  12 , 12 A and the GPS satellites about the assumed event location vector E*. The linearized pseudoranges are used with the estimated unit vectors U* and the estimated velocity vectors V* for computing a position offset vector ΔE, the time receiver time error, and the replica clocking offset. The position offset vector ΔE is the difference between the position computed from the linearized pseudoranges and the estimated location vector E*. 
     This resolution process may need to be iterated several times by substituting improved assumptions and estimates. The GPS time calculator  64  calculates the GNSS-based clock time of the event from the resolution of time-of-transmission of the GPS signal received at the device  12 , 12 A that is resolved by the GPS matrix resolver  60 . The receiver time error is resolved through the process described herein but is not necessarily issued as an explicit result. 
     The GPS matrix resolver  60  may use an altitude Z from a positioning source other than a GNSS positioning satellite in order to reduce the required number of positioning satellites from five to four for resolving the location of the event or with five GNSS positioning satellites for overdetermining the location. The altitude Z may be carried in the date file  14 , 14 A or entered at the computer  16 . In some circumstances the altitude Z may be assumed to be sealevel. 
       FIG. 6  is a flow chart of a method for using an event data file medium at an arbitrarily later time or date in an arbitrarily different geographical location for determining the location and time for an event (or picture) from GPS signal samples taken at the location and time of the event. The steps of the method may be embedded and stored as instructions on a computer-readable medium  100  that may be read by a computer for carrying out the steps. 
     A digital camera device or other event capture device takes a picture or otherwise captures an event in a step  102 . In a step  104  the device receives GPS signals at the time and place of the event or picture. In a step  106  the device samples the GPS signals. In a step  108  the GPS signal samples are time tagged for the time of reception of the GPS signals. In a step  112 , the device writes the event or picture with the GPS signal samples and time tags into an event data file medium. The time tags may have errors up to at least ±6 hours for GPS C/A code. Altitude may also be written into the medium. Then, time passes of seconds, minutes, hours, days, weeks, months, or even years. 
     An arbitrary time later the event or picture data file medium is read in a step  122  by a computer apparatus. In a step  124  the computer estimates velocities of the GPS satellites having GPS signals represented in the GPS signal samples. In a step  126  the GPS signal samples are measured by correlating and timing the samples against GPS replicas for providing correlation levels and time or phase offsets. The correlation levels are used to determine code phase offsets for the GPS replicas for signal power for the GPS signals and to determine data bit timing. The code phase offsets are determined relative to a replica clocking offset. In a step  128  the code phase offsets, data bit timing and satellite velocity estimates for typically five GPS satellites are used with matrix arithmetic for a mutual resolution of the location of the event or picture, the replica clocking offset and the receiver time error. The use of the estimated satellite velocities and the signal samples  18  for the extra GPS satellite effectively compensates for large receiver time errors. The GPS-based clock time may be computed in a step  132  from the time tag, the replica clocking error and the receiver time error. 
       FIG. 7  is a timing chart, not drawn to scale, showing an exemplary receiver time error that is local to the device  12 , 12 A and an exemplary replica clocking offset that is local to the GPS sample processor  40 . The receiver time error is the time difference between the event time tag and the correct GPS clock time (unknown in the system  10  until the unknowns are resolved). The acceptable range of uncertainty of the event time tag with respect to the GPS clock time is the receiver time uncertainty window. The replica clocking offset is the time difference between local replica code time ticks in the GPS signal processor  40  and the correct GPS code time ticks (unknown in the system  10  until the unknowns are resolved). The code phase offset, measured in time, is the difference between the time of the GPS code carried in the GPS signal samples  18  and the time of the local replica code generated by the GPS sample processor  40  where signal power of the GPS signals  19  is acquired and tracked. The DBE integer times are added to the time of the code phase offset for providing pseudoranges. For the GPS C/A code, the replica clocking offset and the code phase offset including the DBE integer time are typically less than twenty milliseconds. The receiver time uncertainty window may be up to at least several hours. 
     Acceptable Range of Receiver Time Error 
     The acceptable range the receiver time error (window of uncertainty of the event time tags with respect to GPS clock time) results from the orbital repetitions of the satellites, and the accuracy of an estimate of the position of the device  12 , 12 A at the start of the resolution process. In a GPS C/A code positioning system, with no estimated position, the system  10  can resolve a correct position with a receiver time error from zero up to about six hours (plus or minus). This twelve hour window results from the approximately twelve hour orbits of the GPS satellites causing the locations-in-space of a constellation to repeat at about twelve hour intervals. At about twelve hour intervals a different position solution having a high integrity will be resolved from this constellation. Therefore, an accuracy of the receiver time error that places time within a single twelve hour window will limit the resolution process to the correct position. When an estimated position is accurate to about 90° (plus or minus) longitude, the GPS orbital characteristics allow a receiver time error of about twelve hours (plus or minus), or a window of about one day, for resolution of the correct position. 
     In a Galileo positioning system, with no starting estimated position, the orbital characteristics of the satellites enable the system  10  to resolve the correct position with a receiver time error from zero up to about seven hours (plus or minus), or within a fourteen hour window. When an estimated position is accurate to about 74.1° (plus or minus) longitude, the Galileo orbital characteristics allow a receiver time error from zero up to about fourteen hours (plus or minus). An estimated position accuracy of about 31.7° (plus or minus) longitude allows a receiver time error from zero up to about 35 hours (plus or minus). An estimated position accuracy of about 10.5° (plus or minus) allows a receiver time error from zero up to about five days (plus or minus). 
     Normally, the uncertainty range or window of the receiver time error is centered so that the receiver time error can be only as great as one-half the total window. However, computer  16  can provide a time to offset the center of the range. For example, a time offset of six hours would provide for a receiver time error of zero to twelve hours. This offset can be calculated in the computer  16  by extrapolating from receiver time errors for other events or pictures in the medium  14 , 14 A. 
     Many data bits, such as almanac orbital parameters, ephemeris orbital parameters, satellite health and framing bits, repeat in successive frames of a GPS signal. The system  10  can resolve the position of the device  12 , 12 A for an event time tag having a window of time uncertainty that is greater than the time span of the frame. A frame for GPS or Galileo has a sequence of repeating bits. Because of the repeating bits, it would be difficult for a GNSS receiver to determine time by comparing and aligning the incoming bits and the stored expected bits unless the time uncertainty window is less than a frame. The system  10  uses a different technique involving satellite velocities and signal measurements for an extra satellite for resolving time without the need for comparing the incoming and stored data bit sequences. For the GPS C/A code, the window of uncertainty of the event time tag for the system  10  is allowed to be greater than the thirty second frame of the GPS signal for resolution of the position of the event. 
     In GPS and Galileo, time stamps are encode in the data bits and inserted in the navigation message at regular intervals by the broadcasting satellite. In Galileo the time stamps identify GST in multiples of the shortest page period of one second. The exact timing of the message frame boundaries will be used to identify fractional GST timing (less than one frame period). This will be measured relative to the leading edge of the first chip of the first code sequence of the first frame symbol. The system  10  does not require decoding of this GST time stamp. 
     Matrix Arithmetic for Mutual Resolution of Unknowns for Position, Replica Clocking Offset and Receiver Time Error 
     The discussion below describes the mutual resolution of the receiver time error RTE local to the device  12 , 12 A, the replica clocking offset, denoted as B, local to the computer apparatus  16 , and the location of the event captured by the device  12 , 12 A. Equation 1 shows that a measured pseudorange PR m  corresponding to a GPS satellite equals the true range R between the event and the satellite plus an unknown variable B for the replica clocking offset that is local to the correlation machine  48 .
 
 PR   m   =R+B   (1
 
     The measured pseudorange PR m  is the measured code phase offset in time plus the time for the data bit edge (DBE) integer multiplied by the speed of light. The replica clocking offset B is expressed as a distance by multiplying the time offset of the replica clocking offset by the speed of light. 
     Equations 2 and 2A show the location-in-space L of the satellite as a three dimensional function x(T),y(T),z(T) using the satellite orbital ephemeris parameters for computing the location-in-space L from a GNSS-based clock time-of-transmission T of the GPS signal.
 
 L=x ( T ), y ( T ), z ( T )  (2
 
 T=t−R/C   (2A
 
     The time-of-transmission T is equal to a time-of-arrival t of the GPS signal minus a transit time R/C where C is the speed of light. The above-described receiver time error RTE is the error the time-of-arrival (signal reception time) t. The location-in-space coordinates computed by x(T),y(T),z(T) are the ECEF coordinates of the GPS satellite. 
     Because the transit time R/C is known to be 75±10 milliseconds (mean of 75 ms plus and minus the variation of 10 ms of the transit time from the satellite to Earth for the satellite between the horizon and overhead), the transit time R/C can be replaced with a nominal value of 75 milliseconds for GPS estimation purposes. This estimation results in a few meters error in the resolution of the variables. If desired, after a solution is found using T and the range R is known to a few thousand meters, the solution can be iterated one more time using time-of-arrival t in place of time-of-transmission T for a more accurate answer. The use of arrival time t place of transmission time T and the additional iteration is unnecessary until the last iteration and is unnecessary if ten meters accuracy is sufficient. 
     Equation 3 shows the relationship of the range R as a function of the time-of-transmission T to the location-in-space x(T),y(T),z(T) of the satellite and an estimate of the coordinates X,Y,Z for the geographical location of the device  12 , 12 A at the event with the approximation that time T equals the reception time t minus 75 milliseconds. The device coordinates X,Y,Z may in some cases be derived from the resolution of the X,Y,Z of a previous event.
 
 R ( T ) 2 =( X−x ( T )) 2 +( Y−y ( T )) 2 +( Z−z ( T )) 2   (3
 
     Equation 4 combines equations 1 and 3.
 
 PR   m =[( X−x ( T )) 2 +( Y−y ( T )) 2 +( Z−z ( T )) 2 ] 1/2   +B   (4
 
     It might be noted that the discussions above listed the five unknowns that required resolution as the three dimensions of location of the device  12 , 12 A, the replica clocking error, and the receiver time error. However, the current discussion lists the five unknown variables that must be resolved as the device event location coordinates (X,Y,Z), the replica clocking offset B (expressed in units of length) and the GPS signal transmission time T. This can be understood by observing that the receiver time error RTE is resolved when the time-of-transmission T is resolved according to the relationship that the RTE is the difference between the time tag of the device  12 , 12 A and the time T (allowing for the transit time R/C). Therefore, the RTE is resolved when the time T is resolved. The elements X,Y,Z,B,T are unknowns that are mutually resolved as described below. The unknowns are said to be mutually resolved because no one unknown is resolved before all the unknowns are resolved. The resolution of a GPS time T provides the resolution of the receiver time error RTE as the difference between a GPS time T and the time tag of the GPS signal samples. 
     Equation 5 shows the satellite velocity vector V as a three dimensional function x′(T),y′(T),z′(T) using the satellite orbital ephemeris parameters for computing the velocity of the GPS satellite from the time-of-transmission T of the GPS signal. The functions x′(T),y′(T),z′(T) are the scalar speeds of the satellite in the three dimensions x, y and z.
 
 V =( x ′( T ), y ′( T ), z ′( T ))  (5
 
     When the device  12 , 12 A is expected to have a high velocity, for example when the device  12 , 12 A is being used in an automobile or an airplane, the three dimensional satellite velocity function x′(T),y′(T),z′(T) should be the difference between the satellite velocity function based on the satellite ephemeris parameters and an expected or estimated velocity of the device  12 , 12 A in the same three dimensions. The 3D device velocity may in some cases be derived from the resolution of the X,Y,Z of previous events. 
     Equation 6 shows the satellite unit vector U as a function of the time-of-transmission T for the direction from the event to the satellite in three dimensions x, y and z as functions of the time T. The unit vector U is computed from the assumed or estimated coordinates X,Y,Z of the event and the location-in-space L of the satellite.
 
 U ( T )=( u   x ( T ), u   y ( T ), u   z ( T ))=( X−x ( T ))/ R ( T ),( Y−y ( T ))/ R ( T ),( Z−z ( T ))/ R ( T ))  (6
 
     Equation 7 shows a satellite range rate rr as a function of the time-of-transmission T in terms of the unit vector U and the satellite velocity vector V for the time T.
 
 rr ( T )=( u   x ( T ) u   y ( T ), u   2 ( T ))·( x ′( T ), y ′( T ), z ′( T ))= U ( T )· V ( T )  (7
 
     Equation 8 shows a calculated pseudorange PR c  as a function of the time-of-transmission T in terms of a calculated range R c  as a function of time T as shown in the equation 4 between an assumed or estimated location X,Y,Z of the event and the satellite location-in-space L.
 
 PR   c ( T )= R   c ( T )+ B   (8
 
     Equations 9 and 9A show a satellite row vector RV derived from the partial derivative of the calculated pseudorange PR c  as a function of (X,Y,Z,B,T) with respect to the time T. The satellite row vector RV has five components: the unit vector U having three components u x ,u y ,u z , one component for a constant G, and one component for the range rate rr. The constant G is the sensitivity of the pseudorange to the replica clocking offset B. It is preferred that both the pseudorange and the offset B are expressed in meters. When meters are used the constant G is one.
 
∂ PR   c /∂( X,Y,Z,B,T )= RV =( u   x   ,u   y   ,u   z   ,G,−rr )  (9
 
 RV =( u   x   ,u   y   ,u   z   ,G ,−(( u   x   ,u   y   ,u   z )·( x′,y′,z ′))  (9 A  
 
     Equation 10 is a satellite velocity positioning matrix [H]. The satellite velocity positioning matrix [H] has K row vectors RV for K satellites, respectively denoted as SV1, SV2, SV3, SV4, SV5 and five columns for the five components of the row vectors RV. 
     
       
         
           
             
               
                 
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     Finally, equation 10A shows that the difference between the measured pseudorange PR m  and the calculated pseudorange PR c  is a pseudorange residual ΔPR.
 
Δ PR=PR   m   −PR   c   (10 A  
 
       FIG. 8  is a flow chart of the steps for a mutual resolution of the five unknown variables X,Y,Z,B,T by the GPS matrix resolver  60 . The variables X,Y,Z are the GPS-based position of the event, the variable B is the replica clocking offset and the variable T is the time-of-transmission of the GPS signal. The receiver time error RTE is resolved by resolving the time T. The steps of the method may be provided in a tangible medium  200  having a code in a computer-readable form that may be read by a computer for carrying out the steps. 
     Step  202 , measure the pseudorange PR m  for a satellite by measuring the code phase offset and DBE integer. 
     Step  203 , estimate an initial position (X 0 , Y 0 , Z 0 ). The initial position (X 0 , Y 0 , Z 0 ) may be estimated from the positions that have been resolved for previous events or pictures, or based on extrapolation or interpolation from the positions resolved for other events or pictures, or entered by a user of the device  12 , 12 A, or entered by a user of the computer  16 . 
     Step  204 , propose an initial (0th) mutual resolution (X 0 , Y 0 , Z 0 , B 0 , T 0 ). Conventional methods for anywhere fix can be used when no estimated position E* is known for initial position (X 0 , Y 0 , Z 0 ). The time T 0  must be known to within an acceptable range of the estimated time tt* based on the time tag for the GPS signal samples  18  and the unknown receiver time error. 
     Step  205 , compute a satellite (SV) location-in-space vector L (x 0 ,y 0 ,z 0 ) from the satellite orbital ephemeris parameters at the time T 0 . 
     Step  206 , compute a satellite velocity vector V (x′ 0 ,y′ 0 ,z′ 0 ) from the satellite orbital ephemeris parameters at the time T 0 . 
     Step  208 , compute the range R at the time T 0  for the device position (X 0 ,Y 0 ,Z 0 ) and the satellite location (x 0 ,y 0 ,z 0 ) as shown in an equation 11.
 
 R   0 =[( X   0   −x   0 ) 2 +( Y   0   −y   0 ) 2 +( Z   0   −z   0 ) 2 ] 1/2   (11
 
     Step  212 , compute a unit vector U (u x0 ,u y0 ,u z0 ) at the time T 0 , for the direction between the event and the satellite as shown in an equation 12.
 
( u   x0   ,u   y0   ,u   z0 )=(( X   0   −x   0 )/ R   0 ,( Y   0   −y   0 )/ R   0 ,( Z   0   −z   0 )/ R   0 )  (12
 
Step  214 , compute a rate of change of the range (range rate) rr at the time T 0  between the event and the satellite as the dot product of the satellite velocity vector (x′ 0 ,y′ 0 ,z′ 0 ) at T 0  and the unit direction vector (u x0 ,u y0 ,u z0 ) at T 0  as shown in an equation 13.
 
 rr   0 =( u   x0   ,u   y0   ,u   z0 )·( x′   0   ,y′   0   ,z′   0 )  (13
 
     Step  216 , calculate a pseudorange PR c  at the time T 0  for the range R 0  and the replica clocking offset B 0  according to an equation 14.
 
 PR   c0   =R   0   +B   0   (14
 
     Step  218 , calculate a satellite row vector RV at the time T 0  according to an equation 15. The satellite row vector RV has five components including the three components u x0 , u y0 , u z0  for the unit vector U 0 , one component for the constant G, and one component for the range rate rr 0 .
 
 RV   0 =( u   x0   ,u   y0   ,u   z0   ,G,−rr   0 )  (15
 
     An equation 16 shows the row vector RV 0  for a kth satellite where the term for the range rate rr 0  is expanded to show that it is a dot product of the unit vector U 0  and the velocity V O .
 
 RV   0   =u   x0   SVk   ,u   y0   SVk   ,u   z0   SVk   ,G ,−( u   x0   SVk   ,u   y0   SVk   ,u   z0   SVk )·( x′   0   SVk   ,y′   0   SVk   ,z′   0   SVk )  (16
 
     Step  222 , repeat the above steps  202  to  218  for K−1 additional satellites (for a total of K satellites) where K is five or more so that K satellite row vectors RV 0  of the equation 16 may be arranged as a satellite velocity positioning matrix [H]. The satellite velocity positioning matrix [H] is arranged as K row vectors corresponding to the K satellites by five column vectors corresponding to the five coefficients of the row vectors RV for the satellites. 
     
       
         
           
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     Step  224 , determine the satellite velocity positioning matrix [H] at time T 0 . The satellite velocity positioning matrix [H] at time T 0  is denoted as [H 0 ]. 
     Step  226  perform matrix arithmetic on the matrix [H 0 ] to determine a solution matrix [S 0 ] according to an equation 17.
 
[ S   0   ]=[[[H   0 ] t   ×[H   0 ]] 1   ×[H   0 ] t ]  (17
 
     In the equation 17, the matrix [H 0 ] t  is the transpose of [H 0 ] t  the matrix [[H 0 ] t ×[H 0 ]] −1  is the matrix inversion of the product of the satellite velocity positioning matrix [H 0 ] and its transpose matrix [H 0 ] t . The solution matrix [[[H 0 ] t ×[H 0 ]] −1 ×[H 0 ] t ] has dimensions of five rows by K columns. 
     The product matrix [[H] t ×[H]] −1  contains the Dilution of Precision (DOP) values. This matrix can be used to assess the accuracy and stability of the process. If the scaled trace of the matrix [[H] t ×[H]] −1  exceeds a certain value, the iterative estimation process can become unstable. The procedure in such a case is to choose a different constellation of satellites for consideration or use more satellites (increase K). Another method is to just examine the square root of the sum of the first four elements of the diagonal of [[H] t ×[H]] −1 . This corresponds to the classical GDOP of GPS; if it is less than a preselected number such as ten, the satellite geometry is such that a least squares solution is reasonably precise. 
     Step  228 , take K pseudorange residuals between the measured pseudoranges PRm and the calculated pseudoranges PR c0  at the time T 0  on a satellite-by-satellite basis as shown in an equation 18.
 
Δ PR   0   =PR   m   −PR   c0   (18
 
     The K pseudorange residuals (PR m −PR c0 ) form a column vector ΔPR 0  of length K. 
     Step  232 , perform matrix arithmetic on the column vector for the initial solution (X 0 , Y 0 , Z 0 , B 0 , T 0 ), the solution matrix [S 0 ] and the pseudorange residual column vector ΔPR 0  for the K (K≧5) satellites for generating a new, next better, solution (X 1 , Y 1 , Z 1 , B 1 , T 1 ) according to an equation 19.
 
[ X   1   ,Y   1   ,Z   1   ,B   1   ,T   1   ]=[X   0   ,Y   0   ,Z   0   ,B   0   ,T   0   ]+[S   0   ]×[ΔPR   0 ]  (19
 
     In the equation 19, a column vector (X 1 ,Y 1 ,Z 1 ,B 1 ,T 1 ) for the new solution is the sum of the column vector (X 0 ,Y 0 ,Z 0 ,B 0 ,T 0 ) for the old solution and the column vector for the matrix product of the solution matrix [S 0 ] times the pseudorange residual column vector ΔPR 0 . 
     Step  234 , compute a convergence distance ΔCD 1  according to an equation 20 for the difference between the new position (X 1 ,Y 1 ,Z 1 ) and the old position (X 0 ,Y 0 ,Z 0 )
 
 ΔCD   1 =(( X   1   −X   0 ) 2 +( Y   1   −Y   0 ) 2 +( Z   1   −Z   0 ) 2 ) 1/2   (20
 
     Step  236 , compare the convergence difference ΔCD 1  to a selected convergence difference threshold. A reasonable threshold is in the range of a one to one-hundred meters. The convergence threshold comparison may be performed for a square of the convergence difference ΔCD 1   2  against a square of the threshold, thereby avoiding the arithmetic operation of taking a square root of the sum of the squares for (X 1 −X 0 ) 2 +(Y 1 −Y 0 ) 2 +(Z 1 −Z 0 ) 2 . 
     When the convergence distance ΔCD 1  is less than the selected threshold for convergence, the solution (X 1 ,Y 1 ,Z 1 ,B 1 ,T 1 ) is the successful mutual resolution of the event location (X,Y,Z), the replica clocking offset B, and the receiver time error RTE (included within the resolution of the time-of-transmission T). The receiver time error RTE is the difference between the time tag of the event from the device  12 , 12 A and the actual time-of arrival t (equation 2A) in GPS clock time of the GPS signal for the event. It should be noted that the solution for (X,Y,Z,B,T) resolves the receiver time error RTE without a requirement for issuing an explicit number for receiver time error RTE. However, the resolution of the receiver time error RTE may be used for reducing the error in the time tt* in order to reduce the time or number of iterations for resolution convergence. 
     Step  238 , when the convergence distance ΔCD 1  is greater than the selected threshold for convergence, the old proposed solution (X 0 ,Y 0 ,Z 0 ,B 0 ,T 0 ) is replaced with the new, next better solution (X 1 ,Y 1 ,Z 1 ,B 1 ,T 1 ) and the above steps  204  to  236  are repeated to compute a new, next better solution (X 2 ,Y 2 ,Z 2 ,B 2 ,T 2 ) for a new, next better position (X 2 ,Y 2 ,Z 2 ,) and so on for n times until a convergence distance ΔCD n  is greater than the selected threshold for convergence. 
     The convergence process is repeated until a solution (X n ,Y n ,Z n ,B n ,T n ) is obtained where the convergence distance ΔCD n  between the nth position (X n ,Y n ,Z n ) and the (n−1)th position (X n-1 ,Y n-1 ,Z n-1 ) is less than the convergence threshold, denoted by C Th , according to equations 21 and 22.
 
[ X   n   ,Y   n   ,B   n   ,T   n   ]=[X   n-1   ,Y   n-1   ,Z   n-1   ,B   n-1   ,T   n-1   ]+[S   n-1   ]×[ΔPR   n-1 ]  (21
 
Δ CD   n =(( X   n   −X   n-1 ) 2 +( Y   n   −Y   n-1 ) 2 +( Z   n   −Z   n-1 ) 2 ) 1/2   &lt;C   Th   (22
 
     When the convergence is acceptable the solution (X n ,Y n ,Z n ,B n ,T n ) is resolved. 
     In general, greater receiver time errors will require more resolution iterations and a longer time to converge on the solution of the unknown variables for the location of the event. A calculation of the square of the sum of the squares of the pseudorange residuals (SSR) may be used as an integrity acceptance rejection criteria. A high integrity is indicated by a low SSR. If time is not well known, a first pass of iterations may use a coarse mesh of time estimates, for example every half hour, then take the most nearly successful solution as judged by the lowest SSR, and then use that time as a iterating within a fine mesh of time estimates. The SSR may use weightings so that pseudoranges corresponding to GPS signals  19  having higher signal strength are weighted more heavily. 
     Step  240 , determine a sum of the squares of the pseudorange residuals (SSR) for determining the integrity of the position according to an equation 23. The solution may have converged to a solution but that solution may not be reliable. A high SSR indicates that the solution should not be relied upon. 
     
       
         
           
             
               
                 
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     When the SSR is acceptable, the solution (X n ,Y n ,Z n ,B n ,T n ) is considered to have integrity. 
     Step  242 , calculate GNSS-based time for the event or picture from the GNSS-based time-of-transmission T that was resolved, the transit time that can be calculated using the position that was resolved, and replica clocking offset B. 
     Step  244 , calculate receiver time error RTE from the time tag for the GPS signal samples, the time T and the transit time. This calculated receiver time error for the current set of GNSS signal samples may be used to offset or extend the acceptable range for the error in the time tags for other sets of GPS signal samples. 
     Although the present invention has been described in terms of the present embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various supersets, subsets and equivalents will no doubt become apparent to those skilled in the art after having read the above disclosure. However, these supersets, subsets and equivalents should not be regarded as limiting the idea of the invention. Accordingly, it is intended that the claims, written below be interpreted as covering the present invention&#39;s true spirit and scope.