EP1949126A2 - Procede et appareil permettant de reconstruire la duree de transmission a partir d'observations de gps assiste ou a faible signal - Google Patents

Procede et appareil permettant de reconstruire la duree de transmission a partir d'observations de gps assiste ou a faible signal

Info

Publication number
EP1949126A2
EP1949126A2 EP06788960A EP06788960A EP1949126A2 EP 1949126 A2 EP1949126 A2 EP 1949126A2 EP 06788960 A EP06788960 A EP 06788960A EP 06788960 A EP06788960 A EP 06788960A EP 1949126 A2 EP1949126 A2 EP 1949126A2
Authority
EP
European Patent Office
Prior art keywords
tot
solution
gps device
satellite
error
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP06788960A
Other languages
German (de)
English (en)
Other versions
EP1949126A4 (fr
Inventor
Eamonn Glennon
Roderick Bryant
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
SigNav Pty Ltd
Original Assignee
SigNav Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by SigNav Pty Ltd filed Critical SigNav Pty Ltd
Priority claimed from US11/460,784 external-priority patent/US20070024500A1/en
Publication of EP1949126A2 publication Critical patent/EP1949126A2/fr
Publication of EP1949126A4 publication Critical patent/EP1949126A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
    • G01S19/256—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS relating to timing, e.g. time of week, code phase, timing offset

Definitions

  • This invention relates to Global Positioning Satellite systems where the time of transmit of satellite signals is not available from the navigation message data. In particular it concerns the reconstruction of time-of-transmit from the course acquisition code of weak signals or where assisted-GPS is employed.
  • Assisted GPS (AGPS) navigation solutions differ from normal GPS navigation solutions due to the use of ambiguous code-phases rather than full time-of-transmits for each GPS satellite observation. As such, it is necessary to reconstruct the full time-of- transmit using a-priori knowledge such as a position estimate within 150 km of the true position and an estimate of the time-of-receipt. Errors arise if the initial time-of-receipt used to construct the time-of-transmit observations is in error.
  • Enhanced-E911 requirement for mobile cellular communications and the subsequent use of GPS in order to fulfill this requirement has necessitated the development of new methods to perform GPS navigation solutions.
  • weak-signal or AGPS is not able to extract all the information from the GPS signal due to extremely weak signal to noise ratios.
  • a weak-signal or AGPS satellite observation generally consists of a 1-ms ambiguous code phase and a measured Doppler frequency compared to a standard GPS observation which consists of a full time-of-transmit (TOT) and a measured Doppler frequency.
  • AGPS is still able to perform a navigation solution through the use of prior information, including a rough estimate of the position of the receiver and a time tag for the time-of-receipt (TOR).
  • the rough estimate of position of the receiver is generally based on the location of the cell-site, although it could be based on the use of previous estimates or calculated using a Doppler based solution.
  • These parameters can then be used to estimate ranges to each satellite which together with the initial TOR estimate can be used to estimate a full TOT for each satellite.
  • the initial TOR typically contains errors, the reconstructed TOTs will all be subject to a common timing error thereby resulting in navigation position errors in the solution process.
  • the invention is embodied in a GPS device having a processor that runs an algorithm for determining TOT, the time of transmit of a satellite signal. Inaccuracies in the receiver clock are the source of errors in the TOT. After adjusting the TOT by the phase shift between a locally generated C/A code and the satellite gold code, a calculation in a single stage is performed in which both the receiver clock error ⁇ t and the TOT error ⁇ T. A measurement matrix is used whose row elements are the partial derivatives of the pseudorange with respect to the unknowns, in this case including an addition timing error term. The solution is obtained by applying the pseudoinverse of the measurement matrix to the difference between the predicted and measured pseudorange vector.
  • the Dilution of Precision is calculated based on the measurement matrix.
  • the DOP provides a prediction of the accuracy of the solution.
  • Other embodiments take use the accurate time determination when time signals are required.
  • Figure 1 depicts the timing for a satellite signal.
  • Figure 2 depicts a satellite configuration for an example used to demonstrate the invention.
  • GPS observations used in AGPS Mobile Station Assisted (MS-Assisted) or MS- Based navigation solutions generally consist of a set of C/A code phases measured at a given time-instant as well as Doppler frequency measurements. Since the C/A code has a repetition frequency of 1 kHz, this means that the C/A code phases indicate the TOT modulo 1 ms and for a navigation solution to be performed, it is necessary to reconstruct the full TOT for each satellite.
  • This process generally starts with an estimate of the TOR 1 the time instant at which the observation was made, which is generally obtained using assistance from the cell phone or real time clock (RTC). Subtracting an estimate for the satellite signal's time-of-flight (TOF) from the TOR then provides an estimate for each satellite TOT.
  • TTC real time clock
  • the time of flight is essentially the phase shift in the C/A signal modulo the length of a C/A code epoch of 1 ms.
  • Each TOT can then have it's sub- millisecond portion replaced by the measured code phase and the resulting TOTs can then be corrected by a integer number of millisecond epochs such that the times are consistent with the specified coarse receiver location.
  • Boundary truncation or round-off problems can be avoided by adjusting the initial TOR such that after application of the measured code phases, at least one satellite has a millisecond adjustment that is exactly zero. This procedure produces a set of reconstructed TOTs that is consistent with the true location at which the measurement was made and that can be then used to perform a navigation solution.
  • a satellite 1 in an orbit 3 transmits a signal to a receiver 5 located on or near the surface of the earth.
  • a portion of the signal typically the start of an epoch, commences its transmission at a time TOT as measured by an accurate satellite clock, and commences its reception at a time TOR as measured by a less accurate receiver clock.
  • the signal arrives at the receiver it is detected by its correlation with a locally generated C/A code.
  • the phase difference between the locally generated code and the received code translates into a time difference modulo the length of a code epoch.
  • the epoch difference is constrained by knowledge of the approximate location 7 of the receiver to an accuracy of about 150 km, since the speed of light is approximately 300km/ms and the epoch length is 1 ms.
  • a problem with the above procedure is that any error in the initial TOR will result in biased TOTs. Where this causes a problem is that each TOT is used to estimate the position of the corresponding satellite in its orbit 3 which means that if the TOT is incorrect then the satellite position will be incorrect as well. Since each satellite range rate lies in the range of ⁇ 1000 m/s, a timing error in the TOT of as little as 0.01 seconds can result in pseudorange predictions that are in error by 10 m, while errors of 1 second can result in 1000 m pseudorange errors. These errors then result in biases in the calculated positions.
  • the problem can be solved if there is redundancy in the number of satellite observations used to perform the navigation solution.
  • four observations are used to solve for four different state variables, namely (x, y, z, cAt), where x, y and z are the WGS84 coordinates of the receiver, c is the speed of light and ⁇ t is the clock error in the TOR.
  • this clock error Ai in the TOR is different to the timing error ⁇ T that is present in the estimated TOTs when AGPS is carried out.
  • the dimension of the state vector is increased by addition of the timing error ⁇ Tand an additional observation used to solve for this additional unknown.
  • the present invention is best understood by comparison with the following two stage algorithm.
  • the pseudorange error for each satellite / is modeled as:
  • PR is the pseudorange for satellite /
  • R - is the range for satellite /
  • R 1 is the range-rate for satellite / '
  • ⁇ t is the TOR clock error
  • ⁇ T is the timing error.
  • R is calculated using the satellite position at the current TOT x ; or (x lt y,, Z 1 ) and the receiver position x or (x, y, z).
  • One method is to substitute the calculated ⁇ t value from the position solution thereby providing a set of five or more equations each with a single unknown, namely the ⁇ T quantity. Using these equations, five or more estimates ⁇ T, are obtained by re- writing (1) as:
  • the range-rate quantity can lie anyway in the range of -1000 m/s to 1000 m/s, including zero.
  • weighted means The effect of these weighted means is to give low weighting to estimates derived from range-rates close to zero and larger weights to values derived from higher range- rates. Estimates derived from values very near to zero were excluded from the solution completely.
  • An alternate method of performing the solution for AT is to recalculate both At and ⁇ T at each position solution step. This can be done by constructing a vector of Pf?;, R, and Jj 1 . as PR, R and R respectively and rewriting (1) as
  • PR -R [l R] C ⁇ 1 (6) where 1 denotes a vector of 1's. This can then be used to solve for c ⁇ t and AT using a standard pseudo-inverse. This method did not seem to be as susceptible to the problem of zero R 1 values as the previous technique, probably because of the presence of the c ⁇ t variable constraining the solution in some way.
  • DOP Dilution of Precision
  • DOP is a term that describes the geometric strength of a satellite configuration. When visible satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Factors that affect the DOP are, besides the satellite orbits, the presence of obstructions which make it impossible to use satellites in certain sectors of the local sky.
  • This reversal of signs requires that the solution process adjust the signs of the input quantities to compensate accordingly. It also makes sense to express the range-rate terms with units of km/s rather than m/s thereby ensuring that the magnitude of all matrix elements are similar. This results in the solved TOR error having units of milliseconds rather than seconds.
  • n is the number of satellites and the sign of ⁇ y has been adjusted to match the DOP friendly H matrix.
  • the correction is applied and the predicted range vector y updated before repeating the process.
  • the process is terminated when the magnitude of the calculated correction ⁇ x becomes sufficiently small. This typically takes less than 10 iterations and is significantly faster than the two-stage algorithm.
  • the algorithm just described may be improved in a number of ways. Firstly, it is often the case that two-dimensional fixes need to be performed during periods of reduced satellite availability, so support for this requirement may be included. One way in which this can be done is to add a 'fake' satellite in the center of the earth and then specify a required pseudorange for that satellite. The corresponding row of the H matrix is then constructed as the partial derivatives of the WGS84 reference ellipsoid evaluated at the estimated position, where the ellipsoid is described as
  • R * is the weighting matrix.
  • R is generally a diagonal matrix in which the noise- variance of each observation is the corresponding diagonal element in R, so observations with a low noise variance end up as large values in the inverted matrix R '1 .
  • Another feature of a preferred embodiment is to permit the use of differenced pseudoranges for solution of the (x, y, z, ⁇ T). This is implemented by modifying the H and R matrices once all the measured satellite observations have been inserted (but before adding the fake 2D observation). The modification involves selecting a reference satellite (usually the highest one) and then subtracting the reference satellite row from each of the other rows. Performing a differenced observation in this way eliminates the clock error c ⁇ t as a solution state. It is also possible to disable the solution of the timing error for situations in which exact TOTs or accurate time-aiding is available omitting the final range-rate column of H thereby resorting to the standard formulation. This is also useful for permitting calculation of the receiver velocity solution since the H matrix required for a velocity solution is the same as a standard position solution. Only the input and output vectors change for this situation, which in this case are pseudorange- rates and receiver velocity respectively.
  • Table 2 shows the resulting satellite positions and range-rates for the constellation at this time, while Figure 1 shows provides a graphical representation of the satellite visibility.
  • the GDOP can then be calculated as the square root of the trace of this matrix, giving a GDOP value of 7.17, which although higher than normal is not excessive. However, if the DOP is calculated using the H matrix as actually used in the solution process, a result with significantly larger magnitude is obtained.
  • the numerical values for the extended DOP are generally similar to the values obtained using the conventional approach. However, in cases where the extended DOP is high, it means that the timing error and navigation solution cannot be reliably calculated.
  • the technology is also applicable to the newly developing field of GPS weak signal timing receivers.
  • this involves the use of GPS for time-transfer, except that the receiver only has access to weak signals and obtains ephemeris, coarse location and coarse time from other sources such as the Internet or wireless links.
  • the main reason for requiring such receivers is either convenience, with many environments not being GPS antenna friendly or the requirement to provide timing in areas of heavy vegetation where GPS signals are obscured.
  • the first option is to formulate the solution in terms of an extended Kalman filter (P. Axelrad and R. G. Brown, "GPS Navigation Algorithms," in Global Positioning Systems: Theory and Applications Volume 1, B. W. Parkinson, J. J. Spilker, P. Axelrad, and P. Enge, Eds.: American Institute of Astronautics and Aeronautics, Inc, 1996.), rather than the single shot solution algorithm just described.
  • the procedure for doing this is straightforward and requires the inclusion of the TOR timing error /17 as an additional element in the Kalman filter state vector.
  • the state covariance must also increase to account for the additional dimension and the new H matrix used instead of the normal H matrix.
  • the Kalman filter can then be operated over a number of GPS measurements until the state- covariance associated with /47 has fallen significantly below one millisecond. Provided the Kalman filter state covariance represents a realistic estimate of the error it is then possible to correct the receiver TOR using the calculated estimate after which this TOR can be locked-in and a switch to a conventional solution process performed.
  • bit sync could be obtained by performing noncoherent correlations over many rounds (typically 50 to several hundred) of 20ms coherent correlations using 20 offsets at 1ms spacing and choosing the alignment that yields the highest correlation.
  • the 1ms ambiguity of the codephase is replaced by a 20ms ambiguity for the bits.
  • the invention can be used to determine time to better than 10ms, by utilizing the TOR-resolving position-Time Kalman filter. By combining this better time resolution with the 20 ms bit ambiguity one can completely resolve the ambiguity leading to precise time resolution using the codephase measurements with no ambiguity.
  • the validity of the time resolution can be tested by performing long coherent correlations across many bit periods after stripping data known in advance. This will yield a very high correlation if the bit ambiguity has been properly resolved. If not, this will yield a very low correlation and one could re-resolve the ambiguity.
  • the accurate time could be further utilized by outputting synchronization pulses at any desired repetition rate with sub-microsecond precision and stamping these with time via a communications port of some sort.
  • one could discipline the receiver's reference oscillator by estimating the frequency bias of the oscillator using the Doppler measurements and the estimated time and position and steering the local oscillator to the correct frequency.

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  • Engineering & Computer Science (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Position Fixing By Use Of Radio Waves (AREA)

Abstract

L'invention porte sur un dispositif GPS résolvant les erreurs de synchronisation qui peut être appliqué à une solution de navigation AGPS. Un algorithme à état unique fournit une convergence rapide, typique d'une performance Newton-Raphson standard. On identifie les problèmes liés à un mauvais conditionnement de la matrice d'entrée par une procédure de calcul DOP augmentée, qui peut servir à identifier les résultats non fiables ou à estimer les erreurs dues à ces résultats.
EP06788960A 2005-07-29 2006-07-28 Procede et appareil permettant de reconstruire la duree de transmission a partir d'observations de gps assiste ou a faible signal Withdrawn EP1949126A4 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US70363705P 2005-07-29 2005-07-29
PCT/US2006/029702 WO2007016474A2 (fr) 2005-07-29 2006-07-28 Procede et appareil permettant de reconstruire la duree de transmission a partir d'observations de gps assiste ou a faible signal
US11/460,784 US20070024500A1 (en) 2005-07-29 2006-07-28 Method and apparatus for reconstructing time of transmit from assisted or weak signal gps observations

Publications (2)

Publication Number Publication Date
EP1949126A2 true EP1949126A2 (fr) 2008-07-30
EP1949126A4 EP1949126A4 (fr) 2010-09-29

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EP (1) EP1949126A4 (fr)
CA (1) CA2617142A1 (fr)
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Publication number Priority date Publication date Assignee Title
US8190536B2 (en) 2008-09-10 2012-05-29 King Fahd University Of Petroleum & Minerals Method of performing parallel search optimization

Citations (2)

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Publication number Priority date Publication date Assignee Title
US6072428A (en) * 1998-06-03 2000-06-06 Trimble Navigation Limited Location determination using doppler and pseudorange measurements from fewer than four satellites
US6215442B1 (en) * 1997-02-03 2001-04-10 Snaptrack, Inc. Method and apparatus for determining time in a satellite positioning system

Family Cites Families (3)

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Publication number Priority date Publication date Assignee Title
US6411899B2 (en) * 1996-10-24 2002-06-25 Trimble Navigation Ltd. Position based personal digital assistant
US6633621B1 (en) * 2000-03-20 2003-10-14 Motorola, Inc. Apparatus and method for synchronizing a clock using a phase-locked loop circuit
US6658048B1 (en) * 2000-04-07 2003-12-02 Nokia Mobile Phones, Ltd. Global positioning system code phase detector with multipath compensation and method for reducing multipath components associated with a received signal

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6215442B1 (en) * 1997-02-03 2001-04-10 Snaptrack, Inc. Method and apparatus for determining time in a satellite positioning system
US6072428A (en) * 1998-06-03 2000-06-06 Trimble Navigation Limited Location determination using doppler and pseudorange measurements from fewer than four satellites

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of WO2007016474A2 *

Also Published As

Publication number Publication date
EP1949126A4 (fr) 2010-09-29
WO2007016474A3 (fr) 2007-06-07
WO2007016474A9 (fr) 2007-04-19
CA2617142A1 (fr) 2007-02-08
WO2007016474A2 (fr) 2007-02-08

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