US 20050021284 A1 Abstract Methods and apparatus for improved position determination of a device using multiple pseudo range measurements from transmitting sources at known locations, such as GPS satellites. A plurality of pseudo range measurements for each transmitting source are processed together to obtain a simplified maximum likelihood estimate for the pseudo range for that transmitting source at a common reference time. The processed pseudo range estimates for all transmitting sources are then combined using conventional position determination algorithms. This technique facilitates removal of raw measurement outliers prior to position determination, which results in improved (i.e., more accurate) position fixes of the device. In addition, improved measurement integrity monitoring of the pseudo range measurements is a feature of this invention.
Claims(31) 1-5. (Cancelled) 6. A method for determining a position of a device, comprising:
receiving a plurality of pseudo range measurements from a transmitting source; adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; dividing the pseudo range interval into a plurality of increments having a plurality of grid points; aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval; selecting a best window based on a maximal number of pseudo range measurements within the sliding pseudo range window; comparing the maximal number with an incidence threshold; and then based on the comparison, determining an average pseudo range value; wherein the sliding pseudo range window width is 300 meters. 7. A method for determining a position of a device, comprising:
receiving a plurality of pseudo range measurements from a transmitting source; adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; dividing the pseudo range interval into a plurality of increments having a plurality of end points; aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval; selecting a best window based on a maximal number of pseudo range measurements within the sliding pseudo range window; comparing the maximal number with an incidence threshold; and then based on the comparison, determining an average pseudo range value; wherein the value of each of the plurality of increments is 5 metes. 8. A method for determining a position of a device, comprising:
receiving a plurality of pseudo range measurements from a transmitting source; adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; dividing the pseudo range interval into a plurality of increments having a plurality of grid points; aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window and then repeating for each of the plurality of grid points within the pseudo range interval; selecting a best window based on a maximal number of pseudo range measurements within the sliding pseudo range window; comparing the maximal number with an incidence threshold; and then based on the comparison, determining an average pseudo range value; wherein the quantity of the plurality of pseudo range measurements equaling N, and wherein the incidence threshold is the larger of: a filtering threshold times N, or 4 times N divided by the sum of 3 and the sliding pseudo range window width in units of GPS chips. 9. The method of 10. The method of 11. The method of 12. The method of 13. The method of 14. The method of 15. The method of 16. The method of 17. The method of 18. The method of 19. The method of 20. The method of 21. A method for determining a position of a device, comprising:
receiving a plurality of pseudo range measurements from a transmitting source; dividing the pseudo range interval into a plurality of increments having a plurality of grid points; aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval; selecting a best window based on a maximal number of pseudo range measurements within the sliding pseudo range window; comparing the maximal number with an incidence threshold; and then based on the comparison determining an average pseudo range value; wherein each of the plurality of pseudo range measurements having an associated Doppler offset and further comprising the step of comparing the associated Doppler offset with a Doppler threshold. 22. (Cancelled) 23. A method for determining a position of a device, comprising:
receiving a plurality of pseudo range measurements from a transmitting source; dividing The pseudo range interval into a plurality of increments having a plurality of grid points; repeating the steps of claim 1 M−1 (i.e., M minus one) times for each of remaining M−1 transmitting sources to determine a plurality of M average pseudo range values. 24. The method of 25. A method for determining a position of a device, comprising:
receiving a plurality of pseudo range measurements from a transmitting source; dividing the pseudo range interval into a plurality of increments having a plurality of grid points; selecting a plurality of best windows based on at least one predetermined criterion and determining a plurality of average pseudo range values wherein each of the plurality of average pseudo range values corresponding to each of the plurality of best windows. 26. The method of 27. The method of 28. The method of 29. The method of 30. A device for determining position, comprising:
a receiver for receiving a plurality of pseudo range measurements from a transmitting source; a processor coupled to the receiver and configured to accept the plurality of pseudo range measurements for processing by:
i) adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints;
ii) dividing the pseudo range interval into a plurality of increments having a plurality of grid points;
iii) aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval;
iv) selecting a best window based on a maximal number of pseudo range measurements within the sliding pseudo range window and comparing the maximal number with an incidence threshold; and then based on the comparison, determining an average pseudo range value.
31. A device for determining a position, comprising:
a receiver for receiving a plurality of pseudo range measurements from a transmitting source; a processor coupled to the receiver and configured to accept the plurality of pseudo range measurements for processing by:
i) adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints;
ii) dividing the pseudo range interval into a plurality of increments having a plurality of grid points;
iii) aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval;
iv) selecting a plurality of best windows based on at least one predetermined criterion and determining a plurality of average pseudo range values wherein each of the plurality of average pseudo range values corresponding to each of the plurality of best windows.
32. A device for determining position, comprising:
means for receiving a plurality of pseudo range measurements from a transmitting source; means for adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; means for dividing the pseudo range interval into a plurality of increments having a plurality of grid points; means for aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval; means for selecting a best window based on a maxim number of pseudo range measurements within The sliding pseudo range window and comparing the maximal number with an incidence threshold; and then based on the comparison, determining an average pseudo range value. 33. A method for determining a position of a device, comprising:
means for receiving a plurality of pseudo range measurements from a transmitting source; means for adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; means for dividing the pseudo range interval into a plurality of increments having a plurality of grid points; means for aligning a sliding pseudo range window having a width over the pseudo range interval at a first of the plurality of grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the plurality of grid points within the pseudo range interval; means for selecting a plurality of best windows based on at least one predetermined criterion and determining a plurality of average pseudo range values wherein each of the plurality of average pseudo range values corresponding to each of the plurality of best windows. 34. (Cancelled) 35. Computer readable media embodying a program of instructions executable by a computer program to perform a method for determining a position of a device, the method comprising:
receiving a plurality of pseudo range measurements from a transmitting source; dividing the pseudo range interval into a plurality of increments having a plurality of grid points; selecting a plurality of best windows based on at least one predetermined criterion and determining a plurality of average pseudo range values wherein each of the plurality of average pseudo range values corresponding to each of the plurality of best windows. Description 1. Field of the Invention The field of the present invention relates generally to determining the location of a device. More particularly, the invention relates to methods and apparatus for providing an improved (i.e., more accurate) estimate of the location of the device based on processing pseudo range values from transmitting sources, particularly GPS satellites. 2. Background of the Invention The desire to determine accurately the location of wireless devices is being driven in part by regulatory forces. In June 1996, the Federal Communications Commission (FCC) mandated support for enhanced 911 (E-911) service with planned phased implementations by the first decade of the 21st century. A common method of locating a device is to determine the amount of time it takes for signals transmitted by known sources to reach the receiver of the device to be located. One such source of transmitted signals is known as the Global Positioning Satellite (GPS) system as shown in One conventional method estimates a position for a particular time using M pseudo range values determined from M GPS satellites at that particular time. For a stationary device, this estimation is repeated at N sequential time intervals to derive N position estimates over the N time intervals. The N position estimates are then processed to determine the estimated position fix of a device. The processing technique can incorporate Kalman filtering, maximum likelihood estimation, weighted averaging, unweighted averaging, and variations of the above-mentioned processing techniques known to one skilled in the art. However, since each of these processing techniques uses computed position estimates to determine a filtered position, there is an inherent difficulty with outlier removal which can result in a less accurate (i.e., more erroneous) position determination estimate. It would be easier to filter out outliers if pseudo range values were processed directly prior to position determination since in the “raw” pseudo range format, outliers are more apparent. An outlier is an aberrant measurement which is statistically inconsistent with other measurements. For example, the GPS receiver is susceptible to occasional measurement outliers. In addition, when position estimates are processed to determine position, accurate signals from at least four GPS satellites are needed to fully determine position. In the event there are more than at least four in-view GPS satellites (i.e., the measurement is over-determined), then measurement integrity monitoring can be performed. Measurement integrity monitoring is the process of ensuring the validity of a set of GPS measurements, which may be achieved by checking the validity of each particular range measurement (taking measurement values in turns) against the position computed based on the remainder of the measurements (i.e., the set excluding the chosen measurement). However, if there are not enough GPS measurements to give an over-determined solution of position determination, then measurement integrity monitoring cannot be performed easily to determine which measurement is inaccurate. Accordingly, it would be desirable to provide methods and apparatus for providing device position determination with improved outlier removal. Additionally, it would be desirable to perform some form of measurement integrity monitoring even in the event when the GPS measurements do not give an over-determined solution of position determination. The methods and apparatus disclosed herein satisfy these needs. The present invention provides an improved (i.e., more accurate) estimate of the location of the device based on processing pseudo range values from transmitting sources at known locations. According to one aspect of the invention, the method includes the following steps: receive a plurality of pseudo range measurements from a transmitting source; adjust each of the plurality of pseudo range measurements for time correction and then arrange each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; divide the pseudo range interval into a plurality of increments with a plurality of grid points; align a sliding pseudo range window with a grid point and count the number of pseudo range measurements within that sliding pseudo range window. Repeat for each of the other grid points within the pseudo range interval and select a best window based on the maximal number of pseudo range measurements within a sliding pseudo range window. Compare that maximal number with an incidence threshold. Based on the comparison, determine an average pseudo range value. In a preferred embodiment, assign an average SNR and an average RMSE (root-mean-square error) estimate to the average pseudo range value. In another aspect of the invention, the method includes the following steps: receive a plurality of pseudo range measurements from a transmitting source; adjust each of the plurality of pseudo range measurements for time correction and then arrange each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints. Divide the pseudo range interval into a plurality of increments having a plurality of grid points; align a sliding pseudo range window with the first grid point, count the number of pseudo range measurements within the sliding pseudo range window and repeat for each of the other grid points within the pseudo range interval. Select a plurality of best windows based on at least one predetermined criterion and determine a plurality of average pseudo range values such that each of the average pseudo range values corresponds to a best window. In another aspect of the invention, the device for determining position includes a receiver for receiving a plurality of pseudo range measurements from a transmitting source, and a processor coupled to the receiver and configured to accept the plurality of pseudo range measurements for processing by: i) adjusting each of the plurality of pseudo range measurements for time correction and then arranging each of the plurality of pseudo range measurements in order of smallest value to largest value to form a pseudo range interval with the smallest value and the largest value as endpoints; ii) dividing the pseudo range interval into a plurality of increments having a plurality of grid points; iii) aligning a sliding pseudo range window with one of the grid points and counting the number of pseudo range measurements within the sliding pseudo range window, and then repeating for each of the other grid points within the pseudo range interval; iv) selecting a best window based on the maximal number of pseudo range measurements within the sliding pseudo range window and comparing the maximal number with an incidence threshold; and then based on the comparison, determining an average pseudo range value. Other and further objects and advantages of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and drawings. The received GPS signal frequency is typically different from the nominal GPS carrier frequency, mostly due to the high velocity of the satellites. This difference (satellite Doppler) can be as high as ±5 kHz. The expected satellite Doppler can be computed even before any measurements are made. This Doppler prediction computation is based on an approximate user location estimate and the satellite orbital data. The predicted Doppler may be conveyed from a base station to the GPS receiver in order to help the receiver speed up searching for the satellite signal. After the search for a satellite signal is complete, the receiver determines a measured satellite Doppler. The receiver also determines the Doppler offset, Δf -
- Doppler measurement error
- The actual receiver location is not the same as the location assumed when calculating the Doppler prediction
- The receiver's non-zero velocity in the direction of the satellite
- The receiver's frequency offset, in the case of a receiver employing a free running oscillator
- The receiver's velocity relative to the base station, in the case of a CDMA receiver, when the receiver frequency is locked on the base station frequency, which in turn is locked on GPS frequency
In step In step In step In step If the number of remaining pseudo range measurements is at least N*T The preprocessing may also involve conventional methods of applying any or all of the following correction terms: -
- Ionospheric delay correction
- Tropospheric delay correction
- Group delay correction
- Satellite clock correction
- DGPS correction
In one embodiment, the preprocessing algorithm is performed by a conventional programmable processor capable of accepting raw data (i.e., pseudo range measurements) and capable of determining position of the device. Additional implementation of the preprocessing algorithm may be achieved with an ASIC, a discrete logic circuit, a state machine or a software application within another network device. The preprocessor may be located within the device or may be part of the base station. Steps In step In step If N In step In an alternative embodiment, the pseudo range averaging algorithm may output more than one average pseudo range values for any of the M transmitting sources. This may be advantageous because of the possible existence of signal repeaters in the case where the transmitting sources are terrestrial base stations, or because of the existence of multipath signal propagation. Further processing, which is well-known in the art, is then used to determine which average pseudo range value corresponds to a base station and which average pseudo range value corresponds to a repeater, or alternatively, which average pseudo range value corresponds to the earliest path amongst the possible multipath average pseudo range values. Note that the input to the pseudo range averaging algorithm may also contain multiple pseudo range values (i.e., both detected base station and repeater signals, or a multitude of detected multipath signals) for any single measurement time. In yet another embodiment, for each transmitting source, multiple best windows are selected based on predetermined criteria known to one skilled in the art, with each best window having an associated weight. In one embodiment, the weight is the sum of the associated SNR values of the pseudo range measurements within the sliding pseudo range window. For each transmitting source i, denote N The advantages of the improved pseudo range estimation algorithm of the present invention include: reduced measurement error (as shown in While the present invention has been described in terms of the preferred embodiments, other variations which are within the scope of the invention as defined in the claims will be apparent to those skilled in the art. Referenced by
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