HK1034571B

HK1034571B - Method for determining the location of a gps receiver using an estimated reference time

Info

Publication number: HK1034571B
Application number: HK01105039.1A
Authority: HK
Inventors: 小W‧O‧坎普
Original assignee: 艾利森公司
Priority date: 1997-12-18
Filing date: 1998-12-14
Publication date: 2006-11-10

Description

Method for determining the position of a GPS receiver using an estimated reference time

Cross reference to related applications

This application relates to commonly assigned International application PCT/US98/21709, entitled "International application for the manufacture of A pharmaceutical compositionFor bees Simplified GPS receiver code shift search space for cellular telephone systems "Application 10/15 of 1997 (to William o. camp, jr., Kambiz Zangi and rajaram Ramesh), which is incorporated herein by reference.

Technical Field

The present invention pertains generally to methods for determining the position of a Global Positioning System (GPS) receiver, and more particularly to methods for determining the position of a GPS receiver when aiding information for determining the position of the GPS receiver is calculated at a different time than a range measurement made by the GPS receiver.

Background

It is desirable, and likely to become a mandatory standard in the near future, for cellular telephone systems to be equipped with means for determining the geographical position of cellular telephones operating in the cellular telephone system. To meet this demand, it has been proposed to equip a cellular phone with a Global Positioning System (GPS) receiver for determining the position of the cellular phone. However, GPS receivers are expensive, increase the size of the cellular telephone, and consume the limited battery power available to the cellular telephone. In addition, GPS receivers do not work well in buildings or other locations where GPS satellite transmissions are attenuated due to attenuation, reflection, etc. from obstacles.

It is well known that GPS receivers can be made smaller, cheaper and more energy efficient by eliminating the function of a particular GPS receiver used to obtain the aiding information that is typically obtained by demodulation of GPS satellite signals. Instead of demodulating GPS satellite signals, another means is employed to provide the required aiding information to the GPS receiver. The assistance information includes various information such as a list of GPS satellites currently in view of the GPS receiver, doppler shifts for the listed GPS satellites, ephemeris data for each listed GPS satellite, and clock correction data for each listed GPS satellite. The elimination of the need for the GPS receiver to demodulate the GPS satellite signal also allows the GPS receiver to integrate the GPS satellite signal over a longer period of time for receiving signals attenuated by obstructions.

However, in order to calculate the aiding information due to the GPS receiver, the approximate location of the GPS receiver must be known. In addition, the closer the actual position of the GPS receiver is to the approximate position used in calculating the aiding information, the less final position search is performed by the GPS receiver. For example, it is known that if a GPS receiver is given aiding information calculated for a location within a one hundred mile radius of the actual location of the GPS receiver, the GPS receiver does not need to measure the actual range to the GPS satellites, but only needs to measure a fraction of a millisecond for each range. Finding the relative code shift position in one millisecond code period greatly simplifies the necessary range measurements. However, to achieve this operation, the GPS receiver still must search all one-thousand-zero twenty-three code-shift positions for all GPS satellites to be used for the positioning solution.

The code-shift search may be performed by combining a fast fourier transform and an inverse fast fourier transform correlator to search all code-shift locations simultaneously. Techniques for finding the code shift position of a cyclic series are described in textbooks, such as digital signal processing by Oppenheim and Shafer. Although this method is more computationally efficient than the direct correlation method, it still requires a large amount of computation, which requires additional functionality and consumes limited battery power. In addition, as information is communicated to the mobile unit to assist it in searching for GPS satellite range, it becomes computationally inefficient as it consumes computational cycles to search for as many code-shifted locations as is not possible.

Another solution to searching all one-thousand-twenty-three displacement code positions is to design dedicated hardware to search multiple code displacement positions simultaneously. However, the solutions of dedicated hardware so far cannot search a small fraction of the code shift positions at the same time, thus requiring multiple searches and long time delays.

In commonly assigned International application PCT/US/98/21709, entitled:for cellular electricity Simplified GPS receiver code shift location search space for a speech systemApplication 10/15 in 1997, which discloses a method for providing a simplified function GPS receiver in a mobile station, which utilizes assistance information calculated for a known geographical position in the cell serving the mobile station. A server connected to the cellular telephone network calculates assistance information based on known locations in cells served by the cellular telephone base stations. In one example, the assistance information includes: a list of GPS satellites in view of the base station, a doppler correction value for each listed GPS satellite, and a code-shifted position for each listed GPS satellite at a time adjusted according to international standard time for a known location. In another example, the assistanceThe information includes a list of GPS satellites in view of the base station, a central location of coverage of the base station, and the location and clock corrections of the listed satellites based on the time adjusted by international standard time.

One disadvantage of this approach is that it depends on the time synchronization between the GPS satellites and the range measurements made by the GPS receiver. However, in many instances, the time at which the GPS receiver performs the range calculation varies due to delays in the cellular telephone network. It would therefore be advantageous to provide a method of performing range calculations based on aiding information that is not time synchronized with GPS satellites.

Disclosure of Invention

The present invention includes a method for determining a position of a satellite receiver. The method begins by selecting a time of attempt for calculating a presumed position using at least four satellites. The assumed position is determined based on the selected trial time. A first range from the presumed location to a fifth satellite is calculated, and a second range from the presumed location to the fifth satellite is measured. The first range is compared to the second range. If the first range is not equal to the second range, the assumed position is not the actual position. A new trial time is selected and the method is repeated. When the first range and the second range are substantially equal, the assumed position is an actual position. Wherein the calculation of the presumed position comprises the steps of:

calculating a current position for each of at least four satellites; calculating a third range of each of the four satellites to a known location;

measuring a third range from each of the four satellites to the unknown position of the satellite receiver;

calculating a correction vector based on the difference between the calculated third range from each of the four satellites to the known position and the measured third range from each of the four satellites to the unknown position; and

the correction vector is added to the coordinates of the known position.

The invention also includes a system characterized by:

means at the known location for calculating satellite aiding information based on the known location;

a cellular telephone network for communicating the assistance information to a cellular mobile station at an unknown location different from the known location of the device;

a satellite receiver, coupled to the cellular mobile station, for determining its position by:

(a) selecting, using the aiding information for the known location, trial times for computing a presumed location using at least four satellites;

(b) calculating a presumed location of the satellite receiver based on the selected trial time (360);

(c) calculating a first range from the presumed location to a fifth satellite;

(d) measuring a second range from the presumed location to a fifth satellite;

(e) comparing the first range to the second range;

(f) if the first range is substantially equal to the second range, determining the assumed position as the current actual position of the satellite receiver, otherwise;

(g) selecting a new trial time using the assistance information of the known location; and

(h) repeating the process from step (b) and

wherein the calculation of the presumed location comprises:

calculating a current position for each of at least four satellites;

calculating a third range of each of the four satellites to a known location;

the correction vector is added to the coordinates of the known position.

Drawings

For a more complete understanding of the present invention, reference is now made to the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a functional block diagram of a system for determining a geographic location in accordance with a preferred embodiment of the present invention;

FIG. 2 shows a plurality of GPS satellites and a known and unknown location for illustrating an example for determining a geographic location in accordance with a preferred embodiment of the present invention; and

FIG. 3 is a flow diagram of a method for determining a geographic location according to one embodiment of the present invention.

Detailed Description

Referring now to FIG. 1, a functional block diagram of a system for determining a geographic location is depicted in accordance with a preferred embodiment of the present invention. A cellular telephone base station 100 located in a cell site 120 of a cellular telephone network 110 serves a cellular telephone 140. The cell site 120 is divided into a first sector 130, a second sector 132, and a third sector 134, with fig. 1 showing the cellular telephone 140 located in the first sector 130. The cellular telephone 140 also includes a simplified function GPS receiver 150 that receives GPS satellite transmission signals from a plurality of GPS receivers 160. The reduced functionality GPS receiver 150 does not include functionality for demodulating signals from GPS satellites 160 and determining aiding information. Alternatively, the aiding information needed to determine the location and reduce the doppler shift and code phase shift range to be searched is provided to the reduced functionality GPS receiver 150 from another source.

Various different sets of information may be included in the side information to achieve a reduction in search space, each of which has certain advantages and disadvantages. The first set includes a list of satellites 160 in view of the reduced functionality GPS receiver 140, doppler frequencies for the listed satellites 160, and expected code phase relationships for the listed satellites 160. However, this information must be transmitted with very short delays because the desired code phase relationship for the listed satellites 160 changes dramatically over time. Also, some form of time synchronization is required to locate the satellites 160 at the time of measurement.

Another aiding information includes a list of satellites 160 in view of the reduced functionality GPS receiver 140, ephemeris data for the listed satellites 160, clock correction data for the listed satellites 160, approximate location of the reduced functionality GPS receiver 140, and timing information. In this case, the delay of the information is less problematic, and the relative code phase between the doppler frequency and the satellite signal can be calculated. However, this time information needs to be known within a few seconds for efficient searching and measurement of pseudoranges. For example, since the satellite 160 needs to be accurately positioned at the time of measurement, the time information needs to be known within 10 milliseconds to solve the positioning problem. The time information may be obtained from a time standard, an internal clock, or a cellular telephone network. Time information can also be obtained from GPS signals, but additional functionality needs to be included in the reduced functionality GPS receiver to perform demodulation of the GPS signals.

The third set of aiding information includes a list of satellites 160 in view of the reduced functionality GPS receiver 140, position data and motion data for the listed satellites 160, clock corrections for the listed satellites 160, and an approximate location of the reduced functionality GPS receiver 140. The set of aiding information considers the position of the satellites 160 at known times and their short-term motion in place of the usual ephemeris and time information. Using the set of aiding information and the method of the present invention for locating a reduced functionality GPS receiver, delays of up to 60 seconds can be tolerated for fast signal search and measurement. In the above case, the summation of the differential GPS corrections may be performed.

The known location is the location of the base station 100 or the center 200 of the sector 130 in which the cellular telephone is located. The known locations in the coverage area of the base station 100 are used to calculate the assistance information. The location may be determined in any manner, including using a GPS receiver 180 located at the base station 100 or at the server 170 to compute aiding information.

The base station 100 obtains and periodically updates the GPS ephemeris information and the clock correction value belonging to the current state of the GPS satellite 160. In a preferred embodiment, the GPS ephemeris information and clock correction values are obtained by the base station 100 from the data service 190 through the cellular telephone network 110. Alternatively, the information may be obtained directly from the transmitted signals of the GPS satellites 160 received by the GPS receiver 180 located at the base station 100 or the server 170.

In the future, when an assisted GPS related system called a wide area incremental system (WAAS) is put into use, the GPS receiver 180 located at the base station 100 or the server 170 will also be able to obtain differential correction information. This differential correction information allows the GPS receivers to calculate their position with greater accuracy.

The server 170, located at the base station 100 or a remote location, calculates aiding information using information obtained from the GPS receiver 180 or the data service 190 and then transmits to the base station 100 and to the reduced function GPS receiver 150 located in the cellular telephone 140. The assistance information includes, for example, a list of GPS satellites 160 in view of the base station 100, clock correction information, three-dimensional coordinates of the location of each listed GPS satellite 160 corresponding to the most likely time that the reduced function GPS receiver will make a range measurement, the three-dimensional velocity and trajectory of each listed GPS satellite 160, and three-dimensional coordinates of a known location for calculating assistance information. The doppler frequency is calculated for each listed GPS satellite using the velocity information. The expected code phase shift for each listed GPS satellite 160 is calculated using the position of the satellite 160, the location of the known position, and the clock correction data. In addition, the aiding information includes a list of GPS satellites 160 in view of the base station 100, ephemeris data for each listed GPS satellite 160, and three-dimensional coordinates of a known location for calculating aiding information.

If the cell location 120 is divided into sectors and the base station 100 can determine the sector in which the cellular phone 140 is located, in this example the first sector 130, the server 170 calculates the assistance information from the central location 200 of the sector 130 as opposed to the center of the cell 120. Calculating the assistance information from the central location 200 increases the accuracy of the assistance information because the cellular phone 140 is more likely to be close to the central location 200 than the base station 100 at the center of the cell. The geographic value of the central location 200 need not be the actual center of the sector 130, but may be the location where the cellular telephone is most likely to be, such as a shopping mall, office, airport or sports venue within the sector. However, if the cell location 120 is divided into sectors, or if the base station 100 is unable to determine the sector in which the cellular phone 140 is located, then the aiding information is calculated based on the geographic location of the base station 100.

In another embodiment, the geographical center location of the commercial transaction area or municipal service area is used in place of the geographical location of the base station 100. Each cell phone service area is identified by a System Identification (SID) read by the cell phone 140. The cellular phone 140 may store the ancillary information pertaining to these locations and reference the information related to the current SID or the ancillary information is stored in the server 170 and the cellular phone provides the SID to the server 170 which provides the ancillary information.

After the server 170 calculates the aiding information, the base station 100 sends the aiding information to the reduced functionality GPS receiver 150 in the cellular telephone 140. The aiding information may be transmitted to the reduced functionality GPS receiver 150 through various means. For example, in cellular telephone networks using the global system for mobile communications (GSM), information may be sent via short message service messages, via packet data messages on traffic channels, or via broadcast messages on control channels. The auxiliary information is sent using means known in the art for transferring information between the cellular telephone network 110 and the cellular telephone 140. The transceiver 141, located in the cellular telephone 140, receives the transmitted signals from the base station 100 and the controller 142, also located in the cellular telephone 140, identifies the information as aiding information and provides the aiding information to the reduced function GPS receiver 150. In addition, the assistance information may be sent to a particular cellular telephone or to multiple cellular telephones over a broadcast channel when needed.

Referring now to FIG. 2, therein is shown a plurality of GPS satellites 220_a-nA known location 230 and an unknown location 240, illustrate examples of determining a geographic location according to a preferred embodiment of the present invention. Each GPS satellite 220 listed in the aiding information_a-nHaving a set of three-dimensional coordinates (X)_a-n，Y_a-n，Z_a-n). A geocentric earth fixation system is typically used for all coordinates. The known location 230 also has a set of three-dimensional coordinates (X, Y, Z). From a known location 230 to each GPS satellite 220_a-nRange R of_a-nCalculated by the following equation:

using this aiding information, the reduced functionality GPS receiver 150 utilizes the known value of the speed of light and the individual clock correction values for each satellite to each satellite 220 in a manner known in the art_a-nAnd calculating the code shift position. The simplified function GPS receiver 150 also provides for each GPS satellite 220_a-nSearching the code shift search space for each GPS satellite 220_a-nThe measured code shift position is determined. The measured code shift position is subtracted from the calculated code shift position to calculate a code shift to each GPS satellite 220_a-nThe range of (1). These vectors of incremental ranges are taken from known locations to each GPS satellite 220_a-nIs multiplied by the inverse of the matrix of the cosine of the unit vector to yield the correction values for the X, Y and Z correction vectors, which are added to the known position 230 to determine the unknown position 240.

Referring now to FIG. 3, shown is a flow chart of a method for determining a geographic location in accordance with a preferred embodiment of the present invention. After the aiding information is received by the reduced functionality GPS receiver, three-dimensional position coordinates of at least four GPS satellites are calculated (step 300). If the auxiliary information is contained at the time T₀By multiplying their speed by time and calculating the coordinates of the GPS satellites at time T, and their speed and trajectory₀And T₁The distance moved along their trajectory in the time period in between, and the current time T is calculated₁The location of the GPS satellites. On the other hand, if the aiding information includes ephemeris data, the position of the GPS satellites is calculated using methods that are well known and commonly used in GPS receivers.

The range from the GPS satellites to the known location is calculated using the representation shown in fig. 2 (step 320), and the range from the GPS satellites to the unknown location is also calculated using the code shift measurements described in fig. 2 (step 330). The difference between the calculated and measured ranges is calculated (step 340) and a correction vector is calculated (step 350), as shown in fig. 2. An assumed position is calculated by adding the correction vector to the known position (step 360). At this point, the position is only assumed, since the exact time of the reference GPS satellite is only an attempted estimate. Thus, it is assumed that the position needs to be confirmed.

To confirm that the presumed location is the actual location, a range from the presumed location to a fifth GPS satellite in the list of satellites in view of the receiver is calculated (step 370), and a range from the presumed location to the fifth GPS satellite is measured (step 380). The difference between the calculated and measured ranges between the hypothetical location and the fifth GPS satellite is calculated (step 390), and it is determined whether the hypothetical location is an actual location (step 400). If the difference between the calculated and measured ranges is zero, the position is assumed to be the actual position. On the other hand, if the difference between the calculated and measured ranges is not zero, then the selected time at step (300) is incorrect and the assumed position is incorrect. In this case, a new trial time is selected (step 410), and the process repeats beginning with step 310. After the selection of two incorrect attempt times, the orientation of the selection time when the third attempt time is selected is determined and any search routine may be used to identify the correct reference time.

It is assumed that one can apply higher order corrections to the orbital motion according to the assumed circular orbit. This enables a simple quadratic correction to be used for the satellite position calculation. Practical effects make this approach useful in cases where the time of the pseudorange measurement is several minutes uncertain.

Although the method of the present invention has been described with reference to the accompanying drawings and the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous changes, modifications and substitutions without departing from the spirit of the invention as defined by the following claims.

Claims

1. A method for determining the position of a satellite receiver, the method comprising the steps of:

computing satellite aiding information from a known location (230) other than the currently unknown location (240) of the satellite receiver (150);

transmitting the calculated aiding information to a satellite receiver at an unknown location; and

determining its position by the satellite receiver by performing the steps of:

(a) selecting an attempt time (300) for calculating a presumed location using at least four satellites using assistance information of known locations;

(c) calculating a first range from the presumed location to a fifth satellite (370);

(d) measuring a second range (380) from the presumed location to a fifth satellite;

(e) comparing (390, 400) the first range to the second range;

(f) if the first range is substantially equal to the second range, determining the assumed position as the actual current position of the satellite receiver, otherwise performing the following steps;

(g) selecting a new trial time using the assistance information for the known location (410); and

(h) the process is repeated starting from step (b), and

wherein the calculation of the assumed position comprises the steps of:

calculating (310) a current position for each of at least four satellites;

calculating a third range of each of the four satellites to a known location (320);

measuring a third range from each of the four satellites to the unknown location of the satellite receiver (330);

calculating a correction vector (350) based on the difference between the calculated third range from each of the four satellites to the known location and the measured third range from each of the four satellites to the unknown location; and

the correction vector is added to the coordinates of the known position.

2. The method of claim 1, wherein the step of calculating a correction vector comprises the steps of: multiplying the difference between the calculated range from each of the four satellites to the known position and the measured range from each of the four satellites to the unknown position by an inverse of a matrix of unit vector cosines from the known position to each of the four satellites.

3. The method of claim 1, wherein the step of transmitting the aiding information comprises the step of transmitting the aiding information to the satellite receiver via a cellular telephone network (110), wherein the satellite receiver forms a portion of a cellular telephone (200).

4. The method of claim 1, further comprising the step of receiving auxiliary information comprising:

a list of satellites with respect to the receiver;

clock correction information;

the original three-dimensional coordinates of each listed satellite;

three-dimensional velocity and trajectory for each listed satellite; and

the three-dimensional coordinates of the location are known.

5. The method of claim 4, wherein the step of calculating the current position of each of the at least four satellites comprises the steps of:

calculating a time difference between the selected trial time and a time when the three-dimensional coordinates of each of the four satellites are determined;

multiplying the time difference by the velocity of each of the four satellites to determine the distance traveled by each of the four satellites within the time difference; and

the position of each of the four satellites is identified based on the distance and trajectory traveled by each of the four satellites.

6. The method of claim 1, further comprising the step of receiving auxiliary information, the auxiliary information comprising:

a list of satellites with respect to the receiver;

clock correction information;

ephemeris data for each listed satellite; and

the three-dimensional coordinates of the location are known.

7. The method of claim 6 wherein the step of calculating the current position of at least four satellites includes the step of extrapolating information from the ephemeris data to calculate the current position.

8. A system, characterized by:

a device (100, 170, 180) at a known location (230) for calculating satellite assistance information from the known location;

a cellular telephone network (110) for communicating the assistance information to a cellular mobile station (200) at an unknown location (240) different from the known location of the device;

a satellite receiver (150) coupled to the cellular mobile station for determining its position by:

(a) selecting, using the assistance information for the known location, a trial time for calculating the presumed location using at least four satellites (300);

(e) comparing (390, 400) the first range to the second range;

(f) if the first range is substantially equal to the second range, the assumed position is determined as the current actual position of the satellite receiver, otherwise

(h) repeating the process from step (b) and

wherein the calculation of the presumed location comprises:

calculating a current position for each of at least four satellites (310);

the correction vector is added to the coordinates of the known position.

9. The system of claim 8, wherein the calculation of the correction vector comprises multiplying a difference between the calculated range from each of the four satellites to the known position and the measured range from each of the four satellites to the unknown position by an inverse of a matrix of unit vector cosines from the known position to each of the four satellites.

10. The system of claim 8, wherein the auxiliary information comprises:

a list of satellites with respect to the receiver;

clock correction information;

the original three-dimensional coordinates of each listed satellite;

three-dimensional velocity and trajectory for each listed satellite; and

the three-dimensional coordinates of the location are known.

11. The system of claim 8, wherein the calculation of the current position of each of the at least four satellites comprises:

12. The system of claim 8, wherein the auxiliary information comprises:

a list of satellites with respect to the receiver;

clock correction information;

ephemeris data for each listed satellite; and

the three-dimensional coordinates of the location are known.

13. The system of claim 11, wherein the calculation of the current position of at least four satellites includes extrapolating information from the ephemeris data to calculate the current position.