WO2001094968A1

WO2001094968A1 - Location procedure for mobile telephone units

Info

Publication number: WO2001094968A1
Application number: PCT/US2001/016196
Authority: WO
Inventors: Joseph Nir; Baruch Shayevits; Hanoch Cohen
Original assignee: Cellguide Ltd.; Friedman, Mark, M.
Priority date: 2000-06-02
Filing date: 2001-05-21
Publication date: 2001-12-13
Also published as: AU2001264697A1

Abstract

A procedure for the location of a receiver of a system of navigation satellites incorporates preliminary location data. In accordance with the present invention the preliminary location data is at least as accurate as a cell sector, and potentially defines the position of the receiver seeking location, in geographical terms more accurate than one cell size of the cellular network. The preliminary data is obtained from a cellular network deployed locally, through either of two categories of data sources. The one category of sources includes the regular management procedures of the cellular network. The other category of sources includes methods of location finding based on cellular BTSs (52, 54) signal travel time measurement.

Description

LOCATION PROCEDURE FOR MOBILE TELEPHONE UNITS

FIELD OF THE INVENTION

The present invention relates generally to satellite navigation, and more specifically to methods for improving accuracy thereof by the use of ancillary data.

BACKGROUND OF THE INVENTION

The GPS navigation satellite network provides a common solution to a variety of navigation requirements. The navigation signals sent by the satellites of that system are received by compatible receivers, and subsequent calculations made, based on data acquired from the navigation signals provide the sought after navigation fix. In Fig. 1 to which reference is now made, the conventional sequence of steps of the process for determining a location of a receiver of the GPS satellite system is described in general terms. The process

is initiated in step 20. Then, navigation signals from at least four satellites are

acquired in step 22 followed by extraction of navigation data from each of the

satellites received, in step 24. In step 26 calculations are performed to determine location of the receiver. The acquisition of the satellite signal is a two dimensional process in that the search which is carried out by the receiver, proceeds along two independent variables in order to find each available satellite's signal. One variable is the C/A code that each satellite transmits uniquely typifying the satellite signal, differentiating it from all other satellites of the GPS constellation. The other variable is the frequency (Doppler) shift of the carrier frequency related to the line of sight (LOS) relative velocity between the receiver and the satellite. If the receiver has zero speed with respect to the earth, the maximum LOS velocity between the satellite and the receiver is about 800 m/sec either way. The Doppler component emanating from the satellite motion relative to the earth can be calculated by using the satellite position data. Thus, any error in defining the position of the of the receiver with respect to a satellite, is bound to cause an error in estimating the Doppler offset of the frequency of the signal sent from that same satellite.

Another source of error that relates to offset in position of the receiver with respect to the satellite stems from disregarded variations in altitude of the receiver, which can account for significant error in navigation. Solving the equations obtained from the four satellites as mentioned above takes account of three dimensional unknowns, but if the position of the receiver in the altitude was accurately known, less one satellite would have been required to solve the navigation. In Fig. 2, to which reference is now made, are described the main

steps of a prior art process for location finding of a receiver of the GPS satellites system, into which process, preliminary location data is incorporated.

In step 30 the location process is initiated, following which the receiver obtains

preliminary location data in step 32. In step 34 the preliminary location is

calculated, and in step 36, the reference satellites for the task of location determination are selected. Doppler offset parameters for each reference satellite, which are each a function of the geometrical relationship between the receiver and the respective satellite, are estimated in step 38. In step 40 the receiver acquires the signal of each of the selected reference satellites, and in step 42, the position of the receiver is calculated from the navigation data contained within the satellite navigation signals.

It is highly desirable, on account of the reasons set forth above, to have a precise as possible assessment of the location of the receiver prior to the acquisition of the signal, in order to produce better and faster results from the navigation signals. In WO 9825157, the contents of which are incorporated herein by reference, an improved GPS based procedure for the determination of location is disclosed. In this disclosure, cellular network data is used to provide preliminary location data with regards to the receiver. Such data is typically geographic data concerning the service area of the cellular network or more precise, of a specific cell site within the service area. In WO 9957575C2, which also deals with location system based on navigation satellites, the preliminary location data used includes altitude parameter of the satellite receiver. Such information may be obtained from a cellular source or from other available sources.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for exploiting preliminary location data for improving satellite based navigation.

A further object of the present invention is to provide preliminary location pertaining to a receiver of navigation satellite system, by which the accuracy obtained is more precise than the size of the cell in the cellular network in which the receiver is operative. The preliminary location data in accordance with the present invention are derived either from the regular activity of the cellular network, or from cellular based location procedures carried out for that purpose.

Preliminary position data is - ^' calculated, in accordance with the invention, by measuring the transmission power of adjacent base stations and correlating the distance of the receiver to the base stations with the measured power. Another method for measuring the distance within a cell, is by taking account the timing advance step assigned to receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates generally a prior art sequence of steps of the process for determining location of a satellite receiver;

Fig. 2 illustrates generally a prior art sequence of steps in which preliminary data is included in the general satellite based location process;

Fig. 3 is a schematic description of a service area of a cellular network, wherein a mobile telephone receives signals from several BTSs of varying distances.

Fig. 4 is a schematic description of a single cell in which the location of a mobile telephone is preliminarily determined within a sector and within a timing - advance step.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In accordance with the present invention, preliminary geographical location data is provided to the procedure that calculates the location, of a receiver of navigation satellites signals. The precision of the preliminary data thus provided is higher than the precision hitherto provided as disclosed in the prior art, consequently facilitating more efficient acquisition of the satellites navigation signals. Following the acquisition of the navigation signals, the navigation data is extracted, and used in the procedure that calculates the receiver's location. Into this procedure, the preliminary location data is incorporated. The enhanced precision of the preliminary location also enhances the accuracy of the location procedure. In accordance with the present invention, data derived from a locally deployed cellular network is utilized for providing such preliminary geographical location data. The preliminary location data can be used for enhancing the signal acquisition stage as explained supra, and it also can be used in the actual calculation of the location.

The derivation of data

The sources for data used for calculating the preliminary geographical

location, fall into either one of two categories:

1. Sources related to regular network management procedures.

2. Sources pertaining to ranging techniques utilizing travel time of

base station signals.

1. Sources related to regular network management: a mobile cellular

telephone is directly controlled by the BTS of the cell in the bounds of which it is operative. The geographical location parameters (longitude, latitude and altitude) pertaining to the same cell can potentially be obtained from the network and thus provide preliminary data regarding the location of the mobile telephone of the network. In some cellular networks, notably the GSM network, the transmission power level of the BTSs of adjacent cells is constantly measured by the mobile telephone. The results of this measurements can be used to assess the location of the mobile telephone within the bounds of cell. In

Fig. 3, to which reference is now made, the relationship between BTSs of the

respective cells are shown with respect to mobile telephone active within one of the cells. Thus, mobile telephone 50, is active within cell 4, receiving signals

from BTS 52. The same telephone, receives signals from BTS 54 of cell 1 and

from BTS 56 of cell 2. The telephone measures the power of the signals from each BTS, which potentially gives indication about the distance of the receiving telephone from the respective BTSs. The distance to BTS 58 at the center of cell 5, shown in dotted line, from telephone 50 is greater than the distances of

the telephone 50 to any of the other BTSs, also shown in dotted lines. In such a

case, the power of the signal transmitted from BTS 58 is weaker than the

signals arriving from the other BTSs depicted above, i.e. BTS 52, 54, 56 and

58. However, since the power of transmission at the BTS may vary over time, as in GSM systems, some statistical averaging may be required over a period of time in order to make the comparisons between the BTS power meaningful.

Another method of obtaining sub - cell size precision location, makes use of the adaptive frame alignment, which is a process of synchronizing the mobile telephone frame sequence with that of the BTS. This process is essential in TDMA (Time Division Multiple Access) cellular networks, such as GSM. In this process, the BTS sends to the mobile telephone instructions as to the TA (timing advance) value to be employed in order to keep the frames matched. In the GSM system there are 64 possible steps of TA. Three such steps are represented in Fig. 4 to which reference is now made. BTS 60 , at

the center of cell 62 sends TA (timing advance) instructions to the mobile

telephone 64, operative within the cell sector delimited by heavy line 66. The

mobile phone is located in a ring delimited by the two circles: the inner circle 68

and the outer circle 70. This ring represents a certain TA (Timing Advance)

step, which is defined by the network. The ring limits and the sector limits together define a place within the cell in which the mobile telephone is located. This definition has geographic implications that may be used in accordance with the present invention as a preliminary location data.

2. The second category_, of data sources is typically associated with prior art techniques of determining location of a receiver of cellular networks, either synchronized or non synchronized, by exploiting the signals of several

BTSs. The fundamental methodology on which such techniques are based, is cellular network signal ranging according to which the time it takes for respective signals to travel from, several base stations (BTSs) to the receiver is measured. In such a technique, reference is made to the geographical position of the BTSs which is known in advance. The distance of the receiver to each

BTS is calculated, by calculating the relative time offset of the signals reaching the receiver as compared to the time it takes for an equivalent signal to reach a reference receiver having known geographic location. In WO- 99 - 21028 a method is disclosed for locating a mobile unit of a digital telephone system, in which a reference receiver positioned at a known location receives signals of BTSs (base transceiver stations) of the telephone system, each having a known location. Another receiver, of unknown location receives the same signals and by calculating the time offsets between the respective reception times in each receiver, location is determined. Another method, disclosed in WO - 99 - 61934 utilizes downlink signals of BTSs of a cellular network, using them as ranging purposes.

Claims

1. A method for determining a location of a receiver of a satellite navigation system, based on navigation signals of reference navigation satellites, comprising:

• obtaining preliminary location data relating to said receiver, from a locally deployed cellular network,

• calculating preliminary location from said preliminary location data wherein said preliminary location is more precise than one cell size,

• selecting reference satellites, and

• using said preliminary location information in a process for finding a location of said receiver.

2. A method for determining a location of a receiver of a satellite navigation system as in claim 1, and wherein said preliminary location information is used for acquiring signals of said selected satellites.

3. A method for determining a location of a receiver of a satellite navigation system as in claim 1 , and wherein said preliminary location information is incorporated into a procedure for finding a position of said receiver.

4. A method for determining a location of a receiver of a satellite navigation system as in claim 1 , and wherein said preliminary location data is obtained from said cellular network, deriving data from its regular management procedures.

5. A method for determining a location of a receiver of a satellite navigation system as in claim 1 , and wherein said preliminary location data is obtained from said cellular network, by performing procedures of cellular signal ranging technique.

6. A method for determining a location of a receiver of a satellite navigation system as in claim 4, and wherein said preliminary location data is obtained by comparative signal power measurement of available BTSs of said cellular network.

7. A method for determining a location of a receiver of a satellite

navigation system as in claim 4, and wherein said preliminary

location data is a timing - advance information relating to a

receiver of said cellular network.

8. A method for determining a location of a receiver of a satellite navigation system as in claim 1 , and wherein said locally deployed cellular network system is TDMA network.