Mobile position determination with error correction utilizing cellular networks

A position determining system takes the form of a cellular radio system including at least one base station having a base station satellite receiver (6) and a mobile unit including a cellular mobile station (50) coupled to a local satellite receiver (58). The base station transmits base station satellite data via a cellular radio link to the mobile unit, the data including data representing a carrier phase measurement derived from a satellite signal (70) received by the base station satellite receiver (6). The mobile unit determines its position relative to the base station using local satellite data received by the local satellite receiver (58) and corrects errors in this position determination using base station satellite data. The relative positions of the base stations are determined automatically with reference to an external positional reference which may be a satellite-based position determining system.

The present invention relates to position-determining apparatus 
particularly but not exclusively for mobile use and has particular 
application in land surveying. 
The infrastructure currently available to surveyors, map makers, GIS data 
collectors and navigators, for example, for high precision positioning is 
historically based on a national triangulation scheme. The infrastructure 
includes a network of markers such as triangulation pillars, whose 
coordinates are known and are usually sold to interested parties through 
the country's national mapping agency. 
Historically, geodetic surveying was performed optically using a device 
such as a theodolite in conjunction with the markers. For this reason, 
many of the markers are located at the tops of hills to ensure that they 
are readily intervisible. This historical system has several drawbacks. 
Firstly, although the positions of the markers have been measured over many 
years and have been computed with additional data such as that provided by 
satellite positioning systems e.g. Global Positioning System (GPS), the 
network of markers and their coordinates frequently include significant 
errors. For example, in the UK the ordnance survey mapping is based on a 
triangulation performed in 1936 (the so-called OSGB36 triangulation). This 
is known to represent the United Kingdom as having a north-south length 20 
metres different from its true length. Various more recent, more accurate 
and therefore different triangulations (such as OSGB72 and OS(SN)80) are 
used in different fields such as engineering. The use of different 
triangulation networks causes confusion. Secondly, the above problem is 
exacerbated by the respective existence of two independent triangulation 
networks for the determination of horizontal and vertical positions. To 
make matters worse, the markers for the two independent networks are not 
necessarily co-located. 
In most countries, the national mapping organisation maintains the 
triangulation network. Income is generated from the network by one-off 
payments for the sale of the computed coordinates of the markers. Thus the 
revenue is limited and arises in significant amounts only sporadically 
e.g. when the positions of the markers have been recalculated. It is not 
uncommon, however, for markers to be destroyed or to move due to ground 
movement. With inadequate maintenance, further inaccuracies are thereby 
introduced into the triangulation networks. 
It is known to perform accurate position determinations using a 
differential GPS-type measurement in which two satellite receivers are 
used, one being placed at a known position and the other being placed at a 
position to be determined. It is possible to use a triangulation marker as 
the known position. However, this has at least two disadvantages. Firstly 
the coordinates of the marker may be inaccurate for the reasons given 
above and secondly the markers are frequently positioned in inaccessible 
areas such as hilltops, as described above. In some countries, such as the 
United Kingdom, new markers suitable for the use of GPS equipment are 
being placed in more accessible areas so that the satellite receiver of 
known position (the so-called master station) can more conveniently be set 
up. For example, in the United Kingdom the new GPS control markers can all 
be accessed with a 2-wheel drive vehicle. However, for a party to take 
advantage of this it is necessary for the party to provide both satellite 
receivers at each respective site and to set them up itself; this being 
relatively expensive and inconvenient. 
Currently there exists a worldwide network of several hundred tracking 
stations incorporating permanently recording dual-frequency geodetic GPS 
receivers. This network is called the IGS (International GPS Service for 
Geodynamics) network. 
Further features of IGS are summarised in document JPL 400-552 6/95 
published by the Jet Propulsion Laboratory of the California Institute of 
Technology and entitled "Monitoring Global Change by Satellite Tracking". 
In particular, the IGS network provides online access to IGS tracking 
station coordinates and velocities, GPS satellite to IGS tracking station 
measurement information, and very accurate GPS satellite ephemerides. 
The so-called Precise Ephemerides, precise GPS satellite orbit and clock 
data are available in near real-time (approximately a day late). The 
tracking station coordinates are regularly computed and published and have 
a relative accuracy of between 3 mm and 1 cm between any two sites around 
the world. 
These published "control" coordinates can be regarded as absolute reference 
positions to which differential GPS measurements can be referred in order 
to define actual coordinates for the newly surveyed points. Any position 
obtained using a differential GPS technique is in fact only a relative 
position with respect to the stations providing the measurement and/or 
measurement correction data. 
Whilst sufficient information exists to use one of these receivers for 
differential GPS measurements, it should be appreciated that due to 
differences in propagation conditions existing between the satellites and 
the master station and the satellites and the position to be determined, 
that the accuracy of differential GPS is diminished as the distance 
between the master station and the position to be determined increases 
(typically at a rate of 0.5 to 1 mm per kilometre separation). Thus it is 
not possible to use an IGS or equivalent receiver for high accuracy 
real-time position determination in all areas. 
Attempts have been made to use the base station of a cellular radio network 
as the reference station in a differential GPS arrangement. Two such 
attempts are described in GB 2264837-A and WO94/12892 respectively. 
However, both of these attempts have been made in order to provide vehicle 
location facilities for vehicle fleet managers. Since the described 
systems do not seek to solve the problem of providing high accuracy 
measurements for surveying, these systems do not provide sufficient 
positioning accuracy for surveying. 
According to a first aspect of the present invention, position determining 
apparatus comprises a cellular radio system including at least one base 
station having a base station satellite receiver, and position 
determination means including a cellular mobile station coupled to a local 
satellite receiver, the base station being arranged to transmit base 
station satellite data including data representing a carrier phase 
measurement derived from a satellite signal received by the base station 
satellite receiver to the mobile station via a cellular radio link and the 
position determination means being arranged to determine its position 
relative to the base station using local satellite data received by the 
local satellite receiver and to correct errors in this position 
determination using the base station satellite data. 
In this application the term "base station" means a cellular radio base 
station forming part of a cellular radio communication infrastructure 
having a plurality of such base stations which are capable of exchanging 
radio signals with a plurality of cellular mobile stations such as mobile 
cellular telephones over cellular radio links. 
Most digital cellular telephone base stations (e.g. GSM base stations) need 
to have access to a precise timing system to ensure the accuracy of the 
TDMA modulation techniques used. A common and cost-effective method of 
providing the necessary accuracy of timing is to use the accurate timing 
built into the GPS system. Each of the GPS satellites has a plurality of 
atomic clocks which are monitored and adjusted from stations on the 
ground. Thus, using conventional techniques it is possible to derive an 
extremely accurate (down to nanosecond accuracies) timing signal from 
received GPS signals. For this reason, many base stations already include 
a base station satellite receiver which is used to steer a high quality 
oscillator for the TDMA timing. This means that the cost of the present 
invention in terms of adapting the cellular radio system hardware can be 
minimal. The base station satellite data transmitted to the mobile station 
may be raw GPS data (typically pseudorange and carrier phase measurements 
in a standard format such as. RINEX-Receiver Independent Exchange) and 
information concerning the precise position of the base station. From this 
information, the position determination means may calculate its own 
measurement corrections. Using this information, the determination means 
may correct for errors such as satellite clock and orbit errors and 
atmospheric propagation effects to improve the accuracy of a position 
determined using the local satellite data. The use of carrier phase 
measurements provides the accuracy required of surveying apparatus. 
Correction data based on a code phase measurement does not provide 
sufficient accuracy. 
The mobile station may be a conventional mobile station having a data 
communication facility. Data communication is built into most digital 
cellular radio specifications (including GSM) and is already used for 
mobile modem connections for laptop computers. 
The relative position determination may be performed in real-time using the 
local and base station satellite data to perform a differential GPS 
measurement or alternatively both signals may be logged to a data recorder 
and the calculations performed later. Data-logging may be performed in 
addition to a real-time calculation so that the position solutions may be 
double-checked later, or the raw data may be archived for QA (quality 
assurance) purposes. 
The accuracy of the relative position determination achieved using 
apparatus according to the invention will depend in part on the quantity, 
or more importantly the duration, of data received from the base station. 
By providing billing means arranged to measure the duration of a 
transmission to the mobile station, the user may be charged for the length 
of time for which data is transmitted to the mobile station which relates 
directly to the quality of the relative position determination. 
The quality of the position determination may further be enhanced by making 
additional relative determinations using base station satellite data 
received from alternative adjacent base stations. This allows 
independently determined position solutions to be compared. 
Each base station may include a local database including information 
relating to its own location, the location of adjacent base stations, the 
telephone number for the service for the adjacent base stations, and/or 
predetermined GPS measurements. The base stations are preferably 
interconnected using existing land lines to permit the interchange of at 
least information related to the relative locations of adjacent base 
stations. Preferably the location information held in the local databases 
is coordinated by at least one central computing centre (CCC), which is 
connected to each base station. 
According to a second aspect of the invention, a cellular radio system 
includes at least one base station satellite receiver and is operable to 
transmit base station satellite data including data representative of a 
carrier phase measurement derived from a satellite signal received by the 
base station satellite receiver for reception by remote position 
determination means comprising a mobile station coupled to a remote 
satellite receiver, for use in computing the position of the remote 
determination means relative to the base station based on the base station 
satellite data received by the remote satellite receiver. 
According to a third aspect of the invention, a method of position 
determination using a cellular radio infrastructure having a plurality of 
base stations are of known position and include a respective satellite 
receiver, wherein the method comprises transmitting from at least one of 
the base stations via a cellular radio link, base station satellite data 
derived from a satellite signal received by the base station satellite 
receiver, wherein the data includes data representative of a carrier phase 
measurement and is transmitted to a cellular mobile station coupled to a 
local satellite receiver, and wherein the method further comprises using 
the base station satellite data received via the cellular radio link to 
determine the position of the mobile station relative to the said base 
station based on local satellite data received by the local satellite 
receiver. 
According to a fourth aspect of the invention a method of operating a 
cellular radio infrastructure having at least one base station including a 
base station satellite receiver comprises transmitting base station 
satellite data including data representative of carrier phase measurement 
derived from satellite signals received by the base station satellite 
receiver for reception by a cellular mobile station and for computation of 
the position of the mobile station relative to the base station based on 
data received by a satellite receiver associated with the mobile station. 
According to a fifth aspect of the invention a method of operating a mobile 
position-determining unit which includes a cellular mobile station coupled 
to a local satellite receiver, receiving at least a carrier phase 
measurement from a cellular base station of known position and forming 
part of a cellular radio infrastructure and computing the position of the 
mobile unit relative to the base station position based on the local 
satellite data and the remote satellite data. 
According to a sixth aspect of the invention, there is provided a method of 
determining the position of a plurality of cellular base stations forming 
part of a cellular radio system, each base station having a base station 
satellite position fixing system, wherein the method comprises determining 
the position of at least one base station relative to the position of a 
reference satellite position-fixing system located at a known position by 
passing correction information derived from the reference position fixing 
system to the said at least one base station to permit correction of the 
base station position determined by the respective base station position 
fixing system thereby to determine the position of the base station more 
accurately, and using the more accurately determined position of the said 
base station to permit the said base station to replace the reference 
satellite position-fixing system as a reference for the more accurate 
determination of the position of another of the base stations. 
The external reference satellite position fixing system may be one of the 
above mentioned IGS receivers. Starting with this known reference, the 
accurate position of each of the base stations may be determined using 
relative GPS carrier phase measurements initially between the external 
reference system and base stations and then between adjacent base stations 
throughout the whole cellular system. Preferably, as many base station 
positional measurements as possible are taken directly from one or more 
external reference satellite position fixing systems to minimize the 
cumulative effect of errors as the base station position determinations 
propagate through the cellular system. 
Each base station may undertake the abovementioned measurement of its own 
position relative to adjacent base stations as a background task over many 
hours or days. It may do this using a differential GPS measurement using 
an adjacent base station as the master station. It will be appreciated 
that since in a differential GPS measurement the position of the master 
station is assumed to be known, any errors in the position of the master 
station will translate directly into an equivalent error in any 
measurement taken using that master station. Thus, in effect, the 
measurement is always made relative to the master station. 
Relative position data (typically in the form of position vectors of a base 
station relative to the external reference system) are preferably 
communicated back to the CCC which can monitor trends in movements of a 
particular base station and can determine the correct functioning of the 
satellite receiving equipment of each base station by checking the 
relative positions of adjacent base stations against their expected and/or 
previous fixes. The check will also highlight movement of the base station 
such as may be caused by mining subsidence. The CCC preferably also 
operates to scale, orient and adjust the determined relative positions of 
the base stations to ensure that their determined absolute position fixes 
fit plausibly within the network of external reference satellite position 
fixing systems. It will be noted that a self-determined relative base 
station position measurement can be cross-referenced with a measurement 
based on any available adjacent base station. Thus the CCC has a margin of 
redundant data with which to determine where errors or movements seem to 
be occurring. The CCC adjustment may be performed using "least squares" 
optimisation algorithms. 
Similarly, when a new base station is installed, its position may be 
determined with reference to adjacent base stations. In this way, the 
system can be expanded and made self-calibrating. Periodically, the 
network-wide and local databases may be updated with revised position 
calculations for each of the base stations. At the same time, the updated 
positions may be published to the surveying community. 
Thus, in summary, the invention provides accurate, reliable and convenient 
positioning information requiring the user to have only a cellular mobile 
station such as a mobile telephone and a satellite receiver (typically a 
GPS receiver). If several base stations are equipped to make their 
satellite data available to mobile cellular stations, the user can roam 
between the coverage areas of the different base stations without having 
to set up differential position determination equipment at different 
locations of known positions. Existing cellular radio system hardware may 
be used in particular the GPS receivers in the base station. Mobile GPS 
receivers and cellular stations having data transmission capabilities are 
already available. 
The GPS unit installed at the cellular base station is preferably of a type 
that can provide both the timing outputs required to time-synchronise the 
cellular communications, and the raw measurements required for the 
differential positioning applications. This arrangement has significant 
cost benefits since the GPS receiver is required for time synchronisation 
of the cellular network and the ability to offer a surveying service may 
therefore be offered by adding features to the GPS receiver at relatively 
small cost. An alternative implementation is to have separate GPS timing 
and position determining units to produce the time synchronisation and the 
above-mentioned data. 
Given the built-in call duration measurement and billing means found in a 
cellular telephone network, and the good correlation between length of 
availability of base station satellite data and accuracy of measurement, 
the presently available infrastructure is well suited to 
position-determination-accuracy related billing. This continuous flow of 
revenue is better suited to a position determination network which 
inevitably requires continuous maintenance rather than the historical 
system in which income was generated sporadically. 
The invention is readily able to monitor its own performance and to 
calibrate new base stations as described above. Thus maintenance and 
expansion can be partly automated. 
Due to the nature of cellular mobile transmissions, it is already necessary 
to have a relatively dense network of base stations in urban areas. Denser 
base station distribution can provide greater accuracy since the distance 
between the base station and position to be determined is less (therefore 
enhancing the effect of differential measurements) and furthermore such a 
dense distribution allows an increased number of adjacent base stations to 
be used for independent accuracy checks to be made. 
A connection to a base station may be made using a conventional dial-up 
connection. This may be automated by the provision by the base station of 
the numbers of suitable adjacent base stations for additional checks on 
the accuracy of the data. 
A mobile unit, regardless of its location within the cellular 
infrastructure, can be arranged to call a unique number corresponding to 
the type of service required (eg raw measurements or measurement 
corrections required) which would then automatically route its call to the 
nearest cellular base station satellite data source. A separate set of 
unique numbers may then be used automatically to route the call to the 
second, third or more closest cellular base stations if independent check 
measurements are required. The cellular infrastructure already stores 
information indicating which cell the mobile unit is in and can therefore 
be automatically arranged to locate adjacent cells. 
An alternative technique is automatically to communicate data between 
adjacent sites and store the adjacent base station coordinates and/or raw 
satellite measurements locally in the adjacent cell data base so that only 
one base station need be accessed by the user to obtain a first 
measurement and one or more independent check measurements. 
It will be appreciated that the IGS network is cited as one example of a 
highly accurate external positional reference network. Other such networks 
may be used to tie the base station positions into an external frame of 
reference, it being necessary only that the positions of the reference 
network nodes be known to an accuracy at least as good as that required of 
the position-determining apparatus. Furthermore, GPS-related terms 
(including measurement types) should be construed to indicate their 
equivalent in other satellite-based navigation systems such as the former 
Soviet Union's GLONASS system. Additionally, the apparatus is not limited 
to use with a GSM cellular radio system.

With reference to FIG. 1, which shows the fixed parts of the system in 
accordance with the invention, a plurality of cellular base stations 2 
comprise a cellular transceiver 4 for controlling and communicating with a 
cellular mobile station, a GPS receiver 6, a local cell database 8 and an 
adjacent cell database 10. 
The GPS receiver 6 is conventionally used to provide a timing and frequency 
reference to the cellular transceiver 4. As part of the derivation of a 
timing solution, the receiver 6 generates time-tagged pseudo-range and 
carrier phase data derived from received GPS satellite signals, given the 
accurate coordinates for the position of the receiver 6 sufficient 
information is then available to compute accurate differential GPS 
corrections for both carrier phase and pseudo-range measurements. The 
receiver 6 preferably has the capability of tracking all satellites above 
the horizon in order to provide high accuracy measurements. Typically the 
measurements are pseudoranges, allowing accuracy to within plur or minus 
10 to 20 cm, and carrier phase, allowing accuracy to within plus or minus 
1 mm or less. 
Computed corrections for the carrier phase and pseudo-range measurements 
are fed into the local cell database 8. When a user wishes to make a 
position determination, the base station is called and its position and 
correction data are relayed from the local cell database 8 to the 
transceiver 4 for onward transmission to the user's mobile station. 
Alternatively, raw GPS measurements and the base station coordinates may 
be transmitted to the mobile station for local measurement correction 
calculations to be performed. 
The GPS receiver 6 also operates, as a background task, to compute its 
position relative to an adjacent base station site, typically to an 
accuracy of between 1 and 3 cm. This computation is performed as a check 
of the correct functioning of the base station systems and therefore can 
be performed over several hours. The computation is performed by receiving 
measurement correction information from adjacent cell base stations via 
the adjacent cell database 10. These corrections are then used to estimate 
a correction for the base station's own position determination and thereby 
to derive a position determination relative to the adjacent base station. 
This relative position vector once calculated is passed via an intercell 
communications land line 12 to a network-wide database 14. The GPS 
receiver 6 may then select an alternative adjacent base station and 
perform an identical computation. The network-wide database 14 then holds 
a series of relative position vector measurements for each adjacent base 
station pair which may be used to calibrate the network as described 
below. 
Having performed a position determination using satellite data (i.e. 
measurement corrections or raw GPS data and coordinate data) from one 
cellular base station 2, a user may wish to repeat the determination using 
an alternative base station 2 as a quality or confidence assessment. To 
facilitate this, the adjacent cell database 10 holds details of adjacent 
cell base stations such as a telephone number and any other necessary 
access information. This information is passed back to the user via the 
transceiver 4. The user may then transfer his call to another base station 
2 using this information and may then perform a further position 
determination. 
It may be desired to provide the determining means with measurement data 
and/or correction data from several cellular base stations simultaneously 
thus allowing the computation of its position relative to the same several 
base stations using either of the known GPS multi-baseline or GPS network 
computation techniques. 
Information related to usage of the base station and in particular the 
duration of any calls made to the base station, are passed to the 
network-wide database 14 and then onto a customer charging system 16. 
A central computing centre (CCC) 18 is connected to the network-wide 
database 14 (typically using an integrated services digital network (ISDN) 
connection). Amongst other functions, the CCC periodically reads the 
computed intercell relative positions computed by each base station 2, 
from the network-wide database 14. The CCC uses these relative positions 
and solution quality information to calculate new solutions for the 
network base station coordinates. The solutions for these coordinates are 
automatically positioned, orientated and scaled to fit within accurately 
known base station position coordinates derived directly from an accurate 
external reference framework such as the positions of the IGS recording 
stations. Periodically (typically every six or twelve months) the 
CCC-computed base station coordinates are published and entered into the 
network-wide database for use by each base station in its own GPS 
measurements. The coordinates are typically derived using least squares 
optimisation techniques. 
The CCC 18 also computes position solutions for those base stations 2 which 
can be computed relative to an IGS tracking station. This is performed by 
downloading IGS tracking data, site coordinates and precise ephemerides 
from the IGS database 20 via an Internet connection Typically this data is 
available in the RINEX format (see above). 
The CCC 18 may when it is reading the relative position measurements 
computed by the base stations monitor the performance of each base 
station- Since the network-wide database 14 contains many interrelated 
relative position measurements and in particular will usually have a 
relative measurement between two base stations performed by each 
respective base station, it is a relatively trivial task for the CCC to 
determine which base station is in error if unlikely relative measurement 
results are produced. This permits the system to continuously monitor its 
own performance and highlight problems automatically. Furthermore, a 
similar technique may be used by the CCC to calibrate a new base station 
site by updating the new stations adjacent cell database 10 to cause it to 
perform repeated relative measurements with its adjacent base stations 
thereby providing a set of data from which the new base station's position 
can be determined. Once such a position has been determined it can be 
published on the network-wide database 14 and the relevant adjacent cell 
databases 10 can be updated. It is expected that each base station may 
have its position determined to a confidence level of between 1 and 2 cm. 
With reference to FIG. 2, a user's mobile position determination means 
includes a cellular telephone 50 which may be of a digital type operable 
directly to transmit digital data to the transceiver 4 or may be of the 
analogue type having a connection via a modem 52 to the base station 2. 
Satellite data received from the base station transceiver 4 is communicated 
to a GPS data processor 54. 
The data processor 54 is connected to a GPS measurement sensor 56 which 
passes locally measured pseudoranges and carrier phase measurements to the 
data processor 54. Depending on the quality of the GPS sensor 56 and the 
quantity of the GPS base station measurement and/or correction data 
received via the cellular telephone 50, a position solution for the 
determination means may vary in accuracy from 10 metres down to a few 
millimetres. The accuracy produced may be tailored to a user's particular 
requirements. Typically the GPS sensor 56 and data processor 54 will be in 
a single GPS positioning system 58. 
User command inputs and outputs such as position and quality may be issued 
and viewed via a control display unit (CDU) interface 60. 
The computed positions and/or the raw GPS data (both local and from the 
base station) may be logged to a data storage device 62 or fed out to an 
external device via a connection 64. This permits additional checks and 
measurements to be made at a later time, or data may be archived for QA 
(quality assurance) purposes. 
The GPS sensor 56, the data processor 54, the CDU interface 60, the data 
storage device 62 and the position output connection 64 may conveniently 
be packaged into a single unit. This may then be connected directly by a 
wire link to a digital cellular telephone for communication with the base 
station. Alternatively, the cellular telephone may be packaged with the 
above-mentioned components to provide a single device capable of 
determining position. This may be packaged in the form of a cellular 
mobile telephone with an integral GPS and cellular radio antenna. 
FIG. 3 shows a schematic diagram of the complete system. A satellite signal 
70 is received by GPS receiver 6. Measurement corrections are passed to 
cellular transceiver 4 for onward transmission to cellular telephone 50. 
The measurement corrections are communicated to the GPS positioning system 
58 which uses these corrections to determine a position relative to the 
base station 6 based on satellite signals 72 received locally by the 
mobile station from GPS satellites 74.