HK1079003B

HK1079003B - Methods and apparatuses for measuring frequency of a basestation in cellular networks using mobile gps receivers

Info

Publication number: HK1079003B
Application number: HK05110811.1A
Authority: HK
Inventors: N．F．克拉斯那
Original assignee: 高通股份有限公司
Priority date: 2002-04-15
Filing date: 2003-04-14
Publication date: 2009-04-24

Description

Method and apparatus for measuring base station frequency in cellular network using mobile GPS receiver

RELATED APPLICATIONS

This application claims priority to U.S. provisional application No.60/372,944, filed 4/15/2002.

Technical Field

The present invention relates to the field of cellular communication systems, and more particularly to those systems in which the location of a mobile cellular communication station (MS) is determined.

Background

For position location in a cellular network (e.g., a cellular telephone network), several schemes for performing triangulation are based on using timing information sent between several base stations and a mobile device such as a cellular telephone. In one scheme, known as Time Difference of Arrival (TDOA), signals received from a mobile device are measured at several base stations and sent to a position determination entity, known as a location server, which uses the times of reception to calculate the position of the mobile device. To perform such a scheme, the exact locations of the base stations need to be known, and the time-of-day (time-of-day) of these base stations needs to be coordinated to provide an accurate measurement of the location. Time coordination is an operation that maintains the time of day associated with multiple base stations within a specified margin of error at a specified time.

Fig. 1 shows an example of a TDOA system, where the time of reception (TR1, TR2, and TR3) for the same signal from mobile cellular telephone 111 is received at cellular base stations 101, 103, and 105 and processed by a location server 115. The location server 115 is coupled to data received from the base stations through the mobile switching center 113. The mobile switching center 113 provides signals (e.g., voice communications) to and from a landline Public Switched Telephone System (PSTS) whereby signals can be transferred from a mobile telephone to other telephones (e.g., landline telephones or other mobile telephones on the PSTS) or vice versa. The location server may also communicate with the mobile switching center over a cellular link in some cases. The location server may also monitor transmissions from several base stations to determine the relative timing between the transmissions.

An alternative method, known as Enhanced Observed Time Difference (EOTD) or Advanced Forward Link Triangulation (AFLT), measures the Time of arrival of signals transmitted from each of several base stations at a mobile device. Fig. 1 is applicable in this case if the arrows TR1, TR2 and TR3 are reversed. The timing data may then be used to calculate the location of the mobile device. These calculations may be calculated at the mobile device itself or at a location server if the timing information obtained by the mobile device is transmitted to a location server over a communication link. Again, the time of day of the base stations must be adjusted and their locations accurately estimated. In other arrangements, the location of the base station may be measured by standard measurement means and stored in the base station, a location server, or some computer memory elsewhere in the network.

A third method of position location includes using a receiver for a Global Positioning System (GPS) or other Satellite Positioning System (SPS) in a mobile device. This approach may be completely autonomous or may use a cellular network to provide assistance data or sharing in location calculation. Examples of such methods are described in U.S. Pat. Nos. 5,841,396, 5,945,944, and 5,812,087. For simplicity, we refer to these different methods as "SPS". In practical low cost implementations, both the mobile cellular communications receiver and the SPS receiver are integrated into the same package (enclosure) and may in fact share common electronic circuitry.

The combination of using EOTD or TDOA on an SPS system is referred to as a "hybrid" system.

It should be clear from the above description that for EOTD, TDOA, or hybrid systems, time adjustments between different cellular base stations are necessary to accurately calculate the location of the mobile device. The accuracy of the time of day required at the base station depends on the details of the positioning method used.

In another variation of the above method, the signal transmitted from the base station to the mobile device and back may have a Round Trip Delay (RTD). Similarly, in another approach, there is also a round trip delay for signals sent from the mobile device to the base station and back. Each round trip delay is divided into two to determine an estimate of the one-way time delay. Knowledge of the location of the base station, plus a one-way delay, constrains the location of the mobile device to a circle on the earth. Two such measurements result in the intersection of two circles, which limits the position to two points on the earth. The third measurement (even the angle of arrival or sector of the cell) disambiguates this ambiguity. With the round-trip delay method, it is important that the adjustment of the RTD measurement must be obtained within a few seconds in the worst case, if the mobile device is moving rapidly, the measurement corresponding to the mobile device will be in the vicinity of the same location.

In many cases, it is not possible to make round-trip measurements for each of the two or three base stations, but only one of them, the primary base station with which the mobile device is communicating. For example, the IS-95 north american CDMA cellular standard IS applied. Or due to device or signaling protocol limitations, it is not possible to make accurate (e.g., sub-millisecond) round trip delay timing measurements at all. This situation arises when the GSM cellular communication standard is applied. In these cases, since only the time differences between the different mobile-base station paths are used, it becomes more important to maintain accurate timing (or relative timing) at the base station transmission if the triangulation operation is to be performed.

The reason for maintaining accurate timing information at the base station is to provide time to the mobile device for assisting in GPS-based location calculations; such information can reduce the time to first fix (first fix) and/or increase sensitivity. U.S. Pat. Nos. 6,150,980 and 6,052,081 include such examples. The accuracy required for these cases may range from a few milliseconds to about 10 microseconds, depending on the performance improvement desired. In a hybrid system, base station timing serves two purposes, improving TDOA (or EOTD) operation as well as GPS operation.

The prior art inter-base station tuning uses a dedicated fixed location timing system, referred to as a Location Measurement Unit (LMU) or a Timing Measurement Unit (TMU). These units typically include a fixed position GPS receiver that enables accurate time of day determinations. The location of the unit is measurable, such as with GPS-based measurement equipment. In an alternative implementation, the LMU or TMU may not rely on absolute time provided by the GPS receiver or other source, but rather may relate only to the timing difference that exists between one base station and the other. However, this alternative (without the use of a GPS receiver) relies on the observation of multiple base stations provided by a single entity. Furthermore, this approach can promote errors that accumulate across the network.

In general, the LMU or TMU observes the timing signals, such as framing markers, present in the cellular communication signals transmitted from the base station and attempts to time-tag (time-tag) these timing signals using the local time found by a GPS group or other timing determination device. Messages may be sent to the base station (or other infrastructure component) in turn, which allows these entities to keep track of these elapsed times. Then, upon command or periodically, a specific message may be sent over the cellular network to the mobile devices served by the network to indicate the signal frame structure associated with the time of day. This is very easy for systems such as GSM where the entire frame structure lasts for a period of more than 3 hours. Note that the local measurement unit may be used for other purposes, e.g. as a location server-i.e. the LUM may indeed make time-of-arrival measurements from the mobile device to determine the location of the mobile device.

One problem with these LMUs or TMUs is that they require the construction of new dedicated fixed equipment at each base station or other sites within communication range of several base stations. This results in high installation and maintenance costs.

Disclosure of Invention

Methods and apparatus for frequency synchronizing base stations in a cellular communication system are desired.

According to one aspect of the present invention, a method for measuring frequencies associated with a base station in a cellular communication system is provided. The method comprises the following steps:

receiving, at a first mobile station, a first cellular signal from a base station, the first cellular signal including a first timing marker;

determining a first time stamp of a first timing marker from at least one satellite positioning system signal received by a first mobile station;

determining a first position of a first mobile station from at least one satellite positioning system signal received by the first mobile station;

transmitting the first time stamp and the first location to a server over a cellular communication link;

receiving, at a second mobile station, a second cellular signal from the base station, the second cellular signal including a second timing marker;

determining a second time stamp of a second timing marker from at least one satellite positioning system signal received by a second mobile station;

determining a second position of the second mobile station from at least one satellite positioning system signal received by the second mobile station;

transmitting the second time stamp and the second location to the server over a cellular communication link; and

combining, at the server, a location of a base station with the first and second time stamps and the first and second locations to calculate a first frequency associated with the base station.

In accordance with another aspect of the present invention, a system for measuring frequencies associated with a base station is provided. The system comprises:

the first mobile station includes:

a first satellite positioning system receiver configured to receive at least one first satellite positioning system signal and determine a first position of a first mobile station from the at least one first satellite positioning system signal; and

a first cellular transceiver coupled to the first satellite positioning system receiver, the first cellular transceiver receiving a first cellular signal containing a first timing marker from a base station; and

first circuitry coupled to the first cellular receiver and the first satellite positioning system receiver, the first circuitry to determine a first time marker of the first timing marker using the at least one first satellite positioning system signal;

the second mobile station includes:

a second satellite positioning system receiver configured to receive at least one second satellite positioning system signal and determine a second position of a second mobile station from the at least one second satellite positioning system signal; and

a second cellular transceiver coupled to the second satellite positioning system receiver, the second cellular transceiver receiving a second cellular signal containing a second timing marker from the base station; and

second circuitry coupled to the second cellular receiver and the second satellite positioning system receiver, the second circuitry to determine a second time stamp of the second timing marker using the at least one second satellite positioning system signal;

a server coupled to the first and second mobile stations via a communication link, the first cellular transceiver transmitting the first time stamp and the first location to the server via the communication link, the second cellular transceiver transmitting the second time stamp and the second location to the server via the communication link; the server combines the location of the base station with the first and second time stamps and the first and second locations to calculate a first frequency associated with the base station.

According to yet another aspect of the present invention, there is provided a method of predicting timing of base station transmissions in a cellular communication system, comprising: receiving a first time stamp for a first timing marker in a first cellular signal transmitted from a base station; receiving a second time stamp for a second timing mark in a second cellular signal transmitted from the base station; and calculating a frequency associated with the base station using the first and second time stamps. Each time stamp is determined using at least one satellite positioning system signal received at the mobile station, which also receives a corresponding timing mark contained in the cellular signal from the base station. In one example in accordance with this aspect, the time stamp is determined from a time of day message in the satellite positioning signal. In another example according to this aspect, the difference between the at least two time markers is determined from a local reference signal, the frequency of which is determined from processing of the satellite positioning signals.

According to another aspect of the present invention, there is provided a method of measuring a frequency associated with a base station, comprising: receiving at least one satellite positioning system signal at a mobile station; determining a reference signal frequency from a local oscillator of the mobile station from at least one satellite positioning system signal; receiving at the mobile station a cellular signal from the base station, the cellular signal being modulated on a carrier; measuring a carrier frequency using a reference signal from a local oscillator; and determining a frequency associated with the base station using the carrier frequency.

The present invention includes apparatus for performing these methods, data processing systems and machine-readable media for performing these methods, which when executed on data processing systems perform these methods as described above.

Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.

Brief Description of Drawings

The present invention is illustrated by way of example and not limited in the accompanying figures in which like references indicate similar elements.

Figure 1 shows an example of a prior art cellular network for determining the location of a mobile cellular device.

Figure 2 shows an example of a mobile cellular communication station that may be used with the present invention and that includes a GPS receiver and a cellular communication transceiver.

Fig. 3 shows a block diagram representing a combined mobile station that may be used with the present invention and that shares a common circuit between a GPS receiver and a cellular communication transceiver.

Fig. 4 shows an example of a cellular base station that may be used in various embodiments of the present invention.

Fig. 5 shows an example of a server that can be used in the present invention.

Fig. 6 illustrates a network topology for measuring signal frequencies of base stations in accordance with an embodiment of the present invention.

Fig. 7 shows a frame structure of a GSM cellular signal.

Fig. 8 shows a flow diagram for determining a frequency of a base station in accordance with an embodiment of the invention.

Fig. 9 illustrates a detailed method for determining the frequency of a base station signal by measuring the time of occurrence (epoch) of a frame of the base station signal in accordance with an embodiment of the present invention.

Fig. 10 illustrates another method for determining the frequency of a base station signal by measuring the time of occurrence (epoch) of a frame of the base station signal in accordance with an embodiment of the present invention.

Fig. 11 shows a detailed method for determining the frequency of a base station signal by measuring the carrier frequency of the base station signal according to an embodiment of the invention.

Detailed Description

The following description and drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, in certain instances, well-known or conventional details are not described in order to avoid obscuring the description of the present invention.

In most digital cellular systems, the numbered frame markers are transmitted as part of the cellular system transmission. In a network such as GSM, time of day information from a GPS receiver may be used to time stamp the frame structure (e.g., framing markers) of a received communication (e.g., GSM) signal. For example, the start of a particular GSM frame boundary, which occurs once every 4.6 microseconds, may be used (see fig. 7). 2,715,648 such frames are included in each superframe (superframe) for 3.48 hours; each boundary of these frames is therefore unambiguous for all purposes achieved. Co-pending U.S. patent application serial No.09/565,212, filed on 5/4/2000, describes a method of time adjustment in which a Mobile Station (MS) including a GPS receiver is used to measure time of day and position with high accuracy. The time stamp information of the cellular frame structure measured at the mobile station is transmitted by ordinary cellular signaling to a Base Station (BS) (e.g., the cellular base station shown in fig. 4) or other network entity (e.g., a server or location server) to determine the time of day of the base station. The delay due to the propagation time from a Mobile Station (MS) (e.g., a mobile cellular communication station shown in fig. 2) to a Base Station (BS) can be determined (typically at the base station or other network entity) by dividing the BS-MS distance by the speed of light, because the mobile station determines its location via a GPS unit and the base station knows its exact location (e.g., via measurements). The base station then determines the timing of its transmitted framing marker by simply subtracting the calculated propagation time from the time stamp of the framing marker provided by the mobile station.

Closely related to the time adjustment between base stations is the frequency adjustment (or synchronization) between the base stations. Once established, it is expected that the time adjustment may be maintained for a long period of time. Otherwise such time adjustments need to be made frequently, which would be a complex and expensive operation. For example, the base stations may adjust the time by sending signals back and forth between them over existing communication channels (e.g., cellular channels). If such signaling requires that on a continuous basis, valuable communication resources are wasted, which may otherwise be used to transmit other voice and data information.

To avoid frequent time adjustments, it is desirable to have accurate measurements of the primary signal resource frequency or frequencies of base station resources associated with other base stations at each base station. If the frequency of the primary signal resource of a base station is known with high accuracy, it can be maintained for a long period of time by using a time interval counter when the time of day at these base stations is adjusted.

At least one embodiment of the present invention seeks to frequency adjustment between base stations. The method according to the invention uses a normal mobile cellular communication receiver equipped with GPS positioning capabilities, without using fixed and expensive network resources.

One embodiment of the present invention uses a cellular transmission timing marker (e.g., a framing marker) for frequency synchronization. Measurements of the base station framing marker transmission frequency are used to provide an estimate of the error between the best and true timing between successive framing markers. The error can be propagated forward in time as a function of the index number by using a standard curve fitting type algorithm. Thus, when the initial framing marker timing is determined and a good estimate of the framing marker rate (or error other than the nominal rate) is determined, the occurrence time of the framing markers can be used as an accurate clock for a long time.

Another embodiment of the invention uses the carrier frequency of the cellular frequency for frequency synchronization. In most cases, the framing markers and carrier frequency of the cellular signal from the basestation are synchronized to the same basestation reference signal generator. Thus, by simple mathematical calculations, the frequency of the framing marker of the basestation signal can be determined from the carrier frequency of the cellular signal.

In at least one embodiment of the invention, the frequency of the framing marker transmitted by the cellular base station transmitter is determined for frequency adjustment. However, the framing markers and signal symbols (assuming digital modulation) and the signal carrier frequency are typically all synchronized to a common master oscillator (e.g., oscillator 413 in fig. 4) in a digital cellular system. In several important cellular systems, including the GSM system, the japanese PDC system, and the WCDMA system, the frequency of the timing signal (e.g., framing markers) and the carrier frequency are derived from the same basic oscillator. Therefore, accurate measurements of such transmission timing marks (symbol rates) or carrier frequencies can be used to achieve the same goal. The frequency of the in-wave may be used to infer the frequency of the transmission and vice versa. The advantages or disadvantages of either measurement relate to the details of implementation and the accuracy of the measurement.

In one embodiment, one or more mobile stations make one or more timing measurements of the received base station signal and transmit these time stamps and optional additional information to a server, which in turn performs frequency calculations.

In another embodiment, one or more mobile stations measure the carrier frequency of the received base station signal and transmit information about the carrier frequency and optionally additional information to a server.

In one embodiment, one or more mobile stations each make at least two timing measurements of a received base station signal, calculate a frequency (or, equivalently, a time interval) measurement based on the measurements, and send the frequency measurement to a server.

In various embodiments, the server collects continuous data from the mobile stations for further processing for better estimation of frequencies, or curve fitting operations on these frequency versus time information (frequency over time information).

It is to be appreciated that the cellular base station transmission frequency can be calculated at a Base Station (BS), a Mobile Station (MS), or a server (e.g., a location server or other network entity).

Thus, in order to time synchronize base stations (equivalently, determine the timing of the markers transmitted from these base stations), the methods according to the present invention determine the frequency of such transmissions from the base stations, which is an important part of the time synchronization problem, as previously described. Details of these methods will be described below.

Fig. 2 shows an example of a mobile station comprising a GPS receiver, which can be used in the present invention. The GPS receiver can determine with high accuracy the time of day the signal was received (e.g., the timing marker at which the cellular signal was received at transceiver 213), the location of the receiver, and the frequency of the externally provided signal. Measurements of time of day, location and frequency may be run in an autonomous mode if the level of the received signal is high, or with the aid of equipment in the infrastructure (server) when the signal-to-noise ratio of the received signal is low (see, for example, U.S. Pat. nos. 5,945,944, 5,841,396 and 5,812,087).

The mobile station 210 shown in fig. 2 includes a GPS receiver 211 connected to a GPS antenna 203 and a cellular communication transceiver 213 connected to a communication antenna 201. Alternatively, the GPS receiver 211 may be contained on another chassis; in this case, the mobile station 210 does not include a GPS receiver and is not required to have it when the GPS receiver is coupled to and co-located with the mobile station 210.

The GPS receiver 211 may be a conventional, hardware correlator based GPS receiver, or may be a matched filter based GPS receiver, or may be a GPS receiver that uses a buffer to store the digitized GPS signals that are processed by the fast convolution, or may be a GPS receiver as described in U.S. patent No.6,002,363, where components of the GPS receiver are shared with components of the cellular communication transceiver (e.g., see fig. 7B of U.S. patent No.6,002,363, which is incorporated herein by reference).

The cellular communication transceiver 213 may be a modern cellular telephone operable with any one of the well-known cellular standards including: GSM standard, or japanese PDC communication standard, or japanese PHS communication standard, or AMPS analog communication standard, or north american IS-136 communication standard, or asynchronous wideband spread spectrum CDMA standard.

The GPS receiver 211 is coupled to the cellular communication transceiver 213 to provide GPS time and location, and in one embodiment, to the cellular communication transceiver 213 (which in turn transmits this information to a base station). In another embodiment, the GPS receiver 211 provides assistance in the precise measurement of the carrier frequency of the cellular signal received by the transceiver 213.

In one embodiment, GPS time may be obtained at the mobile station 210 by reading GPS time of GPS signals from GPS satellites. Alternatively, a technique for determining time as described in U.S. Pat. No.5,812,087 may be used. In such an arrangement, samples of the GPS signals received at the mobile station may be transmitted to a location server or some other server where the signal samples are processed as described in U.S. patent No.5,812,087 to determine the time of reception. In addition, the time of day may also be calculated using one of one or more of the various methods described in U.S. Pat. No.6,215,442.

In addition, the cellular communication transceiver 213 may provide assistance information, such as Doppler (Doppler) information or time information, to the GPS receiver, as described in U.S. Pat. Nos. 5,841,396 and 5.945,944. The coupling between the GPS receiver 211 and the cellular communication system transceiver 213 may also be used to transmit GPS data to or from a cellular base station for matching the record with another record to determine the time at the GPS receiver, as described in U.S. patent No.5,812, 087. In those cases or embodiments where a location server is used to provide assistance data to the mobile cellular communication system for determining the location or time at the mobile station 210, or where a location server is shared in the information processing (e.g., location server determines the time or final location calculation for the mobile station 210), it is recognized that a location server such as that shown in fig. 5 and described further below is connected to a cellular base station over a communication link to assist in the processing of the data.

The position of the mobile station 210 is typically not fixed and is typically not predetermined.

Fig. 3 illustrates a block diagram representing a combined mobile station that may be used with the present invention and in which common circuitry is shared between a GPS receiver and a cellular communication transceiver. Combined mobile station 310 includes circuitry for performing the functions required for processing GPS signals as well as the functions for processing communication signals received over cellular communication link 360 to and from base station 352.

The mobile station 310 is a combined GPS receiver and cellular communication transceiver. The acquisition and tracking circuit 321 is coupled to the GPS antenna 301 and the communication transceiver 305 is coupled to the communication antenna 311. Oscillator 323 provides a reference signal to circuit 321 and to communication receiver 332. The GPS signal is received by the GPS antenna 301 and input to the circuit 321, and the circuit 321 obtains signals received from a plurality of satellites. Processor 333 processes data generated by circuitry 321 for transmission by transceiver 305. The communication transceiver 305 includes a transmit/receive switch 331, the transmit/receive switch 331 routing communication signals (typically RF signals) to and from the communication antenna 311. In some systems, a band-split filter, or "diplexer", is used in place of the T/R switch. The received communication signal is input to the communication transceiver 305 and transmitted to the processor 333 for processing. Communication signals to be transmitted from processor 333 are propagated to modulator 334, frequency converter 335, and power amplifier 336. U.S. patent No.5,874,914, which is incorporated herein by reference, describes details regarding a mobile station that includes a GPS receiver and a cellular transceiver and that uses a communication link.

The carrier frequency of the cellular signal from the base station can be measured using a GPS receiver and using a variety of methods. In one approach, the communication receiver 332 is frequency or phase locked to a carrier received from a base station. This is typically done with the aid of a voltage controlled oscillator (VDO) (e.g., oscillator 323) in a phase or frequency locked loop configuration, which may be controlled by a signal from the communication receiver on line 340. The long period frequency of the VCO may be proportional to the carrier frequency transmitted by the base station (after the doppler frequency offset due to the velocity of the mobile station is removed). The output of the VCO may then be used as a frequency reference for the GPS receiver down-conversion circuitry (e.g., used by the acquisition and tracking circuit 321). As part of the signal processing in the GPS receiver, a frequency error is determined for each received GPS signal received from several GPS satellites. Each such received signal may also contain a common component of such frequency error, such as due to VCO error relative to an ideal value. The frequency error from the VCO (referred to as the "offset" frequency) may then be determined and scaled (scaled) to determine the base station frequency after the doppler induced frequency offset due to the movement of the mobile station is removed.

It is well known that such a "common mode" frequency offset can be obtained in the GPS processing. The received frequency error is due to a combination of receiver motion and common mode offset. User movement is described by a three-component velocity vector. Thus, including the common mode offset, a total of four relevant frequencies are unknown and need to be addressed. Signals received from four different GPS satellites will generally be able to solve the four equations and thus the common mode offset due to VCO error. Performing multiple sets of frequency measurements in a single cycle can further reduce the number of GPS satellite signals that must be received. Similarly, limiting the speed of the receiver (e.g., assuming a slight amount of movement in the Z-axis) may further reduce the number of satellite signals required to be received.

As an alternative to the above, a GPS receiver may have a reference signal that is independent of that used by the cellular transceiver VCO. In this case, the GPS receiver again determines the frequency of its reference signal (typically from a crystal oscillator). The output of the cellular transceiver VCO and the reference signal for the GPS receiver may both be sent to a frequency counting circuit that determines the frequency ratio of the two reference signals in a manner well known in the art. Since the reference signal for the GPS receiver has been determined, the frequency of the VCO of the cellular transceiver can be determined from the frequency ratio. Since the VCO is phase or frequency locked to the carrier of the arriving base station signal, the carrier frequency can then be determined from a simple scale conversion process. In order to eliminate the doppler frequency offset due to the motion of the mobile station relative to the base station, the position of the base station is generally required in addition to the velocity of the mobile station. The server that performs the final base station frequency calculation typically knows the location of the base station.

Fig. 4 shows an example of a cellular base station that may be used in various embodiments of the present invention. The base station 410 comprises a cellular transceiver 411 connected to at least one antenna 401 for communicating signals to and from mobile cellular communication stations currently located within the service area of the cellular base station 410. For example, mobile stations 210 and 310 may be mobile stations served by cellular base station 410. The cellular transceiver 411 may be a conventional transceiver for transmitting and receiving cellular signals, such as GSM cellular signals or CDMA cellular signals. The oscillator 413 may be a conventional system oscillator that controls the signal frequency of the base station. The frequency of the oscillator can be measured according to the method of the invention for frequency synchronization. In many cases, the oscillator 413 may be highly stable, but over time a small error in the oscillator frequency will cause the clock phase of the base station to drift a significant amount from the ideal value. An accurate measurement of the frequency of the oscillator can be used to predict the error in the base station clock and the error in the timing of the framing markers transmitted by the base station. The cellular base station 410 also typically includes a network interface 415, the interface 415 transferring data to and from the cellular transceiver 411 to couple the cellular transceiver with a mobile switching center 421, as is well known in the art. The cellular base station 410 may also include a co-located (co-located) data processing system 423. Alternatively, the data processing system 423 may be remote from the base station 410. In some embodiments, the data processing system 423 is coupled to the oscillator 413 to adjust or recalibrate the time of the clock to synchronize the clock with other clocks in other base stations, in a manner described in U.S. patent application No.09/565,212, filed on 5/4/2000. In many cases, clock 413 is highly stable but free running, but it can affect network operation to actually change the clock's epoch (epoch). The time associated with the clock epoch is adjustable. This is called "recalibration". Thus, there may be no connection between the data processing system 423 and the oscillator 413 for frequency synchronization. The data processing system 423 is coupled to the network interface 415 to receive data from the cellular transceiver 411, such as time stamp information of the framing markers measured by the mobile system for synchronization with other cellular base stations or to calculate the transmission frequency of the framing markers. In practice, a base station may include a physical tower structure, one or more antennas, and a set of electronics.

FIG. 5 illustrates an example of a data processing system that may be used as a server in various embodiments of the invention. For example, as described in U.S. patent No.5,841,396, the server may provide assistance information, such as doppler or other satellite assistance information, to a GPS receiver in the mobile station 210. Additionally or alternatively, the location server may perform a final location calculation in place of the mobile station 210 (after receiving pseudoranges (pseudoranges) from the mobile station or data from which pseudoranges may be calculated) and then transmit the location determination to the base station to enable the base station to calculate the frequency. Alternatively, the frequency may be calculated at a location server, other server, or other base station. The data processing system, which is a location server, typically includes a communication device 512, such as a modem or network interface, and is optionally coupled to a co-located GPS receiver 511. The location server may be coupled to several different networks through a communication device 512 (e.g., a modem or other network interface). These networks include a cellular switching center or multiple cellular switching centers 525, a land-based telephone system switch 523, cellular base stations, other GPS signal sources 527, or other processors of other location servers 521.

A plurality of cellular base stations are typically arranged to cover a geographical area with a radio coverage area and the various base stations are coupled to at least one mobile switching center as is well known in the art (e.g., see fig. 1). Thus, multiple instances of the base station 410 are geographically separated but coupled together by a mobile switching center. Network 520 may be connected to a network of reference GPS receivers that provide differential GPS information and may also provide GPS ephemeris (ephemeris) data for use in calculating the position of the mobile system. The network is coupled to the processor 503 through a modem or other communication interface. The network 520 may be connected to other computers or network components (through an optional interconnection not shown in fig. 4), such as the data processing system 423 in fig. 4. Also, the network 520 may be connected to a computer system operated by an emergency operator, such as a public safety answering point for answering 911 telephone calls. Several examples of methods of using a location server are described in several U.S. patents, including: U.S. Pat. Nos. 5,841,396, 5,874,914, 5,812,087, and 6,215.442, all of which are incorporated herein by reference.

The location server 501, which is one form of a data processing system, includes a bus 502 coupled to a microprocessor 503, a ROM 307, a volatile RAM505, and a non-volatile memory 506. The microprocessor 503 is coupled to a cache 504, as shown in the example of fig. 5. A bus 502 interconnects these various components. While the non-volatile memory is shown in FIG. 5 as a local device coupled directly to the other components in the data processing system, it will be appreciated that the present invention may utilize a non-volatile memory which is remote from the system, such as a network storage device which is coupled to the data processing system through a network interface such as a modem or Ethernet interface. The bus 502 may include one or more buses connected to each other through various bridges, controllers and/or adapters, as is well known in the art. In many cases, the location server may do its operations automatically without human assistance. In some designs requiring human intervention, the I/O controller 509 may communicate with a display, keyboard, and other I/O devices.

Note that fig. 5 illustrates various components in a data processing system and is not intended to represent any particular architecture or manner of interconnection between the components, as such details are not germane to the present invention. It will also be appreciated that network computers and other data processing systems which have fewer components or perhaps more components may also be used with the present invention.

It will be apparent from this description that aspects of the invention may be embodied, at least in part, in software. That is, the techniques may be performed in a computer system or other data processing system that executes sequences of instructions in response to a processor included in a memory, such as ROM 507, volatile RAM505, non-volatile memory 506, cache 504, or a remote storage device. In various embodiments, hard-wired circuitry may be used in combination with software instructions to implement the invention. Thus, the techniques are not limited to any specific combination of hardware circuitry and software or to any particular source for the instructions executed by the data processing system. Furthermore, throughout the description, various functions and operations are described as being performed by or caused by software code to simplify description. However, those skilled in the art will recognize what is meant by such expressions is that the functions result from execution of the code by a processor, such as the processor 503.

In some embodiments, the methods of the present invention may be performed on computer systems that may be used for other functions as well, such as cellular switching, messaging services, and the like. In these cases, some or all of the hardware in FIG. 5 would be shared by several functions.

Fig. 6 shows the topology of a general system that can be used for the present invention. For the sake of illustration, the figure is quite concise, however, it illustrates a number of different scenarios that may be used in practice.

In fig. 6, three mobile stations (615, 616, and 617), two cellular base stations (613 and 614), three satellite GPS constellations (610, 611, and 612), and one location server 618 are shown.

The location server 618 communicates with other infrastructure, typically via a wired link 622, cellular infrastructure links 619 and 620, typically wired, and communication infrastructure 621, typically wired. Transmissions from the GPS satellites 623 and 625 are shown as unfilled (no fill). Transmissions from base station 613 have shadows (e.g., 626); while the transmission from the base station 614 is filled with solid fill (e.g., 627). The signals received by the mobile station (with the SPS receiver) follow the same coding scheme. Thus, it can be seen in FIG. 6 that MS 615 receives signals from GPS satellites and BS 613; MS 616 receives signals from GPS satellites and BS 613 and BS 614; and MS 617 receives signals from GPS satellites and BS 614.

For simplicity, all mobile stations receive signals from all GPS satellites, although this is not necessary in practice. In practice, there may be multiple location servers, more base stations and mobile stations; and each mobile station may observe transmissions from one or more base stations. Likewise, the location server may be co-located with the base station (co-located) or remote from the base station (as shown in fig. 6).

In the example of fig. 6, mobile station 616 is typically in two-way communication with only one of the base stations from which it receives signals. For example, MS 616 may be in two-way communication with basestation 613 but it may also receive transmissions from basestations 613 and 614 simultaneously. Thus, MS 616 may perform synchronization operations on both basestations 613 and 614 in this case, although in this example it will only communicate synchronization information with basestation 613. It is well known in the art that in addition to the primary or "serving" site, the cellular telephone monitors transmissions from other base stations in preparation for future communications, or "handoffs" to different base stations.

Fig. 6 also illustrates that data may be communicated to and from a location server of the mobile station via the communications infrastructure as well as the cellular infrastructure. The location server may be located at a base station, but is typically remote from the base station and may in fact communicate with multiple base stations. The synchronization information provided by the mobile station is typically sent to one or more location servers, which process the information and determine the relative or absolute timing of the base station transmissions.

Fig. 7 shows a frame structure of a traffic channel of a GSM cellular signal. In a GSM traffic signal, a superframe (superframe) occurs every 6.12 seconds; while a superframe (superframe) occurs once every 2048 superframe or once every 3.4816 hours. Therefore, a superframe is a valid time unit (epoch) of the gap size measured by the time interval. Alternatively, an integer number of frames, multiframes, etc. may be used as they occur at times uniquely determined by multiples of the bit duration.

In one embodiment of the invention, the duration of a transmission is measured between two frame markers contained in a cellular communication signal transmitted by a cellular base station. A set of measurements is made by one or more mobile stations to determine the duration, i.e., the timing, of a subsequent framing marker relative to a previous framing marker. The measured duration is compared to an expected timing (typically by a server). The result is used to determine the error in the frequency of the base station oscillator relative to the desired value.

The error in the measurement may be specified as a fraction of the true value and may be expressed in terms of Parts Per Million (PPM). For example, if the time between specified framing markers is designed to be 1 second and the measurement is 1 second plus 1 microsecond, the error can be expressed as 1 microsecond/1 second-1 PPM. This is a convenient way of specifying the error, since it applies equally to errors in other synchronisation epochs (e.g. bit rate) and in the carrier frequency of the base station, assuming (most often) that the transmitted carrier frequency is synchronised with the framing marker.

Assume that the duration of time for one or more mobile stations to measure the base station signal corresponds to 98 transmitted superframes, approximately 10 minutes. The specified time of measurement may correspond to the start of a number of multiframes. The mobile station keeps track of the number of the multiframe explicitly by means of signalling information carried in the baseband transmission. The ideal period of the measurement is therefore accurately known, expressed in units of transmitted bit duration (one bit period equal to 48/13 microseconds). The ideal measurement period is 98 times each superframe, i.e., 599.76 seconds. However, the true time measurement can be affected by errors in the transmitter clock and various measurement related errors.

When the duration between two predetermined framing marks is maintained for 600 seconds and an error of less than 1 microsecond is measured, the error in the frequency of transmission of the measured framing marks is less than 0.00167 PPM. This accuracy is very consistent with short term accuracy and long term frequency stability of ovenized crystal oscillators, which are typically used in cellular basestations, although the absolute accuracy of such oscillators is often poor. Indeed, in many cases, the frequency of framing markers may be measured with greater accuracy. Although the maximum absolute error of the GSM base station reference oscillator frequency is specified to be 0.05PPM, the stability of these oscillators is generally much better than this specification.

The duration to be measured may extend over a period of even a few hours to achieve better accuracy in the measurement, provided that the short term stability of the base station oscillator supports a certain quasi-certainty and the long term drift characteristics (e.g. due to aging) follow a smooth curve. As an example, a measurement period extending to one hour with an accuracy of 1 microsecond refers to a frequency accuracy of 0.000278PPM, which again conforms to the short term stability of a high quality oven-type (overized) crystal oscillator. Typically, the accuracy of a high quality crystal oscillator will be ten times better than this.

Thus, using a mobile station to measure the duration of the transmission period between two framing markers may provide a very accurate measurement of the transmission of the framing markers, which is related to the frequency of the oscillator of the base station.

Fig. 8 shows a flow diagram for determining a frequency of a base station transmission in accordance with an embodiment of the invention. In operation 801, the arrival times of cellular signals transmitted by base stations are measured at different time instants. The time of arrival of a frame marker (e.g., the boundary of a frame) is measured using one or more mobile stations (e.g., MS 210, MS 310, or MS 615) with a GPS receiver. Next, the frequency of the transmissions of the base station may be calculated using the time of arrival of these cellular signals. The frame markers may be calculated by dividing the number of frame markers known within the current duration by the duration. Since the carrier frequency of the base station signal and the frequency of the transmission of the framing marker are synchronized with the frequency of the master oscillator of the base station, the frequency of the master oscillator of the base station and the carrier frequency of the base station signal can be determined. In some embodiments, it may be computationally more convenient to calculate the period of transmission from the base station.

As previously mentioned, the determination of the cellular transmitter frequency is typically made at a server, or at what is referred to as a Position Determination Entity (PDE), rather than at the cellular base station, although the PDE may be co-located with the cellular base station. The server or PDE is a set of devices located in the cellular or communication network infrastructure that can communicate messages to and from the mobile stations over the communication network, cellular network, and wireless links. That is, when the mobile station makes timing related measurements of base station transmissions, these measurements are transmitted over the cellular link to the serving base station and then over the infrastructure terrestrial lines to the PDE. The PDE would then use these measurements to calculate the time and frequency associated with future frame markers. This information may then be communicated to the mobile station or other network entity that wishes to use the information to improve system performance. Indeed, in one embodiment, this timing information is used as assistance data to allow the mobile station to further GPS reception and measurement operations in a more efficient manner. This embodiment next provides a "bootstrap" scheme in which GPS measurements made by some mobile stations largely aid in the performance of subsequent GPS measurements. Performance enhancements in this manner include greatly increased sensitivity, reduced time to first fix, and increased availability, as described in U.S. patent nos. 5,841396 and 5,945,944.

Fig. 9 illustrates a detailed method for determining the frequency of a base station signal by using a measurement of the frame occurrence time (epoch) of the base station signal according to an embodiment of the present invention. In operation 901-; finding a frame marker contained in such cellular signal; using a GPS receiver to search the time of day and the position of the GPS receiver; assigning a time stamp to the frame marker using the found time of day in operation 905; and send its location (or information determining its location) and the time stamp (or information determining the time stamp) to a server, such as a location server.

It is to be understood that operation 905 may precede operations 901 and 903, or occur concurrently with operations 901 and 903. The transmission path over which the location and time stamp information is sent typically includes a cellular link followed by additional terrestrial links (e.g., telephone lines, local networks, etc.).

The cellular signal received in operation 901 may come from a different communication link than that used to transmit data in operation 909. That is, it is observed in operation 901 that the base station may not be the "serving" base station for the mobile station. It may be a brief observation by the mobile station to determine one of a list of "neighbor" base stations that may be used in a future handoff operation. Typically, a mobile station may observe 10 base stations or more, as is well known in the art.

The second base station (or even the same base station) performs operations 911-919 in a manner similar to operations 901-909. Generally, operations 911-. It is to be understood that operations 911-.

At operation 921, a server (e.g., a location server) processes the time stamp received from the mobile station, the location of the mobile station, and information regarding the location of the base station to calculate a frequency associated with the base station, such as a frequency associated with a framing marker or any other frequency of the base station synchronized to the rate. The frequency may be expressed in terms of nominal (ideal or theoretical) frequency plus error, for example the latter may be in PPM units of infinite steel. Since the time stamp corresponds to the time at which the framing marker arrives at the mobile station being measured, the location of the mobile station and the base station is required for converting the time stamp into a time measurement at the same location in order to calculate an accurate transmission duration. This is obtained by subtracting the delay of the cellular signal from the transmitting base station to the mobile station being measured from the time stamp.

In operation 923, the occurrence time of the frame marker of the future base station may be predicted using the measured frequency of the transmission. Such predictions may be transmitted to a different network entity, such as a base station or mobile station, in operation 925.

Since the information provided to the server in operations 909 and 919 also allows the time of day associated with the framing marker to be determined, the time adjustment may also be performed in accordance with the method described in co-pending application No.09/565,212, U.S. patent application filed on 5/4/2000.

In operation 927, the predicted epoch timing may be used by the mobile station or base station to assist with SPS measurements or TDOA or EOTD operations.

Fig. 9 illustrates a method of determining a transmission frequency of a base station using two mobile stations and one base station, and in practice, more mobile stations are generally included. In addition, each mobile station can view (view) the timing epochs of several base stations simultaneously or sequentially. Thus, multiple operations similar to operations 901-909 may occur in parallel to correspond to multiple base stations. The process shown in fig. 9 may be performed continuously. As previously described, the operations of fig. 9 may be performed by a single mobile station observing one or more base stations.

Errors in the time of occurrence prediction can be reduced by modeling (modeling) the long-term frequency versus time (drift) characteristics of the base station. In many cases, the long term drift is smooth and fairly easily predictable for a high quality base station oscillator. Thus, the drift characteristics may be determined from a plurality of measurements transmitted by the base station over a very long period of time. A curve fitting process may be used to predict future drift from the drift characteristics. A typical curve fitting algorithm may use a polynomial.

In the method shown in fig. 9, the same mobile station is not required to make continuous timing measurements. In practice, each timing measurement corresponding to a given base station may be made by a different mobile station. When a large number of measurements are taken over a period of time, a different averaging operation, such as Least Mean Square (LMS) averaging, may be performed. Taking a large number of measurements not only significantly reduces measurement errors, but also allows measurements containing abnormally high errors due to spurious effects such as multiple receptions to base station transmissions (spurious effects) to be discarded. This discarding of "outliers" can be performed by first making an initial estimate of the frequency using all measurements, then discarding those measurements that are significantly further away from the initial measurement, and finally recalculating the estimate using the measurements that were not discarded. Other methods such as those using order statistics may also be used to discard outliers.

The cellular signal arriving at a mobile station may be the result of a reflection of the primary signal or the presence of multiple direct and reflected received signals, a result known as "multipath". In most cases, multipath results in a positive excess delay, i.e., a longer signal transmission delay than a direct line-of-sight (line-of-sight) transmission. The delay of the line-of-sight transmission may be obtained by dividing the distance between the base station and the mobile station by the speed of light. Since multipath rarely produces negative excess delays, simple averaging is not the best way to reduce errors due to multipath.

Excessive delay due to multipath can be compensated for by using a weighted average. One approach is to select, or otherwise weight, measurements derived from high quality signals, e.g., signals with high intensity (high signal-to-noise ratio) and signals with narrow and well-defined signal shapes. Some type of autocorrelation (autocorrelation) analysis that analyzes the shape of a received signal may be used to determine the quality of the received signal. A high quality signal may come more from line-of-sight transmissions or from an environment with minimal reflections and therefore exhibit less excessive delay than a low quality signal. In some cases, with sufficiently high received signal levels, signal processing algorithms may be used to estimate the number, strength, and relative delay of signals received from a given base station. In which case the minimum delay may be selected to minimize the effect of excessive delay.

Fig. 9 illustrates a method where the duration of the transmission is calculated at the server and fig. 10 illustrates another method where the duration of the transmission is determined at the mobile station. In operation 1001-; finding a frame marker contained in the cellular signal; using its GPS receiver to find its position and time; the time of day found in operation 1005 is used to assign a time stamp to the frame marker. Similarly, the time stamp of the second frame marker is determined in operation 1011-. In operation 1019, the mobile station calculates a duration of the transmission time using the time stamp. In this case, information about the position of the mobile station and the base station is generally required, since the mobile station may move between measurements and therefore the occurring base station-mobile station distances have to be compensated for. This information is not needed if the mobile station is known to be stationary. The transmission frequency of the framing markers of a base station may be determined and used to predict the timing of future framing markers of the base station. The duration or measured frequency may be transmitted to a server and the prediction of timing may also be made at the server. At operations 1022 and 1023, the prediction may be provided to the mobile station or base station to assist in SPS measurements, or EOTD or TDOA operations. The first and second cellular signals in fig. 10 generally correspond to two portions of a cellular signal received at different times during the same telephone "call". However, these signals may also correspond to signals received from the base station during a separate call.

Fig. 11 shows a detailed method for determining the frequency of a base station signal using a measurement of the carrier frequency of the base station signal according to an embodiment of the invention. In operation 1101, a mobile station receives a cellular signal transmitted from a base station. Which is synchronized to the carrier frequency of the received cellular signal in operation 1103. This may generally use a Phase Locked Loop (PLL) or an Automatic Frequency Control (AFC) circuit, either of which includes a voltage controlled oscillator (e.g., VCO 323). The synchronization process results in the VCO having a proportional relationship with the phase or frequency of the received carrier.

In operation 1105 the mobile station uses a GPS (or SPS) receiver to determine its position, velocity, time of day, and frequency from the local oscillator reference signal. For the determination of the base station frequency, the measurement of the local oscillator reference frequency is the main information of interest; however, position, velocity, and time-of-day information are typical by-products of GPS processing. Position and velocity are required to determine the effect of movement of the MS on the frequency measurements. As previously mentioned, the local reference signal used by the GPS receiver may be provided by the VCO of the cellular transceiver or may be provided by a separate crystal oscillator.

In operation 1107, the mobile station determines the received base station carrier frequency from the VCO signal and from the GPS reference frequency measurement. As described earlier, if the VCO is used as its frequency reference, this is a direct byproduct of the GPS process. Alternatively, separate frequency counting circuits may be used to determine the frequency ratio of the VCO and GPS reference signals. The ratio and value of the GPS reference frequency determined when processing the GPS signal provides an accurate estimate of the VCO frequency and hence the carrier frequency of the received base station signal.

In operation 1109, frequency information is transmitted to a base station along with assistance data (e.g., timing, base station identification information, and others). In operation 1111, the carrier frequency information, which may be expressed using PPM units or other units, may be used to calculate the base station oscillator frequency, and/or other frequencies (e.g., framing mark frequency). The position and velocity of the mobile station are used together with the base station position to determine the frequency error due to the relative movement of the mobile station. This error must be eliminated to obtain an accurate estimate of the base station frequency. The server may combine several such frequency measurements together to further improve the estimation of the base station frequency. Finally, in operation 1113-.

Fig. 11 illustrates a case where only one mobile station and one base station are included, and in practice, more mobile stations may be included. Each mobile station may view the transmissions of several base stations simultaneously or in sequence. Thus, multiple sequences of operations (such as operations 1101-1109) may occur in parallel corresponding to multiple base stations. It is also to be understood that the process as shown in fig. 11 may be performed on a continuous basis.

Various other variations of the methods of fig. 8-11 will be apparent to those skilled in the art. For example, if the position of the base station is received, the mobile station may perform the calculations 1111-. In fig. 10, as an alternative to the time of day measurement in operations 1005 and 1015, the mobile station may calculate the elapsed time after calibrating its clock by the method of 1101-.

Base station frequency calibration may allow accurate prediction of the time of occurrence of future timing markers transmitted by the base station when the base station oscillator is sufficiently stable. Typically, the stability of the base station oscillator is sufficient to allow accurate timing predictions over long periods of time when time adjustments are made.

Base stations typically use high quality oven-box type crystal oscillators as frequency references. Some base stations further lock their reference to the signal transmitted from the GPS satellite, and in some cases, the long term stability of the base station transmission is locked to Cesium (Cesium) type stability and is suitable for accurate timing measurements. In the following discussion, we assume that such GPS locking is not used. In this case, two main sources of base station oscillator stability are: i) short term frequency stability, typically marked by a short term frequency stability measurement such as a noise spectral density method or an Allan variance (charcterized); and ii) a long term frequency drift, generally associated with aging effects. Long term frequency drift tends to be 0.001PPM or better per day and therefore should not represent a significant source of error over relatively short time periods (e.g., 15-30 minutes).

Most base station oscillators use oven-box (overized) crystal oscillators. Small temperature variations in the tank or variations in the voltage supplied to the tank will cause an increase in the frequency error. In addition to this short-term Frequency stability characteristic, such as random-walk Frequency effects, an error that grows as a function of observed timing is generated (see J.Rutman and F.L.Walls, Characterization of Frequency stability in Precision Frequency resources, Proc.IEEE, Vol.79, No.6, 6 months 1991, page 952 and 959). It is therefore important to examine the magnitude of these effects from the point of view of both the device and the system.

Short-term frequency stability as considered herein is measured over a time interval of seconds to hours. Measured over these periods, high quality ovenized oscillators typically have a short term stability (partial frequency deviation, otherwise known as Allan variance) on the order of 0.00001 PPM. With such stability, timing signals from the base station can be predicted with accuracy up to 6 nanoseconds in a period of 10 minutes in the future and up to 36 nanoseconds in a period of 1 hour in the future.

The long term stability of a high quality oven-type oscillator may be on the order of 0.001PPM per day or better, corresponding to about 0.00004PPM per hour. (refer to Fundamentals of Quartz Oscillators, Hewlett Packard Application Note 200-2). Thus, the influence of the aging behavior is decisive for the prediction of one hour or more.

From a measurement point of view, Pickford takes into account the frequency drift between two base stations based on round-trip measurements (cf. Andrew Pickford, BTS Synchronization Requirements and LMU Update Rates for E-OTD, Technical sub-transmission to Technical Subcommittee T1P1, 10.8.1999). He found that the net RMS time error was on the order of 66 nanoseconds even over a period of more than 1 hour, when the linear phase (or time) drift (i.e., fixed frequency offset error) was eliminated. He also demonstrated that using measurements over a 1 hour period and continuing them for the next hour produced the same accuracy. Furthermore, examination of his curve (curve) shows that the error remaining after the mean drift is eliminated is dominated by randomly occurring errors. This indicates that the remaining error is mainly measurement error, or additive noise, rather than actual oscillator jitter. Note that measurements over a 1 hour period have an error of 66 nanoseconds RMS, which is equivalent to a frequency stability of about 0.000018PPM, which is typical of a high quality crystal oscillator.

A similar paper by t.rantalaine et al, provides results similar to those described above (see t.rantalaine and v.rutu, RTD Measurements for E-OTD Method, technical submissions to T1P1.5/99-428R0, 8/6/1999). However, in this paper, several phase versus time fits (fits to phase vs. time) require a second order polynomial to maintain a low residual error. Typical treatment intervals are 1500 to 2200 seconds. No explanation is given of the non-linear behavior of the phase versus time curve (plot). As indicated above, this may be very good due to the aging characteristics of the crystal oscillator. Since the aging characteristic is predictable and smooth, the polynomial adaptation algorithm works well. For example, fitting a second order polynomial to the frame period versus measurement time will compensate for linear frequency versus time drift.

Other factors that may cause small changes in frequency with respect to time include voltage and temperature fluctuations of the frequency reference. These factors may manifest themselves in small frequency variations. The base station should have a regulated voltage and temperature to ensure high reliability.

When there is significant user movement, it is important that any doppler related effects do not unduly affect the timing and frequency measurements described above. In particular, if the mobile station measures time at one time and predicts the time of day associated with the occurrence of a cellular signal boundary at a different time, errors may be generated due to movement of the mobile station, particularly when the mobile station is moving rapidly and/or when the distance between the times is large. There are many ways to solve this type of problem. For example, when the mobile station can determine its velocity, data regarding the velocity of the mobile station can be provided to the server to compensate for errors due to doppler effects associated with the range rating (range) between the mobile station and the base station. This method is illustrated in fig. 11. As described above, the GPS signals may be processed to estimate the velocity of the receiving station. This information can be used to compensate for any errors due to movement of the mobile station.

Residual errors such as multipath delays and transmission delays through the mobile station hardware may remain. However, the mobile station and/or base station may often determine the level of such degradation and weight those measurements with less error.

The effective time of transmission (i.e., time of arrival) is determined at the face of the base station antenna. Using a large number of mobile stations may reduce errors by the averaging process. This assumes that the base station offset can be eliminated or reduced by appropriate measurement selection or other estimation procedures.

Focusing on sufficient mobile station activity to support timing (e.g., earlier morning hours) may be improved by placing the mobile station in a different location and making periodic calls. However, this does not need to be a fixed resource.

Typical timing errors due to the GPS processor at a single mobile station may be on the order of 10-30 nanoseconds. Thus, errors from other sources, such as multipath, may be dominant.

The stability of the base station oscillator affects how often timing measurements need to be made and propagated. Not only can the instantaneous frequencies of the base station oscillator be accurately determined by using multiple measurements from the base station, but higher order moments such as the rate of change of these frequencies can also be determined. As discussed above, a simple curve fit of the base station frequency versus time can generally be maintained with extremely high accuracy over a long period of time.

Although the method and apparatus of the present invention have been described with reference to GPS satellites, it will be appreciated that these principles are equally applicable to positioning systems using pseudolites or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters that broadcast a PN code (similar to a GPS signal) modulated on an L-band carrier signal, typically synchronized with GPS time. Each transmitter may be assigned a unique PN code to allow it to be identified by a remote receiver. Pseudolites are useful in environments where GPS signals from an orbiting satellite are not present, such as tunnels, mines, buildings, or other enclosed areas. The term "satellite" is used herein to mean including pseudolites or equivalents of pseudolites, and the term GPS signals is used herein to mean including GPS-like signals from pseudolites or equivalents of pseudolites.

In the foregoing discussion, the invention has been described with reference to application to the united states global positioning satellite system. It is apparent, however, that these methods are equally applicable to similar satellite positioning systems, such as, in particular, the russian Glonass system and the proposed european Galileo (Galileo) system. The Glonass system differs from the GPS system primarily in that transmissions from different base stations are distinguished from each other by using slightly different carrier frequencies, rather than using different pseudo-random codes. In which case substantially all of the circuitry and algorithms previously described may be applied. The term "GPS" as used herein includes such other satellite positioning systems, including the Russian Glonass system and the proposed European Galileo (Galileo) system.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be apparent that various changes may be made in these embodiments without departing from the principles and scope of the invention, which is defined in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims

1. A method for measuring frequencies associated with base stations in a cellular communication system, the method comprising the steps of:

2. The method of claim 1, wherein the combining step further comprises:

the transmission time of the cellular signal from the base station to the mobile station is calculated.

3. The method of claim 2, wherein the difference in transmission time is inversely proportional to the first frequency.

4. The method of claim 1, wherein the server is located at the base station.

5. The method of claim 1, wherein the server is located at a location remote from the base station.

6. The method of claim 1, wherein the cellular communication system uses one of the following standards:

a) the GSM communication standard;

b) japanese PDC communication standard;

c) the Japanese PHS communication standard;

d) AMPS analog communication standard;

e) the North American IS-136 communication standard; and

f) the asynchronous wideband spread spectrum CDMA standard.

7. A method as claimed in claim 1, characterised in that common circuitry in the first mobile station is used for processing cellular signals and satellite positioning system signals.

8. The method of claim 1, wherein the first frequency is associated with a carrier frequency of a cellular signal from the base station.

9. The method of claim 1, wherein the first frequency is related to a symbol rate of a cellular signal from the base station.

10. The method of claim 1, wherein the first mobile station and the second mobile station are the same station.

11. The method of claim 1, wherein the first mobile station and the second mobile station are different, separate mobile stations.

12. The method of claim 1, wherein the first cellular signal and the second cellular signal correspond to different portions of the cellular signal at different times from one another.

13. A system for measuring frequencies associated with a base station, the system comprising:

the first mobile station includes:

the second mobile station includes:

14. The system of claim 13 wherein the first satellite positioning system receiver and the first cellular transceiver are integrated in a package of the first mobile station.

15. The system of claim 13 wherein the first satellite positioning system receiver and the first cellular transceiver share at least one common component.

16. The system of claim 13, wherein the first timing marker is a frame sync epoch in the first cellular signal.

17. The system of claim 13, wherein the base station uses one of the following criteria:

a) the GSM communication standard;

b) japanese PDC communication standard;

c) the Japanese PHS communication standard;

d) AMPS analog communication standard;

e) the North American IS-136 communication standard; and

f) the asynchronous wideband spread spectrum CDMA standard.

18. The system of claim 13, wherein the server is located at the base station.

19. The system of claim 13, wherein the server is located at a location remote from the base station.

20. The system of claim 13, wherein the first and second mobile stations are the same station.