System and method for correlating traffic channel signal measurements in a communications system

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SYSTEM AND METHOD FOR CORRELATING TRAFFIC CHANNEL SIGNAL MEASUREMENTS IN A COMMUNICATIONS SYSTEM

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to wireless communication systems and networks. More particularly, the present invention relates to a method and apparatus for coordinating measurements taken at different locations or by different transceivers in a communications system. The invention further relates to making such measurements in a satellite communication system.

IL Description of the Related Art

A typical terrestrial wireless communications system comprises at least one terrestrial base station and a number of mobile stations (for example, a mobile telephone). A base station can link a mobile station to other mobile stations or to a terrestrial network, such as a public telephone system. A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway or hub), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway (GW) provides links from a user terminal (UT) to other user terminals or communications systems, such as a terrestrial telephone system.

Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Patent No. 4,901,307, issued February 13, 1990, entitled Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,” and U.S. Patent 5,691,974, issued November 25, 1997, entitled Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy, both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.

In satellite communication systems employing CDMA, separate communication links are used to transmit communication signals, such as data or traffic, to and from a

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PCT/US00/08629 gateway. The term forward communication link refers to communication signals originating at the gateway and transmitted to a user terminal. The term reverse communication link refers to communication signals originating at a user terminal and transmitted to the gateway. The transfer of signals to and from users over these links generally occurs over channels referred to as “traffic” channels. However, additional channels are used for access requests and paging messages, pilot signals, and synchronization signals.

On the forward link, information is transmitted from a gateway to a user terminal over one or more beams. These beams often comprise a number of so-called subbeams (also referred to as frequency division multiplex (FDM) or CDMA channels in a CDMA communication system) covering a common geographic area, each occupying a different frequency band.

More specifically, in a conventional spread-spectrum communication system, one or more pre-selected pseudorandom noise (PN) code sequences are used to modulate or “spread” user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. On the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different gateways or over different beams, as well as between multipath signals. These codes are often shared by all communication signals within a given beam or subbeam.

In a conventional CDMA spread-spectrum communication system, channelizing codes are used to discriminate between different user terminals within a satellite subbeam on a forward link. That is, each user terminal has its own orthogonal channel provided on the forward link by using a unique channelizing orthogonal code. Walsh functions are generally used to implement the channelizing codes, also known as Walsh codes. The channelizing codes divide a subbeam into orthogonal channels, also known as Walsh channels. A majority of the Walsh channels are traffic channels that provide messaging between a user terminal and a gateway. The remaining Walsh channels often include pilot, sync, and paging channels. Signals sent over the traffic channels are typically meant to be received by only one user terminal, except for broadcast schemes where a set of users utilize a single channel. In contrast, paging, sync, and pilot channels may be monitored by multiple user terminals.

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While the advent of the mobile user terminal has freed users to take their wireless devices, such as mobile phones, wherever they go, it has also imposed upon the satellite communications earner the burden of determining the location of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. One aspect of this consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory or even a service gateway, and handles all calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries or contractual services, or for determining position related charges such as fees for “roaming.” In addition, some emergency services may require position information.

Several methods of determining the location of the user terminal involve measuring the distance between the user terminal and the satellite. When this distance is combined with other data, such as the position of the satellite, the location of the user terminal can be determined with great precision. Techniques for determining user terminal location using satellite-user terminal range are disclosed in commonly-owned, copending U. S. Patent Applications entitled Position Determination Using One LowEarth Orbit Satellite, Serial No. 08/723,751; Passive Position Determination Using Two Low-Earth Orbit Satellites, Serial No. 08/723,722; and Unambiguous Position Determination Using Two Low-Earth Orbit Satellites, Serial No. 08/723,725, which are incorporated herein by reference.

Theoretically, the user terminal-satellite range can be determined by combining certain measurements of the communications signals received at the gateway and the user terminal. However, in order to use this technique the relative timing of these measurements must be considered. Without an accurate assessment of this relative timing, the accuracy of position location measurements or computations suffers. This can lead to unacceptable errors, or delays created by requiring collection of additional measurements.

What is needed is a way to correlate measurements of communications signals received at the gateway and user terminal to improve the accuracy of range ' measurements.

SUMMARY OF THE INVENTION

In a first aspect, the present invention accordingly provides a wireless communication system having a first station and a second station exchanging communications signals, a method for correlating measurements of the communications signals at the first and second stations, the method including the steps of:

repeatedly taking and storing a measurement of the communication signals received at the first station;

taking a measurement of the communication signals received at the second station at a particular time; and selecting at least one of said measurements taken at the first station that was taken at a time nearest said particular time.

In a second aspect, the present invention accordingly provides a wireless communications system having a first station and a second station exchanging communications signals, a method for taking synchronized measurements of the communications signals at the first and second stations, the method including the steps of:

repeatedly taking and storing a measurement of the communication signals received at the first station;

taking a measurement of the communication signals received at the second station at a particular time;

selecting two or more of said measurements taken at the first station that were taken at times nearest said particular time; and interpolating between said two or more measurements taken at the first station based on said particular time.

4a

In a third aspect, the present invention accordingly provides a wireless communications system having a first station and a second station exchanging communications signals, a system for correlating measurements of the communications signals at the first and second stations, the system including:

means for repeatedly taking and storing a measurement of the communications signals received at the first station;

means for taking a measurement of the communications signals received at the second station at a particular time; and means for selecting the one of said measurements taken at the first station that was taken at the time nearest said particular time.

In a fourth aspect, the present invention accordingly provides a wireless communications system having a first station and a second station exchanging communications signals, a system for taking synchronized measurements of the communications signals at the first and second stations, the system including:

means for repeatedly taking and storing a measurement of the communications signals received at the first station;

means for taking a measurement of the communications signals received at the second station at a particular time;

means for selecting two or more of said measurements taken at the first station that were taken at the time nearest said particular time; and means for interpolating between said two or more measurements taken at the

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BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the 5 drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 illustrates an exemplary wireless communication system in which the present invention is useful;

FIG. 2 illustrates an exemplary wireless transceiver useful for user terminals in the 10 system of FIG. 1;

FIG. 3 illustrates an exemplary wireless transceiver useful for gateways or base stations in the system of FIG. 1;

FIG. 4 is a flowchart depicting the operation of the present invention according to a preferred embodiment; and

FIG. 5 depicts a timeline showing the relative timing of several gateway measurements and a user terminal measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Introduction

The present invention is particularly suited for use in wireless communications systems employing Low Earth Orbit (LEO) satellites. However, as would be apparent to one skilled in the relevant art, the concept of the present invention can also be applied to 25 other satellite systems, or to non-satellite wireless systems in which improved correlation of signal measurements would be useful. Before discussing the embodiments of the present invention, a typical environment in which the invention can operate is presented.

Π. A Typical Communication System

An exemplary wireless communication system in which the present invention is useful is illustrated in FIG. 1. It is contemplated that this communication system uses CDMA type communication signals, but this is not required by the present invention. In a portion of a communication system 100 illustrated in FIG. 1, one base station 112, two

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PCT/US00/08629 satellites 116 and 118, and two associated gateways or hubs 120 and 122 are shown for effecting communications with two remote user terminals 124 and 126. Typically, the base stations and satellites/gateways are components of separate communication systems, referred to as being terrestrial- and satellite-based, although this is not necessary. The 5 total number of base stations, gateways, and satellites in such systems depends on desired system capacity and other factors well understood in the art.

User terminals 124 and 126 each include a wireless communication device such as, but not limited to, a wireless telephone, a data transceiver, or a paging or position determination receiver, and can be hand-held or vehicle-mounted as desired. However, it 10 is also understood that the teachings of the invention are applicable to fixed units such as where remote wireless service is desired. User terminals are sometimes also referred to as subscriber units, mobile stations, mobile units, or simply as users or subscribers in some communication systems, depending on preference.

Generally, beams from satellites 116 and 118 cover different geographical areas in 15 predefined patterns. Beams at different frequencies, also referred to as FDMA channels or CDMA channels (where CDMA type signals are used) or sub-beams, can be directed to overlap the same region. It is also readily understood by those skilled in the art that beam coverage or service areas for multiple satellites, or antenna patterns for multiple base stations, might be designed to overlap completely or partially in a given region 20 depending on the communication system design and the type of service being offered, and whether space diversity is being achieved.

A variety of multi-satellite communication systems have been proposed with an exemplary system employing on the order of 48 or more satellites, traveling in eight different orbital planes in LEO orbits for servicing a large number of user terminals. 25 However, those skilled in the art will readily understand how the teachings of the present invention are applicable to a variety of satellite and non-satellite communication systems and base station/gateway configurations, including other orbital distances and constellations, or methods of creating moving signal sources. In addition, those skilled in the art will readily understand how the teachings of the present invention are applicable 30 to a variety of terrestrial or other wireless communication systems where improved correlation of the signal measurements performed at base stations and mobile stations is desired.

F* FIG. 1, some possible signal paths are illustrated for communications being established between user terminals 124 and 126 and base station 112, or through satellites

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116 and 118, with gateways 120 and 122. The base station-user terminal communication links are illustrated by lines 130 and 132. The satellite-user terminal communication links between satellites 116 and 118, and user terminals 124 and 126 are illustrated by lines 140, 142, and 144. The gateway-satellite communication links, between gateways 120 and 122 and satellites 116 and 118, are illustrated by lines 146, 148, 150, and 152. gateways 120 and 122, and base station 112, may be used as part of one or two-way communication systems or simply to transfer messages or data to user terminals 124 and 126.

The preferred embodiment of the invention is discussed in detail below. While specific steps, configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other steps, configurations and arrangements can be used without departing from the spirit and scope of the present invention. The present invention could find use in a variety of wireless information and communication systems, including those intended for position determination. A preferred application is in CDMA wireless spread spectrum communication systems for wireless or mobile telephone service.

ΠΙ. Communication System Transceivers

An exemplary transceiver 200 for use in a user terminal 106 is illustrated in FIG- 2. Transceiver 200 uses at least one antenna 210 for receiving communication signals which are transferred to an analog receiver 214, where they are downconverted, amplified, and digitized. A duplexer element 212 is typically used to allow the same antenna to serve both transmit and receive functions. Digital communication signals output by analog receiver 214 are transferred to at least one digital data receiver 216A and at least one digital searcher receiver 218. Additional digital data receivers 216B216N can be used to obtain desired levels of signal diversity, depending on the acceptable level of unit complexity, as would be apparent to one skilled in the relevant art.

At least one user terminal control processor 220 is coupled to data receivers 216A-216N and searcher receiver 218. Control processor 220 provides, among other functions, basic signal processing, timing, power and handoff control or coordination, and selection of frequency used for signal carriers. Another basic control function often performed by control processor 220 is the selection or manipulation of PN code sequences or orthogonal codes to be used for processing communication signal

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PCT/US00/08629 waveforms. Control processor 220 signal processing can include a determination of relative signal strength and computation of various related signal parameters, and may also include the use of additional or separate dedicated circuitry to provide increased efficiency or speed in measurements or improved allocation of control processing resources.

The outputs of digital data receivers 216A-216N are coupled to digital baseband circuitry 222 within the user terminal. User digital baseband circuitry 222 comprises processing and presentation elements used to transfer information to and from a user terminal user. That is, signal or data storage elements, such as memory; input and output devices such as display screens, speakers, and keypads; A/D elements, vocoders and other signal processing elements; etc., all form parts of the user terminal baseband circuitry using elements well known in the art. If diversity signal processing is employed, user digital baseband circuitry 222 can comprise a diversity combiner and decoder. Some of these elements may also operate under the control of, or in communication with, control processor 220.

When voice or other data is prepared as an output message or communications signal originating with the user terminal, user digital baseband circuitry 222 is used to receive, store, process, and otherwise prepare the desired data for transmission. User digital baseband circuitry 222 provides this data to a transmit modulator 226 operating under the control of control processor 220. The output of transmit modulator 226 is transferred to a power controller 228 which provides output power control to a transmit power amplifier 230 for final transmission of the output signal from antenna 210 to a gateway.

At least one time reference element 230 is used to generate and store chronological information such as date and time, which can be used to assist in determining satellite positions within known orbits. The time can be stored and updated periodically, and a Universal Time (UT) signal from a GPS receiver can be used as part of this process in some applications. The time may also be supplied to the user terminal periodically by a gateway or base station. In addition, current time or a set of time measurements can be stored in a memory device or storage element 232 such as when a user terminal enters an inactive mode, for example when it is turned off, and used to determine various time dependent signal parameters, or when simply measuring time itself. The time information can be used to generate “time stamps” as discussed below. In combination with other elements such as the controller.

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Information or data corresponding to one or more measured signal parameters for received communication signals, or one or more shared resource signals, and/or the time” can be sent to the gateway using a variety of techniques known in the art. For example, the information can be transferred as a separate information signal or be appended to other messages prepared by user digital baseband circuitry 222. Alternatively, the information can be inserted as predetermined control bits by transmit modulator 226 or transmit power controller 228 under control of control processor 220.

Data receivers 216A-N and searcher receiver 218 are configured with signal correlation elements to demodulate and track specific signals. Searcher receiver 218 is used to search for pilot signals, or other relatively fixed pattern strong signals, while digital receivers 216A-N are used to demodulate other signals associated with detected pilot signals. A data receiver 216 can also be assigned to track or demodulate a pilot signal after acquisition. Therefore, the outputs of these units can be monitored to determine the energy in or frequency of the pilot signal or other signals. These receivers employ frequency tracking elements that can be monitored to provide current frequency and timing information, to control processor 220 for signals being demodulated.

An exemplary transmission and reception apparatus 300 for use in a gateways 120 and 122, or a base station, is illustrated in FIG. 3. Such apparatus is known in the art and discussed in the patents referenced above. For example, additional details on the operation of this type of apparatus are found in U. S. Patent No. 5,103,459, issued April 7, 1992, entitled “System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone,” assigned to the same assignee as the present invention and incorporated herein by reference.

The portion of gateway 120, 122 illustrated in FIG. 3 has one or more analog receivers 314 connected to an antenna 310 for receiving communication signals which are then downconverted, amplified, and digitized using various schemes well known in the art. Digitized signals are provided as inputs to at least one digital receiver module, indicated by dashed lines generally at 324. One analog receiver 314 can provide inputs for many digital receiver modules 324, and a number of such modules are typically used in gateways to accommodate all of the satellite beams and possible diversity mode signals being accommodated. Each digital receiver module 324 has one or more digital data receivers 316 and searcher receivers 318. A searcher receiver 318 generally searches for appropriate diversity modes of signals other than pilot signals. Where implemented in

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PCT/US00/08629 the communication system, multiple digital data receivers 316A-316N are used for diversity signal reception.

The outputs of data receivers 316 are provided to subsequent baseband processing elements 322 comprising apparatus well known in the art and not illustrated in further detail here. Exemplary baseband apparatus includes diversity combiners and decoders to combine multi-path signals into one output for each user. Exemplary baseband apparatus also includes interface circuits for providing output data, typically to a digital switch or network. A variety of other known elements such as, but not limited to, vocoders, data modems, and digital data switching and storage components may form a part of baseband processing elements 322. These elements operate to also control or direct the transfer of data signals to one or more transmit modules 334.

Signals to be transmitted to user terminals are each coupled to one or more appropriate transmit modules 334, with the number of transmission modules 334 used by gateways 120, 122 being determined by factors well known in the art, including system complexity, number of satellites in view, user capacity, degree of diversity chosen, and the like.

Each transmit module 334 includes a transmit modulator 326 which spread spectrum modulates data for transmission and has an output coupled to a digital transmit power controller 328, which controls the transmission power used for the outgoing digital signal. Digital transmit power controller 328 applies a minimum level of power for purposes of interference reduction and resource allocation, but applies appropriate levels of power when needed to compensate for attenuation in the transmission path and other path transfer characteristics. At least one PN generator 332 is used by transmit modulator 326 in spreading the signals. This code generation can also form a functional part of one or more control processors or storage elements used in gateways 122, 124, or base station 112, and may be time shared.

The output of transmit power controller 328 is transferred to a summer 336 where it is summed with the outputs from other transmit power control circuits, which are for transmission to other user terminals 124, 126 at the same frequency and within the same beam. The output of summer 336 is provided to an analog transmitter 338 for digital-toanalog conversion, conversion to the appropriate RF carrier frequency, further amplification, filtering, and output to one or more antennas 340 for radiating to user terminals 124, 126. Antennas 310 and 340 may be the same antennas depending on the complexity and configuration of the system.

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Timing adjustments are generally accomplished by having the control processor adjust code generation and timing or other signal parameter timing when a signal is generated at baseband and prior to output by power controller 328. Controller 320, for example, determines code timing and application, as well as when signals are transmitted to various satellites and user terminals by power controller 328.

As shown in FIG. 3, a time and frequency unit (TFU) 348 provides reference frequency and timing signals for various stages or processing elements within gateway or base station 300, such as analog receiver 314 and the correlators in digital receivers 316A-N and 318, or transmit modulator 326, and control processor 320. This information is used to establish the frequency or timing adjustment for each satellite being used by the gateway at any given time. A Universal Time (UT) signal from a GPS receiver can be used as part of this process in some applications. TFU 348 is also configured to retard or advance the relative timing of (clock) signals, under processor control, by predetermined amounts, as desired.

ThU 348 or other well known time reference elements, operating alone or in combination, are also used to generate and store chronological information such as date and time, which can be used to assist in determining satellite positions within known orbits. The time information can be stored in and retrieved from one or more memory device or storage elements 340. This data can also be provided from other data sources, as desired. A variety of well known devices such as RAM and ROM circuits, or magnetic storage devices can be used to construct storage elements 340, where information can be stored and updated periodically, as desired. The time information can be used to generate “time stamps” as discussed below.

At least one gateway control processor 320 is coupled to receiver modules 324, transmit modules 334, and baseband circuitry 322; these units may be physically separated from each other. Control processor 320 provides command and control signals to effect functions such as, but not limited to, signal processing, timing signal generation, power control, handoff control, diversity combining, and system interfacing. In addition, control processor 320 assigns PN spreading codes, orthogonal code sequences, and specific transmitters and receivers or modules for use in user communications. Control processor 320 also controls the generation and power of pilot, synchronization, and paging channel signals and their coupling to transmit power controller 328.

While control processor 320 can be coupled directly to the elements of a module, such as transmit module 334 or receive module 324, each module generally comprises a

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PCT/US00/08629 module-specific processor, such as transmit processor 330 or receive processor 321, which controls the elements of that module. Thus, in a preferred embodiment, control processor 320 is coupled to transmit processor 330 and receive processor 321, as shown in FIG. 3. In this manner a single control processor 320 can control the operations of a large number of modules and resources more efficiently. Transmit processor 330 controls generation of, and signal power for, pilot, synchronization, paging signals, and traffic channel signals, and their respective coupling to power controller 328. Receiver processor 321 controls searching, PN spreading codes for demodulation and monitoring received power.

For certain operations, such as shared resource power control, gateways 120 and 122 receive information such as received signal strength, frequency measurements, or other received signal parameters, or the user terminal “time”, from user terminals in communication signals. This information can be derived from the demodulated outputs of data receivers 316 by receive processors 321. Alternatively, this information can be detected as occurring at predefined locations in the signals being monitored by control processor 320, or receive processors 321, and transferred to control processor 320. Control processor 320 uses this information to control the timing and frequency of signals being transmitted and processed using transmit power controllers 328 and analog transmitter 338, among other things.

IV. Preferred Embodiment

As discussed above, a need exists for a technique that can correct for the time difference between the taking of a measurement of a communications signal at the gateway and the taking of a corresponding measurement at the user terminal. Once this time difference is accounted for, the user terminal and gateway measurements can be combined to calculate various useful parameters, such as user terminal location and correction factors for the user terminal local clock, or for local oscillator frequencies.

In a preferred embodiment of the present invention, the user terminal and gateway measurements are performed on signals transmitted and received over one or more traffic channels. The user terminal measurement is then reported to the gateway. The user terminal measurement can then be combined with a corresponding gateway measurement. However, measurements related to time of arrival and other signal parameters can also be performed on paging, synchronization, and access channel signals, and some time or

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PCT/US00/08629 measurement information can also be transferred using signals over these channels, as desired for some system configurations.

However, because the time or timing of the transfer of information to specific users over traffic channels is somewhat arbitrary or random, in terms of absolute system control, the time at which a user terminal measurement will be taken cannot be predicted with any accuracy. That is, traffic channels are used only when a user terminal desires to have a communication link established such as to make or accept a “call” to a wireless phone. The establishment or existence of such calls is of course not a fixed periodic function and occurs at somewhat arbitrary times according to the desires of human operators or callers. Therefore, the timing of a measurement made by a user terminal is not known at the gateway until the user terminal measurement is received at the gateway. By then, of course, the user terminal measurement has already occurred and it is too late to take a simultaneous gateway measurement.

The present invention solves this problem by periodically or regularly taking time measurements either at the gateway or the user terminal, establishing an historical measurement record, and later using this historical data to select or calculate a value that corresponds to a reported user terminal measurement. According to a preferred embodiment of the present invention, gateway measurements are taken at regular intervals using known techniques for measuring signal parameters such as code phase or timing in a spread spectrum signal, and time. The measurement or values are stored in a pre-selected location or memory device for later retrieval, or used immediately to make a determination. Alternatively, as discussed below, the user terminal makes periodic time measurements, and either stores or transfers them.

Gateways utilize known control processors and computer programs to effect many operations. Therefore, there are well known memory elements such as RAM and ROM circuits along with magnetic storage devices and associated storage and retrieval apparatus or devices available for storing such information, as discussed above. Furthermore, measurements are stored using one of many known techniques which allow association or knowledge of the time at which the measurements were taken with the measured value. For example, a memory can be structured such that it stores the measured value and an associated time in pairs. Alternatively, memory locations are indexed or assigned to certain times on some pre-selected time scale. Values are then stored according to the memory location corresponding to the time of the measurement.

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The gateway memory used for this process can be very transient, that is, the data is preferably deleted or erased after a predetermined length of time which could be fairly short. Generally, there is a period after which a measurement is either considered not useful, or not likely to have been taken and still remain useful in this process. For example, measurements arriving many days or even many hours after being taken may be of no value, as having low accuracy or providing no useful position data. The corresponding gateway measurements might not be stored for that long, or could be stored in a different type of memory for alternative types of lower accuracy computations.

Then, when a user terminal measurement is reported to the gateway, the gateway selects one or more stored gateway measurements that were taken nearest the time at which the user terminal measurement was taken. The gateway then applies an interpolation process to the gateway measurements to obtain a gateway measurement that corresponds to the user terminal measurement. For example, the gateway can apply linear interpolation by selecting two measurements that are nearest but on either side of the user terminal measurement, and then generating a measurement value that would have been at the same time. In the alternate embodiments where the user terminal periodically stores measurements and the corresponding time, the gateway reports measurements it takes to the user terminal which then applies interpolation to obtain a user terminal measurement that corresponds to the gateway measurement.

It should be understood that using two points and linear interpolation is not a limiting feature of the present invention. That is other known techniques for establishing a probable data value can be used. For example, the two values can simply be averaged or used to form some mean value and so forth. The two values can be on the same “side” of the time provided by the user terminal, or gateway, and used to extrapolate the desired value. In addition, multiple values can be used, such as 3 or more data points with two or more on each side of the reported measurement that is being analyzed. The use of multiple values and the specific technique should be applied consistently and have some statistically or historically relevant value for the accuracy being sought..

FIG. 4 is a flowchart depicting the operation of the present invention according to a preferred embodiment. The gateway takes measurements of the signals received at the gateway, as shown in a step 402. Representative measurements are described in commonly-owned, co-pending U. S. Patent Applications entitled Position Determination Using One Low-Earth Orbit Satellite, Serial No. 08/723,751; Passive Position Determination Using Two Low-Earth Orbit Satellites, Serial No. 08/723,722;

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PCT/US00/08629 and Unambiguous Position Determination Using Two Low-Earth Orbit Satellites, Serial No. 08/723,725, all of which are incorporated herein in their entirety by reference.

In a preferred embodiment, the gateway measurements are taken at regular intervals. These intervals are selected in accordance with known factors such as the anticipated length of or variation in path delay for signals, anticipated movement of user terminals, or the amount of circuitry and processing power needed to handle larger numbers of measurements as this interval is shortened. Clearly, the usefulness of the storage technique diminishes for those situations in which the user terminal measurement is arriving at the gateway very close to the time of the gateway measurements, and the minimum interval length should be set accordingly.

In one embodiment, the duration of the interval is 20ms. FIG. 5 depicts a timeline showing the relative timing of several gateway measurements GWi through GW₆. As discussed above, the timing of the gateway measurements must be known with accuracy. In a preferred embodiment, this is accomplished by time-stamping each measurement. The gateway stores each gateway measurement for future reference.

At some arbitrary time, the user terminal measures the signals it receives, as shown in a step 404 in FIG. 4. The timing of the user terminal measurement is shown in the timeline of FIG. 5 as UTi. This measurement is time-stamped and reported to the gateway.

When the gateway receives the user terminal measurement, the gateway selects the pair of gateway measurements that are nearest in time to the user terminal measurement, as shown in a step 406. In other words, the gateway selects the pair of gateway measurements that were taken immediately before and immediately after, or nearest in time before and after, the user terminal measurement. In the example of FIG. 5, for user terminal measurement UTj, the gateway selects gateway measurements GW3 and GW4. In a preferred embodiment, this selection is made by comparing the time stamp of the user terminal measurement to the time stamps of the stored gateway measurements. This can be accomplished for example by comparison to time data stored with the gateway measurements or to an index value for the stored data. Finally, the gateway interpolates between the selected gateway measurements to obtain a value that most nearly corresponds to the reported user terminal measurement, as shown in a step 208. In a preferred embodiment, linear interpolation is used. However, other modes of interpolation or choosing a close value may be used where improved accuracy is achieved, or desired, as would be known. For example, one can use simple averaging or mean values discussed above.

In another embodiment of the present invention, when a user terminal measurement is reported to the gateway, the gateway selects the single stored gateway measurement that was taken nearest the time at which the user terminal measurement was taken. This approach is less accurate, but avoids the need for interpolation or other processing discussed above, which can reduce computational requirements for circuitry within a gateway and increase the speed of processing.

The procedures described above account for the difference in time between user terminal and gateway measurements with a high degree of accuracy. The calculated or selected gateway value can be combined with the corresponding user terminal measurement to determine many useful parameters such as user terminal

15¹ position.

V. Conclusion . The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. While the • invention has been particularly shown and described with reference to preferred ₉ embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

, It will be understood that the term comprise and any of its derivatives (eg.

• comprises, comprising) as used in this specification is to be taken to be inclusive of ’ features to which it refers, and is not meant to exclude the presence of any additional ‘ features unless otherwise stated or implied.