Satellite and local system position determination Number:7,385,554 from the United States Patent and Trademark Office (PTO) owispatent

Title: Satellite and local system position determination

Abstract: In a local positioning system, augmentation of the land-based system is provided by receiving signals from a GNSS. The signals from the land-based positioning system have a code phase accuracy better than one wavelength of a carrier of the signals from the GNSS. Different decorrelation may be used for signals from a satellite than from a land-based transmitter, such as using a digital decorrelator for signals from the satellite and an analog decorrelator for signals from a land-based transmitter. The receivers may include both a GNSS antenna and a local antenna. The phase centers of the two antennas are within one wavelength of the GNSS signals from each other. The local antenna is sized for operation in the X or ISM-bands of frequencies. The GNSS antenna is a patch antenna where the microwave antenna extends away from the patch antenna in at least one dimension.

Patent Number: 7,385,554 Issued on 06/10/2008 to Zimmerman,   et al.

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Inventors:	Zimmerman; Kurt R. (Mountain View, CA), Cobb; H. Stewart (Palo Alto, CA), Montgomery; Paul Y. (Menlo Park, CA), Lawrence; David G. (Santa Clara, CA)
Assignee:	Novariant, Inc. (Fremont, CA)
Appl. No.:	11/515,547
Filed:	September 5, 2006

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Related U.S. Patent Documents

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	Application Number	Filing Date	Patent Number	Issue Date
	10909184	Jul., 2004	7271766

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Current U.S. Class:	342/464 ; 342/357.02; 342/357.14; 342/456; 342/463
Field of Search:	342/357.02,357.06,357.04,450,456-457,463-464

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References Cited [Referenced By]

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Primary Examiner: Tarcza; Thomas H.
Assistant Examiner: Mull; Fred H.
Attorney, Agent or Firm: Brinks, Hofer, Gilson & Lione

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Parent Case Text

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CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/909,184, filed Jul. 30, 2004 now U.S. Pat. No. 7,271,766, the entire disclosure of which is incorporated herein by reference.

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Claims

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We claim:

1. A system for determining a position of a receiver from transmitted signals, the system comprising: a digital de-correlator operable to de-correlate first signals from at least one satellite; an analog de-correlator operable to de-correlate second signals from at least one land based transmitter; and a processor operable to determine the position of the receiver as a function of the outputs of the digital and analog decorrelators; wherein the second signals have a modulation rate of a second code being at least 30 MHz carried in the X-band, JSM band or combinations thereof.

2. The system of claim 1 wherein the second signals from the at least one land based transmitter have a code phase range accuracy better than one wavelength of a carrier of the first signals from the at least one satellite.

3. The system of claim 1 further comprising: first and second analog-to-digital converters; wherein the digital de-correlator comprises: a first code generator; and a digital mixer operable to mix a first code output by the first code generator with the first signals output by the first analog-to-digital converter; wherein the analog de-correlator comprises: a second code generator; and an analog mixer operable to mix a second code output by the second code generator with the second signals; wherein the second analog-to-digital converter is operable to convert the mixed second code and second signals from analog to digital.

4. The system of claim 1 wherein the first signals have a carrier with a wavelength greater than 15 cm, and measurements of the second signals have code phase range accuracy better than 2 cm.

5. The system of claim 1 wherein the modulation rate of the second code of the second signals is at least ten times a first code modulation rate of the first signals.

6. The system of claim 1 wherein the first and second signals are direct-sequence, spread spectrum signals responsive to a first code and the second code, respectively, the first code different than the second code.

7. The system of claim 1 wherein the first signals have a carrier with a wavelength greater than 15 cm, and measurements of the second signals have accuracy better than one-half wavelength of the carrier of the first signals.

8. A method for determining a position of a receiver from transmitted signals, the method comprising: (a) digital de-correlating first signals from at least one satellite; (b) analog de-correlating second signals from at least one land based transmitter; and (c) determining the position of the receiver as a function of the de-correlated first and second signals; wherein (a) comprises de-correlating the first signals, the first signals having a carrier with a wavelength greater than 15 cm; and wherein (b) comprises de-correlating the second signals, the second signals having a better than 2 cm code phase range accuracy.

9. The method of claim 8 wherein (b) comprises de-correlating the second signals from the at least one land based transmitter, the code phase range accuracy better than one wavelength of a carrier of the first signals from the at least one satellite.

10. The method of claim 8 wherein (a) comprises mixing a first code with the first signals after converting the first signals from analog to digital; and wherein (b) comprises mixing a second code with the second signals prior to converting the mixed second code and signals from analog to digital.

11. The method of claim 8 wherein (b) comprises de-correlating the second signals, the second signals having a modulation rate of a code being at least 30 MHz carried in the X-band, ISM band or combinations thereof.

12. The method of claim 8 wherein (b) comprises de-correlating the second signals, the second signals have a second code modulation rate at least ten times a first code modulation rate of the first signals.

13. The method of claim 8 wherein (a) and (b) comprise de-correlating direct-sequence, spread spectrum signals responsive to different codes.

14. The method of claim 8 wherein the code phase range has an accuracy better than one-half wavelength of the carrier of the first signals.

15. A system for determining a position of a receiver from transmitted signals, the system comprising: a digital de-correlator operable to de-correlate first signals from at least one satellite; an analog de-correlator operable to de-correlate second signals from at least one land based transmitter; and a processor operable to determine the position of the receiver as a function of the outputs of the digital and analog decorrelators; wherein the first signals have a carrier with a wavelength greater than 15 cm, and measurements of the second signals have code phase range accuracy better than 2 cm.

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Description

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BACKGROUND

The present invention relates to range or position determination. In particular, signal structures, transmitters, receivers, other components and/or methods of operation of a ranging or positioning system are provided.

Global navigation satellite systems (GNSS) allow a receiver to determine a position from ranging signals received from a plurality of satellites. Different GNSS systems are available or have been proposed, such as the global positioning system (GPS), Gallileo or GLONASS. The GPS has both civilian and military applications. Different ranging signals are used for the two different applications, allowing for different accuracies in position determination.

Position is determined from code and/or carrier phase information. A code division multiple access code is transmitted from each of the satellites of the global positioning system. The spread spectrum code is provided at a 1 MHz modulation rate for civilian applications and a 10 MHz modulation rate for military applications. The code provided on the L1 carrier wave for civilian use is about 300 kilometers long. The codes from different satellites are correlated with replica codes to determine ranges to different satellites. Using civilian code phase information, an accuracy of around one or two meters may be determined. Centimeter level accuracy may be determined using real-time kinematic processing of carrier phase information. A change in position of the satellites over time allows resolution of carrier phase ambiguity.

In addition to satellite based systems, land-based transmitters may be used for determining a range or position. Land based transmitters may include pseudolites. Pseudolite systems have been proposed for landing aircraft and determining a position of a cellular telephone. Pseudolites typically use GPS style signals or codes. For example, a GPS spectrum code is transmitted on a same or different carrier frequency as used for GPS. Code division multiple access (CDMA) may be over-laid with time division multiple access (TDMA) methods to increase a dynamic range of the GPS style coded signals. Some pseudolites systems are arranged for use with GNSS. As a result of using GNSS types of signals, psuedolite systems may be limited to several meters of accuracy based on code phase measurements.

BRIEF SUMMARY

The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include methods and systems for a land-based range or position determination. To provide sub-meter accuracy, ranging signals with a high modulation rate of code, such as 30 MHz or more, are transmitted. Code phase measurements may be used to obtain the accuracy without requiring relative motion or real time kinematic processing to resolve any carrier cycle ambiguity. The ISM bands or X-band is used for the carrier of the code to provide sufficient bandwidth within available spectrums. The length of codes used is at least about a longest length across the region of operation, yet less than an order of magnitude longer, such as about 15 kilometers in an open pit mine, but other lengths may be used. The spread spectrum codes from different land-based transmitters are transmitted in time slots pursuant to a time division multiple access scheme for an increase in dynamic range. The dynamic range is a range of power over which a receiver can track a signal, to distinguish from "range" as in distance measurement. To avoid overlapping of code from different transmitters, each time slot includes or is separated by a blanking period. The blanking period is selected to allow the transmitted signal to traverse a region of operation without overlap with a signal transmitted in a subsequent time slot by a different transmitter. Differential measurements of signals received at a base station and a mobile receiver may allow for improved accuracy. Any one or more of the signal structure characteristics summarized above may be used independently or in combination with other signal characteristics in a land-based transmitter system.

In addition to or for use independently of the signal structure characteristics discussed above, the land-based transmitters include free running oscillators or oscillators free of clock synchronization with any remote oscillator. A reference receiver receives the ranging signals from different transmitters and generates timing offset information, such as code phase measurements. The timing offset information is then communicated back to transmitters. The temporal offset information indicates relative timing or phasing of the different transmitted ranging signals to the reference receiver. The transmitters then transmit the temporal offset information with the ranging signals, such as modulating the transmitted code by the timing offset information. A mobile receiver is operable to receive the ranging signals and timing offset information in a same communications path, such as on a same carrier. Alternatively, the timing offset information is communicated from the reference directly to the mobile. A system with oscillators that are synchronized with GNSS or any other synchronization source may be used. Position is determined with the temporal offset information and the ranging signals. The temporal offset information for the various transmitters allows the mobile receiver to more accurately determine position than in an unsynchronized system. Various aspects of the synchronization of the system discussed above may be used independently of each other or in combination.

In addition to or for use independent from the above described synchronization and signal characteristics, other features are provided in a land-based ranging system. For example, augmentation of the land-based system is provided by receiving signals from a GNSS. The signals from the land-based positioning system have code phase accuracy better than one wavelength of a carrier of the signals from the GNSS. Different decorrelation may be used for signals from a satellite than from a land-based transmitter, such as using a digital decorrelator for signals from the satellite and an analog decorrelator for signals from a land-based transmitter. The receivers may include both a GNSS antenna and a higher frequency microwave antenna, also referred to as a local antenna. The term "microwave" is used here to include frequencies from about 900 MHz to 300 GHz. The phase centers of the two antennas are within one wavelength of the GNSS signals from each other. The microwave antenna is sized for operation in the X or ISM-bands of frequencies. The GNSS antenna is a patch antenna where the microwave antenna may extend away from the patch antenna in at least one dimension. Any of the various characteristics of an augmented GNSS and land-based ranging system may be used independently or in combination.

In addition to or for use independent of the signal characteristics, synchronization characteristics or augmentation characteristics discussed above, a receiver is adapted for receiving signals from a land-based transmitter. The receiver includes an analog decorrelator for decorrelating the transmitted spread spectrum signals. A down converter connected with an antenna may be spaced away from other portions of the receiver. The down converter down converts received ranging signals and provides them to the remotely spaced receiver portions. A signal line connecting the down converter to the receiver may be operable to transmit any two or more of a reference signal provided to the down converter, the down converted intermediate frequency signals provided to the receiver, and power provided to the down converter. The receiver may be positioned adjacent to or as part of a land-based transmitter. By determining positions of two or more antennas, the location of the associated transmitter is determined. Any of the various receiver characteristics described above may be used independently or in combination.

In a first aspect, a method is provided for determining a position from transmitted signals. At least one range is measured from signals for a GNSS system. At least another range is measured from signals for a local positioning system. A position is determined from the measured ranges. The signals of the local positioning system, as measured by a receiver, have code phase range accuracy better than one wavelength of a carrier of the signals from the GNSS system.

In a second aspect, a system is provided for determining a position from transmitted signals. A GNSS receiver is operable to measure at least one range from signals from a satellite. A local positioning system receiver is operable to measure another range from signals from a land-based transmitter. The signals from the land-based transmitter, when measured by a receiver, have code phase range accuracy better than one wavelength of a carrier of the signals from the GNSS system. A processor is operable to determine a position from the measurements of the GNSS receiver and the local positioning system receiver.

In a third aspect, a system is provided for determining a position from transmitted signals. A digital decorrelator is operable to decorrelate signals from a satellite. An analog decorrelator is operable to decorrelate signals from a land-based transmitter. A processor is operable to determine a position as a function of the digital and analog decorrelated signals.

In a fourth aspect, a method is provided for determining a position from transmitter signals. Signals from a satellite are decorrelated in a digital domain. Signals from a land-based transmitter are decorrelated in an analog domain. A position is determined as a function of the decorrelated signals.

Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments. The further aspects and advantages may be later claimed independently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

FIG. 1 is a graphical representation of one embodiment of a local positioning system with GNSS augmentation in an open pit mine;

FIG. 2 is a graphical representation of one embodiment of characteristics of a code and carrier of radio frequency ranging signals;

FIG. 3 is a graphical representation of time division multiple access transmissions used in a local positioning system of one embodiment;

FIG. 4 is a graphical representation of time division multiple access transmissions of another embodiment;

FIG. 5 is a graphical representation of the distribution of local transmitters and receivers for differential positioning in one embodiment;

FIG. 6 is a graphical representation of the distribution of local transmitters and receivers for differential positioning in another embodiment;

FIG. 7 is a block diagram of one embodiment of a land-based transmitter;

FIG. 8 is a block diagram of one embodiment of a receiver using two different ranging methods;

FIG. 9 is a block diagram of another embodiment of a receiver using two different ranging methods;

FIG. 10 is a block diagram of one embodiment of digital logic implemented in a receiver;

FIG. 11 is a block diagram of one embodiment of a receiver with a separated or remote down converter module;

FIG. 12 is a graphical representation of one embodiment of position solutions based on a number of available satellites and land-based transmitters;

FIG. 13 is a flow chart of one embodiment of a method of solving for a position using local positioning and a GNSS;

FIG. 14 is a graphical representation of an embodiment of combined GNSS and local positioning receive antennas;

FIGS. 15A and B graphically represent another embodiment of combined GNSS and local positioning antennas;

FIG. 16 is yet another embodiment of combined GNSS and local positioning antennas;

FIG. 17 is a graphical representation of one embodiment of self-surveying transmitter antenna of a local positioning system;

FIG. 18 is a block diagram of one embodiment of a receiver;

FIG. 19 is a flow chart diagram of one embodiment of a method for determining range information in a local positioning system;

FIG. 20 is a flow chart diagram of one embodiment for determining a position of a transmitter;

FIG. 21 is a flow chart diagram of one embodiment for decorrelation to determine a position; and

FIG. 22 is a flow chart diagram of one embodiment of a method for remote down conversion of received local positioning system ranging signals.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

GNSS relies on access to a plurality of satellites at any given location on the globe. For example, access to at least five satellites allows for position solution with carrier phase based centimeter accuracy. Some locations lack sufficient access to satellites. For example, FIG. 1 shows a system 10 with a plurality of satellites 12A-N relative to an open pit mine. A reference station 18 and mobile receiver 22 have lines of sight 14B, 14C to two satellites 12B, 12C but the walls of the mine block access to signals from other satellites 12A, 12N. In order to provide accurate positioning, a plurality of land-based transmitters 16A-N are positioned within, encircling, around, or combination thereof the mine.

The land-based transmitters 16, reference station 18 and/or mobile receiver 22 are a local positioning system. The local positioning system is operable without the satellites 12, but may be augmented with the satellites 12. Additional, different or fewer components may be provided, such as providing a greater or less number of land-based transmitters 16. As another example, the local positioning system may use a mobile receiver 22 without a reference station 18. A receiver may use signals from the local positioning system to determine a position or range. For example, the range from any one or more of the land-based transmitters 16 to either the reference station or the mobile receiver 22 is determined. A position may be determined from a plurality of ranges to other land-based transmitters 16. Using the reference station 18, additional accuracy in determining the position of the mobile receiver may be provided.

The land-based transmitters 16 are positioned at any of various locations within or around the mine. The land-based transmitters 16 include transmitters on poles, towers, directly on the ground, on stands, or other locations where the transmitter is maintained in a substantially same position relative to the ground. The land-based transmitters 16 are positioned such that most or all locations in the mine have line-of-sight access to four or more land-based transmitters 16. Access to a fewer number of transmitters may be provided.

The mobile receiver 22 is positioned on a piece of equipment, such as a truck, crane, excavator, vehicle, stand, wall or other mobile or possibly moving piece of equipment or structure. The reference station 18 is a land-based receiver, such as a receiver on a pole, tower, stand, directly on the ground or other position maintained in a substantially same location relative to the ground. While the reference station 18 is shown separate from the land-based transmitter 16, the reference station may be located with one or more of the land-based transmitter 16. More than one reference station 18 may be used. Both of the reference station 18 and mobile receiver 22 are operable to receive transmitted ranging signals from at least one of the land-based transmitters 16. FIG. 5 shows a top view of FIG. 1. As shown in FIGS. 1 and 5, a differential solution technique may be used. The ranging signals from one or more of the land-based transmitters 16 or other transmitters are received by both the reference station 18 and the mobile receiver 20. By communicating information on link 20 from the reference station 18 to the mobile receiver 22, additional accuracy in determining a position may be provided.

The local positioning system uses GNSS, such as GPS, ranging signals for determining the position of the mobile receiver. For example, the ranging signal is transmitted at the L1, L2, or L5 frequencies with a direct-sequence, spread spectrum code having a modulation rate of 10 MHz or less. A single cycle of the L1 frequency is about 20 centimeters in length, and a single chip of the spread spectrum code modulated on the carrier signal is about 300 meters in length. The code length is about 300 kilometers. The transmitters 16 continuously transmit the code division multiple access codes for reception by the receivers 18, 22. In the absence of movement by the mobile receiver 22, integer ambiguity of the carrier phase may be unresolved. As a result, code based accuracy less accurate than a meter is provided using GPS signals. Given movement of the mobile receiver 22, carrier phase ambiguity may be resolved to provide sub-meter or centimeter level accuracy.

In an alternative embodiment, different ranging signals are used by the local positioning system. FIG. 2 shows one embodiment of a ranging signal. The carrier wave of the ranging signal is in the X or ISM-bands. The X-band is generally designated as 8,600 to 12,500 MHz, with a band from 9,500 to 10,000 MHz or other band designated for land mobile radiolocation, providing a 500 MHz or other bandwidth for a local transmitter. In one embodiment, the carrier frequency is about 9750 MHz, providing a 3 centimeter wavelength. The ISM-bands include industrial, scientific and medical bands at different frequency ranges, such as 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz. The frequency and the bandwidth may be limited by government regulatory constraints. Other factors that affect the frequency and bandwidth include signal propagation properties. Different frequency bands for the carrier wave may be used, such as any microwave frequencies, ultra wide band frequencies, GNSS frequencies or other RF frequencies.

The ranging signals have a Direct-Sequence Spread Spectrum (DSSS) code. For example, a direct sequence code such as a Maximal Length Linear Finite Shift Register (MLFSR), a Gold or other pseudo-random noise (PN) code is provided. Other codes may be used. The code is modulated with a carrier at a modulation rate. For DSSS codes, the modulation rate is called the chipping rate. The modulation rate of the code is at least 30 MHz, at least 60 MHz, at least three times a GNSS modulation rate or other modulation rate. Given high bandwidth available at the chosen carrier frequency, greater or lesser modulation rates may be provided, such as 200 MHz. In one embodiment, the modulation rate is less than 250 MHz. Greater modulation rates may be used, such as rates categorized under Ultra Wide Band regulation. Given a 200 MHz modulation rate of the code, the width of each chip is 1.5 meters. In yet another alternative embodiment, a modulation rate of the code less than 30 MHz, such as 10 MHz or fewer, is used. The modulation rate is the same for each of the transmitters. The code used by each transmitter may be the same or different. In alternative embodiments, the modulation rate may also be different or the same for different transmitters.

The accuracy of the system to a first order is proportional to the code modulation rate. The rate is provided as high as possible to meet the desired accuracy but bounded by the available bandwidth, carrier frequency, hardware and other constraints. The modulation rate must be below or equal to the carrier frequency. In one embodiment, the nominal modulation rate is set to half of the available bandwidth, but may be greater and filtered to meet constraints of the available bandwidth, or lesser. The high bandwidth or modulation rate of the code may provide code based accuracy for range or position less than one wavelength of an L1 or L2 GPS frequencies. The accuracy of a signal is the accuracy of a measurement of the signal made by a receiver. The accuracy is sub-meter, such as being better than 19 or 24 centimeters. In one embodiment, the code-based range accuracy is better than 4 centimeters. In one method, accuracy is calculated with RMS code tracking errors. The RMS code tracking error for a single land-based transmitter 16 is computed from the radio navigation signal power present at the input terminals of a receiver. The ranging signal in one example is a pseudo random Binary Phase Shift Key (BPSK) signal centered at 9,750 MHz with a modulation rate of 30 million chips per second, so that the length of one chip in space is about 10 meters, and a peak transmit power of 1 Watt. The ranging signal is pulsed on and off in a duty cycle of 4 percent. A plus 10 dBIC gain Right Hand Circularly Polarized (RHCP) transmit antenna and a 0 dB gain RHCP receive antenna are assumed. The propagation environment is assumed to be free space with a maximum distance of 15 kilometers. Using a Friis transmission formula, the power available to the receiver is -110 dBm. The RMS code tracking jitter with the input power level of -110 dBm is computed. X-band receiver noise is assumed to be about 4 dB, resulting in a -170 dBm/Hz of the thermal noise power at the receiver input. The Delay Lock Loop (DLL) loop bandwidth is assumed to be 10 Hz, the predetection integration time is assumed to be 40 microseconds, the correlator chip spacing is assumed to be one chip, and no carrier smoothing is implemented. The RMS code tracking error resulting from these assumptions is 3.2 centimeters, or about 1/300 of the length of a single code chip. This fraction ( 1/300 of a single code chip) is one way to quantify the accuracy of a code signal. The 3.2 centimeters provides the 1-standard-deviation (1-Sigma) error on range for one transmitter 16. Accuracy is provided for 1-standard-deviation or better, such as 2- or 3-standard-deviations (e.g. 2-Sigma, 3-Sigma), but not worse, such as less than 1-standard-deviation. To estimate accuracy for a three-dimensional position solution, the 1-Sigma error is multiplied by the Dilution of Precision (DOP). Assuming a worst case DOP of 4.0, a 1-Sigma position error of 12.8 centimeters is provided. 12.8 centimeters is well within one wavelength of a GPS carrier. Better than 12 cm position accuracy may be provided. The accuracy was calculated above based on a number of assumptions and a particular ranging signal. Other ranging signals may be used with the same or different accuracy and using different assumptions. Using only a code phase measurement of the ranging signals shown in FIG. 2, centimeter level or sub-meter level position accuracy may be provided. When computed for a modulation rate of 200 MHz with the same assumptions, about 8.6 millimeters code phase accuracy is provided.

Accuracy may be improved by providing differential measurements of the ranging signals, particularly in the case of transmitters free of synchronization. For example, the reference station 18 and the mobile receiver 22 both measure a same ranging signal transmitted by a same land-based transmitter 16. The differential between phase measurements performed by the reference station 18 and the mobile receiver 22 systematically removes the unknown clock of the transmitter, as well as other common mode errors possibly including cable and receiver circuitry biases, and results in one unknown time difference between the reference and mobile receivers. Additional differential phase measurements from additional transmitters have the same one unknown time difference between the reference and mobile receivers. The fewer unknown variables and reduced common mode errors due to differential phase measurements results in improved accuracy of the mobile receiver relative to the reference. Accuracy may be further improved by placing the transmitter in a location relative to other transmitters (and satellites) that improves the dilution of precision (DOP). Generally, DOP is improved by placement in a region most orthogonal to other transmitters/satellites, as viewed from the receiver.

The ranging signals are further characterized by the length of the code. The local positioning system is operated within a region. The chip width and code length are set as a function of a longest dimension over which a ranging signal from a particular land-based transmitter 16 will traverse within the region of operation. In one embodiment, the chip width in space is much shorter than up to approximately equal to the longest dimension of the region of operation. The chip width of the code is directly related to the modulation rate by the speed of light. Therefore, selecting the chip width according to the region of operation is an alternative way to select a minimum modulation rate. In the embodiment shown in FIG. 2, the chip width is 1.5 meters, less than ten meters, or another value much less than the longest dimension of a likely region of operation.

In one embodiment, the code length in space is approximately equal to or slightly longer than a longest dimension of the region of operation. In another embodiment, the code length may be shorter. The code length in bits is equal to the modulation rate times the code length in space divided by the speed of light. Most easily generated codes have lengths close to powers of 2. For example, an N-bit MLFSR or Gold code register generates a code (2^N)-1 bits long. Given a desired initial code length in space and initial modulation rate, the nearest available code length in bits that is a power of two may be chosen. The code length in space or the modulation rate or both are adjusted accordingly. "Approximately equal" may include shorter code lengths than the region of operation. The code length in bits is calculated from the desired length and chip rate, and then adjusted to the next highest power of 2. The code length in space is then recalculated to provide the code length used. For example, the longest dimension of the region of operation is 10 kilometers in an open pit mine. The length of a 10 km code in bits at a 200 MHz chip rate is 10 km*200 MHz divided by the speed of light, which results in 6667 bits. The nearest easily generated code is 8191 bits long, so the code length in space is lengthened to 12.3 km. A lesser or greater code length may be provided for a 10 kilometer region of operation. Other code lengths may be used for the same sized or other sized regions of operation.

The length of code ensures that each measured code phase defines a unique range within the region of operation. A set of four measured code phases define a unique three-dimensional position and time within the region. The region of operation is the open pit mine or the region of the open pit mine associated with line of sight from a particular land-based transmitter 16. The region of operation may be the same or different for each land-based transmitter 16.

The local positioning system may have a code length set in common for any of a number of different uses. Alternatively, the code length is programmable to be configured depending on the different use or relative size of a region of operation for a given use.

As shown in FIG. 2, another possible characteristic of the ranging signal for each given transmitter is that the ranging signal is transmitted in a time slot. The Direct-Sequence Spread Spectrum (DSSS) code is turned on and off periodically pursuant to a time division multiple access scheme. The DSSS code is a ranging code, or more concisely here, a "code." Time division multiple access for the local positioning system increases the dynamic range. The number of time slots corresponds to the number of transmitters 16. The length and number of the time slots set the repeat period for each transmitter 16. The repeat period may be set based on the mobile user dynamics. The mobile receiver 22 makes range measurements and calculates position from the range measurements. The maximum acceleration and velocities of the mobile receiver 22 within the region of operation are taken into account by setting the repeat period. For faster moving mobile receivers 22, the local positioning system makes more rapid measurements over a shorter repeat period. A new measurement may be made each time the transmitted code is repeated. For faster mobile user dynamics, shorter repeat periods of the code are provided. For civilian GPS code, a repeat period of one millisecond is used. The fastest possible range measurement update is 1 KHz with a typical position measurement update of 10 Hertz or every 100 milliseconds. Update rates and repeat periods may be slower for users that move slower but desire greater accuracy. For example, tracking earth movement and deformation modeling or earthquake prediction may use much greater repeat periods, but update rates may be faster for faster moving mobile receivers, such as race cars on a track. Various methods may be used to predict the user dynamics to allow for longer repeat period. A nominal setting for the repeat period provides 100 times the bandwidth of the user dynamics. Other weighting values than 100 may be used, such as greater or lesser values. In the open pit mining embodiment, one millisecond repeat period is used, but other repeat periods may be provided.

The number of time slots available to the local positioning system is equal to the repeat period times the speed of light divided by the code length in space. Using time division multiple access, only one transmitted signal is desired to be present in the operating space for any given location at a given time. A ranging signal with a code length approximately equal to the size of the operating space is transmitted in one time slot. An additional time slot or a longer time slot period is provided to allow the transmitted ranging signal to traverse the operating space. This blank period is about as long a code length in space, but may be longer or shorter. Each transmitter 16 transmits in a different time slot with an associated blanking period. Given the code length and corresponding blanking period, the transmitter capacity is equal to half the number of time slots calculated as discussed above. The desired transmitter capacity may include 4, 10, 100 or other numbers of transmitters. Through iteration, ranging signals with the desired capabilities may be provided for various types of operation.

The ranging signal transmitted in each of the time slots is synchronized to within at least three microseconds of other ranging signals, but longer or shorter time periods may be used. Using the open pit mine example discussed above, 12 time slots are available where each time slot is twice the length of the code. The time slot includes a period for transmission of the code and a subsequent blanking period. The transmitted code is aligned to the beginning of the time slot and the time slot is maintained to a global time to better than a few microseconds, such as synchronized to within at least 3 microseconds. The slot timing accuracy is provided from received GPS signals, a radio pulse, a common cable connected between different components, cable modem connections between different components, programmed clock settings, combinations thereof or other mechanisms for generally aligning time slots. In one embodiment, the timeslots are arbitrarily selected and fixed per transmitter. In another embodiment, a user may remotely configure the transmitter timeslot. In yet another embodiment, a transmitter may have a sensor to detect energy transmitted by other transmitters, and dynamically select an empty timeslot.

FIG. 3 shows one representation of the TDMA/DSSS ranging signals transmission and reception scheme. Multiple DSSS signals are transmitted in unique TDMA time slots. Each transmitter transmits two unique DSSS codes or two instances of the same code, designated C11 and C12 for transmitter 1, C21 and C22 for transmitter 2 and so on. The codes from any one transmitter may be different to enable a receiver 18, 22 to easily distinguish between a time period to accumulate the detection measurements and another time period to accumulate tracking measurements.

The process of tracking a DSSS signal in TDMA uses two control signals, a detection signal for detecting correlation power between an incoming signal and an internally generated replica, and a tracking signal for feedback in a delay lock loop control scheme. The tracking signal is generated by correlating the incoming signal with an internally generated early-minus-late replica. One transmitter transmits two codes in immediate succession, such as two codes taking a total time of about 41 microseconds. A blank period of similar length follows the transmission of the ranging signal. A different transmitter subsequently transmits two codes in immediate succession, with the process repeating for all transmitters and each of the different time slots. Within the mobile receiver 22, channels are switched between different internally generated digital codes to control which code is sent to a mixer. Another switch prior to the channel switch switches between detection and tracking signals. When the receiver tracks a transmitter code, the internally generated replica of the code is delayed by a period of time that corresponds to the range of the receiver 18, 22 from the transmitter 16. The receiver's internally generated C11 detection and C12 tracking codes are shown in FIG. 3 for tracking the ranging signals transmitted by the first transmitter 16, delayed by the time of flight over the range from the first transmitter 16 to the mobile receiver 22. In the second time slot, the mobile receiver 22 internally generates the C21 detection and C22 tracking codes to track the ranging signal transmitted by the second transmitter 16, delayed from the start of the second time slot by the time of flight of the range between the second transmitter 16 and the mobile receiver 22. The process is repeated for each of the transmitter and receiver channels. After a 1 millisecond or other repeat period, the process is repeated for the same transmitters.

FIG. 4 shows an alternative scheme for transmitting signals than shown in FIG. 3. In the scheme shown in FIG. 4, a transmitter 16 transmits the two different codes in two different repeat cycles. For example, a first code C11 by the first transmitter 16 is transmitted in a first cycle and a second code C12 is transmitted in a second repeat cycle of the time division multiple access scheme. The pair of repeat periods is then repeated. Correspondingly, the receiver generates the replica prompt and tracking codes on alternate repeat periods to correspond to the transmitted signal. In another embodiment, a single code is used by each transmitter 16. In yet another embodiment, continuous, non-TDMA transmission is used by each land-based transmitter 16.

The local ranging signals and positioning system having any one or more of the signal characteristics may be used in many different environments. In the example of FIG. 1, the local positioning system is used in an open pit mine. The local positioning system is used to track and guide machinery such as haul trucks, drills, shovels, or bulldozers. The positioning information is used for machine guidance, machine-tool guidance, geographic surveying, operation scheduling, determining deformation of a wall or other structure to predict collapse and avoiding hazards. Other uses may be provided. Centimeter level accuracy allows these uses to be more versatile. With a real time update rate of 10 Hz or higher, vehicle speeds of about 50 miles per hour may be accurately tracked. Greater or lesser update rates and associated speeds may be provided. By allowing the interaction of a multiple land-based transmitter 16, such as 10 to 100 transmitters, an entire open pit mine may be covered by ranging sign