WO2010016783A1 - Space based augmentation system ranging signal applied to l1 real time kinematic - Google Patents

Abstract

Methods and apparatus, including computer program products, for Space Based Augmentation System (SBAS) ranging signal applied to L1 Real Time Kinematic (RTK). A method includes enabling Space Based Augmentation System (SBAS) ranging data received from SBAS satellites in L1 radio signal (L1) Global Positioning System (GPS) Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

Description

SPACE BASED AUGMENTATION SYSTEM RANGING SIGNAL APPLIED TO Ll

REAL TIME KINEMATIC

BACKGROUND

[001] The present invention relates to data processing by digital computer, and more particularly to Space Based Augmentation System (SBAS) ranging signal applied to Ll Real Time Kinematic (RTK).

[002] In general, RTK satellite navigation is a process where Global Positioning System (GPS) signal corrections are transmitted in real time from a reference receiver at a known location to one or more remote rover receivers. The use of an RTK capable GPS system can compensate for atmospheric delay, orbital errors and other variables in GPS geometry, increasing positioning accuracy up to within a centimeter.

[003] "Normal" satellite navigation receivers compare a pseudo random signal being sent from a satellite with an internally generated copy of the same signal. Since the signal from the satellite takes time to reach the receiver, the two signals do not "line up" properly, the satellite's copy is delayed in relation to the local copy. By progressively delaying the local copy more and more, the two signals will eventually line up properly. The delay is the time needed for the signal to reach the receiver, and from this the distance from the satellite can be calculated.

[004] The accuracy of the resulting range measurement is generally a function of the ability of the receiver's electronics to accurately compare the two signals. In general, receivers are able to align the signals to about 1% of one bit-width. For instance, the C/A signal sent on the GPS system sends a bit every 1/1 Oth of a microsecond, so a receiver is accurate to 1/lOOth of a microsecond, or about 3 meters in terms of distance. The military-only P(Y) signal sent by the same satellites is clocked ten times as fast, so with similar techniques the receiver will be accurate to about 30 cm. Other effects introduce errors much greater than this, and accuracy based on an uncorrected C/A signal is generally about 15 m.

[005] RTK follows the same general concept, but uses the satellite's carrier as its signal, not the messages contained within. An improvement possible using this signal is potentially very high if one continues to assume a 1% accuracy in locking. For instance, the GPS C/A signal broadcast in the Ll signal changes phase at 1.023 MHz, but the Ll carrier itself is 1575.42 MHz, over a thousand times faster. This corresponds to a 1% accuracy of 19 cm using the Ll signal, and 24 cm using the lower frequency L2 signal.

[006] One difficulty in making an RTK system is properly aligning the signals. The navigation signals are deliberately encoded in order to enable them to be aligned easily, whereas every cycle of the carrier is similar to every other. This makes it extremely difficult to know if you have properly aligned the signals, or are "off by one" and thus introducing an error of 20 cm or a larger multiple of 20 cm. This integer ambiguity problem can be addressed to some degree with statistical methods that compare the measurements from the C/A signals and by comparing the resulting ranges between multiple satellites. However, none of these methods can reduce this error to zero.

[007] In practice, RTK systems use a single base station receiver and a number of mobile units. The base station re-broadcasts the phase of the carrier that it measured, and the mobile units compare their own phase measurements with the ones received from the base station. This enables the units to calculate their relative position to millimeters, although their absolute position is accurate only to the same accuracy as the position of the base station.

SUMMARY

[008] The present invention provides methods and apparatus, including computer program products, for Space Based Augmentation System (SBAS) ranging signal applied to Ll Real Time Kinematic (RTK).

[009] In general, in one aspect, the invention features adding a SBAS ranging signal into GPS RTK solution improving thereby the time to achieve cm level accuracy.

[0010] In another aspect, the invention features a number of special SBAS related processes implemented to enable adding SBAS seamlessly to a GPS RTK process improving thereby solution robustness.

[0011] In another aspect, the invention features a method including enabling Space Based Augmentation System (SBAS) ranging data received from SBAS satellites in Ll radio signal (Ll) Global Positioning System (GPS) Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

[0012] In another aspect, the invention features a system including a number of Global Positioning System (GPS) satellites, each of the GPS satellites transmitting a periodic satellite GPS Ll signal, a number of Space Based Augmentation System (SBAS) satellites, each of the SBAS satellites transmitting a periodic SBAS satellite Ll signal, a land-based reference receiver configured to receive one or both of the GPS Ll signal and SBAS Ll signal, and a land-based rover receiver communicatively coupled to the land-based reference receiver with an airwave link, the land-based rover receiver enabled to use either the SBAS Ll signal or the GPS Ll signal in Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

[0013] Other features and advantages of the invention are apparent from the following description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a block diagram of an exemplary system of a positioning satellite system. [0015] FIG. 2 is a block diagram showing satellite positions. [0016] FIG. 3 is a block diagram of a receiver system.

[0017] Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0018] As shown in FIG. 1, an exemplary coordinate system of a positioning satellite system 10 includes a center (0,0,0) that is made to be coincident with the center of the earth. Only two satellites SV.sup.j and SV.sup.k are shown, for simplicity, and they are indicated to be traveling in the same orbit 12. The satellite orbits are actually elliptical in shape but shown in this example to be circular for ease in illustration.

[0019] Receivers are positioned at points A (position X.sub.A, Y.sub.A, Z.sub.A of the satellite system) and B (X.sub.B, Y.sub.B, Z.sub.B), which, in this example, are stationary locations on earth. It is the relative position of points A and B that is to be measured from the simultaneous receipt of signals from multiple satellites. The satellites include Global Positioning System (GPS) satellites and Space Based Augmentation System (SBAS) satellites. In other examples, the relative positions of three or more points may be determined, and one or more of them may be moving with respect to the others. The principles of the relative positioning techniques described herein are used in these examples as well.

[0020] Data of the respective ranges .rho..sub.R.sup.j and .rhc.sub.R.sup.k of the two satellites shown are transmitted on respective satellite signals as a function of each satellite's internal clock. Data of the path of travel of each satellite is updated periodically, as often as once each hour, by transmissions from a ground station as a function of some common time, such as GPS time. The ranges .rhc.sub.A.sup.j and .rhc.sub.A.sup.k are measured from data acquired by the receiver A. The ranges .rho..sub.B.sup.j and .rhc.sub.B.sup.k are measured from data acquired by the receiver B. This enables the difference between position vectors .rhc.sub.R.sup.A and .rhc.sub.R.sup.B to be calculated, and a relative position vector b. sub. AB of the points A and B to be determined. It is the relative position vector b.sub.AB, the difference in location of the points (X.sub.A, Y.sub.A, Z.sub.A) and (X.sub.B, Y.sub.B, Z.sub.B) that is a final result.

[0021] It should be appreciated that the quantities such as pseudo-range and carrier phase may be computed from measurements at a location that is other than the location of the receiver corresponding to the measurements.

[0022] Since the relative positions of the satellites with respect to the ground points A and B are constantly changing, a signal measurement is made at one instant of time (in one epoch). Additional measurements are generally made in subsequent epochs in order to obtain redundant information that is processed in order to improve a signal-to-noise ratio of the resulting composite measurement.

[0023] As shown in FIG. 2, a position of the satellite SV.sup.J at one point in time t is SV.sup.J (t). At a subsequent time (t+1), after traveling over a path 14, that satellite is at the position SV.sup.J (t+1). Similarly, the range of that satellite from the point A at time t is .rho..sub.A.sup.j (t), and at time (t+1) is .rho..sub.A.sup.j (t+1).

[0024] As shown in FIG. 3, in order to measure the relative positions of points A and B, a receiver antenna is positioned at each of these points and connected to respective receivers 16, 18. Characteristics (observables) of each of the multiple satellite signals received by each of the receivers 16, 18 are measured. Data of those observables are then sent by each receiver 16, 18 to a post processor 20, which is a computer programmed to calculate the relative position vector b.sub.AB from these observables. The post processor 20 is typically placed in a location that is different from either of the receivers 16, 18.

[0025] In one example, the post processor 20 can be co-located with one of the receivers 16, 18. In other examples, the post processor 20 may be co-located with one of the receivers 16, 18 and post-processing functions performed in near real time. Communication between the receivers 16, 18 and the post processor 20 can be via a radio link, a telephone data link, a dedicated data cable, and so forth.

[0026] In this example, signals from five satellites SV.sup.j, SV.sup.k, SV. sup.1, SV.sup.m, and SV.sup.n, are shown. Theses satellites can be any combination of GPS satellites and SBAS satellites.

[0027] Kinematic real-time (RTK) relative positioning can be accomplished using combined GPS and SBAS (Space Based Augmentation System) signals. SBAS' primary role is to deliver to a Global Navigation Satellite System (GNSS) user global ionosphere, orbit and Satellite clock corrections. These corrections are provided through a GPS-like ranging signal. In general, a ranging signal is an electromagnetic signal originating from an operational satellite that is used to measure the distance to the satellite. Along with correction decoding, a GNSS receiver can potentially generate pseudo-range and carrier phase measurements from a SBAS ranging signal. These measurements can be used in receiver positioning along with other GNSS measurements like GPS, GLONASS and Galileo.

[0028] A challenging part of using the SBAS ranging signal is its application to a Ll RTK (Real Time Kinematics) process that delivers centimeter level accuracy. SBAS measurements can be used in RTK positioning as an extra "GPS-like" Ll CA signal. Adding more satellites (e.g., SBAS Satellites) onto RTK process, one can noticeably improve satellite geometry especially in partly shaded and shaded areas. This improved geometry makes ambiguity initialization (time to centimeter) faster and more reliable. A SBAS ranging signal, being quite a similar to a GPS ranging signal, has nevertheless some negative differences that require more care when making GPS+SBAS RTK instead of GPS RTK. These differences include, for example, different SBAS orbits and clock computation and validation, insuring the same SBAS tracked on base and rover, and possible SBAS measurements calibration.

[0029] SBAS satellites provide not only long, fast and ionosphere corrections, but also GPS- like signals, the pseudo range and carrier phase measurements of which can be used in positioning, together with GPS measurements, i.e., SBAS measurements can augment GPS Ll RTK.

[0030] The SBAS ranging signal is the same as the GPS ranging signal. This means that corresponding pseudo-range and carrier phase measurements are equivalent to GPS Ll CA measurements. A difference between SBAS and GPS is different navigation data, e.g., SBAS orbits and SBAS clock corrections are computed differently from GPS.

[0031] Currently the operating SBAS constellations include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), the MTSAT Satellite based Augmentation System (MSAS) and the Indian SBAS GPS Aided Geo Augmented Navigation (GAGAN). WAAS includes two satellites covering North and South America and parts of the Pacific ocean. EGNOS includes three satellites covering Europe and Africa and some nearby countries. MSAS includes two satellites covering Japan, China and Australia. Signal quality and maturity of orbital information in WAAS and MSAS satellites are better than in EGNOS but it is a challenge to perform GPS+EGNOS RTK and actually achieve improvement.

[0032] Fixed RTK usually concerns a time needed to fix carrier ambiguity and to achieve a centimeter (cm) level solution, insuring at the same time, preset reliability. The GPS+SBAS RTK process enables use of SBAS measurements in the RTK process, thus making it a true GNSS technology. SBAS gives extra GPS-like measurements, which improves satellite geometry and enables achieving centimeter (cm) level accuracy faster than GPS alone. [0033] SBAS ranging data (i.e., pseudo range and carrier phase measurements) appear to be very similar to GPS ranging data. SBAS ranging data follow the very same observation model and can be absorbed into the GPS RTK positioning process as extra GPS Satellites. However, the following must be taken into account.

[0034] First, SBAS navigation information is not always accurate. This is clearly seen with EGNOS, which often provides low quality ephemeris and no acceptable clock corrections.

[0035] Second, a SBAS signal is not always stable.

[0036] Third, SBAS constellations have changed many times and are still not fixed for EGNOS.

[0037] Fourth, short term SBAS clock stability is poorer than that in GPS, which does not enable extrapolation of SBAS data effectively in time.

[0038] Fifth, there are some receiver related issues that can lead to some SBAS measurement biases between receivers of different types/manufactures.

[0039] Sixth, most current GPS+SBAS receivers are not "SBAS-all-in-view," because the primary function of SBAS is to provide corrections and not measurements, e.g., there is no any need to have more than two SBAS tracking channels.

[0040] This GPS+SBAS RTK method described herein uses SBAS ranging and carrier data while taking care that a possible SBAS immaturity and failure does not spoil RTK behavior.

[0041] This GPS+SBAS RTK process includes adaptive SBAS usage, SBAS data calibration, and SBAS tracking synchronization.

[0042] Adaptive SBAS usage includes detecting incorrect SBAS measurements and/or orbit measurements and stopping their usage in the RTK process. One example is poor ephemeris information. In this example, transmitted User Range Accuracy (URA) is not always adequate because SBAS with bad URA can often be effectively used in RTK process (i.e., orbital and/or clock errors can be acceptable for RTK positioning), but not for standalone positioning.

[0043] SBAS measurements (especially when base and rover data are provided by different receiver types) can have biases which must be accounted for. This process described herein estimates the possible SBAS biases in real time and compensates for them in the RTK processing.

[0044] Usually a receiver is equipped with only two channels to track SBAS, i.e. it is not an all-in-view SBAS receiver. In some cases, two plus SBAS satellites can be seen, so it is desirable to track in the rover receiver those SBAS satellites for which the base transmits data. Such a process has been implemented, which insures matched SBAS tracking on base and rover.

[0045] To enable GPS+SBAS RTK processing, a base station should send SBAS reference data. With standardized protocols, this is possible when using Radio Technical Commission for Maritime Services Standard 10403.1 for Differential GNSS Service, Version 3 (RTCM-3) format, where room for SBAS data is reserved.

[0046] The following SBAS-related methods are applied to insure seamless SBAS usage in RTK process. Adaptive SBAS usage is very important for insuring robust GPS+SBAS RTK. SBAS (mainly EGNOS) provides very inaccurate orbit/clock information and set corresponding flag URA = 15. This means that SBAS orbits/clocks cannot be used in a positioning process. However, an adaptive method is implemented that enables using SBAS with poor orbits in RTK process.

[0047] First, SBAS clock error does not matter for RTK because is canceled when making differences between base and rover. Second, orbit error (even large) is reduced noticeably when making differences between base and rover. The shorter the baseline, the less residual orbital error one experiences in RTK process. Orbital error can be transformed into baseline differential error. A problem is that one does not know a priori what the orbital error is. However, experience has shown that one can approximate orbital error by analyzing orbit jumps at the moment when SBAS IODE (Issue-Of-Data term for Ephemeris) changes (SBAS IODE typically changes each 3 minutes). Having estimated this error it can be transformed it to a corresponding sigma-value of baseline residual error. If given a sigma- value is less than a cm, it means that one can safely work with current SBAS orbits for current baseline. If it is larger than approximately 1 cm, it means that it is better not to use a given SBAS in RTK process. This enables using WAAS Satellites up to 10-20 km baseline length, while in many occasions with EGNOS limit SBAS RTK range by 1-2 km only. [0048] SBAS measurement calibration is important to insure robust GPS+SBAS RTK. SBAS pseudo range observables are affected by different biases. These biases are caused by different methods of computing SBAS pseudo ranges in different receivers/firmware. Because of current immaturity of SBAS ranging, one can expect some inter-receiver biases related with SBAS pseudo-range. Another important issue is slow multipath error which static receivers can experience. Any static receiver can experience slow multipath error, the period defined by satellite movement on the sky and nearby reflectors. For GPS, this period can vary from few minutes to ten minutes. At the same time, for SBAS this period can be as long as hours, because SBAS position is almost unchanged with time. This can result in very large pseudo range biases due to slow multipath. While it is not possible generally to calibrate multipath for GPS, it is possible (and desirable) to calibrate that for SBAS. Here a pseudo-range bias calibration procedure is implemented to mitigate the two negative effects above, i.e., different computation of SBAS pseudo-range by different receivers and multipath bias.

[0049] Calibration starts with an assumption that the initial bias is very large and corresponding pseudo-range is used in RTK process with very low weight. Once calibration is in progress, the estimated bias value becomes more and more accurate and corresponding measurement is taken into processing with higher weight. Once calibration converges, SBAS pseudo range biases can be used in RTK with weighting similar to GPS weights. The calibration method is guarded to escape possible bias mis-calibration in the effect when bias can be fast varied or have occasional jumps.

[0050] Synchronous SBAS tracking on base and rover has an important role to increase SBAS availability in RTK process. Most of SBAS-enabled receivers assign only two channels for SBAS. In many areas one can potentially see more than two SBAS signals. So there is a probability that base and rover receivers will track different SBAS signals. Here synchronous SBAS tracking between base and rover is implemented that in not all-in-view receivers can enable using the maximum possible number of SBAS signals.

[0051] Validation of adaptive SBAS orbit technology can be seen in the results for three baselines collected in Nantes when one of the WAAS satellites was seen in Europe. In all cases, two SBAS were tracked, i.e., SBAS#35 (WAAS) - good with URA = 0-2, and SBAS#33 (EGNOS) - bad with URA = 15.

[0052] Three comparable schemes were used, i.e., always using only one good SBAS#35, always using two SBAS (good and "bad"), and trying to use two SBAS (good and "bad") with adaptive auto-detection bad SBAS.

[0053] The RTK engine was validated as follows. The RTK engine was executed with default settings and an external script made to complete RTK reset each 300 seconds. Each test lasted for a duration of approximately 24 hours, which gave almost three hundred independent trials. After reset, the RTK engine was initialized and started with a float position. Some time later (in T seconds), fixed position was achieved. This time (T) is measured for each 300 second trial. Additionally, the accuracy of a fixed solution was measured against a true reference. Position is considered as correctly fixed if its horizontal error is below 7 cm. Otherwise, position is considered as wrongly fixed. The data from all trials were statistically processed and the following figures were derived:

[0054] Availability = the percentage of fixed trials over all the trials

[0055] Reliability = the percentage of correctly fixed trials over all fixed trials

[0056] 50% TTFF = the minimum time needed to fix ambiguity in at least 50% of trials

[0057] As seen in the above table, for baseline 1 km, using two SBAS (good and bad) is noticeably better than using only one good SBAS. Adaptive de-selecting works in such a way that "bad" SBAS is not deleted. As a result, with adaptive mode, the statistic is very close to that with two SBAS and much better than that with one SBAS.

[0058] For baseline 2 km, using two SBAS (good and bad) is a little better than using only one good SBAS. Adaptive deselecting works in such a way that "bad" SBAS is deleted some time. As a result, with adaptive mode, the statistic is very close to that with two SBAS and better than that with one SBAS.

[0059] For baseline 5 km, using two SBAS (good and bad) is much worse than using only one good SBAS. Adaptive de-selecting works in such a way that "bad" SBAS is always deleted. As a result, with adaptive mode, the statistic is very close to that with one SBAS and much better than that with two SBAS.

[0060] Accordingly, adaptive de-selecting logic behaves adequately, i.e., uses "bad" SBAS for short baselines and improves statistic compared to using only 1 good SBAS, and deselects "bad" SBAS for long baselines and make statistic similar to that with manual disabling "bad" SBAS.

[0061] SBAS can help in shaded areas. Three data sets, each at least 24 hours long, for blocked sky baselines were used. All the data were collected with Magellan® ProMark3 receivers on the west coast of the United States. At the given location, even with a shaded environment, at least one (and often two) common SBAS satellites were available for each baseline.

[0062] Baseline lengths were 1 km, 3.6 km (both partly shaded) and 2 meters (mostly shaded). The diagram below shows the availability for each data set, where availability refers to the percentage of fixed 300 sec trials over all the trials. Availability with and w/o SBAS

data set

[0063] One can see that for partly shaded baselines, SBAS performs very well. With heavy shading, the value of SBAS is very difficult to overestimate.

[0064] The GPS+SBAS RTK method is implemented in two Magellan products, i.e., the Magellan® DG 14 OEM board and the Magellan® ProMark3 hand held Surveyor. While RTK source code is exactly the same in both products, items related to deriving raw SBAS measurements, generating and decoding RTCM corrections, are different for these receivers. Therefore, the compatibility between the two different SBAS-enabled RTK receivers should be checked.

[0065] As stated above, commercial RTK bases do not send SBAS ranging data. So the Magellan® ProMark3 and Magellan® DG RTK rovers cannot take advantage of GPS+SBAS RTK processing in conjunction with third party RTK bases. They can effectively work with each other.

[0066] Eight open sky short baseline RTK tests (from one to ten meters) were performed in Nantes, France, Santa Clara, (CA) and Moscow, (Russia). These tests included the following configurations:

ProMark3 - ProMark3 DGRTK - ProMark3 DGRTK - DGRTK ProMark3 - DGRTK [0067] Receivers were operated with default settings. [0068] The PM3 and DG 14 are different hardware designs and use different SBAS tracking strategies. With features such as SBAS code bias calibration and synchronous SBAS satellite tracking, GPS+SBAS RTK between these two receivers cannot give any advantage over GPS only RTK. The below data shows what time to first fix (TTFF) performance can be achieved with each of receiver combinations using SBAS technology.

[0069] Each data set includes more than twenty- four hours of RTK data. Since receivers can track no more than two SBAS simultaneously, while in some cases (e.g. in Santa Clara) four SBAS can be potentially seen, tests were conducted under the control of special script with forced base receiver (DGRTK or ProMark3) to switch from one 2-SBAS combination to another each two hours. This enabled one to validate interoperability and performance for any SBAS configurations. Rover (DGRTK or ProMark3) always performed synchronous (with base) SBAS tracking.

[0070] As described above, the same RTK auto-reset methodology was used to derive TTFF performance. The diagram below gives the summary TTFF statistics.

data set

[0071] One can see that all the ProMark3/DGRTK combinations are compatible and provide excellent short baseline GPS+SBAS Ll RTK performance.

[0072] Embodiments of the invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Embodiments of the invention can be implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine readable storage device or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

[0073] Method steps of embodiments of the invention can be performed by one or more programmable processors executing a computer program to perform functions of the invention by operating on input data and generating output. Method steps can also be performed by, and apparatus of the invention can be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[0074] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry.

[0075] It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims. [0076] What is claimed is:

Claims

1. A method comprising: enabling Space Based Augmentation System (SBAS) ranging data received from SBAS satellites in Ll radio signal (Ll) Global Positioning System (GPS) Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

2. The method of claim 2 wherein enabling comprises: detecting incorrect SBAS measurements and/or orbit; excluding the detected incorrect SBAS measurements and/or orbit; estimating possible SBAS biases in real time compensating for the estimated possible SBAS biases; and tracking in a rover receiver those SBAS satellites for which a base receiver transmits data to ensure matched SBAS tracking on base receiver and rover receiver.

3. The method of claim 1 wherein the received SBAS ranging data is in a RTCM-3 format.

4. A system comprising: a plurality of Global Positioning System (GPS) satellites, each of the GPS satellites transmitting a periodic satellite GPS Ll signal; a plurality of Space Based Augmentation System (SBAS) satellites, each of the SBAS satellites transmitting a periodic SBAS satellite Ll signal; a land-based reference receiver configured to receive one or both of the GPS Ll signal and SBAS Ll signal; and a land-based rover receiver communicatively coupled to the land-based reference receiver with an airwave link, the land-based rover receiver enabled to use either the SBAS Ll signal or the GPS Ll signal in Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

5. The system of claim 4 wherein the RTK processing comprises: detecting incorrect SBAS measurements and/or orbit; excluding the detected incorrect SBAS measurements and/or orbit; estimating possible SBAS biases in real time; compensating for the estimated possible SBAS biases; and tracking in the rover receiver those SBAS satellites for which the reference receiver transmits data to ensure matched SBAS tracking on reference receiver and rover receiver.

6. A system comprising: one or more Space Based Augmentation System (SBAS) satellites, each of the SBAS satellites configured to periodically send a SBAS Ll signal containing SBAS ranging data; a base station adapted to receive the SBAS ranging data and send the ranging data in a specified format over a communication link to rover station; and the rover station enabled to use the received SBAS ranging data in Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

7. The system of claim 6 wherein the specified format is RTCM-3.

8. The system of claim 6 wherein the RTK processing comprises: detecting incorrect SBAS measurements and/or orbit; excluding the detected incorrect SBAS measurements and/or orbit; estimating possible SBAS biases in real time; compensating for the estimated possible SBAS biases; and tracking in the rover station those SBAS satellites for which the base station transmits data to ensure matched SBAS tracking on base station and rover station.

9. A computer program product, tangibly embodied in an information carrier, for determining a relative position between at least two locations, the computer program product being operable to cause data processing apparatus to: enabling Space Based Augmentation System (SBAS) ranging data received from SBAS satellites in Ll radio signal (Ll) Global Positioning System (GPS) Real Time Kinematic (RTK) processing for resolution of integer ambiguity in relative positioning.

10. The computer program product of claim 9 wherein enabling comprises: detecting incorrect SBAS measurements and/or orbit; excluding the detected incorrect SBAS measurements and/or orbit; estimating possible SBAS biases in real time compensating for the estimated possible SBAS biases; and tracking in a rover receiver those SBAS satellites for which a base receiver transmits data to ensure matched SBAS tracking on base receiver and rover receiver.

11. The computer program product of claim 9 wherein the received SBAS ranging data is in a RTCM-3 format.