WO2011010299A1

WO2011010299A1 - Satellite based positioning system

Info

Publication number: WO2011010299A1
Application number: PCT/IB2010/053387
Authority: WO
Inventors: Ake Gustafson
Original assignee: Datagrid Inc.
Priority date: 2009-07-24
Filing date: 2010-07-26
Publication date: 2011-01-27

Abstract

Satellite positioning system comprising a common stable clock, or a set of synchronized clocks, and multiple broadcast stations (SNGSs) on known Earth-fixed coordinates to broadcast timing signals toward the satellites, said SNGSs being adapted to allow the satellites to self-determine their position and time; said timing signals being broadcast from said common stable clock or from said set of synchronized clocks. A Tamperproof Systematic Demarcation with Land Management Database generation and maintenance on National scales suitable for the Developing World can be performed based on the proposed apparatus. The invention includes means to collect and transfer data using secure satellite data transfer with checks on the operation of the equipment and provisions for the security of the operators as well as total survey integrity.

Description

Satellite based positioning system

FIELD OF INVENTION

This invention generally relates to satellite systems of the GPS type.

STATE OF THE ART

The Global Positioning System (GPS) is an example of a space-based radio positioning network designed to provide users who are equipped with a suitable receiver with position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space-based segment of GPS comprises a constellation of NAVSTAR satellites in inclined orbits around the earth with each satellite carrying its own precision clock or timing system. A ground segment works out the satellite trajectory and forecasts ephemerides that the satellites rebroadcast alongside clock corrections also determined by the ground segment and from which the forecasted satellite position and the time can be calculated by receivers of the GPS signal. A GPS receiver can determine PVT using this information from three satellites and its own clock or may eliminate the need for a precise clock at the receiver end by tracking four or more satellites. FIG. 1 shows an exemplary constellation of navigation satellites 100 in orbit around the earth. The navigation satellites 100 of the Global Positioning System (GPS) are placed in six orbital planes 101 with four satellites 100 in each plane plus a number of "on orbit" spare satellites (not shown) for redundancy. The orbital planes 101 of the GPS satellites 100 have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles); each satellite completes one orbit in approximately 12 hours. This configuration positions the GPS satellites 100 so that a minimum of five of the GPS satellites 100 are normally observable (above the horizon) by a user anywhere on earth at any given time. Suitable satellite configurations depend on the orbital radius and desired coverage, the configuration can be optimized using standard methods by a person skilled in the art. GPS provides PVT information based upon a concept referred to as time-of-arrival (TAO) ranging. The orbiting GPS satellites 100 each broadcast spread-spectrum microwave signals encoded with time of broadcast and ephemerid data. The signals are conventionally broadcast at a number of known frequencies; for example, Ll at 1575.42 MHz, L2 at 1227.60 MHz, and (in the near future) L5 at 1176 MHz, with satellite ephemeris (satellite orbit data that allows its position to be computed in an earth-centered, earth-fixed, coordinate system) clock correction, and other data modulated using bi-phase shift keying, pseudo-random noise, or other techniques. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise times of transmission. A user receives the signals with a GPS receiver, which is designed to time the signals and to demodulate the satellite orbit data contained in the signals. Using the clock-correction data, the GPS receiver determines the time between transmission by the satellite and reception by the receiver. Multiplying this amount by the speed of light gives what is termed the pseudorange measurement for that satellite. The dominant error comes from imperfection of the receiver clock that causes the received measurement to differ from the satellite data by the time offset between actual time and receiver time. Thus, the measurement is called a pseudorange rather than a range. The time offset is common to the pseudorange measurements of all the satellites tracked by that receiver. By determining the pseudoranges of four or more satellites, the GPS receiver is able to determine its location in three dimensions, as well as the time offset. Thus, a user equipped with a proper GPS receiver is able to determine his PVT and to use this information to navigate from point to point, among other uses. But this requires highly stabilized clocks on the satellites.

FIG. 2 illustrates the signal processing 200 for a conventional navigation satellite. The signal processing center 200 permits the satellite to receive TOF information from other satellites which allows determination of relative position, velocity and time (PVT) information, from neighboring satellites through cross-links 201 established between the satellite and neighboring satellites as long as there are at least three cross-links. Cross-link technologies are well known in the satellite art. Two examples of cross-link technology are shown and described in U.S. Pat. No. 5,971,324 to Williams et al. (Oct. 26, 1999), and U.S. Pat. No. 6,125,261 to Anselmo et al. (Sep. 26, 2000), the disclosures of which are hereby incorporated herein by reference. As shown in the block diagram of FIG. 2, conventional navigation satellite technologies also include a signal processor 202 and a transmission antenna 203 to transmit information from the satellite to a ground station or other satellite signal user. An atomic frequency standard ("AFS") 209 is used to maintain a satellite clock time nearly identical to the master time for all of the satellites in a constellation. If each satellite's time (including the clock corrections in the navigation data) is not maintained close to the common master time, there may be significant errors in user PVT solutions. The AFS 209 drives the carrier frequency generator BB ("base band") 204 at GPS frequencies and through countdown circuits for GPS code generators NDS 205 at lesser frequencies. After comparing the phases 206 of the AFS 209 and local signals, the CPU modulates the GPS codes, which include the GPS navigation data, onto the carrier at each of the transmitted frequencies. The cross-links may be used to keep the AFS 209 synchronized to the master time of the constellation.

Satellite navigation systems operating on the same principle include the Russian GLONASS system as well as the emerging European Galileo and Chinese Compass systems.

The GPS system has become expensive to operate and maintain. In 2009 the US Government Accountability Office noted that the Air Force has struggled in recent years to stay within cost and scheduling constraints while building GPS satellites. So far, one satellite program has incurred cost overruns of $870 million and the launch of its first satellite has been pushed back three years to November 2009. In addition China, Russia and the US have demonstrated the ability to "shoot down" satellites in earth orbit, or more accurately to disrupt the satellites.

In addition, the GPS system has presently many uses beyond those for which it is designed and optimized for. One such use is cadastral surveying with sub-meter to cm-range accuracy. While the required performance can be obtained using carrierphase measurements on GPS Navstar satellite broadcasts or broadcasts from any combination of GPS with GLONASS, Galileo, Compass or any other GNSS system satellites (using for example the DGRx receivers by DataGrid Inc.), this imposes high demands both on users and on user equipment. The user equipment must be capable of integer wave ambiguity resolution and maintenance of cycle slip information. To work properly the user must maintain antenna exposure that allows the receiver to avoid cycle slips or monitor and count the slips, else fix is lost resulting in much reduced reliability and compromised accuracy. The operator should be aware of the possibility of compromised data integrity that may result in incorrect ambiguity fixes or loss of fix. Existing positioning and navigation systems capable of cadastral accuracy therefore require expensive rover equipment and trained operators. Cost and logistics of training become major hurdles as well as the increased safety risk to operators and equipment when valuable rover equipment is used in remote locations or locations of low security. This drives up costs of large scale survey and may render them impractical or impossible with current positioning systems. A cadastral survey may also be compromised when rover equipment is used incorrectly or data may be deliberately manipulated.

Thus, there is a need for a GPS satellite system which can be deployed quickly and at low cost and can replace the main functions of the GPS system. This system may be used as a supplement or backup in case of delays or malfunctioning of the traditional GPS system or it might be developed into optimized systems for tasks such as land survey and management. There is also a need for a system that reduces the demand on the operators and lowers the desirability of the rover equipment (preferably no street value) and therefore reduces insurance costs and increases operator safety while improving data integrity. It is important that the system is corruption resistant, at least for applications such as cadastral surveys which currently allow an operator/surveyor direct access to the data and only offers few and costly means of data integrity checking. Because of its comparatively low cost, the proposed system may be accessible to organizations such as the UN that may prefer independence, and to industrial or governmental associations such as transportation logistics, disaster management, or Land Management projects.

GENERAL DESCRIPTION OF THE INVENTION

This invention generally relates to a low cost satellite positioning and navigation system that can be fully deployed using six or fewer launches and more specifically to a satellite system where all satellites share the most expensive electronics and may locate them on the ground. It may as an alternative use the resources of other positioning or navigation systems when available to eliminate the need for a system dedicated clock and/or to determine satellite positions. A GPS satellite reception can be turned into a local clock signal using the procedure described in the diagram of Figure 3.

In this regard the object of the invention is a satellite navigation system comprising multiple broadcast stations (SNGSs) on known Earth- fixed coordinates to broadcast timing signals toward the satellites. The satellites are adapted to self-determine the satellites' position and time based on timing signals that are broadcast from suitably positioned SNGS using a common stable clock or from a set of synchronized clocks similar to the GPS system, the common clock being ground based.

The invention in a preferred embodiment is characterized by the fact that the SNGSs are structurally and functionally adapted to intercept broadcasts from the satellite and compare the range calculated from the coordinate difference between the known SNGS coordinates and the broadcasted satellite self-determined position to the pseudorange determined based on the signal normally used by a rover to determine its position. The thus calculated range and/or timing errors may be broadcast to the satellite which rebroadcasts it in a correction message.

The range correction message may be iteratively improved through inclusion of the correction in the calculation of the pseudorange based on the previous epoch in an automated procedure that generate corrections in near real-time.

Corrections to the ionospheric delay, tropospheric delay, or any other message may be similarly improved.

The satellite navigation system according to the invention may be based on a small satellite standard, e.g. the cube sat standard. Advantageously the timing signal is rebroadcast by transceivers on the satellites so that the signal is filtered or otherwise reconstructed and amplified (Fig. 3) to eliminate the error introduced by interpreting the time and regenerate the signal. The route of the retransmitted signal or a message indicating the corresponding delay is then also encoded in the transmitted signal to allow satellite to rover range determination.

The satellites may use onboard cameras to estimate or verify its position based on known landmarks on the ground, or using the stars, or a combination thereof. This may serve as an integrity check and a backup procedure to determine satellite position in case of failing support infrastructure.

Timing signals are thus broadcast e.g. from a common ground-based stable clock using multiple broadcast stations on known Earth-fixed coordinates. On-board each satellite is then a receiver and processor combination used for observing and evaluating the signals sent from the ground to determine the position of the satellite and time. This is achievable when four or more broadcast stations are received simultaneously. Each navigation satellite broadcasts this information and a timing signal which is then used by an end-user receiver to similarly locate its position when it tracks the broadcast from four or more satellites. This last step is analogous to a GPS receiver using the NAVSTAR satellite navigation system. Uplinks as well as downlinks use signals at multiple frequencies to assist in compensating for signal interference and ionospheric delays.

As indicated above the present invention advantageously relates to a navigation satellite system based on a small satellite standard such as the cube sat system. This is to take advantage of standardized satellite vehicles, space craft control and maintenance hardware / software, manufacturing and launch infrastructure including methods to quickly launch satellites to orbit in sets or clusters using a single launch.

One aspect of this invention is cost reduction through the use of a single high stability clock shared throughout the system. Further, this common clock may be located on the ground where it can be maintained, protected and can even be upgraded or replaced if required. As shown previously the invention provides several advantages with respect to existing satellite positioning system. Three of them are briefly discussed hereafter:

The first is the use of a quality "stamp" to the determined position and procedures used to obtain it. It is this quality control alone that makes the system usable by operators with very little training and the data from correctly operated survey equipment can be recognized while data from incorrectly operated equipment can be rejected. Practically this issue is of great importance because it eliminates or considerably reduces the risk of errors or cheating among the operators.

The second is that the positional data is dependent first on a procedure where stakeholders have a common interest in agreeing or their claims cannot be supported with the issue of a land certificate or similar. The system is resistant against the generation of fraudulent claims or claims that ignore stakeholder disagreements. The stakeholders (for example claimants to adjacent lots or claimants to land use that may be overlapping so that one stakeholder claims for example the right to cattle grazing on a piece of land while another claims the right to gathering firewood) are issued reprogrammable RFID tags who's programmed ID code is associated with the claimants' statements in the form of a list of claims that may include narratives. The claimants then stake their claims by physically locating their RFID equipped markers on the ground. The location of the markers is then recorded using either a GPS/GNSS receiver that may transmit rover data to one or more of our satellites or if sufficient numbers of satellites are available to allow positioning determination there is no need for an expensive GPS/GNSS unit. Either way, the rover unit determines the approximate distance (and direction depending on the implementation) to all RFID tags within its range. This way a map can be produced using data collected by the satellites and combined at the ground control. This map shows all claims and reveals any conflicting claims. The stakeholders have an interest in locating their stakes such that no conflicts are recoded or the certificates cannot be issued. All stakeholders in adjacent or overlapping land have a common interest in the integrity of the survey and the absence of conflicts and this interest is also shared throughout the region to be demarcated. Once the data is recorded the markers may be moved and the RFIDs reprogrammed and reused. The coordinates and claims can then be used to recuperate a geographical location using standard survey equipment at any future time should that be needed. We note that data beamed to the satellite from a location and time combination different than the expected location and time can be recognized and rejected to further reduce the risk of data tampering.

The third is the possibility to use equipment with low street value on the ground. The usual high market value of the operator equipment has already resulted in attacks and robberies in Africa. Such a situation will be mitigated thanks to the present invention both because of a low cost and even lower or vanishing value since the equipment can only be used with the proposed satellite system and the result is only available to the operator of the satellite system. Results are not available to the operator so the equipment has zero utility.

The reason for this is that the satellites according to the present invention have bidirectional communication and their own ranging capability.

A cluster of satellites can therefore determine the position of a beacon on the ground. The beacon advantageously uses a clock similar to those used in a low cost modern watch and is inexpensive. The position is determined at the satellite system ground control through the ranging performed by at least four satellites (the fourth satellite allows differencing using standard ranging procedures). Use of a virtual base station program (that may use the SNGS station data) allows double differencing for improved accuracy. All data needed to calculate a position is converging at the ground station. Positioning is therefore not known except centrally at ground control where it may be generated without human intervention which eliminates the possibility of operator manipulation of the data and several other problems with data transfer and possible distortion or loss in the process.

Use of low cost ground equipment and non-specialist operation allows cadastral data bases to be updated and maintained even where travel is difficult and expensive so that surveyors cannot be deployed in timely manner to remote villages or rural regions. Instead it becomes possible to consider the use of bacon transmitters that can be distributed and stored until needed and operated following simple instructions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be better understood with more specific considerations and with some figures.

Brief description of the figures

Figure 1 (already discussed) shows a state-of-the-art constellation of navigation satellites in orbit about the earth. Figure 2 (already discussed) illustrates the signal processing for a state-of-the-art navigation satellite.

Figure 3 illustrates the cleaning of a received signal from a Navstar GPS satellite.

Figure 4 illustrates how the timing information, in the present invention, is distributed to multiple SNGSs. Figure 5 illustrates how a satellite that is below the horizon as seen from a SNGS still can get position and time propagated to it by way of a network of broadcasting satellites according to the invention.

Figure 6 shows a ground equipment (pole) which can be used with a system according to the present invention.

Figure 3 shows a diagram depicting how the received signal from a Navstar GPS satellite may be "cleaned" using a squaring procedure (which is well known in the literature) then encoded with new messages before being transmitted. The receiving and transmitting antennas may be one and the same and the time delay in the loop containing the demodulator and modulator may be calibrated.

Fig 4 illustrates how the timing information from the AFS 301 which now is located on the ground is distributed to multiple (four minimum) ground based satellite navigation ground stations (SNGSs) 303 using over the air transmissions, optical fiber links or other similar transmission link 302 with calibrated transmission delays. Transmission delays through optical fiber or an electric wire can be routinely measured at the transmission end based on a return echo and a correction may be transmitted. Each SNGS 303 is located on "fixed" coordinates for which Earth Fixed Earth Centered (EFEC) coordinates are known. We note that solid Earth tides can cause diurnal shifts in the meter range and tectonic drifts in the centimeter to decimeter per year range. This shift propagates through the system so that single difference positioning solutions may have errors shifted by this same magnitude. The error is less in differential positioning using rover/base combinations. However the SNGS 303 motion is usually predictable to a high level of reliability (centimeter to sub-centimeter range) and may be corrected for to reduce this error drastically even for single difference solutions. The SNGS 303 transmits its EFEC coordinates and the system time obtained from the common AFS clock 301 after applying corrections for the known delays due to the transmission 302 from the central clock 301 to the SNGS 303, alternatively it transmits the uncorrected time and a code to identify the time shift. The SNGS 303 transmissions 304 include the same essential information that traditional navigation satellites transmit (from which position and time at the transmitter may be calculated). The SNGS broadcasts can therefore be used to calculate the position of a satellite 305 using a multi- frequency high precision GPS receiver 306 such as the DataGrid DGRx® or similar receiver with adjusted broadcast frequencies and chip rates if they differ from those of the GPS system. This is in analogy to satellite navigation systems such as GPS that are used to determine a point on the ground based on transmitters in space that have known positions and time. The direction of information flow ground to space or space to ground is not relevant. We note that the transmission delays cause position determination latency. While such latency may not be acceptable for military use or for machine control, latency is easily tolerable in a cadastral survey.

Older navigation systems such as LORAN (Long Range Aid to Navigation) and the Russian CHAYKA used broadcast stations on the ground. In the system according to the invention the SNGS ground stations are not used directly for end user position determination but to determine the satellite position and time. The reason is geometry and the curvature of the Earth's surface as well as ground interference and obscurations on the surface that limit ground based systems range and accuracy. Ground to space broadcasts are much less affected than ground to ground and should allow for the same accuracy as the current GPS or better since the broadcast station positions are precisely known, the ground stations use a common clock and the stations can be located at sites with minimal ground interference and multipath. Optimal conditions allow increases in the chip rate on multiple carrier frequency bands which in turn improves ranging and timing measurements. Multiple radio transmission bands can be used to estimate and correct for ionospheric delays, the same as with GPS Lie and L2c broadcasts except with higher accuracy because of the above listed improvements. Carrier phase measurements and known satellite dynamics (based on a Geoid model) allow use of standard solution improvements (see standard text books on the subject, e.g., van Sickle 2001). Fig. 5 illustrates how a satellite 405 that is below the horizon as seen from a SNGS 403 still can get position and time propagated to it by way of a network of broadcasting satellites 401 each of which have their time and position determined either from SNGS broadcasts or propagated to them from other satellites.

The satellites now have position and time known to them to an accuracy that is similar to the current GPS satellites when they are in direct view of SNGSs or the position error may be less. At other times accuracies may be reduced. Either way, the satellite PVT is determined in real time and therefore is not depending on an ephemerid generated by a ground segment. The expensive AFS 209 is replaced by the combination of low cost crystal oscillator present in the receiver 306 and time corrections obtained by the above described procedure of differentiating the TOF signals from four or more SNGSs 403 or satellites with known positions and times 401 and its feedback loop. The satellite signal processing is in other respects the same as in a typical Navigation satellite, shown in Figure 2.

The satellites transmit data from which their position and time can be calculated thus mimicking GPS satellites. The system is similar to the GPS navigation satellite system in the sense that an end user may determine the position of a receiver using standard methods developed for GPS although for example the operating frequencies and chip rates may be different. The most common method to derive a position is single differencing and for a more accurate position, double differencing. In single differencing GPS satellite signals are differentiated at the receiver which eliminates most of the receiver clock error allowing the use of relatively inexpensive clocks in the receiver. By differentiating these differentiated signals between stations, most of the satellite clock errors are eliminated and satellite position errors also tend to chancel. Station differencing results in the determination of a baseline which is a vector with its origin at the base station. Ionospheric delays and Solid Earth tide effects also tend to chancel for sufficiently short baselines so that centimeter range accuracy relative positions can be determined even as the satellite positions are known to lesser accuracy.

Despite techniques like the ones mentioned above that reduce errors, satellite position and time errors still propagate to the position errors especially in the case where station differencing cannot be done, so it is desirable to reduce the errors. To this effect the present invention preferably provides an automated near real-time feedback system. Using a receiver on the SNGS stations, the SNGSs monitor the satellite broadcasts constantly checking the range it determines based on the broadcast from each satellite noting the rage error. To separate the ionospheric delays, this is repeated at two or more frequencies. By broadcasting these ranging errors in the messages to the satellites with ionospheric delays separated out, a processor at the satellites can correct the satellite's three-dimensional spatial coordinates and the time as long as it receives ranging errors from at least four stations. The corrected position and time, both of them propagated to account for any data latency will be broadcast as part of the following message. The errors are again determined at the SNGS and relayed back to the satellite which completes the feedback loop. The goal and expectation is to reach sub-meter accuracy range from standalone receivers without need for station differencing (no base-station data). For this the present invention advantageously takes into account the need to broadcast the derived ionospheric delays and remaining ranging error which is likely to be dominated by tropospheric delays. Receivers sufficiently close to a SNGS are then able to apply these corrections for improved range measurements. One can also anticipate interpolation or modeling of errors to cover distances.

The system according to the present invention provides the following advantages compared to the GPS system:

Ground station number and ground coverage: As described above, the satellites must be within view of at least four SNGSs to determine their positions and system time. This requirement may be relaxed. The satellite position is normally obtained from differentiating the SNGS signals at the satellite. When SNGSs are not in view of the satellite, ranging measurements from satellites that could determine their position can propagate the positions throughout the entire satellite network as long as every satellite has at least four other satellites with known positions within view, albeit at reduced accuracy. When at least four ranging stations (SNGSs or satellites) are in common, satellite to satellite differentiation also gives the satellite to satellite baseline. These baselines can be used in a network adjustment procedure to reduce positional errors. (This procedure uses algorithms normally used to minimize errors from a network of ground stations by minimizing least square residuals in their relative position vectors to which this problem is analogous.) This method may also currently be used by the latest generation GLONASS satellites. Additionally, when available, GPS, GLONASS, or other navigation systems may also be used for the satellite positioning and even to generate the timing signal. It is anticipated that this feature will originally be used as a consistency check.

- Broadcast power: The power available for broadcasting is expected to be less than the nominal 70 Watt of a NAVSTAR satellite and may be 7 Watt. Lower orbit satellites may facilitate the concentration of a beam. Ground coverage of similar intensity to the GPS signals can be achieved across the visible part of Earth from the satellite using approximately 1 Watt from the much lower altitude of Low Earth Orbits (LEOs). In addition, signals of just a few percent of the unobstructed GPS signal intensity can be tracked by several existing GPS receivers and even lower intensity signals are tracked by the emerging category of indoors capable GPS receivers. Professional grade high precision receivers normally require higher signal levels but DataGrid's high precision DGRx receiver can track these low signals. It is therefore conceivable to broadcast a lower intensity signal. Alternatively, attitude control equipped satellites may aim a concentrated beam to an area of interest or there may be burst modes when satellites broadcast only at certain epochs or with varying power. Aimed beams may necessitate higher carrier frequencies for convenient antenna geometry or multiple space vehicles may be used to generate a synthetic antenna aperture. The aperture may be controlled by manipulating the relative positioning of space vehicles or in a simpler procedure by knowing the position of the space vehicles and their antenna to an accuracy of a fraction of a carrier wavelength one may adjust a broadcast delay or advance to generate the desired synthetic aperture.

Orbit configuration: While achieving Low Earth Orbit (LEO) is less expensive than the much higher orbits of the NAVSTAR configuration, a smaller area of the globe will be covered as satellites spend a reduced fraction of time above the horizon as seen from a location on the Earth's surface the lower the orbit is. This lower coverage per satellite from LEO is in part offset by the high angular velocity which results in reduced occupation time for a given accuracy position determination using carrier phase measurements. This is reducing the probability of incorrect ambiguity resolution in carrier phase measurements. This is similar to having additional satellites in a given time period since the satellites shift their positions in the sky. Satellites may be placed in eccentric orbits to achieve best coverage at a given time for a given region. Ionospheric delays will be less and more stable (less scintillation) than for transmissions from satellites in higher orbits. Tropospheric delays remain the same.

Systems Optimization for specific tasks: The low cost of the system opens the possibility to tailor systems for specific tasks rather than as an all purpose system or a defense system. Advantages include the ability to tailor message codes and the chip rate. In addition GPS and other navigation systems are strictly one way broadcast systems while the proposed system may have two-way capabilities. In an exemplary embodiment the task is recovery of for example the "black box" of a downed airliner. The box broadcasts short codes that may be tracked by the satellites. This is reminiscent of the pings broadcast by the boxes today. These would be tracked from the satellites that in turn send a message to a ground control station that the message was received and the precise time of receipt. Knowing that the message was sent from the black box to all satellites simultaneously it is now possible to determine the location as long as four or more satellites receive the broadcast. This is subject to the timing error of the satellite's time determination which has the advantage of originating from a common clock and to have atmospheric delay corrections. In the exemplary embodiment of land survey satellites, a similar pinging message device can be made at very low cost and can be in the possession of local authorities who now can schedule a measurement to take place. These may be control measurements in case of dispute or for example the relocation of a parcel corner point, or a parcel may be divided in smaller parts. This simplifies and reduces the cost of maintaining a land database once it has been established. The local authorities need just locate the beacon on the point of interest and "fire" (presumably under the supervision of the stakeholders). The new or updated coordinates can be determined and incorporated in the data base without the need to deploy a land surveyor. Additional tasks such as coordination and tracking of surveyors are trivial but important tasks that are best accomplished through the satellite link.

Any position measurement performed using the satellite broadcasts can be improved based on a return broadcast that is then known to be sent simultaneously to all satellites.

Error estimates can now be obtained from as few as four satellite ranging measurements. It should also be noted that a low mass satellite is intrinsically more resistant to a nearby detonation in space such as used to incapacitate satellites than are larger more massive satellites. A sufficiently small satellite tends to be pushed into a slightly different orbit rather than to disrupt. We note that our navigation satellites are not seriously affected by such orbit changes. This is because of the system of dynamic real time satellite position/time self determination that the satellites encode in their broadcasts. The system may therefore keep operating while a system based on NAVSTAR or traditional navigation satellites would be knocked out or at least malfunctioning temporarily due to invalid ephemerides. We note that this hardened potential is in addition to the low cost and quick response replacement or redeployment capability of the satellite system. There is much reduced burden on the control segment including the fact that no satellite ephemerides need to be derived. However for enhanced (compared to GPS) precision implementations, ground station tidal and tectonic motion needs to be given. We note that station differencing will give a base line and result in relative positioning so that solid earth tides and tectonic motion tend to average out over sufficiently short distances

(approximately 10 km or less) but they figure in single difference measurements. The accuracy of standalone measurements using the proposed system may be sufficiently accurate to warrant corrections. Those are likely to be generated in the form of models with periodic terms corresponding to solar and moon tides and a linear component to represent tectonics. Such models would be regional.

In embodiments of dedicated land survey satellite systems remote sensing cameras may be used. They aid in planning large scale demarcation tasks such as currently undertaken in Uganda under World Bank auspices and United Nations monitoring. Land use as well as spread of pollution, drought, or diseases will be tracked. In embodiments of navigation satellites (or more correctly "positional satellites") for Land Management, satellites may be equipped with attitude control such as that developed at the University of Florida (Norman Fitz-Coy) that allow the entire satellite to adjust its orientation and aim onboard equipment. Onboard equipment besides the positioning and timing receiver 306 (that get its time from the AFS 301 either directly from the SNGS 303 with position and time correction based on TOA or indirectly by way of satellites that have their time and position already determined 401) is an Infrared (IR) and possibly visual camera. The data stream from the SNGs will beside the described signals 404 and housekeeping data include messages from the ground control station that can be passed on to the signal modulator 204 and be imbedded in the GPS digital broadcasts or sent at separate frequencies. These will include messages used to coordinate the surveyors on the ground and messages that can be decoded by receivers acting like radios that may be used to inform the stakeholders on the land. This is important in practice since rural areas are not always covered by radio broadcasts. The low broadcast power available as well as low power receivers may be used for digital messages that are decoded into analogue voice using digital to voice hardware decoders. The messages sent to the surveyors are used to coordinate the surveyors on the ground. The ground equipment depictured in Fig. 6 consists of a receiver unit 501 with a GPS receiver 502 such as the DataGrid DGRx adapted to receive the frequencies of the new system and their messages including the chip rate. It may receive these messages in addition to other Navigation Satellite messages (that may be GPS and GLONASS, Galileo, etc..) for faster position determination and improved coverage where the sky is partially covered. The receiver sends the raw data to a processor 503 that checks the data for integrity and estimates the error from standalone and from postprocessing with a short base line. If a radio or other communication link with a base is added to the system it will be possible to obtain Real Time Kinematic RTK solutions from the system. However it is anticipated that the system may also operate without a link to a base station. In this case the processor 503 estimates the postprocessing error without access to actual base station data based on empirical criteria. If the error estimate is within preset values this is indicated on the display 510, else the display may indicate if sufficient satellite information is tracked (four at minimum for a position fix) and a time estimate to reach to reach the preset accuracy requirement (collecting data for a longer time period reduces the error).

In an embodiment for land demarcation it is anticipated that no base data will be sent to the rover. Instead rover data will be broadcast to the satellite and relayed to a ground control station where a synthetic base station data stream is generated using software similar to the Trimble RTKNet software. A high precision solution with a few centimeters range accuracy can now be produced but is never in a format at the rover where it can be distorted, manipulated or serve unintended use. The broadcast may be in a standard format such as the open format RTCM by The Radio Technical Commission for Maritime Services or any other format carrying the same information. The messages may be encrypted and for added security we may broadcast a time code and make use of the ranging capability of the satellites to make sure that the broadcast station to satellite range agrees with the calculated coordinates and time at the rover. When all integrity tests are passed a "measurement completed" message is sent as soon as the transmitted rover data yield a position to the desired accuracy. The rover operator can now progress to the next location to be surveyed.

The receiver unit 501 with the navigation receiver 502, processor 503 and display 510 is mounted on an extendable pole 504 (typically extendable from 2 meters to 7 or 8 meters) with a reader that allows the processor 503 to always know the length of the pole. The processor 503 also reads the deviation angle from vertical at which the pole is held from a tilt sensor 506 and it reads the RFID code of a tag embedded in the monument 507 using a reader mounted at the tip of the pole 508. The monument 507 may be a temporary marker staked out by a claimant and equipped with reprogrammable RFID tags or may be made in the presence of stakeholders by a separate crew trained in conflict resolution procedures not to be discussed here. The RFID tag can operate as a regular passive tag taking its energy from the transmission emitted by the RF reader or it can transmit a stronger signal for a short time based on its own power source. The RFID tag may draw its power from the curing special cement or concrete mixture used to make the monument. Therefore during the curing time it will be possible for a surveyor's pole equipped with a direction sensitive loop antenna as part of the RT reader 508 to receive the RFID signals from completed monuments at a distance and make a rough direction and distance estimate based on the signal strength or by other means.

The RFID can be made such that when the RF reader 508 is brought sufficiently close, the tag is able to sense a signal emitted from the reader and the tag responds by sending a message. This can be accomplished in several alternative ways including the use of a RFID tag that is active or passive or a combine tag where part of the tag circuitry is passive. A distance estimate may be obtained either based on the signal level or by tuning the sensitivity of the passive tag (or passive part of a combined tag) to the desired distance. A tag is then deemed to be on the point surveyed only if the passive tag signal is recorded by the reader, else the active tag serves to record that the claim corresponding to that tag is out of acceptable range. A separate position measurement then needs to be made for the location of that tag. If the active tag signal cannot be recorded it is assumed to be either missing or located sufficiently distant from the other tags so a major disagreement is evident. The processor 503 knows when the pole is positioned on top of the monument 507 based on its ability to read RFID tags and records a unique code associated with the tag or monument 507 given by the ID broadcast by the associated RFID tag. The processor 503 also knows if the pole is held vertically within a preset tolerance based on the tilt sensor 506 and the vertical distance between the phase stable antenna 505 and the monument based on the reader 504 of the pole extension. The processor 504 also has access to all navigation data from the satellites and can therefore evaluate if the position determination is valid and within error tolerances. It also knows the vertical offset between the antenna (that the satellite positioning relates to) and the monument. This data is encoded using an encryption key and saved by the processor in nonvolatile memory. The processor indicates on the display 510 (and optionally with an audio signal) when the data has passed the tests and been stored which signals completion and the display 510 indicates the estimated direction and distance to any other monuments who's active RFID broadcasts it is able to receive. It may also indicate such information as the projected completion time of a schedule, if the operator is behind on his/her schedule etc The power source may be located anywhere but is expected to be in the form of batteries 509 mounted at the bottom of the pole for optimal balance.

In the case that adequate data integrity cannot be established, the pole is tilted on purpose by approximately 10 to 15 degrees and the top of the pole is swept in a circle or a minimum of a 90 degrees arc (quarter circle). This procedure is designed to identify the presence of multipath or other error and will in most cases confirm the coordinate obtained from the data collected. In case that valid position indication is still not obtained above a given monument (valid position is not indicated on the display 510), the processor 510 can estimate when additional satellites may come into view based on approximate orbits (satellite position data that may be extrapolated from previous satellite passages). As an alternative and for very difficult locations one or more positions will be established nearby and the offset recorded. This is a separate procedure that we do not elaborate on here.

One purpose with systematic demarcation is to provide land certificates that can be used as collateral in financial transactions. The remote sensing cameras on the proposed system for Uganda will be capable of determining if certain crops have been harvested or are still on the field and if they are healthy to help evaluate the collateral. The remote sensing camera function may also be used to plan land survey campaigns including estimates of tree canopy or other difficult for satellite signals conditions. The satellite orbits may therefore be eccentric so that some satellites are near apogee (at a high altitude) above the region of interest while other satellites are near perigee (low altitude) but fast motion. This combination allows a combination of overview pictures as well as close up higher resolution imaging depending on the satellite. It also allows for wide ground coverage navigation signal from satellites near apogee and fast motion across the sky (fast changing geometry) that help establish a position fix under difficult circumstances.

The demarcation procedures and apparatus described above can be used to perform Tamperproof Systematic Demarcation with Land Management Database generation and maintenance on National scales suitable for the Developing World which we specifically describe below.

Consider stakeholders that lay claims on one or more plots in the form of ownership, partial ownership or usage claims. Usage claims can be full or partial such as fire wood gathering, cattle grazing, etc... while ownership claims may be customary, MaIo, or any other type of full or joint claim.

While most claims are undocumented, a specific claim may be documented or undocumented. Either way, the value of the claim is only as strong as the ability to enforce it. Stakeholders usually in an entire nation or major regions of a nation are offered to have their claims documented in a systematic land demarcation such as proposed and promoted by the UN and the WB in their drive to mitigate poverty through conflict reduction. This process offers titles or certificates to document all claims in a uniform body, the integrity of which is the common interest of all title and certificate holders.

A possible procedure that takes advantage of our invention is that the stakeholders are informed of the imminent demarcation in their area (usually a village) and provided with markers. These are to be placed at the corner points of their property knowing that their claim will be based on the location of those markers and will only be valid if their claim is free from overlapping claims. The stakeholders are informed that markers will be surveyed during an agreed upon time period. A surveyor is not needed, instead an operator of the data-collecting rover can be a person with minimal training and may be a volunteer. This operator brings the rover depicted in Figure 5 to each marker. The RFID signal confirms the proximity of the actual marker to be surveyed and also the approximate distance to nearby markers that may mark the opinion of other stakeholders including stakeholders of adjacent parcels. The rover is turned to "on" which starts data logging. When a marker is sensed to be sufficiently close by the RFID reader at the tip of the measuring pole 508 and the tilt sensor indicates that the pole is held vertical satellite data logged are flagged as representing the point corresponding to the RFID code. The rover indicates "measurement completed" once sufficient data is logged to allow position fix at the prescribed accuracy. The tags on the ground have now served their purpose and may be discarded or reprogrammed with some other ID code for reuse. It is up to the stakeholders to maintain a monument on the ground or not. Either way, the official demarcation is virtual and is now securely stored in a digital cadastral database. It is possible to recover the point on the ground in all perpetuity using the coordinates stored in the database as long as the map datum is well defined and accounts for tectonics.

In the case of at least one two-way satellite in view, the recorded data is transferred to the satellite using messages following the RTCM standard or similar that is usually used to transfer base station data. In this embodiment the rover data is joined with its corresponding base data at a remote site using a virtual base software (similar to the Trimble RTKNet software) and rover data. A signal may then be sent back to the rover indicating when sufficient rover data is obtained and the fix was generated and available at the database site. By including the RFID tag code the rover will know if the "mission accomplished" signal broadcast by satellite is indeed the one corresponding to the RFID its measurement refers to. This embodiment has many advantages including the fact that the position data is never calculated at the rover and cannot be manipulated by an incompetent or corrupt operator. In addition the rover is useless for any other application than in the intended system which means that its street value is null or at least low. In an exemplary embodiment the rover data which includes GPS and/or any other GNSS data is sent to a satellite capable of the two-way communication, the same as in the preceding description, but now both ambiguity resolution and the data integrity is improved by the two-way communication capable satellite also performing a code phase ranging measurement with the rover echoing the satellite message with a known delay on one polarization branch while encoding its identity on the perpendicular branch. This ranging determination has the advantage that only one clock is involved and it can now be used to make sure that the transmitter is indeed at a location consistent with the distance to the satellite. If the message is repeated the ranging measurements must correspond to the satellite's motion over the relevant time period. There are only two such points, the actual location and its mirror symmetry point in the satellite's orbital plane. Finding the mirror point requires precise knowledge of the satellite orbit which is presumably not general knowledge. We also note that the mirror point is usually not on the Earth's surface and is therefore either difficult to reach and hold or impossible in case of the subsurface location. Not even a controlled delay loop would mimic this range variation if two or more ranging satellites were in use. This specific transmission strategy allows for a secure transmission from any specific point and has many uses besides position determination.

Claims

1. Satellite positioning system comprising a common stable clock, or a set of synchronized clocks, and multiple broadcast stations (SNGSs) on known Earth-fixed coordinates to broadcast timing signals toward the satellites, said SNGSs being adapted to allow the satellites to self-determine their position and time; said timing signals being broadcast from said common stable clock or from said set of synchronized clocks.

2. Satellite positioning system according to claim 1 adapted to use Global Navigation satellite Systems (GNSS) or any local or global navigation system to self determine the satellite position which is then encoded in the satellites' broadcast.

3. Satellite positioning system according to claims 1 or 2 wherein said SNGSs are structurally and functionally adapted to intercept satellite position messages broadcast from at least one satellite and compare the range calculated from the coordinate difference to the pseudorange determined based on the signal normally used by a rover to determine its position, the thus calculated range error being broadcast to the satellite which rebroadcasts it in a correction message.

4. Satellite positioning system according to one of the previous claims comprising a ground- based common stable clock.

5. Satellite positioning system according to any of the preceding claims including processing means to iteratively improve the range correction message through inclusion of the correction in the calculation of the pseudorange based on the previous epoch, providing thereby corrections in near real-time and without the need for human intervention.

6. Satellite positioning system according to any of the previous claims whereby corrections to the ionospheric delay, tropospheric delay, or any other message is similarly improved.

7. Satellite positioning system according to any of the previous claims which is based on a small satellite standard.

8. Satellite positioning system according to claim 7 wherein said standard is the cube sat system.

9. Satellite positioning system according to any of the previous claims for cadastral surveying and wherein each satellite comprises attitude control means that allow the entire satellite to adjust its orientation and aim onboard equipment.

10. Satellite positioning system according to claim 9 wherein said onboard equipment comprise an Infrared and/or a visual camera.

11. Use of a satellite positioning system according to any of the previous claims in which the timing signal is reconstructed and used as a satellite clock.

12. Use of a satellite positioning as defined in any of the previous claims 1 to 10 in which the timing signal is reconstructed and rebroadcast as a timing signal.

13. Use according to claim 12 whereby the route of the retransmitted signal or a message indicating the corresponding delay is also transmitted.

14. Use of a satellite positioning system as defined in any of the previous claims 1 to 10 in which the satellites use an onboard camera to estimate or verify its position based on known landmarks on the ground, or using the stars, or a combination there-off.

15. Use of a satellite positioning as defined in any of the previous claims 1 to 10 for cadastral surveying.

16. Use of ranging satellites in a satellite positioning system as defined in any of the previous claims 1 to 10 to determine the location of a transmitter on the ground.

17. Use of ranging satellites in a satellite positioning system as defined in any of the previous claims 1 to 10 to authenticate a broadcast using the ranging and time information to determine that the broadcast was made from the expected transmitter and that the position it transmits agrees with the determined range or set of ranges if multiple satellites receive the broadcast.

18. Use of RFID tags (that may be reprogrammable) in a satellite positioning system as defined in any of the previous claims 1 to 10, in a procedure that allows determination of the distance to a stake or marker that incorporates the tag.

19. Use of a loop antenna in a tag reader to determine the direction to an RFID tag.

20. Use according to claim 18 and procedures where stakeholders make their own stakeout of their claim to a geographical area.

21. Use of a combination of ranging RFID measurements with a satellite positioning system as defined in any of the previous claims 1 to 10.

22. Use according to claim 21 to generate cadastral data bases revealing overlapping or conflicting claims.

23. Use, in a satellite positioning system as defined in any of the previous claims 1 to 10, of encoded digital messages transmitted by the satellites that may be translated into text or voice messages to coordinate the demarcation crew, inform stakeholders or others on the ground.

24. Use according to claim 23 where the messages are used for educational or other informational purposes.

25. Use of a satellite positioning system according to any of the previous claims 1 to 10 where the rover data is sent to a central processing unit for authentification, merger with base data when available and generation of the rover position.

26. Use according to claim 25 where a confirmation message indicating the completion of a data record that meets set requirements is sent to the rover.