Integrated vehicle positioning and navigation system, apparatus and method

Abstract: Systems and methods for positioning and navigating an autonomous vehicle (102,310) allow the vehicle (102,310) to travel between locations. A first position estimate (112) of the vehicle (102,310) is derived from satellites (132-170,200-206) of a global positioning system (100A) and/or a pseudolite(s) (105). The pseudolite(s) (105) may be used exclusively when the satellites (132-170,200-206) are not in view of the vehicle (102,310). A second position estimate (114) is derived from an inertial reference unit (904) and/or a vehicle odometer (902). The first and second position estimates are combined and filtered to derive a third position estimate (118). Navigation of the vehicle (102,310) is obtained using the position information (414), obstacle detection and avoidance data (416), and on board vehicle data (908,910).

Description: The present invention relates to positioning systems, and more particularly, to a positioning system and method for determining the terrestrial position of an autonomous vehicle on or near the planet Earth's surface. Several national governments, including the United States (U.S.) of America, are presently developing a terrestrial position determination system, referred to generically as a global positioning system (GPS). In a GPS, a number of satellites are placed in orbit around the planet Earth. The GPS satellites are designed to transmit electromagnetic signals. From these electromagnetic signals, the absolute, terrestrial position (position with respect to the Earth's center) of any receiver at or near the Earth's surface can ultimately be determined. The U.S. government has designated its GPS the "NAVSTAR." The NAVSTAR GPS will be declared operational by the U.S. government in 1993. Moreover, the government of the Union of Soviet Socialist Republics (U.S.S.R.) is currently developing a GPS known as "GLONASS," which is substantially similar to the NAVSTAR GPS. In the NAVSTAR GPS, it is envisioned that four orbiting GPS satellites will exist in each of six separate orbits. A total of 24 GPS satellites will be in orbit at any given time with 21 GPS satellites in operation and 3 GPS satellites serving as spares. The three GPS satellite orbits will have mutually orthogonal planes relative to the Earth. The GPS satellite orbits will be neither polar orbits nor equatorial orbits. Moreover, the GPS satellites will orbit the Earth once every 12 hours. Using the NAVSTAR GPS, the relative position of orbiting GPS satellites with respect to any Earth receiver can be determined from the electromagnetic signals. The relative position is commonly referred to as a "pseudorange." Moreover, the relative position can be calculated by two methods. One method is to measure the propagation time delays between transmission and reception of the emanating electromagnetic signals. In the NAVSTAR GPS, the electromagnetic signals are encoded continuously with the time at which the signals are transmitted from the GPS satellites. Needless to say, one can make note of the reception time and subtract the encoded transmission time in order to derive time delays. From the calculated time delays and from knowing the speed at which electromagnetic waves travel through the atmosphere, pseudoranges can be accurately derived. Pseudoranges computed using the foregoing method are referred to in the context of this document as "actual" pseudoranges. Another method involves satellite position data that is encoded in the electromagnetic signals being transmitted from the orbiting satellites. Almanac data relating to the satellite position data of the NAVSTAR GPS is publicly available. Reference to this almanac data in regard to data encoded in the electromagnetic signals allows for an accurate derivation of pseudoranges. Pseudoranges computed using the foregoing method are referred to in the context of this document as "estimated" pseudoranges. However, with respect to the previous method of deriving estimated pseudoranges, it should be noted that the satellite position data is updated at the GPS satellite only once an hour on the hour. Consequently, an estimated pseudorange decreases in accuracy over time after each hour until the next hour, when a new estimated pseudorange is computed using updated satellite position data. Furthermore, by knowing the relative position of at least three of the orbiting GPS satellites, the absolute terrestrial position (that is, longitude, latitude, and altitude with respect to the Earth's center) of any Earth receiver can be computed via simple geometric theory involving triangulation methods. The accuracy of the terrestrial position estimate depends in part on the number of orbiting GPS satellites that are sampled. Using more GPS satellites in the computation can increase the accuracy of the terrestrial position estimate. Conventionally, four GPS satellites are sampled to determine each terrestrial position estimate because of errors contributed by circuit clock differentials among the Earth receiver and the various GPS satellites. Clock differentials could be several milliseconds. If the Earth receiver's clock were synchronized with that of the GPS satellites, then only three GPS satellites would need to be sampled to pinpoint the location of the Earth receiver. In the NAVSTAR GPS, electromagnetic signals are continuously transmitted from all of the GPS satellites at a single carrier frequency. However, each of the GPS satellites has a different modulation scheme, thereby allowing for differentiation of the signals. In the NAVSTAR GPS, the carrier frequency is modulated using a pseudorandom signal which is unique to each GPS satellite. Consequently, the orbiting GPS satellites in the NAVSTAR GPS can be identified when the carrier frequencies are demodulated. Furthermore, the NAVSTAR GPS envisions two modes of modulating the carrier wave using pseudorandom number (PRN) signals. In one mode, referred to as the "coarse/acquisition" (C/A) mode, the PRN signal is a gold code sequence having a chip rate of 1.023 MHz. The gold code sequence is a well-known conventional pseudorandom sequence in the art. A chip is one individual pulse of the pseudorandom code. The chip rate of a pseudorandom code sequence is the rate at which the chips in the sequence are generated. Consequently, the chip rate is equal to the code repetition rate divided by the number of members in the code. Accordingly, with respect to the coarse/acquisition mode of the NAVSTAR GPS, there exists 1,023 chips in each gold code sequence and the sequence is repeated once every millisecond. Use of the 1.023 MHz gold code sequence from four orbiting GPS satellites enables the terrestrial position of an Earth receiver to be determined to an approximate accuracy of within 60 to 300 meters. The second mode of modulation in the NAVSTAR GPS is commonly referred to as the "precise" or "protected" (P) mode. In the P mode, the pseudorandom code has a chip rate of 10.23 MHz. Moreover, the P mode sequences are extremely long, so that the sequences repeat no more than once every 267 days. As a result, the terrestrial position of any Earth receiver can be determined to within an approximate accuracy of 16 to 30 meters. However, the P mode sequences are classified and are not made publicly available by the United States government. In other words, the P mode is intended for use only by Earth receivers authorized by the United States government. In order for the Earth receivers to differentiate the various C/A signals from the different orbiting GPS satellites, the Earth receivers usually include a plurality of different gold code sources for locally generating gold code sequences. Each locally-derived gold code sequence corresponds with each unique gold code sequence from each of the GPS satellites. The locally-derived gold code sequences and the transmitted gold code sequences are cross correlated with each other over gold code sequence intervals of one millisecond. The phase of the locally-derived gold code sequences vary on a chip-by-chip basis, and then within a chip, until the maximum cross correlation function is obtained. Because the cross correlation for two gold code sequences having a length of 1,023 bits is approximately 16 times as great as the cross correlation function of any of the other combinations of gold code sequences, it is relatively easy to lock the locally derived gold code sequence onto the same gold code sequence that was transmitted by one of the GPS satellites. The gold code sequences from at least four of the GPS satellites in the field of view of an Earth receiver are separated in this manner by using a single channel that is sequentially responsive to each of the locally-derived gold code sequences, or alternatively, by using parallel channels that are simultaneously responsive to the different gold code sequences. After four locally-derived gold code sequences are locked in phase with the gold code sequences received from four GPS satellites in the field of view of the Earth receiver, the relative position of the Earth receiver can be determined to an accuracy of approximately 60 to 300 meters. The foregoing approximate accuracy of the NAVSTAR GPS is affected by (1) the number of GPS satellites transmitting signals to which the Earth receiver is effectively responsive, (2) the variable amplitudes of the received signals, and (3) the magnitude of the cross correlation peaks between the received signals from the different GPS satellites. Because multiple PRN signals are received simultaneously at the Earth receiver, a common time interval exists wherein some of the codes can conflict. In other words, the codes cause a degradation in measurements of the time of arrival of each received PRN because of the cross correlations between conflicting received signals. The time of arrival measurement for each PRN signal is made by determining the time of a peak amplitude of a cross correlation between the gold code sequence of the received PRN signal and the locally- derived PRN signal. When a locally-derived PRN signal is superimposed over a received PRN signal thereby increasing the averaging time of their cross correlation, the average noise contribution decreases. However, because the cross correlation errors between the received PRN signals are periodic, increasing the averaging time also results in increases to both the error signal and the cross correlation value between the received PRN's alike. Consequently, errors relating to the time of arrival of PRN signals are not reduced by cross correlation. In addition to the GPS, it is known in the conventional art to use inertial systems in navigation systems to obtain position estimates of vehicles. Such an inertial reference unit (IRU) obtains specific-force measurements from accelerometers in a reference coordinate frame which is stabilized by gyroscopes, or gyros. An IRU can be of several types, including for example, laser, mechanical, or fiber optic. In an unaided navigation system using an IRU, the specific force (corrected for the effects of the Earth's gravity) as measured by an accelerometer is integrated into a navigation mathematical equation to produce the vehicle's position and velocity. The instrument measurements of the IRU may be specified in a different rectangular coordinate frame than the reference navigation frame, depending on the platform implementation. The most commonly used reference navigation frame for near Earth navigation is the local-level frame (east-north-vertical). Several gimballed platform implementations exist with the forgoing reference navigation frame. In a gimballed, local level-north seeking IRU, the gyroscopes and accelerometers are mounted on a platform which is torqued to maintain the platform level and azimuth pointing to the north. The platform is the reference plane. In contrast, in a gimballed, local-level azimuth-wander IRU, the platform is maintained level, but is not torqued about the vertical axis. Furthermore, in a strap-down IRU, the gyroscopes and the accelerometers are directly mounted on the vehicle body. They measure the linear and angular motion of the vehicle relative to inertial space. The motion is expressed in vehicle coordinates. Therefore, in a strap-down IRU, it is necessary to first compute the altitude of the vehicle to the referenced navigation frame. Then, the computed altitude is used to transform the accelerometer measurements into the reference frame. After the accelerometer data of a strap-down IRU has been extrapolated into the reference frame, the solution of the navigation equations mentioned previously is identical in both the gimballed IRU and the strap-down IRU. In the strap-down IRU, the altitude computations, which are required to resolve accelerometer measurements, are usually carried out at a high rate. The computations suffer from numerical errors because of the limited computer byte size and throughput availability. These computation errors depend on the frequency response of the sensor loop, data rate, and resolution and magnitude of the sensor output at the sampling time. However, significant benefits arise from using the strap-down IRU, rather than the gimballed IRU. The strap-down IRUs are less costly. Moreover, the strap-down IRUs are generally smaller in physical size. Thus, the potential to realize size and cost savings in IRUs can make strap-down IRUs attractive for both military and commercial applications. The performance of navigation systems using IRUs is primarily limited by errors contributed by the various constituent sensors within the IRUs. Gyroscopes drift. Accelerometers have inherent biases. Further, errors are contributed from improper scale factors and improper IRU alignment angles. Typically, the preceding errors cause inaccuracies in the estimates of vehicle positions, velocity, and altitude, which accumulate over time as a vehicle mission progresses. To some extent, the errors are dependent on user dynamics. If a very accurate navigation system is required for a vehicle, high precision gyroscopes and accelerometers can be utilized to satisfy that need. However, such high precision equipment increases the complexity and costs of the vehicle. EP-A-0181012 discloses a vehicle position estimating system combining GPS and inertial navigation systems. IEEE Position Location and Navigation Symposium, 4-7 Nov 1986, discloses an inertial navigation system with GPS comparison. DE-A-3310111 discloses a navigation system with drift compensation. US-A- 3630079 discloses a navigation system utilizing multiple sensors and error correction. US-A-4786164 discloses an obstacle detecting scanning system with cyclical scanning. According to the present invention, there is provided a scanning system for detecting obstacles in a path of a vehicle, mounted on the vehicle, comprising: means for producing a beam of energy; means for scanning the beam of energy through a desired range of angles relative to the vehicle in cycles; and means for detecting reflected energy; characterized by: means for storing information associated with the reflected energy detected, during each of the scanning cycles; and means for calculating the distance the reflected energy detected has travelled by processing the associated information which is stored during each of the scanning cycles; and means for producing a beam of energy, during a portion of the scanning cycle, so that only a fraction of the total field of view is scanned, wherein the total amount of data transferred is minimized. The present invention also provides a scanning method of detecting obstacles in a path of a vehicle, using a scanner mounted on the vehicle, comprising the steps of: producing a beam of energy; scanning the beam of energy through a desired range of angles relative to the vehicle in cycles; measuring distances between the vehicle and objects in the path of the vehicle, comprising the steps of: detecting reflected energy; and characterized by the steps of: storing information associated with the reflected energy detected, during each of the scanning cycles; calculating the distance the reflected energy detected has travelled by processing the associated information which is stored during each of the scanning cycles; and producing a beam of energy, during a portion of the scanning cycle, so that only a fraction of the total field of view is scanned, wherein the total amount of data transferred is minimized. Further features and advantages of the present invention will become apparent to one of skill in the art upon examination of the following drawings and detailed description. It is intended that any additional features and advantages be incorporated herein. The present invention as defined in the claims can be better understood with reference to the text and to the following drawings. Figure 1 illustrates a block diagram of the preferred embodiment of the present invention; Figure 1A is a block diagram 100A of the operational GPS satellites in the NAVSTAR GPS; Figure 2 illustrates four, simultaneous, navigation equations regarding four GPS satellites of the NAVSTAR GPS; Figure 3 is a block diagram of a typical autonomous work site; Figure 4 is a block diagram of the interrelationships between a navigator, a vehicle VPS architecture, and vehicle control system; Figure 6 is a block diagram of the operation of a GPS; Figure 7 is a block diagram of a GPS processing system of the preferred embodiment; Figure 8 is a flow diagram of the GPS processing system of figure 7; Figure 9 is a block diagram of an MPS, including an odometer 902 and an inertial reference unit (IRU) 904; Figure 10 is a block diagram of the VPS; Figure 11 is a block diagram of the VPS architecture of Figure 10; Figure 12 is a diagram of vehicle route definitions; Figure 13 is a diagram showing the replanning of a path; Figure 14 is a diagram showing how an error vector including curvature is computed; Figure 15 is a diagram showing how an error vector including curvature is computed with the vehicle path included; Figure 16 is a context diagram of the navigator of the present invention; Figure 17 is a context diagram of a path tracking structure; Figures 18A-18D are data flow summaries of the navigator 406; Figures 19A is an illustration of a vehicle mounted scanner 404; Figure 19B is an illustration of an autonomous vehicle scanning for an obstacle; Figure 20 is a diagram of selected scan lines in a laser scanner system; Figure 21 is a diagram of an autonomous vehicle avoiding obstacles; Figure 22 is a diagram of obstacle; Figure 23 is a block diagram of a laser scanner system used for obstacle detection; Figure 24 is a block diagram 4300 of a control system for an autonomous mining vehicle; Figure 25 is a state diagram showing the transitions between modes of operation of the control system of Figure 24; Figure 26 is a communications diagram showing tasks of a navigator; (1) "Absolute position" in the context of this document refers to a position relative to the center of the Earth. Generally, an absolute position will be in reference to a vehicle or the base station, both on or near the Earth's surface. First, second, and third position estimates are all absolute positions. (2) "Actual pseudorange" means an approximation of the distance between (1) a reference point and (2) a source of a terrestrial position determination system. In this document, actual pseudoranges usually refers to an approximation of the distance between (1) an Earth receiver and (2) GPS satellites and/or psaudolites. Actual pseudoranges are approximated by first measuring the propagation time delays between transmission and reception of the electromagnetic signals emanated from the GPS satellites and/or pseudolites. Actual pseudoranges can be readily calculated by multiplying the calculated time delays by the speed of light, or 2.9979245898 * 10 8 m/s. (3) "Anti-selective availability" refers to a method/technique/process for detecting and compensating for corrupted GPS data in the coarse/acquisition (C/A) mode of modulation. (4) "Autonomous" is used in this document in its conventional sense. It indicates operation which is either completely automatic or substantially automatic or without significant human involvement in the operation. Generally, an autonomous vehicle means an unmanned vehicle in operation, or a vehicle in operation without a human pilot or co-pilot. However, an autonomous vehicle may be driven or otherwise operated automatically and also have a human passenger(s) as well. (5) "Base correlator bias" means a spatial bias derived in accord with the flowchart 1700A of Figure 17A. (6) "Base correlator bias technique" means a method/process for computing base correlator biases. (7) "Base estimated position" or "BEP" refers to the relative position of the base station with respect to a vehicle. The BEP is used in the base correlator bias technique. (8) "Base known position" or "BKP" is the absolute position of the base station (used as a reference point) which is known. The BKP can be an estimate itself, derived from any accurate positioning system. The BKP is assumed to be a more accurate estimate of the base station's absolute position than any other position estimate. (9) "Base position estimate" means the absolute position estimate of the base station as derived from the GPS processing system within the host processing system. The base position estimate is substantially similar to the first position estimate derived by the GPS processing system at the vehicle. The base position estimate is computed in the base residuals bias technique. (10) "Base residuals bias" means a spatial bias which is the effective difference in the base known position (BKP) of the base station and the position estimate of the base station which is computed by the host processing system. (11) "Base residuals bias technique" refers to a method for deriving base residuals biases. (12) "Bias" refers to a differential between two measurements, usually position estimates (spatial bias) or clock rates (clock bias). Because one measurement is usually known to be more accurate than another, the bias is often times referred to as an "error." (13) "Clock bias" means the difference in the clock times between (1) the transmission circuitry of GPS satellites and/or pseudolites and (2) the reception circuitry of an Earth receiver. When using a clock bias in the computation of a spatial bias, the clock bias is multiplied by the speed of light, or 2.998 * 10 8 meters per second. Consequently, the clock bias is transformed into units of length. (14) "Constellation" refers to a group comprised of GPS satellites and/or pseudolites whose signals are utilized to derive an absolute position estimate of a point on or near the Earth's surface. See "optimal constellation" below. (15) "Constellation effects method" means a technique or process by which an optimal constellation of GPS satellites is selected from a larger group of GPS satellites in view of a vehicle. (16) "Data radio" refers to a transmitter, receiver, transceiver, or any combination thereof, for communicating data at radio frequencies (RF). (17) "Earth receiver" refers to any apparatus or device, or any part thereof, which receives and processes signals from a GPS and/or pseudolites. Earth receivers may be situated on or near the Earth's surface. Moreover, earth receivers may take the form of, for example, a vehicle or a base station. (18) "Estimated pseudorange" refers to an approximation of the distance between (1) a reference point and (2) a source of a terrestrial position determination system. In this document, actual pseudoranges usually refers to an approximation of the distance between (1) an Earth receiver and (2) GPS satellites and/or pseudolites. Estimated pseudoranges are computed from GPS data encoded on the electromagnetic signals being transmitted from the GPS satellites and/or the pseudolites. Almanac equations for computing estimated pseudoranges from the GPS data of the NAVSTAR GPS are publicly available. (19) "First position estimate" or "FPE" or "FPE(i)" refers to an estimated absolute position of any vehicle which is outputted, in any form, from the GPS. The first position estimate and a second position estimate are independently derived in the present invention. Subsequently, these estimates are combined and filtered to derive a third position estimate. Consequently, the accuracy of the first position estimate affects the accuracy of the third position estimate. (20) "GLONASS GPS" refers to the GPS which has been designed and which is currently being deployed by the U.S.S.R. (21) "Global positioning system" or "GPS" is a type of terrestrial position determination system. In a GPS, a number of satellites are placed in orbit around the planet Earth. The GPS satellites are designed to transmit electromagnetic signals. From these electromagnetic signals, the absolute, terrestrial position (position with respect to the Earth's center) of any receiver at or near the Earth's surface can ultimately be determined. The U.S. government has designated its GPS the "NAVSTAR." The government of the U.S.S.R. has designated its GPS the "GLONASS." (22) "GPS data" means all data encoded on signals transmitted from GPS satellites of a GPS. GPS data includes, for example, ephemeris data and time data. (23) "GPS processing system" refers to a system for receiving signals from a terrestrial position determination system and for deriving first position estimates of vehicles from the received signals. The GPS processing system may receive electromagnetic signals from GPS satellites of a GPS and/or from pseudolites. (24) "Host processing system" refers to a computer system which is operating at the base station for performing methods and techniques which increase the accuracy of position estimates of vehicles. Data derived from these methods and techniques is transmitted to vehicles so that the vehicles can use the data when computing first, second, and third position estimates. The architecture/hardware of the host proceasing system is substantially similar to the architecture/hardware of the VPS. (25) "Inertial reference unit" or "IRU" refers to a system, usually on-board a vehicle, for aiding in the derivation of a second position estimate of the vehicle. An IRU obtains specific-force measurements from accelerometers in a reference coordinate frame which is stabilized by gyroscopes, or gyros. An IRU can be of a laser type or a mechanical type. In an unaided navigation system using an IRU, the specific force (corrected for the effects of the Earth's gravity) as measured by an accelerometer is integrated into a navigation mathematical equation to produce the vehicle's position and velocity. The IRU may be part of the MPS. (26) "Kalman filter" is used in its conventional sense. It refers to a software program for filtering out noise or errors in data. A GPS Kalman filter is utilized to filter out noise or errors in the GPS processing system in order to enhance the accuracy of first position estimates. Also, a VPS Kalman filter is utilized to filter out noise in the VPS in order to enhance the accuracy of second position estimates. (27) "Motion positioning system" or "MPS" means a system comprising at least an IRU and a vehicle odometer. The MPS derives the second position estimate of any vehicle on or near the Earth's surface. Moreover, an MPS need not be present at the base station due to its stationary nature. (28) "Optimal constellation" means a satellite constellation in which the relative positions of the GPS satellites in space affords superior triangulation capabilities in order to derive the most accurate estimate of a point on or near the Earth's surface. (29) "Original bias" means a spatial bias calculated by subtracting both estimated pseudoranges and clock biases (in units of length) from actual pseudoranges. Clock biases are transformed into units of length by multiplying them by the speed of light, or 2.9979245898 * 10 8 meters per second. (30) "Original bias technique" is a method for computing original biases. (31) "NAVSTAR GPS" means the GPS which has been designed and which is currently being deployed by the U.S. government. (32) "Navigation system" refers to any systems and/or methods for guiding any vehicle on or near the Earth's surface. The navigation system can be on-board a vehicle. The VPS of the present invention can supply the navigation system of the vehicle with a very accurate, third position estimate of the vehicle so that the navigation system can thereby precisely guide the vehicle. (33) "Parabolic bias" is a spatial bias computed by constructing parabolic models for the actual pseudoranges of each observed GPS satellite and extrapolating values from the parabolic models. The parabolic biases are the actual pseudoranges minus the value extrapolated from the constructed parabolic models and minus the clock biases (in units of length, via multiplying by the speed of light). (34) "Parabolic bias technique" is a method for computing parabolic biases for each of the GPS satellites that are utilized. (35) "Preferred embodiment" refers to the best mode of implementing the present invention. The preferred embodiment is merely exemplary. The present invention should not be interpreted as being limited by the preferred embodiment. (36) "Pseudolite" refers to a radiating system on or near the Earth's surface for emulating a GPS satellite. Electromagnetic signals, similar to those from GPS satellites, are transmitted from land-based pseudolites. One or more pseudolites can 92 be used to emulate GPS satellites to enhance the computation of first position estimates. (37) "Pseudolite data" means all data encoded on signals received from pseudolites. Pseudolite data resembles GPS data in many respects and includes similar information. (38) "Pseudorange" means the distance between a source of a terrestrial position determination system and a point on or near the Earth's surface. Sources can be GPS satellites and/or pseudolites. The terrestrial position determination system can be a GPS used with pseudolites, if any. Further, the point on or near the Earth's surface can be the base station and/or vehicles. (39) "Satellite position predictor" is a method for determining the future positions of GPS satellites. The method allows for the selection of optimal constellations ahead of time. (40) "Second position estimate" or "SPE" refers to an estimated absolute position of any vehicle which is outputted, in any form, from the MPS. Second position estimates include at least position information from an IRU. The second position estimate could include position information from a vehicle odometer situated on a vehicle. (41) "Spatial bias" refers to a bias related to approximations of positions in two-dimensional or three-dimensional space. Spatial biases are used to offset a position estimate to enhance the accuracy of the position estimate. Spatial biases can be computed by a number of different methods of the present invention. Included in these methods are, for example, an original bias technique 1500, a parabolic bias technique 1600, a base residuals bias technique 1700, and a base correlator bias technique 1700A. (42) "System" is used for shorthand purposes to mean apparatus, method, or a combination of both apparatus and method. Moreover, it could include software, hardware, or a combination of hardware and software. (43) "Position determination system" means any system having sources which emanate signals which can be used by a receiver of the signals to estimate the relative distance between the sources and the receiver. The signals may be in the form of, for example, electromagnetic waves, percussion waves, and/or sound waves. (44) "Terrestrial position determination system" means any position determination system which can be used to ultimately estimate the terrestrial position of an Earth receiver. The signals may be in the form of, for example, electromagnetic waves, percussion waves, and/or sound waves. In the preferred embodiment, the terrestrial position determination system is the NAVSTAR GPS. (45) "Third position estimate" or "TPE" refers an estimated absolute position of any vehicle that is outputted, in any form, from the VPS. Third position estimates are more accurate position estimates of vehicle positions than the first and second position estimates. Third position are derived by the VPS processing system from the first and second position estimates. (46) "Vehicle" means any carrier for the transportation of physical things. Vehicles may take the form of mining trucks, construction trucks, farm tractors, automobiles, ships, boats, trains, balloons, missiles, or aircraft. In the preferred embodiment, a Caterpillar Inc. 785 off-highway truck is utilized. (47) "Vehicle positioning system" or "VPS" refers to the system deriving position estimates of any vehicle. The position estimates from the VPS are extremely accurate and can be used by a navigation system on any vehicle to accurately guide the vehicle. Position estimates from the VPS are referred to as third position estimates. (48) "VPS processing system" means the processing system of the VPS. The VPS processing system derives third position estimates from the first and second position estimates. The architecture is depicted in Figures 10 and 11. (49) "Weighted combiner" refers to a particular software program which processes data. Inputted data is assigned a predetermined weighing factor based on the estimated accuracy of the data and the technique used to gather the data. For example, the first position estimate of the GPS signal 716 is weighted heavier than the second position estimate of the IRU signal 910 because the former is inherently more accurate. Furthermore, the velocity measured by the IRU can be weighted heavier than the velocity measured by the GPS processing system because the former is more accurate. The velocity measured by the GPS processing system is not used at all, but could be used in other implementations. (50) "Weighted path history technique" is a method or process for increasing the accuracy of first position estimates outputted from the GPS processing system. The technique uses previous first position estimates to derive a vehicle path model for testing the validity of future first position estimates. Use of the weighted path history technique results in a reduction to wandering of first position estimates and in enhanced immunities to spurious position computations. Figure 1 illustrates a high level block diagram 100 of an example positioning system. To provide for the accurate autonomous operation of a vehicle 102 on or near the Earth's surface, the example includes both a vehicle positioning system (VPS) 1000 and a navigation system 1022. Both of these systems include apparatus, methods, and techniques which, when integrated together, provide for highly accurate control of unmanned vehicles. The task of guiding the autonomous vehicle 102 along a prescribed path requires, among other things, an accurate estimate of the vehicle's current position relative to some reference point. Once the current position is known, the vehicle 102 can be commanded to proceed to its next destination. Using the VPS 1000 position estimates of the vehicle 102 can be determined with extreme preciseness. The VPS 1000 receives GPS data from GPS satellites 104 of a GPS, such as the NAVSTAR GPS or the GLONASS GPS. The NAVSTAR GPS may be utilized. Figure 1A illustrates the NAVSTAR GPS. GPS satellites 130-168 travel around the Earth 172 in six orbits 174-184. Referring back to Figure 1, the VPS 1000 also may receive pseudolite data from a pseudolite(s) 105. The term "pseudolite" in the context of this document means a radiating device on or near the Earth's surface for emulating a GPS satellite. From the GPS data and/or the pseudolite data, the VPS 1000 derives accurate estimates of position of the vehicle 102. The GPS data and/or the pseudolite data is significantly enhanced via numerous inventive techniques and methods to enhance the accuracy of vehicle position estimates. More specifically, the VPS 1000 is a positioning system based on the incorporation of GPS data from the NAVSTAR GPS 104 and from a motion positioning system 900. The motion positioning system 900 comprises an inertial reference unit (IRU) 904 and/or a vehicle odometer 902. The IRU 904 comprises a laser gyroscope(s) 106 and an accelerometer(s) 108 which can be used to produce position, velocity, roll, pitch and yaw data. The vehicle odometer 902 produces data on the distance travelled by the vehicle 102. A first position estimate of the vehicle 102 is derived by the GPS processing system 700 from GPS data received from the GPS satellites 104 and from the pseudolite data received from the pseudolite(s) 105. One of a number of methods may be implemented to increase the accuracy of the first position estimate. In addition, a second position estimate is derived by the MPS intercommunications processor 906 of the motion positioning system 900, which comprises the IRU 904 and/or the vehicle odometer 902. As shown by respective arrows 112 and 114, the first position estimate and the second position estimate are then combined and filtered by a VPS processing system 116. The result as shown by an output arrow 118 is a more accurate, third position estimate. The navigation system 1022 receives the third position estimate from the VPS 1000. The navigation system 1022 uses the precise, third position estimate to accurately navigate the vehicle 102. A primary purpose of the navigation system 1022 is to guide the vehicle 102 between points along pre-established or dynamically-generated paths. The navigation system 1022 is situated on the vehicle 102 itself. In other words, it is essentially an "on-board" system. Moreover, the navigation system 1022 may be designed to be retrofitted into the vehicle 102. So that the navigation system 1022 can guide the vehicle 102 to follow the pre-established or dynamically-generated paths, various models or conceptual representations are generated and utilized. For example, lines and arcs may be used to establish vehicle paths between objective points. Mathematical B-splines or clothoid curves may be used to model the actual path where the vehicle 102 is to navigate. Using the above modelling or representational techniques provides for enhanced data communications, storage, and handling of the vehicle 102. The techniques further allow for simplification of supervisory tasks by providing a hierarchy of control and communication. The higher that a level of control exists on the hierarchical control scheme, the simpler the task and the more compact the commands. The navigation system 1022 further provides for controlling the vehicle's mechanical systems, such as brakes, steering, and engine and transmission, to effect the necessary physical acts required to move, stop, and steer the vehicle 102. The navigation system 1022 also checks the actual position of the vehicle 102 against the desired position to correct vehicle control in accord with the desired position. The navigation system 1022 may run multi-state models to enhance this checking capability. The navigation system 1022 also checks for errors or failures in the system itself and vehicle components. If errors or failures are detected, the navigation system 1022 can provide for fail-safe shutdown by bringing the vehicle 102 to a complete stop. The navigation system 1022 further provides for different modes of controlling the vehicle 102. These include (1) a fully autonomous mode, where navigation of the vehicle 102 is automatically handled by the navigation system 1022; (2) a tele or remote control mode, where a remote human operator (not shown) may control the direction and motion, and so on, of the vehicle 102; and (3) a manual mode, where a human operator sitting in the vehicle 102 can take control of the vehicle 102 and drive it manually. In the autonomous mode, obstacle detection is critical because if the vehicle 102 is not under control, then it could cause great damage to property and great injury to life. The navigation system 1022 can efficiently detect obstacles. Boulders, animals, people, trees, or other obstructions may enter the path of the vehicle 102 unexpectedly. The navigation system 102 is capable of detecting these obstacles, either stopping or plotting a path around the obstruction, and returning the vehicle 102 to its original route when the route is deemed safe. Accurately tracking the desired route is another function of the navigation system 1022. The functioning and architecture of the navigation system 1022 has been designed for real time tracking of vehicle paths at speeds of up to approximately 30 miles per hour (mph). The example can comprise a host processing system 186 at a base station 188. The host processing system 186 performs functions for both the VPS 1000 and the navigation system 1022. With respect to the VPS 1000, the host processing system 186 receives GPS data and/or pseudolite data, as shown by respective arrows 190 and 192. In effect, the host processing system 186 as well as the base station 188 can serve as a known reference point to improve the accuracy of vehicle position estimates as discussed in detail below. The following discussion relative to the VPS 1000 will make specific reference to Figures 7 through 10. Figure 10 shows the architecture/hardware of the VPS 1000. The VPS 1000 is a highly accurate position determination system for a moving or stationery vehicle 102 on or near the Earth's surface. Recall that the VPA 1000 includes the GPS processing system 700 and the MPS 900, which are shown in respective Figures 7 and 9. Further recall that the MPS 900 includes the IRU 904 and the vehicle odometer 902, which are both shown in Figure 9. In effect, these systems have been enhanced and integrated to produce a highly effective position determining system. Referring to Figure 7, the GPS processing system 700 includes an antenna 702 connected to a GPS receiver 706. When the GPS satellites 104 in view of antenna 702 comprise multiple GPS satellites 200-206 as shown in Figures 2 and 3, the GPS receiver 706 reads each of their GPS data along with any pseudolite data from any pseudolite(s) 105 in view of antenna 702. The GPS receiver 706 is responsible for computing the first position estimate of the vehicle 102 from the GPS data and/or the pseudolite data. To increase the accuracy of the first position method, a satellite position predictor method 1800 is implemented by a GPS processor 710 of the GPS processing system 700. The satellite position predictor method 1800 predicts the position of any GPS satellite at the current time or any future time. Using the satellite position information, the GPS processing system 700 can determine the optimum GPS satellite constellation to recognize by using a constellation effects method 1300. The constellation effects method 1300 is also implemented by the GPS processor 710. Pursuant to the constellation effects method 1300, a best constellation is selected from the data sources comprising the GPS satellites 200-206 and pseudolite(s) 105. The GPS processor 710 computes a first position estimate of the vehicle 102 based on the best constellation and geometry/triangulation methods. The accuracy of the first position estimate is, in part, dependent on the number of GPS satellites used in the computation. Each additional GPS satellite used can increase the accuracy of the first position estimate. After the computation, the first position estimate of the vehicle 102 is transmitted to a VPS main processor 1002 of Figure 10. Referring to Figure 9, the IRU 904 comprises laser gyroscopes and accelerometers which produce position, velocity, roll, pitch, and yaw data. The IRU 904 combines this information into a second position estimate of the vehicle 102. The odometer 902 can be implemented to measure the distance travelled by the vehicle 102. The data from the IRU 904 and the odometer 902 is also transmitted via the MPS intercommunications processor 906 to the VPS main processor 1002, as shown in Figure 10. The VPS main processor 1002 combines the second position estimate from the MPS 900 (the IRU 904 and perhaps the odometer 902) with the first position estimate from the GPS processing system 700 to produce a more accurate third position estimate. The VPS 1000 further implements a method of eliminating erratic or spurious, third position estimates which can cause vehicle "wandering." This method is called the weighted path history method. Essentially, the path history of the vehicle 102 is used to statistically determine the accuracy of future estimates of the vehicle 102's position. Referring now to Figures 1 and 3, a base station 188 provides a geographic proximate reference point for the VPS 1000. The base station 188 includes a host processing system 186. The host processing system 186 comprises a similar architecture and performs the same functions as the GPS processing system 700. However, the host processing system 108 performs additional functions for increasing the accuracy of first position estimates. The satellite position predictor method 1800 is implemented by the host processing system 186, in addition to the GPS processing system 700 as discussed above. Accordingly, the host processing system 186 will recognize the same GPS satellite constellation that is observed by the vehicle 102 or include the same GPS satellite in a larger constellation. Calculations are performed on the GPS data and/or pseudolite data to derive biases, including spatial biases and clock biases. To compute spatial biases, the host processing system 186 implements a number of methods. The GPS processing system 700 utilizes vehicle position data from a terrestrial position determination system to derive the first position estimate of the vehicle 102. The terrestrial position determination system comprises the NAVSTAR GPS, which is currently being developed by the U.S. government, and/or Earth- based pseudolites. As shown in Figure 1A, 24 man-made electronic GPS satellites 132-170 in six orbits 174-184 are currently envisioned for the NAVSTAR GPS. They are planned for deployment by 1993. As currently envisioned, the GPS satellites 132-170 will orbit the Earth 172 at an altitude of approximately 14,000 miles and encircle the globe twice a day. Using the C mode of the NAVSTAR GPS, as will be discussed below, it will be possible to determine terrestrial positions within 15 meters in any weather, any time, and most areas of the Earth 172. As of the date of the filing of this document, there are known to be six experimental and seven operational GPS satellites in orbit around the Earth 172. Further, several manufacturers are known to be designing and building GPS receivers, such as the GPS receiver 706 of Figure 7. As more and more GPS satellites are deployed and operational, the time periods increase when three or more of the experimental GPS satellites are available each day for position tracking. Moreover, the location of the experimental GPS satellites (and all others once deployed) is very predictable. The relative position, or "pseudorange," of these GPS satellites with respect to the GPS receiver 706 on the vehicle 102 can be determined from the electromagnetic signals by two methods. One method is to measure the propagation time delays between transmission and reception of the emanating electromagnetic signals. In the NAVSTAR GPS, the electromagnetic signals are encoded continuously with the time at which the signals are transmitted from the GPS satellites. Needless to say, one can make note of the reception time and subtract the encoded transmission time in order to derive time delays. From the calculated time delays and from knowing the speed at which electromagnetic waves travel through the atmosphere, pseudoranges can be accurately derived. Pseudoranges computed using the foregoing method are referred to in the context of this document as "actual" pseudoranges. Another method involves satellite position data that is encoded in the electromagnetic signals being transmitted from the orbiting GPS satellites. Almanac data relating to the GPS satellite position data of the NAVSTAR GPS is publicly available. Reference to this almanac data in regard to data encoded in the electromagnetic signals allows for an accurate derivation of pseudoranges if the receiver location is known. Pseudoranges computed using the foregoing method are referred to in the context of this document as "estimated" pseudoranges. However, with respect to the previous method of deriving estimated pseudoranges, it should be noted that the satellite position data is updated at the GPS satellite only once an hour on the hour. Consequently, an estimated pseudorange decreases in accuracy over time after each hour until the next hour, when a new estimated pseudorange is computed using updated satellite position data. Reference is again made to Figure 1A of the drawings wherein the configuration of the fully- operational NAVSTAR GPS is schematically illustrated. Each of the 24 GPS satellites 132-170 transmits electromagnetic signals which can be used to determine the absolute terrestrial position (that is, longitude, latitude, and altitude with respect to the Earth 172's center) of the vehicle 102. Specifically, by knowing the relative position of at least three of the orbiting GPS satellites 132-170, the absolute terrestrial position of the vehicle 102 can be computed via simple geometric theory involving triangulation methods. The accuracy of the terrestrial position estimate depends in part on the number of orbiting GPS satellites 132-170 that are sampled by the vehicle 102. The sampling of more GPS satellites 132-170 in the computation increases the accuracy of the terrestrial position estimate. Conventionally, four GPS satellites, instead of three, are sampled to determine each terrestrial position estimate because of errors contributed by circuit clock differentials among the circuitry of the vehicle 102 and the various GPS satellites 132-170. In the NAVSTAR GPS, electromagnetic signals are continuously transmitted from all of the GPS satellites 132-170 at a single carrier frequency. However, each of the GPS satellites 132-170 has a different modulation scheme, thereby allowing for differentiation of the electromagnetic signals. In the NAVSTAR GPS, the carrier frequency is modulated using a pseudorandom binary code signal (data bit stream) which is unique to each GPS satellite. The pseudorandom binary code signal is used to biphase modulate the carrier frequency. Consequently, the orbiting GPS satellites in the NAVSTAR GPS can be identified when the carrier frequencies are demodulated. Furthermore, the NAVSTAR GPS envisions two modes of modulating the carrier wave using pseudorandom number (PRN) signals. In one mode, referred to as the "coarse/acquisition" (C/A) mode, the PRN signal is a gold code sequence having a chip rate of 1.023 MHz. The gold code sequence is a well-known conventional pseudorandom sequence in the art. A chip is one individual pulse of the pseudorandom code. The chip rate of a pseudorandom code sequence is the rate at which the chips in the sequence are generated. Consequently, the chip rate is equal to the code repetition rate divided by the number of members in the code. Accordingly, with respect to the coarse/acquisition mode of the NAVSTAR GPS, there exists 1,023 chips in each gold code sequence and the sequence is repeated once every millisecond. Use of the 1.023 MHz gold code sequence from four orbiting GPS satellites enables the terrestrial position of the vehicle 102 to be determined to an approximate accuracy of within 60 to 300 meters. The second mode of modulation in the NAVSTAR GPS is commonly referred to as the "precise" or "protected" (P) mode. In the P mode, the pseudorandom code has a chip rate of 10.23 MHz. Moreover, the P mode sequences that are extremely long, so that the sequences repeat no more than once per 276 days. As a result, the terrestrial position of the vehicle 102 can be determined to within an approximate accuracy of 16 to 30 meters. However, the P mode sequences are classified and are not made publicly available by the United States government. In other words, the P mode is intended for use only by Earth receivers authorized by the United States government. Figure 2 illustrates navigation equations 212 regarding four GPS satellites 200-206 of the NAVSTAR GPS. The four GPS satellites 200, 202, 204, and 206 have respective pseudoranges R0, R2, R4, and R6 and comprise the current constellation of GPS satellites 132-170 recognized by the vehicle 102. The navigation equations 212 include the clock bias C b between the GPS satellites 200-206 and the vehicle 102. The navigation equations 212 are used to compute the longitude and latitude of the vehicle 102 using the pseudoranges R0, R2, R4, and R6. As is shown in the description block 208, each of the GPS satellites 200, 202, 204, and 206 transmits GPS data that includes timing data (GPS time) and ephemeris data. Using the navigation equations 212, which are well-known in the conventional art and the foregoing timing data, the pseudoranges R0, R2, R4, and R6 can be estimated (called actual pseudoranges) by the GPS processing system 700. Furthermore, using the foregoing ephemeris data and almanac data on the Earth 172, the pseudoranges R0, R2, R4, and R6 can be estimated (called estimated pseudoranges) by the GPS processing system. Turning now to Figure 6, a representative GPS constellation is shown in operation. Four GPS satellites 200, 202, 204 and 206 are transmitting GPS data. Both the vehicle 102 and the base station 188 are receiving these signals from each of these GPS satellites 200, 202, 204, and 206 on their respective GPS antennas 312 and 316. Both the C/A code and the carrier frequency are received at GPS antennas 312 and 316 for processing. In addition to the four GPS satellites shown in the Figure 6 is the pseudolite 105. The pseudolite(s) 105 can be strategically placed around the perimeter of any mine pit and can emulate the GPS satellites 200, 202, 204, and 206 as shown in Figure 6. This arrangement can be extremely useful in situations such as a mine pit, cavity, or the like, in which mining vehicles may be out of view of one or more of the GPS satellites 200, 202, 204, and 206, because of topographic features such as high mine pit walls. The ground-based pseudolite(s) 105 provides additional ranging signals and can thus improve availability and accuracy of the positioning capability in the present invention. The pseudolite(s) 105 is synchronized with the GPS satellites 200, 202, 204, and 206 and has a signal structure that, while different, is compatible with the GPS satellites 200, 202, 204, and 206. Moreover, the distance (range) between the vehicle 102 and the pseudolite(s) 105 is calculated similarly as the distance between the vehicle 102 and one of GPS satellites 200, 202, 204, and 206. With pseudolite(s) 105, the ranging error does not include selective availability nor ionospheric errors. However, other errors must be accounted for such as tropospheric, pseudolite clock error and multipath errors. In a deep pit surface mining operation, the view of the sky from a vehicle 102 in the mine can be limited by the walls of the mine. Consequently, an adequate number of GPS satellites may not be in view for the GPS processing system 700 to properly derive a first position estimate. In such a case one or more pseudolites 105 can serve as secondary sources. The pseudolite(s) can be placed on the rim of the mine or elsewhere. The pseudolite(s) 105 can be used by the vehicle 102 in conjunction with any visible GPS satellites to obtain accurate first position estimates. It is also envisioned that other forms of secondary sources could be implemented to aid GPS satellites or to completely eliminate the need to receive GPS data from the GPS satellites. Moreover, a laser scanning technique may utilized to give localized ranging data to the vehicle 102 from a secondary reference source. Communication channel 618 represents the communications link between the base station 188 and the vehicle 102. The communication channel 618 comprises an electromagnetic link established by data-radios 620 and 622 which are transceivers. The communication channel 618 is used to transfer data between the base station 188 and the vehicle 102. It is envisioned that other forms of communication media may be utilized. For example, a laser scanning technique may utilized to convey information from the base station 108 to the vehicle 102. The data radios 620 and 622 are located at the base station 188 and vehicle 102 respectively. The radios 620 and 622 are responsible for exchanging data between the base station 188 and the vehicle 102. The type of data exchanged will be discussed further below. A radio transceiver which functions appropriately as the data radios 620 and 622 is commercially available from Dataradio Ltd. of Montreal, Canada, Model Number DR-4800BZ. Turning now to Figure 7, a GPS processing system 700 is shown. The GPS processing system 700 on the vehicle 102 includes a GPS antenna 702. The GPS antenna 702 is receptive to the radio spectrum of electromagnetic radiation. However, the example system contemplates reception of any signal by which GPS satellites 132-170 might encode data. The GPS antenna 702 is the commercially available antenna having Model No. CA3224 from Chu Associates Inc. of Littleton, Massachusetts. The GPS antenna 702 is coupled to a preamplifier 704 so that the signals received at the GPS antenna 702 can be transmitted to the preamplifier 704. The term "couple" in the context of this document means any system and method for establishing communication. Coupling systems and methods may include, for example, electronics, optics, and/or sound techniques as well as any others not expressly described herein. Coupling is commonly electronic and adheres to any one of numerous industry standard electronic interfaces. The preamplifier 704 amplifies and down converts the GPS data received from the GPS antenna 702 so that the GPS data can be processed, or decoded. The example system contemplates any method by which the received signals can be amplified. The preamplifier 704 is the commercially available preamplifier having Model No. 5300, Series GPS RF/IF from Stanford Telecommunications Inc. (STel) of Santa Clara, California. The preamplifier 704 is coupled to a GPS receiver 706. The GPS receiver 706 processes the GPS data sent from the GPS satellites 200, 202, 204, and 206 in view of the GPS antenna 702. The GPS receiver 706 computes actual pseudoranges for each of the GPS satellites 200, 202, 204, and 206. Actual pseudoranges are defined in this document as an estimate of the pseudoranges R0, R2, R4, and R6 which is derived from the time delay between the transmission of electromagnetic signals from the GPS satellites and the reception of the electromagnetic signals by the GPS processing system 700. Moreover, the GPS receiver 706 can process in parallel all of the actual pseudoranges for the GPS satellites 200, 202, 204, and 206. The GPS receiver 706 produces this data when four or more GPS satellites are visible. Using the differential correction techniques described in of this document, the GPS processing system 700 can compute (at GPS processor 710) the first position estimate with an accuracy of approximately 25 meters when an optimal constellation of four GPS satellites 200, 202, 204, and 206 is in view. When an optimal constellation of five GPS satellites (not shown) is in view, the GPS processing system 700 can compute the first position estimate with an accuracy of approximately 15 meters. An "optimal" constellation is one in which the relative positions of the GPS satellites in space affords superior triangulation capability, triangulation technology being well known in the art. The GPS receiver 706 outputs actual pseudoranges and the number of GPS satellites 132-170 currently being sampled. In cases in which the number of GPS satellites 132-170 viewed for a series of first position estimates is less than four, the VPS weighted combiner 1204 does not use the first position estimates received from the GPS processing system 700 (specifically, the GPS processor 710) in the computation of the third position estimate. The GPS receiver 706 comprises a Model Number 5305-NSI receiver, which is commercially available from Stanford Telecommunications Inc. However, any receiver which is capable of providing actual pseudoranges and the number of sampled GPS satellites may be utilized. The GPS receiver 706 is coupled to a GPS intercommunication processor 708. The intercommunication processor 708 is the commercially available 68000 microprocessor from Motorola Inc., of Schaumburg, Illinois, U.S.A. Any processor alone or in combination with the GPS receiver 706 for accomplishing the same purpose as described below may be utilized. The GPS intercommunication processor 708 is further coupled to a GPS processor 710 and a GPS Console 1 712. The GPS intercommunication processor 708 coordinates data exchange between these three devices. Specifically, the GPS intercommunication processor 708 receives pseudorange data from the GPS receiver 706 which it passes on to the GPS processor 710. The pseudorange data includes, for example, the actual pseudoranges computed by the GPS receiver 706, the number of GPS satellites 200, 202. 204, and 206 currently being viewed by the GPS receiver 706, and other GPS data needed by the GPS processor 710 to compute the estimated pseudoranges for each of the GPS satellites 200, 202, 204, and 206. The GPS intercommunication processor 708 also relays status information regarding the GPS receiver 706 and the GPS processor 710 to the GPS Console 1 712. The GPS intercommunication processor 708 transmits the above information to the GPS processor 710. The GPS processor 710 comprises the 68020 microprocessor, which is commercially available from Motorola Inc. Figure 8 is a low level flow diagram 800 illustrating the functioning of the software in the GPS processor 710. The GPS process 710 uses a number of algorithms and methods to process the data it receives including, for example, a GPS Kalman filter 802, which is shown in Figure 8. The Kalman filter 802 is well known in the conventional art. The GPS Kalman filter 802 is a module in the software of the GPS processor 710. In part, function of the Kalman filter 802 is to filter our noise associated with the pseudorange data. The GPS processor 710 then computes the estimated pseudoranges, the first position estimate, and the vehicle velocity (from Doppler shift) using the above current state and any biases, including the clock biases and the spatial biases. However, the GPS processor 710 discards the computed velocity data when the C/A code, rather than the carrier frequency, is utilized by the GPS receiver 706 to derive the vehicle velocity. The rationale for discarding the vehicle velocity is that experimentation has shown that it is not adequately accurate when derived from the C/A code. Vehicle velocities derived from the carrier frequency (Doppler shift) are much more accurate than the velocities derived from the C/A code. The first estimated position (and vehicle velocity if derived from the carrier frequency) are encoded on GPS Signal 716 and sent on to the VPS main processor 1002 shown on Figure 10. As previously discussed, the GPS processor 710 analyzes both the carrier frequency and the C/A code. Unlike data demodulated from the C/A code, data may be retrieved from the carrier frequency by the GPS receiver 706 at approximately 50 Hz (not approximately 2 Hz, as is the case for demodulating C/A code). This increased speed allows the present invention to produce more precise position and velocity determinations with less error. Figure 8 illustrates other functions of the GPS processor 710. However, the present invention contemplates any method by which GPS data can be processed to determine pseudoranges. As shown at a flowchart block 816, a console function controls the operation of the GPS console 2. This console function regulates the operation of the GPS Kalman filter 802 by providing a user interface into the filter. The VPS communications function 818 controls the outputs of the GPS Kalman filter 802 which are directed to the VPS 1000. At a flowchart block 806, it is shown that the GPS Kalman filter 802 requests and decodes data from the GPS receiver 706, which data is routed through an IPROTO function 804 shown at a flowchart block 804. As shown, the IPROTO function 804 resides in the GPS intercommunications processor 708 and executes tasks associated with the GPS intercommunications processor 708. The IPROTO function 804 is the model number XVME-081, which is commercially available from Xycom Inc. The GPS console 712 is well known in the art. Many types of devices are commercially available which provide the desired function. One such device is commercially available from Digital Equipment Corporation of Maynard, Massachusetts Model Number VT220. The GPS console 712 displays processor activity data regarding the GPS intercommunications processor 708 and the GPS processor 710. The GPS processor 710 is coupled to a GPS console 722 and a GPS communications interface processor 720. The GPS console 722 is well known in the art. Many types of devices are commercially available which provide the desired console function. One such device is commercially available from Digital Equipment Corporation of Maynard, Massachusetts Model Number VT220. The GPS console 722 provides the user interface from which the GPS processor 710 can be activated and monitored. The GPS communications interface processor 720 is essentially an I/O board. It is coupled to a data-radio 714 and a GPS data collection device 718. The GPS communications interface processor 720 coordinates data exchange between the GPS processor 710 and both the data-radio 714 and the GPS data collection device 718. The communications interface processor 720 in the example system is the model no. MVME331, which is commercially available from Motorola Inc., U.S.A. The data-radio 714 establishes a communication link between the GPS processor 710 (through the GPS communications interface processor 720) at the vehicle 102 to a similar data-radio 714 located at the base station 188 (see Figure 6). The data-radio 714 communicates synchronously at 9600 baud using RF frequencies. The data-radio 714 at the base station 188 provides periodic updates on the amount of spatial bias and clock bias for each satellite to the data-radio 714 at the vehicle 102 at a rate of 2 Hz (twice per second). Spatial and clock biases computed by the base station 188 will be discussed further below. The GPS data collection device 718 can be any of numerous common electronic processing and storage devices such as a desktop computer. Any personal computer (PC) manufactured by the International Business Machines Corporation (IBM) of Boca Raton, Florida, U.S.A., can be implemented. The MPS 900 is illustrated in Figure 9. The MPS 900 derives the second position estimate of the vehicle 102. Usually, this second position estimate is combined and filtered with the first position estimate to thereby derive a more accurate third position estimate. However, it is envisioned that in some instances the second position estimate may be utilized exclusively as the third position estimate, when the first position estimate is deemed to be drastically inaccurate. For the MPS 900, the example system envisions the combination of the odometer 902 and the IRU 904. However, the IRU 904 could be utilized without the odometer 902. The odometer and the IRU 904 are coupled to an MPS intercommunications processor 906 to thereby comprise the MPS 900. IRUs and odometers are well known in the art and are commercially available, respectively, from Honeywell Inc. of Minneapolis, Minnesota, Model Number HG1050-SR01 and from Caterpillar Inc. of Peoria, Illinois, Part Number 7T6337. The IRU 904 comprises ring-laser gyroscopes and accelerometers of known design.. The IRU 904 is a replica of the system used by Boeing 767 aircrafts to determine aircraft position, except that the IRU 904 has been modified to account for the lesser dynamics (for example, velocity) that the vehicle 102 exhibits relative to that of a 767 aircraft. The IRU 904 can output vehicle position at 5 Hz, velocity at 10 Hz, roll at 50 Hz, pitch at 50 Hz, and yaw data at 50 Hz. Furthermore, the vehicle odometer 902 can output the distance travelled by the vehicle 102 at 20 Hz. The laser gyroscopes of the IRU 904, in order to function properly, must at first be given an estimate of the vehicle 102's latitude, longitude and altitude. Using this data as a baseline position estimate, the gyroscopes then use a predefined calibration in conjunction with forces associated with the rotation of the Earth 172 to determine an estimate of the vehicle 102's current position. This information is then combined by the IRU 904 with data acquired by the IRU 904 accelerometers to produce a more accurate, second position estimate of the vehicle's current position. The second position estimate from the IRU 904 and the data from the vehicle odometer 902 are transmitted to the MPS intercommunications processor 906 as shown by respective arrows 910 and 908 of Figure 9. Arrow 114 of Figure 1 includes arrows 908 and 910. Upon experimentation, it has been determined that the IRU 904 may provide erroneous second position estimates of the vehicle 102 due to imprecise constituent parts. More specifically, in the example system, it has been observed that the directional output of the IRU 904 has drifted counterclockwise from the direction north during operation. The drift is dependent upon the direction in which the vehicle 102, and consequently the IRU 904, is travelling. Moreover, the drift can be defined by an IRU drift equation. The IRU drift equation can be derived similar to the construction of path equations described in regard to the weighted path history technique or similar to the construction of parabolic equations described in regard to the parabolic bias technique. After derived, the IRU drift equation can be utilized to extrapolate more accurate second position estimates. The intercommunications processor 1002 comprises the commercially available 68000 microprocessor from Motorola Inc. The intercommunications processor 1002 coordinates exchange of data between the MPS 900 and the VPS 1000. Any processor with similar function as described herein may be utilized. Turning now to Figure 10, the architecture of the VPS 1000 is depicted. Figure 11 shows in detail a diagram of the VPS 1000 connected to the GPS processing system 700 and MPS 900. GPS processing system 700 and MPS 900 are independently coupled to the VPS main processor 1002. The independent coupling is an important novel feature of the present invention. Because they are independent, the failure of one of the systems will not cause the other to become inoperative. Thus, if the GPS processing system 700 is not operative, data can still be collected and processed by the MPS 900 and, consequently, the VPS 1000. The GPS processing system 700 and the MPS 900 transmit signals 716, 908, 910 to the VPS main processor 1002, as shown. These signals contain position, velocity, time, pitch, roll, yaw, and distance data (see Figures 7 and 9 and associated discussions). The VPS main processor 1002 is coupled to the VPS I/O processor 1004. The VPS main processor 1002 transmits a signal 1008 to a VPS I/O processor 1004, as shown. The signal 1008 comprises the third position estimate. The third position estimate is derived from the GPS, IRU, and odometer data noted above, and more specifically, the first and second position estimates of the vehicle 102. The example system contemplates any system and method by which the signals indicated by arrows 716, 908 and 910 can be received by the VPS main processor 1002 from the GPS processing system 700 and MPS system 900 and forwarded to the VPS main processor 1002. The VPS main processor 1002 is the 68020 microprocessor, which is commercially available from Motorola Inc., U.S.A. Referring now back to Figure 10, the VPS I/O processor 1004 is coupled to a VPS communications interface processor 1020. The communications interface processor 1020 is the MVME331 processor, which is commercially available from Motorola Inc., U.S.A. Any processor accomplishing the same purpose as described below may be utilized. The VPS communications interface processor 1020 is coupled to three different devices: (1) a VPS console 1012, (2) a data collection device 1014, and (3) the navigation system 1022. The VPS communications interface processor 1020 routes the data, including the third position estimate, contained in output 1016 to the above three devices at a rate of 20 Hz. The VPS console 1012 is well known in the art, and is commercially available from Digital Equipment corporation, of Minneapolis, Minnesota, Model Number VT220. This VPS console 1012 is used to display the current status of the VPS I/O processor 1004. The VPS data collection device 1014 can be any of numerous commercially available electronic processing and storage devices, for example, a desktop PC. Any MacIntosh PC available from Apple Computer of Cupertino, California, can be used successfully to achieve this purpose. The navigation system 1022 comprises the features associated with the navigation of the vehicle 102. The VPS 1000 transmits the third position estimate to the navigation system 1022, so that the navigation system 1022 can accurately and safely guide the autonomous vehicle 102. With reference to Figure 7, the host processing system 186 at the base station 188 comprises the GPS processing system 700 of Figure 7. The purposes of the host processing system 186 at the base station 188 are to (1) monitor the operation of the vehicle 102, (2) provide a known terrestrial reference point from which spatial biases can be produced, and (3) provide any other information to the vehicle 102 when necessary over the high-speed data communication channel 618. The base station 188 will be located close to the vehicle 102, preferably within 20 miles. The close geographical relationship will provide for effective radio communication between the base station 188 and the vehicle 102 over the communication channel 618. It will also provide an accurate reference point for comparing satellite transmissions received by the vehicle 102 with those received by the base station 188. A geographically proximate reference point is needed in order to compute accurate spatial biases. Spatial and clock biases are, in effect, the common mode noise that exists inherently in the NAVSTAR GPS and the GPS processing system 700. Once computed at the base station 188, the spatial and clock biases are then sent to the vehicle 102 using the data-radio 714, as shown in Figure 7. The spatial biases are computed using various methods which are discussed further below. The host processing system 186 at the base station 188 further coordinates the autonomous activities of the vehicle 102 and interfaces the VPS 1000 with human supervisors. The example system improves the accuracy of the position estimates of the vehicle 102 via a number of differential correction techniques. These differential bias techniques are used to enhance the first, second, and third position estimates. Several of these differential correction techniques are designed to directly remove errors (noise or interference) in the calculation of pseudoranges R0, R2, R4, and R6 (both actual and estimated pseudoranges). The removal of these errors results in a more precise first position estimate, which is outputted by the GPS processing system 700 to the VPS 1000, and ultimately, in a more precise third position estimate, which is outputted by the VPS 1000 to the navigation system 1022. The host processing system 186 at the base station 188 is responsible for executing these differential techniques and for forwarding the results to the vehicle 102. Recall that the host processing system 186 comprises the GPS processing system 700, just as the vehicle 102. The term "differential" is used because the base station 188 and the vehicle 102 use independent but virtually an identical GPS processing system 700. Furthermore, because the base station 188 is stationary and its absolute position is known, it serves as a reference point from which to measure electronic errors (noise or interference) and other phenomena inducing errors. The example system includes a method by which the future positions of the GPS satellites 132-170 can be predicted with respect to a known absolute position of the base station 188 and/or the vehicle 102. The future positions are based upon estimated pseudoranges calculated by the GPS processor 710 at the host processing system 188 and/or the VPS 1000. Moreover, the computations can be performed at the base station 188 and/or vehicle 102 and transferred anywhere, if necessary. By predicting the future positions of the GPS satellites 132-170, optimum satellite constellations for the vehicle 1-2 can be determined well in advance, Thus, the example system can provide for the prediction of satellite availability and unavailability in a systematic manner. It further allows for future planning related to the operation, service, and maintenance of the vehicle 102. In considering implementation of an autonomous navigation system, there are some basic questions which any autonomous system must be able to answer in order to successfully navigate from point A to point B. The first question is "where are we (the vehicle) now?" This first question is answered by the positioning system, as discussed above in section III. The next or second question is "where do we go and how do we get there?" This second question falls within the domain of the navigation system portion of the present invention, discussed in this section (IV). A further (third) question, really a refinement of the second one, is "how do we actually physically move the vehicle, for example, what actuators are involved (steering, speed, braking, and so on), to get there?" This is in the domain of the vehicle controls subsystem of the navigation system, also discussed below. As has been discussed implicitly above, autonomous navigation, of a mining vehicle as an example, may provide certain significant advantages over conventional navigation. Among them is an increased productivity from round the clock, 24 hr. operation of the vehicles. The problems presented by dangerous work environments, or work environments where visibility is low, are particularly well suited to solution by an autonomous system. There are, for instance, some mining sites where visibility is so poor that work is not possible 200 days of the year. There are other areas which may be hazardous to human life because of being contaminated by industrial or nuclear pollution. An area may be so remote or desolate that requiring humans to work there may pose severe hardships or be impractical. The application could foreseeably include extraterrestrial operations, for example, mining on the Moon, provided that the necessary GPS satellites were put in Moon orbit. In a typical application, as shown in Figure 3, with regard to the navigation of a mining vehicle at a mining site, there are three basic work areas: the load site, the haul segment, and the dump site. At the load site, a hauling vehicle may be loaded with ore in any number of ways, by human operated shovels for instance, controlled either directly or by remote control, or by autonomous shovels. The hauling vehicle then must traverse an area called the haul segment which may be only a few hundred meters or may be several km's. At the end of the haul segment is the dump site, where the ore is dumped out of the hauling vehicle to be crushed, or otherwise refined, for instance. In the example system autonomous positioning and navigation may be used to control the hauling vehicle along the haul segment. Autonomously navigated refuelling and maintenance vehicles are also envisioned. Referring now to Figures 4 and 5, navigation of the AMT (Autonomous Mining Truck) encompasses several systems, apparatus and/or functions. The VPS 1000 subsystem of the overall AMT system as described above, outputs position data that indicates where the vehicle is located, including, for example, a North and an East position. Referring now to Figures 4 and 5, position data output from the VPS is received by a navigator 406. The navigator determines where the vehicle wants to go (from route data) and how to get there, and in turn outputs data composed of steer and speed commands to a vehicle controls functional block 408 to move the vehicle. The vehicle controls block then outputs low level commands to the various vehicle 102 systems, such as the governor, brakes and transmission. As the vehicle is moving towards its destination, the vehicle controls block and the VPS receive feed-back information from the vehicle indicative of, for example, any fault conditions in the vehicle's systems, current speed, and so on. Navigation also must include an obstacle handling (detection and avoidance) capability to deal with the unexpected. A scanning system 404 detects obstacles in the vehicle's projected trajectory, as well as obstacles which may be approaching from the sides and informs the navigator of these. The navigator may be required to then decide if action is required to avoid the obstacle. If action is required, the navigator decides how to avoid the obstacle. And after avoiding the obstacle, the navigator decides how to get the vehicle back onto a path towards its destination. Referring now to Figure 16, titled the context diagram, and Figure 18A-18D definitions of the communications, which are shown as circles with numbers in them, are provided below: 502. Host commands & queries: Commands given by the host to the vehicle manager. These commands could be of several types: initiate/terminate; supply parameters; emergency actions; and directives. Queries inquire about the status of various parts of the navigator. 504. replies to host: These are responses to the queries made by the host. 432. position data: This is streamed information provided by the VPS. 416. Range data: This is range data from the line laser scanner. 432. VPS control: These are commands given to the VPS to bring it up, shut it down and switch between modes. 416. scanner control: These are commands sent to the laser scanner to initiate motion and set follow velocity profile. 420. steering & speed commands These are commands given to the vehicle to control steering and speed. These commands are issued at the rate of 2-5 Hz. Referring to Figure 5, as described above, both the VPS and the navigator are located on the vehicle and communicate with the base station 188 to receive high level GPS position information and directives from a host processing system 186, discussed below. The system gathers GPS position information from the GPS satellites 200-206 at the base station and on-board the vehicle so that common- mode error can be removed and positioning accuracy enhanced. Alternatively, portions of the VPS and navigator may be located at the base station. The host at the base station may tell the navigator to go from point A to point B, for instance, and may indicate one of a set of fixed routes to use. The host also handles other typical dispatching and scheduling activities, such as coordinating vehicles and equipment to maximize efficiency, avoid collisions, schedule maintenance, detect error conditions, and the like. The host also has an operations interface for a human manager. It was found to be desirable to locate the host at the base station and the navigator on the vehicle to avoid a communications bottleneck, and a resultant degradation in performance and responsiveness. Since the host sends relatively high-level commands and simplified data to the navigator, it requires relatively little communication bandwidth. However, in situations where broad-band communication is available, this may not be a factor. Another factor in determining the particular location of elements of the system, is the time-criticality of autonomous navigation. The navigation system must continually check its absolute and relative locations to avoid unacceptable inaccuracies in following a route. The required frequency of checking location increases with the speed of the vehicle, and communication speed may become a limiting factor even at a relatively moderate vehicle speed. However, in applications where maximum vehicle speed is not a primary consideration and/or a high degree of route following accuracy is not critical, this communication factor may not be important. For example, in rapidly crossing large expanses of open, flat land, in a relatively straight path, it may not be necessary to check position as often in the journey as it would be in navigating a journey along a curvaceous mountain road. Conceptually, the navigation aspects of the present invention can be arbitrarily divided into the following major functions: route planning/path generation; path tracking; and obstacle handling. Autonomous vehicle navigation conceptually consists of two sub problems, path generation and path tracking, which are solved separately. Path generation uses intermediate goals from a high level planner to generate a detailed path for the vehicle 102 to follow. There is a distinct trade-off between simplicity of representation of such plans and the ease with which they can be executed. For example, a simple scheme is to decompose a path into straight lines and circular curves. However, such paths cannot be tracked precisely simply because of discontinuities in curvature at transition points of segments that require instantaneous accelerations. Following path generation, path tracking takes, as input, the detailed path generated and controls the vehicle 102 to follow the path as precisely as possible. It is not enough to simply follow a pre-made list of steering commands because failure to achieve the required steering motions exactly, results in steady state offset errors. The errors accumulate in the long run. Global position feedback 432 may be used to compensate for less than ideal actuators. Methods have been developed which deviate from traditional vehicle control schemes in which a time history of position (a trajectory) is implicit in the plan specified to the vehicle 102. These methods are appropriately labelled "path" tracking in that the steering motion is time decoupled; that is, steering motions are directly related to the geometric nature of the specified path, making speed of the vehicle 102 an independent parameter. Referring now to Figure 3, an autonomous vehicle 102 may be required to traverse a haul segment 320 to a dump site 322, and after dumping its load, traverse another haul segment to a service shop 324, under the direction of the host processing system 186. The host processing system 186 determines the vehicle 102's destinations, which is called "cycle planning." The determination of which routes to take to get to a desired destination must be accomplished by "route planning." "Route planning" is the determination of which path segments to take to get to a desired destination. In general, a route can be thought of as a high-level abstraction or representation of a set of points between two defined locations. Just as one can say to a human driver "take route 95 south from Lobster, Maine to Miami, Florida," and the driver will translate the instruction into a series of operations (which may include starting the vehicle 102, releasing the brake, engaging the transmission, accelerating to the posted speed limit, turning the steering wheel, avoiding obstacles 4002, and so on), the autonomous navigation system of the present invention performs similarly. A "route" is a sequence of contiguous "segments" between the start and end of a trip. An autonomous vehicle 102 may begin at any position in the sequence and traverse the route in either direction. A "segment" is the "path" between "nodes." A "node" is a "posture" on a path which requires a decision. Examples of nodes are load sites 3318, dump sites 322, and intersections 326. There are various types of segments. For instance, there are linear and circular segments. Linear segments. (lines) are defined by two nodes. circular segments (arcs) are defined by three nodes. "Postures" are used to model parts of a route, paths and nodes for instance. Postures may consist of position, heading, curvature, maximum velocity, and other information for a given point on the path. A "path" is a sequence of contiguous postures. A segment is, therefore, a sequence of contiguous postures between nodes. All segments have a speed associated with them, which specifies the maximum speed with which the vehicle 102 is to traverse that segment. The navigator 406 can command slower speeds, if necessary, to meet other requirements. Determining which postures are required to define a path segment by analytical, experimental or a combination of both, is called "path planning" To bring the discussion full circle, a sequence of contiguous routes, as mentioned above, is referred to as a "cycle," and a vehicle 102's work goals determine its "cycle." Therefore, to define a route one must first define the nodes and segments. Next, the nodes and segments must be ordered. Finally the routes must be defined by specifying where in the ordered set a route is to begin, and in which direction the ordered set is to be traversed (See Figure 12 which illustrates these concepts of the present invention). The aforementioned method of defining routes was developed for memory efficiency. It is also a convenient way to define many routes on a specific set of nodes and segments. In a real world example, picture a site where there are many intersecting roads 326. A route programmer would define nodes at the intersections, and segments to define the roads between the intersections. Routes would therefore be determined by the roads and intersections. There will however, be many ways to get from point A to point B (many routes) with a fixed set of intersections and roads. The path-tracking method uses route curvature to steer the vehicle. Methods of route definition using lines and arcs do not provide for continuous curvature. Clothoid curves are another way to define routes. Another method of defining routes developed by the inventors, fits B-splines to the driven data. B-splines provide continuous curvature and therefore enhance tracking performances. In addition, since B- splines are free form curves, a route may be defined by a single B-spline curve. By using free form curves, a more robust method (semi-automatic) for fitting routes to data collected by driving the vehicle over the routes is produced. Referring to Figures 4 and 12 in operation, the host processing system 186 from the base station 188 commands an identified vehicle 102 to take route N from its present location. The navigator 406 functions to generate a path by translating "route 1" into a series of segments, each of which may have a "posted" or associated maximum speed limit, which together form a generated path for the vehicle to attempt to follow. By specifying routes and commanding the autonomous vehicle 102 with high-level commands this way, enormous data requirements and inefficiencies are avoided in giving directions. The navigator 406 stores the routes as a linked-list of path segments, rather than the set or series of sets of individual points. These segments are also abstractions of the set of points between defined locations or nodes. A LINKER then takes given path segments and generates a linked-list of control points, allowing for flexibility and efficiency. Path segments are shared by different routes, as is shown in Figure 12. The path segments are stored in a memory called the TARGA 5302 as a set of arcs, lines, and postures. For instance, an analytical generator function generates paths using these arcs, lines and postures. In another example, B-splines are used as a mathematical representation of a route, as mentioned above. In another example, "clothoid" curves are used in generating path segments. In order to create routes for a site 300, data may first be collected from the VPS 1000 and stored while a human drives the vehicle 102 over the road system of the work site 3000. Nodes and segments can then be fitted to the stored driven data, and organized into routes as per the aforementioned procedure. An application on an APOLLO Computer (now HEWLETT-PACKARD of Palo Alto, California) work station (a graphics display system, not shown) was developed to graphically fit route data to the stored driven data and to further define routes (that is, speeds, sequences, starting point, traversal direction). Any graphics work stations equivalent to the APOLLO could be used. Once the routes for a site are defined, the route data is written to a permanent storage device. The storage device used may be a bubble memory cartridge 5302 with an associated reader/writer. The bubble memory device 5302 is durable and retains the data when power is disconnected. The APOLLO application is capable of writing data to a cartridge 5302 and reading data from a cartridge 5302. As implied above, routes may be predefined, or they may be generated dynamically. In mining applications, generally a site 300 is surveyed and roads are pre-planned, carefully laid out and built. The routes used by the navigation system may then either be obtained from a manually created computer data base (created specifically to be used by the navigation system), or alternately, a vehicle may be physically driven over the actual routes on site to learn the routes as described above. In the learning method, several trips over a given route may be made. Then the variations in the data (due for instance to driver weaving) are averaged, and a smoothed-out best fit developed. The tracking method of the present invention, requires certain information about the route it is tracking. The information is contained in a packet called a "posture" 3314. A single posture 3314 may contain position (that is, north and east coordinates), heading, and curvature data, for a specified location on the route. Therefore, a way of producing posture data from the route specification is required. Among the navigator tasks, (discussed below) is a task which reads the route information and produces postures at intervals (one meter for instance) along the route which are used by the tracking method. In an example system , each posture requires 36 bytes of memory which translates to about 36k of memory for each kilometer of route. To reduce the memory requirements, the navigator buffers posture data. The task which produces the postures reads the current position of the vehicle 102, finds the nearest point on the route to the current position, then generates a specified number of postures ahead of the vehicle 102. The number of postures generated is dependent on the maximum stopping distance of the vehicle 102. That is, there should always be enough postures in the buffer 3000 to guide the vehicle 102 to a stopping point. In the B-spline approach to route definition, the need for a posture buffer is eliminated, since the tracking method is able to directly produce posture information from the B-spline curve. Path tracking or following is a critical aspect of vehicle navigation in the example system. The technique of the present invention uses position based navigation (rather than vision based navigation used in conventional navigation systems) to ensure that the correct autonomous vehicle path 3312 is followed. The present invention is also innovative in that it provides for separate control of steering angle 3116 and vehicle speed 3118. Figure 17 graphically illustrates a path tracking system 3102, For an autonomous vehicle 102 according to the present invention to track specified paths, it is necessary to generate referenced inputs for the vehicle servo-controllers. Thus, path tracking can be considered as a problem of obtaining a referenced steering angle and a reference speed for the next time interval in order to get back to the referenced path ahead from the current deviated position. In general terms, path tracking is determining the autonomous vehicle commands (speed, steer angle) required to follow a given path. Given a pre-specified steering angle, driven wheel velocity values and error components, the command steering and driving inputs are computed. Obstacle handling involves at least three major functions: detection of obstacles 4002, avoidance of obstacles 4002, and returning to path 3312. The returning to path function is similar to path generation and tracking, as described above. In addition to path tracking (following), successful navigation of vehicle 102 requires that vehicle 102 be able to detect obstacles 4002 in its path, thus allowing the vehicle to stop or otherwise avoid such an obstacle before a collision occurs. In one embodiment of the present invention, a single line infra-red laser scanner 404 (See Figure 19) is used in a configuration where the scan is horizontal (not shown). The scan line 3810 does not contact the ground, so any discontinuities in the range data can be attributed to objects 4002 in the environment. Since a reference path 3312 is available and the vehicle position is known relative to the reference path, only the range data and a region bounding the reference path 3312 is processed for threatening objects 4002. Objects outside of this region, or boundary zone, are ignored. The width of the boundary zone (not shown) is equal to the vehicle width plus some selected safety buffer to allow for tracking and positioning errors. This method is limited in its usefulness and is referred to as "clearance checking." In the simplest case of the present invention, the laser 404 may be used in a single line scan mode with successive range measurements being made at regular angular intervals as the laser scans over the field of view. Again for simplicity, these scans can commence at regular time intervals. The term "clearance checking" has been used to describe this method. In this version of the present invention, the method has been limited to processing only two dimensional data. This type of obstacle method is limited to checking to see if the path 3312 is clear using a single line scan mode with successive range measurements being made at regular angular intervals as the scanner 404 scans over the field of view. It does not include any methods to establish the existence of any obstacle 4002 or to create a path around it if the path is not clear. This type of method is not deemed to be a particularly useful obstacle detection method, except in very rigidly controlled environments, such as on a factory floor. A second obstacle detection embodiment of the present invention uses a multiple-line scanner 3804 (See Figure 19), whose scan 3810 contacts the ground at some distance in front of the vehicle 102. Since the scan line contacts the ground, discontinuities in range data can no longer be attributed to threatening objects 4002. For example, profiles from natural objects such as hills and banked or crowned roads can cause discontinuities in range data. This technique of the present invention can discern discontinuities in range data between threatening objects 4002 and natural objects (not shown). In this embodiment of the present invention, a filtering scheme is used to decrease the amount of data processed and is independent of the scanner configuration used. The edges of the boundary zone are found by transferring the range data to an image plane representation 3900 (See Figure 20), where each range value is located by a row number 3908 and a column number 3910 (a matrix representation). Processing load is minimized by selecting a relatively small number of the scan lines available in a range image representation 3900. The scan lines are selected by vehicle speed, and are concentrated at, and beyond, the vehicle stopping distance. The selected scan lines from successive frames of data can overlap. In this method, if the vehicle 102 is moving fast, the selected scan lines 3906 are far in front of the vehicle (near the top of the range image representation 3900). In contrast, when the vehicle is traveling slowly, the selective scan lines 3906 are closer to the vehicle (near the bottom of the range image representation 3900). Each scan line is made up of many pixels of data. Each pixel has two parameters associated with it. First, the actual value of the pixel is the range value returned by the scanner 3804. Second, the location of the pixel on the scan line gives an indication of the angle, relative to the vehicle centerline, at which the range value was recorded. This corresponds to a cylindrical coordinate frame (R,THETA,Z) description. Given the cylindrical description and the known scanner location with respect to the vehicle 102, the range values can be converted to a Cartesian coordinate (X,Y,Z) system. The result is a road profile description which can be used by a novel filtering scheme to determine if threatening objects 4002 are present in the vehicle path 3812, while ignoring effects due to natural hills and valleys in a typical roadway. After the scanner data is converted to Cartesian coordinates, the data is processed to determine which part of the scan is actually on the road 3312 and which part of the scan line is outside of the vehicle path and therefore safely ignored. Given the vehicle position and the width of a boundary (which is equal to the vehicle width plus some safety margin), the coordinates of the boundary on either side of the vehicle path can be determined. The coordinates of the boundary can be compared to the coordinates of each pixel on the current scan line. The pixels which have coordinates outside of the boundary are ignored. The filtering scheme builds an expectation of the road profile from previously sensed road profiles. This expectation is based on three parameters which were found to adequately describe typical paved roads. These three parameters are: road crown: the curvature of the road cross section (perpendicular to the road centerline). road bank: the 'tilt' of the road profile (perpendicular to the centerline). road height: the height of the road centerline above a reference plane described by the location of the four tires of the vehicle 102. Expected values of the road crown and the road bank are determined by performing a standard, least-squares Kalman filtering technique on previously sensed scanner data. The Kalman filter basically keeps a type of running average of the two parameters based on the values determined from the previous data. The expected road height for a particular scan can be determined through one of two similar methods. One is to average the road height at each pixel within the current scan line to determine a characteristic height of the scan line in question. The second method is to filter the road height using the standard Kalman filter similar to that used when determining crown and bank expectations. These three parameters can be used to determine a second order equation which describes the expected road profile. This expected profile is compared to the actual road profile. Any deviations between the two which exceed a preset threshold value are assumed to be threatening objects. This scheme is viable given the assumption that any detected objects 4002 are small in comparison to the width of the road. Then, when these averaging or least squares methods are used, the effects due to objects are negligible in comparison to natural road data. This filtering scheme also includes a very simple edge detection method which convolves the selected range data with a simple seven point weighing function. An additional technique processes an entire range image representation 3900 from a multi-line scanner 3804 for objects. This method accomplishes three goals: 1. Do not detect obstacles 4002 when none exists, 2. Detect obstacles 4002 when obstacles do exist, and 3. Detect the correct obstacles 4002 when obstacles exist. Obstacle extraction is obstacle detection through the use of blob extraction. Blob extraction is well known in the art of computer graphics. Obstacles are found by clustering similar pixels into groups, called blobs. The goal of obstacle extraction is to store and process obstacles as units rather than as individual pixels. The obstacle extraction may be done by preforming the following steps in the image plane 3901: 1. Project the vehicle path into the image plane 3901, 2. Transform range data into height data, 3. Fit a curve to the height at the center of the road (this represents the expected road height at each row), 4. Threshold the actual road height against the height expectation, and 5. Extract the obstacles (indicated by differences in actual and expected road heights which exceed the threshold). Once an obstacle 4002 is deleted in the path of the vehicle 102 (see Figure 21), the vehicle must then avoid a collision with the object. Certain assumptions are made concerning the obstacle avoidance problem: 1. The obstacle environment is populated with obstacles 4002 that can be represented by convex-polygons or convex lines; 2. The navigation methods only have access to the local environment information in the form of a local map representing all of the visible faces of the obstacle from the position of the vehicle 102, which can be obtained from unprocessed laser range data or from data processed through blob-extraction; 3. The vehicle 102 is a conventionally steered type which has constraints on its speed and acceleration and constraints on its steering angle and the rate of change in the steering angle. To deal with the obstacle avoidance problem, it is divided into two sub-problems. First, to decide if any obstacles are in the way, and if so, which side should the vehicle pass on. Then select a sub-goal 4006, which will lead the vehicle 102 around the obstacle 4002, leading towards a higher level goal 4008, which is to get back on the desired path. Second, once a sub-goal 4006 is selected, make a steering decision which drives the vehicle 102 towards the sub-goal 4006, while steering clear of the obstacle 4002. A sub-goal selection method and a steering decision method. can. solve these two sub-problems. The example system includes a method, as shown diagrammatically in Figure 21, whereby a safe path around a detected object 4002 will be plotted and navigated so that the vehicle 102 will reacguire the reference path after avoiding the object 4002. Referring to Figures 19 and 23, the present invention also includes a laser scanner system 404. The scanner 404 is used to find obstructions 4002 (See Figure 21) that randomly crop up in the vehicle 102 path, as previously discussed. Sources of such obstructions 4002 may be varied and numerous depending on the particular work site. They may include fallen trees and branches, boulders, moving and parked vehicles, and people. The scanner 404 gives the autonomous vehicle 102 the ability to detect and deal with the external world as conditions require. The major components of the laser scanner system 404 are depicted in Figure 23. A laser range finder 3804 uses an infra-red beam 3810 to measure distances between the range finder unit 3804 and the nearest object 4002. A brief pulse is transmitted by the unit 3804 and the time for the beam 3810 to reflect off an object 4002 and return gives the distance. The beam 3810 from the range finder 404 is reflected by a rotating mirror 4222 giving the range finder 404 a 360° view of the world. Mirror rotation is accomplished through a motor 4206. The motor speed is controlled via a terminal 4210, which communicates with a motor amplifier/controller 4220 through a standard RS232C serial link 4224. Synchronization between laser firings and mirror angular position is done with an encoder. Distance data on a line 4226 from the laser range finder 404 is taken by an interface circuit 4228, which transmits the data differentially to a buffer circuit 4214. Individual pieces of data are collected by the buffer circuit 4214 until the mirror 4222 makes one full revolution. This set of data comprises one scan. When a scan is complete, the buffer circuit 4214 signals a processor 4212, whereupon data for the entire scan is transferred to the processor 4212 for processing. The interface circuit 4228 has three functions. First, it acts as a safety monitor. A situation could occur where the mirror 4222 would stop rotating, as in the case of the drive belt 4230 between the motor 4206 and mirror 4222 breaking. Under this condition, the laser 4204 would continue to fire, and since the mirror 4222 is stationary, it would fire at a single point (dangerous for anyone looking directly into the laser beam). The interface circuit 4228, however, senses when the angular velocity of the mirror 4222 falls below half a revolution per second, and disables the laser 4204 if such a condition occurs. The second function is to disable the laser 4204 from firing for part of the 360 degrees scan area. Typically, the laser scanner unit 404 will be mounted in front of a vehicle 102, and the field of interest is in the 180 degree area in front of the vehicle. The vehicle itself will block the back portion of the 360 degree scan area. In this case, the circuitry 4228 will prevent the laser 4204 from firing into the vehicle, extending the life of the laser diode while receiving range data for the area in front of the vehicle. The enabling and disabling of the laser range-finder 4204 is done through two sensors (not shown) mounted near the mirror housing 4222. For testing purposes, or for applications where a 360 degree scan is desirable, the disable feature can be turned off through a DIP switch. The third function of the circuit 4228 is to convert signals between single ended and differential form. TTL signals from the laser unit 4204 are differentially transmitted to the buffer circuit 4214, and differentially transmitted signals from the buffer circuit 4214 are converted to TTL levels. This prevents noise contamination along the cable 4226 connecting the two circuits. The function of the buffer circuit 4214 is to synchronize laser 404 firings with the angular position of the mirror 4222, to collect data for one complete scan, and to transmit the scan to computer 4214 for processing. The angular position of the mirror 4222 can be determined through signals sent by the encoder 4208. The buffer circuit 4214 uses two signals from the encoder 4208: the Z and A channels. The Z channel is the encoder index; it gets asserted once per revolution of the encoder 4208, and is used to signal the beginning of the scan area. The A channel is one line of the two line quadrature output of the encoder 4208, and pulses 1000 times per revolution of the encoder. This channel is used to trigger laser firings. One additional signal is needed to fully synchronize the scan field with the encoder signals. There is a gearing ratio of 2:1 between the encoder/motor 4206 and the mirror 4222. Two revolutions of the encoder 4208 rotates the mirror 4222 once. This translates to 2Z channel pluses and 2000 A channel pulses per revolution of the mirror 4222, and the inability to differentiate the beginning of the first half of the scan with the beginning of the second half. To fully synchronize the scan field, the DB (dead band) signal generated by the interface circuit 4222 is used. The DB signal, used to disable the laser 4204 from firing in the back half of the scan, allows the differentiation of the front and back halves of the scan. The Z and DB signal together signal the beginning of the scan area. The second task of the buffer circuit 4214, to collect data for one complete scan, is accomplished through the A channel of the encoder 4208. The 2000 pulses of the channel are divided by either 2, 4, 8, or 16, selected through DIP switches (not shown) on the circuit board 4228. This allows the number of data points per scan to be varied between 1000, 500, 250, and 125. The divided signal is used to trigger the laser range-finder 4204 at appropriate angular intervals, and to store the resulting range data in memory 4214. The sequence of events is as follows. W (write) is asserted one clock cycle upon a rising edge on the divided A signal. At this point, data from a previous T (laser trigger) is available and is stored in memory 4214. T is asserted the following clock cycle, triggering the laser and putting the resulting range data onto the memory input bus 4226. This data is written on the next W pulse, repeating the cycle. The final task of the buffer circuit 4214 is to transmit the scan data to a computer 4212 for processing. Completed scans are signaled by the Z and the DB signals (the beginning of a scan is also the end of a previous one). Upon a completed scan, an interrupt request line is asserted, and remains asserted until either the mirror 4222 has made half a revolution, or the processor 4212 acknowledges the interrupt. In the first case, the half revolution of the mirror 4222 is signaled by a subsequent Z pulse and indicates a timeout condition; the processor 4212 has failed to respond and the data is lost. In the normal case, the interrupt is acknowledged. Upon receipt of the acknowledgement, STR (data strobe) is asserted and held until IBF (input buffer full) is received. During this time, data is put on the data bus 4230 and may be ready by the computer 4212. Data is valid on the bus 4230 until IBF is asserted, at which time STR is deasserted and the data removed from the bus 4230. Once the processor 4212 detects the de-assertion of STR, it de-asserts IBF. This causes STR to be asserted for the next piece of data, repeating the cycle. Scan data is collected and stored in two memory banks 4214. This avoids shared memory and synchronization problems between scan storage and scan transmission. Data for a new scan is stored in one bank, while the previous scan is being transmitted from the other bank. The buffer circuit 4214 removes from the processor 4212 the responsibility of synchronizing laser findings with mirror position and collecting individual pieces of data. It allows more efficient use of CPU time, as data is received in scan sized chunks. The processor 4212 spends its time processing the data, not in collecting it. Referring now to Figure 24, the vehicle controls are comprised of four, low-level functional blocks. One is called a "vehicle manager" (4302). A second is called a "speed control" (4304). The third is called a "steering control" (4306). The fourth is called a "monitor/auxiliary control" (depicted as two separate blocks 4310 and 4308. They are all tied together with a high-speed serial data bus 4314. The bus 4314 is a data collision detection, packet passing system. Each of these functional blocks have separate microprocessors, for instance of the Motorola 68000 16 bit series. Each of these microprocessors talks to and listens to the others over the bus 4314. While each functional block has a more or less specific function, the vehicle manager 4302 functions as a communications hub. It sends to and receives messages from the navigator 406 via an RS-422, 9600 Baud serial link 4316. It is also listening to and sending to the remote control or "tele" panel 410 via an FM radio communications link 4318.

Claims: A scanning system (404) for detecting obstacles (4002) in a path (3312) of a vehicle (102), mounted on the vehicle (102), comprising: means (4222) for producing a beam of energy; means (4206) for scanning the beam of energy through a desired range of angles relative to the vehicle in cycles; and means (4222) for detecting reflected energy; characterized by : means (4212, 4214, 4226, 4228, 4230) for storing information associated with the reflected energy detected, during each of the scanning cycles; and means (4212) for calculating the distance the reflected energy detected has travelled by processing the associated information which is stored during each of the scanning cycles; and means (4222) for producing a beam of energy, during a portion of the scanning cycle, so that only a fraction of the total field of view is scanned, wherein the total amount of data transferred is minimized. A scanning system (404) according to claim 1, further comprising means (4228) for disabling the means (4222) for producing a beam of energy if the operation of the means (4206) for scanning the beam of energy through a desired range of angles relative to the vehicle in cycles is substantially impaired. A scanning system (404) according to claim 1, wherein the means for measuring distances (4212, 4214, 4222, 4228) between the vehicle (102) and objects (4002) in the path of the vehicle (102) further comprises: means (4228) for detecting substantial impairment of the means (4206) for scanning the beam of energy through a desired range of angles relative to the vehicle (102) in cycles. A scanning system (404) according to claim 1, further comprising means (4210) for varying the scanning cycles in number of scans per unit time. A scanning system (404) according to claim 1, further comprising means (4210) for varying distance measurements per scan, thereby varying scanner resolution. A scanning system (404) according to claim 1, wherein the means (4212, 4214, 4226, 4228, 4230) for storing information associated with the reflected energy detected further comprises: means (4228) for converting signals from TTL to differential type signals; means (4226) for transmitting the differential type signals to a temporary storage buffer (4214); and means (4230) for transmitting the differential type signals from the storage buffer (4214) to a host processing system (4212). A scanning method (4150) of detecting obstacles (4002) in a path (3312) of a vehicle (102), using a scanner (404) mounted on the vehicle (102), comprising the steps of: producing a beam of energy; scanning the beam of energy through a desired range of angles relative to the vehicle in cycles; measuring distances between the vehicle and objects in the path of the vehicle, comprising the steps of: detecting reflected energy; and characterized by the steps of: storing information associated with the reflected energy detected, during each of the scanning cycles; calculating the distance the reflected energy detected has travelled by processing the associated information which is stored during each of the scanning cycles; and producing a beam of energy, during a portion of the scanning cycle, so that only a fraction of the total field of view is scanned, wherein the total amount of data transferred is minimized.

Inventors: BRADBURY WALTER J, CHRISTENSEN DANA A, CLOW RICHARD G, DEVIER LONNIE J, FRIEDRICH DOUGLAS W, GUDAT ADAM J, KEMNER CARL A, KLEIMENHAGEN KARL W, KOEHRSEN CRAIG L, KYRTSOS CHRISTOS T, LAY NORMAN K, PETERSON JOEL L, RAO PRITHVI N, SCHMIDT LARRY E, SENNOTT JAMES W, SHAFFER GARY K, SHI WENFAN, SHIN DONG HUN, SINGH SANJIV J, STAFFORD DARRELL E, BRADBURY, WALTER J., CHRISTENSEN, DANA A., CLOW, RICHARD G., DEVIER, LONNIE J., FRIEDRICH, DOUGLAS W., GUDAT, ADAM J., KEMNER, CARL A., KLEIMENHAGEN, KARL W., KOEHRSEN, CRAIG L., KYRTSOS, CHRISTOS T., LAY, NORMAN K., PETERSON, JOEL L., RAO, PRITHVI N., SCHMIDT, LARRY E., SENNOTT, JAMES W., SHAFFER, GARY K., SHI, WENFAN, SHIN, DONG HUN, SINGH, SANJIV J., STAFFORD, DARRELL E., WEINBECK, LOUIS J., WEST, JAY H., WHITTAKER, WILLIAM L., WU, BAOXIN (NMI)
Applicants: CATERPILLAR INC, CATERPILLAR INC.
Application year: 1990
Main classification: G05D1
IPC codes: G01S17, G01S5, G01C21, B60K31, G05D1, G01S7, G08G1, G01S1