An Evaluation of INS and GPS for Autonomous Navigation

Copyright © IFAC Intelligent Autonomous Vehicles. Espoo. Finland. 1995

AN EV ALUA nON OF INS AND GPS FOR AUTONOMOUS NA VIGA nON Carl D. Crane rn, Arturo L. Rankin, David G. Annstrong 11, Jefl'rey S. Wit, David K. Novick

Center for Intelligent Machines and Robotics University of Florida. Gainesville. Florida 32611

Abstract: This paper focuses on the evaluation of two types of positioning systems commonly used for outdoor autonomous navigation: an Inertial Navigation System (INS) and a Global Positioning System (GPS) . The INS used for these tests was the H-726 Modular Azimuth Positioning System (MAPS) marketed by Honeywell Inc .• Clearwater. Aorida. Two Z-12 GPS receivers manufactured by Ashtec Inc.• Sunnyvale, California, were also evaluated while operating in kinematic differential mode and carrier phase differential mode . The results obtained from these tests illustrate the marked improvement in autonomous navigation when an INS is integrated with GPS, operating using either the kinematic differential or the carrier phase differential corrections. Keywords: Positioning systems. global positioning systems. inertial navigation. inertial sensors, autonomous vehicles, satellite applications. intelligent control, strapdown systems, robot navigation, computer controlled systems.

I. INTRODUCTION

1be concept of autonomous surveying was demonstrated in August 1993 at Wright Laboratory. The survey vehicle consisted of a Kawasaki Mule 500 all-terrain Navigation Test Vehicle (NTV) modified for computer control (Fig. I). Actuators and encoders were placed on the vehicle to perform the four functions of steering. throttle control. braking. and transmission control. Closed loop control is exercised on each of the four actuators via a VME based computer which is operating under the VxWorks operating system. Autonomous navigation was accomplished by calculating a survey path based on the coordinates of the surveying site boundaries

The U.S. Army Environmental Center has identified the need for environmental cleanup of various Department of Defense facilities containing buried unexploded ordnance (UXO) . This ordnance must be located and removed prior to the land being declared safe for other use. The U.S . Navy Explosive Ordnance Disposal (EOD) Technology Division has been tasked with the demonstration and evaluation of the various subsystems which are required to accomplish this cleanup. An engineering program has been established at Wright Liboratory, Tyndall Air Force Base. Aorida in support of

the Navy EOD Technology Division program. The objective of this program is to autonomously accomplish two tasks: first to locate the buried munitions and second. to remotely uncover and remove each buried ordnance. The Center for Intelligent Machines and Robotics (CIM:AR), at the University of Aorida, Gainesville. Aorida, has been contracted to develop the navigation system for the vehicles required to locate and remove the buried ordnance. Autonomous operation removes the operator from a potentially hazardous environment. In addition it offers a means of quality control which ensures that 100% of the site is efficiently searched.

Fig. 1. Navigation Test Vehicle (NTV). 193

and utilizing an Inertial Navigation System (INS) and a Global Positioning System (GPS) to detennine the vehicle's location relative to this path. The survey path was constructed by dividing the polygonal site into a set of parallel rows separated by 2 meters, allowing for a 1 meter overlap when using a 3 meter wide ordnance detection system. Analysis of the results from the August 1993 testing showed that over 96% of the five acre surface area of the test fields was surveyed (Annstrong, et ai., 1994; Rankin, et al., 1994). Subsequent improvements to the control algorithms and to the software integration of the INS and the GPS positioning data have increased the survey performance to 99%.

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The focus of this paper is the evaluation of the two positioning systems used on the NTV to accomplish autonomous navigation at 4.8-5.6 kmIh. The INS used on the NTV is the H-726 Modular Azimuth Positioning System (MAPS) marketed by Honeywell Inc., Clearwater, Aorida. The GPS used by the NTV consists of two Z-12 receivers marketed by Ashtec Inc ., Sunnyvale, California.

Fig.2. An example survey path using the Half-Field method.

distance collision-free paths from one positIOn and heading to another and the planning of a field survey path that guarantees 100% field coverage. An A * search algorithm is used to generate collision-free paths in traveling from the vehicle storage facility to a position near the field to be swept, from a field that has just been swept to another field, or back to the storage facility (Rankin and Crane, 1994). The field survey path planner divides the polygon shaped field into a set of k parallel rows separated by 2 meters. The current sweep pattern used is the "Half Field Method" . Row #1 is swept followed by the middle row m, where m=k12. Next, row #2 is swept followed by the row just after the middle row (m+ 1). This pattern is continued until the entire field has been swept (Armstrong, et ai., 1994).

The GPS receivers are operated in differential mode. One GPS receiver is placed at a known location to serve as a base station. The second GPS receiver and the INS are mounted on a NTV and used to accomplish autonomous navigation along a predefined path. The random error in the positioning signals obtained from orbiting satellites is determined by the base station GPS receiver and the corrections are transmitted to the remote GPS receiver. A recent prerelease of updated software runs on both GPS receivers using carrier phase differential corrections, improving the estimate of random errors in position, and allowing centimeter level positioning accuracy during real-time operation.

Two types of paths were used to evaluate the two positioning sensors' ability to achieve accurate path following. For the first path, the NTV was manually driven over a chalked square path at 4.8-5.6 kmIh. The second path was a field survey path similar to.that shown in Fig. 2, generated by the path planning software.

Of particular interest in this evaluation is detennining the increased benefit of using the INS integrated with the GPS operating in the new carrier phase differential mode as compared to using solely the INS or the INS integrated with the GPS operating in the kinematic differential mode . The focus of this paper is thus the evaluation of three ways in which the data from the two positioning systems can be used during autonomous navigation. To establish the expected error in vehicle positioning, static and dynamic tests were completed using solely the INS, the INS integrated with the GPS operating in the kinematic differential mode, and lastly, the INS integrated with the GPS operating in the carrier phase differential mode. The following sections will discuss the method used to detennine the path to be followed, the vehicle positioning sensors, the integration of the vehicle positioning sensors, experimental results, and finally some conclusions.

3. POSITIONING SENSORS To successfully navigate along a preplanned path, the vehicle must have some means of accurately and consistently detennining its position and orientation. lbis problem is being addressed by the application of an Inertial Navigation System (INS) and a differential Global Positioning System (GPS) . Each system is evaluated independently and integrated as they are applied to autonomous navigation. Inertial navigation systems have been used extensively in both commercial and military applications. They are completely self contained, non-jamrnable, and nonradiating. Further they deliver highly accurate results. An INS is capable of detennining position, angular orientation, velocities and angular rates in three

2. PATII PLANNING There are two types of off-line path planning normally used by the survey vehicle: the planning of shortest194

dimensions without requiring any beacons, landmarks or external sensing devices of any kind .

Post-processed GPS data is typically accurate to within 2 cm and is therefore considered to be the actual path traversed by the vehicle.

Inertial navigation systems typically use an internal set of inertial sensors, consisting of accelerometers and gyroscopes. The combined accelerometer and gyro systems are generally arranged in a cluster which is then either gimballed or strapped down. With a gimballed system, the sensor cluster is housed in a mechanical device which is free to rotate in three dimensions. The accelerometer system can then be maintained in a stable reference frame. TIlis is accomplished by using the gyros to measure the angular rotation about the axes, then applying the appropriate response to the gimballed system. With a strapped down system the inertial sensor assembly is fixed and cannot move. A stable reference frame is then detennined computationally using software.

The GPS receivers used for evaluation are the Z-12 models manufactured by Ashtec, Inc. The Z-12 receiver is designed to make full use of the Navstar Global Positioning System. It has twelve independent channels and can track all of the satellites in view automatically. Data from the Z-12 is made available to the on-board controlling computer via an RS-232 serial port at a rate of 1.0 Hz. There are two ways in which the two GPS receivers can be used simultaniously to determine the vehicle's position in real-time: the standard kinematic differential mode and the carrier phase differential mode. Ashtec has recently provided CIMAR with a prereleased version of software that enables the receivers to operate in the carrier phase differential correction mode. For both of these methods, one of the GPS receivers remains stationary at a known location. TIlis receiver is referred to as the base receiver. The other receiver is placed on the moving vehicle and referred to as the remote receiver. The purpose of the base receiver is to provide correction data for the remote receiver . This is accomplished via a 9600 baud radio modem. The position data is made available by the remote receiver to the on-board computer with a message which conforms to the National Marine Electronics Association (NMEA) standard.

The inertial navigation system selected for use on the N1V is the H-726 Modular Azimuth Position System or MAPS . The MAPS is a completely self contained, strapped down, inertial navigation system manufactured by Honeywell, Inc. Given only an initial position, the MAPS makes use of its three ring laser gyros and three accelerometers to accurately determine relative position, angular orientation, and velocities in real time. Position and orientation data from the MAPS are made available to the controlling computer at a rate of 12.0 Hz. The MAPS makes use of zero-velocity updates, as well as odometer data, to damp velocity errors that cause drift in the position accuracy over time. The position of the MAPS unit is calculated relative to where the system is initialized (Armstrong, 1993). The second positioning system that is utilized on the N1V is a global positioning system. Global positioning systems are a rapidly emerging technology with many systems now in use. GPS offers enormous potential for military and civilian applications such as air traffic control, land mapping, transportation, and space operations. A single GPS unit would be very accurate, however the military introduces two forms of random errors into the signal known as selective availability and anti-spoofing. Under these conditions, the GPS only delivers absolute position accurate to within 80.0 meters. To increase the system's accuracy, a method known as differential GPS can be applied as follows : A GPS receiver is placed at a known pre-surveyed location called the base station. A second remote GPS receiver is placed on the moving vehicle. Using a priori knowledge of its position, the base station receiver can determine the systematic or bias errors from the incoming signal. The corrections are then transmitted to the remote vehicle, where they are used to reduce positioning errors (Wintec, 1993).

The nature of the GPS makes it a global system able to provide three dimensional position data worldwide. This eliminates the need for setting up a custom system at each location as with a beacon or landmark strategy. Another advantage of the GPS is that its accuracies do not drift with time, allowing for its use as an update mechanism to other positioning systems such as the INS. Although GPS offers many strengths it is important to note it also has several weaknesses. First, it requires line-of-sight with at least four orbiting satellites to resolve position. In many cases the signal may be lost due to tree coverage or tall buildings. Secondly, the position accuracy depends on the changing geometry of the satellites in view . The satellite geometry is relayed in the GPS data message as horizontal dilution of precision (HDOP), where a lower HDOP number indicates better satellite geometry. Thirdly, a slow data rate of 1.0 Hz is unacceptable for accurate path tracking. In addition, the output data tends not to be smooth and contains no heading information. By itself, GPS does not offer a comprehensive solution to positioning for this application. However, when combined with an inertial navigation system such as described previously, the overall resulting system performs very well.

The most accurate method of determining vehicle position is available from the GPS receivers in non-realtime or post mission . This method is performed by postprocessing the GPS data gathered by each receiver during a test run with software provided by the manufacturer.

The INS and GPS units have been integrated so that the INS provides accurate orientation information and is also used to calculate an interpolated position of the N1V between GPS readings. The next section will outline the method by which the INS is integrated with the GPS . 195

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4. INTEGRAnON OF POSmONING SENSORS

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In general, the INS is used as the prime navigator because of its smooth, continuous output, while the GPS is used to periodically correct the INS drift when "good" GPS data is available. INS drift corrections are achieved as follows: Prior to navigation, a common clock is established and set to Greenwich Mean Time (GMl) using a data message from the GPS . As the system runs, INS data is time stamped using the GMT clock. A history of INS data is then stored in a circular buffer. When "good" GPS data becomes available, its time stamp is used to find a corresponding INS time stamp in the circular buffer. When a time match is found the two sets of position data can be compared. The difference between the INS position and the GPS position is considered to be a result of INS drift, and thus an INS drift correction is calculated. The calculated output of the system, which is used to drive the vehicle, is equal to the INS position added to the INS corrections. Some limitations are placed on the corrections. For example, a new correction can not exceed an old correction by more than 15 cm. This limitation ensures a smooth output rather than fluctuating with the GPS . It also prevents a large jump in the calculated output as a result of spurious GPS data.

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Integration of the INS with the GPS has greatly increased the overall system performance. The two systems complement each other well in that the INS provides continuous data at high rates while the GPS is not subject to drift Results from navigating with solely the INS and the INS integrated with the GPS is provided in the next section.

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Data from a static test of the MAPS is shown in Fig. 3. A dynamic test of the MAPS was also performed where the reported position of the MAPS was compared to the post-processed GPS position. This post-processed GPS position is treated as the true position of the vehicle. The results of the dynamic test are shown in Fig. 4. It is apparent from the figures that the INS drifts with time. The velocity error of the unit becomes significant after approximately five minutes. To compensate for this, the MAPS can be operated in a zero velocity update mode in which the INS must periodically remain stationary for about thirty seconds. While stationary, the velocity of the unit is recalibrated to zero. A static test of the INS was performed when a zero velocity update was accomplished repeatedly every minute. For this static case, the position of the MAPS unit was found to drift by no more than 6

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cm after thirty minutes of operation. Fig. 5 shows the difference between the calculated MAPS position and the post-processed GPS position for a dynamic test where zero velocity updates were performed every five minutes. Performing a zero-velocity update significantly improves the accuracy of the INS. However it is not desirable to periodically stop the survey vehicle.

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computer. The VME computer is housed in an air-cooled box on the NTV and operates under the VxWorks operating system. The vehicle's current heading was obtained from the INS for each test. Estimates of the vehicle's current position, as required for autonomous navigation, was determined in three different ways: first using only the INS, secondly, using the INS integrated with the GPS operating in kinematic differential mode, and lastly, using the INS integrated with the GPS operating in the carrier phase differential mode. Static and dynamic tests were accomplished for each of these categories. The GPS position data was stored in flies on both the remote and base receivers and post-processed using ·software developed by Ashtec to determine the actual path of the vehicle to within 2 cm.

A series of tests were also performed to evaluate the performance of the GPS . A static and dynamic test was performed as the GPS was operated in kinematic differential mode and then carrier phase differential mode. For the static tests, the reported position was compared to the actual pre-surveyed location of the unit. For the dynamic tests, the reported position was compared to the post-processed GPS position which is treated as the "true" position of the vehicle. The results of the GPS tests are shown in Figures 6 through 9. It is apparent from the static test that the GPS is much more accurate when operated in carrier phase differential mode . The improvement in performance is not as obvious in the dynamic test. It is believed, however, that the errors shown in Fig. 9 are in large part due to the fact that the time stamp of the position data is slightly different for the post-processed data and the carrier phase differential data. The time stamp was used to correlate the two sets of position data and Fig. 9 implies that there may have been a constant time differential.

Both positioning systems have inherent weaknesses that makes each difficult to use independent of another positioning system for autonomous navigation. The MAPS position data has been shown to drift over time due to round-off errors in integrating three-axis accelerations to obtain three-axis velocities. Without occasionally stopping to allow the MAPS to dampen velocity errors, it is possible to accurately navigate only over short periods of time. A GPS system does not provide vehicle heading information. Further, the availability and accuracy of data obtained from a GPS is dependant on a line-of-sight view of several satellites and the current satellite geometry.

6. CONCLUSIONS The navigation algorithms were developed in-house on a SUN SPARC 2 workstation and run on a VME based

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Fig. 10 shows the results of a field survey. The latitude and longitude of the corner points of the field were input to the path planner and a field sweep pattern was determined. The figure shows the calculated position based upon integrating the INS with the carrier-phase differential GPS . The post-processed GPS position is also displayed. It is apparent from the figure that the integrated INS and GPS provides accurate real-time position and orientation of the vehicle. The availability of this accurate data makes it possible to effectively control the remote vehicle autonomously .

Arrnstrong, D.G. (1993). Position and Obstacle Detection Systems for an Outdoor, Land-Based, Autonomous Vehicle. Masters Thesis, University of Florida, Gainesville, Florida. Rankin, AL., D.G. Arrnstrong, and C.D. Crane (1994). Navigation of an Autonomous Robot Vehicle. In: Robotics for Challenging Environments (L.A. Demsetz and P.R. Klarer (Ed.», 44-51. American Society of Civil Engineers, New York. Rankin, AL. and C.D. Crane (1994). Off-line Path Planning Using an A * Search Algorithm. Proceedings of the 1994 Florida Conference on Recent Advances in Robotics, Gainesville, Florida, 218-225.

REFERENCES Armstrong, D.G., C.D. Crane, AL. Rankin, A Nease, and E. Brown (1994). Autonomous Location of Hazardous Waste and Unexploded Ordnance. In: Intelligent Automation and Soft Computing: Trends in Research, Development, and Application (M . Jamshidi, J. Yuh, c.c. Nguyen, R. Lurnia (Ed.», 263-268. TSI Press Series, Albuquerque, New Mexico.

Wintec, Inc. (1993). GPS as a Navigation Aid to Rapid Runway Repair. Final Project Report, Ft. Walton Beach, Florida.

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