A Navigation/Positioning Service Based on Pseudolites. Installed on Stratospheric Airships. Toshiaki Tsujii, Chris Rizos, Jinling Wang, Liwen Dai, Craig Roberts.
A Navigation/Positioning Service Based on Pseudolites Installed on Stratospheric Airships Toshiaki Tsujii, Chris Rizos, Jinling Wang, Liwen Dai, Craig Roberts School of Geomatic Engineering, University of New South Wales, Australia Masatoshi Harigae National Aerospace Laboratory, Japan
BIOGRAPHY Toshiaki Tsujii is a senior researcher at the Flight Systems Research Center, National Aerospace Laboratory (NAL), Japan, where he has been investigating aspects of satellite navigation and positioning for ten years. In February 2000 he commenced a 2-year visit with the Satellite Navigation and Positioning Group, at the School of Geomatic Engineering, The University of New South Wales (UNSW), as a JST postdoctoral research fellow. He holds a Ph.D. in applied mathematics and physics from Kyoto University. Chris Rizos is a Professor at the School of Geomatic Engineering, UNSW, and leader of the Satellite Navigation and Positioning (SNAP) Group. Chris holds a B.Surv. and Ph.D., both obtained from The University of New South Wales, and has published over 100 papers, as well as having authored and co-authored several books relating to GPS and positioning technologies. He is secretary of Section 1 'Positioning', of the International Association of Geodesy. Jinling Wang holds a Ph.D. from the Curtin University of Technology, Perth. He is currently an Australian Research Council Postdoctoral Fellow in the School of Geomatic Engineering, UNSW, and is a member of the Editorial Advisory Board of the journal GPS Solutions, and Chairman of the Working Group "Pseudolite applications in Engineering Geodesy", the International Association of Geodesy's Special Commission 4. His research interests are GPS modelling, GPS and INS integration, and pseudolites. Liwen Dai received a B.Sc. and M.Sc. in Geodesy in 1995 and 1998 respectively, from the Wuhan Technical University of Surveying and Mapping, P.R. China, and then joined the SNAP group, UNSW, as a Visiting Fellow in November 1998. Since the start of 2000 he has been a full-time Ph.D. student where his current research interests are software and algorithm development for rapid static and kinematic positioning (and attitude determination) using integrated GPS, Glonass and pseudolite systems. Craig Roberts graduated with a B.Surv. from the South AUstralian Institute of Technology (now the University of South Australia) in 1988. He has worked as a private surveyor, a geodetic engineer with UNAVCO in Colorado, USA, and as an orbit specialist with the GeoForschungsZentrum, Germany. He is currently a Ph.D. student at UNSW with an interest in developing and testing permanent GPS systems for volcano deformation monitoring.
Masatoshi Harigae is head of the Flight Instrumentation Laboratory at NAL. For more than ten years he has been engaged in developing a GPS/INS hybrid navigation system for automatic take-off and landing of airborne vehicles. He holds a Ph.D. in aerospace engineering from Tokyo University, and his current interest is in evaluating the integrity performance of GPS/INS systems.
ABSTRACT Recently, some countries have begun conducting feasibility studies and R&D projects on High Altitude Platforms Systems (HAPS). Japan has been investigating the use of an airship system that will function as a stratospheric platform (altitude of about 20km) for applications such as environmental monitoring, communications and broadcasting. It is planned that such an airship network would cover all of Japan. Remote sensing from such an airship would be very effective because it floats above the same ground area, permitting continuous monitoring of the surface. However, the precise positioning of the airship is one of the most important technical challenges for such a project. If pseudolites were mounted on the airships, their GPS-like signals would be stable augmentations that would improve the accuracy, availability, and integrity of GPS-based positioning systems. The accuracy of the pseudolite positions would be a limiting factor for such a service since the PL 'ephemeris error' is more serious than GPS due to the lower height of the airship. In this paper, a conceptual design of the airship-based augmentation system is first introduced. Then some schemes for estimating the pseudolite position are described. A preliminary experiment involving 'inverted-GPS' shows excellent positioning stability in the static mode, while a kinematic test suggests the need to mitigate multipath effects on ground receivers.
INTRODUCTION The transmitters of GPS-like signals, which are called pseudolites (PL), or "pseudo-satellites", have been widely investigated as additional ranging sources to enhance the performance of GPS (see, e.g., Cobb, 1997). Ground-based GPS augmentation systems using pseudolites have been investigated for several applications such as vehicle navigation in downtown urban canyons (Altmayer, 1998), positioning in deep open-cut pits and mines (Stone & Powell, 1999), Figure 1: Navigation/positioning service using attitude determination (Wang et al., pseudolites on stratospheric platforms. 2000), and precision landing of aircraft (Barltrop et al., 1996; Pervan & Parkinson, 1997). The application of airborne pseudolites was suggested by Raquet et al. (1995). However, their purpose was the positioning of mobile pseudolites installed on military aircraft (Pachter &MaKay, 1998), not the augmentation of a navigation/positioning system. Recently, some countries have
begun conducting feasibility studies and R&D projects on High Altitude Platforms Systems (HAPS). Although some authors suggest the use of HAPS as navigation aids, their main function would be providing (pseudo-range) DGPS correction data (Dovis et al., 2000), not transmitting extra ranging signals. Japan has been investigating an airship system that will function as a stratospheric platform (SPF) at an altitude of about 20km for applications such as environmental monitoring, communications and broadcasting (Yokomaku, 2000). Because of the airship ’s station-keeping characteristics, the SPF can be considered as a signal source for a navigation/positioning service (Figure 1). If pseudolites were mounted on the airships, their GPS-like signals would be stable augmentations that would improve the accuracy, availability, and integrity of GPS-based positioning systems across all of Japan. In this paper, the concept of an innovative GPS navigation/positioning system augmented by SPF-based pseudolites is introduced (referred to as GPS/PL system hereafter). Some schemes for estimating the pseudolite position are described, because the precision of the PL position ('PL ephemeris') would be a limiting factor for such a service. Finally, the results of PL positioning experiments based on the 'inverted-GPS' concept are presented and technological issues are discussed.
GPS/PL POSITIONING SYSTEM Advantages of Pseudolites on SPF Although many applications using pseudolites have been proposed, an operational system has not been established due to problems specific to pseudolites. These include the socalled 'near-far' problem, multipath, and time synchronization. However, these are not serious problems for pseudolites on a SPF. Since the height of the SPF is about 20km, and the distance between the PL and the user is from 20km to 100km, the dynamic range is much less than for ground-based pseudolite applications. The multipath of PL signals would be less because the elevation angle is rather high compared with ground PLs. Furthermore, the PL clock on the SPF can be referenced to the GPS receiver installed on the SPF, for the navigation of the SPF itself. An Example of a SPF Constellation In the early stage of the SPF project, approximate 200 platforms were planned to be deployed in order to cover all of Japan. The average distance between SPFs would therefore be about 50km. This plan has since been revised and the number of platforms is to be reduced. It would be reasonable to distribute the SPFs densely over the metropolitan areas – where the users are concentrated, and earthquake/volcano zones – where the continuous and high resolution monitoring is necessary. Therefore, a rather dense distribution of nine platforms above the Tokyo metropolitan area is assumed in this paper, and the technical issues are discussed based on this scenario. The height of the platforms is about 20km and the separations are about 0.5 degree (about 55km) in latitude and longitude as shown in Figure 2. The GDOP variation at Chofu city in Tokyo (N35h40m, E139h33m, indicated as × in Figure 2) is shown in Figure 3. The elevation mask angles are set at 10 degrees in the top figure, and 20 degrees in the bottom figure (note that the scales are different). The GDOP varies greatly for the case of GPS (blue thin line), while it is very stable if the GPS is augmented by stratosphere-based pseudolites (blue bold line). This tendency is distinguished in the case of mask angle 20 degrees. Since the other satellite positioning system, GLONASS, is considered as an augmentation system for GPS, the GDOP of GPS/GLONASS integrated systems was
also computed, and is shown in Figure 3 (red line). However, since only seven GLONASS satellites are operating (as of May 2001), the GPS/PL system is indeed superior to a GPS/GLONASS system. Although the availability, continuity, and integrity would be improved as well by increasing the number of signals, the descriptions are beyond the scope of this paper.
Mask angle = 10 (deg) 5
GDOP
4 3 2 1 0 0
5
0
5
10 15 Mask angle = 20 (deg)
20
GDOP
10
5
0 10
15
20
GPS time (hour)
Figure 2: An example of a SPF constellation ( : SPF, × : user).
Figure 3: GDOP variation (Blue bold: GPS/PL, Red: GPS/GLONASS, Blue: GPS).
Precise Positioning By GPS/PL System Although the GPS/PL system can be used for standalone and code differential positioning, the feasibility of carrier phase-based differential positioning, in which the technical requirement is most demanding, is investigated here. When the signal from a PL on a SPF is measured by the user on the ground, or in the troposphere, some measurement errors, which may be negligible for GPS signals, can be significant for PL signals, and vice versa. Table 1 summarizes the effects of errors on both the GPS and PL double-differenced measurements, when the baseline length is assumed to be a of the order of a few tens of kilometres. While the GPS satellite clock error cancels by taking single-differences between satellites, the drift error of PL clocks should be calibrated by some external method. This is required because of the lower stability of PL clocks, which is approximately 10-6sec/sec, compared to that of the GPS atomic clocks, which is typically at the level of 10-12sec/sec. Since the timing difference between the reference and the user receiver is normally within one millisecond, the ranging error due to the PL clock drift would be 0.3 m (=10−6 × 10 − 3 × 3× 108 ) . The calibration of clock drift error would be easy if both reference and user receivers tracked the GPS satellites, because the time difference of signal reception can be estimated. For some PL applications, such as indoor positioning, in which satellite signals are normally unavailable, a time synchronization scheme would be necessary to accomplish precise positioning (Zimmerman & Cannon, 1995; Kee et al., 2000).
The signal from a PL on a SPF does not suffer from the ionospheric delay since the SPF is located far below than the ionospheric layer (altitude 70 – 1000km). This characteristic would be a significant advantage of the PL signal for the cases where the solar activity is extremely high or the baseline length is rather long. The tropospheric delay of the PL signal is similar to that of the GPS signal because the tropospheric layer is under the stratosphere. However, the tropospheric delay of PL signal does not cancel by taking the single-difference between receivers due to the lower altitude of the SPF, which makes the propagation paths differ considerably. Although the effect of wet delay would be more serious on the PL signal than on the GPS signal, the tropospheric delay estimate from a continuous ground-based GPS array would mitigate such errors. In fact, the delay information from PL signals could be useful for the estimation of the precipitable water.
Table 1: Error sources in double-differenced measurement of GPS and PL on a SPF. Receiver clock error Satellite clock error
Ionospheric delay Tropospheric delay Orbit error Multipath Noise
GPS Cancels by SD between satellites Cancels by SD between receivers
PL on SPF Cancels by SD between satellites Mostly cancels by SD between receivers, but drift error should be calibrated Partly cancels by SD None between receivers Partly cancels by SD Not cancelled between receivers Mostly cancels by SD Not cancelled between receivers Exists Similar to GPS Exists Similar to GPS
The accuracy of the pseudolite positions would be a limiting factor for such a GPS/PL system since the PL 'ephemeris error' is more serious than for GPS due to the lower height of the SPF (Dai, et al., 2000, Wang, et al., 2001). This PL ephemeris error does not matter in the case of ground-based PLs because the position of ground-based PL antennas can be determined precisely by conventional positioning techniques (including GPS). However, since the SPF is always moving, real-time SPF positioning and frequent broadcast of its coordinates to the user would be necessary. Decimetre-level positioning accuracy of the SPF could be easily achieved using an onboard GPS receiver and the attitude information of the platform. This level of ephemeris error would not be serious for GPS/PL stand-alone positioning, but it cannot be neglected for code/carrier differential positioning. Therefore, the precise positioning of the PL antenna is one of the most challenging issues for such a service, and is discussed in the next section. For the real-time user of the kinematic GPS/PL service, an update rate of ephemeris more frequent than 1Hz would be necessary, depending on the motion of the platform due to the wind in stratosphere. On the other hand, for the off-line user the publication of a ‘precise PL ephemeris’, similar the precise IGS GPS ephemeris would be practical.
Precise Positioning of PL Installed on a SPF Three kinds of methodologies and system configurations for the precise positioning of a PL antenna on a SPF are discussed below. KGPS and Attitude Information (Figure 4) The position of a GPS antenna, which is nominally installed on the fuselage of a SPF for the navigation of the platform itself, can be estimated with an accuracy of a few centimetres using the kinematic GPS method (Tsujii et al., 2000). However, some complicated calibrations would be necessary to estimate the position of a PL antenna underneath the platform. In addition to the attitude of the platform, information on the variation of the platform’s shape due to structural flexure and changes in atmospheric pressure must be considered. Inverted-GPS (Figure 5) The position of a PL antenna underneath a SPF can be estimated directly by the inverted-GPS method (Raquet et al., 1995), where the ranging information is obtained by a receiver on the ground. The complicated calibration from the 'top' GPS antenna and 'bottom' PL antenna is then not necessary. Also, the position of earth observation sensors, which may require precise position information, can be determined using the attitude of the SPF payload. However, in order to perform the double-difference processing for the inverted-GPS approach an additional transmitter, such as a GPS satellite or ground-based PL, is required. The use of a GPS satellite would affect the positioning accuracy because ionospheric delay exists only in the GPS signal (not in the PL signal). If a ground-based PL is used as a reference transmitter, it should be located on a high mountain or a tower to ensure line-of-sight from all ground receivers.
Figure 4: KGPS & attitude information method.
Figure 5: Inverted GPS method.
Figure 6: GPS transceiver method.
GPS Transceiver (Figure 6) Recently a GPS transceiver, which combines the function of a GPS receiver and PL, has been proposed (Stone et al., 1999). Such GPS transceivers could communicate and synchronize each other, and then estimate relative positions using the ranging information among them. If only one transceiver observes the GPS satellites, all transceivers can be referred to the precise GPS time. In this method, the onboard PL and all ground receivers used in the inverted-GPS configuration are replaced by the GPS transceivers. Since the reference transmitter is not required, this disadvantage of the inverted-GPS approach can be overcome.
EXPERIMENT USING INVERTED GPS Although the transceiver-based method seems to be the best among the three methods, the inverted-GPS experiment was conducted as a preliminary test to clarify the potential problems of the proposed GPS/PL system because the off-the-shelf transceiver described is currently not available. Measurement Equation The measurement equation for the carrier double-differenced observable for the 1 st /i-th receivers and the airborne/base transmitters of the GPS/PL system can be written as follows: ∇∆ Φ 1ab = X1 − X a (t1a ) − Xi − X a ( tai ) − X1 − Xb (tb1 ) + Xi − Xb (tbi ) i
+∇∆N1abi + ∇∆dion + ∇∆dtrop + ∇∆dmulti + ∇∆ε
, (i=1,2, …,n) (1)
where the Xi , X a ,Xb denote the position vectors of i-th receiver on the ground, which is constant, the airborne transmitter, and the base transmitter respectively. The t ia ,tbi represent the signal transmitting time to the i-th receiver from the airborne/base transmitters, referred to each of the transmitter clocks. With GPS positioning, the signal reception time for all the observed satellites is the same. However, in the case of inverted-GPS, the signal transmitting time to the receivers, which is analogous with the reception time in conventional GPS, may differ depending on the clock biases of the receivers and the distances between the transmitter and the receivers. These time differences may degrade the PL positioning accuracy because the motion of a PL on a SPF is difficult to predict (in contrast to GPS satellites). The GPS receivers normally synchronize to GPS time to within 1msec. Assuming that the motion of a SPF is less than 1m/sec, the position change of a PL is less than 1mm ( = 1 m /sec × 1 m sec) . Since the distance difference between the PL and the receivers is less than 150km in the example of the proposed Japanese configuration of SPFs, the PL position change is less than 0.5mm ( = 1 m /sec × 150km /speed of light) . This effect would have to be investigated further if the station-keeping performance of SPFs were found to be worse. The ionospheric delay term can be neglected if the reference PL is used instead of a GPS satellite, or if GPS transceivers were used. For the test configuration, described below, both the ionospheric and tropospheric delay terms were neglected because of the close proximity of the receivers. Also, the effect of nonsimultaneous transmitting time can be neglected due to the close proximity of the PL and receivers, and the slow motion of the rover PL. The measurement equation can be simplified as:
1 ∇∆ Φ abi = X1 − X a (t) − Xi − Xa (t) − X1 − Xb (t 1b ) + Xi − Xb (tbi )
+∇∆ N1iab + ∇∆d multi + ∇∆ε
, (i=1,2,…,n)
(2)
where t is the signal transmitting time from the PL, referred to the PL clock. The position of the receivers, X i , and the base transmitter, Xb , are precisely determined before the test if a PL is used as the base transmitter. If a GPS satellite is used as the base, the ephemeris error can be neglected because the receivers and the rover PL are close to each other relative to their distance from the satellite (Raquet et al., 1995). All data processing were performed using a modified version of the KINGS software developed at NAL (Tsujii et al., 1998). Configuration of the Inverted-GPS A miniature configuration of the GPS/PL system was constructed as shown in Figures 7 and 8, and the inverted-GPS experiment in static/kinematic modes was conducted on 20 April 2001. Six NovAtel GPS receivers were placed on the roof of the Electrical Engineering building, on the campus of The University of New South Wales. An antenna connected to a IntegriNautics IN200C PL was set on a wooden rail fixed between the two pillars. The pseudolite is slid on the wooden rail for the kinematic test. In addition, an Ashtech GG24 antenna was mounted on the PL antenna in order to determine the correct position of the PL antenna, and was able to slide with the PL antenna. The satellite with the highest elevation was used as the reference transmitter since no other PL was available in these experiments. As reported by other investigators (Raquet et al., 1995; O’Keefe et. al., 1999; Dai et al., 2000), three-dimensional positioning was indeed difficult due to the poor geometry of the receiver constellation. However, it would be easy to establish a good geometry for the GPS/PL system because a receiver can be placed directly underneath the PL on the SPF. In this experimental GPS/PL configuration the constellation of receivers was carefully designed in order to establish the best geometry (Dai et al., 2001b). The GDOP, HDOP, VDOP values for relative positioning were 1.23, 0.70, 1.01 in the static test, and similarly good in the kinematic test.
Figure 7: Miniature configuration of the GPS/PL system for UNSW test.
Figure 8: Coordinates of PL and GPS receivers.
Static Test The PL signals may contain some multipath errors due to the many metal objects on the roof of the building. However, the multipath errors should be constant in the static mode, so they can be calibrated in advance (Dai et al., 2001a). Since the true position of the PL antenna was known, the residuals of the double-differenced carrier phase were easily estimated (Figure 9). The middle receiver (No.6) was chosen as the reference receiver, and the highest satellite (PRN13) was chosen as the reference transmitter. The residuals do not seem to be constant. This is probably due to the time-variant multipath error of the reference transmitter (GPS satellite). If an additional PL was used as a reference transmitter, the residual would be more constant, and the positioning solution would be improved at the same time. The constant biases were computed by averaging the residuals and were subtracted from the carrier phase double-differences before the positioning computations were carried out. The initial value of the PL position is important because the mirror image of the PL with respect to the receiver constellation can also be a solution. Figure 10 shows the difference between the inverted-GPS solution and the true coordinates. The average should be zero because the biases were computed using the same data. However, it can be noted that the standard deviations (2.1, 4.5, 4.7 mm in North, East, Height) were better than the variation of the GG24 antenna computed by the normal KGPS method (5.5, 2.8, 7.2 mm).
Figure 9: DD residuals of L1 carrier phase in the static test.
Figure 10: Positioning error of the L1 carrier inverted-GPS solution in the static test.
Kinematic Test In the kinematic mode, the PL antenna was slid on the wooden rail together with the GG24 antenna by pulling a rope tied to the antenna mount. The trajectory of the GG24 antenna computed by the standard KGPS method is shown in Figure 11. The antennas were moved very slowly so as not to lose signal tracking. However, some receivers did lose signal lock, as shown in Figure 12, even though the movement was very slow. This will aspect of PLs will be investigated further.
Figure 11: Trajectory of GG24 antenna (above the PL antenna) in the kinematic test.
Figure 12: No. of receivers tracking PL signals (top), and the tracking status (1: tracked, 0: not tracked) in the kinematic test.
If the residual errors shown in Figure 9 were due to multipath, it can be expected that ambiguity resolution in the kinematic inverted-GPS approach would be difficult. In order to analyse further the characteristics of the residuals, the true trajectory of the GG24 antenna (shown in Figure 11) was used to compute the true position of the PL antenna, and the residuals of the double-differenced carrier phase were computed (an example is shown in Figure 13). The residuals change very quickly due to the motion of the PL antenna compared to the static period (time 449679-449803, 450123-450530). This is a reasonable result considering the characteristics of the multipath, because the angle between the PL antenna and the ground antenna changes drastically in such an experimental configuration, even if the motion of the PL is very slight.
Figure 13: DD residuals of L1 carrier phase between receiver No.4 and No.6 in the kinematic test.
Figure 14: Positioning error of the L1 carrier inverted-GPS solution in the kinematic test.
With such large residuals, ambiguity resolution would be very difficult. Even if the ambiguities were resolved by some other method, the position solution would be degraded, as shown in Figure 14 (where the ambiguities were computed using the true position of PL antenna). However, in the actual GPS/PL system, the residual seems to be rather constant because the angles between the ground receivers and the SPF do not change drastically as in the case of this experimental configuration. Also, the multipath error can be mitigated by using a chokering or other multipath-surpressing antenna. The possibility of other residual sources, such as the phase polarization and antenna phase centre variation, will need to be investigated further (Zimmerman, 1996).
CONCLUSIONS A new concept for a navigation/positioning service based on pseudolites installed on stratospheric platforms, currently under investigation in Japan, has been introduced. The precise positioning of the transmitting antenna on the platform is one of the most significant technological challenges for such a service. Although precise positioning using 'GPS transceivers' seems to be the best method, the inverted-GPS approach was investigated in a preliminary feasibility study. The static test showed excellent positioning stability (standard deviations less than 5mm), while the kinematic test suggested the need to investigate further the mitigation of residual errors such as multipath, phase polarization, and antenna phase centre variation.
ACKNOWLEDGMENTS The authors gratefully acknowledge the assistance of Dr. Joel Barnes, Mr. Michael Moore and other members of the Satellite Navigation and Positioning group (SNAP) of the School of Geomatic Engineering, UNSW. The first author would like to acknowledge support from the Japan Science and Technology Corporation (JST) postdoctoral fellowship scheme.
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