Recommendations on Differential GNSS

Transcription

1 Recommendations on Differential GNSS Mr. Joseph W. Spalding USCG Research & Development Center Dr. Jacques Beser S Navigation Inc. Dr. Frank van Diggelen Ashtech, Inc. BIOGRAPHY Mr. Joseph Spalding is radionavigation project manager at the US Coast Guard R&D Center. Past efforts include development of CG DGPS technology. Current projects include testing of GNSS receivers and new CG vessels. Mr. Spalding served on the NRC Committee on the Future of GPS. He chairs the RTCM SC- working group on version and is a member of IEC TC working groups on Integrated Navigation Systems and GPS receivers. He holds a BEEE from SUNY Maritime College, MSCS from the University of New Haven and is a licensed Merchant Marine officer. Dr. Jacques Beser has worked on GPS-related projects for over twenty years. He is currently VP and Director of S Navigation, responsible for Satellite Navigation products. Dr. Beser pioneered the differential use of GPS and has extensive experience in the design and development of GPS and GPS/GLONASS receivers. He holds degrees from CALTECH and Stanford University. He is a member of ION and IEEE. Dr. Frank van Diggelen is Product Manager for OEM & Navigation products at Ashtech Inc. He has over years navigation experience, including years in the GPS industry. Frank was a Navigation Officer in the South African Navy, he holds a Bachelors Degree in Electrical Engineering from the Witwatersrand University in Johannesburg, South Africa and a Ph.D. in Automatic Control Theory from Cambridge University, England. ABSTRACT The use of differential navigation techniques combined with GLONASS and GPS, collectively referred to as Global Navigation Satellite Service (GNSS), presents [Ed. Note: Views expressed in this paper are those of the authors and should not be construed as official or reflecting the views of Commandant or of the U. S. Coast Guard.] maritime authorities with an opportunity to provide highly precise navigation service for harbor entrance and approach areas with a reduced dependence on any one system. The use of satellites ( GPS and GLONASS) also provides improved availability in situations of partial satellite signal blockage due to terrain, structures, and large vessels. Receiver autonomous integrity monitoring techniques also become highly reliable with this increased number of signals for comparison. Differential techniques for GPS are well understood and have been standardized across the industry. The Radio Technical Commission for Maritime Services (RTCM) Special Committee has developed a proposed format for messages containing differential GLONASS information. In addition to the data formats to transmit GLONASS corrections a differential GNSS service must resolve the technical issues in combining the separate systems. When GLONASS and GPS measurements are to be combined there are two technical issues that must be resolved. First, the difference in the WGS and PE 9 datum must be adjusted, and second, how to handle the difference between GPS time and GLONASS time. This paper presents the results of this USCG R&D study to determine the best method of integration and recommendations on implementing DGNSS service. INTRODUCTION Increased reliance on differential navigation services along with a desire to lower dependence on any one system will push differential GPS service providers around the world into considering integrating differential GLONASS into their operations. Such a differential GNSS service will provide users with significant benefits in terms of availability, integrity and reliability. While the Radio Technical Commission for Maritime Services Special Committee (RTCM SC-) has developed several new message types to provide GLONASS corrections and deal with the integration issues of datum and time, there has not been any committee recommendations in this area. This paper is making an attempt to resolve these issues as manufacturers currently have different implementations and methods of combining the two

2 systems that could lead to a loss in differential performance when interoperability is required. This paper will provide the objectives of the integration, outline the test scenarios and present the results. The intended result of this work are recommendations on the combination of DGPS and DGLONASS into a DGNSS that promotes interoperability between manufacturers and provides the users with the most options. INTEGRATION OBJECTIVES Our first objective in integrating DGPS and DGLONASS is to develop methods that provide the accuracy at least as good as either single system in good conditions. Good conditions would include unobstructed satellite visibility, no RF interference in satellite signals and high quality differential corrections. In these conditions many high quality GPS receivers achieve - meters 9% using pseudorange corrections as per RTCM SC-. The second objective is to keep the integration simple to implement, that is, not require new standards or algorithms be developed, universally accepted, and implemented. This objective helps to promote interoperability between the manufacturers. TEST SCENARIOS These test scenarios were planned to try to test the merits of the different ways to combine GPS and GLONASS. The S GNSS- and Ashtech GG were used to conduct these scenarios. We were somewhat limited by the current implementations in the equipment. Not all of the subject equipment had the flexibility in its configuration to test every conceivable combination. Scenario Scenario was designed to evaluate the use of datum transformations as part of the differential process. DGPS corrections were calculated in their normal fashion using WGS-, DGLONASS was calculated by transforming the PE-9 satellite position into WGS- before calculating the corrections. There was a single reference position in WGS-. The user set was also set to transform the GLONASS satellite positions before calculating its pseudoranges. A S GNSS- was used as the reference station and an Ashtech GG was used as the user equipment in this scenario. Scenario Scenario was designed to evaluate the use of the DGNSS corrections to perform the datum transformation. DGPS corrections were calculated in their normal fashion using WGS-, DGLONASS was calculated by using the PE-9 satellite position to calculate the corrections. These GLONASS corrections, which were referenced to the WGS- reference position, then include the change in datum from PE-9 to WGS- for the user. The user set was set to not transform the GLONASS satellite positions before calculating its pseudoranges. This is similar to situations where a DGPS reference station may use an alternate datum for its reference position, in order to accomplish a coordinate transformation. A S GNSS- was used as the reference station and an Ashtech GG was used as the user equipment in this scenario Scenario Scenario was designed to evaluate different options in handling the difference in GPS and GLONASS time as calculated by the reference station and the userõs receiver. In this scenario, the datum was handled as described in scenario. The intention here was to implement the RTCM type correction that gives the time offset between what the reference station calculates GLONASS and GPS time to be. We had some limitations in the equipment however, and the scenario was divided into two parts. First, was a test of the concept of fixing the time offset in both the reference station and the user equipment. Second, was testing the use of type by cold starting a user set and restricting the number of satellites it could track to see how well the type allowed the measurements to be combined. RESULTS All of the results presented here were obtained using combinations of the S Navigation GNSS- and Ashtech GG receivers. Data was collected in real time using the NMEA- message output from each receiver. RTCM SC- output from the reference stations was directly connected to the user sets. The update rate for the RTCM corrections was one Type (GPS) and one Type (GLONASS) every seconds. Results for scenario and are presented below under the datum issue. Scenario is presented in the timing issue section. Datum issue In order to test the two scenarios presented for compensating for the differences in the GPS and GLONASS datum, real time navigation data was collected concurrently using two identical sets of equipment. Two S GNSS-s using a single antenna were used as the reference stations. Two Ashtech GG receivers using a single antenna were used as remotes or user equipment. In one setup, the manufacturerõs default coordinate transformations were used to shift the positions of the GLONASS satellites into WGS- (scenario ). Scenario was implemented in the other reference station, user equipment pair by setting the transformation parameters to in the GG and by disabling the transformation in the S reference station.

3 Scenario (Figure ) demonstrates the current weakness of trying to do datum transformations across different vendors equipment. In this scenario a S GNSS- was used as the reference station and the Ashtech GG was the user set. The manufacturerõs default datum transformations were used to adjust the PE-9 satellite positions to WGS-. S and Ashtech use different values in their default datum transformations. Ashtech offsets the XYZ origin by +. meters in Y and does a rotation of -.99Ó about the Z axis. S offsets the XYZ origin by +. meters in Z and does a rotation of -.Ó about the Z axis. This difference accounts for the poor differential performance at the times that the number of GLONASS satellites is at its highest. Total SVs GLONASS SVs Number of Satellites //9 : //9 : //9 : //9 : //9 9: //9 : //9 : //9 : //9 : //9 9: Scenario Figure Satellite Datum Adjustment

4 Scenario (Figure ) demonstrates the ability of the DGNSS corrections to account for the difference in the GLONASS satellites positions. In this scenario a S GNSS- was used as the reference station and the Ashtech GG was the user set. The manufacturers datum transformations were disabled or set to. This hour period showed excellent positioning performance and the interoperability of the GNSS equipment. Timing Issue Scenario was designed to test the concept of controlling the time offset in the DGNSS reference station and reporting that time offset to the user so that the user could properly combine the DGPS and DGLONASS corrections Total SVs GLONASS SVs Number of Satellites //9 : //9 : //9 : //9 : //9 9: //9 : //9 : //9 : //9 : //9 9: Scenario Figure No Datum Adjustment

5 without the need for a time calculation in the user set. This technique could assist users that may be obstructed by terrain or buildings. In the first part of this test a S GNSS- was used as the reference station and the Ashtech GG was the user set. The reference station was transmitting type and was keeping the time offset between GPS and GLONASS fixed at. seconds for the duration of the test. The GG was allowed to use all of the available satellites in order to calculate its own time offset, then was commanded to fix its offset using its most recently calculated value. The number of satellites in use by the GG was then controlled to allow D navigation with only satellites, GPS and GLONASS. The test successfully demonstrated the principle that as long as the time offset between the GPS and GLONASS is controlled then accurate DGNSS navigation is possible. As Figure shows the GG was able to maintain accuracy better than meters for over two hours. Figure shows two areas of poor performance. The first at for a duration of minutes is due to poor satellite selection yielding a of about. The second at for minutes occurred due to the configuration of the altitude hold function. One of the selected satellites became unavailable, so the receiver reverted to D mode using the last computed altitude, which was in error by about meters. This error condition persisted until another satellite was selected allowing D navigation again. 9 Number of Satelites Total SVs GLONASS SVs 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : 9 RadialError 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : Figure Time Offset Fixed

6 Num.SVs GLONASS.Svs Number of Satellites 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : RadialError 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : 9//9 : Figure Type on Start-up The second part of scenario involved powering up a receiver and selecting just enough satellites (i.e. ) to allow D navigation when using type messages. In this part of the test, a S GNSS- was used as the reference station and as the remote or user set. By disabling the automatic tracking function on startup we were able to command the receiver to track specific satellites. The receiver was never allowed enough observables to calculate the GPS-GLONASS time offset. Instead, the receiver had to use the type time offset message that was being broadcast by the reference station. The receiver started navigating at with a poor operator choice of satellites, the was between and. One satellite was changed dropping the to. From there navigation performance was acceptable, in the - meter range. At 9 the receiver was shut off and powered up again. Satellites were selected yielding an of about and the unit began navigating at

7 with GLONASS and GPS satellite with - meter accuracy. Conclusions Datum adjustment of the GLONASS into WGS- or GPS into PE-9 during the differential process has been proven to work well by several manufacturers. The transformation parameters and application of them must be done the same way in the reference station and user set. Currently, there is no universally accepted datum transformation. Interoperability between manufacturers will be a problem if different transformations are being used. Using no transformation and allowing the DGNSS corrections to perform this adjustment eliminates the need for a standard coordinate transformation to allow DGNSS operation. The first part of scenario proved that the concept of fixing the time offset between GPS and GLONASS in both the reference station and the user set allows the user to navigate with one less satellite. Communicating this offset to the user was demonstrated in the second part of scenario through the first use of RTCM SC- Type time offset message. Fixing the time offset allows users to navigate with one less satellite when combining the two systems. D Navigation with GPS and GLONASS satellites was successful as well as GPS, GLONASS and fixed altitude for D navigation. Although the time offset may be fixed in a reference station, the users can either choose to use this information or not depending, on their application. Recommendations No datum transformations should be used when integrating GPS and GLONASS. The reference station should leave the satellite positions in PE-9 and let the corrections account for the change. Reference Stations should be designed to keep the time offset between GPS and GLONASS time fixed. The reference station should report this offset to users with message type. Users can then use type to take advantage of the fixed offset in the reference station and fix their own clock when required or ignore this method altogether and calculate their own time offset. References ÒRTCM Recommended Standards for Differential Global Navigation Satellite Systems, Version. DRAFTÓ, RTCM Special Committee Number, Radio Technical Commission for Maritime Services, Washington DC. ÒIntegrated GPS/GLONASS receivers: The Key to a New World of Possibilities!Ó, Beser, J., RTCM Annual Assembly, St. Petersburg, FL, May 99.