Embed Size (px)
DESCRIPTION
LONASS (Russian: ГЛОНАСС; IPA: [ɡlɐˈnas] - Глобальная Навигационная Спутниковая Система), acronym for Globalnaya Navigatsionnaya Sputnikovaya Sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Aerospace Defence Forces. It both complements and provides an alternative to the United States' Global Positioning System (GPS) and is the only alternative navigational system in operation with global coverage and of comparable precision.
Citation preview
1
Comparing Long Baseline Results from GPS and GPS/GLONASS
Junqiang Wang and Jinling Wang
School of Surveying & Spatial Information Systems, University of New South Wales, Sydney, NSW 2052, Australia
Tel: +61 2 9385 4185 Fax: +61 2 9313 7293 Email: [email protected], [email protected]
Abstract: The availability of GLONASS would bring significant benefits to geodetic applications of global positioning systems. GPS and GLONASS observations could be combined directly in the data processing; as a result, the geometry of observed satellites could be enhanced by increasing the number of available satellites. Up to now, with the recent revitalization of GLONASS, more high precision geodetic GLONASS receivers are brought to market, and GPS/GLONASS receivers have been equipped in some International GNSS Service (IGS) tracking stations all over the world. The short latency precise IGS orbits for GLONASS, consistent GPS/GLONASS CODE orbits as well as combined GPS/GLONASS observations are available. So it becomes worthwhile to investigate the advantages and disadvantages of combined GPS/GLONASS solutions for geodetic applications. In this paper, the GPS and GPS/GLONASS baselines of two networks have been analysed, aiming to assess the influence of GLONASS data to the GPS-only long baseline solution. The IGS final GPS precise orbits and fully consistent GPS/GLONASS orbits from CODE are used in all baseline solutions. Stations of the networks are equipped with GPS/GLONASS receivers, and all of them are IGS tracking stations, which are located in Australia and Europe. GPS-only and GPS/GLONASS observations are processed, respectively, using the same processing strategy. The results of the GPS-only baselines and GPS/GLONASS baselines have been compared.
Keywords: GPS, GPS/GLONASS, Long Baselines
1. Introduction
As a second GPS-like global positioning system, the GLObal NAvigation System (GLONASS), which is operated by Russian Federation, has increased its constellation recently, with an expectation of 18 satellites by the end of 2007. The availability of GLONASS would bring two significant benefits to geodetic applications of global positioning systems. First, the solution from GLONASS could be employed as an independent verification of GPS solution to improve quality control. Second, GPS and GLONASS observations could be combined directly in the process of solution; as a result, the geometry of observed satellites could be enhanced by increasing the number of available satellites. Early studies have shown the benefits of integrated GPS/Glonass solutions and have also addressed some modelling and ambiguity resolutions issues over short baselines (e.g., Dai et al., 2003; Wang, 2000; Wang et al., 2001).With the recent revitalization of GLONASS, more high precision geodetic GLONASS receivers are brought to market, and GPS/GLONASS receivers have been equipped in some International GNSS Service (IGS) tracking stations all over the world. So it becomes worthwhile to investigate the advantages and disadvantages of combined GPS/GLONASS solution in geodetic application.
In order to process combined GPS/GLONASS observations, we should address the problems that arise from the difference in the coordinate systems of the satellite systems, as well as the time and frequency systems employed by GPS and GLONASS. Due to the different signal
2
frequencies for different GLONASS satellites, a strategy, which scales GLONASS L1 and L2 observations from each satellite to a common frequency, is commonly used in combined GPS/GLONASS data processing. In practice, GLONASS L1 and L2 carrier phase observations are scaled to GPS L1 and L2 frequencies respectively. While some clock errors can be removed in the double difference procedure, negative side effects will arise simultaneously. Investigations show that whilst the number of observations increases in combined GPS/GLONASS baseline, additional parameters will be added to the solution by the similar percentage.
Due to the differences of GPS and GLONASS, the conventional GPS data processing techniques should be modified to incorporate the new satellite systems GNSS with the major problems of different satellite signals, different reference frames and different time scale. As one of the major scientific data processing software, Bernese 5.0 software has the capability to process GPS and GLONASS observations, offering the precise and improved solutions (Dach et al., 2007). In this study, GPS/GLONASS observations are processed using the Bernese 5.0 software, and the results of GPS-only and GPS/GLONASS are compared.
2. Data Processing
In order to investigate the advantages and disadvantages of combined GPS/GLONASS long baseline solutions, the observations from two networks have been processed using the Bernese 5.0 software. The first network consists of four IGS tracking stations located in Australia, and all of them are equipped with GPS/GLONASS receivers. Most of the baselines are longer than 2000 km. Another network includes eight IGS tracking stations located in Europe, and GPS/GLONASS observations are available in these stations. The lengths of baselines from this network are less than 1000 km.
The GPS only and combined GPS/GLONASS observations were processed using the Bernese 5.0 software. Baselines were formed between the stations with a maximum number of common dual frequency GPS/GLONASS observations. Firstly, GPS only observations were processed, and then combined GPS/GLONASS observations were considered. The same baselines were formed in both of the process, and the QIF (Quasi Ionospheric Free) method was employed in ambiguity resolution. IGS final orbits and clocks weree used in processing GPS only observations, whilst CODE fully consistent GNSS orbits were used for combined GPS/GLONASS observations.
2.1 Results from the network in Australia
Figure.1 The Locations of GNSS Stations in Australia.
3
The first network consists of four stations, which are located in the coastlines of four states of Australia. All of the four stations are IGS tracking stations equipped with GPS/GLONASS receivers; they are DARR, STR2, SUNM and YARR, as displayed in Figure.1. Table.1 shows the details of equipped GPS/GLONASS receivers.
In this network, GPS and GLONASS observations were selected for 24 hours on day 0 of week 1387. Three baselines were formed in processing GPS only observations and the same baselines were processed with combined GPS/GLONASS observations. In these three baselines, two of them have the length of more than 2000 km, and the other one is nearly 1000 km; the details are displayed in Table.2.
Table.1 GPS/GLONASS Receivers of the Stations in the First Network.
Table.2 The Details of the Baselines in the First Network.
As a result of introducing GLONASS data, the amount of observations increased dramatically (see Figure.2), at the percentages of 42%, 26% and 73% respectively (Figure.3). However, the number of parameters, which are GLONASS ambiguities, was increased at the similar level corresponding to the increase of observations (Figure.4).
Number of Observations
05000
10000
150002000025000
DASU DAYA STSU
GPS/GLONASSGPS
Figure.2 Number of Observations in the Baselines
Name Receiver Type Antenna Type DARR TPS LEGACY ASH700936D_M STR2 TPS E_GGD TPSCR3_GGD SUNM JAVAD LEGACY JPSREGANT_SD_E YARR ASHTECH Z18 ASH701073.1
No Baseline Station1 Sation2 L(km) 1 DASU DARR SUNM 2774 2 DAYA DARR YARR 2412 3 STSU STR2 SUNM 948
4
Percentage of Increased Observations
0%10%20%30%40%50%60%70%80%
DASU DAYA STSU
Figure.3 Percentage of Increased Observations
Percentage of Increased Ambiguities
0%10%20%30%40%50%60%70%80%90%
100%
DASU DAYA STSU
Figure.4 Percentages of Increased Ambiguities
RMS of Baselines
1.3
1.4
1.5
1.6
1.7
DASU DAYA STSU
mm
GPS/GLONASSGPS
Figure.5 RMS of Baselines
5
Coordinates difference between GPS & GPS/GLONASS
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
DARR STR2 SUNM YARR
mm
∆X∆Y∆Z
Figure 6 Coordinates Differences
It can be seen from Figure 5, the RMS of GPS and combined GPS/GLONASS baselines are at the same level. The differences between station coordinates carried out from GPS only observations and combined GPS/GLONASS observations are not significant except the station of SUNM. More than 35 percent of bad observations were reported at the station of SUNM, when combined GPS/GLONASS data were processed. It can be seen from Figure.7 that the RMSs of three components of station coordinates derived from the combined GPS/GLONASS observations, are better than those derived from GPS only observations.
RMS Y
012345
DARR STR2 SUNM YARR
mm
GPS/GLONASSGPS
RMS Z
012345
DARR STR2 SUNM YARR
mm
GPS/GLONASSGPS
Figure.7 RMSs of X, Y, Z Coordinates
RMS X
0
2
4
6
8
DARR STR2 SUNM YARR
mm
GPS/GLONASSGPS
6
2.2 The results from the network in Europe
The second network consists of nine stations in Europe, and all of them are equipped with GPS/GLONASS receivers. The locations of these stations are displayed in Figure.8. In processing this network, 24 hours observations were selected on day 0 of week 1387. The details of receivers equipped in these stations are shown as Table.3. The data processing strategy employed in this network is the same as that used in the first network. In the processing of this network, 8 baselines were formed and the longest baseline is 971 km, while the shortest baseline is 68 km (see Table.4).
Figure.8 The Location of Stations in Europe
Table.3 GPS/GLONASS Receivers of the Stations in the Second Network
Name Receiver Type Antenna Type BOGI JPS E_GGD ASH701945C_M HERT Ashtech Z18 ASH701946.2 MAR6 JPS E_GGD AOAD/M_T ONSA JPS E_GGD AOAD/M_T SASS JPS LEGACY TPSCR3_GGD SPT0 JPS LEGACY AOAD/M_T VENE ASHTECH Z18 ASH701941.B VIS0 JPS E_GGD AOAD/M_T WARN JPS LEGACY TPSCR3_GGD
7
Table.4 The Details of the Baselines of the Second Network.
Observations of Baselines
0
5000
10000
15000
20000
25000
30000
BOSA HEWA MAVI ONSA ONSP SAWA SPVI VEWA
GPS/GLONASSGPS
Figure.9 Number of Observations in the Baselines
In this network, the number of observations in these baselines increased dramatically for the reason of involving GLONASS satellites, and the number exceeds 25000 in more than half of the stations (see Figure.9).
The percentages of increased observations are displayed in Figure.10. However, the number of parameters, which are GLONASS ambiguities, increased at the larger percentages than that of observations (Figure.11).
The RMS of the baselines stays at the same level in both GPS only solutions and combined GPS/GLONASS solutions (Figure.12), the differences of station coordinates in both cases are within 5mm (Figure.13). It can be seen that the accuracies of station coordinates derived from combined GPS/GLONASS solutions are higher than that derived from GPS only solutions (Figure.14). These results are consistent with the analysis in Bruyninx (2007).
No Station1 Station2 L (km) 1 BOGI SASS 540 2 HERT WARN 877 3 MAR6 VIS0 334 4 ONSA SASS 338 5 ONSA SPT0 68 6 SASS WARN 107 7 SPT0 VIS0 327 8 VENE WARN 971
8
Percentage of Increased Observations
0%10%20%30%40%50%60%70%80%90%
100%
BOSA HEWA MAVI ONSA ONSP SAWA SPVI VEWA
Figure.10 Percentage of Increased Observations
Percentage of Inceased Ambiguities
0%10%20%30%40%50%60%70%80%90%
100%
BOSA HEWA MAVI ONSA ONSP SAWA SPVI VEWA
Figure.11 Percentage of increased Ambiguities
RMS of Baselines
0
0.2
0.4
0.6
0.8
1
1.2
BOSA HEWA MAVI ONSA ONSP SAWA SPVI VEWA
mm
GPS/GLONASS
GPS
Figure.12 RMS of Baselines
9
Coordinate Difference between GPS & GPS/GLONASS
0.00.51.01.52.02.53.03.54.04.55.0
BOGI HERT MAR6 ONSA SASS SPT0 VENE VIS0 WARN
mm
∆X
∆Y
∆Z
Figure.13 Coordinates Difference
RMS X
00.20.40.60.8
11.21.41.61.8
BOGI HERT MAR6 ONSA SASS SPT0 VENE VIS0 WARN
mm
GPS/GLONASS
GPS
RMS Y
00.20.40.60.8
11.21.41.61.8
2
BOGI HERT MAR6 ONSA SASS SPT0 VENE VIS0 WARN
mm
GPS/GLONASS
GPS
RMS Z
00.20.40.60.8
11.21.41.61.8
2
BOGI HERT MAR6 ONSA SASS SPT0 VENE VIS0 WARN
mm
GPS/GLONASSGPS
Figure.14 RMS of X, Y, Z Coordinates
10
3. Conclusion
In this study, two networks, which are equipped with GPS/GLONASS receivers, have been analyzed, in order to investigate the advantages and disadvantages of combined GPS/GLONASS long baseline solutions. One day GPS/GLONASS observations, precise GPS final orbits and fully consistent GNSS orbits have been retrieved from the IGS and CODE websites. In addition, Bernese 5.0 software has been employed in the process of data processing.
From the results generated from the two networks, it can be seen that the accuracy of long baseline can be improved by introducing GLONASS observations. The differences of station coordinates between GPS only solutions and combined GPS/GLONASS solutions can affect the coordinates at a level of up to 5mm in three coordinate components over the long baselines in Australia, and the differences are relatively smaller in the second network within Europe, which the length of the baselines is less than 1000km.
Acknowledgement All of the observation data and orbit products are from IGS and CODE websites. Ocean Tidal Loading Tables are from the free ocean tide loading provider (http://www.oso.chalmers.se/~loading/).
References
Bruyninx, C. (2007) Comparing GPS-only with GPS+GLONASS positioning in a regional permanent GNSS network. GPS Solution, 11(2), 97-106.
Dach, R., Hugentobler, U., Fridez, P., Meindl, M. (eds) (2007) Bernese GPS Software Version 5.0. Astronomical Institute, University of Berne, Switzerland, available at http://www.bernese.unibe.ch (accessed on 16/May/2007)
Dai, L., Wang, J., Rizos, C., & Han, S. (2003) Predicting atmospheric biases for real-time ambiguity resolution in GPS/GLONASS reference station networks. Journal of Geodesy, 76, 617-628.
Wang, J. (2000) An approach to GLONASS ambiguity resolution. Journal of Geodesy, 74(5), 421-430.
Wang, J., Rizos, C., Stewart, M.P., & Leick, A. (2001) GPS and GLONASS integration: Modelling and ambiguity resolution issues. GPS Solutions, 5(1), 55-64.
