Online GPS processing services: an initial study
- First Online:
- Cite this article as:
- Ghoddousi-Fard, R. & Dare, P. GPS Solut (2006) 10: 12. doi:10.1007/s10291-005-0147-5
- 253 Views
There are a number of online Global Positioning System (GPS) processing services that provide GPS processing results to the user free of charge and with unlimited access. These services provide solutions for a user-submitted Receiver Independent Exchange Format (RINEX) file based on differential methods using reference stations or precise point positioning using precise GPS orbit and clock data. Different data sets varying in time and location were submitted to the online services and their results compared. Although the quality of results depends on many factors, in most cases the users can expect reliable online processing results for a 10-h data set made by a geodetic dual frequency receiver anywhere in the world.
Over the last few years a number of organizations have developed online Global Positioning System (GPS) processing services. These services provide GPS processing results to the user free of charge and with unlimited access. The user sends a Receiver Independent Exchange Format (RINEX) file to the service and within a short period of time, the estimated position of the receiver used to collect the RINEX data is sent back to the user. Organizations that provide these free services include: Geohazards Division of Geoscience Australia, the Geodetic Survey Division (GSD) in Canada, the United States’ National Geodetic Survey (NGS), Scripps Orbit and Permanent Array Center (SOPAC) at the University of California and the Jet Propulsion Laboratory (JPL) at National Aeronautics and Space Administration (NASA).
The objective of this paper is to evaluate these online services and compare their position results with expected values. A comparison has also been made between the results obtained using data sets with varying observation time intervals. Furthermore, the results of the services are evaluated using data collected in different parts of the world.
Online services: an overview
Each of the above-mentioned organizations provides their own free online GPS processing service. The basic requirements that the user needs to take advantage of these different services are almost the same: access to the Internet and a valid email address. The following sections will give a brief description of each service.
The Geoscience Australia [formerly the Australian Surveying and Land Information Group’s (AUSLIG)] Online GPS Processing Service (AUSPOS) was officially launched in late 2000 (Dawson et al. 2004), and has been in continuous operation since then processing data for dual frequency geodetic GPS receivers located anywhere on earth. The AUSPOS positioning is by differential GPS to the nearest three International GNSS Service (IGS) stations and uses the IGS precise orbit information. This service is accessible via the Geoscience Australia website at: http://www.ga.gov.au.
The Scripps Coordinate Update Tool (SCOUT) was developed by the Scripps Orbit and Permanent Array Center (SOPAC). This service also uses by default the three nearest IGS stations. However, this service allows the user to choose up to four different reference stations. The SCOUT uses the GAMIT processing software. This service is accessible from the SOPAC website at: http://sopac.ucsd.edu.
The United States’ National Geodetic Survey developed the Online Positioning User Service (OPUS). This service generates coordinate results by using data from three Continuously Operating Reference Stations (CORS). The CORS sites are chosen not according to closest proximity but picked according to compatibility between the user’s data and the CORS site (OPUS Team 2004). There is also an option that allows the user to choose the CORS stations to be used. The service can be found at: http://www.ngs.noaa.gov.
Auto-GIPSY is an e-mail/FTP interface to the GPS Inferred Positioning System (GIPSY) developed by JPL. This service performs single point positioning, and is therefore not dependent on the proximity or availability of CORS/IGS data (Macdonald 2002). The FTP address of user’s data should be submitted by email to: firstname.lastname@example.org.
The Geodetic Survey Division (GSD), Canada, developed the Canadian Spatial Reference System (CSRS) Precise Point Positioning (PPP) service. Single point positioning is provided for users operating in static or kinematic modes using precise GPS orbits and clocks (GSD 2004). This service is available via the GSD website at: http://www.geod.nrcan.gc.ca.
Online services: a preliminary assessment
An overall assessment of online GPS processing services
Name of service
Data transfer method
Elapsed time to receive results (min)
Restrictions on length of GPS data set
Minimum of 1 h
Via anonymous FTP
No. of RINEX files (maximum 7)
Via anonymous FTP
Minimum of 1 h
Upload the file to Scripps FTP site
Selection of reference stations
PPP* (*An “expert” version is also available with more options)
Mode of processing (static or kinematic)
Reference system (NAD 83 or ITRF)
Maximum 6-day long providing uncompressed RINEX file is less than 100 MB (GSD 2004)
Minimum of 2 h (recommended by the service)
24 h maximum
Additional options: selection of state plane and base stations, extended output, set user profile
Only available for use in Central and North America
Via sending the anonymous FTP address
At least an hour, preferably more (Zumberge 1999)
Data within 15 h of GPS noon of obs. day will be analyzed
One results validation method is to process observed GPS data at known points and compare the resultant coordinates with the known position values. Also evaluated was the relationship between the accuracy of the results and the observation time span.
Analysis of solutions for UNB1 RINEX data
As it can be seen in Figs. 6, 7 and 8 OPUS3, SCOUT3 and SCOUT4 provided closer latitude and longitude results to the known values than OPUS and SCOUT for data sets up to 6 h. After 6 h no significant difference can be seen. For the height results, however, SCOUT3 and SCOUT4 provided closer results to the known value than SCOUT for up to 8 h. In the case for OPUS3 the height results with respect to OPUS were improved for up to 2 h.
Results in different parts of the world
Except for OPUS, which is limited to Central and North America, all of the online services provide GPS processing results for observations made anywhere in the world. The PPP and Auto-GIPSY processing are based on precise GPS orbit and clocks products that are global in nature while SCOUT and AUSPOS use differential methods to the nearest three or four reference stations. However, these reference stations are not uniformly distributed in the world. In order to investigate the effect of reference station proximity on online services results two further tests have been done, as explained in the following sections.
Analysis of solutions from commercial RINEX data
Processing of observations from an IGS point in Africa
Further investigation on PPP results
As mentioned in the previous section, the results of PPP did not converge for point MALI on January 1, 2004. To investigate this further, IGS points were selected in different locations and their data for January 1, 2004 were processed by PPP. These points are: ALGO (Ontario, Canada), UNB1 (New Brunswick, Canada), STJO (Newfoundland, Canada), STR1 (Australia), BAHR (Bahrain) and RIOG (Argentina). The location of these points is indicated in Fig. 9.
Analysis of the results
On average, Auto-GIPSY was found to produce the closest horizontal and vertical coordinates at the investigated points. Furthermore, the quality of this service and the PPP service are independent of site location due to using precise GPS orbit and clock data in point positioning mode. These two services are also the fastest to return the results.
Ellipsoid height results of SCOUT and Auto-GIPSY show some unusual changes as can be seen in Fig. 5 at hour 4 and at hour 20 for SCOUT and at hour 6 for Auto-GIPSY. However, the SCOUT3 and SCOUT4 results do not show such unusual changes, even though the average baselines length in SCOUT3 and SCOUT4 are more than SCOUT.
For short data sets, user selected reference stations in SCOUT3/4 and OPUS3 scenarios provided closer results to the known values than SCOUT and OPUS. This may be due to reference station quality and geometric configuration. A significant change in vertical accuracy of SCOUT can be seen at point MALI, where on the first day the vertical accuracy was about 3 cm but on the next 2 days was more than 10 cm (Figs. 13, 14 and 15).
At DODOLA, the latitude converges as it did for UNB1. The longitude solutions from SCOUT and PPP, however, continue to show variations at approximately 2–3 cm and the convergence pattern is not as clear as UNB1. The AUSPOS produced identical results for the last 8 h and Auto-GIPSY for the last 12 h. In the solution for height (Fig. 12), Auto-GIPSY provided the closest results to the assumed value while it had a systematic difference of 12 cm with other services. After 10 h, the height solutions for the services (ignoring Auto-GIPSY) vary by about 4 cm.
The PPP results did not converge for points MALI and STR1 on January 1, 2004 (both of these points located in the southern hemisphere). On January 1, 2004 PRN 23 experienced failure in its atomic frequency standard (Sigmond 2004). However it does not seem that the two mentioned reasons caused the failure of the PPP results in MALI and STR1 because although the point RIOG is also in the southern hemisphere accurate results were provided by the PPP (Fig. 16). Furthermore, other services provided reliable results on the same day for point MALI (Fig. 13).
Online GPS processing services can help GPS users all over the world to take advantage of precise point positioning or differential methods with one single receiver, and without requiring detailed knowledge of processing software. Solution quality depends on the availability, proximity and quality of base station data, and the availability of precise satellite orbits and clock corrections. Performed tests in this paper indicate that users can expect reliable results from online services, although some problems have occurred, such as those mentioned for PPP.
The resultant coordinates converged after almost 10 h of observations using default-processing parameters. This shows that users can expect almost the same results for a 10-h data set as for a 24-h data set. With user-selected reference stations for the examples used, the data set length could be reduced by a few hours.
Preliminary work on this research was carried out by two undergraduate students at UNB. Their work has been published in Leslie (2004) and Hatch (2003). We thank Duncan Moss and Neil Stuart of the University of Edinburgh, Scotland, for providing the GPS data they collected at Dodola, Ethiopia. Paul Jamason is thanked for providing comments that improved the quality of this paper. We also acknowledge Canada’s ‘Natural Science and Engineering Research Council’ (NSERC) for providing funds to enable this research to be carried out.