Comparative Study of Some Online GNSS Post-Processing Services at Selected Permanent GNSS Sites in Nigeria Comparative Study of Some Online GNSS Post-Processing Services at Selected Permanent GNSS Sites in Nigeria

Many applications in surveying and mapping have been made simpler and more precise due to the advent of GNSS, and thus, the demand for using cutting-edge GNSS techniques in surveying and mapping applications has become indispensable. Online GNSS post- processing services are now available to provide support for users in need of precise point positioning or conventional differential positioning services and without requiring a prior knowledge of GNSS processing software. This study evaluates the performance of some online GNSS facilities with emphasis on observation duration (i.e. 1hr, 2hr, 6hr 12hr and 24hr observations). Three of these online facilities (AUSPOS, GAPS and magic-GNSS) were chosen based on their mode of operation and were evaluated at the location of five permanent GNSS stations in Nigeria. The study cut across two epochs in the year 2014 (i.e. seven days each in the months of January and July). Results in this study indicate that users can expect reliable results from these online services and their accuracy is within allowable limits for mapping applications in Nigeria. The similarity of the results between all of the services used is amazing, thus further demonstrates the robustness of the algorithms and processes employed by the different online facilities.


Introduction
Global Navigation Satellite Systems (GNSS) is generic term for a composition of different satellite navigation technologies such as American GPS (Global Positioning System); its Russian equivalent, GLONASS (GLObal Navigation Satellite System); the Chinese system, BeiDou; the Japanese regional system, QZSS; the Indian regional system IRNSS (Indian Regional Navigation Satellite System); finally, is the European Galileo system. The GPS and GLONASS has since attained full operational status. The BeiDou, is expected to achieve completion for worldwide service in 2020, although a limited version of its signal has already been available since December 2012. The QZSS, is at present providing a limited service in the form of an augmented signal for GPS, but should be progressively upgraded and achieve full impartiality in 2023. The IRNSS, is at a final point operation as well. The Galileo system is expected to attain full operational capability in 2020 [1,2].
Global Navigation Satellite System (GNSS) is one of the most innovative and practical technology developed in recent times. Since its inception it has grown to provide not only world-wide, all weather navigation, but precise position determination capabilities to all manner of users especially for surveying and geodetic applications. In surveying and mapping, this represents a revolutionary departure from conventional surveying procedures, which relied on observed angles and distances for determining point positions [3,4].
Traditionally, it was necessary to obtain positioning with GNSS using at least two receivers, and the collected data processed for high accurate positioning using the GNSS data processing software whether scientific or commercial. However, the usage of such software is also quite difficult because they generally require deep knowledge of the GNSS, experience in the processing and they mostly need a licencing fee [4][5][6][7].
GNSS processing services. These services provide GNSS 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. Organisations 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) [7].
The only requirement for using these services is a computer having an internet connection and web browser. These services are designed to be as simple as possible for the user and with minimal input. Users of such systems have to perform uploading/sending of their collected data in RINEX format by using the web site of these services, e-mail or ftp sites to the system and selecting a few processing options. Some of these services process not only the GPS but also the data of other systems, particularly those of GLONASS, and provide resilience and a higher accurate positioning service in certain cases to their users [5].
Currently, there are several online GNSS post-processing services, and are best categorised base on their adopted approach of processing the RINEX files. Categorically, there are those that use the Precise Point Positioning (PPP) approach (see [11][12][13] for documentation). Those in this category include Canadian Spatial Reference System-Precise Point Positioning (CSRS-PPP), magicGNSS, (APPS) and GPS Analysis and Positioning Software (GAPS). PPP based services used the GNSS data collected with only a single receiver with precise satellite ephemerides and clock data by taking into account corrections like carrier phase wind-up, satellite antenna phase offset, solid and ocean tides. The category of the GNSS online processing services that adopted the conventional relative approach, where user's RINEX files are processed relative to other GNSS continuously operating reference stations (CORS). The Trimble RTX, Australian Surveying and Land Information Group Online GPS Processing Service (AUSPOS) and Online Positioning User Service (OPUS) are based on this approach [5].
The application/usage of these facilities are gaining global acceptance and numerous studies have evaluated the accuracy of different online GNSS processing in different part of the world (e.g. Australia, Egypt, etc.). The results of such studies have demonstrated inherent limitations, the accuracies, conveniences of online post processing of GNSS observations, and have also identified a wide range of uses within the surveying community (e.g., see [13][14][15]). This chapter is dedicated to the report on the accuracy of three online GNSS processing facilities (magic GNSS, GAPS, and AUSPOS) over the territory of Nigeria. The major objective of the study is to investigate the effects of the variation in the duration of GNSS observation sessions on the positional accuracy when using online processing facilities.
The structure of the paper is as follows: first a general description and status of the different online GNSS post-processing services is presented in Section 2. Section 3 explains the methods used in the data acquisition, processing and evaluation of results. Section 4 describes the results. Lastly, the concluding remarks were presented and additionally, the paper gives insight into possible future expansion of GNSS infrastructures in Nigeria.

Overview of GNSS data processing and online services
Currently, there exist several online facilities for GNSS post processing applications. The different facilities or services are provided by different organisations and thus their mode of processing, restrictions, processing options, and format/latency of results varies. Table 1 gives a summary of the comparison of the different facilities.
Each of the above-mentioned organisations have different technical specifications with respect to service features such as membership requirement, storage limitation of the GPS/GNSS RINEX data to be uploaded, process in static/kinematic modes, evaluation the data collected by single/dual or multi frequency receiver, GPS/GNSS antenna type selection, etc. 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 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. Solution quality from the various processing services depends on the availability, proximity and quality of base station data, and the availability of precise satellite orbits and clock corrections.

Methodology
Three online GNSS processing software were selected for this study. The selection was based on their mode of processing. One out of the selected three used the relative solution approach (i.e. AUSPOS) and the remaining two utilises the PPP technique (i.e. magicGNSS and GAPS).
The study utilised data from the new Nigerian GNSS Network (NIGNET) [16,17] for the evaluation of the selected online GNSS services. Daily GNSS data in Hatanaka-compressed ASCII format were downloaded from the NIGNET site at www.nignet.net. The files were uncompressed with the freely available CRX2RNX software. The GNSS data were downloaded at the location of five different stations in the NIGNET (see Figure 1) for the year 2014. These stations include: ABUZ (Zaria); BKFP (Birnin-Kebbi); CLBR (Calabar); FUTY (Yola); and UNEC (Enugu). The stations were selected based on the data available per day (data consistency) from each station as the NIGNET is often characterised by large data gaps [18].
The GNSS data were collected at two epochs corresponding to GPS weeks 1774 and 1800, respectively. The data were collected for all 7 days in each week, it cuts across two different seasons of the year (months of January and July). The reason for this was to identify possible seasonal variations in the estimated coordinates from the different online facilities. The daily (24 h) RINEX files (observation data files) at each station were then decimated into 2, 6 and 12 h using the TEQC analysis software. This was done in order to check the effect of the length of observation session on the output of the different online GNSS processing services. The 24 h files and the decimated files were submitted to the three GNSS online processing services (magicGNSS, GAPs, and AUSPOS). After submission, both the 24 h and decimated files were processed and all the results were received via e-mail. To compare the results from the online GNSS post processing facilities with known station coordinates which were originally obtained from long time station average using BERNESE software, the residuals (differences) in northing, easting and heights components were computed for all observations in the two epoch and were employed in subsequent analysis. Consequently, the root-mean-square error (RMSE) in both the vertical and horizontal directions were computed from the differences using Eqs. (1) and (2). Similarly, the Horizontal RMSE (HRMSE) and vertical RMSE (VRMSE) were calculated using Eqs. (3) and (4); In Eqs. (1), (2), and (4); P i is the known station coordinates for the NIGNET stations and the estimated coordinates from the different online GNSS services are denote by O i , and n is the total number of observations.

Results and discussions
The coordinate of the NIGNET stations were obtained in geographic unit and were converted to equivalent Universal Traverse Mercator (UTM) coordinate system with projection on the WGS 84 ellipsoid. The coordinates of the selected five NIGNET station for this study in UTM (Northing, Easting and Height) system is presented in Table 2. Similarly, all 3D coordinates obtained from the magicGNSS, GAPS, and AUSPOS were converted to UTM system for easy comparison. Appendices A.1-A.5 contain the average 3D coordinates of the stations at the 2, 6, 12, and 24 h observation sessions.
To compare accuracy of magicGNSS, AUSPOS, and GAPS online services, the coordinates of the selected permanent GNSS site which were originally computed using BERNESE software are taken as reference. The coordinate differences of each online services subtracted from reference coordinates of all the stations and RMSE, HRMSE, and VRMSE have been computed by Eqs. (1)-(4). The combined results of the performance measures (RMSE, HRMSE, and VRMSE) is presented in Table 3 for observations at all the permanent GNSS stations in January 2014 (first epoch).
The RMSE values for the east and north components are typically less than 0.3 m for the magic GNSS and GAPS services; while those of the AUSPOS service were higher and greater than 0.3 m in all instances as seen in Table 3. Accordingly, the HRMSE values for the magicGNSS and GAPS were also less than those from AUSPOS; also, the VRMSE values for AUSPOS are higher than those of magicGNSS and GAPS which is an indication that AUSPOS results are less accurate when compared to magicGNSS and GAPS. Figure 2 is a plot of the different performance measures, it very evident form Figure 2 that AUSPOS performs less than the other two services. Also, it can be seen the 24 h file do not always give the best results. However, AUSPOS did gave some deterrent messages on the use of 2 h files for processing.
Again, the combined results of the performance measures (RMSE, HRMSE, and VRMSE) is presented in Table 4 for observations at all the permanent GNSS stations in July 2014 (second epoch).  The results from Table 4 are in very good agreement with those in earlier discussed ( Table 3 for the first epoch of observations). Figure 3 is a plot of the different performance measures for the second epoch of observation.

S/no
From Figure 3 it is evident that the 24 h observation files and the decimated files (2, 6 and 12 h), produce results with millimetre (mm) to a centimetre (cm) level of accuracy when processed with magicGNSS and GAPS. It is again evident from Figure 3 that magicGNSS produces the best results, followed by GAPS and then AUSPOS. This is the same for the two epochs.
The AUSPOS is the only one of the three facilities that utilises the relative approach, its results were not pleasing, the poor performances of AUSPOS is attributed long baselines in the processing because of non-availability of nearby IGS stations for the processing. Thus, baselines of shorter lengths will increase the quality of data, the reliability and dependability of the online AUSPOS facilities. As earlier stated, AUSPOS again gave a warning message in processing the 2 h files indicating that the precision of estimated coordinates are outside the confidence level but the situation was different with magicGNSS and GAPS.
All the three services investigated in this study return results to users via email. Time delay on receiving the results depends on several factors including the traffic on the Internet and the number of users accessing the service at the same time. The displayed times in Table 5 are only a rough estimates in order to compare the speed of each of the services and were obtained by submitting the same 24 h data set to each of the service.
The AUSPOS is the fastest to return results, followed by GAPS and then magicGNSS; again it was found to be more user friendly, followed by magicGNSS (e-mail version) and then GAPS. The GAPS facilities has some security features which sometimes exasperate the process of submitting files for processing. Also, the advanced mode of processing in GAPS gives room to decimate files automatically by just giving the range of observation without going into the tedious processes of doing it with TEQC software.

Concluding remarks
In this work, a comparative analysis of some online GNSS post-processing services at locations of permanent GNSS stations in Nigeria has been made. Online GNSS processing services can help users either using precise point positioning (PPP) or differential method, and without requiring a prior knowledge of GNSS processing software. Results in this project indicate that users can expect reliable results from these online services. The similarity of the results between all of the services used is amazing. That they differ only by a few millimetre (mm) or centimetre (cm) demonstrates the robustness of the algorithms and processes they employ in processing GNSS observations. Results for decimated daily RINEX files also show that users can process data sets of less than 24 h observation period and expect almost the same results  Table 5. Latency results from magicGNSS, AUSPOS, and GAPS online GNSS post-processing services.
(or better results in some cases) when compared to the 24 h data set. Among the three online facilities examined in this study, the AUSPOS seems to have the most flexible and user friendly interface, followed by magicGNSS and then GAPS. As mentioned earlier, magicGNSS produces the best result, followed by GAPS and then AUSPOS. When selecting a faster means of obtaining result from these software, AUSPOS is the fastest, followed by GAPS and then magicGNSS. The reason why AUSPOS did not perform as GAPS and magicGNSS is due to the effect of long baselines in the processing and this again affirm the advantage of the PPP techniques. Regardless of the problem that might be encountered in the return of results (processed coordinate values), magicGNSS is undoubtedly the best of the three. Undoubtedly, the online GNSS facilities have brought a paradigm shift in GNSS positioning applications, in view of the accuracy and efficiency (saving cost of buying and operating a second receiver) they offer to users. It is therefore necessary that if any of these facilities (including those not considered in this study) is to be used for processing, the need for reliability and accuracy must first be considered. Finally, creating awareness among surveyors and other professionals on the functionality and dependability of online GNSS post-processing services is needed so that they can fully explore the potential of these facilities in mapping and possibly cadastral applications in Nigeria and other parts of the world.