It acquires 2 GPS receivers to carry out instantaneous observations that are to be processed together. Accordingly, a baseline is created between them, where the coordinates of one of them are known [7]. In addition, its accuracy ranges between decimeters “dm” and mm [6, 8]. GPS user has relative positioning techniques to attain highly accurate coordinates [9]. However, in Egypt, GPS technology is acknowledged by mapping bodies.

It is relatively economical and easier to utilize, in reference to the Differential Global Positioning System “DGPS,” as it acquires one receiver only. It utilizes the trilateration concept that acquires 4 satellites to determine the user’s position [10]. It is categorized into Single Point Positioning “SPP” and Precise Point Positioning “PPP,” where SPP utilizes a pseudo-range that has lower accuracy of a meter and is implemented for navigation purposes [11].

PPP is a technique that utilizes an analogous carrier phase as a pseudo-range in order to determine the user position, where PPP achieves DM accuracy [12]. PPP has measurement errors (i.e. ionospheric delay-tropospheric delay-relativity, delay-ocean tide loading-code biases-antenna phase offset-antenna phase windup and satellite clock error), that are modeled and minimized by models, where listed the impact of each error [13].

On the other hand, highlighted that PPP utilizes accurate satellite position and satellite clock, where their information is available in ephemerides (i.e., data of places of celestial bodies provided on regular intervals basis) [14]. Ephemerides is a product of IGS Analysis Centers “AC” that utilizes a global Continuous Operating Reference Station “CORS,” [15].

IGS classifies the ephemerides into four (i.e., broadcast ephemerides-ultra rapid ephemerides-rapid ephemerides-final ephemerides) [16]. as follows:

GNSS online PPSs are utilized worldwide to substitute traditional processing methods, as their utilization became widely popular due to the ease of its implementation, which has low-cost fees and does not require a license or knowledge of a GPS-processing [18]. GNSS data should be converted by the users to Receiver Independent EXchange (RINEX) format and sent via email or by uploading them to a specific website [19]. The uploading process after the coordinates could be easily obtained via the user email. However, it is possible to process data of positioning modes (i.e., static and kinematic) by a free online engine, which is web-based processing. Fig. (2) presents the processing method, which is web-based processing. Fig. (2) presents the processing method, which is online-based GNSS [20, 21].

Fig. (1). IGS network.

Fig. (2). Online GNSS processing.

PPSs encompass many services, where 3 were implemented in this study (i.e., AUSPOS, CSRS-PPP and Trimble Center-Point RTX). These are elaborated as follows:

Several site visits were carried out to Al-Azhar University. Data was assembled for 15 days from 4 points with different materials (i.e., concrete, grass, asphalt and sand), where Fig. (4) presents the 4 points together with the M1 point that was considered the reference point; Fig. (4).

During the 15 days, data was assembled throughout 5 SDs (i.e., 1, 2, 3, 4 and 5 hr.), where the assembled data were GPS observations undertaken by 2 dual-frequency receivers (i.e., Trimble R4 GPS system).

Fig. (3). Research methodology.

Fig. (4). Study area in al-azhar university with the 4 observation points and the M1 reference point.

The GPS operators introduced the RINAX file (i.e., in RINEX format) into the online system and the GPS receiver Coordinates location is obtained within a short time, where Table 1 outlines these services as software.

The GPS data were processed by TBC software. The IGS accurate orbits were obtained from CDDIS. NASA.GOV, whereas the reference stations of ELAT, DRAG, RAMO and BSHM were selected as control points.

Moreover, the web-based services were implemented to process observed data, where accurate data (i.e., from Trimble Center Point RTX “trimblertx.com,” AUSPOS “Geoscience Australia,” and CSRS-PPP “Natural Resources Canada”) were obtained.

Table 2 is provided, which presents the TBC processing results and lists the coordinates of the 4 observation points (i.e., before and after processing).

However, it is of significance to mention that:

• The Position Dilution of Precision “PDOP” and the precise ephemeris were used for post-processing by the Trimble business center in the UTM zon36 and WGS 84.

• The 4 points were connected to a known coordinate point, “M1”, where the distances between M1 and the 4 points were not equal (i.e., asphalt, grass, sand and concrete were at distances 83.644, 126.341, 261.538 and 122.400 m, respectively).

• Two receivers were utilized simultaneously, where one receiver occupied the M1 known coordinate (i.e., reference or base) and 2^nd occupied unknown points (i.e., rove).

Figs. (5-7) are provided, where they present the designated RMSE, after processing the observations and extracting the correct coordinates, for the 5 SDs, for the Easting, Northing and elevation, respectively.

Fig. (5). Easting RMSE of the 4 observed points for the different SDs.

Fig. (6). Northing RMSE of the 4 observed points for the different SDs.

Fig. (7). Elevation RMSE of the 4 observed points for the different SDs.

However, it is significantto mention the following:

• After recording the 4 points observations for the different SDs (i.e., 1, 2, 3, 4 and 5hr) using a dual-frequency receiver (i.e., Trimble R4 GPS) and extracting observations files, TBC software was utilized to produce an accurate coordinate system, in terms of Worldwide/UTM zone 36 WGS-84, where all observations data were introduced to TBC Software (i.e., with .0T02 extension).

• Precise satellite orbits were downloaded to TBC. However, IGS reference stations (i.e., BSHM-ELAT-DRAG-RAMO) were used as control points from the SOPAC site with RINEX format.

Figs. (5-7) indicate the following:

• For all SDs, no significant difference was detected with respect to the Easting and Northing.

• Relatively better RMSE values were obtained in the Easting (i.e., 0.02 m at 3hr-SD and 5hr-SD).

• The RMSE of the elevation was very big for the 1hr-SD (i.e., 0.25 m). However, this value decreased for the 2, 3, 4 and 5hr-SDs (i.e., 0.06 m)

• The 3hr-SD could be sufficient to obtain better accuracy in the Easting and Northing coordinates.

• As for the elevation, 5hr-SD is recommended to attain elevation accuracy.

Figs. (8-10) are provided. They present the RMSE of the online processing results for the 5 SDs, for the Easting, Northing and elevation, respectively.

Fig. (8). Easting RMSE for four points with different SDs from online service.

Fig. (9). Northing RMSE for four points with different SDs from online service.

Fig. (10). Elevation RMSE for four points with different SDs from online service.

However, it is important to mention the following:

• After online results processing, adjustment of the observations was achieved, and final accurate coordinates, X, Y, and Z, from TBC software were obtained.

• All the observed data files were converted into RINEX format and introduced to online services (i.e., AUSPOS, Trimble RTX and CSRS–PPP).

• Each online service provided some products (i.e., results, graphics, and final coordinates, with their standard deviation). These products were emailed to the researcher.

• The corrected observations from online services were subtracted from the relative coordinates, and the RMSE was obtained.

Figs. (8-10) indicate the following:

• RMSE values were influenced by the SD.

• Significant variations were observed in the Easting, Northing and elevation RMSE values.

• The RMSE values order of AUSPOS, CSRS-PPP and Trimble-RTX were descending (i.e., 0.05, 0.03 and 0.02 m, respectively).

• Online services provided variable RMSE values in Easting.

• All online services provided similar RMSE values in Northing, with averages of 0.014, 0.13 and 0.13 for Trimble-RTX, CSRS-PPP and AUSPOS.

• For the elevation, the obtained RMSE values from Trimble-RTX, CSRS-PPP and AUSPOS were close, with an average value of 0.1, 0.1, and 0.012, respectively.

Figs. (11 and 12) are provided. They compare the obtained RMSE values for the TBC and AUSPOS, CSRS-PPP so as Trimble-RTX, at the 5 SDs, for the Easting so as Northing and elevation, respectively.

However, it is of significance to mention the following:

• At this stage, a comparison between the IGS and online service was achieved.

• A comparison was held between the TBC and online services results.

Fig. (11) indicates the following:

• The Easting and Northing (i.e., 2-D) RMSE values differed by varying the SD.

• Trimble-RTX provided the highest 2-D accuracy for the 4hr-SD (i.e., 0.008 m).

• CSRS-PPP provided an RMSE, for the 2-D, of 0.01 m, for the 5hr-SD.

• AUSPOS recorded an accuracy for the 2-D, of 0.01 m, for the 5hr-SD.

• IGS network provided the lowest accuracy, for the 2-D, of 0.026 m 5hr-SD.

• Utilizing Trimble-RTX corrections yields superior accuracy in east-north coordinates.

Fig. (12) indicates the following:

• IGS network provided least accuracy for the 1hr-SD.

• IGS network provided better accuracies for the 2hr-SD or more.