*3.3. ZTD Evaluation Using Single-Frequency PPP with SEID Modelling*

Table 7 shows ZTD results using L1 data combined with SEID modelling of L2. The Table shows results using the original RINEX L1 data, elevation-corrected L1 data and azimuth-elevation corrected L1 data as input. The IGS ANTEX file is used to provide the satellite PCO/PCVs. In case of the original RINEX L1 data, the receiver PCO/PCV is used as well in case there is an entry in the IGS ANTEX file. In case of the elevation and/or azimuth corrected data the RINEX L1 data is modified for the L1 PCV (no receiver entries from ANTEX are used).

**Table 7.** ZTD PPP results using original L1 data, elevation-corrected L1 data and azimuth-elevation corrected L1 data together with the original IGS ANTEX type mean entries using SEID. For the reference target antennas only satellite PCO/PCV ANTEX corrections were utilized. For the SEID processing, the Dutch Permanent GNSS Array (DPGA) stations APEL (Apeldoorn, Netherlands) and IJMU (IJmuiden, Netherlands) as well as the EUREF station DENT (Dentergem, Belgium) were used. The stations are located approximately 110, 55 and 130 km from the experimental setup.


Applying SEID and using the original RINEX L1 data and IGS ANTEX file, the overall performance of the geodetic quality antennas (LEIAR25.R3 LEIT and TRM55971.00 NONE) remained at a high level. It is also evident that noise and bias were significantly decreased for the antennas Trimble AV28 and U-blox ANN-MB-00 for both mounting types compared to the satellite-only PCO and PCV corrected results from Table 6. The bias was almost zero for the U-blox ANN-MB-00 (circular plane) and −6 mm for the rectangular bracket dataset. The AV28 data were slightly more biased with values of about 3 mm for the circular plane data and −5 mm using the rectangular bracket. The RMSE values were between about 4 mm and 7.5 mm. Notably, the ANN-MB-00 (circular plane) dataset is already comparable to the reference results from Table 6. Applying elevation and azimuth-elevation dependent corrections to the RINEX L1 observations further improved the results with biases between almost zero and 4 mm and RMSE values of about 4 to 6 mm ZTD. The performance of the higher grade antennas (LEIAR25.R3 LEIT, TRM55971.00 NONE and Trimble GA530) remained at a stable level which is similar to the results from Table 6.

The results from all conducted evaluations are summarized in the boxplots shown in Figure 7. The boxplots show the differences between the IGS ZTD reference data and the obtained goGPS PPP ZTD estimations. The boxplots on the left depict the results using the original L1 + L2 RINEX data with the original IGS ANTEX, without supplying ANTEX corrections and using only satellite PCO and PCV. One should note that these graphs in the left box illustrate the ZTD results with the original

recorded dual-frequency data and different IGS ANTEX constellations. It is evident that supplying the original IGS ANTEX corrections resulted in the smallest errors, but these are not available for the low cost antennas. It also shows that applying no ANTEX corrections to the data yield high standard deviations and offsets in the ZTD estimations. Supplying only satellite PCO/PCV corrections provide reasonable results for the antennas TRM55971.00 NONE and GA530. The antennas LEIAR25.R3 LEIT, AV28 (circular) and ANN-MB-00 (circular) are at a comparable level. Although the standard deviation decreased for the data from the ANN-MB-00 and AV28 antennas, a significant bias was still present for the rectangular ground plate datasets.

**Figure 7.** Boxplots of goGPS PPP ZTD estimations against IGS reference data from DLF1. The figure on the left depicts the combinations with the original dual-frequency RINEX data using the original IGS ANTEX file, supplying no ANTEX file at all, or only satellite PCO/PCV corrections. The boxplots on the right illustrate the references (original ANTEX, only satellite PCO/PCV), the results with original RINEX L1 + L2 and modified ANTEX entries (elevation modified ANTEX, azimuth-elevation modified ANTEX) and SEID results with original, elevation corrected and azimuth-elevation corrected L1 data.

The boxplots on the right in Figure 7 depict the reference data (original ANTEX and satellite PCO/PCV) and summarize the results for the corrected datasets (the first four groups) and applied SEID experiments (the rightmost three groups). One can note that the corrected data illustrate values in the range between −30 and 20 mm compared to −70 and 45 mm when not using the IGS ANTEX file. The presented results demonstrate a similar performance of all antennas with their most significant differences being the biases. Notably, the biases of the rectangular and circular mounting types are of opposite sign using only satellite PCO/PCV corrections. After correction, the biases are significantly reduced for the LEIAR25.R3 LEIT antenna as well as for the low-cost antennas AV28 and ANN-MB-00 using both, rectangular bracket and circular plane. It also shows that results for the single-frequency SEID experiments are comparable to the dual-frequency results.
