*3.2. Evaluation of ZTD Estimates from Low-Cost Dual-Frequency Receiver and Different Quality Antennas*

Table 5 shows the ZTD bias and RMSE values, computed without supplying an ANTEX file (no ANTEX), applying only satellite PCO and PCV corrections (Only satellite PCO/PCV), using the original IGS ANTEX entries (IGS ANTEX), elevation-only corrected ANTEX entries (El. Corrected) and azimuth-elevation dependent corrected ANTEX entries (Az.-el. Corrected). For the latter two runs, the existing ANTEX entries from the original IGS ANTEX file were removed and replaced with the generated antenna corrections from our experiment. For all evaluations, the IGS final ZTD estimations from the IGS station DLF1 located at approximately 10 m distance served as reference.


**Table 5.** ZTD estimation based on antenna PCV corrections with no ANTEX corrections, only satellite PCO/PCV corrections, original IGS type mean ANTEX, elevation corrected ANTEX and azimuthelevation corrected ANTEX entries on the reference antennas LEIAR25.R3 LEIT and TRM55971.00 NONE. Bias and RMSE refer to the PPP ZTD estimations compared to the IGS ZTD reference.

The PPP runs without supplying an ANTEX file yielded high RMSE of about 15 mm for both antennas and a bias of 7.5 mm for the LEIAR25.R3 LEIT antenna. No significant bias is evident for the TRM55971.00 NONE antenna. Applying only the satellite PCO and PCV corrections, the bias of the LEIAR25.R3 antenna slightly decreased to about 6 mm and the RMSE to about 8 mm. The bias of the TRM55971.00 antenna remained at about the same level while the RMSE decreased drastically to about 4 mm which already agrees well with the expected standard deviation of about 4 mm from the official IGS ZTD final product [3,34]. Both reference antennas performed well using the original IGS ANTEX file resulting in no significant biases. The RMSE is in an expected range of about 4 mm. Replacing the existing ANTEX entries with the generated ANTEX entries resulted in a similar performance to the IGS reference results with RMSE values between 3 and 5 mm. Remarkably, compared to the LEIAR25.R3 antenna, the bias of the TRM55971.00 antenna increased up to about −2.3 mm in the azimuth- elevation corrected results.

The same evaluation is done for the antennas without official ANTEX entries, yet without the IGS receiver antenna corrections that are unavailable for this case. Table 6 shows the results for the ANTEX corrected PPP-based ZTD estimations with the antennas GA530, AV28 and ANN-MB-00.

**Table 6.** ZTD estimation based on antenna corrections with no ANTEX corrections, only satellite PCO/PCV corrections, elevation corrected ANTEX and azimuth-elevation corrected ANTEX entries on the antennas Trimble GA530, Trimble AV28 and U-blox ANN-MB-00. Bias and RMSE refer to the PPP ZTD estimations compared to the IGS ZTD reference.


By supplying no ANTEX file or only correcting for the satellite antenna PCO and PCV, the results in Table 6 depict a generally better ZTD quality for the Trimble GA530 antenna compared to the Trimble AV28 and U-blox ANN-MB-00 antennas. A considerable difference is evident when using a circular ground plane and a rectangular bracket for the latter two. While the bias and RMSE of the data with rectangular brackets remained at a constant high level (>20 mm), employing a circular plane reduced the errors down to about 7 and 10 mm RMSE. Remarkably, applying the elevation-only or azimuth-elevation dependent ANTEX entries for these antennas resulted in a significant improvement for both antennas and mounting types. The offsets are lowered to between −0.47 and −3.91 mm and the ZTD RMSE are between 3.77 and 6.64 mm. Whilst the RMSE of the Trimble GA530 remained at the same level of about 5 mm, the bias increased slightly compared to the satellite-only PCO and PCV corrections.
