*2.4. Evaluation of ZTD Estimates from Dual-Frequency PPP*

This subsection describes the PPP processing configuration and ZTD evaluation procedure with dual-frequency GNSS measurements.

To estimate absolute ZTD values and to evaluate the newly obtained antenna PCVs, the open source tool goGPS [29] was used. Its ZTD estimations are comparable to existing reference datasets [6,7]. The ZTD results are computed for all available DOYs and compared to IGS final ZTD estimations from the IGS station DLF1 [24] in approximately 10 m distance. The closest available data point from the high rate ZTD estimations was used to match the IGS reference (5-min) interval.

We used goGPS PPP with the same configuration for all runs but with different ANTEX file configurations for the different antennas. The goGPS configuration used for our comparisons can be found in Table 3. The concept of the ZTD evaluation is summarized in Figure 3.

For all goGPS ZTD estimations, data from GPS-only satellites were used. An elevation cutoff angle of 10 degrees and ocean loading effects obtained from the FES2004 model [30] were applied. The IGS antenna calibration, final orbits, 30-s satellite clocks and earth rotation products were used [31].

The ZTD analysis consisted of up to five different cases. Two of our tested rover antennas, the LEIAR25.R3 LEIT and TRM55971.00 NONE, had already calibration entries in the IGS ANTEX file. This provided another set of PCVs for testing with PPP and serves as a verification of our estimations. The first case consists of the verification antennas (LEIAR25.R3 LEIT and TRM55971.00 NONE antennas). For this, a goGPS PPP run is performed using the original IGS I14.ATX ANTEX file. To investigate the impact of not using an ANTEX file at all, another run without the ANTEX file was performed in a second run. The third case applies only satellite PCO/PCV corrections by removing the receiver antenna-specific entries. The last two cases evaluate the elevation-only and azimuth-elevation corrected ANTEX entries obtained from the calibration step. One has to note that the full observation period (up to ten days observed data) is used as RINEX input for the ZTD estimation, while only two or three days are utilized to compute the PCVs.


**Table 3.** goGPS settings for Precise Point Positioning (PPP) ZTD estimation.

**Figure 3.** PPP ZTD evaluation concept. Original dual-frequency RINEX files are utilized as file input. Several runs with different antennas and ANTEX configurations are tested. The absolute ZTD estimations are evaluated against International GNSS Service (IGS) final ZTD estimations from DLF1.

If one runs PPP software, for example, goGPS, over which one has full control, one can use the newly generated ANTEX entries directly in the software. However, when PPP is performed in a server environment, one does not have the possibility to modify the ANTEX files. In these cases it is more convenient to correct the original RINEX phase data for the antenna patterns. To achieve this, we corrected the original RINEX phase data for the PCV estimations retrieved from the calibration step by the following formula:

$$
\tilde{L}\_i = L\_i - \frac{q\_j^r(el, az)}{\lambda\_i},
\tag{2}
$$

where *L*˜*<sup>i</sup>* is the corrected phase data on frequency *i* and *Li* the original RINEX phase observation. *ϕr <sup>i</sup>*(*el*, *az*) is the azimuth and elevation dependent antenna phase pattern on frequency *i*. *λ<sup>i</sup>* denotes the wavelength on each frequency. A comparison between ANTEX-corrected and RINEX-corrected PPP ZTD comparisons resulted in no or negligible differences.
