*2.5. Evaluation of ZTD Estimates from Single-Frequency PPP*

This subsection illustrates the single-frequency ZTD evaluation using the Satellite-specific Epoch-differenced Ionospheric Delay model (SEID) from a surrounding dual-frequency network. PPP uses the ionosphere-free linear combination to eliminate the first order effect of the ionospheric delay error. The drawback of this method is that the errors and multipath on L1 is increased by a factor 2.546, and on L2 by a factor 1.546. The effect is that, when the errors are not correlated, the noise of the ionosphere free linear combination is increased by a factor 3. Also for antennas that are sensitive to multipath, or are placed in a multipath-prone environment, this effect may cause the performance to decrease significantly. In these cases the performance of using L1-only measurements may outperform the dual-frequency ionosphere-free linear combination results (see also Reference [32]).

Instead, the SEID algorithm [33], combined with L1 data, generates a synthesized L2 measurement by utilizing data from a network of existing dual-frequency receivers. The synthesized signal contains in principle the same information as L1 but is adjusted to the ionospheric delay on that frequency based on the dual-frequency measurements from the surrounding network. This effectively reduces the noise effect. As part of the evaluation, we apply the SEID model on L1-data using a network of three receivers. By removing the second frequency observations from the RINEX data and correcting the L1 data for elevation and azimuth-elevation patterns, we also investigate the performance of the

SEID algorithm on L1-corrected data. Though stations in the close proximity to DLF1 are available, the outside station network distances are chosen between 55 and 130 km to simulate a more realistic scenario to densify existing networks. The SEID evaluation process is depicted in Figure 4.

**Figure 4.** Satellite-specific Epoch-differenced Ionospheric Delay model (SEID) PPP methods. The original (dual-frequency) RINEX data is trimmed to L1-only observations and the RINEX correction after Equation (2) is utilized. SEID is applied with the stations APEL, IJMU and DENT. The subsequent PPP process is conducted once with the original I14.ATX file and once without the LEIAR25 and TRM55971.00 entries, hence providing satellite Phase Center Offset (PCO)/Phase Center Variation (PCV) corrections only. The resulting absolute ZTD estimations are evaluated against final IGS ZTD estimations from DLF1.

The following section describes the results of the experiment by analyzing the satellite phase residuals and ZTD estimations.
