**3.2. The minimum distance of the SARAL/AltiKa retracked SLAs to the coastline**

The quality of the AltiKa retracked SLAs over the coastal region is further investigated by computing the minimum distance of the available data to the coastline. **Figure 10** shows an example of the minimum distance of retracked AltiKa data computed from several passes over the Southeast Asia region. In general, the AltiKa SLAs data becomes available within 2 km from the coastline.

**Figure 11** shows the mean minimum distance of the retracked AltiKa data computed over the five experimental regions. The total number of satellite passes included in the calculation is similar to the number of passes utilized in the computation of data availability. As shown in **Figure 11**, the AltiKa data over the South China Sea region have the lowest mean minimum distance compared to the other regions. The data are available within a distance of ≤3.2 km from the coastline. The Sulu Sea shows the opposite result, with the highest mean minimum distance ≥4 km from the coastline.

The finer spatial resolution with ~174 m along-track sampling in the Ka-band AltiKa (compared to the Jason-2 with ~300 m along-track sampling) enables high-density coastal observations, thus bringing the AltiKa measurements closer to the coastline. Based on previous research [21], this has confirmed that the AltiKa provides a significant improvement in accuracy and data availability up to ~3 km from the coastline. This is overwhelmingly better than the Jason-2 and Envisat which generally provide data beyond ~7–10 km from the coastline [18, 41, 42].

The overall inspection shows that the Ice-1 retracker performs better than other retrackers within a minimum distance of ≤4 km from the coastline, except for in the Gulf of Thailand. Over this region, the Ice-2 retracker is better than the other retrackers. The Ice-1 retracker is based on the OCOG, which is well-adapted to the rapidly changing surface like the continental sea shelf [8]. The MLE-4 retracker underperforms when compared to the other retrackers, with a mean minimum distance of ≥3.2 km from the coastline. The MLE-4 retracker is the standard ocean retracker, which relies on the waveform's physical shape to fit the waveform to the theoretical Brown-shape [8, 12]. The Ice-2 retracker underperforms when compared to Ice-1 for most of the regions. However, the Ice-2 is better than the MLE-4 retracker. This is because the Ice-2 retracker is an adaptation of Brown's model, with slight modifications, and was purposely developed for continental ice regions, making it more adaptable for coastal waveforms.

**Figure 11.** Mean of minimum distance of AltiKa for MLE-4, Ice-1 and Ice-2 retracked SLAs to the coastline over five

Validation and Quality Assessment of Sea Levels from SARAL/AltiKa Satellite Altimetry over the…

http://dx.doi.org/10.5772/intechopen.74399

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To validate the AltiKa retracked SLA, the temporal correlation and RMS error between the SLAs from the retracking algorithms and tide gauges are calculated. In this study, the mean value of temporal correlation and RMS errors are computed for distances ≤5 km from the coastline. **Figure 12** shows the result for retracked SLAs from the MLE-4 Ice-1 and Ice-2 cor-

For the validation near Bintulu tide gauge, the MLE-4 retracker has better performance than those of Ice-1 and Ice2 retrackers with a correlation of 0.53 and RMS error of 8 cm. The performance of Ice-1 and Ice-2 retrackers is significantly low with correlation ≤4 and RMS error ≥13 cm. Near Geting station, the performance of three retrackers are nearly similar, with

**4. Validation against tide gauge**

experimental regions, computed from 19 cycles.

responding to the six tide gauge stations.

**Figure 10.** Mean of minimum distance of AltiKa MLE-4, Ice-1, and Ice-2 retracked SLAs to the coastline for several passes over the Southeast Asia coastal regions, computed from 19 cycles.

**Figure 11.** Mean of minimum distance of AltiKa for MLE-4, Ice-1 and Ice-2 retracked SLAs to the coastline over five experimental regions, computed from 19 cycles.

The overall inspection shows that the Ice-1 retracker performs better than other retrackers within a minimum distance of ≤4 km from the coastline, except for in the Gulf of Thailand. Over this region, the Ice-2 retracker is better than the other retrackers. The Ice-1 retracker is based on the OCOG, which is well-adapted to the rapidly changing surface like the continental sea shelf [8]. The MLE-4 retracker underperforms when compared to the other retrackers, with a mean minimum distance of ≥3.2 km from the coastline. The MLE-4 retracker is the standard ocean retracker, which relies on the waveform's physical shape to fit the waveform to the theoretical Brown-shape [8, 12]. The Ice-2 retracker underperforms when compared to Ice-1 for most of the regions. However, the Ice-2 is better than the MLE-4 retracker. This is because the Ice-2 retracker is an adaptation of Brown's model, with slight modifications, and was purposely developed for continental ice regions, making it more adaptable for coastal waveforms.
