**4. Validation against tide gauge**

It is comprehended that the Ice-1 retracker always outperforms the MLE-4 and Ice-2 retrackers, except for the Andaman Sea where the performance of the Ice-1 and Ice-2 retrackers are similar. In contrast, the MLE-4 always underperforms compared to the other retrackers, except in the Andaman Sea where the percentage is slightly superior than those of other two retrackers.

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

**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

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

**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.

Envisat which generally provide data beyond ~7–10 km from the coastline [18, 41, 42].

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

2 km from the coastline.

distance ≥4 km from the coastline.

56 Multi-purposeful Application of Geospatial Data

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 corresponding to the six tide gauge stations.

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

as close as 0.9 km from the coastline. Nevertheless, the amount of data that can be recovered through the retracking algorithm depends on the coastal topography and sea states, in which different coastal characteristics have different impacts on the altimetric signals. Thus, this

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

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

59

The research is supported by Research University Grant Tier 1 (vot 17H59). We would like to acknowledge the Ministry of Higher Education Malaysia for providing research funding. This acknowledgement also goes to the Achieving, Validating, and Interpretation of Satellite Oceanography (AVISO) data team for providing SARAL/AltiKa data, the Department of Survey and Mapping Malaysia (DSSM) and the University of Hawaii Sea Level Centre (UHSLC) for providing tide gauge data. Special thanks to Dr. Angela Maharaj from University

of New South Wales, Australia for her constructive comment about this book chapter.

1 Faculty of Geoinformation and Real Estate, Universiti Teknologi Malaysia, Johor Bahru,

[1] Cipollini P, Snaith H. A short course on altimetry. In: 3rd ESA Advanced Training on Ocean Remote Sensing; 23-27 September. Cork, Ireland; European Space Agency; 2013

[2] Anzenhofer M, Shum CK, Rentsh M, O.S. University. Coastal Altimetry and Application. Vol. 1. Columbus, Ohio: Department of Civil and Environmental Engineering and Geo-

[3] Challenor PG, Read JF, Pollard RT, Tokmakin RT. Measuring surface current in drake passage fron altimetry and hydrography. Journal of Physical Oceanography. 1996;**26**:

[4] Fu LL, Cazenave A, Fu LL, Cazenave A. Satellite Altimetry and Earth Sciences: A Handbook of Techniques and Applications. San Diego, California: Academic Press; 2001

and Nurul Hazrina Idris1,2\*

2 Geoscience and Digital Earth Center, Research Institute for Sustainability and

Environment, Universiti Teknologi Malaysia, Johor Bahru, Malaysia

\*Address all correspondence to: nurulhazrina@utm.my

makes the performance of each retracker differ between different locations.

**Acknowledgements**

**Author details**

Malaysia

**References**

detic Science; 1999

2784-2758

Noor Nabilah Abdullah1

**Figure 12.** The temporal correlation and RMS error for MLE-4, Ice-1, and Ice-2 for distance within 5 km from coastline.

temporal correlation of 0.64 for MLE-4 and Ice-2 retrackers and 0.66 for Ice-1 retracker, and RMS error of 34 cm for all retrackers. Similar situation is shown near the Ko Taphao Noi tide gauge. The performance of three retracker are nearly similar with temporal correlation between 0.87 and 0.89 and RMS error between 8 and 12 cm. Near Vung Tau tide gauge, there is no data available for distance within 5 km from coastline. This is because, within this distance, satellite track crosses over a bay, thus producing complicated waveforms. The data over this area are unable to be retrieved from the three retracking algorithms. Near Langkawi tide gauge, the Ice-1 is the only retracker that can recover data. The temporal correlation and RMS error are 0.63 and 52 cm, respectively. Near Lubang tide gauge, Ice-2 is the optimum retracker based on the highest temporal correlation of 0.72 and RMS error 15 cm. The MLE-4 retracker seems to be the most inferior for this region, with a temporal correlation of 0.27 and an RMS error of 22 cm.
