Akurasi estimasi tinggi muka laut (SSH) dari satelit altimeter sangat dipengaruhi oleh kondisi perairan dan daratan disekitar perairan tersebut. Estimasi SSH di laut lepas umumnya sudah akurat. Namun, pada daerah pantai, estimasi SSH kurang akurat karena gangguan pantulan sinyal dari daratan. Penelitian ini bertujuan untuk melakukan analisis retracking waveform satelit altimeter pada perairan yang kompleks di Laut Halmahera. Data yang digunakan pada penelitian ini yaitu data waveform dari Sensor Geophysical Data Record type D (SGDR-D) Jason-2 dan Jason-3 tahun 2017. Algoritma retracking yang digunakan yaitu Offset Centre of Gravity (OCOG), Iced, Threshold, dan Improved Threshold. Hasil retracking waveform menunjukkan semua retracker memberikan perbaikan data SSH yang signifikan kecuali OCOG. Retracker yang paling cocok diaplikasikan di Laut Halmahera pada teluk dangkal dan sempit yaitu Threshold 10%, pada teluk dalam dan lebar yaitu Threshold 50%, serta pada perairan dekat pulau pulau kecil yaitu Threshold 10% dan Threshold 20%. Secara umum, Non-Brown waveform lebih banyak ditemukan di perairan teluk dangkal dan sempit (rata-rata=63,49%) dibandingkan dengan teluk dalam dan lebar (rata-rata=11,51%) dan perairan pulau-pulau kecil (rata-rata=9,57%). Namun demikian, tingkat perbaikan data SSH di perairan teluk dangkal dan sempit lebih tinggi dibandingkan dengan teluk dalam dan lebar serta perairan pulau-pulau kecil dan laut dalam. Persentase peningkatan perbaikan data (IMP) tertinggi yaitu 96,71% dengan algoritma Improved Threshold 10% pada Jason-2 pass 164.
The accuracy of sea surface heights (SSHs) estimation from satellite altimeters is strongly influenced by the microwave reflected signals (or waveforms). Waveforms in open oceans generally have ideal shapes following the Brown (1977) model. However, in coastal and shallow waters, the signals are disturbed by lands, thus resulting in complicated waveforms (non-Brown). Non-Brown waveforms produce inaccurate SSH estimations; therefore, specialized protocols such as waveform classification and retracking are crucial when attempting to produce accurate estimations. In this study, waveforms of Jason-2 and Jason-3 satellite altimeters in the Halmahera were classified and retracked using several algorithms, such as Offset Centre of Gravity (OCOG), Ice, Threshold, and Improved Threshold. The results showed that waveforms in the Halmahera Sea had ten generic classes with dominant class of the Browns. The validation results showed that all retrackers (except OCOG) had the value of correlations exceeding 0.75, and Root Mean Square Error (RMSE) smaller than 25 cm at a distance of 5-20 km from the land. The Threshold 10% was the most common retracker that appeared with the highest improvement percentage (IMP), meanwhile the Ice retracker consistently produced the best correlation (0.86) and the lowest RMSE (16cm). The retracking results showed that waveform retracking generally can improve SSH estimation accuracy from ocean (standard) retracker.
This study examines the accuracy of the sea surface height anomaly (SSHA) altimetry data products of Copernicus, Colorado University (CU), and X-TRACK-Centre for Topographic studies of the Ocean and Hydrosphere (X-TRACk-CTOH). The SSHA derived from altimetry accuracy was tested by comparison with tide gauge (TG) observations. Taking measurements along the IMC coast demonstrates the excellent agreement between the SSHA derived from altimetry and the TG observations, with an average root mean square deviation (RMSD) as low as 10 cm and a strong correlation. The study’s findings revealed that the Copernicus data products could be used to monitor sea-level variability and trends in the IMC accurately. The 25-year time series data from SSHA demonstrated that the sea-level trend in the IMC is higher than the global trend.
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