Retracking satellite radar altimetry using a new approach 'ExtR method'; Case study: Persian Gulf

Monitoring of the water levels within the seas and oceans has been enhanced by application of satellite radar altimetry missions, compared to the traditional in-situ tide gauge measurements, due to their vast coverage and better spatial resolution. Satellite radar altimetry, which is originally designed to measure global ocean surface height, has been applied to inland surface water hydrological studies. Satellite radar altimetry, well known as TOPEX/POSEIDON, JASON, ENVISAT, which have been originally designed to measure global ocean surface height, nowadays, also demonstrated a great potential for the applications of inland water body studies. Altimetry was designed to determine the sea surface height based on spatial technology, electronic technology and microwave technology and basically work with sending and receiving electromagnetic pulse. Waveform is actually a curve, which shows the power of mentioned pulse reflected back to the altimeter. Altimeter on board of the satellite measures the range by sending and receiving a short pulse and calculating its travel time. The most important output of this procedure is the altimeter range. Due to the effect of topography and heterogeneity of reflecting surface and atmospheric effects, the expected waveform for altimeter returns over land differs from that over the ocean surfaces and subsequently the range is not as accurate. As a result, sea surface height values derived from altimetry over ice sheets and inland water bodies (particularly close to the coast lines) represent more errors in compared to the waveforms returned from other part of the ocean surface and may include missing data. We have developed a water-detection algorithm based on statistical analysis of decadal TOPEX/POSEIDON and JASON-1 height measurement time series and also their ground passes of the sea surface height in Persian Gulf. The Persian Gulf is certainly one of the most vital bodies of water on the planet; as gas and oil from Middle Eastern countries flow through it, supplying much of the world's energy needs.This algorithm contains a noise elimination process that includes Outlier Detection and Elimination of Unwanted Waveforms (ODEUW), an unsupervised classification of the satellite waveforms, and finally a retracking procedure. An unsupervised classification algorithm is implemented to classify the waveforms into consistent groups for which the appropriate retracking algorithms are performed. On the other hand the waveforms belong to the same group which refer to almost the land with common properties. The waveform retracking method is mainly used to calculate the offset between the practical middle point of waveform leading edge and the designed gate, based on which the retracked distance correction can be computed. Four different methods are implemented for retracking the waveforms. This includes the three previously introduced algorithms, including off center of gravity, threshold retracking and optimized iterative least squares fitting, after some improvements. We also introduce a new method based on edge detection and extracting extremum point, which is called 'ExtR retracking method'. At the end two different methods for validation of our results are examined, first consider the SSH time series before and after retracking then compare those with in situ data, the second, retrack the ground pass track lines data from two satellites and compare them with the geoid data.