استفاده از روش نوین «بازیابی اکسترمم» جهت اصلاح داده‌های ماهواره‌های ارتفاع‌سنجی، منطقه مطالعاتی: خلیج فارس

استفاده از داده‌های ماهواره‌های ارتفاع‌سنجی امروزه در تعیین نوسانات سطح دریاها، به‌خصوص آب‌های بسته به دلیل پوشش بالا، در دسترس بودن و دقت داده‌ها افزایش یافته است. با وجود این، در برخی نواحی نظیر آب‌های بسته، دریاچه‌ها، نواحی یخ‌زده، به‌خصوص خطوط ساحلی از دقت این داده‌ها کاسته می‌شود. مقابله با این کاهش دقت هدف این تحقیق است که از طریق مطالعه و اصلاح موج‌های بازگشتی از سطح آب صورت می‌پذیرد. داده‌های ارتفاع‌سنجی ماهواره‌های topex/poesidon  و jason1 در مناطق ساحلی و کم‌عمق خلیج فارس پرنوسان و نویزی‌ است، لذا نیازمند اصلاح است. چهار روش مختلف برای اصلاح موج‌های دریافتی ماهواره‌ها استفاده شده است، شامل سه روش موجود تکنیک مرکز ثقل، حد آستانه، روش مبتنی بر تکرار کمترین مربعات برای انطباق موج و روش جدید که براساس بازیابی اکسترمم‌ها عمل می‌کند. سه بخش اصلی این مقاله شامل حذف موج‌های نویزی در دو مرحله، طبقه‌بندی موج‌ها و بازسازی و اصلاح آن‌ها، در نهایت استخراج ارتفاع صحیح سطح دریاست. در نهایت، برای بررسی دقت عملکرد میزان خطا و نویز با استفاده از داده‌های تاید گیج، همچنین ژئوئید محاسبه می‌شوند. این مقایسه بیانگر بهبود تا 0/3092 متر در خطا و 0/4573 متر در نویز اطلاعات ارتفاعی با استفاده از الگوریتم جدید است.

Using new approach ‘ExtR method’ to retrack satellite radar altimetry; case study: Persian Gulf

Monitoring of water levels within the seas and oceans has been enhanced by application of satellite radar altimetry missions, compared to the traditional insitu 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 hydrologic 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 with great potential for 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 outputs of this procedure are the altimeter range. Due to the effect of topography and heterogeneity of reflecting surface and atmospheric propagation, the expected waveform for altimeter returns over land differs from that over ocean surfaces and subsequently range is not 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 water body and include missing data. We have developed a waterdetection algorithm based on statistical analysis of decadal TOPEX/POSEIDON and JASON1 height measurement time series and also their ground passes 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 include Outlier detection and Elimination of Unwanted Waveforms, 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 almost refer to 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 get to work, first consider the SSH time series before and after retracking then compare those with in situ data, second retrack the ground pass track lines data of two satellites and compare with geoid data.