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Satellite altimetry for inland waters is inherently intricate: the technique was mainly developed to monitor open waters, meaning that radar pulses would interact with lakes and rivers over relatively big and non-representative footprints that, for example, also include surrounding land. While the more recent missions offer the possibility of unfocussed and focused SAR data processing, height retrieval for inland waters still suffers from a range of limitations, ambiguities and challenges – e.g., the size and disposition of the effective footprint, ambiguities in across-track reflections, reliability of the applied atmospheric and geophysical corrections, and establishing multi-mission altimetry products. It is therefore very important to have clear answers to two primary questions: 1) What is the overall uncertainty associated with water level products from satellite altimetry, and 2) What are the major contributors to this overall uncertainty? Application-oriented studies provide answers to the first question by comparing results against available in situ datasets. A more dependable answer is however given by calibration/validation studies and via collecting in situ measurements which meet the requirements for Fiducial Reference Measurements (FRMs). According to ESA, ‘FRMs are a suite of independent, fully characterized, and traceable sub-orbital measurements that follow the guidelines outlined by the GEO/CEOS Quality Assurance framework for Earth Observation (QA4EO) and have value for space-based observations.’ St3TART is one of the current ESA projects which provides roadmaps for establishing FRMs for validating inland altimetry products. The second question however has to do with the satellite data processing. We need to quantify any source of uncertainty in deriving altimetric water level and propagate them through to a specific measurand to be compared to FRMs. This is where the concepts of Fundamental Data Records (FDRs) and Thematic Data Products (TDPs) are required. FDRs/TDPs are physical/geophysical variables of sufficient duration and known uncertainty for Earth observation satellites. Another ESA-funded project FDR4ALT considers developing a set of FDR for various altimetry applications including inland altimetry. One of the goals in this project is to define the uncertainty budget for satellite altimetry products to have one uncertainty value associated with each measurement. Having been involved in both St3TART and FRD4ALT, we would like to argue that studies on FDRs/TDPs and FRMs are naturally intertwined. Through such an acknowledgement not only can we eliminate a major source of uncertainty in our understanding of uncertainties, but we can also design more effective calibration/validation scenarios. Moreover, we would like to elaborate on a fully metrological approach on how to derive uncertainties for altimetric inland water level (TDP) and in situ measurements (FRM) and compare them against one another.
Topic : Theme 1: Oceans and Hydrology.
Reference : T1-B18
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