The Exploration of Spaceborne Coherent GNSS Reflectometry for High Resolution Hydrological and Ice Observation

低地球轨道卫星可以收集微波频率GNSS信号（GPS, Galileo）在地球外的反射，并将其用作雷达源，以感知土壤湿度、洪水、永久冻土冻结/解冻状态、冰和生物量；更精确的信息可以通过使用信号处理来获得，该处理寻找保持相位相干的反射信号。土壤湿度是一个基本气候变量（ECV），与水文、天气、农业和气候变化密切相关，具有改进空间测量的潜力。GNSS反射测量已被证明是一种新的地球观测技术，Surrey公司的GNSS反射测量仪器已在TechDemoSat-1、DoT-1和NASA CYGNSS上用于测量海风，并在陆地和冰面上检索反射。SSTL在ESA Scout HydroGNSS任务概念中领导一个联盟。在亚马逊上空检索到的GNSS反射显示出在雨林树冠下突出显示河流的独特属性。假设GNSS反射的分辨率在海洋上空25公里左右，但当有平坦的表面时，反射变得相干，并且分辨率接近菲涅耳区约500米。提高的分辨率可以帮助绘制丛林河流的河岸过度（甲烷的重要来源）和海冰边缘。相干反射测量可以改善对永久冻土的冻结/解冻监测，并为使用GNSS进行测高以及对具有反射表面的物体进行目标探测打开大门。目前在仪器上使用的处理方案假设信号不是相干的，但是是不相干的，并且没有收集载波相位等关键信息。该博士研究生将研究从GPS和更宽带宽的伽利略信号中收集相干信号的替代处理方案。TechDemoSat-1收集的原始数据可用于测试新的信号处理方案，并且有可能在DoT-1和HydroGNSS卫星任务中参与和实施算法。挑战包括：相干信号的开环捕获，测量的辐射校正，允许噪声，根据原位或其他数据源校准和验证测量，以及在可在轨道上运行的嵌入式信号处理仪器中实际实施。

Reflections of microwave-frequency GNSS signals (GPS, Galileo) off the Earth can be collected by a satellite in low Earth orbit and used as a radar source to sense soil moisture, flooding, permafrost freeze / thaw state, ice and biomass; more precise information can be gained by using signal processing that looks for reflected signals that retain their phase coherency.Soil moisture is an Essential Climate Variable (ECV) closely associated with hydrology, weather, agriculture and climate change, and that has potential for improved measurement from space. GNSS Reflectometry has been demonstrated as a new Earth Observation technique, and Surrey's GNSS-Reflectometry instrument has been used on TechDemoSat-1, DoT-1 and NASA CYGNSS to measure ocean winds, and reflections are also retrieved over land and ice. SSTL leads a consortium in the ESA Scout HydroGNSS mission concept. GNSS Reflections retrieved over the Amazon show the unique property of highlighting rivers underneath the rain forest canopy. The resolution of GNSS reflections is assumed to be around 25 km over the ocean, but when there is a flat surface, the reflections become coherent, and the resolution approaches the Fresnel zone of about 500 metres. The improved resolution could help map jungle river over-banking (an important source of methane), and sea ice edges. Coherent reflectometry may improve freeze / thaw monitoring over permafrost, and open the door for altimetry using GNSS, and for target detection of objects with reflective surfaces.The processing scheme currently used on the instruments assumes that signals are not coherent, but are incoherent, and crucial information such as carrier phase is not being collected. This PhD studentship will investigate alternative processing schemes for collecting coherent signals from GPS and the wider bandwidth Galileo signals. Raw data collected by TechDemoSat-1 can be used to test new signal processing schemes, and there is the potential for involvement and implementation of algorithms on the DoT-1 and HydroGNSS satellite missions. Challenges include: open loop capture of coherent signals, radiometric correction of measurements, allowing for noise, and calibration and validation of measurements against in-situ or other sources of data and practical implementation in an embedded signal processing instrument that can be operated in orbit.