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

微波频率GNSS信号的反射（全球定位系统、伽利略系统）在地球外的数据可以由低地球轨道上的卫星收集，并用作雷达源，以感知土壤湿度、洪水、永久冻土冻/融状态、冰和生物量;更精确的信息可以通过使用信号处理来获得，该信号处理寻找保持其相位相干性的反射信号。土壤湿度是基本气候变量（ECV）这与水文、天气、农业和气候变化密切相关，有可能改进从空间进行的测量。全球导航卫星系统反射仪已被证明是一种新的地球观测技术，萨里的全球导航卫星系统反射仪仪器已被用于TechDemoSat-1、DoT-1和美国航天局CYGNSS，以测量海洋风，还可以检索陆地和冰上的反射。SSTL在欧空局Scout HydroGNSS使命概念中领导着一个联合体。在亚马逊河上空检索到的GNSS反射显示了突出显示雨林树冠下河流的独特属性。全球导航卫星系统反射的分辨率假定在海洋上空约为25公里，但当有平坦表面时，反射变得连贯，分辨率接近约500米的菲涅尔区。改进后的分辨率可以帮助绘制丛林河流溢出（甲烷的重要来源）和海冰边缘的地图。相干反射仪可以改善冻土层的冻融监测，并为使用全球导航卫星系统的测高和具有反射表面的物体的目标检测打开大门。目前仪器上使用的处理方案假设信号不是相干的，而是不相干的，并且没有收集载波相位等关键信息。这个博士研究生将研究从GPS和更宽带宽的伽利略信号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，并有可能在DoT-1和HydroGNSS卫星任务上参与和实施算法。挑战包括：开环捕获相干信号，测量的辐射校正，考虑到噪声，以及根据现场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的实际实施。

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.