Development and application of GNSS remote sensing techniques for Earth Observation

无线电掩星（RO）技术使用天基低地球轨道（LEO）卫星接收器接收GPS/GNSS信号，并对地球大气层和电离层进行临边探测。由于由六颗微型LEO卫星组成的FormoSat-3/ Constellation Observing System for Meteorology，Ionosphere and Climate（FS 3/COSMIC）的成功，美国和台湾的联合RO团队，分别由美国国家海洋和大气管理局（NOAA）和台湾国家空间组织（NSPO）领导，决定推进COSMIC后续使命（称为FS 7/COSMIC 2.名为Tri-G GNSS无线电掩星系统（TGRS）的GNSS RO有效载荷将接收多通道GPS，GLONASS和Galileo卫星信号，并能够在低倾角和高倾角星座完全部署后每天跟踪10，000多个RO观测。更密集的RO闪烁观测将被用于精确地构造和模拟地球大气层和电离层，此外，从地球表面反射的特殊类型的GNSS多路径延迟可以用于感测地球表面环境，例如海洋高度测量和海况。这就需要设计和开发适当的接收器，以便实时跟踪和处理反射和散射的全球定位系统/全球导航卫星系统信号，避免储存大量原始数据。我们还提出了将现场可编程门阵列（FPGA）应用于GPS/GNSS反射仪中，以实现高同步性和最大限度地利用现有硬件资源。使用Simulink/Matlab，FPGA还可以通过相关基带信号的同相和正交分量来实时计算复杂的延迟-多普勒映射（DDM）数据。实时高分辨率DDM反射计可以在GPS L1和L2频率下实现。这项研究将探讨全球导航卫星系统在大气层、电离层、海洋测高和海况方面的新目标和遥感结果，以及未来FS 7/COSMIC 2使命的新机会。该项目将在柏林工业大学大地测量和地球信息科学研究所完成（IGG TUBerlin）与德国地理研究中心（GFZ）波茨坦和国立中央大学台湾GPS科学与应用研究中心（GPSARC）的科学家密切合作。（1）利用全球定位系统/全球导航卫星系统反渗透大气数据，并为低对流层和气候调查开发先进的算法，（2）反演和监测偶发的E（Es）层、振荡和相关效应（包括垂直耦合）;（3）开发基于FPGA的实时GPS/GNSS反射计，用于海洋测高和海况观测。

The radio occultation (RO) technique uses a space-based Low Earth Orbit (LEO) satellite receiver to receive GPS/GNSS signals and perform limb sounding on the Earth atmosphere and ionosphere. Due to the success of FormoSat-3/ Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) consisting of six micro-LEO satellites, the joint U.S. and Taiwan RO team, lead separately by the National Oceanic and Atmospheric Administration (NOAA) and Taiwan National Space Organization (NSPO), have decided to move forward with a COSMIC follow-on mission (called FS7/COSMIC2. The GNSS RO payload, named Tri-G GNSS Radio-occultation System (TGRS) will receive multi-channel GPS, GLONASS, and Galileo satellite signals and will be capable of tracking more than 10,000 RO observations per day after both low- and high-inclination constellations are fully deployed. It is expected that denser RO scintillation observations will be used to accurately structure and model the Earth atmosphere and ionosphere.In addition, the special kind of GNSS multi-path delay reflected from the Earth surface could be used to sense the Earth surface environments such as ocean altimetry and sea state. This has brought out the need of designing and developing appropriate receivers in order to track and process reflected and scattered GPS/GNSS signals in real-time to avoid the storage of huge volumes of raw data. We also propose to apply the Field Programmable Gate Array (FPGA) to the GPS/GNSS Reflectometer Instrument achieving a high synchronism and a most benefit of the available hardware resources. Using Simulink/Matlab, the FPGA can also compute complex Delay-Doppler Map (DDM) data in real-time by correlating the in-phase and quadrature components of the baseband signals. A real-time high resolution DDM reflectometer can be implemented at both GPS L1 and L2 frequency. This study will address new objectives and results of GNSS remote sensing in the atmosphere, ionosphere, ocean altimetry and sea state as well as new opportunities for the future FS7/COSMIC2 mission. The project will be accomplished at Institute of Geodesy and Geoinformation Science of Technische Universität Berlin (IGG TUBerlin) in close connection with scientists at Deutsches GeoForschungs Zentrum (GFZ) Potsdam and Taiwan GPS Science and Application Research Center (GPSARC) at National Central University (NCU).Objectives of the project can be summarized as:(1) using GPS/GNSS RO atmosphere data and developing advanced algorithms for the lower troposphere and climatological investigations,(2) retrieving and monitoring sporadic E (Es) layer, scintillations and related effects including vertical couplings, and(3) developing real-time FPGA based GPS/GNSS reflectometer for applications on ocean altimetry and sea state observations.