Wang Yang, Yang Rong, Morton Yu T
Ann & H.J. Smead Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, CO, USA.
Shanghai Jiaotong University, China.
IEEE Trans Aerosp Electron Syst. 2020 Oct;56(5):3384-3393. doi: 10.1109/taes.2020.2972248. Epub 2020 Feb 7.
GNSS radio occultation (RO) signals have been demonstrated as a viable means to retrieve atmospheric profiles. Current GNSS-RO observations rely on open-loop (OL) processing of the signals, especially for signals propagating through the lower troposphere. The reason is that GNSS signals at low elevations are adversely affected by multipath effects due to propagation through lower troposphere structures and reflections and scattering from the Earth surface. The low-elevation RO signals are characterized by deep and fast amplitude fading and rapid signal carrier phase fluctuations, collectively referred to as signal scintillation. The conventional phase-lock loop (PLL) may lose lock of these signals. While OL tracking is known for its robustness, its accuracy is determined by the climatological models used to create the reference for the GNSS signal carrier tracking loop. The wide bandwidth typically associated with OL tracking also introduces large errors in signal parameters estimations. In this paper, we present an adaptive Kalman filter-based closed-loop (KFC) tracking method, which takes into consideration the tropospheric scintillation, platform vibration, and real-time C/N estimation of the RO signals. The KFC method has comparable robustness with and improved accuracy over the OL tracking, which are demonstrated through comparison using real GPS RO data collected on an airborne platform. Analysis of the excess Doppler estimation, retrieved bending angles and impact parameters also confirms the improved performances of the proposed algorithm over OL tracking.
全球导航卫星系统无线电掩星（GNSS-RO）信号已被证明是获取大气廓线的一种可行手段。当前的GNSS-RO观测依赖于信号的开环（OL）处理，特别是对于穿过低层对流层传播的信号。原因是低仰角的GNSS信号在穿过低层对流层结构以及从地球表面反射和散射时，会受到多径效应的不利影响。低仰角RO信号的特征是幅度深度快速衰落以及信号载波相位快速波动，统称为信号闪烁。传统的锁相环（PLL）可能会失去对这些信号的锁定。虽然OL跟踪以其鲁棒性而闻名，但其精度取决于用于为GNSS信号载波跟踪环创建参考的气候模型。通常与OL跟踪相关的宽带宽也会在信号参数估计中引入较大误差。在本文中,我们提出了一种基于自适应卡尔曼滤波器的闭环（KFC）跟踪方法，该方法考虑了对流层闪烁、平台振动以及RO信号的实时载噪比（C/N）估计。通过使用在机载平台上收集的真实GPS RO数据进行比较，证明了KFC方法与OL跟踪相比具有相当的鲁棒性且精度更高。对多余多普勒估计、反演的弯曲角和碰撞参数的分析也证实了所提算法相对于OL跟踪具有更好的性能。
