A method for monitoring aircraft height above ground across diverse landforms using GNSS reflected signals: a field study

Abstract Altitude above Ground Level (AGL) is an important parameter in aircraft flight operations and safety. Conventional aircraft altitude measurement techniques are largely constrained by their dependence on active signal transmission, sensitivity to environmental conditions, limited operational coverage, and inherently localized single-point measurement geometry. This paper presents a method that leverages Global Navigation Satellite System-Reflectometry (GNSS-R) to acquire AGL height, providing a complementary solution to current measurement systems. The proposed system employs an upward-Right-Hand Circular Polarization (RHCP) antenna and a downward-Left-Hand Circular Polarization (LHCP) antenna together with an Intermediate Frequency (IF) collector for raw data collection. The collected data are subsequently processed offline using a Software Defined Receiver (SDR). This paper proposes the Short‑Time Shortest‑Path (STSP) method to retrieve high‑accuracy GNSS‑R AGL, which uses only 25 incoherent accumulations and selects the shortest path delay when the reflected waveform exhibits multiple peaks. For a feasibility assessment, we installed the system on an Aurora SA60L aircraft and conducted an approximately 288-km flight from Dongying City to Tai’an City of Shandong Province, China. Dual-frequency Global Navigation Satellite System (GNSS) IF data on the shared frequency bands were collected during the flight over diverse terrains, including rural, urban, suburban, and mountainous areas. BeiDou Navigation Satellite System (BDS) B1C/B2a, Global Positioning System (GPS)/Quasi-Zenith Satellite System (QZSS) L1 C/A/L5, and Galileo E1C/E5a signals were leveraged to compute the code- level path -delay differences between direct and reflected signals. The measurements were used to retrieve the aircraft's heights above multiple distinct specular reflection points. The results demonstrate the capability of GNSS-R technology to obtain reliable AGL height estimates under various surface conditions. The GNSS-R derived AGL solutions were compared with the GNSS-Digital Elevation Model (DEM) for accuracy assessment. The results show that the proposed method achieves a height estimation accuracy of better than 4 m in flat terrain and about 15 m in mountainous areas.