GLORI: A GNSS-R Dual Polarization Airborne Instrument for Land Surface Monitoring

Abstract

Global Navigation Satellite System-Reflectometry (GNSS-R) has emerged as a remote sensing tool, which is complementary to traditional monostatic radars, for the retrieval of geophysical parameters related to surface properties. In the present paper, we describe a new polarimetric GNSS-R system, referred to as the GLObal navigation satellite system Reflectometry Instrument (GLORI), dedicated to the study of land surfaces (soil moisture, vegetation water content, forest biomass) and inland water bodies. This system was installed as a permanent payload on a French ATR42 research aircraft, from which simultaneous measurements can be carried out using other instruments, when required. Following initial laboratory qualifications, two airborne campaigns involving nine flights were performed in 2014 and 2015 in the Southwest of France, over various types of land cover, including agricultural fields and forests. Some of these flights were made concurrently with in situ ground truth campaigns. Various preliminary applications for the characterisation of agricultural and forest areas are presented. Initial analysis of the data shows that the performance of the GLORI instrument is well within specifications, with a cross-polarization isolation better than −15 dB at all elevations above 45°, a relative polarimetric calibration accuracy better than 0.5 dB, and an apparent reflectivity sensitivity better than −30 dB, thus demonstrating its strong potential for the retrieval of land surface characteristics.

Potential Applications of GNSS-R Observations over Agricultural Areas: Results from the GLORI Airborne Campaign

“…In the present analysis, the potential use of three different GNSS-R observables (apparent reflectivity (Γ RL ), reflected signal (SNR), and the reflectivity polarization ratio) for the monitoring of the soil and vegetation is discussed, which was not possible in the context of previous campaigns. A large ground-truth database, including data corresponding to a wide range of soil moisture values and various types of vegetation cover, was collected during the GLORI'2015 campaign to confirm several important results retrieved from smaller databases, such as those reported in [30].…”

“…The airborne GNSS-R data presented in this study were recorded with the GLORI instrument during the GLORI'2015 Campaign [30]. The GNSS-R GLORI instrument, developed by the CESBIO, is a polarimetric device with four channels.…”

“…The cross-polarization isolation is better than 15 dB up to 45 • from boresight (45 • elevation) for both LHCP and RHCP ports. Below 15 dB of cross-polarization, the performance is considered insufficient to perform polarimetric measurements [30].…”

“…Three synchronized streams of raw data, centered on the GPS L1 frequency (1.57 GHz), were continuously acquired at 10 MSPS: one from the zenith-pointing RHCP antenna and two from the nadir-pointing antenna, in the RHCP and LHCP polarizations. The data were then processed offline to extract the GPS L1 signal waveforms, with a coherent integration time of 20 ms. After correcting for the antenna radiation pattern and receiver noise [30,31], the individual waveform maxima were averaged incoherently for 240 ms, allowing the GNSS-R observables to be computed.…”

Low-Cost GPS Receivers for the Monitoring of Sunflower Cover Dynamics

“…In the specific case of vegetation measurements, several GNSS-R techniques have demonstrated their potential: a single antenna approach with dedicated linearly polarized receivers [10] or commercial geodetic receivers [11]. A dual antenna approach can also be used [12,13].…”