Laurent Lestarquit

Detection of Soil Moisture Variations Using GPS and GLONASS SNR Data for Elevation Angles Ranging From 2° to 70°

We propose a Global Navigation Satellite System-Reflectometry (GNSS-R) interference pattern technique method to estimate the temporal variations of the soil moisture content of the ground surrounding a single geodetic antenna. Three parameters can be inverted from GNSS signal-to-noise ratio (SNR) acquisitions: amplitude/phase of the multipath contribution to SNR and effective antenna height. Our method is applied to determine the surface moisture of a bare soil at Lamasquère, France, from February 5 to March 15, 2014. First, only data from low satellite elevation angles (<; 30°) are taken into consideration and are compared with independent 2-cm depth soil moisture records. The combination of the measurements from all GPS satellites, tested for the first time, improves the quality of the results with a correlation coefficient reaching 0.95, with a 10-min sampling rate. Our study shows that it is also possible to take high satellite elevation angles into account, even if the sign of the correlation appears to be reversed w.r.t. data from low satellite elevation angles. The cutoff angle where the sign of the correlation reverses seems to be around 30°. With regard to the effective antenna height, only a very low correlation is observed for high satellite elevation angles. We propose a new inversion method taking the pseudo-dynamic of the surface into account, which increases the correlation from 0.39 to 0.82. By normalizing and inverting the time series obtained from either low or high satellite elevation angles, it is possible to combine them, which enhances the results (correlation = 0.95).

Reflectometry With an Open-Source Software GNSS Receiver: Use Case With Carrier Phase Altimetry

An open-source GNSS software receiver allows to have full access to the signal processing and to make add-ons to the source code in order to obtain the desired GNSS reflectometry processing. The direct signal is processed in the standard way, its tracking loops replica are tapped to have a robust processing of the reflected signal in a master-slave configuration, with the very same carrier replica used to correlate the reflected signal. In addition, the data bit sign is wiped off, which allows to extend the coherent integration time (CIT) well beyond the usual 20-ms limit on the reflected way. This allows having a straightforward and accurate measurement of the Amplitude Ratio and Differential Carrier Phase between the direct and reflected signals. The possible applications are precise carrier phase altimetry and any application requiring signal amplitude ratio, or reflected signal Delay Map, with single or dual polarization, this include code altimetry, humidity, biomass, soil roughness, ocean surface wind and wave height, and snow and ice characteristics retrieval. This software is intended to be used as a research tool. It has been tested for carrier phase altimetry on real data sets collected in rather calm water conditions: at the 60-m Cordouan Lighthouse, and during a 600- m high ATR42 flight over a lake. In both cases, continuous carrier phase measurement with a centimeter level precision was obtained when extending the CIT up to 500 ms. Increasing the CIT beyond 20 ms is the key to improve carrier phase altimetry robustness.

Modeling of GNSS multipath signals for altimetry applications using signal to noise ratio data

Multipaths are one of the major source of error for positioning applications using Global Navigation Satellite Systems (GNSS). During the last decade, several studies demonstrated the potential of the inversion of Signal to Noise Ratio (SNR) due to multipaths for estimating environmental parameters such as sea level, soil moisture, and snow depth variations, vegetation growth and biomass quantification. Here, we propose i) a modeling approach contribution of the multipath signals to the SNR, ii) an inversion technique of the receiver height based on the modeled SNR.

First results from the GLORIE polarimetric GNSS-R airborne campaign dedicated to land parameters estimation

The GLORIE GNSS-R airborne campaign was conducted in the late spring 2015 with the GLORI polarimetric receiver. More than 15 hours or raw data was gathered during 5 flights that spanned over a 3-week period. The aircraft flew over several areas if interest including: 1) agricultural plots with coincident in-situ measurements of soil moisture, vegetation biomass and roughness, 2) in situ monitored forest plots with a wide range of above ground biomass values and 3) inland water bodies in order to test phase altimetry retrievals. Apparent reflectivity was computed from the data, showing a good dynamics above various types of terrains. Phase altimetry was performed over calm water, showing a precision in the range of the centimeter level.