PAU/GNSS-R: Implementation, Performance and First Results of a Real-Time Delay-Doppler Map Reflectometer Using Global Navigation Satellite System Signals

Abstract

Signals from Global Navigation Satellite Systems (GNSS) were originally conceived for position and speed determination, but they can be used as signals of opportunity as well. The reflection process over a given surface modifies the properties of the scattered signal, and therefore, by processing the reflected signal, relevant geophysical data regarding the surface under study (land, sea, ice…) can be retrieved. In essence, a GNSS-R receiver is a multi-channel GNSS receiver that computes the received power from a given satellite at a number of different delay and Doppler bins of the incoming signal. The first approaches to build such a receiver consisted of sampling and storing the scattered signal for later post-processing. However, a real-time approach to the problem is desirable to obtain immediately useful geophysical variables and reduce the amount of data. The use of FPGA technology makes this possible, while at the same time the system can be easily reconfigured. The signal tracking and processing constraints made necessary to fully design several new blocks. The uniqueness of the implemented system described in this work is the capability to compute in real-time Delay-Doppler maps (DDMs) either for four simultaneous satellites or just one, but with a larger number of bins. The first tests have been conducted from a cliff over the sea and demonstrate the successful performance of the instrument to compute DDMs in real-time from the measured reflected GNSS/R signals. The processing of these measurements shall yield quantitative relationships between the sea state (mainly driven by the surface wind and the swell) and the overall DDM shape. The ultimate goal is to use the DDM shape to correct the sea state influence on the L-band brightness temperature to improve the retrieval of the sea surface salinity (SSS).

UAV-Based GNSS-R for Water Detection as a Support to Flood Monitoring Operations: A Feasibility Study

“…Esterhuizen and Akos [49] also presented a miniaturized receiver based on two GPS L1 front ends and a Nano-ITX Single Board Computer (SBC) to store raw signal samples, which were analyzed in post-processing [50]. Marchan-Hernandez et al [51] on the other hand, designed an FPGA-based GNSS reflectometery that is capable of computing the Delay Doppler Maps (DDMs), with update rate of 1 ms.…”

Description and Performance of an L-Band Radiometer with Digital Beamforming

“…The Passive Advanced Unit (PAU) for ocean monitoring [10] is an instrument that combines, in a single receiver and without time-multiplexing, an L-band microwave radiometer (PAU-RAD) [11] and a GPS-reflectometer (PAU-GNSS/R) [12][13][14] which, in conjunction with an infra-red radiometer (PAU-IR) [15], simultaneously provide the sea state information and surface temperature needed to accurately retrieve the SSS. Furthermore, PAU-RAD, as it has been designed, is a concept demonstrator ground-based instrument, which is expected to measure multi-angular incidence ocean brightness temperature from a terrain elevation, for instance a cliff.…”

GNSS Application in Retrieving Sea Wind Speed

“…It consists of the PAU/RAD which is an L-band radiometer to measure the brightness temperature of the sea surface, the RAU/GNSS-R which is a reflectometer to measure the roughness of the sea surface and the PAU/IR which is two infrared radiometers used to observe the temperature of the sea surface. PAU/GNSS-R was designed based on FPGA to synchronously process the reflected GPS signal from different satellites in real-time and output corresponding to 2-D DDM [35]. To reduce the hardware resources of the payload, hardware reuse technique was adopted based on two RAM-like registers that change their respective input and output connections to allow the DDM generator to be implemented at a higher clock rate.…”