Airborne wind retrieval using GPS delay-Doppler maps

Global Navigation Satellite System Reflectometry (GNSS-R) has emerged recently as a promising remote sensing tool to retrieve various geophysical parameters of Earth's surface. GNSS-reflected signals, after being received and processed by the airborne or space-borne receiver, are available as delay correlation waveforms or as delay-Doppler maps. In the case of a rough ocean surface, those characteristics can be related to the RMS of L-band limited slopes of the surface waves, and from there to the surface wind speed. The raw GNSS-reflected signal can be processed either in real time by the receiver, or can be recorded and stored onboard and post-processed in a laboratory. The latter approach leveraging a software receiver allows more flexibility while processing the raw data. This work analyzes Delay Doppler Maps (DDM) obtained as a result of processing of the data collected by the GPS data logger/software receiver onboard the NOAA Gulfstream-IV jet aircraft. Thereafter, the DDMs were used to retrieve surface wind speed employing several different metrics that characterize the DDM extent in the Doppler frequency-delay domain. In contrast to previous works in which winds have been retrieved by fitting the theoretically modeled curves into measured correlation waveforms, here we do not rely on any model for the determination. Instead, the approach is based on a linear regression between DDMs observables and the wind speeds obtained in simultaneous GPS dropsonde measurements.