Reflectometry and Occultation Sounding Using GNSS and Other Opportunity Signals

Dear Colleagues,

Global Navigation Satellite System reflectometry (GNSS-R) and GNSS radio occultation (GNSS-RO) were originally proposed about two decades ago, and significant advances have been made since then. The use of Global Positioning System (GPS) signals as signals of opportunity to perform ocean scatterometry was originally proposed in 1988, and then in 1993 for mesoscale altimetry. The first evidence that GPS navigation signals could be collected and tracked after being scattered on the sea surface dates back to 1991, when a French aircraft was testing a GPS receiver. In 1996, it was proposed to use the reflection of the GPS signals from the oceans to extend ionospheric measurements in satellites carrying single-frequency radar altimeters.

On the other hand, since the GPS/MET satellite mission in 1995, GNSS radio occultation technique has emerged as a powerful approach for sounding the global atmosphere with high resolution and precision in all weather conditions. Various RO space missions have proved successful in addressing a broad range of scientific questions on climate monitoring, operational weather prediction, ionospheric research, space weather forecasting, calibrating other observing systems, and geodesy.

With the advent of other Global or Regional Navigation Satellite Systems (GNSS or RNSS) such as GLONASS, Galileo, Beidou, IRNSS, and QZSS, and satellite-based augmentation systems including WAAS, EGNOS, MSAS, etc., the number of transmitting satellites is dramatically increasing, thus providing potentially even more simultaneous observations.

Since the early days of GNSS Reflectometry, different techniques have been developed to make better use of the signal properties. Besides the originally proposed applications (wind speed and altimetry), many new applications have been developed, including wind speed and direction, ice altimetry, soil moisture, vegetation height and biomass, snow depth, etc. The NASA CYGNSS mission, launched in December 2016, measures the ocean surface wind field with unprecedented temporal resolution and spatial coverage, and also opens new applications for wetlands and soil moisture monitoring, etc.

In the past years, the use of other opportunity signals away from L-band has also taken off, such as P- and X-band, using satellite radio and TV signals, but also ground transmitters. This opens even more new applications given the larger bandwidth and higher signal-to-noise ratio, and/or the longer wavelength signal capable of deeper penetration into the ground. Correspondingly, the sensor technologies for various opportunity signals are developed including the experimental setup.

This special issue seeks to publish original contributions in theory, methodology, sensors, instrumentation, experiments, retrieval techniques, and scientific applications in both the reflectometry and occultation from ground-based, drone-based, airborne, or space-borne platforms.

Dr. Yu-Ming Yang
Dr. Hyuk Park
Dr. Feiqin Xie
Guest Editors

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