GNSS remote sensing
It is well known that Global Navigation Satellite Systems can also be used to characterize media where they propagate in. In the last decade, GNSS atmospheric and Earth’s surface remote sensing become more and more important, thanks to technical improvements applied to the processing of such “free-of-charge”, everywhere available and weather insensitive signal.
Remote sensing of wet part of troposphere is possible “extracting” the atmospheric delays from carrier phases of GNSS observations. In the phase observations adjustment (a standard GNSS signal pre-processing) it is possible to estimate the wet contribution to atmospheric total delay mapped into the zenith direction. From the remote sensing point of view, this Zenith Wet Delay may be then transformed into the so-called Integrated Precipitable Water Vapour (IPWV), which allows meteorologists to know the evolution of total water vapour content in atmosphere, which is one of the variable operatively used in Numerical Weather Prediction Models.
A second important application allows to add vertical variability information to the atmospheric parameter distribution with respect to the previous one, which represents an “integrated” quantity. The amplitude and phase variations experienced by GNSS signal crossing the atmospheric “limb” and received on-board a Low Earth Orbit satellite, can be used to infer temperature and water vapor profiles, thanks to the GNSS Radio Occultation technique.
A third approach may be followed to add a further spatial variability characterization with respect to that given by IPWV and Radio Occultation. It deals with the three-dimensional reconstruction of atmospheric refractivity and, thus, water vapour density, applying tomographic techniques to phase delays measurements collected by small (but dense) networks of GPS receivers.
Another application is the most recent and maybe the most challenging one. It foresees the use of GNSS signals reflected off from lands and oceans for characterizing the Earth’s surface at L-band frequencies. The reflected signal contains many differences with respect to the direct one, in terms of delay, Doppler shift, power strength and polarization. Among the possible remote sensing applications we list: ocean altimetry (from delay); wind speed and ocean scatterometry (from shape and spreading); ice topography and monitoring (from delay and magnitude); soil moisture (from magnitude).
All the previous research areas are related to the monitoring of the Neutral Atmosphere or the ground. Another field of activities on which the NavSAS group is active is the Ionosphere Monitoring. We are actually developing solutions to mitigate ionosphere threats both at the receiver level, their impact on augmentation systems and for a better characterization of the geophisical parameter which is monitored (in particular, considering the effects of Ionosphere on Radio Occultation derived products).
All these topics are within the research activities performed by the NAVSAS group.