ROHP-PAZ is a mission of opportunity: The Spanish Earth Observation PAZ satellite, ready for launch, was initially designed to carry a Synthetic Radar Aperture (SAR) as primary and sole payload. It included an advanced Global Navigation Satellite System (GNSS) receiver for precise orbit determination. The design of this particular GNSS receiver allows the tracking of 'occulting' signals, that is, signals transmitted by navigation satellites setting below the horizon of the Earth (or rising above it). The Spanish Ministry for Science and Innovation (MICINN) approved a proposal aimed to modify the original plans of PAZ, by including a polarimetric GNSS Radio-Occultation (RO) payload, the ROHP-PAZ experiment.

ROHP-PAZ is a proof-of-concept experiment: for the first time ever, GNSS RO measurements will be taken at two polarizations, to exploit the potential capabilities of polarimetric radio occultation for detecting and quantifying heavy precipitation events and other de-polarizing atmospheric effects (e.g. cloud ice). If the concept is proved, PAZ will mean a new application of the GNSS Radio-Occultation observations, by providing coincident thermodynamic and precipitation information with high vertical resolution within regions with thick clouds.

ROHP-PAZ potential impact: Coincident thermodynamic and precipitation information with high vertical resolution within regions with thick clouds might help understanding the thermodynamic conditions underlying intense precipitation, which is relevant because these events remain poorly predicted with the current climate and weather model parametrization. A better understanding of the thermodynamics of heavy precipitation events is necessary towards improving climate models and quantifying the impact of climate variability on precipitation. The particular advantage of GNSS polarimetric RO is that their signals are in the microwave spectrum which, unlike infrared sensing technology, is little influenced by clouds, not even by the thick clouds that are typically associated with heavy precipitation.
Polarimetric observables obtained in ROHP-PAZ ground-based campaign for one particular GNSS satellite and day, as function of the elevation angle, the gray shades represent 1 and 2 sigmas of all the days without any rain. (bottom) Polarimetric observables corresponding to the same GNSS transmitter as on the top panel, but acquired during a heavy rain episode. The observable clearly overpass the statistics of dry conditions. Figure from [Padullés et al., 2016].
Aim of our participation

The ICE team conceived the polarimetric-RO measurement concept to be proved with this experiment. The ICE team proposed the experiment and leads the mission and its International Science Team. ICE is also in charge of searching the required funding and collaboration agreements with external agencies to maximize the dissemination of ROHP-PAZ data and its impact in the science community and return to the society. Once in orbit, ICE will be responsible of the storing and distributing the data sets, as well as to generate and distribute the novel products obtained with polarimetric observables.
(left) Example of simulated rain cell in the radio-occultation plane. (centre) Polarimetric observable obtained with the simulated rain scenario on the left. (right) Inversion of the polarimetric observables in the centre, using the technique developed in [Padullés, Cardellach and Rius 2016]. Figures from [Padullés, Cardellach and Rius 2016].
Recent developments

During 2016 the final results of the ground-based experiment have been published, together with a new approach to invert the observables. During this year, the team has also developed the algorithms which will be used in actual data to separate the hydrometeor component of the observables (of interest) from other undesired components (systematic effects).

Recent publications
Padullés, R., Cardellach, E., de la Torre Juárez, M., Tomas, S., Turk, F. J., Oliveras, S., Ao, C. O., Rius, A., Atmospheric polarimetric effects on GNSS Radio Occultations: the ROHP-PAZ field campaign,Atmospheric Chemistry and Physics, 16, pp. 635-649, 2016, jan, doi:10.5194/acp-16-635-2016

Padullés, R., Cardellach, E., Rius, A., Untangling rain structure from polarimetric GNSS Radio Occultation observables: a 2D tomographic approach, European Journal of Remote Sensing, 49, pp. 571-585, 2016, doi:10.5721/EuJRS20164930

Senior Institute's members involved

E. Cardellach, S. Ribó, A. Rius
Institute of Space Sciences (IEEC-CSIC)

Campus UAB, Carrer de Can Magrans, s/n
08193 Barcelona.
Phone: +34 93 737 9788
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An institute of the Consejo Superior de Investigaciones Científicas

An institute of the Consejo Superior de Investigaciones Científicas
Affiliated with the Institut d'Estudis Espacials de Catalunya

Affiliated with the Institut d'Estudis Espacials de Catalunya