Excess Path Model for Space-based GNSS Reflectometry: A PRETTY mission concept study

Abstract

Signals of Global Navigation Satellite Systems (GNSS) became an important source for remote sensing over the last decades. Signal delays introduced either by atmospheric refraction or by Earth surface reflection are used to monitor the Earth system. Neutral gas properties and electron density in the atmosphere are retrieved, for example, by radio occultation techniques. Sea surface altimetry and ocean wind retrievals can be based on respective reflectometry techniques. The GNSS signals experience an excess path delay when propagating in atmospheric media (neutral and electron gases) relative to propagation delay in vacuum. It is an essential observable for atmospheric sounding and a major bias for sea surface altimetry. The ESA satellite mission PRETTY (Passive REflecTo- and dosimeTrY) shall be launched end of 2022. The three-unit cubesat is equipped with a designated GNSS reflectometry receiver for altimetry and dosimeters for space weather monitoring. In a current study the scientific opportunities of the reflectometry payload are investigated. The excess path model contributes to the altimetric mission goal by providing corrections for the space-based scenario. The model has two major parts. The first part is a purely geometric model to determine the specular point of the Earth's surface reflection (Sp) and the three distance vectors connecting Sp with the receiver (Rx) and with the transmitter (Tx), as well as, the vector connecting Tx and Rx, directly. The second part involves the atmospheric correction. Neutral gas and ionospheric refractivity profiles are derived along the three vectors (between Sp, Rx and Tx). Dedicated atmospheric input data are obtained from: ERA5 (Reanalysis products of the European Centre of Medium-Range Weather Forecast) and NEDM (Neustrelitz electron density model of the German Aerospace Center). Spatial and temporal variability of refractivity is resolved down to 30 km horizontally in 1h intervals (neutral gas by ERA5) and down to about 250 km horizontally in the ionosphere with semi-diurnal scale and longer. In a final step the excess phase path (delay in neutral gas and advance in ionosphere) is computed using the refractivity profiles. It is achieved either by slant path integration (disregarding bending effects) or by ray tracing calculations (considering ray bending). First tests of the model were performed for mountain-based observation scenarios (receiver height at 1430 m above Mediterranean Sea). In this scenario the excess path delay of direct signals in grazing angle geometry (5° elevation) varies between 14 and 16 m depending on atmospheric conditions. The relative excess path (of the reflected signal relative to the direct one) lies between 8 and 10 m under the same conditions. The excess path variability of up to 20% (2 m) indicates the necessity of a precise model considering spatial and temporal variations in refractivity, especially as variability in the space-based scenario will still increase.