PRETTY Mission Preparations: Steps to foster Grazing-Angle Reflectometry

Abstract

For the ESA mission PRETTY a three-unit cubesat is foreseen to be launched in March 2023. The satellite is designed for a low earth orbit (LEO) with a high inclination (97.66°), a low orbit excentricity and a mean orbit height of 564 km. The satellite layout comprises two scientific payloads: A passive reflectometer to acquire Earth-reflected signals of Global Navigation Satellite Systems (GNSS) and novel dosimeters to measure the Total Ionizing Dose (TID). This presentation concentrates on the preparation of the GNSS reflectometer measurements (reflectometry) for altimetric application over the global ocean and cryosphere. Reflectometry data from satellites have been used earlier for altimetric retrievals: over sea-ice [1], over the Caribbean [2] and the Seas of Indonesia [3]. The TDS-1 satellite mission and the CYGNSS constellation of reflectometry satellites were important milestones in this respect. A crucial challenge for the altimetric application of reflectometry is the retrieval of precise observations over the global oceans. High precision in GNSS is achieved observing the carrier phase delay. Respective retrievals in reflectometry reach centimeter level precision, resolving sea surface topography [4]. This precise information, however, is lost over rough ocean areas due to the dominance of diffuse scatter and the absence of coherent reflection, see limitation in [2]. Group delay observations persist under diffuse scatter conditions. However, limits in code bandwidth and group delay sampling of the previous space-borne scenarios provided only meter-level precision (tracking error) [3]. The surface roughness and the observation geometry play an important role for the altimetric application. Reflectometry satellites often acquire signals in a near-nadir geometry (seen from the receiver) that corresponds to high elevation angles at the specular surface point (usually > 45°). High elevation angles imply a high altimetric sensitivity, however, they also imply a high sensitivity to surface roughness [5]. The PRETTY mission will shift the observation geometry to grazing angles with elevations between 5° and 15° at the specular point. This shift is expected to increase the number of coherent reflections and provide better conditions for carrier phase altimetry, as shown with airborne data [4]. PRETTY’s focus on grazing angles is one step to mitigate the ocean roughness impact. The reduced height sensitivity in this case should still be sufficient to resolve decimeter-sized features of sea-surface topography, cf. simulations in [5] and feature detected in CYGNSS data [2]. The focus of the PRETTY mission on the L5 carrier frequency (1191.795 MHz) is another step to mitigate the ocean roughness impact. Compared to the commonly used L1 carrier (1575.42 MHz), L5 has a longer wavelength (lower frequency) and is more robust against roughness, cf. the Rayleigh criterion [6]. Finally, the interferometric concept of signal correlation is a third step of the PRETTY mission to improve altimetric precision. The reflected and the direct signal are correlated without using modelled replica of the signal. The interferometric approach can increase the bandwidth for group delay observations beyond the replica bandwidth. It may reach sub-meter precision in group delay waveforms [7].

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