Remote sensing using coherently reflected signals of Global Navigation Satellite Systems: Opportunities and Challenges

Abstract

Signal reflection, in the sense of multipath, is one of the most critical error sources in precise radio-based navigation. Intensive research evolved during the last two decades to apply reflected signals of GNSS (Global Navigation Satellite Systems) for remote sensing (reflectometry) of ocean and land surfaces. We conducted a series of reflectometry experiments since 2010 to study the occurrence of coherent GNSS reflection and the radio wave's interaction with water and sea-ice surfaces. These studies, led by GFZ, comprised airborne and ship-based measurements. The airborne measurements were realized with a Zeppelin NT airship over Lake Constance, with the HALO (High Altitude Long range) research aircraft over the Mediterranean Sea and in cooperation with ULCO along the channel coast in Northern France using a gyrocopter aircraft. The ship-based measurements were conducted during expeditions of Norwegian research vessels (Lance, Kronprins Haakon), led by NPI, to Arctic sea ice in Fram Strait, Barents Sea and aboard the German research vessel Polarstern during the MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate) to the central Arctic. Two main objectives were defined for these investigations: on the one hand, to measure the carrier phase of the reflected signal for surface height retrieval and on the other hand to estimate reflected signal power as a parameter for sea-ice remote sensing. The results demonstrated that coherent GNSS reflections can be used to resolve variations of the water surface (geoid undulation and sea surface topography) and to resolve the permittivity contrast of sea ice under variable conditions (ice concentration and ice type). We will give a review on these experiments and the results in combination with a glance on the evolution of GNSS reflectometry research in general. Finally, challenges that arise for coherent signal requirement, e.g., due to surface roughness and atmospheric irregularities will be discussed.