New altimeter concept for next-generation global Earth topography microwave instruments

摘要

Pulse limited radar altimeters (Geosat, ERS1/2, Topex/Poseidon) have demonstrated excellent ability in performing measurements of the ocean topography from space with a high degree of accuracy. Data continuity will be ensured through follow on missions like TOPEX-POSEIDON Follow- on and ENVISAT RA2 (developed by ALENIA AEROSPAZIO under ESA contract) in this case providing also the chance for a global Earth topography mapping not more limited to ocean but extended to land and ice regions thanks to innovative design features like resolution adaptivity and robust on board tracking. Earth sciences are now demanding for systems with extensive capability to get topographic measurements over non- ocean surfaces (ice and land regions) but with improved spatial resolution, in the order of 100 - 300 meters respect to the several hundreds of meters provided by nadir looking pulse limited systems. A real step forward in high resolution topography with microwave instrumentation is represented by the application of synthetic aperture and interferometric techniques to the conventional pulse limited altimeter concept, a solution proposed in the literature and extensively exploited by ALENIA AEROSPAZIO in the frame of the ESA studies TOS (Topography Observing Systems) and HSRRA (High Spatial Resolution Radar Altimeter) and proposed in late 1998 for the Earth Explorer Opportunity Mission CRYOSAT. In the high spatial resolution altimeter synthetic aperture processing applied along the direction of motion will allow to improve the resolution in the along track while dual antenna observation geometry will enable reconstruction of surface topography within each synthesized Doppler filter from the phase difference between the radar returns at the two antennas. Thanks to a proper baseline selection, a unique interference fringe can be generated within the observed swath thus avoiding the troubles of phase unwrapping otherwise required in conventional interferometric processing. Aim of this paper is to review the key concepts of the proposed measurement technique and to present the design of the high spatial resolution altimeter operating according to the outlined measurement principle and suitable for a global ocean/ice/land topography mission.