Spaceborne P-band radar for ice-sheet sounding: design and performances

The Antarctic ice sheet is one of the most prominent physical features on our planet. It represents an area of about 14 millions km/sup 2/. The average ice thickness being around 2200 m (and up to 4500 m), the Antarctic ice sheet contains a 30 million km/sub 3/ ice volume corresponding to about 70 m of sea level. However, this huge reservoir of water is still partially unknown and due to its large extent, only observations from space can give some global insight into the structure of the ice sheet. The Antarctic ice sheet is an exceptional archive of the past climates and a major actor of the Earth water cycle. Each ice layer corresponding to one period of time keeps its own chemical characteristics linked to past atmospheric events. The typical temporal scale involved allow for a description of past climate over a few climatic cycles, or a few 100000-year events. A dedicated mission has been proposed to European Space Agency as a candidate for an Earth Explorer Opportunity mission. This mission is based on a P-band radar instrument with a nadir-looking geometry, working at 435 MHz with a bandwidth of 6 MHz (imposed by ITU regulation), allowing to sound the vertical structure of the ice from the surface down to the bedrock. A feasibility study of the mission was done in Alcatel Space under ESA contract with an emphasis on the radar instrument. For coherent reflection and deep sounding (corresponding to the so-called Fresnel zone), the useful signal coming from any ice layer or from the bedrock overlap with lateral incoherent surface scattering. The surface clutter is reduced by the antenna pattern in the across-track direction (typical aperture is lower than 5/spl deg/) and by Doppler filtering in the along-track direction. These beam requirements impose to have a very long antenna in the across-track direction. The typical dimension of the antenna is 10 m /spl times/ 1 m. This work summarizes the principle of the instrument, and the associated antenna solution. Main performances are also presented.