1. Simulation of D2P radar echoes from CryoVex 2003 Scanning Laser Measurements D. Wallis 1, D. J. Wingham 1 and R. Cullen 2 1. CPOM, Space and Climate Physics, The Pearson Building, University College London, Gower Street, London WC1E 6BT 2. EOP-PY, ESTEC, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands. Results Abstract Method Conclusions The DEM, shown at the top, is generated from scanning laser altimetry, collected during CryoVex 2003, by interpolating the laser measurements over a regular grid. The data are over sea ice at 18.8 degrees North, 10.26 degrees West, recorded on 15th April 2003. The blue line shows the aircraft ground-track. Two notable features are the flat area of low elevation between 400m and 700m, and the raised, linear feature (probably a pressure ridge) between 800m and 900m. The D2P echoes recorded over the ground-track in the top figure are shown in the centre figure, and the corresponding simulated echoes are shown at the bottom. The flat area between 400m and 700m can be seen on the simulated and real echoes, and the ridge is also visible in both, represented as a reduction in range of about 2m. The radar echo expected from some surface can be calculated if the topography is known. The calculation is shown to the right (see box ‘The Radar Equation’). Scanning laser measurements were transformed to cartesian coordinates using an oblique Mercator projection and converted to a regular grid with 1m horizontal resolution by interpolation, creating a grid of 60x60 scattering elements. Antenna positions were calculated by interpolating the D2P longitude and latitude positions to give locations corresponding to the Pulse Repetition Frequency of the radar, and the range to each scattering element was calculated for each antenna position. Each echo was then calculated at 128 time steps, with a sampling interval of 1/B. A modified version of the CryoSat processor was used to process the echoes generated by the simulator in the along-track direction. The CrySat processor takes time domain echoes and performs a 2D FFT on bursts of 16 echoes to form along-track beams. Echoes from the simulator were grouped into bursts of 16 echoes and an inverse FFT applied in the range direction to produce time domain echoes suitable for the CryoSat processor. The CryoSat processor forms the synthetic aperture beams. Surface locations are chosen and the beams are ‘steered’ to these locations. This ensures that beams in adjacent bursts correspond to the same sorface locations. The echoes are then corrected in range to compensate for the difference in range to each beam (slant-range correction) producing a ‘stack’ of echoes. These are summed to produce a multi-looked echoes. The simulator/processor combination was tested with simulations over a single scatterer (point target). The results of this simulation can be predicted easily. The figures to the right show a slant-range corrected stack of echoes and the corresponding mult-looked echo from the stack. The echoes in the stack are centred on range- bin 64, which corresponds to the range to the scatterer. This shows that the range, and slant-range correction are correct. We have developed a simulator for the D2p radar altimeter that can calculate the expected echo from a surface with a known topography. Preliminary results show that some features that appear in D2P radar echoes collected during the CryoyVex 2003 experiment can be seen in the simulated echoes. Simulated echoes will provide a means of comparing the radar and laser data collected during the CryoVex 2003 campaign. The CRYOVEX 2003 field campaign has provided simultaneous d2p radar and scanning laser elevation measurements over sea ice in the Fram Strait region. Obtaining a meaningful comparison between the laser and radar measurements is not straightforward. Basic methods of comparison could be envisaged, for example using the laser measurement closest to nadir, or using the mean laser elevation within the radar footprint. Our approach is to use scanning laser elevation measurements, from within the radar footprint, to calculate the expected radar echo from the measured surface. Our simulator produces coherent, pulse limited radar echoes (level 1 data), using the characteristics of the d2p radar and a DEM made from interpolated laser measurements. Along-track (synthetic aperture) processing with a processor based on the CryoSat and ASIRAS processors provides level 1b data (multi-looked echo powers) suitable for comparison with the d2p echoes. We describe the basis of the radar simulator, the along-track processing, and present some preliminary results of comparisons of laser and radar measurements from CRYOVEX 2003. The echoes are a sinc squared function in the range direction, which corresponds to the transmitted power envelope. In the beam direction, the maximum echo power follows the antenna pattern. The figure at the bottom shows the echoes shown in the stack summed in the beam direction, corresponding to a multi-looked echo. The echo is a sinc squared function centred on range bin 64. The Radar Equation