Simulation of D2P radar echoes from CryoVex 2003 Scanning Laser Measurements

1
Simulation of D2P radar echoes from CryoVex 2003
Scanning Laser Measurements
D. Wallis1, D. J. Wingham1 and R. Cullen2
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.
Abstract
Results
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.
Method
The Radar Equation
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.
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 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.
Conclusions
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.