Analysis of Radar Altimeter Waveform Retrackers for Geodynamics Studies
Hyongki Lee1, C.K. Shum1, Chung-Yen Kuo1 and Yuchan Yi1
1
Laboratory for Space Geodesy & Remote Sensing, The Ohio State University, USA
It has been demonstrated that the ice sheet mass balance, hydrology and land/ice digital
topography studies require development of specific radar altimeter waveform retrackers to
accommodate peculiar radar backscatter over different land covers.
Here we describe
methodologies to attempt the difficult problem of detecting solid Earth deformation with an
accuracy of <1 mm/yr level, which has not been achieved before. Although our technique
focuses on the use of TOPEX/POSEIDON radar altimetry with circularly polarized antennae, it is
potentially applicable using other satellite radar altimetry. Even though TOPEX can maintain
lock over moderately smooth land surface, the return waveforms over different land covers and
conditions, including ice/snow, land and vegetations are complicated limiting the accuracy of
topographic height recovery typically only at the meter level. We developed a technique to
improve topographic height by retracking over various land cover and conditions. We first
categorized each waveform according to its shape and number of ramps and interpreted the
corresponding distinct surface features, and then applied several waveform retracking algorithms
to assess their respective appropriateness. These algorithms include the NASA V4, the ESA/UCL
COG, the threshold method, the cross-correlation technique and ESA’s ICE2 algorithms. We
have generated a mean height profile using each of the available retracking algorithms, and then
calculated deviations to assess the best method. We found most of the returned waveforms are
more or less specular and contain a pre-leading edge bump which is likely due to the topography.
Since NASA V4 fits a waveform to a function whose variables are estimated to achieve the best
fit, it performed well on these topographically noisy waveforms and waveforms with fast
decaying trailing edge. Unlike the 10% threshold algorithm, which has been used to calculate
ice-sheet height change at crossover points, a 20% threshold level has been proven to be adequate
since it reduces the effect of this pre-leading edge noise. Another model-independent crosscorrelation technique calculates retracking differences from the non-zero-lag of the correlation
function between a pair of waveforms. The advantage of this algorithm is that it can work for
any pair of similar waveforms. Since the peakiness and power distribution of the waveform will
differ due to the different surface condition which will vary seasonally, different retracking
algorithms should be chosen to generate a decadal time series from TOPEX/POSEIDON data.
Our initial study region covers a relatively flat land area where the maximum solid Earth crustal
uplift of ~1 cm/yr due primarily to the Glacial Isostatic Adjustment centering in the Hudson Bay
over the former Laurentide ice sheet.
This paper provides examples of the resulting land
deformation signals and results comparing with ENVISAT ICE2 measurements and incidental
ICESat laser altimeter data. Finally, we provide an assessment of the feasibility of the technique
to be applicable to land regions with larger slope, for example, Patagonia.