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Interferometric swath processing of Cryosat data for glacial ice topography

Cryosphere (2013) 7(6) 1857-1867
DOI: 10.5194/tc-7-1857-2013
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Abstract

We have derived digital elevation models (DEMs) over the western part of the Devon Ice Cap in Nunavut, Canada, using "swath processing" of interferometric data collected by Cryosat between February 2011 and January 2012. With the standard ESA (European Space Agency) SARIn (synthetic aperture radar interferometry) level 2 (L2) data product, the interferometric mode is used to map the cross-track position and elevation of the "point-of- closest-approach" (POCA) in sloping glacial terrain. However, in this work we explore the extent to which the phase of the returns in the intermediate L1b product can also be used to map the heights of time-delayed footprints beyond the POCA. We show that there is a range of average cross-track slopes (~ 0.5 to ~ 2°) for which the returns will be dominated by those beneath the satellite in the main beam of the antenna so that the resulting interferometric phase allows mapping of heights in the delayed range window beyond the POCA. In this way a swath of elevation data is mapped, allowing the creation of DEMs from a sequence of L1b SARIn Cryosat data takes. Comparison of the Devon results with airborne scanning laser data showed a mean difference of order 1 m with a standard deviation of about 1 m. The limitations of swath processing, which generates almost 2 orders of magnitude more data than traditional radar altimetry, are explored through simulation, and the strengths and weaknesses of the technique are discussed. © Author(s) 2013. CC Attribution 3.0 License.

Figures

  • Fig. 1. Cryosat is oriented such that the baseline formed by the two receiving antennas is oriented perpendicular to both the along-track direction and to the normal to the WGS84 ellipsoid. Each crosstrack scan line will contain a unique estimate of the POCA on the ice surface, followed in delay time by the sum of returns from both sides of the POCA. With a suitable geometry the returns from the ambiguous region can be much weaker than those from the main beam underneath the satellite which allows the mapping of position and height of main-beam footprints. The dashed magenta line illustrates diagrammatically the dB variation in cross-track two-way gain. Note that the angles have been exaggerated for clarity; the first side lobe in the pattern occurs at ∼ 2◦ from nadir and represents a two-way power level of −40 dB with respect to the power at nadir.
  • Fig. 2. (Top): the surface area illuminated by a burst of 64 pulses extends fore–aft and side-to-side of the sub-satellite point. A locally circular Earth model has been used to illustrate the relative positions of some of the circular iso-range contours and half of the 32 fore Doppler beam positions. The central (dark blue) semicircular contours illustrate the first nine iso-range contours after the POCA. Subsequently, the blue semi-circles indicate the positions of every 25th iso-range contour. The 512 samples in each waveform or scan line are separated in t e L1b files used i this work by 0.47 m in slant range. The cros -track footprint dimensi n chang rapidly fr the large size at POCA (∼ 1.5 k ) to ∼ 2 m for the 400th footprint beyond the POCA. The positions of the Doppler beams extending across-track are shown by dotted magenta lines, for clarity every second beam is shown. Both distance and angular coordinates in degrees are given for the across- and along-track dimensions with respect to the POCA. (Bottom): the relative two-way antenna gain is shown for cross-track cuts at five different fore angles corresponding to the central and the 8th, 16th, 24th and 32nd beams. These are the dashed lines while the solid blue line corresponds to the effective two-way pattern G2-way(θ) given in Eq. (6). Note that the cross-track angle extent in Fig. 2 (bottom) is much wider than that in Fig. 2 (top).
  • Fig. 4. The western part of the Devon Ice Cap with topography illustrated in colour. The sub-satellite position of the 23 February descending pass is shown as a black line and the positions of the irborne laser scanning altimeter data are shown in white. The dotted black lines indicate the sub-satellite tracks of the 25 descending passes which were used to create th DEM illustrated in Fig. 8. The position of the Devon Ice Cap in t Ca adian Arctic is shown in the insert. In this figure the topog hy is derived from the Canadian digital levation databa e me ged with the CS swath processed results shown in Fig. 8.
  • Fig. 3. Ratio of the interferogram power in the ambiguous beam to that in the main beam for cross-track slopes between 0.5 and 2.0◦ based n a locally spherical Ear model. The minimum in the suppression f th ambiguous power occurs when the ambiguous footprints are illuminated by the low antenna gain region between the main lobe and the first side lobe at ∼ 1.5◦ from nadir.
  • Fig. 6. Top: profile of the return power for the scan line at the position of the white arrow in the illustration of return power in the upper part of Fig. 5. Note the leading edge is relatively weak, indicative of the low two-way antenna gain at the POCA cross-track angle. The strong delayed returns reflect the fact that the delayed main-beam returns have a much stronger two-way antenna gain. Second from top: profile of the smoothed coherence for the same scan line. Note that the dip in the coherence at ∼ 30 m in the delay window corresponds to the situation when the baseline decorrelation affects the “cross terms” discussed in the text. The high coherence for the range window between∼ 60 and 180 m implies that the interferogram can be represented by Eq. (5) and that the returns are dominated by those from the main footprints. Third from top: the original phase from the L1b file is shown for the scan line in blue. The region of the scan line for which processing was done is shown in red. Note that the phase has been low pass filtered, the boxed region in this figure has been blown up in the bottom figure and illustrates the degree of smoothing; the red line is the unwrapped smoothed phase and the blue dots are the original values. Although it is slope dependent the sampling and resolution in the across-track direction is normally better than in the along-track direction, even with this smoothing.
  • Fig. 5. Illustration of the return power for the central part of the February 23 pass (top), the filtered coherence (middle), and the differential phase (bottom), all as a function of the delay window expressed as a slant range distance. Note that the phase has not been unwrap ed at this stage. The white arrow in the upper image of return power indicates the position of the profiles shown in Fig. 6.
  • Figure 8. Results from 25 descending passes have been combined to create a DEM of the western slopes of the Devon Ice Cap. Colour has been used to indicate height in meters. The uneven edges of the DEM at the north and south edges are due to the relatively large alongtrack slope and the resulting poor solution often indicated by lower data coherence and higher phase noise.
  • Fig. 7. Illustration of the swath processed geocoded elevations for the 23 February descending pass. The oblique white line indicates the sub-satellite track and the position of the maximum antenna gain. The two white arrows indicate the position of the E–W pass of the airborne ALS data which were used as a height reference.

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Gray, L., Burgess, D., Copland, L., Cullen, R., Galin, N., Hawley, R., & Helm, V. (2013). Interferometric swath processing of Cryosat data for glacial ice topography. Cryosphere, 7(6), 1857–1867. https://doi.org/10.5194/tc-7-1857-2013

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