Speaker
Description
Spaceborne differential SAR interferometry has been demonstrated to potentially allow for snow water equivalent (SWE) change measurements on a spatial scale, resolution, and accuracy unprecedented by other sensor concepts [1][2]. However, its operational use is hindered mainly because of: i) low coherence areas resulting from temporal decorrelation, largely complicating a robust phase unwrapping, ii) inaccurate density estimates introducing systematic errors in the phase-to-SWE inversion, and iii) an unknown phase offset due to the $2\pi$ ambiguity of the interferometric measurement and therefore a strongly biased SWE estimate.
We present strategies to tackle these shortcomings by exploiting simultaneously acquired interferograms with different squint angles. The different line-of-sights result in differential phase delays introduced by a SWE change due to the varying path length through the snow cover. The phase difference between the interferograms may be exploited to produce a low-resolution SWE estimate without the need for phase unwrapping and to resolve the $2\pi$ phase ambiguity of the single interferogram, following a similar rational as delta-k approaches for absolute phase estimation [3][4]. In contrast to delta-k approaches, the ratio between the interferograms acquired with different squint angles is a direct measure of the dielectric permittivity of the snow and can be related to the snow density. Estimates of the snow density and permittivity are required for an unbiased SWE estimate and may be relevant for other sensor modalities or snow modelling approaches.
The performance of the strategies is evaluated for the Harmony mission (ESA's Earth Explorer 10), demonstrating great potential given the large squint diversity of the Harmony constellation. By means of simulated D-InSAR acquisitions of the Sentinel-1 and Harmony satellites that are based on real Sentinel-1 repeat-pass acquisitions covering a snow accumulation event in northern Alaska, we show that convincing inversion results of both SWE change and density may be obtained for realistic assumptions on coherence levels and spatial SWE variability. Beyond Harmony, also the Co-Flier concepts of NASA JPL with receive-only companions for upcoming L-band missions may be candidates to implement the proposed techniques.
[1] T. Guneriussen, K. Hogda, H. Johnsen, and I. Lauknes, “InSAR for estimation of changes in snow water equivalent of dry snow,” IEEE Transactions on Geoscience and Remote Sensing, vol. 39, no. 10, pp. 2101–2108, 2001.
[2] K. Belinska, G. Fischer, T. Nagler, and I. Hajnsek, “Snow water equivalent estimation using differential SAR interferometry and co-polar phase differences from airborne SAR data,” in IGARSS 2022 – 2022 IEEE International Geoscience and Remote Sensing Symposium, 2022, pp. 4545–4548
[3] G. Engen, T. Guneriussen, and Y. Overrein, “Delta-K interferometric SAR technique for snow water equivalent (SWE) retrieval,” IEEE Geoscience and Remote Sensing Letters, vol. 1, no. 2, pp. 57–61, 2004
[4] S. Madsen, H. Zebker, and J. Martin, “Topographic mapping using radar interferometry: processing techniques,” IEEE Transactions on Geoscience and Remote Sensing, vol. 31, no. 1, pp. 246–256, 1993
