| 08 May 2026
Dual frequency airborne SAR experiment on snow water equivalent retrieval in Alpine terrain
Abstract. In March 2021 a field experiment was conducted in the high-Alpine test site Woergetal of the Austrian Alps, exploring the feasibility and performance of snow water equivalent (SWE) and snow depth retrievals by means of C- and L-band SAR data. Multiple repeat-pass acquisitions were acquired along two flight tracks with an airborne SAR system. The SAR acquisitions spanned two snowfall events of different intensity. In situ data on physical snow properties were collected throughout the campaign. Main objective was the development and evaluation of procedures for SWE monitoring by measuring the interferometric path delay in snow, with focus on the future Radar Observation System for Europe at L-band (ROSE-L), the Sentinel-1 Next Generation mission, and the proposed geosynchronous C-band SAR mission Hydroterra. The response of the VH/VV backscatter ratio and the co-polarized phase differences (CPD) to snow accumulation was also studied. These two parameters do not show a reproducible relation with snow depth and SWE of the snowfall events. CPD measured after snowfall shows an increase with the incidence angle that can be attributed to radar wave propagation through a refrozen crust formed before the snowfall. For the interferometric SWE retrievals three input data configurations were applied: (1) C- and L-band repeat-pass SAR data at full spatial resolution; (2) differential wavenumber (Delta-k) interferograms obtained by split-bandwidth processing; (3) simulated geosynchronous C-band SAR data reflecting the properties of the Hydroterra mission. Coherence, interferometric phase and computed snow accumulation (ΔSWE) images and their properties are documented for these configurations. L-band case 1 shows high coherence and good retrieval performance for both snowfall events. C-band case 1 and case 3 show high coherence and good performance for the low intensity snowfall but low coherence and 2π phase ambiguity prohibit the application for the intense snowfall. This deficiency can be overcome by the Delta-k approach which delivers suitable results for both frequencies. Options for complementary use of the different approaches are discussed.
First, I really appreciated this paper. The methodology and dataset is well presented and described. In terms of results, when looking at other L-Band InSAR SWE retrieval work, the discussion on the coherence is often lacking or completely missing, which I think is a key aspect to enabling the retrieval of SD or SWE using InSAR. This is well presented here, and I really like the discussion on the different challenges between the three different approaches (C-band or L-Band alone, and together). I also like the mention of InSAR using lower frequency overcomes the issue of sensitivity to SWE and microstructure, which is a well known challenge to retrieve SWE using higher frequency backscatter measurements.
One thing I would appreciate if the authors could expand upon is the applicability of these results to current/future EO missions. I am currently investigating NISAR retrievals in Canada with the early released datasets, and I see challenges that were not necessarily addressed in past work, that I feel these results could (not dismissing NISAR data here, I know the team is working on improving it, and looking forward to using it). For instance, satellite L-Band SAR can require atmospheric corrections of several fringe cycles for a single image, and the results shown here present a phase change of 1/4pi between two scenes. This issue is not relevant to this study since atmospheric corrections would be minimal at the given altitude, but how realistic is it to assume that it'll be possible to detect those phase changes when the atmospheric corrections can be several orders of magnitude stronger. How confident is the community on the atmospheric corrections that often rely on modeling. Also, how does scaling up to a space-based platform impact the coherence of the signal. The authors do compare results with changes in resolution, which I think is key when developing an EO mission.
Since the authors already make links with future ESA missions (ROSE-L, Hydroterra), I think it would be nice to push the discussion a bit further with these results, and potentially highlighting challenges for these missions that could be mitigated in the development phases, or expanding upon those challenges that might simply be rapidly mentioned in the conclusion. I know the authors have the knowledge to add a paragraph on how these results translate to the satellite scale.
Thanks