the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2026
Simulating SAR altimeter echoes from cryospheric surfaces with the Snow Microwave Radiative Transfer (SMRT) model version sarm-v0
Abstract. Radar altimeters are essential tools for observing the cryosphere, especially for estimating ice-sheet elevation change and sea-ice thickness. However, retrieving these quantities remains challenging, and progress depends on physically based numerical simulations of the recorded waveforms to understand their sensitivity to the geophysical parameters of the medium. Such models can also guide the design of future satellite missions. Accurate simulations require a balanced combination of a realistic description of the medium, precise calculation of wave–medium interactions, and an accurate representation of the altimeter measurement process, including downstream processing. The Snow Microwave Radiative Transfer (SMRT) model has addressed the first two aspects for a decade and includes an altimetric Low Resolution Mode (LRM) module, but has, until now, lacked a delay-Doppler (SAR) altimetric capability used by most modern sensors. This study introduces the new SMRT SAR altimetry module, which operates in three steps. First, it calculates the backscatter of all layers and interfaces using existing SMRT modules. Next, it models the waveforms of each layer and interface using a delay-Doppler approach. Finally, these components are combined to produce the final waveform. The user selects the delay-Doppler model from one of eight formulations reviewed, implemented, and compared in the literature. The validation first assesses these models under simple conditions, confirming they produce consistent results but differ in computational efficiency and flexibility. Subsequently, the new module is compared with external models to confirm its accuracy. Finally, it is applied to Antarctic conditions, where the simulations reproduce observed Sentinel-3 waveform variability linked to surface roughness. The open-source module, equipped with the eight options, now enables a wide range of numerical experiments, from studying penetration bias to exploring the potential for snow retrieval on sea ice and lake ice thickness.
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Status: open (until 04 May 2026)
- RC1: 'Comment on egusphere-2025-6056', Christopher Buchhaupt, 18 Mar 2026 reply
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CEC1: 'Comment on egusphere-2025-6056', Juan Antonio Añel, 26 Mar 2026
reply
Dear authors,
Unfortunately, after checking your manuscript, it has come to our attention that it does not comply with our "Code and Data Policy".
https://www.geoscientific-model-development.net/policies/code_and_data_policy.html
You have archived the SMRT code on GitHub. However, GitHub is not a suitable repository for scientific publication. GitHub itself instructs authors to use other long-term archival and publishing alternatives, such as Zenodo. In addition, it is not clear to me if you have published in the data repository the data used from the BC-005 ESA Land Ice Thematic Products. If not, please add it to the repository.
The GMD review and publication process depends on reviewers and community commentators being able to access, during the discussion phase, the code and data on which a manuscript depends, and on ensuring the provenance of replicability of the published papers for years after their publication. Please, therefore, publish your code and data in one of the appropriate repositories and reply to this comment with the relevant information (link and a permanent identifier for it (e.g. DOI)) as soon as possible. We cannot have manuscripts under discussion that do not comply with our policy.
The 'Code and Data Availability’ section must also be modified to cite the new repository locations, and corresponding references added to the bibliography.
I must note that if you do not fix this problem, we cannot continue with the peer-review process or accept your manuscript for publication in GMD.
Juan A. Añel
Geosci. Model Dev. Executive EditorCitation: https://doi.org/10.5194/egusphere-2025-6056-CEC1 -
AC1: 'Reply on CEC1', Ghislain Picard, 26 Mar 2026
reply
Dear Chief Editor,
Multiple iterations with Copernicus staff have been done to conform the Code and Data policy between the submission and the publication in preprint.
The “code and data availibility” section, page 35, indicates:
- The code published under the license LGPL-3.0-or-later and the documentation are available from the archive at https://doi.org/10.5281/ZENODO.17808241 . The link is working.
- The in-situ measurements are available at: https://doi.org/10.18709/PERSCIDO.2022.05.DS367. As of today, the link is temporarly not working, but the manuscript provides an alternative link for an easier access, and this link is working. In addition, I provide here the zenodo link to this repository is https://doi.org/10.5281/zenodo.6519037
Regarding the BC-005 ESA Land Ice Thematic Products data, they are publicly available from ESA. The extracted data are now copied to the archive: https://doi.org/10.5281/zenodo.19231174
Citation: https://doi.org/10.5194/egusphere-2025-6056-AC1 -
CEC2: 'Reply on AC1', Juan Antonio Añel, 26 Mar 2026
reply
Dear authors,
Thanks for the quick reply. I understand that you can feel frustrated regarding the compliance with the Code and Data policy of the journal, but it is crucial, and the very first issue to address when submitting a manuscript to the journal. At best, I would say that the text in the "Code and data availability" section of your manuscript is misleading. I would like to clarify that the goal of such section is to provide the information for the permanent repositories containing the assets necessary to ensure the provenance and replicability of the work presented. The GitHub links that you provide do not serve such purpose, and they clearly create confussion. Therefore, in this case, I would ask you to remove them from the section, as any other site that does not comply with the policy. This is the case of PERSCIDO, which we can not accept as a trusted long-term repository. Currently, it does not even work, and moreover it does not appear to have a published policy for data preservation over many years or decades (some flexibility exists over the precise length of preservation, but the policy must exist), and it does not appear to have a published mechanism for preventing authors from unilaterally removing material. Archives must have a policy which makes removal of materials only possible in exceptional circumstances and subject to an independent curatorial decision.
Therefore, thanks for clarifying that the SMRT code is stored in the first mentioned repository (something that I failed to spot, my apologies), and please, I would kindly request you to reply to this comment with a modified "Code and Data Availability" section that refers only to the strictly necessary repositories to avoid confusion.
Juan A. Añel
Geosci. Model Dev. Executive Editor
Citation: https://doi.org/10.5194/egusphere-2025-6056-CEC2 -
AC2: 'Reply on CEC2', Ghislain Picard, 26 Mar 2026
reply
Dear Chief Editor,
Thank you for the recommendations. We propose the following new “code and availabiliy section”:
The code published under the license LGPL-3.0-or-later and the documentation are available from the archive at \url{https://doi.org/10.5281/ZENODO.17808241} \citep{smrt_sarm_v0}, the Antarctic in-situ measurements from the archive at \url{https://doi.org/10.5281/zenodo.6519037 } \citep{picard_smrt_notebooks_2022} and the BC-005 ESA Land Ice Thematic Products extracted at the in-situ sites from the archive: \url{https://doi.org/10.5281/zenodo.19231174}.
Citation: https://doi.org/10.5194/egusphere-2025-6056-AC2
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AC2: 'Reply on CEC2', Ghislain Picard, 26 Mar 2026
reply
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CEC2: 'Reply on AC1', Juan Antonio Añel, 26 Mar 2026
reply
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AC1: 'Reply on CEC1', Ghislain Picard, 26 Mar 2026
reply
Data sets
Snow properties in Antarctica, Canada and the Alps for microwave emission and backscatter modeling Ghislain Picard et al. https://doi.org/10.18709/PERSCIDO.2022.05.DS367
Model code and software
SAR altimetry module in SMRT version sarm-v0 Ghislain Picard https://github.com/smrt-model/smrt/releases/tag/sarm-v0
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- 1
The authors present a novel snow microwave radiative transfer model for SAR altimetry signals including surface scattering and snow-volume scattering. Overall, the manuscript is well written but sometimes lacks some clarity. Especially, the volume and interface backscattering function implementation on DDMs was hard to understand from the equations and text presented. Therefore, I believe the manuscript requires major revision.
Page 2 line 31: I personally would write cm/yr instead of cmyr^{-1}
Page 3 line 59: I am not sure about the formulation “an inverse method”. I guess you mean a nonlinear optimization approach fitting a measured waveform with a modelled waveform maximizing the likelihood?
Table 1: I guess AN means analytical (okay it is probably analytical numerical based on table 2, but then I do not understand why Ray is AN and Dinardo is A)? The antenna pattern in Ray et al. (2015) was really a free function? I thought it was Ellip. Gaussian as well. Okay in the paper he did not seem to define the antenna pattern to be Gaussian, but in the SAMOSA based retrackers I am very sure that the Ellip. Gaussian approximation is used. The surface backscatter in SAMOSA should be Gaussian as well? Please check the Halimi retracker as well for the surface backscatter
Table 2: The caption should be changed to match with figure 1. Figure 1 is early models and Figure 2 is models. Maybe say something like newer or recent approaches? Or make one table for ocean and one for ice-sheets and one for sea-ice and soil (not sure to be honest)? Buchhaupt (2018) is a bit outdated but fine to use. Maybe add in the text that an update for stack retracking (10.3390/rs15174206, 10.1016/j.asr.2022.12.034 ) and an antenna pattern update of that approach ( 10.1016/j.asr.2025.02.056 ). The missing yes or no in terrain slope means that e.g. Boy and Dinardo don’t consider the terrain slope? The PTRs in Buchhaupt 2018 is btw arbitrary the paper only uses the sinc^2 PTR as an example.
Table 3: Jack Landy is a coauthor, so I guess he checked it, but wasn’t the facet based retracker allowing an arbitrary antenna pattern and PTR or do you refer to the final LARM implementation here? If yes, I think that pitch and roll only had a limited support which might be worth mentioning? Maybe check with Jack.
Page 9 line 178 needs some clarification as I do not understand why they do not account for slope variations. For open ocean and sea-ice those are considered in the backscatter function as far as I know. Or do you mean non random variations?
Page 9 line 180: The ocean PDF for SAR altimetry is actually 2D (one dimension is the elevation and one dimension the vertical velocity) and the 1D PDF is an approximation yielding SWH errors for open ocean surfaces. However, for sea-ice it is indeed 1D. Not sure if you want to discuss this here of not since your focus is sea-ice.
Line 185: The height and slope are correlated for non-Gaussian surfaces? Please provide a reference for this claim.
Section 3.1: The DDM and \sigma^0 terms need a clearer definition, preferably with equations defining each. Otherwise, it is unfortunately very hard to follow the authors intention in this section.
Eq. 3: Is \sigma^0_{interface i} a function of the incident angle? In the text below q. 3 it is not. It would also be helpful to define \sigma^0_{volume} and DDM_{volume}. In my (still unpublished) work I just assumed DDM_volume == DDM \conv \sigma^0_{volume}(\tau)
Line 517: A horizontal correlation length of 10 cm sounds very short. Are you sure that this is not a typo?
Line 557: A more detailed description of your roughness scales would be helpful if I did not overread it. Is the roughness w.r.t. the air-snow interface?
Figure 6: What extinction coefficient would that correspond to?