| 10 Oct 2025
One-day repeat pass interferometry highlights the role of temporal baseline on digital elevation models retrieved from Sentinel-1
Abstract. Digital Elevation Models (DEMs) derived from Synthetic Aperture Radar (SAR) interferometry are a key data source for numerous geospatial applications, from hydrological modelling to environmental monitoring. The launch of Sentinel-1C in late 2025 introduces a new sensor into the Sentinel-1 constellation. This study evaluates the vertical accuracy of DEMs generated from interferometric image pairs acquired during the satellite’s calibration phase. The analysis uses a set of image pairs with temporal baselines of 1, 6, and 12 days, over a test site in Angola, validated against ICESat-2 elevation measurements. The workflow includes interferometric processing, coherence assessment, and statistical error evaluation. Results indicate high accuracy for the 1-day pair (RMSE ≈ 14.7 m) and moderate degradation for the 6-day pair (RMSE ≈ 16.4 m), but a pronounced loss of accuracy for the 12-day pair (RMSE ≈ 49.4 m), primarily linked to coherence loss in vegetated areas. Coherence and elevation error distributions reveal clear land cover and slope dependencies, with lower performance in forested and steep terrain. These findings should be regarded as indicative due to the limited number of suitable image pairs for the calibration phase. However, this early assessment provides an important reference point for future Sentinel-1A/C DEM generation studies, informing both methodological refinement and application planning in SAR-based topographic mapping.
Review of the paper “One-day repeat pass interferometry highlights the role of temporal baseline on digital elevation models retrieved from Sentinel-1” by Andreas Braun.
The paper describes the evaluation of a digital elevation model generated from three different pairs of Sentinel-1 acquisitions with a temporal lag of 1, 6 and 12 days. The results show, that the DEMs with 1 and 6 days temporal distance are comparable, while the DEM with 12 days is much worse. The paper is written very clearly and easy to read.
General comments:
I understand, that there is only one feasible data set available to perform this analysis. However, a similar performance for 1 and 6 days temporal lag, and a such worse performance for 12 days is very unexpected in my opinion. The author does not provide a stringent explanation for this, but rather guessing, that it might be due to atmospheric effect (why should there be a dependency of seven days?) or whatever.
A more thorough error analysis analyzing the differences between the pairs is required to make this paper complete. In addition, an analysis comparing the performance for 6 days and 12 days repeat with similar measures for Sentinel-1A versus Sentinel-1B (best over the same area) should be easily possible and would confirm these massive performance deterioration after 12 days.
In my opinion, the conclusion (12 days very bad, 1 and 6 days good -> I really would also expect some differences between 1 and 6 days) is very miss-leading for designer of future interferometric missions.
I would recommend a major revision for this paper.
Specific comments:
Line 101: Please specify the 14 m wide ground track in more detail. Data points with a diameter of 14 m? Distance in along track?
Figure 3: The elevation value in the legend should be 1,225 m and 1,435 m correct? It seems, that there is a large ramp in flight direction on pair 1. Could this be calibrated to improve the results?
Line 190: The analysis on these effects should be intensified as they drive the conclusion.
Line 212ff: What about shadow/layover regions? How are these considered?
Line 276/277: “This is consistent with previous studies…”: I could not find a satisfactory consistent analysis considering the importance of the temporal baseline of < one week in (Wu and Madson, 2024) except the reference to (Braun, 2021). In my opinion it would be really important to cite independent studies analyzing the difference between 6 and 12 days Sentinel-1 interferograms/DEMs.
Line 290: “correcting the 20 m bias… would bring its accuracy considerably closer to the shorter-baseline results.”: Please perform this correction and re-evaluate the results.
Line 323: “additional examples under similar geometrical conditions would have likely refined the magnitude of error differences but not the central pattern observed here”: I would definitely disagree with this statement. The author has only assessed one single data set. This can be deteriorated by whatever. To conclude, that the “central pattern” would be valid for most other measurements is really mis-leading in my opinion.
Minor comments / editorials:
Line 31: Sentinel-1 instead of “Sentinel 1”
Line 57: Abbreviation IW not introduced
Line 110: “Figure 2 <space> shows”
Line 125: “Comparison of coherence”
Line 133: “at the bottom. Pair 1”
Line 143: German data format
Line 144: “surface. Pair 3”
Figure 3: All histograms from “Delta Pair 3” as the title claims?
All over: Percentages should be uniquely written (50 % vs. 50%)
Line 239: “. ThesE were retrieved…”
Line 244: “grassland” twice
Line 365: Capital letters for “Forests, Shrublands, Wetlands”
Line 306: This instead of “Tis”