Conference Agenda

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This paper discusses the repeat-pass SAR Interferometry (InSAR) capabilities of the Sentinel-1 Constellation, focusing on the results of the Sentinel-1C & Sentinel-1D In-Orbit Commissioning (IOC) phases. In particular, we analyze the cross-SAR interferometry (cross-InSAR) performance using InSAR image pair combinations between Sentinel-1A (S1A), Sentinel-1C (S1C), and Sentinel-1D (S1D) acquired during the S1C and S1D IOC phases.

The Sentinel-1 TOPS IW mode cross-InSAR capability is demonstrated by mapping the ground deformation caused by the powerful earthquake, which struck central Myanmar on March 28^th, 2025.

In addition, we show the repeat-pass capability for the Sentinel-1 Extra Wide swath (EW) mode using EW Single-Look Complex (SLC) images which are available during the S1C and S1D IOC phases.

The Sentinel-1 mission is implemented through a constellation of identical C-band SAR satellites [1], which comprised initially comprising the A and B units, launched on April 3^rd, 2014 and on April 25^th, 2016, respectively.

The Sentinel-1 constellation’s operational 6-day repeat orbit interval along with small orbital baselines enables InSAR coherent change detection applications, such as the monitoring of cryosphere dynamics (e.g., glacier flow) and the mapping of surface deformation, caused by tectonic processes, volcanic activities, landslides or ground subsidence [2].

Sentinel-1C was launched on December 5th, 2024 to replace Sentinel-1B in its orbital node, which ceased operations at the end of 2021.

During the S1C IOC phase, S1C was temporarily positioned for four (4) orbital repat cycles on an orbital node that has a 1-day separation with respect to Sentinel-1A (S1A). This enabled unique opportunities for the implementation of cross-InSAR using S1C/S1A image pairs having a 1-day repat-pass interval.

After completion of four (4) orbital cycles, Sentinel-1C was transferred to its nominal orbital node that is 180 deg. phased (i.e., former Sentinel-1B orbital node) with respect to Sentinel-1A, to complete the remainder of the S1C IOC phase. respectively.

Sentinel-1D was launched on November 4^th, 2025 to eventually replace Sentinel-1A after the successful completion of the S1D IOC phase.

During the entire S1D IOC phase, S1D is positioned on an orbital node that has a 1-day interval with respect to S1C and a 5-day interval with respect to S1A. This enables the implementation of cross-InSAR configurations using S1D/S1C, S1D/S1A and S1C/S1A image pairs.

The generation of high-quality Interferometric Wide Swath (IW) mode cross-interferograms and coherence maps requires achieving an optimum azimuth spectral alignment, i.e., maximizing the common Doppler bandwidth. Consequently, this requires an accurate time synchronization of the TOPS azimuth scanning patterns (i.e., bursts) and a very stable satellite platform and SAR antenna azimuth pointing leading to small differences in Doppler centroid frequencies only [2].

Furthermore, an accurate ground-track repeatability of the Sentinel-1 units involved for cross-InSAR is needed, i.e. within a small orbital tube, to achieve small orbital cross-InSAR baselines.

In this paper, we discuss the Sentinel-1 constellation’s cross-InSAR performance, involving S1A, S1C and S1D, by analysing the impact of burst synchronization and satellite platform and SAR antenna pointing on the achievable common Doppler bandwidth considering specific TOPS scaling and mutual compensation effects.

Furthermore, we report on the orbital baselines for the different cross-InSAR configurations considering that the S1A ground-track deadband is not controlled at higher latitudes since February 23^rd, 2024 due to an underperformance of the S1A thrusters.

For the case of the Myanmar earthquake, we discuss the unique capability of Sentinel-1’s advanced SAR imaging mode known as Terrain Observation with Progressive Scans (TOPS) [3] to measure coseismic ground motion in both East-West and North–South directions [4]. The latter is achieved by applying a technique referred to as ‘burst overlap interferometry’ [5], which exploits the squint angle diversity in the burst overlap region to measure the along-track component of the ground motion using the Enhanced Spectral Diversity (ESD) method [6] [7] [8]. In this context, we discuss the challenges for the related InSAR processing and calibration.

In addition, we discuss the results of the cross-interferogram range spectrum analysis, i.e. delta-k ionosphere estimation [9], which was performed to verify the phase correction applied to the range chirp for each SAR mode, sub-swath, and polarization to correct for an asymmetry in the SAR impulse response function (IRF) during SAR processing.

Literature

[1] R. Torres, R., et al., “GMES Sentinel-1 Mission”, Special Issue of Journal of Remote Sensing of Environment “The Sentinel Missions – New Opportunities for Science”, Vol. 120, pp. 9-24, May 2012.

[2] D. Geudtner, D., et al., “Sentinel-1A/B SAR and InSAR Performance”, Proc. EUSAR 2018, Aachen, Germany.

[3] F. De Zan and A. Monti Guarnieri, TOPSAR: Terrain Observation by Progressive Scans, IEEE Transactions on Geoscience and Remote Sensing, Vol. 44, No. 9, September 2006, pp 2352-2360.

[4] ESA - Sentinel-1 captures ground shift from Myanmar earthquake

[5] Grandin, R., E. Klein, M. Métois, and C. Vigny (2016), Three-dimensional displacement field of the 2015 Mw8.3 Illapel earthquake (Chile) from across- and along-track Sentinel-1 TOPS interferometry, Geophys. Res. Lett., 43, 2552–2561.

[6] N. Yague-Martinez, Prats, P., Gonzalez, F, R., Brcic, R., Shau, R., Eineder, M., Geudtner, D. and Bamler, R., “Interferometric Processing of Sentinel-1 TOPS Data”, IEEE Trans. Geoscience and Remote Sensing, Vol. 54, No. 4, pp. 2220-2234, 2016.

[7] P. Prats-Iraola, R. Scheiber, L. Marotti, S. Wollstadt, and A. Reigber, “TOPS interferometry with TerraSAR-X,” IEEE Trans. Geosci. Remote Sens., Vol. 50, No. 8, pp. 3179–3188, Aug. 2012.

[8] R. Scheiber and A. Moreira, “Coregistration of interferometric SAR images using spectral diversity,” IEEE Trans. Geosci. Remote Sens., Vol. 38, no. 5, pp. 2179–2191, Sep. 2000.

[9] G. Gomba, A. Parizzi, F. De Zan, M. Eineder, R. Bamler, “Toward operational compensation of ionospheric effects in SAR interferograms: the split-spectrum method”, IEEE Trans. Geosci. Rem. Sens., 54 (2016), pp. 1446-1461.

Along-track Interferometry (ATI) is a technique to measure the radial velocity of moving ground targets. A quasi-simultaneous observation is obtained with two phase centres separated along-track, usually implemented with a dual-channel receive system. The along-track baseline between phase centres is a determinant parameter for the ATI sensitivity, trading-off with the lag at which the surface scatterers decorrelate [1].
Even though optimal ATI implementations require dual-channel receivers, single-channel implementations are also possible by exploiting Aperture Switching (AS) schemes [2]. The idea is time-multiplexing two different beams operating at a high pulse repetition frequency (PRF), toggling beams between consecutive pulses. An adequate AS implementation can achieve the phase centre separation that is required for ATI, at the expense of a reduced swath width and increased ambiguities and noise equivalent sigma zero (NESZ).
In order to support the development of Sentinel-1 Next Generation (S1NG) oceanic products [3], an ATI mode demonstrator was implemented with Sentinel-1C&D and executed during their respective in-orbit commissioning phases. Past experiments have shown that ATI can potentially provide lower-variance estimates of the surface velocity fields compared to the Doppler Centroid Anomaly (DCA) estimator [4], making it an interesting option especially for future multi-channel missions like S1NG. Former single-antenna spaceborne ATI implementations can be traced back to TerraSAR-X with a half-antenna on receive AS mode [5] and Radarsat-2 MODEX modes including a dual-channel receiver [6].
Implementing ATI on an AS scheme can be done in several ways, as any toggling of beams providing spatial diversity can potentially be suitable for it. This poses an interesting optimization problem that can lead to different results depending on the targeted application. For the purpose of measuring the surface velocity fields of oceanic currents, we opted for implementing a phase-only toggling on receive that benefits from an increased receive gain with respect to toggling half-antenna.
The existing SAR architecture imposes constraints on the implementation possibilities and achievable ATI performance. Specifically, a single SAR antenna restricts the along-track baseline to being less than its physical length, resulting for Sentinel-1 about an order of magnitude shorter than the C-band optimal of 50–100 m for the observation of ocean currents [7]. However, even with such a short along-track baseline it is still possible to accurately measure and obtain distinguishable signatures of the oceanic currents, provided a large enough number of independent looks is available for the estimation.
This work presents the experimental Along-Track Interferometry (ATI) mode implemented and tested on Sentinel-1C and Sentinel-1D using a phase-only aperture switching technique on receive. Acquisitions were conducted over ocean regions characterized by strong surface currents and the presence of ships, concurrently with the Automatic Identification System (AIS) onboard Sentinel-1C&D and near-simultaneously with Radarsat-2 operating in MODEX mode. The derived surface velocity fields are compared with those from Radarsat-2 and validated against the AIS reported vessel velocities, showing an excellent agreement. The ATI mode design, signal model, data processing methodology and experimental results including the mitigation of azimuth ambiguities will be presented.

[1] R. M. Goldstein and H. A. Zebker, “Interferometric Radar Measurement of Ocean Surface Currents,” Na-ture, vol. 328, pp. 707–709, 1987.
[2] R. Romeiser and H. Runge, "Theoretical Evaluation of Several Possible Along-Track InSAR Modes of TerraSAR-X for Ocean Current Measurements," in IEEE Transactions on Geoscience and Remote Sensing, vol. 45, no. 1, pp. 21-35, Jan. 2007.
[3] R. Torres et al., "Sentinel-1 Next Generation: En-hanced C-band Data Continuity," EUSAR 2022; 14th European Conference on Synthetic Aperture Radar, Leipzig, Germany, 2022, pp. 1-3.
[4] U. I. Ahmed, B. Rabus, D. Geudtner, M. Rashid and C. Gierull, "Along Track Interferometry (ATI) versus Doppler Centroid Anomaly (DCA) Estima-tion of Ocean Surface Radial Velocity using RA-DARSAT-2 Modex-1 ScanSAR Data," EUSAR 2022; 14th European Conference on Synthetic Aper-ture Radar, Leipzig, Germany, 2022, pp. 1-5.
[5] Romeiser, R., Suchandt, S., Runge, H., Steinbrecher, U., Grünler, S., “First Analysis of Ter-raSAR-X Along-Track InSAR-Derived Current Fields,” in IEEE Trans. Geoscience Remote Sensing, vol. 48, no. 2, pp. 820-829, 2010.
[6] Chiu, S., Dragošević, M.V., “Moving Target Indica-tion via RADARSAT-2 Multichannel Synthetic Ap-erture Radar Processing,” in EURASIP J. Adv. Signal Process. 2010, 740130 (2009).
[7] S. Wollstadt, P. López-Dekker, F. De Zan and M. Younis, "Design Principles and Considerations for Spaceborne ATI SAR-Based Observations of Ocean Surface Velocity Vectors," in IEEE Transactions on Geoscience and Remote Sensing, vol. 55, no. 8, pp. 4500-4519, Aug. 2017.

The Copernicus Sentinel-1 mission provides unique capabilities for comprehensive monitoring of ice sheet flow dynamics. Since 2015 Sentinel-1 has delivered continuous 6-day and 12 day repeat observations over the peripheral zones of the Antarctic and Greenland ice sheets, complemented by ice sheet–wide mapping campaigns. These acquisitions form the basis for operational products of ice velocity, grounding line position and ice discharge.

The 6- and 12-day repeat intervals enable routine applications of InSAR, significantly improving ice velocity retrievals for slow-moving regions and provide higher accuracy than offset tracking. InSAR allows also accurate grounding line mapping. However, repeat intervals of 6 days or longer are often affected by temporal decorrelation in shear margins and on fast-flowing glacier sections, and are exposed to temporal decorrelation by snow drift, snow accumulation, and surface melt. In the 1990s the ERS Tandem mission demonstrated the strong potential of 1-day repeat-pass interferometry for investigating ice dynamics and grounding line positions.

A new opportunity arose during the commissioning phase of Sentinel-1D (launched in November 2025) in conjunction with Sentinel-1C. In the first quarter of 2026, Sentinel-1C and -1D operated in a 1-day repeat configuration, acquiring a unique short-repeat pass data set over Antarctica, Greenland and polar ice caps.

This data set provides an excellent basis for retrieving ice velocities and grounding line positions at high accuracy and with heigh spatial detail, also on fast-moving and dynamically complex areas by using interferometric techniques. The 1-day repeat interval substantially reduces temporal decorrelation, even in highly dynamic regions such as West Antarctica and the Antarctic Peninsula, enabling robust velocity estimates also in areas with strong flow variations. Improved coherence compared to 6-day interferograms enables grounding zone mapping with higher spatial detail. The interferometric processing accounts for phase discontinuities between adjacent TOPS bursts by incorporating during burst co-registration ice displacement information derived from multi-temporal Sentinel-1 ice velocity maps. Furthermore, the combination of improved coherence and multi-track acquisitions at high southern latitudes enables to obtain 3D ice displacement by integrating interferograms from tracks with different heading angles.

This presentation will showcase first results from InSAR-based ice velocity and grounding line mapping using the Sentinel-1C/D 1-day repeat-pass dataset. Results will be compared with contemporaneous 6-day products derived from Sentinel-1A and -1C, highlighting the potential of integrating short- and medium-repeat interferometric observations in support of enhanced polar ice monitoring.