The two-satellite Copernicus Sentinel-1 (S1) constellation became operational in Sep 2016, with the successful in-orbit commissioning of the S1B unit. During, the commissioning phase and early operational phase it has been confirmed that the interferometric performance of the constellation is excellent, with no observed phase anomalies. In this work, we show an analysis of selected performance parameters for the S1 constellation, as well as initial results based on the available data from the first months of operations

Dehls, JF

Hooper, AJ

Larsen, Y

Marinkovic, P

Perski, Z

Wright, TJ

Yngvar Larsen

Petar Marinkovic

John F. Dehls

Zbigniew Perski

Andy J. Hooper

Tim J. Wright

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The Sentinel-1 constellation for InSAR applications: Experiences from the InSARAP project

White Rose Research Online

This is a repository copy of The Sentinel-1 constellation for InSAR applications: Experiences from the InSARAP project.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/128104/Version: Accepted VersionProceedings Paper:Larsen, Y, Marinkovic, P, Dehls, JF et al. (3 more authors) (2017) The Sentinel-1 constellation for InSAR applications: Experiences from the InSARAP project. In: International Geoscience and Remote Sensing Symposium (IGARSS). 2017 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 23-28 Jul 2017, Fort Worth, TX, USA. IEEE , pp. 5545-5548. ISBN 9781509049516 https://doi.org/10.1109/IGARSS.2017.8128260(c) 2017 IEEE. Personal use of this material is permitted. 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Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. THE SENTINEL-1 CONSTELLATION FOR INSAR APPLICATIONS— EXPERIENCES FROM THE INSARAP PROJECTYngvar Larsen1, Petar Marinkovic2, John F. Dehls3, Zbigniew Perski4, Andy J. Hooper5, Tim J. Wright51 Norut, P.O. Box 6434, NO-9294 Tromsø, Norway2 PPO.labs, Zilvermeeuwlaan 189, 2496PC Den Haag, The Netherlands3 Geological Survey of Norway, P.O. Box 6315 Sluppen, NO-7040 Trondheim, Norway4 Polish Geological Institute, Rakowiecka 4, 00-975, Warszawa, Poland5 University of Leeds, Leeds, LS2 9JT, UKABSTRACTThe two-satellite Copernicus Sentinel-1 (S1) constellation be-came operational in Sep 2016, with the successful in-orbitcommissioning of the S1B unit. During, the commissioningphase and early operational phase it has been confirmed thatthe interferometric performance of the constellation is excel-lent, with no observed phase anomalies. In this work, weshow an analysis of selected performance parameters for theS1 constellation, as well as selected results based on the avail-able data from the first months of operations.Index Terms— Copernicus, Sentinel-1, InSAR1. INTRODUCTIONThe Copernicus Sentinel-1 constellation brings a paradigmshift to the field of InSAR, with its operational character-istics: mission configuration, acquisition planning, and datadistribution policy[1]. The objective of the ESA SEOM IN-SARAP project is to perform a qualitative and quantitativeevaluation of the S1 constellation for interferometric analy-sis. The evaluation encompasses both algorithmic robustnessand scientific applications [2].In this work we first present an early evaluation of theinterferometric compatibility of the S1A and S1B units. Weconduct an analysis of basic InSAR performance metrics, andpresent some early scientific case studies. Finally, we discussthe initial findings and outline the analysis to be presented inthe final manuscript.This work was supported by ESA contract no 4000110680/14/I-BG: SEOM INSARAP Sentinel-1 constellation study, and ESA contractno 4000115064/15/NL/MP: Sentinel-1B commissioning – SAR Cross-calibration Technical Assistance2. SELECTED RESULTS2.1. First interferogramAlready the day before the S1B unit reached the nominal or-bit, a 6-day interferometric pair was available, see Figure 1and [3]. The perpendicular baseline was still about 500 m,leading to volumetric decorrelation in vegetated areas. How-ever, in the coherent areas, we can observe that no phase arti-facts are present.Fig. 1. First S1A/S1B cross-interferogram. The interfero-gram is based on IW data from 2016-06-09 (S1A) and 2016-06-15 (S1B), acquired in descending track 7 over Romania.The interferogram phase is mixed in with the correspondingbackscatter data in the incoherent areas.2.2. InSAR performanceBasic InSAR performance metrics, including• doppler centroid• perpendicular baseline• burst synchronizationwhere followed closely during the S1B commissioning phase.Initialle, the S1A and S1B units had slightly misaligneddoppler bands due to mispointing, but this was corrected afew weeks before the first products where publicly released.Since early Sep 2016, the two satellites show no statisticaldifference in parameters relevant for InSAR. Further analysisof these parameters is outside the scope of this work.In the following, we present two other metrics relevant forInSAR. First, we show a quantitative analysis of coherence asa function of temporal baseline. Then the potential effects ofradar clock frequency errors are discussed, and results fromestimation of the S1A/S1B clock frequencies presented.Fig. 2. Example 6-day interferogram from ascending track142 over Eastern China.2.2.1. CoherenceTo quantize the effect on coherence of the doubled temporalsampling from one image every 12 days (S1A) to one im-age every 6 days (S1A+S1B), a stack of thirteen 1900 km0.0 0.2 0.4 0.6 0.8 1.0Coherence  Occurence 6 days12 days18 days24 days30 daysFig. 3. Large-scale summer-time coherence statistics for 6-30 days temporal baselines, for a multilook factor of 64x16.Due to slight oversampling in the original data products, theeffective factor smaller by a roughly 40%.datatakes over Eastern China has been analysed. The datacover a large variety of land cover classes. One example 6-day interferogram is shown in Figure 2.From the stack, all interferogram combinations up to 30days are generated, using a multilook factor of 64 in rangeand 16 in azimuth. This corresponds to a 200-m ground res-olution. Then histograms are formed over all coherence val-ues for 6-, 12-, 18-, 24- and 30-day interferograms, respec-tively. The results are shown in Figure 3. The part of thehistogram around its peak represents the incoherent targets.The peak is not at zero coherence due to the inherent bias instandard coherence estimators. Notic how the fraction of in-coherent targets (as measured by the value of the histogrampeak), decreases significantly from 18 to 12 days, and evenmore from 12 to 6 days. For the 6-day coherence histogram,notice that the histogram is now dominated by the coherentpixels, which means that there is at least some coherence al-most everywhere.This case study is large in spatial scale, but limited in timeand to a single track. Nevertheless, it provides a good indica-tion that 6- and 12-day temporal sampling are good choicesfor a C-band interferometric mission. Increasing the temporalsampling interval beyond 12 days has an impact on the abilityto observe, e.g. discrete tectonic events in areas with densevegetation.2.2.2. Effects of radar clock errorsAn important requirement of a multi-satellite interferometricSAR mission is the stability of the radar hardware clock fre-quencies, used to generate relevant radar parameters, includ-ing• Carrier frequency• Range sampling rate• Pulse repetition frequencyAcceptable relative offset from nominal clock frequency is fora SAR instrument typically specified to be within 10−7–10−9,i.e. 1-100 parts per billion (ppb).Constant absolute errors of this level for one or both satel-lites have no noticable effect on the InSAR performance. Onthe other hand, if there is an unknown linear drift over time forone or both satellites, this will result in a time dependent rangedirection phase ramp that will be misinterpreted as linear tiltof the entire swath. For the ENVISAT ASAR instrument, thiseffect was significant [4].And if there is an constant unknown difference in the clockfrequency between two satellites, there will be a constantrange direction phase ramp in all interferograms, independentof temporal baseline. This effect is in principle possible toestimate and remove in the post processing.To estimate the clock frequencies, we exploit the fact thatin addition to the radar clock, all SAR missions necessarily in-clude a maintained source of absolute time, used for time tag-ging of instrument data. By analyzing the relative differencebetween actual time tags with respect to the ones expectedfrom the nominal radar timing parameters, it is possible toestimate the radar clock frequency with very high precision.Such an analysis is only using the time tags of each acquiredline, not the actual data. Thus, it can be carried out based onL0 annotation products if available, or else from standard L0products.Table 2 shows preliminary estimated clock errors duringthe commissioning phase, based on more than 1000 L0 anno-tation products per instrument. Both clocks show spectacular∆fS1A ≈ −fref · 10.1× 10−9≈ −0.38Hz∆fS1B ≈ −fref · 16.8× 10−9≈ −0.64HzTable 1. Preliminary estimated clock frequency offsets for theS1 mission, based on L0 annotation data from Jun-Aug 2016.Here, fref = 37534722.24Hz is the nominal clock frequency.performance, with errors less than 1 Hz. To further checkfor a possible long-term drift of one or both of the clocks,we analysed time tags for all datatakes in track 139 descend-ing over Norway during the entire mission lifetime. Figure 4shows the estimated clock error (squares), together with mod-elled linear trends (dotted lines). The estimated errors as ofJuly 2016 are identical to the preliminary results, confirmingthe initial analysis. In addition, we now have estimated de-cay rates of -1.0 ppb/year for S1A, and -1.1 ppb/year for S1B,respectively. The fact that the decay rates are virtually identi-cal, means that the relative error between S1A and S1B clocksDec 2014 Jun 2015 Jan 2016 Jul 2016 Feb 2017 Sep 2017-20-15-10-50LO relative errror [ppb]Fig. 4. Estimated clock error of S1A (black squares) and S1B(orange squares) based on long datatakes in track 139D. Thedotted lines show the estimated linear trend.will remain constant. The decay rate is slow enough that evenafter a decade of operation, the clock errors will remain closeto the current values.∆fS1A ≈ −fref · (9.9 + 1.0t)× 10−9∆fS1B ≈ −fref · (16.3 + 1.1t)× 10−9Table 2. Estimated clock frequency offset drift for the S1mission, see Figure 4. Here, fref = 37534722.24Hz is thenominal clock frequency. The time variable t is the numberof years since 2016-01-01.2.3. InSAR deformation analysisIn this Section, we present two case studies, one earthquakeanalysed with single S1A/S1B interferograms, and one two-year time series analysis based on S1A/S1B data.2.3.1. Amatrice earthquakeA shallow M6.2 earthquake hit Central Italy, the area of cityof Amatrice, on 24 August 2016. The quake epicentre wassoutheast of Norcia, Italy, in an area near the borders of theUmbria, Lazio, Abruzzo and Marche regions. The earthquakeoccured in the final part of the commissioning phase of S1B,and ESA released a number of S1B scenes to the scientificcommunity as part of the relief effort. Figure 5 shows a 2Ddecomposition based on ascending and descending 6-day in-terferograms, both acquired on Aug 21 and 27, 2016, in tracks117 and 22, respectively. Note that this analysis was madeavailable only 4 days after the disaster, demonstrating the op-erational capability of the S1 constellation. The data set canbe found in [5].Fig. 5. Left panel: Vertical component, with color scale range [-250 mm, 250mm], red is down, green is zero. Right panel: EastWest component, with color scale range [-100mm, 100mm], red is East, blue is West.Fig. 6. Sentinel-1 PSI analysis using data from S1A and S1B,descending track 139, Dec 2014 – Nov 2016.2.3.2. Subsidence of Oslo train stationThe area around the train station in Oslo is reclaimed landand has experienced subsidence for decades or more. We per-formed a PSI analysis, based on a 2-year long S1 stack with43 images from descending track 139. The linear velocitycomponent is shown in Figure 6 (see also [6]). The observeddeformation rates agree well with existing results from otherSAR missions.3. DISCUSSIONTo summarize, the S1 constellation already shows an excel-lent interferometric performance, with no unexpected techni-cal problems. In particular, the coherence for 6-day interfer-ograms has been shown to improve the signal quality signifi-cantly compared to only 12-day interferograms. We have alsoconfirmed that both radar clocks are extremely stable, with noexpected long-term impact on the InSAR performance.4. REFERENCES[1] Ramon Torres et al, “GMES Sentinel-1 mission,” RemoteSensing of Environment, vol. 120, pp. 9 – 24, 2012,[2] “ESA SEOM InSARap project web page (Team B),”http://www.insarap.org/, Accessed: 2017-01-09.[3] “Sentinel-1 satellites combine radar vision,” http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1/Sentinel-1_satellites_combine_radar_vision/, Accessed:2017-01-09.[4] Petar Marinkovic and Yngvar Larsen, “On resolving thelocal oscillator drift induced phase ramps in asar and ers1/2 interferometric data [poster presentation],” FRINGE-2015. http://www.slideshare.net/petarmar/local-oscillator-marinkoviclarsenfringe2015poster[5] Petar Marinkovic and Yngvar Larsen, “Mapping andanalysis of the Central Italy Earthquake (2016) withSentinel-1 A/B interferometry [Data set],” https://doi.org/10.5281/zenodo.61133, Aug. 2016,Accessed: 2017-01-09.[6] “Oslo train station on the move,” http://www.esa.int/spaceinimages/Images/2016/11/Oslo_train_station_on_the_move, Accessed:2017-01-09.

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The Sentinel-1 constellation for InSAR applications: Experiences from the InSARAP project

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