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The Intermittent Small Baseline Subset (ISBAS) Method


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The first major publication describing differential interferometric SAR (DInSAR) was by Gabriel, Goldstein and Zebker (1989) where L-band Seasat data was used to describe surface motion over a soil surface.  Since then, there have been countless instances in the literature where meaningful DInSAR interferograms have been generated over vegetated surfaces, even using C- and X-bands.  However, generating a meaningful profile of deformation over long periods of time has proven evasive as soils and vegetation are very dynamic with the quality of the results (coherence) changing with the weather, over a growing season etc.  There have been no clear solutions to this problem until now...

The Intermittent Small Baseline Subset (ISBAS) method is an adapted version of the established low resolution SBAS DInSAR time series algorithm (Berardino et al., 2002). It has been designed to improve the density and spatial distribution of survey points to return measurements in vegetated areas where DInSAR processing algorithms habitually struggle.


The algorithm can retrieve measurements for targets that are intermittently coherent throughout the period of observations and, as a consequence, the density of survey points returned is higher and their distribution not limited to urban centres.


The ISBAS method has been successfully validated with ground truth over an area of gas production and geostorage in North Holland, the Netherlands. There is also a growing application of the technique to the monitoring of peatland surfaces.  The ISBAS method is entirely compatible with Sentinel-1 IW products and it is possible to form wide-scale mosaics from individual surveys.


These results, and others, have demonstrated that the ISBAS technique can be used with confidence over locations where traditional ground-based survey measurements are not available and without recourse to the use of ground infrastructure, such as corner reflectors.


It has been fully assessed by the British Geological Survey for the baselining of land motion prior to shale gas activities in the UK.

For more detailed information, please see our list of peer-reviewed publications (below).

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Unique Selling Points

Other InSAR Techniques

Only works over urban and rocky terrain (Persistent Scatter Interferometry – PSI methods; Small Baseline methods). 

Example: UK.  In theory, this is limited to 7% coverage as this is the extent of urban  classes.


Wide areas cannot be easily covered as high point densities only found in urban areas.

Sometimes difficult to interpret as only a partial view (urban area only) is available.

Result is in point format which is hard to view and interpret on a GIS.

Not yet operational – still in research domain.

Manually intensive.


Requires control points to provide relative motion over wide areas.

Can provide surveys at full resolution.

Time-series can be provided.

Many different methods available, some open-source.

Most PSI/Small Baseline methods are non-exclusive.


Can provide dense land motion measurements over all vegetated and non-vegetated terrain types.



Example: UK.  In theory we could get 100% coverage.

Can easily cover wide areas due to high density of points everywhere.

Easy to interpret as a high density of points gives a full picture of the deformation.

Result is in raster format making it easy to view and interpret on a GIS.

Routinely capable of regional/national mosaics.

High degree of automation.

Fast turnaround (once data has been downloaded, processing can take less than 2 days).

Requires no ground control to provide relative motion over wide areas.

Medium resolution.  (Sentinel-1/ENVISAT/ERS – 90m.  TerraSAR-X/CSM/Radarsat - <10m, depending on mode.)

Time-series can be provided with ‘movies’ of spatial motion.

Innovation - University of Nottingham have applied for a patent for the ISBAS method.  Andy Sowter named as the inventor.

The University of Nottingham have granted GVL an exclusive license for ISBAS.

Accuracy of ISBAS Products

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A full validation was performed over a gas storage site in the Netherlands and it was found that ISBAS products were accurate to within 1.52mm/year and 1.12mm/year when using ERS and ENVISAT SAR data sets, respectively.

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Technical Limitations

  1. Resolution: To generate meaningful measurements over vegetated areas, the ISBAS method degrades the spatial resolution in order to reduce noise.  For example, although the spatial resolution of Sentinel-1 data is better than 25m, the resolution of an ISBAS survey is around 90m.

  2. Coverage and Errors: The density of ISBAS measurements, and the error of the measurements, improves with the number of images used as input.  Therefore, we insist that more than 30 images are used as input to the ISBAS processing.

  3. Structural Monitoring:  The relatively low resolution of our surveys, and the method that we employ, is excellent at identifying ground motion over wide areas.  However, it is impossible to use these results to identify whether a specific building or structure within an area or pixel is subject to structural damage, or not.

Time to Delivery

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ISBAS processing is relatively automated, so we can generally turnaround a survey from order to delivery in less than one week.


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Monitoring tropical peat related settlement using ISBAS InSAR, Kuala Lumpur International Airport (KLIA).

Marshall, C., Large, D., Athab, A., Evers, S., Sowter, A., Marsh, S. and Sjörgesten, S.

Engineering Geology; doi:10.1016/j.enggeo.2018.07.015.


Long-Term Peatland Condition Assessment via Surface Motion Monitoring using the ISBAS DInSAR Technique over the Flow Country, Scotland.

Alshammari, L., Large, D., Boyd, D., Sowter, A., Anderson, R., Andersen, R. and Marsh, S.

Remote Sensing, 10, 1103; doi:10.3390/rs10071103.



Exploitation of the Intermittent SBAS (ISBAS) algorithm with COSMO-SkyMed data for landslide inventory mapping in north-western Sicily, Italy.

Novellino, A., F. Cigna, A. Sowter, M. Ramondini, and D. Calcaterra.

Geomorphology, 280, pp. 153-166.

The relationship between intermittent coherence and precision of ISBAS InSAR ground motion velocities: ERS-1/2 case studies in the UK.

Cigna F.; Sowter A.


Remote Sens. Env. 2017, DOI: 10.1016/j.rse.2017.05.016

Ground Motion in Areas of Abandoned Mining: Application of the Intermittent SBAS (ISBAS) to the Northumberland and Durham Coalfield, UK.

Gee, D.; Bateson, L.; Sowter, A.; Grebby, S.; Novellino, A., Cigna, F.; Marsh, S.; Banton, C.; Wyatt, L.


Geosciences. 7(3), 85.

Supporting energy regulation by monitoring land motion on a regional and national scale: A case study of Scotland. 

Sowter, A.; Athab, A.; Novellino, A.; Grebby, S.; Gee, D.  


Proc IMechE Part A: J Power and Energy.  2017.  DOI: 10.1177/0957650917737225



Mexico City land subsidence in 2014–2015 with Sentinel-1 IW TOPS: Results using the Intermittent SBAS (ISBAS) technique. 

Sowter, A., Amat, M.B.C., Cigna, F., Marsh, S., Athab, A. and Alshammari, L.


International Journal of Applied Earth Observation and Geoinformation, 52, pp.230-242.

Monitoring land motion due to natural gas extraction: Validation of the Intermittent SBAS (ISBAS) DInSAR algorithm over gas fields of North Holland, the Netherlands.

Gee, D., Sowter, A., Novellino, A., Marsh, S. and Gluyas, J.


Marine and Petroleum Geology, 77, pp.1338-1354.


The application of the Intermittent SBAS (ISBAS) InSAR method to the South Wales Coalfield, UK.

Bateson, L., Cigna, F., Boon, D., & Sowter, A.

International Journal of Applied Earth Observation and Geoinformation, 34, 249-257.

Intermittent small baseline subset (ISBAS) InSAR analysis to monitor landslides in Costa Della Gaveta, Southern Italy.

Novellino, A., Cigna, F., Sowter, A., Syafiudin, M.F., Di Martire, D., Ramondini, M. and Calcaterra, D.


In 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS) (pp. 3536-3539). IEEE.

Intermittent Small Baseline Subset (ISBAS) InSAR of rural and vegetated terrain: a new method to monitor land motion applied to peatlands in Wales, UK.

Cigna, F., Rawlins, B. G., Jordan, C. J., Sowter, A., & Evans, C.


EGU General Assembly.


Intermittent SBAS (ISBAS) InSAR with COSMO-SkyMed X-band high resolution SAR data for landslide inventory mapping in Piana degli Albanesi (Italy).

Cigna, F., Novellino, A., Jordan, C.J., Sowter, A., Ramondini, M. and Calcaterra, D.


In SPIE Remote Sensing (pp. 92431B-92431B). International Society for Optics and Photonics.


DInSAR estimation of land motion using intermittent coherence with application to the South Derbyshire and Leicestershire coalfields.

Sowter, A., Bateson, L., Strange, P., Ambrose, K., & Syafiudin, M. F.


Remote Sensing Letters, 4(10), 979-987.

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