Demonstration of a tropospheric correction for Sentinel 1a

David Bekaert is a PhD student based in the School of Earth and Environment at the University of Leeds. David’s research involves using space based remote sensing technologies and atmospheric corrections for the detection of small magnitude ground movements. In this guest blog, David provides some technical details showing how weather models can be used to estimate the scale of atmospheric delays in the new European Space Agency‘s Sentinel 1a satellite radar data.

Sentinel 1a is a C-band Synthetic Aperture Radar with a repeat acquisition rate of 12 days. In combination with Sentinel 1b (estimated launch in 2016), the acquisition rate will decrease to 6 days (click here for more Sentinel 1 information). This high repeat rate together with the large illuminated tracks (200 km wide) make the Sentinel 1 constellation an attractive source of data to study the cryosphere and solid earth.

An example Sentinel 1 interferogram over Italy is shown in Figure 1a (by NORUT and PPO.Labs, see http://insarap.org). This data is not continuous, but wrapped, were each color cycle represents 2.6 cm of displacement in the radar line of sight (LOS). Unwrapping this correctly and the correction for tropospheric artefacts are two of the main challenges within the InSAR community.

A) Sentinel 1a IW interferogram over Italy. Copyright by NORUT and PPO.labs (http://insarap.org), as part of ESA InSARap using Copernicus data. B) Our estimated tropospheric correction using the 75 km ERA-I weather model product. Courtesy of Bekaert David. ERA-I data were provided by ECMWF. Each color cycle represents 2.6 cm displacement in the radar line of sight (LOS).

The high Sentinel 1 repeat rate does not necessary implies smaller atmospheric signals, as the atmosphere varies from day to day, and in space. In Figure 1b we demonstrate that the tropospheric correction estimated from the ERA-I (75 km) weather model outputs, from the European Centre for Medium- Range Weather Forecasts (ECMWF), shows promising results in reducing tropospheric arctifacts over a large region as imaged by Sentinel 1a. We find good agreement between the Sentinel 1a interferogram A) and our estimated tropospheric delay B) (e.g north-east region, south-west region, the volcanoes Etna and Vesuvius, and other locations).

In general we find that the weather model is capable of estimating the long-wavelength features well. While the weather model is capable of estimating the magnitudes correctly, the timing and thus location in space might be off. The problem of the troposphere will be further reduced in future, with a longer time-series of sentinel 1 acquisitions. These long-term observations together with tropospheric corrections will allow us to detect smaller magnitude surface deformations.

More Information:
[1] More of David’s research on his website: http://davidbekaert.com
[2] http://www.see.leeds.ac.uk/people/d.bekaert

Workshop: Using Satellites to Map Earthquake Hazards

Earlier this year scientists from the University of Leeds met with international colleagues to advance global earthquake hazard mapping capabilities.

The University of Leeds recently held a focused international workshop on using satellite radar to map global earthquake hazard. Scientists and remote sensing experts from Europe, China, and America met to discuss how best to exploit data from the European Space Agency’s new Sentinel-1 satellite, which is due to be launched in early 2014.

Interferogram showing the surface deformation after the 2003 Bam earthquake.
(ESA)

In earthquake-prone regions, the ground slowly and steadily warps in the time period between earthquakes at a rate of a few millimetres to a few centimetres per year. This tectonic warping or strain can be measured using a satellite radar technique known as InSAR and can tell scientists which regions worldwide are most at risk from earthquakes in future.

This technique has been developed over the last decade, but the upcoming launch of Sentinel-1, a new InSAR satellite, presents a major opportunity to map and understand global earthquake hazard better than ever before.

The workshop focused broadly on two themes: assessing the current methods and techniques used to map tectonic strain with InSAR, and discussing the best ways to combine these data with complementary GPS measurements of the same deformation.

The keynote speech at the workshop was given by Corné Kreemer from the University of Nevada, the lead scientist on the Global Strain Rate Model (GSRM) project. This project currently uses only GPS measurements to produce a global map of tectonic strain, and is a major input into the Global Earthquake Model (GEM), an international public-private partnership which aims to produce comprehensive maps of seismic hazard. A major outcome from the meeting was the development of plans to integrate InSAR data into GSRM, which will not only improve spatial resolution of the strain model, but will also improve geographical coverage in remote areas where it is difficult to make GPS measurements.

The Global Earthquake Model

GSRM presents a good opportunity for InSAR data to feed into GEM, and to therefore make the results of research into InSAR strain maps available to a wide variety of users, both inside and outside the scientific community.

Another outcome from the workshop was the compilation of a feedback document for the European Space Agency to help shape the Sentinel-1 mission plan. This is hoped to ensure that the new satellite’s ability to map earthquake hazard is maximised. In addition, the attending scientists made plans for further international collaboration, including between industry and academia. A particular focus for the planned collaboration is the testing and comparison of different methods for InSAR data processing and strain mapping.

The Leeds workshop was hosted at the School of Earth and Environment and was funded by the School’s Climate and Geohazard Services (CGS). 22 experts attended the meeting from the UK, France, Spain, Italy, Germany, the Netherlands, China, and the US, representing not just academia, but also the remote sensing and space industries, and the European Space Agency.