Influence and Correction of Ionospheric Effects on Sentinel-1 TOPS Interferometry

Abstract

Synthetic aperture radar (SAR) and interferometric SAR (InSAR) measurements are disturbed by the propagation velocity changes of microwaves that are caused by the high density of free electrons in the ionosphere. Most affected are low-frequency (L- or P-band) radars although higher frequency (C- or X-band) systems, as the recently launched Sentinel-1, are not immune. Since the ionosphere is an obstacle to increasing the precision of SAR systems needed to remotely measure the Earth’s dynamic processes, as ground deformation, it is necessary to estimate and compensate ionospheric propagation delays in SAR signals. In this work we work discuss about the influence of the ionosphere on interferograms and the possible correction methods. The ionospheric error, when measuring ground motion with C-band InSAR systems, is often considered small enough to be ignored. In this work we assess the average ionospheric error level occurring in non-compensated interferograms by using global ionospheric measurements, to show that the correction of ionospheric effects can sensibly increase the measurement accuracy. A statistical analysis of IGS global ionospheric TEC maps is used to calculate the standard deviation of the LOS and along-track error caused by ionospheric effects. IGS global TEC maps are generated assimilating a network of GPS-based TEC measurements with ionospheric models. The resolution and accuracy of these maps are too low to allow the correction of interferograms. Nevertheless, we use them for the statistical analysis to obtain a reasonable assessment of the possible ionospheric error when no correction is applied to interferograms. Firstly, we produce a histogram of the differential ionospheric TEC level considering all possible 12-days interferograms of one year (2015). In fact, a different absolute ionospheric level during the two acquisitions generates a linear phase term in the interferogram range direction due to the incidence angle change. This additional phase term introduces a measurement error. A global map of the expected LOS error can then be produced; an example is reported in Figure 1. Such a map can be used to predict the ionospheric error to ground deformation measurements. Solar cycle, diurnal, seasonal, and geographical variations of the ionosphere influence the error level for different satellites with different orbits, acquisition times, and for different geographical regions. For example, the result shows how the standard deviation of the LOS deformation error for a single Sentinel-1 interferogram in ascending geometry is, in low latitude regions, about 4 cm every 100 ground range km. The latter considers only the effect due to the incidence angle change; a similar analysis has been also realized for the ionospheric gradients in the range and azimuth directions. The analysis indicates that the ionosphere can sensibly reduce the accuracy of ground deformation measurements. To increase such accuracy, the split-spectrum method can be used to estimate and remove the ionospheric phase screen from interferograms. In the second part of the work, the processing workflow of the split-spectrum method, applied to the special case of TOPS images, will be presented. Practical examples of successful correction of ionospheric disturbances, as well as possible issues, will also be presented. Figure 2 shows a disturbed interferogram and its compensated version. The phase screens estimated with the split-spectrum method will then be compared to the ones derived from the global TEC maps, to verify the quality of the statistical analysis. Finally, other ionospheric effects on Sentinel-1 interferograms, such as ionosphere-induced azimuth shifts will also be discussed with some examples, and possible correction strategies proposed.