Analyzing changes of Sentinel-1 SAR imagery causedby epipolar geometry and satellite moving direction Study area: Baghestan region of Tehran

The complexity of interpreting SAR radar images makes target recognition difficult despite many studies performed in this regard. Various factors including material and dimensions of the target, radar frequency, polarization, target shape, and vision geometry affect the response received from SAR radar. Investigating these characteristics facilitate target recognition. Synthetic Aperture Radar sensors are widely used in both airborne and space-borne systems. Space-borne systems equipped with Synthetic Aperture Radar sensors are side-looking and because of their nature as a radar, many parameters such as vision geometry will affect their ability (or disability) in seeing the target and change the resulting images. Therefore, it is very important to study the effect of this parameter to identify the target and interpret these images. The visibility geometry includes incidence angle, look angle, and the direction of the imaging. 

The present study investigates visibility geometry in revision images and ascending and descending scenes. To reach this aim, a single scene captured by Sentinel-1 from a residential area is examined in different images with different directions, incidence angles, and imaging time. Results indicate that incidence angle changed slightly (4 degrees) and thus, left a negligible effect on the image. Moreover, there was a 5-day time interval between the captured images and therefore, this factor had the least effect on Synthetic Aperture Radar images. Unlike optical images, the direction of imaging had the greatest effect on SAR images. For an instance, a single ramp behaves differently in two images captured from different directions. Therefore, direction of imaging and its effects on seeing (or not seeing) the target are analyzed in ascending and descending images.

The effect of vision geometry on radar images has been rarely investigated in similar studies, and the present paper has taken a step forward in this regard. Fallahpour et al., (2016) have simulated the effect of incidence angle, which is a parameter of visibility geometry and the shape of the targets in SAR images. Shapes such as cones, cylinders, and cubes were used in this simulation representing real buildings, niches, tree trunks, etc. which are very common in SAR images. Moreover, behavioral pattern of the aforementioned geometric shapes were simulated at different landing angles (30, 40, 45, 50, and 60 degrees) from the viewpoint of SAR imaging systems to reach a more comprehensive result. Then, various studies investigating the effects of incidence angle and direction on radar images have been reviewed. Some of these studies have dealt with the effect of these parameters on the classification of radar images. Dumitru et al. have examined the effects of resolution, pixel spacing, patch size, path direction, and incidence angle on the classification of TerraSAR-X images. To reach this aim, they have selected an optimal TerraSAR-X product and then specified the number of classes. They have finally investigated the effects of incidence angle and path direction on the classification results. Results indicated that images captured in ascending direction were 80% better than the descending images. Moreover, images captured from an incidence angle near the upper wing showed better results.

The present study has investigated the effect of usually neglected parameter of visibility geometry on SAR images. Images were captured by Sentinel-1 in both ascending and descending directions. Following speckle noise reduction and geometric correction, incidence angle and its effects on the detected changes were investigated. The slight 4-degree changes of this parameter have not caused the resulting changes. Moreover, there was a 5 day time interval between these two images and thus, time could not be an effective parameter too. Results indicate that detected changes in the residential area were due to a change in the direction of imaging. Changes of this parameter can result in seeing (or not seeing) the target, and therefore, it is very important to investigate the effects of this parameter and correct it.