Space-borne imaging radars

The imaging radar is an active remote sensing instrument, i.e., it transmits radiation towards the target area and receives backscattered echoes. As the imaging radar flies over the target area a two-dimensional image-like representation of the illuminated area is created. One can think of the operation of the imaging radar as a flash light. However, the major difference from a conventional camera is that radars use the microwave part of the electro-magnetic radiation for which the clouds are mostly transparent. In case of satellites, the most important benefit of the imaging radar is that the target area can be imaged in all-weather circumstances and independently from the sun’s radiation. Therefore, imaging radars provide wide-area and up-to-date information about the dynamic processes of our environment.

Synthetic Aperture Radar (SAR) is a special case of imaging radars. At its best, SAR satellites provide us with images with a resolution of about few tens of centimetres on the Earth’s surface. Nowadays, nearly all imaging radars on satellites are SAR sensors. SAR satellite images can be used in various Earth observation applications, such as in the monitoring of sea ice and snow conditions, detecting of land surface movements, and mapping of natural and built-up environments.

The Finnish Geodetic Institute has actively participated in such studies, where the feasibility of the SAR satellite images in practical remote sensing applications has been studied. In the following, two examples of the SAR satellite image products by the Active Sensing research team are presented.

Figure 1. Fusion image product derived from the ERS satellite images of the European Space Agency (Original data © 1995-1996, European Space Agency).

In the figure above, a fusion of 16 individual SAR satellite images is shown. First, the input images were precisely registered with each other, and then the information content of the input images were condensed pixel-by-pixel as a false colour image with three image channels. In the false colour image, the red channel corresponds to the average interferometric coherence, the green channel to the average intensity of backscattering, and the blue channel to the standard deviation of the backscattering intensity. Red areas are open and mainly non-vegetated areas, which are low-backscattering areas with high interferometric coherence. Vegetated areas, especially forests, are shown in green, indicating that the backscattering intensity is relatively higher compared to the other image channels. Bluish areas correspond to the water bodies, where the variation of the backscattering intensity is high due to the sea waves.

Figure 2. Automatically extracted 3D points from TerraSAR-X satellite images (radargrammetric method). (Original data © 2009, German Aerospace Center).

SAR satellite images can also be used to measure elevation differences (topography) in the imaged area. In principle, there are two main approaches, interferometry and radargrammetry. Interferometry is a technique in which the pixel-by-pixel phase difference between two or more SAR images acquired from slightly different perspectives can be converted into elevation differences of the terrain. On the other hand, radargrammetry can be seen as equivalent to the stereoscopic vision of human eyes and the method can be used to measure 3D coordinates for the tie-points between SAR images with different look angles. An example of the radargrammetric point cloud for the Espoonlahti area is shown in the figure. Areas having no 3D measurements are shown in white; they are water bodies and open non-vegetated areas, where an automatic tie-point search typically fails. The colour scale from blue to light green corresponds to the elevation values from 0 to about 60 meters above sea level. In favourable conditions, radargrammetry can be used to extract remarkably accurate 3D information nearly globally, which can be useful in remote areas of the world where other measurement techniques are not feasible.

Mika Karjalainen

D.Sc. (Tech.)
Research Manager