What is InSAR?
InSAR is an abbreviation for Interferometric Synthetic Aperture Radar (SAR) and is a radar imaging technique that allows terrain to be imaged in all weather conditions. With conventional SAR one can obtain a high-resolution image of the terrain radar reflectivity but the image will contain distortions if there are variations in terrain elevation. With InSAR an extra receive antenna is used to allow one to solve for the terrain pixel locations including pixel height above a reference ellipsoid.
The airborne system that was used to collect these images can, in a single flight, produce an accurate map of a swath on the ground 20km wide, and as long as necessary. The output image contains information on two facts. It contains the altitude of the terrain, and it contains the radar "brightness" of the terrain.

How does InSAR work?

An airborne InSAR system collects data from a pair of radar antennas mounted on the aircraft. Each antenna collects a complete image of the scene. The angle between the baseline and the image point is given by the phase difference between these two images. The distance to the image point is given by the return time of the radar pulse. The location of the image point along the track is given by the focussed SAR image. These three measurements provide enough information to locate the image point in three dimensions relative to the aircraft. Thus from a single pass of an aircraft, it is possible to produce a precise digital elevation model describing the region's topography.

Terrain Mapping Using InSAR

Using across-track interferometry, it is possible to measure the position in three dimensions of each image pixel. This makes it possible to automatically generate a Digital Elevation Model (DEM). This model is usually stored in Universal Transverse Mercator (UTM) coordinates, with both altitude and radar backscatter information for each pixel. The accuracy of the DEMs produced at CCRS is comparable to that of a 1:50 000 scale map. In the case of Bathurst Island, no 1:50 000 scale maps exist for the northern part of the island, and so CCRS has the most detailed topographic information available.

In addition to simple topography, the radar image can provide valuable information. The radar backscatter is highly dependent on surface cover, producing different results from first-year ice and multi-year ice, for example. In regions with little or no vegetation, the geology of a region affects the surface roughness, which is clearly visible in the radar backscatter. In regions with vegetation, InSAR clearly shows tree cover both in radar backscatter and in elevation. Where trees have been cut, the clearings can be visible in the backscatter, and a lower elevation is frequently noted. This makes it possible (in some cases) to directly measure tree height.

Visualizing InSAR data

The three dimensions of terrain, as recorded by InSAR have to be appreciated in three dimensions. Yet most data presentations are on paper or a computer monitor, which provide only two spatial dimensions. Several tricks have been developed to allow the eye-brain to see the three spatial dimensions available. Three of these are colour, stereo viewing and perspective viewing.

• Geology: mapping surficial geological features.
• Hydrology: defining drainage patterns and watersheds.
• Agriculture: determining soil erosion potential.
• Soil science: landslide susceptibility mapping.
• Forestry: access road planning.
• Recreation: planning trails for hiking, skiing, snowmobiling.
• Civil engineering: cut and fill estimates, route planning, reservoir design.