SAR Interferometry for DEM Generation

Sar Interferometry (InSAR)
Basically, InSAR can be categorised into three parts,
  • Single or simultaneous pass interferometry 
    In this, two images are simultaneously acquired from two antennas separated by a distance known as baseline. The SRTM is based on this principle where receiving antennas are located at two ends of a boom of 60 meters as baseline length.

  • Repeat or dual pass interferometry
    It is also known as repeat track interferometry. Here, two images of the same area are taken in different passes of satellite. The SAR data acquired from satellites namely ERS-1 and ERS-2, JERS-1 and RADARSAT may be used in this fashion to produce SAR interferograms (Rao and Rao, 1999).

  • Three pass interferometry or differential InSAR 
Here, three SAR images of the same area are acquired in three different passes to generate two interferograms. The arithmetic difference of the two interferograms is used to produce a differential interferogram. From this differential interferogram, a millimeter level accuracy in elevation measurement can be achieved.


Fig. 2: A depiction of fringes
(Lines of constant colour
correspond to constant phase
difference and therefore
constant terrain altitude; i.e.
height contours)
(Source: http://www.earth.esa.int/)
Sar Interferometric Processing

Geometry of InSAR
The geometry of InSAR may be understood with the help of Fig. 1. In this figure, A1 and A2 are the two radar antennas that simultaneously view the same surface and are separated by a baseline vector B with length B and angle á with respect to horizontal. A1 is located at height h above some datum. The distance between A1 and the point on the ground being imaged is the range , while is the distance between A2 and the same point. The aim is to determine the elevation h of each point in the image. The topography z(y) can be inferred from the phase measurement to a precision of several meters, assuming that the 2 ambiguity inherent in any phase measurement can be solved (Equation 1).

Z (y) = h - rcosq    (1)

where q is the look angle of the radar. 

A SAR interferogram, viewed as a fringe pattern, shows the relative difference between phases of two images. The phase difference depends on the geometry of the two antenna tracks and the image point and thus is proportional to the difference in path delays from two antennas and is given by,

j = 4p (r1 - (r + d r) / l    (2)

where l is the wavelength.

To determine h, the interferometric processing steps that are generally followed can now be enumerated as:

  • Selection of suitable pair of SAR images
  • Geometric registration
  • Interferogram generation
  • Phase unwrapping
  • Extraction of elevations from phases
Selection of SAR images
The first requirement is the availability of two SAR images in complex form. Complex SAR data refer to a set of data that has a real (cosine) and an imaginary (sine) component. The two values combine as vectors to provide the overall phase and intensity of a wave. Both these components of backscattered signals are measured by the SAR sensor onboard the satellite. This provides two resulting data streams, namely ‘I’ (representing In-phase / intensity / cosine component) and ‘Q’ (representing Quadrature / phase / sine component) data streams (http://www/asf/alaska.edu/).

The selection of the images is made on the basis of baseline length and the time period between two image acquisitions. Depending upon the application and the spatial resolution of the data, the baseline length can be chosen. For example, in the case of ERS–1 and 2, the baseline may be taken as 150 to 300m for topographic applications, 30 to 50m for surface change detection and up to 5m for surface feature movement studies such as crustal deformations, lithospheric movements, movement of glaciers etc. Also, the time gap between two passes of satellite may not be kept large as there may be some changes in the scene that may lead to temporal de-correlation. However, the temporal de-correlation, in the case of ERS-1 and 2 may be taken care of by tandem operation of two satellites at a small temporal resolution of as low as one day (Rao and Rao, 1999). 

The selected raw data are then processed to convert SAR signals to image products like, Single Look Complex and GTC Geocoded Terrain Corrected with the help of DEM. This processing requires knowledge about the precise orbit and calibration parameters such as time reference and intervals of each image, and the chosen spatial and temporal resolutions of the images. 

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