Method
The
nearly exact repeat orbit allows formation of an interferometric baseline as
shown in the figure. The ERS1 sensor provides a platform to record SAR data
which is useful for interferometry.
This exploitation of the orbit repeat feature is known as multipass INSAR.
For a point target that is at (x, y, z), the phase difference f between the
signal s1 and s2 received at r1 and r2 is
Where is the radar signal wavelength. The interferometric phase is obtained
by complex correlation of the first complex SAR image, obtained during the
first overpass, relative to the second image, belonging to the second pass,
after precise coregistration.
The interferometric phase can be used to determine the precise look angle by
first solving the cosine of the angle between the baseline vector and the look
vector:
The height z at the location r1 is determined from:
Where h is the altitude of the platform above the reference plane.
The phase in the interferogramme is known only modulo 2p, therefore it is necessary
to determine the correct multiple of 2p to add to the phase to obtain
consistent height estimates.
The correlation coefficient g of the complex backscatter intensities s1 and
s2 at r1 and r2 is defined by:
In the following we will call g the interferometric correlation. The correlation
coefficient g in terms of the baseline correlation factor a is equal to
where the geometrical baseline correlation factor a is given by:
where SNR is the signal to noise ratio, dr the slant range resolution, and
alpha is
the radar incidence angle.
The ERS1 radar sensor has a slant range pixel spacing of 7.9m, nominal incidence
angle of 23 degrees, and nominal range of 853 Km to the center of the
image swath.
For an 8 look image with a SNR of 20, the theoretical height resolution is better
than 2.5 m for a wide range of correlation values y.
Keywords: ESA European
Space Agency  Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,
atmospheric chemistry
