714
Acknowledgment
impulse energy to clutter ratio can be formed
Elg d ] °o A ‘
(32)
This expression has no dependence on relative velocity
scaling terms, so that no correction for the orbital
geometry needs to be applied. The only other parameter
needed is a very well known and stable number, the inter
pulse spacing at the Earth’s surface V B /f a .
Velocity Ratio
Figure 6. Velocity Ratio (in dB) for a range
of radar satellite altitudes.
CONCLUSIONS
This paper has explored some of the consequences of the
spherical geometry essential to orbital SAR systems. It
has been shown that parametric dependence on the
relative spacecraft and beam velocity parameters may
become significant for quantitative image analysis
purposes such as calibration. The approach has been
through a discrete azimuth model of the SAR to represent
the pulse repetition of the radar, and the digital
implementation of most SAR processors used in
calibration applications. Analysis is based on propagation
of appropriate correlation properties of signals through
the system. Processor gain has been identified as a
natural consequence of the fact that the system impulse
response width is generally larger than one inter-pixel
distance. The results have been cast in terms of known
and robust parameters, rather than secondary parameters
such as resolution. For orbital SAR systems, it is
recommended that correction for the velocity ratio effect
be included in calibration methodology based on the peak
point scatterer method. The integral method of point to
clutter calibration is not sensitive to the effect, although
the beam footprint velocity (rather than the spacecraft
velocity) is the correct parameter to calculate inter-pulse
spacing at the Earth’s surface.
The author is grateful for constructive discussions on
issues relating to the work of this paper with L.M.H.
Ulander while he was a visiting scientist at the Canada
Centre for Remote Sensing, on leave from Chalmers
University of Goteborg, Sweden.
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