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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