Stationary in local meaning, Under this assumption Castleman (1979) 
Proposed using the geometric mean filter. Here the power spectra of 
the object image (and/or the noise) are functions of the position in 
the image (position variant filtering), 
5.2 Signal Dependent Noise 
The restoration techniques presented in this paper were based on a 
signal independent degradation model (eq. 4-1). The signal dependence 
may be ignored for common restoration techniques, hut for high levels 
of accuracy it must be taken into account, The signal dependent 
property may be handled using homomorphic restoration techniques 
(Stockham 1972, Oppenheim 1975), Consider the multiplicative noise de- 
gradation model 
g(x) = £(x) n(x) (5-1) 
where the original image f(x) is degraded by a multiplicative noise 
term n(x), Taking the logarithm of eq. (5-1) yields the additive 
result 
log(g(x)) = log(f(x)) + log(n(x)) (5-2) 
Conventional linear filtering can now be used to estimate log(f(x)) and 
by exponentiation we obtain the estimate of the original image f(x). 
5.3 Conclusions 
It is reasonable to assume, that the accuracy of matching of 
stereoscopic aerial photos will increase with better image resolution. 
Better resolution can be obtained using a suitable restoration 
technique, The technique should be based On à priori knowledge of the 
most significant degradation sources, This raises the question which 
degradation sources are the limiting factors in the restoration process 
of aerial photos. Three important degradation sources are: 
- motion 
- film grain noise 
- atmosphere 
There exist several nonrecursive and recursive methods to remove both 
Space invariant and space variant motion blur (Sawchuk 1972, Frieden 
1975, Rosenfeld and Kak 1976, Sondhi 1972, Gonzales and Wintz 1977), 
Recently, however, new types of metric aerial cameras (Carl Zeiss Jena, 
GDR and Zeiss Oberkochen, FRG) have been developed that have built-in 
compensation for image motion in the flight direction. As a consequence 
of this, longer exposure time and finer grained film can be used, thus 
increasing spatial resolution, However, the problem of motion blur may 
Still remain, the reason being that the aircraft with the equipment may 
be subject to angular motion, the effect of which is in a direction 
other than the flight direction.