Neshyba 



Bandwidth 



Unfiltered 330-600 millimicrons (S-11) 



Filtered 1 millimicron, centered at 470 



millimicrons 



Dark Current 0. 002 microamperes 



Antenna 5" telescope 



Beamwidth 0. 5 milliradians, circular 



Effective Aperture 1 . 26 x 10"^ square meters 



Such a system might be used to map the ocean floor, em- 

 ploying a scanning technique similar to that used in side-looking 

 aircraft radar, or by side-looking sonar mounted in a submersible. 



Noise Considerations 



In an optical receiver employing a photomultiplier tube, 

 statistical fluctuations in the mean rate of electron emission from 

 the photocathode surface are a noise source. In the absence of 

 extraneous radiation, the thermionic emission, commonly called 

 dark current, limits receiver sensitivity. The noise power depends 

 upon the work function of the cathode emissive surface and repre- 

 sents an inherent limit to system performance. 



The background radiation field results from the natural 

 and stimulated bioluminescence and from near field backscatter 

 from the transmitter. The latter source is usually eliminated by 

 gating the receiver to the actual target return. 



For any radar system, there exists a minimum useful value 

 of the signal-to-noise ratio, S/N, at the receiver. The probability 

 of detection of a target is the probability of the signal-plus-noise 

 exceeding some threshold. This is a statistical value based upon 



(a) the mean rate of thermionic emission from the cathode; 



(b) the mean rate of photon arrival, at the photomultiplier 

 cathode, of both the signal and the radiation background. 



The problem has been treated in the literature, a good example beirg 

 that given by Flint (1964). For mapping radar, a usually selected 

 minimum useful signal-to-noise ratio is one. 



Minimum Detectable Signal 



For a receiver employing a photomultiplier sensor, the in- 

 herent receiver noise is that noise generated by dark level emission 

 from the photocathode. The dark current of the typical RCA 6199 is 

 2 X 10"" amperes and the gain is 6 x 10^. The mean rate of photo- 

 cathode emission due to dark current is given by 



431 



