ANDERSON: VERTICAL NOISE DISTRIBUTION 



DISCUSSION 



Dr. John Hanna (Office of Naval Research) : What exactly is the 

 common mode noise and where does it originate? 



Dr. Anderson: I wish we could say what it is. It apparently is 

 a broadband noise that was introduced on all channels coherently, and 

 it is associated with the interface between the array electronics and 

 the beamformer electronics. It has an analog interface between the 

 two. 



On board FLIP there must have been some noise picked up in that 

 region, and it seemed to have a fairly uniform spectrum. If you look 

 at the levels of noise that appear at the three frequencies they are 

 within a couple of dB of the same level. If you look at the peak of 

 those responses and correct it for the pre-processing filters, the 

 signal-conditioning filters that are ahead of the beamformer, there seems 

 to be just broadband noise arriving in phase on all elements. 



Dr. Gordon Raisbeck (Arthur D. Little, Inc.): We did some 

 analysis of ambient noise measurements made with an ITASS array and 

 found the same phenomenon. There is no way to make a uniform, consis- 

 tent graphical presentation of the average noise level on each beam 

 without having the broadside beam stick out like a sore thumb. No 

 matter how often you calibrate and recalibrate that, there is always 

 a residual amount of energy which seems to be several times as great 

 in linear units than the energy in the adjacent beams. 



Mr. R. L. Martin (New London Laboratory, Naval Underwater Systems 

 Center) : Is there no way of getting rid of that type of energy 

 adaptively by using an adaptive process? 



Dr. Anderson: We looked at the possibility of doing that. We 

 gave it up because we found that if we looked at all the data we had. 



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