ANDERSON: VERTICAL NOISE DISTRIBUTION 



INSTRUMENTATION 



The ability to interpret the measurements was severely limited 

 by the characteristics of the instrumentation system. Specific 

 problems encountered were associated with: 



• Angular sampling 



• Frequency filtering 



• Data quantization 



• Common-mode noise. 



Figures 4, 5 and 6 illustrate the response of the 128, 1.5- 

 degree spaced, preformed beams to a horizontally arriving signal at 

 frequencies of 23, 36, and 100 Hz, respectively. The beams were formed 

 without shading, and since the array elements were equispaced, the 

 sin x/x pattern is obtained. At 23 Hz (Figure 4) , this pattern is 

 fairly adequately sampled by the selected beams. At 36 Hz (Figure 5), 

 they are somewhat undersampled, and by 100 Hz (Figure 6) , they are 

 undersampled to the extent that the near-horizontal beams are steered 

 to the nulls in the sin x/x pattern. At 100 Hz, the array is under- 

 populated, resulting in grating lobes at + 30 degrees from the horizon- 

 tal direction (90 degrees in the figures) . These beam responses play 

 an important role in interpreting the measured data, especially at 

 100 Hz, as described subsequently. 



To obtain data at prescribed frequencies, the basic time series 

 of broadband (digitized at 1 kHz) time-multiplexed beamformer output 

 was multiplied by sine and cosine functions of the center passband 



863 



