SPINDEL: PHASE FLUCTUATIONS, COHERENCE AND INTERNAL WAVES 



The technique used to form these apertures and to compensate 

 them for motion of the receiving hydrophone has been reported in a 

 previous paper (Porter, Spindel and Jaffee, 1973) . In brief, receiving 

 hydrophones are suspended within range of a bottom moored navigation 

 net consisting of three acoustic sources emitting continuous tones in 

 the 12 to 13 kHz region. Receiver motion is manifest as Doppler shifts 

 in these tones. Doppler shifts are translated into equivalent motion. 

 With the current version of the system, receiver motion is tracked to 

 within 1/4 wavelength at 12 kHz, about 3 centimeters. Long-range 

 acoustic transmissions at 220 and 406 Hz are simultaneously received 

 by the moving hydrophone. Doppler shifts due to receiver motion are 

 resolved into equivalent phase shift at 22 and 406 Hz. This shift 

 is subtracted from total accumulated phase leaving a residual phase 

 variation in the long-range transmission resulting solely from 

 variations in the intervening water mass. 



Figure 1 shows the deployment of a typical navigation net and the 

 generation of five distinct apertures labelled 130, 131, 132, 133, 135 

 from 3 to 8 km in length. The time span of each aperture is indicated 

 by time in minutes along each drift path. 



Figure 2 is a schematic illustration of received low-frequency 

 transmission. The carrier at 220 or 406 Hz is received at frequency 

 f , displaced from f by the Doppler shift due to receiver drift. 

 Spread about f results from variation in drift rate, and from vari- 

 ations in the transmission medium. When scattering from the sea 

 surface is significant, it appears as sideband energy about the 

 carrier with peaks at multiples of the peak frequency of the surface 



wave spectrum. The Doppler correction scheme removes variations f ,. 



d 



The signal is then heterodyned down to dc , and variations resulting 

 from surface scatter are removed by filtering around the carrier. 



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