1052 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBEK 1957 



is the heart of the indicating and recording equipment. However, 

 many types of echo sounders have poor timing mechanisms which depend 

 upon mechanical governors, poorly regulated AC power supplies, and 

 friction drives. Recently, a Precision Depth Recorder (FDR) havmg an 

 instrument accuracy of better than one fathom in 3,000 has been de- 

 veloped.^ The equipment is used with standard deep-sea sounding equip- 

 ment. This PDR will perform the timing function with an accuracy bet- 

 ter than one part in a million. 



After obtaining a record of the time between the outgoing and re- 

 ceived pulses the data must be converted to depth, employing velocity 

 corrections and slope corrections. True sound velocity is obtained by 



SURFACE 



SEA BOTTOM-' 



Fig. 1 — Slope correction of echo soundings. 



reference to tables, or by computation based on simultaneous sea water 

 temperature and salinity measurements. The distance traveled by the 

 echo, obtained from the velocity-corrected data, is converted to depth 

 by the slope correction. 



The PDR sends out a ping and records an echo once each second. If 

 the sounding vessel is under way at 10 knots, the individual soundings 

 are about 17 feet apart. When the outgoing ping exactly coincides with 

 the returning echo, a gating circuit reduces the frequency of soundings 

 to once in 2 seconds corresponding to a spacing of about 34 feet at 10 

 knots. 



Fig. 1 illustrates the proljlem of slope correction on an ideal slope, 

 where the ship is steaming at right angles to the slope. At sounding 

 position A, the echo is returned from B, the nearest point on the bottom. 

 On the uncorrected profile, this would be interpreted as a vertical sound- 

 ing AC. To obtain the true vertical depth at A, an amount CD must 



