port and starboard. If the ship's motion is 

 forward, the frequency shift of the beam 

 angled forward will be positive and the one 

 angled aft negative. The difference between 

 the two return frequencies produces speed 

 readings which average out the effects of the 

 pitch of the ship in the water. Each beam is 

 angled from the vertical and is narrow (ap- 

 proximately 3° wide). 



The receiving unit contains four receiving 

 hydrophones, also similarly angled and reso- 

 nant at the same nominal frequency. Speeds 

 fore-aft and athwartship are measured inde- 

 pendently to read the true velocity compo- 

 nents of the ship's motion, and thus the 

 ship's actual speed and drift angle. Integrat- 

 ing speed over a period of time provides a 

 reading of distance traveled over the bottom. 

 Pitch, roll and heave, unless purposefully 

 done, are usually not a problem submerged, 

 but drift due to currents — which can equal or 

 even exceed the submersible's speed — is an 

 important factor in maintaining a desired 

 course. To satisfy the drift problem, the cur- 

 rent Doppler systems not only sense drift, 

 but display it in such a fashion that the 

 operator may compensate for it to follow a 

 specific track over the bottom. 



Doppler sonar has been employed for some 

 time in the docking and piloting of merchant 

 ships. It was only in the late sixties that it 

 became available for deep submersibles. The 

 U.S. Navy performed tests with a General 

 Applied Science Laboratory JANUS series 

 Doppler navigator on hoard ALUMINAUT in 

 1968 (26), and as a part of this work Sperry 

 Rand Corp. performed both laboratory and 

 field tests on the same Doppler models. The 

 tests showed promise and prompted Sperry 

 to produce its inodel SRD-101 Doppler Navi- 

 gator, the transducer of which is shown on 

 JOHNSON SEA LINK in Figure 10.24. A 

 thorough account of its development, design 

 and testing is contained in reference (27). 



The SRD-101 operates on a frequency of 

 400 kHz. Because of high acoustic absorption 

 at this frequency, it is limited to operations 

 no more than 250 feet off the bottom and not 

 closer than 4 feet to the bottom. Kritz (ibid.) 

 points out that a basic accuracy limitation in 

 heading reference resides in the fact that 

 virtually all Doppler systems rely on either a 

 magnetic compass or gyrocompass. To 



achieve a V2 percent (of heading) accuracy a 

 heading input from either source accurate to 

 V4 degree is required. He further comments 

 that small gyrocompasses of the type suita- 

 ble for submersibles do not provide the nec- 

 essary accuracy. At 1-degree accuracy nei- 

 ther do magnetic compasses. 



In spite of these limitations several vehi- 

 cles (e.g., TRIESTE, DEEP QUEST, PC-9, 

 DSRV-1 & 2) employ a Doppler system; one 

 of these, DEEP QUEST, feeds the Doppler 

 output into an x-y plotter which provides a 

 real-time, continuous trace of the vehicle's 

 course. No published accounts of the Doppler 

 sonar's use under operational conditions are 

 available, but personal communications with 

 Mr. Roger Cook, Field Operations Manager 

 and Chief Pilot of the JOHNSON SEA LINK, 

 revealed that the SRD-101 has done all the 

 manufacturer said it would and has shown 

 100-percent reliability — a rare feat in deep 

 submergence. Undoubtedly, for dead reckon- 

 ing the Doppler system is superior to any 

 others now in use. But, remember: While the 

 distance of the line traversed is accurate to 

 within 1 percent of the distance traveled, the 

 geographic position of the beginning and end 

 of the line is still extrapolated from the 



Fig. 10.24 Doppler sonar transducers (right) aboard JOHNSON SEA LINK. The two 



smaller transducers to the left are for the downward-looking echo sounder and are not 



a part of the Doppler system. 



S09 



