Mode Gain set at Xi and Gain set at mid- 

 scale, total gain from the Gifft was about 40 

 db. Thus, with a maximum gain of 100 db 

 from the amplifiers, total gain for receiver 

 and recorder was 140 db. A speaker was 

 installed at the hydrophone which allowed 

 for aural reception of the pinger, and its 

 strength predicated the direction of training. 



Transponder Subsystem — Manufactured by Al- 

 pine Geophysical Associates, the transpon- 

 der transmits at 16 kHz and receives at 18 

 kHz. The interrogating transducer (Straza 

 SP75 CT) was located on the surface ship and 

 had a transmitting sensitivity of +48 db at 

 18 kHz and a receiving sensitivity of -82 db 

 at 16 kHz. 



A transmitter was used in the subsystem 

 to drive the Straza and was manually trig- 

 gered at all times when updated slant range 

 information was required. Although these 

 transponder signals were not automatically 

 recorded, they were displayed on an oscillo- 

 scope. The time delays on the oscilloscope 

 were converted to slant ranges and manually 

 noted on the graphic pinger record as re- 

 quired. 



The general tracking procedure was for 

 the support ship to steam upstream over 

 BEN FRANKLIN to a slant range of about 

 5,000 feet. Here, the engines were idled and 

 the ship drifted back over the submersible to 

 5,000 feet downcurrent and repeated the cy- 

 cle. A photograph of the record displayed by 

 the Gifft recorder is presented in Figure 

 10.18 and clearly shows the opening and clos- 

 ing of slant range to the submersible. 



Fagot and Merrifield (ibid.) analyzed a va- 

 riety of factors that influenced this system. 

 It is interesting that the theoretical range of 

 this tracking system was 25 miles, but only 

 about 1 mile was ever realized at sea. 



More recently, the Wesmar Scanning 

 Sonar has been applied in surface tracking. 

 By using the sonar from a surface ship to 

 interrogate a transponder on the submers- 

 ible, ranges and bearings are obtained and 

 displayed to the surface controllers. 



Short Base Line Systems: 



Of all the surface tracking systems, the 

 short base line system, as termed by Rainnie 

 (19), is the most accurate in determining the 

 submersible's position relative to the support 



craft. With an accurate surface positioning 

 system, it can provide quite accurate geo- 

 graphic positioning and high order repeata- 

 bility. The system was used in the 1966 Span- 

 ish bomb hunt with USNS MIZAR as the 

 surface craft and was instrumental in the 

 mission's success. 



A base line, according to Bowditch, is the 

 line between two transmitters (or receivers 

 in this application) operating together to pro- 

 vide a line of position, or a line serving as the 

 basis for measurement of other lines. The 

 length of a base line should be at least one- 

 fifth that of the average side of the principal 

 network of lines of the survey or search area 

 of interest. Accurate measurement of the 

 base line is critical; the Naval Oceanographic 

 Office specifies a maximum error of one part 

 in 150,000 or about half an inch per nautical 

 mile. Short or long is a relative term, but the 

 length of the base line with MIZAR is less 

 than the ship's length of 266 feet. 



MIZAR's system consists of four hydro- 

 phones spaced at known locations on the hull 

 and either a timed-pinger or a transponder 

 on the submersible. Similar in principle to 

 the acoustic tracking systems discussed, the 

 difference resides in the measurement of the 

 difference in arrival time of the signal at 

 each hydrophone. From these differences the 

 depth, range and bearing to the vehicle can 

 be calculated. A general arrangement of this 

 system is shown in Figure 10.19. 



Rainnie {ibid.) lists several advantages of 

 this system: There is no action required on 

 the submersible's part; several vehicles can 

 use the same system concuri-ently; it works 

 best over the object or area of interest and 

 looks mainly "down" which helps avoid 

 shadow zones; and it is potentially one of the 

 most accurate systems available. On the 

 other hand, he cites the need for a large ship 

 (i.e., long base line), stabilized horizontal 

 reference plane, stringent processing re- 

 quirements, loss of tracking near the surface 

 and bulkiness — all of which lead to a complex 

 and expensive ($0.5 million in 1971) system. 



Excluding the Short Base Line System, all 

 of the foregoing systems provide not much 

 more than an indication of where the sub- 

 mersible is in relation to the surface craft. 

 As we have seen from ALUMINAUT's experi- 

 ence, the one slant range and bearing posi- 



502 



