a strip chart recorder within the hull. The 

 system is simply a conventional echo soun- 

 der which looks ahead instead of down. In 

 operation an acoustic pulse is transmitted 

 from the transducer. Conceptually, objects in 

 the pulse's path which are capable of reflect- 

 ing the pulse will do so and the echo or 

 return pulse will be received by the trans- 

 ducer. The interval between signal output 

 and return of its echo is measured electroni- 

 cally and the distance to the object is com- 

 puted and displayed as a trace on the strip 

 chart recorder. On 23-kHz frequency the DS- 

 4000 system ranges out to 5,400 feet. Rela- 

 tive to CTFM sonar it is inexpensive, less 

 complex and requires less weight and inter- 

 nal volume. There are, however, disadvan- 

 tages to this system: In order to search or 

 scan in any direction but forward the vehicle 

 itself must be reoriented, and no information 

 is given as to form or shape of the object. 

 Furthermore, the cone or beam angle is wide 

 and, owing to beam spreading, there is no 

 selectivity in targets. In other words, the 

 closest reflecting object will produce the first 

 return or trace. Nonetheless, the forward 

 looking echo sounder served quite ade- 

 quately as an object detector on DS-4000, 

 and only the need for a better search capabil- 

 ity on a different task caused its replace- 

 ment. 



Scanning Sonar — Westem Marine Electronics 

 (WESMAR) manufactures a scanning sonar 

 used on all International Hydrodynamics- 

 built vehicles which serves for both obstacle 

 avoidance and search. The unit shown in 

 Figure 10.1 is an earlier model (SSlOO); a 

 later model (SS150) is aboard the Canadian 

 Armed Forces' SDL-1 . The SS150 has a 

 trainable transducer within an oil-filled 

 dome located on the bow (as in Fig. 10.1) 

 which can be elevated from down-vertical to 

 four degrees above the horizontal. The trans- 

 ducer can scan 360 degrees and a scan con- 

 trol feature allows the operator to scan a 

 particular sector anywhere within the 360 

 degrees. A 160-kHz pulse of 0.6 millisecond is 

 transmitted; 300 watts of power are used at 

 long range, and 100 watts at short range. 



CTFM Sonar — In this sonar the transmitted 

 frequency is varied continuously in a linear 

 sawtooth pattern and the received frequency 



from an echo-producing object arrives after a 

 short delay proportional to the range of the 

 object (1). Being a sonic device the principles 

 of ranging are similar to the echo sounder, 

 but there are important exceptions. To ob- 

 tain range with a conventional echo sounder 

 one must wait until each transmitted pulse 

 (all of the same frequency) is received before 

 the next one can be sent. Consequently, the 

 number of "looks" per unit time is governed 

 by distance to the object. With continuous 

 scanning at varying frequencies each re- 

 flected pulse is distinctly recognizable, and 

 therefore, the number of looks over a given 

 time period can be and is much greater. To 

 this is the added capability to train or rotate 

 the transmitting/receiving element through 

 360 degrees laterally about the vehicle and a 

 narrow beam pattern which is highly direc- 

 tional. These last two features can be incor- 

 porated into conventional echo sounders (the 

 WESMAR for example), but the use of vary- 

 ing and continuous frequencies at high angu- 

 lar scan rates is unique to the CTFM. The 

 range information for CTFM is presented 

 both visually on a cathode ray tube (CRT) 

 and aurally — the latter overriding as an ad- 

 ditional means of determining the echo char- 

 acteristics. Frequencies of CTFM's can be 

 low (20 kHz) or high (1,000 kHz), but are 

 generally in the 70- to 90-kHz range. 



The predominant CTFM found on Ameri- 

 can submersibles is manufactured by Straza 

 Industries, whose model 500 CTFM is used 

 on the U.S. Navy's SEA CLIFF and TURTLE. 

 It operates on a frequency of 72 to 87 kHz, 

 scans at a rate of 25 degrees per second and 

 has a range of 10 to 1,500 yards. The details 

 of this system are presented in Table 10.2, 

 not as an endorsement of this particular 

 CTFM, but to provide an idea of CTFM's 

 characteristics and capabilities. An earlier 

 Straza CTFM (model SM502A) is shown 

 aho&rd ALV Mils AVT in Figure 10.2. 



The Straza model 500 also has the ability 

 to receive and indicate bearing to marker 

 signals at 37 kHz. and in another mode can 

 trigger a sonar transducer to respond in the 

 40- to 50-kHz frequency band. This latter 

 feature indicates range and bearing to the 

 transponder(s) and may be used for undersea 

 navigation. 



469 



