P*»OJECTO« 



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CELTIC 

 ANILTIER 

 a DISPLAY 



Figure 1. --Block diagram offlsheryCTFM sonar system. 



I, ■ RECEIVED 



f, ' TRANSMITTED 



Figure 2. — CTFM frequency and time. 



corresponds to this range, then the slope of 

 the frequency-time plot in figure 2 is given by. 



df^ 

 dt 



F C 

 2R ~.v 



where c is the propagation velocity of sound in 

 water and ft is the projected frequency. 



Practical limits exist for the generation of 

 sawtooth frequency functions, and therefore 

 the frequency sweep period T and end point 

 frequencies fi and fa must be considered as 

 design limitations in any CTFM sonar. The 

 time dependence of f t can be shown to be, 



ft= f2 



2Rt 



t, 4 t 4 T 



or, written in terms of the frequency sweep 

 period, T, 



2(f 1 - fa) 



T = Rmax, 



C r inax 



the echo, is A t df/dt = A f, except for the 

 period of time A t immediately after the re- 

 cycle of the sawtoothfrequency variation which 

 is discussed later. This expression also neg- 

 lects Doppler effects. 



The beat note Af is obtained at the output of 

 the balanced modulator and made available for 

 aural observation by the operator as well as 

 fed to a DELTIC (Delay Line Time Compres- 

 sion) frequency analyzer (discussed below) for 

 visual observation. The output of the frequency 

 analyzer is displayed on a CRT (Cathode Ray 

 Tube) as an "A" scan (i.e., target strength 

 versus range) frequency versus tinne, or a 

 combination of both. This analyzer performs 

 the operation of a group of bandpass filters 

 with adjacent filter characteristics inter- 

 secting at the 3 db points. The effective filter 

 bandwidth A F S* 40 Hz and represents a typical 

 number of filters of 100 running from the 

 lowest center frequency of 500 Hz to the 

 maximunn range A f for this system of 4,500 

 Hz. A sloping response is included in the sonar 

 receiver to accentuate the high-frequency beat 

 notes, since the beat note frequency is pro- 

 portional to range for the type of transmission 

 shown in figure 2. The attenuation due to range 

 (spreading loss) follows the inverse fourth 

 power law (R"* ). 



Controlling Relationships 



If Rmax is the maximum range andFmax is the 

 effective center frequency of the highest 

 frequency capability of the DEL TIC filter that 



The mininrmm range, R mjn , which is now 

 determined by previous parameters, is ex- 

 pressed by, 



^mln R max 



where Fmin is the center frequency of the lowest 

 effective frequency bandpass filter of the 

 DEL TIC analyzer. Table 2 lists values 

 used for the fishery sonar in target analysis. 



Doppler System in Block Diagram 



According to classical physics, any sound 

 source that moves withrespect to the observer 

 changes pitch as its velocity changes with re- 

 spect to the observer. The same is true in a 



Table 2. — Sonar parameters 

 Typical parameters for a fishery sonar 



fj = -is kHz 



f = 59 kHz 

 I 



T =50 msec, 

 ^max "" '^° ^^' (^2.2 m. ) 

 «min = '^ "• (1-2 m.) 

 Range to target 30 ft. (9.1 m.)— typical 



