frequency regulation to express depth variations. By using FM, 

 small variations in frequency due to circuit instabilities are 

 minimized as sources of error. The frequency range chosen for 

 the carrier is approximately 21 to 36 kilocycles (kc), which 

 allows an instability of + 150 cycles per second (c.p.s.) as a 

 + 1 percent error. Temperature information is transmitted as 

 amplitude modulation of the carrier with a sine wave tone whose 

 frequency varies approximately as the temperature, 



A Navy- type HBA-6 low-frequency radio receiver was available 

 for use as a telemetering receiver. It was felt that a 1-percent 

 system accuracy could be maintained if frequent calibration checks 

 were made and if extremely high stability were achieved in the 

 transmitter. 



As first planned, the transmission mechanism was to be electri- 

 cal; that is, the carrier signal was to be propagated electrically 

 up the uninsulated trawl cable, using the water as a return path. 

 However, a preliminary trial indicated that this system was impracti- 

 cal because of a high electrical noise level aboard the M/V GERDA 

 and because a failure of electrical connections between the tele- 

 meter transmitter and the trawl cable. Being pressed for time, 

 this method was not pursued further and, after consultation with 

 Dow, an acoustic link was employed. A choice of transducers allowed 

 the incorporation of directional units with their attendant effective 

 transmitting and receiving power gain. In addition, the directional 

 receiving transducer was able to discriminate against undesirable 

 sounds, such as screw noises. Dow outlined a system in which the 

 receiving transducer was kept aimed in the general direction of the 

 transmitter by sliding it a short distance down the trawl wire, using 

 the wire angle for alignment. (See fig. 1.) 



SHIPS WAKE 



0IRECTI0NAL ~~ 

 RECEIVING / TRAWL CABLE 



TRAWSOUCEF^— ~- _^_ TEl LHETER MIOWATCIf 



~ __ ^ ^_$ TRANSMITTER WET 



3I6NAL CONE ^c*"^"^^ 



DIRECTIONAL 



TRANSMITTING 



TRANSDUCER 



Fig, 1, Diagram of Telemeter System 

 2 



