432 MECHANICAL AND ACOUSTICAL SENSES 



1 m/min will occur, 2 therefore the range uncertainty after 1 h will be about 

 60 m (less any correction that might be applied). This amount may seem ex- 

 cessive for close-in positioning but would allow very useful estimations near 

 maximum transmitter ranges of several kilometers. Furthermore, it would 

 often be possible to resynchronize the system periodically, at least roughly, 

 by other estimates based on visual contacts or by passing over the animal and 

 noting the minimum time delay. Although the accuracy of absolute range 

 estimates decline with time, the accuracy of short-term changes in range is 

 not affected. Thus, small relative movements toward or away from the track- 

 ing boat can be determined, provided the short-term stability of the clock 

 remains satisfactory. 



A timefix circuit for the CSULB Mark V single-channel transmitter is 

 shown in Figure 7. At the receiver, the reference clock oscillator is adjust- 

 able in phase, permitting easy initial synchronization and subsequent re- 

 synchronization of the system. The pulse-delay times are output on either 

 an LED display or on a miniature storage oscilloscope. 



In its simplest form, a timefix system does not allow pulse-interval coding 

 of sensor data. However, there are ways of multiplexing such sensor data 

 along with the time-synchronized clock pulses. One way is to switch alter- 

 nately between the sensor and the timefix oscillator at some reasonably 

 slow rate, e.g., once every 10 s, thereby permitting manual stopwatch de- 

 coding of the sensor data. Another, perhaps better, way would be to follow 

 each timefix pulse with a sensor pulse using the interval between to encode 

 sensor data, as described in Figure 7. In this case the pulse intervals must 

 be measured automatically, or with an oscilloscope. 



A clock-controlled transmitter having both timefix capability and pro- 

 grammable on-off duty cycles is currently under development at CSULB. 

 The crystal-clock module provides both the 1-Hz timefix pulse to the trans- 

 mitter circuit and an accurate time base to the 24-h on-off timer. The timer 

 resets at exactly 24 h and can be set for various on-off schedules, e.g., 1 h 

 on and 1 h off; 1 h on and 3 h off; 1 h on and 7 h off; 1 h on and 23 h off; 

 etc. In the off mode, power is consumed only by the very-low-drain clock 

 and timer circuits. 



In comparison to a transponding system, the primary advantage of a 

 timefix system lies in its simpler circuitry and operation, for both shark and 

 boat units. Its 1-Hz oscillator also facilitates the addition of other clock-time 

 functions. In other respects, transponding holds the advantage, providing 

 maximum range accuracy throughout the tracking and permitting interroga- 

 tion (or other commands) at any time during the tracking. 



Clock-timed functions may be used for a variety of other purposes aboard 

 the transmitter, such as scheduled "dumping" of stored sensor data or de- 

 taching the unit from the shark at a precise time. A further potential benefit 

 of a clock-timed transmitter is that its extremely periodic pulse rate would 

 lend itself to a receiver design using the principle of signal averaging, which 



2 Under good conditions, much better accuracies are obtainable. 



