McClure and Hove 



A Beacon transmits a continuous 10-kc signal, modulated by 100 cycles 

 which is received by four Hydrophones located on the corners of the drill ves- 

 sel. If the acoustic path length between the beacon and each hydrophone is dif- 

 ferent, a small signal phase (time) difference in the modulating frequency is 

 apparent between each pair of hydrophones. The received signals are amplified 

 and detected in the Receiver and sent to the Tilt Compensation Unit, where a 

 correction is applied to compensate for vessel roll and pitch. This correction 

 is derived from the Two-Axis Tilt Sensor. After correction, appropriate signals 

 are selected by the Mode-Selection Relays and supplied to the Phase-Compare 

 and Compute circuits, which generate an analog voltage to drive the positioning 

 computer. The magnitude of this voltage is a function of the displacement of the 

 vessel-oriented coordinate system from the drill hole. Displays on the bridge 

 showing vessel displacement in feet from the drill hole are derived from the 

 analog voltage by the use of an Analog-to -Digital Converter. 



Only one bottom beacon is needed to provide a position signal. In normal 

 operation, three hydrophones and receivers are used, and only two phase - 

 compare and compute circuits are on line. The remaining elements are utilized 

 to provide continuous system cross-checks, or to compare beacon position if a 

 new alternate beacon is placed on the bottom. 



Accuracy of this subsystem is about 40 ft in the depths of water anticipated. 



SHORT-BASELINE ACOUSTIC REFERENCE SUBSYSTEM 



Short-Baseline Subsystem is an acoustic system which provides accurate 

 vessel displacement from bottom -mounted transponder located in the vicinity of 

 the drill hole. 



An acoustic interrogation pulse, with a center frequency of 16 kc and a pre- 

 determined width, is transmitted to the sea floor transponder. 



The pulse is intercepted and decoded by the Transponder. If the pulse is of 

 the proper duration, the Transponder transmits a 4-ms reply pulse at 16 kc. 

 This return pulse is detected by four Hydrophones located on arms suspended 

 below each corner column of the platform. Due to the Hydrophone separation, 

 the return pulse does not arrive at each location at the same time. 



Pulses detected by the four Hydrophones are routed to four independent 

 signal-processing channels, where they are detected, amplified, and filtered. 

 Timing marks are generated from these incoming signals and a differentiation 

 technique is used which eliminates amplitude imbalance between channels. 



The coordinate computation consists primarily of digital counters and 

 storage registers. 



The four coordinate values stored in the register are converted to an ana- 

 log voltage, and corrected for platform inclination by the Tilt Compensator. 

 The compensated coordinate values are sent to the computer, as well as for 

 display purposes at the vessel bridge. Accuracy of this reference subsystem is 

 expected to be approximately 34 ft under anticipated operating conditions. 



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