Project Mohole Drilling Platform 



These manual modes also provide smooth handling of the vessel for hole reentry. 

 Further descriptive material on this dynamic positioning system has been pub- 

 lished by H. W. Ragland.* 



LONG-BASELINE ACOUSTIC REFERENCE SUBSYSTEM 



The Long-Baseline Sonar Subsystem is an acoustic ranging system that 

 provides accurate ranges to each of four sea-floor mounted transponders posi- 

 tioned on a radius from the drill hole location approximately equal to the water 

 depth. 



The Interrogator generates an acoustic pulse which is transmitted to the 

 water from the Transducer suspended below the drill vessel. This signal is re- 

 ceived by each of the four Transponders and a signal returned. Each Trans- 

 ponder operates on a different frequency to permit positive identification. The 

 return signals are received at the transducer at some later time. The interval 

 between the original signal and the return signal is a function of the slant range 

 between the vessel and the transponder. 



The received signals are amplified and filtered and then passed to the 

 Range -Measurement and Display Group, where the Range Counter generates and 

 displays on the ship's bridge the true slant range in feet. At the same time, the 

 Range -Deviation Counter subtracts the portion of the slant range outside the 

 vessel operating limits and supplies the resultant to the Digital-to-Analog Con- 

 verter, which provides an analog voltage to the positioning computer. Ancillary 

 to the basic system is an Alarm circuit which provides a warning if a malfunc- 

 tion occurs or if widely divergent range information is being received. In addi- 

 tion, Precision Depth Recorders are provided to permit visual presentation of 

 the signals both for monitoring and emergency use in the event of failure in all 

 digital range -determining circuits. 



In the Long-Baseline Sonar Subsystem, the positioning computer normally 

 utilizes range differences between three of the four received ranges. One of the 

 four channels is, therefore, always available as a standby. 



An error analysis indicates that the maximum repeatability error of this 

 subsystem will be less than 35 ft in 18,000 ft of water. 



PHASE -COMPARISON ACOUSTIC REFERENCE SUBSYSTEM 



The Phase -Comparison Subsystem is an acoustic system that provides ac- 

 curate ranges from a beacon on the sea floor in proximity to the drill hole. 



*H. W. Ragland, Honeywell, Inc., "A Dynamic Positioning System for the Mohole 

 Drilling Platform," in "Ocean Science and Ocean Engineering, 1965," p. 1145, 

 published by Marine Technology Society and the American Society of Limnology 

 and Oceanography, Washington, D.C. 



443 



