50 



acoustic pulse at regular intervals. This pulse is received by a shipboard 

 hydrophone whose three receiving elements are arranged in a known orientation 

 with respect to the vessel's X, Y, and Z axes. As the acoustic wavefront 

 reaches the hydrophone, the three elements measure the relative phase of the 

 signal. When the vessel is directly over the beacon, an acoustic signal 

 received by two hydrophone elements in the same axis will be in phase. As 

 the vessel moves farther from the beacon, the relative phase of the signal 

 increases. The microcomputer uses the difference in phase, together with 

 the vertical separation between the beacon and the hydrophone, to determine 

 the vessel's apparent position with respect to a specific subsea reference 

 point. 



To establish the vessel's true position, the computer then adjusts the apparent 

 position for various offsets. The factors for which adjustments must be made 

 are: vessel pitch and roll (The VRU reference sensor determines the degree 

 of tilt of the vessel relative to the true horizontal plane at the instant 

 the acoustic signal is received.); hydrophone offset (The distance between the 

 vessel's reference point and the hydrophone is entered at the time of system 

 installation. This offset translates the hydrophone's position so that it 

 appears to be at the vessel's reference point.); and beacon offset (If it is 

 impossible to place a beacon at the desired subsea reference point -for example, 

 on a well-head - beacon offsets (X, Y, Z) may be entered before operations 

 begin. Again, these adjust the apparent position to make the beacon appear 

 to be located at the subsea reference point.) 



The RS/902 can track an ROV and display its position relative either to the 



surface vessel or to a si±)sea reference point. For ROV tracking relative to 



a surface vessel, only one beacon is required. This beacon is mounted on 



the vehicle. The vertical separation between the beacon and the hydrophone 



may be obtained in one of three ways - by cable from the submersible, acoustically 



by depth telemetry, or manually by operator entry. For vehicle tracking relative 



to a subsea reference point, two beacons are required - a submersible-mounted 



beacon, assigned to one receiver, and a bottom-mounted beacon, assigned to 



the other receiver. 



The system operates with one-pulse-per-second position reference beacons in 

 the 22 to 30 kHz band (9 channels of 1 kHz intervals). It is accurate within 

 1 percent of slant range for horizontal ranges up to 100 percent of water depth 

 and within 2 percent of slant range for horizontal ranges up to 200 percent 

 of water depth (excluding the effects of the acoustic environment) . 



RS/904 System - This system is used by UDI, Ltd., Aberdeen, Scotland. The 

 system shipboard components are: a hydrophone/projector, power amplifier 

 signal processor, display console and a vertical reference unit. Sub-surface 

 components are a bottom-mounted beacon (pinger or transponder) and a vehicle- 

 mounted beacon (pinger or transponder) . Similar to the RS/902 system, the 

 RS/904 can track an ROV without the need of a bottom-mounted beacon. 



The beacon generates the acoustic signals that are used to measure position.- 

 Three types can be provided: pinger, transponder, and responder. Pinger- type 

 beacons are free-running and transmit continously. Transponders, in contrast, 

 transmit only in response to an acoustic signal (interrogation) sent from the 





