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shipboard projector - and then only a single pulse. Responders transmit only 

 in response to an electric signal sent by cable. A typical transponder for 

 the RS/904 system consists of a battery pack, receiving and transmitting 

 electronics, and a transducer. The transducer receives interrogation signals 

 from the surface and transmits response signals to the hydrophone. The 

 hydrophone projector transmits interrogation pulses to the transponders, 

 receives the response pulses sent back by these units, and conditions these 

 pulses for transmission to the signal processor. The projector portion of 

 the unit consists of an omnidirectional transducer mounted above the hydrophone. 

 The hydrophone portion contains a multisensor array, preamplifiers, and other 

 signal-conditioning electronics packaged in a watertight housing, allowing the 

 hydrophone/projector to be mounted on the hull of the surface vessel below 

 the water line. The signal processor is similar to the RS/902 system, but 

 also includes a transponder controller. The vertical reference unit and the 

 display console are essentially similar in function to the RS/902 system. 



The RS/904 measures vehicle position using acoustic pingers , transponders 

 or responders - separately or combined. The type of beacon used determines 

 the operating mode. Because the RS/904 is an ultrashort-baseline system, it 

 can determine a vessel's position with only one beacon and one hydrophone. 

 Additional beacons may be used, however, to provide position data over broader 

 areas, or to furnish tilt and depth information. The system can pre-store 

 as many as 31 sets of beacon data. The pinger mode of operation is identical 

 to that described for the RS/902 system. Pinger-mode operation is useful in 

 two types of situations: 1) when beacons are located in a noisy environment 

 the noise may prevent a transponder from responding to an interrogation signal; 

 and 2) when very rapid position-updating is required, since signals can be 

 sent continuously. Transponder- mode operation, in contrast, involves a delay 

 between the time that the response signal is sent and the time that the new 

 interrogation signal is received. 



When operating with transponders, the RS/904 determines position by interrogating 

 one or more transponders. When it receives the transponder's response signal, 

 it determines the vessel's position from both phase information and slant-range 

 measurements. To determine relative phase the three sensing elements in the 

 hydrophone array receive the transponder's response pulse and process it in 

 the same way as a pinger signal. To determine slant range, the signal processor 

 uses the two-way propagation time between transmittal of the interrogation 

 pulse and receipt of the response signal. The microcomputer uses both sets 

 of data - phase comparison and slant range - to calculate the vessel's apparent 

 position. It then establishes the vessel's true position by adjusting for 

 the same three factors as in pinger operation: vessel pitch and roll, hydro- 

 phone offset, and beacon offset. The interrogation rate for transponder-mode 

 operation is operator-selectable, and can vary from four times per second to 

 once every two minutes. The interrogation rate depends on the maximum slant 

 range anticipated - the greater the slant range, the less frequent the 

 interrogation can be, due to the increased travel time for the signal. The 

 RS/904 can display the positions of four transponders simultaneously, assigning 

 two to each receiver. The principal advantage of transponder-mode operation 

 over pinger mode is its greater accuracy over broad areas. For horizontal 

 offsets beyond 100 percent of vertical separation, transponder-mode operation 

 is more accurate than pinger mode. 



