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To begin the at-sea procedures, the bottom- mounted transponders are installed 

 and rough (surface) estimates of their positions are put into the mini- computer . 

 The surface craft then cruises over the transponder network and interrogates 

 the transponders at 10 to 15 random locations. The onboard surface equipment 

 now has the capability of determining the transponder position relative to each 

 other to within approximately ±0.6m (2 ft). 



The navigation operation begins with interrogation of the bottom transponders 

 from the surface craft. Each transponder replies individually to the surface 

 and their slant range from the ship is automatically determined. Immediately 

 thereafter, the transponder on the vehicle is interrogated. The reply from 

 the vehicle transponder performs two functions: 1) it permits determination 

 of the slant range from vehicle to ship, and 2) the same reply interrogates 

 the bottom transponder to provide the' total ranges from the ship to the vehicle 

 to each transponder and back to the ship. The range data is processed to 

 determine the positions of both the ship and the vehicle relative to the 

 bottom transponders. The resulting track information is displayed on the 

 x-y plotter and printed out by the teletype. 



There are five standard transponders available for use in the ATNAV system; 

 three are releaseable and two are not. The recommended interrogate/respond 

 frequencies of the submersible transponder is 11 kHz and 9 kHz, respectively. 

 The bottom- mounted transponders will respond to 9 kHz on one of 13 selectable 

 frequencies ranging from 9 through 15 kHz at increments of 500 Hz. The range 

 of frequencies, when using three or more transponders concurrently, must stay 

 within a 4 kHz band in order to be compatible with the standard shipboard 

 Ranging Receiver. The standard transponder battery life is one or two years, 

 or 100,000 interrogations. The depth of transponder operation is 914m 

 (3,000 ft) or 6,096m (20,000 ft). 



An updated ATNAV system, ATNAV II, is used with the U.S. Navy's DEEP DRONE. 

 This system offers larger area coverage (16 transponders) , data recording, 

 dead reckoning capability, and more rapid program loading. 



ELA System - The ELA system is used by COMEX Services. The ELA Corporation 

 no longer produces sub-surface navigation systems. Another French corporation, 

 Oceano Instruments, Clamart, manufactures a navigation system which is, 

 essentially, the successor to ELA. The navigation system's (series 20) 

 underwater components consist of three bottom-mounted and one vehicle-mounted 

 transponder; the surface craft components consist of an acoustic Ranger (PA21) 

 with two interrogation channels, a computer and interfacing electronics. 

 (A display unit, x-y plotter and data recorder are optional.) The deployment, 

 net calibration, navigation procedures and mathematics are, to all intents, 

 similar to that described for the ATNAV system. 



The ELA system can also be used by a surface craft to determine the vehicle's 

 position, or it may be used by the vehicle alone. When used by the submersible 

 only, a position accuracy (relative to three transponders) of -1 meter (3.2 ft) 

 is obtainable. When the surface ship uses the system to obtain the vehicle's 



