47 



position, a variety of factors, e.g., celerity and the ship's own movements 



introduce inaccuracies. The manufacturers state that, if the surface ship 



is moving (drifting) at less than 1 knot, the position error will be in the 

 order of ±5 meters (16.4 ft). 



The Ranging components (which can be used on the surface or on the vehicle) 

 consist of a unit for computing and displaying up to four distances (PA21) , and 

 a transducer for interrogating/receiving the transponders. The unit transmits 

 on 16 kHz and can receive from 8.5 to 15 kHz at 500 Hz steps. The transducer 

 can withstand a pressure of 200 atmospheres or approximately 2,012m (6,600 ft) 

 of sea water. The weight in water of the transducer is 10kg (22 lbs) and the 

 weight in air of the compute/display unit is 13kg (30 lbs) . The transponders 

 (Model AT21) are omnidirectional and can be interrogated on 9 , 11 or 16 kHz 

 and will respond from 8 to 15 kHz selectable at 500 Hz steps. The standard 

 battery pack provides a life of two months (on standby) or 100,000 responses. 

 The maximum depth capability of the transponders is 600 meters (1,969 ft). 



Several options are available in the basic ELA System: 



1. Navigation Display (PA22) : displays the position of a moving craft 

 with respect to four transponders on a CRT; 



2. Data Recorder (PA23) : data recorder with a digital clock and display, 

 an eight-digit printer and a digital cassette recorder; 



3. Cassette Reader (PA24) : used in conjunction with a computer (Hewlett- 

 Packard HP 21, 12 or comparable) and an x-y plotter for processing of results. 



Mesotech System - The Mesotech system is used by the Canadian National 

 Water Resources Institute. It consists of from 4 to 16 bottom-mounted 

 transponders, two transmit/receive transducers, one receiver and one interrogator. 

 A minimum of four transponders are required;, these are installed by the support 

 ship and their positions located to within 1km (O.Snm). Sixteen frequencies 

 from 7 kHz to 16 kHz are used by the transponders. The remaining two frequencies 

 are used by the interrogator and transducers to: 1) initiate transponder 

 reply, 2) initiate a transponder into a 64 pulse pinger mode, or 3) to 

 selectively operate the acoustic release on the transponders. The interrogator 

 and receiver will either display the four closest or four selected transponders. 

 The interrogator can be computer-controlled to interrogate at any rate with 

 intervals of between 5 and 60 seconds, or automatically in 5 - second steps 

 between 5 to 30 seconds or it can be triggered manually. To correct for sea 

 water sound velocity variations, temperature and conductivity sensor information 

 is fed into the computer, as is depth of the vehicle from an onboard quartz 

 pressure transducer. 



All data in the Mesotech system is stored on discs and displayed on a tele- 

 printer. A left-right track indicator box provides the vehicle pilot with 

 course corrections to initially enter the transponder array or proceed to a 

 chosen location, or to steer a selected track. The system provides a relative 

 final accuracy of ±30m (98 ft) when correlated with a surface electronic aid 

 to navigation (e.g., DECCA, SATNAV) ; overall geographic accuracy of 50 to 200m 

 (164 to 656 ft) is attainable. The system is designed to provide relative 

 position accuracy to within -3m (10 ft) and io.3m (1 ft) depth. 



