tages of XBT's versus mechanical BT's, as suggested in point (4) above, 

 is given in Reference 23. As a result of the evaluation, the Navy chose 

 the XBT system of Francis Associates. 



The Francis Associates 1 probe is capable of obtaining water 

 temperatures to 500 meters with an accuracy of ±0.1°C over a range of 

 -2°C to +30°C. Depth accuracy is said to be ±3% over the range. Thus 

 at a depth of about 600 meters the error in velocity due to the error 

 in the pressure correction term approximately equals that due to 

 temperature error. The recorder is a modified strip chart null balance 

 positioning system. Two compensating lead wires and a sea water path are 

 used to the probe. Measurements can be made with a ship underway at 

 speeds up to 30 knots as demonstrated in sea tests carried out by the 

 Navy. Sea tests have been performed to test XBT capability to 1500 

 meters and the results are currently being analyzed. 



A detailed discussion of the adequacy of the XBT to satisfy 

 current accuracy requirements for technical active sonar is given in 

 Reference 3. 



E. S.T.D. Instrument 



Twenty years ago Jacobson (Reference 25) described an S.T.D. 

 developed at W. H.O.I., which included a conductivity cell, a resistance 

 thermometer, and a pressure transducer connected by multiple conductor 

 cable to a deckside unit containing amplifiers, salinity computing cir- 

 cuits, and a three-channel recorder. Salinity was +0.3% o over the range 

 of 20 to 40% o ; temperature covered a range of 28° to 90°F. , and depth to 

 1200 feet. This became a "production" instrument and several were built. 



24 



Arthur ZB.IUttkKnr. 



