Changes in water temperature as 

 the probe descends cause a 

 corresponding change in the 

 resistance of the probe's thermis- 

 tor. The temperature information 

 is converted into an audio range 

 frequency. This frequency is 

 transmitted up the hard wire link to 

 the surface electronics package, 

 where it is used to frequency 

 modulate the transmitted carrier 

 signal. It takes approximately 3.3 

 minutes for the probe to complete 

 its 300 meter descent (approxi- 

 mately 8.5 minutes for the "deep" 

 probe). About 1 minute later, the 

 surface package scuttles itself. 



The radio frequency (RF) signal 

 from the AXBT is received on the 

 aircraft via the aircraft's VHF-FM 

 antenna. The audio signal from 

 the receiver can either be ana- 

 lyzed in real-time using the 

 Sippican MK-9 Data Acquisition/ 

 Processing System, or recorded 

 on audio tapes for later playback 

 and analysis. The MK-9 Data 

 Acquisition/Processing System 

 consists of a Sippican MK-9 Digital 

 Interface and a Hewlett-Packard 

 desktop computer. The MK-9 

 Digital Data Interface requires an 

 AXBT PC board containing an RF 

 demodulator in order to convert 

 the AXBT's audio frequency 

 signal. 



Once the MK-9 interface is 

 modified, the AXBT data are 

 analyzed in the same fashion as 

 ship-deployed XBT data. A typical 

 AXBT 300 meter raw data file 

 contains approximately 2,000 

 depth/temperature points. By 

 selecting only those significant 

 points which are required to 

 reproduce the temperature profile, 

 the number of data points is 

 reduced to 20-30. 



To prevent interference between 

 AXBT's using the same transmis- 

 sion frequency, 4-5 minutes 

 should elapse between AXBT 

 deployments. If a finer data 

 sampling resolution is required, 

 AXBT's using different transmis- 

 sion frequencies will have to be 

 deployed. IIP has not yet fully 

 developed a multi-channel AXBT 

 system, and multi-channel AXBT 

 operations will not be addressed 

 here. 



Test Results and Discussion 



The AXBT system was tested on 

 three Coast Guard aircraft: the 

 HH-3F helicopter; and the HC- 

 130H (a four-engine turboprop) 

 and HU-25A (a twin engine jet) 

 fixed wing aircraft. The AXBT 

 system was first tested from the 



HH-3F to gain experience with the 

 equipment before testing it on the 

 fixed wing aircraft. The primary 

 testing was from the HC-130H, 

 since it is HP's primary aircraft for 

 iceberg reconnaissance. Since 

 the HU-25 is MP's planned backup 

 for the HC-130H, testing was also 

 performed from the HU-25. 



The receiving and recording 

 equipment used for the test were 

 loaned to International Ice Patrol 

 by Sippican, Inc. The receiver 

 was a single channel VHF wide 

 band receiver. Sippican also 

 provided sixteen AXBT's for the 

 testing and evaluation. Sippican's 

 support and helpful advice are 

 gratefully acknowledged. 



The AXBT data for all tests were 

 recorded on audio cassettes for 

 later playback and analysis. The 

 Hewlett-Packard (HP) 85 desktop 

 computer was used to process the 

 AXBT data. The audio AXBT data 

 had to be played back through the 

 MK-9 to the HP-85 computer to be 

 processed and analyzed. 

 Sippican's AXBT program for the 

 HP-85 computer was used to 

 process the data. The time 

 required to play back the audio 

 recording of each AXBT drop 

 through the MK-9, to process the 

 data on the HP-85 computer, and 

 to analyze the temperature profile 

 was approximately 30 minutes per 

 AXBT drop. 



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