The On Top Position Indicator (OTPI) is a VHF receiver system capable of providing relative bearing information from the aircraft to any of the 31 different channels of sonobuoys. It must be used in conjunction with direction finding equipment. The system consists of the following components:

* UHF antenna which is common to both the UHF/DF and the OTPI systems of the aircraft. It is located behind a protective cover aft of the electronics compartment on the bottom of the fuselage.

* AM-3969 amplifier. This is a radio frequency amplifier designed to amplify the received signals and pass them to the receiver for processing. It is located on the upper shelf of the electronics compartment.

* R-1047A/A receiver . The receiver  permits the selection of any sonobuoy channel and uses received signal strength to turn the ARA-25 antenna to its maximum signal position. It is located on the upper shelf of the electronics compartment. Frequency range 162 to 174 MHz  FM.

* C3540 Control Unit. It controls the receiver.

* Indicators . OTPI bearing information is displayed on the number one needle of the pilot, co-pilot and TACCO's BDHI (Bearing Distance Horizontal Indicator)

* Power: Requires 115 VAC 400 HZ or 28 VDC

SWITCHES and CONTROLS

a)  Press for Control Button. Not used in the Sea King aircraft.
b) Channel Knob - A rotary dual range switch which permits selection of the desired sonobuoy channels.
c) OFF/1-16/17-31.  In addition to being a power switch, this control selects either the low or high range of sonobuoy channels.
d) Signal Intensity Lamp. Not used in the Sea King aircraft.

Operating procedure for the OTPI is as follows:

a) Pilot selects either T/R and G or T/R on the UHF radio remote control panel;
b) Set OFF/1-16/17-31 switch to desired range.
c) Set channel knob to desired channel.
d) Select UHF/TAC on BDHI selector panel; and
e) Number one pointer will indicate the bearing to the sonobuoy.

Note: When "UHF ADF" is selected on the UHF control panel, the OTPI system is disabled and the number one needle will display the UHF bearing.

C3540 Control Unit (From an RCAF training manual)

R-1047A receiver (From a RCAF training manual)

Sonobuoy channels.(Courtesy T-pub.com)

Back To Sea King Equipment Listing

Credits and References:

1)  Ian Snow  RCAF/CF retired.<va3qt-4(at)sympatico.ca> . Training Manual OTPI excerpt.
2) http://www.tpub.com/content/MIL-SPEC/MIL-R/MIL-R-24008/MIL-R-2400800006.htm

June 27/10