SEAKING HELICOPTER C3540 OPTI VHF RECEIVER

Installed in the SH-3H SeaKing helicopter as well as the P3 Orion, the On Top Position Indicator (OTPI) was a VHF receiver system capable of providing relative bearing information from the aircraft to any of the 31 different channels of sonobuoys launched across the suspected path of a submerged submarine. It was used in conjunction with direction finding equipment with the system consisting of: 

UHF antenna - This was common to both the UHF/DF and the OTPI systems of the aircraft and located behind a protective cover aft of the electronics compartment on the bottom of the fuselage.

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

R-1047A/A receiver - The receiver enabled the selection of any sonobuoy channel and used received signal strength to tune the ARA-25 antenna to its maximum signal position. It was located on the upper shelf of the electronics compartment. and had a frequency range 162 to 174 MHz  FM.

C3540 Control Unit - Controling the receiver it enabled channel selection between each sonar buoy.

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

The controls on the C3540 consisted of the following:

Press for Control Button -  (This was not used in the SeaKing aircraft)
Channel Knob - The rotary dual-range switch enabled the selection of the desired sonobuoy channels. 
OFF/1-16/17-3 -  In addition to being a power switch, this control selected either the low or high range of sonobuoy channels. 
Signal Intensity Lamp - (Disabled in the SeaKing aircraft).

Operating procedures for the OTPI were as follows:

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

Here is an extremely informative youtube video of the launch and activation of one of the SSQ-801E BARRA Sonobuoy deployment which are an updated version to those carried by the SeaKing and later SeaHawk helicopters:  https://www.youtube.com/watch?v=eidMDdMK38s

This Seaking Helicopter Instrument comes complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit.

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