VIOLET PICTURE / U.H.F. HOMING / A.D.F

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DESCRIPTION

A system that rather confusingly has three names... The Ministry of Supply Rainbow code name for the system is Violet Picture (V.P.). In the aircraft (and documentation) it is referred to as V.P., A.D.F. (Automatic Direction Finding), and U.H.F (Ultra High Frequency) homing.

The purpose this system is to find a left/right direction of another transmitting radio, such as Victor tanker, airfield, or personal locator beacon.

This short video clip extracted from a 1967 Pathe video shows a Westland Whirlwind helicopter equipped with a U.H.F. homing system locating a SARBE beacon. The principle is the same in the Lightning, while the transmitter is transmitting, a direction can be found. Also note the sensitivity switch same as in the Lightning, suggesting the Whirlwind may also have been fitted with the same Violet Picture system.

The principle components of the system are the two "azimuth" whip aerials on top of the spine behind the cockpit. When active, this system uses these two aerials to receive the U.H.F. signal, but as they are spaced apart by a few inches one aerial receives the U.H.F. radio signal fractionally before the other aerial. Other equipment then looks at this phase difference between the two signals to establish the direction of the transmission.

The system required the installation of a PTR-177 or PTR-175 VHF/UHF T/R unit, an AF homing unit and an RF homing unit. The indication of the homing system was a simple left/right displayed by the I.L.S. display bars on the navigation display.

Violet Picture is active when the C1607 control unit is set to ADF, the I.L.S - V.P. switch is set to V.P. and the navigation display mode switch is set to ILS to display bearing information.

A further switch can set the sensitivity of the system, when close to the transmitter can reduce sensitivity.

Accuracy of the indicated direction improves as the aircraft points toward the transmitting radio signal, away from dead-on the indication accuracy diminishes, but left/right indication is still valid.

Homing A.F. unit

The homing A.F. unit acts as a main junction point in the VHF/UHF system - it joins the main T/R unit to the: power supply; C1607 control unit; Mic/Tel and panel lighting circuits; (but critical to Violet Picture) the homing R.F. unit. It provides part of the U.H.F. homing functionality alongside the homing R.F. unit.

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REFERENCES

T Mk.5 electrical "Vol. 1":
A.P.101B-1005-1B, Sect.8, Chap.1, A.L.66, Sept. 1975 - U.H.F./V.H.F., Intercommunication and Telebriefing - Paragraph 29

F Mk. 3, T Mk. 5 and F Mk. 6 aircrew manual:
A.P.101B-1003, 5 & 6A, Part 1, Chapter 13, A.L.2, Nov 84, Communications Equipment - Paragraphs 7, 11 & 12

T Mk. 5 wiring diagrams:
AP101B-1005-10, Sheet 55, A.L.3, Nov 1981, VHF, UHF, Intercommunication and Telebriefing

F Mk. 53 and T Mk. 55 BAC Warton course notes:
BAC Avionics and Communication Systems, Course Notes, Section 4, Violet Picture Homer

External Links

Wikipedia:
Rainbow Code ↗
Automatic Direction Finder ↗
Direction Finding ↗