PEL-6 : Diego Garcia

PRECISION NAV-AID

3.5 MILLION CANDELA PEL-6 LIGHT 2001

THE TECHNICAL CHALLENGE

  • Remote coral atoll in the Indian Ocean
  • Dredged access channel 228 metres wide
  • Safe anchorage inside for naval vessels
  • Vessels up to 41 metres beam would transit daily
  • Underwater channel invisible from above surface
  • Significant drop off in sea floor just outside the reef
  • Unpredictable ocean currents strike vessels side-on
  • Entrance buoys would drift and were not accurate enough
  • A direction-indicating sector light was desirable
  • Only place for a nav-aid was 7.3NM beyond entrance
  • Any light had to be visible at 10NM by day
  • No commercial power was available at the site
  • Site access awkward by both land and sea

SOLUTIONS

  • Use existing PEL-6 Sector Light as starting point
  • Design and manufacture a new “projection” section
  • Beam would be just 1.6° wide, 3 separate colours
  • Central white sector 0.2° wide = 47 metres at entrance
  • Solar power system, with on-demand remote control
  • Remote monitoring incorporated in remote control
  • Lightweight stainless steel for high torsional stiffness
  • Designed for 20-year life (with annual servicing)

OUTCOME

PEL-6 Sector Light Condenser System

The standard PEL-6 condenser system has been in service for almost 20 years. This uses a 250 Watt halogen lamp with a filament configuration to provide maximum pickup angle with the smallest diameter of optic.

New 600 mm diameter Projection Optic

Prior to this project the narowest beam for a PEL-6 light was 3.5° subtense, giving a range of almost 6 Nm by day. Reducing this subtense to 1.6° allowed the intensity to increase 4.8 times to 3,500,000 candela in the white sector. But this required an optic 600mm diameter. To reduce projector length a doublefolded optical system was deployed, with two Mangin mirrors.

Stainless Steel Tower

To achieve the required resolution the tower was required to be torsionally stiff, with less than 0.01° deflection allowable at the top. The tower was designed using marine-grade Avesta stainless steel to achieve the high stiffness and long life in the marine environment without painting or other maintenance. No security fencing was required because site access is controlled.

Solar Power Supply

There was no option but to use solar power. With only 1 transit per day a significant saving was made by using a radio controlled switch with a 2-hour timeout. The solar power system was installed entirely on the top platform to minimise voltage losses in cables and susceptibility to lightning that would be a problem with long vertical cable runs.

Remote Monitoring and Control

Since a radio link was to be in place, for a small additional cost a complete remote monitoring system could be supplied. This monitors parameters such as the state of battery charge, and number of lamps remaining in the lampchanger. An emergency over-ride switch is provided on the tower in the event of complete radio failure.