Port Louis is the principal port of Mauritius, east of Madagascar in the Indian Ocean. A busy commercial area had developed on the land beside the harbour.
The front mark was located on Quay A, from where the photo above was taken (facing south-east). The rear leading mark (and light by night) was located on Moneron Hill behind the city.
An intermediate mark was more recently placed on the MCB building after construction of this building, because the view to the rear mark was obstructed from certain directions. But most of the sensitivity was lost when using just front and intermediate marks because they were so close together.
Most of the building sites in both photos were height-restricted because the “airspace” was reserved by the port authority for the view-line to the rear mark. Developers were seeking a new navigation aid which would be equal or better to what was installed, but would free up airspace reserved for the view corridor.
A PEL Sector Light on the intermediate station met all requirements. A secondary PEL Light (red only) was installed on the front mark to provide alignment information if required.
Resolution with the optional “oscillating boundary” system typically exceeds that available from a 2station range. In addition, the edges of sectors can be used to mark lateral limits to channels or manoeuvring areas, as was done here.
A PEL-6 Sector Light of 10° horizontal subtense was proposed for the top of the MCB building. This gave a good balance between peak intensity and angle of coverage of the beam.
By day the viewing background was dark coloured earth and green jungle, which was more favourable than back-lit sky. This produced a greater viewing range than would otherwise be the case.
As a backup, a second identical PEL Sector Light was installed on the MCB building, and a fail-tostandby system fitted. The uninterruptible power supply and lightning protection system of the MCB building were utilised.
A red light was mounted on the front tower (Quay A) to make a 2-station lead. This provided an easy transition to the new signal format.