The fundamental requirement of a Helicopter Visual Landing Aid and more specifically an SGSI is to provide the pilot of an aircraft with clear unambiguous visual signals that positively contribute to the safe landing on board the deck of a ship.

In benign sea states and clear visibility the pilot will be able to aquire many visual cues from the ship and utilise these to control the aircraft during the initial approach deceleration period and final hover stages of the landing procedure.

However in order to maximise the capability of the aircraft and the Navy in performing its role as a protective force, the ships helicopter must be able to operate in severe and demanding circumstances that fully utilise the capabilities of both the aircraft and the pilot.

In these circumstances the original inherent cues of the vessel will no longer be available to the pilot and instead he will become solely reliant upon the Ship Helicopter Visual Landing Aid package. It is therefore paramount that the aids provided can not only match but must exceed the operating conditions in which they are being used, this ensures that every possible assistance is provided, without compromise to the pilot, ship or aircraft.

The most basic functions of a Glide Slope Indicator are projector beam intensity, beam geometry and stabilisation against the effects of ships pitch and roll motion.

The system proposed for this Project encompasses the following features.

BEAM INTENSITY

The SGSI projector beam has been designed to meet the minimum requirements of STANAG 1236 for Glide Slope Indicators, and produce a nominal 32,000cdm2 beam intensity.

A comparative table illustrating the extended visual range of the AGI System

From an operational perspective, it is relevant to note that in poor visibility conditions the deceleration distance required by an approaching helicopter is much larger than in good visibility. A typical figure of 0.5nm / 925m has been shown to be typical of the distance needed to decelerate. The resultant pitch attitude of the aircraft (helicopter nose up) will result in a loss of contact with the visual cues from the vessel. This is unacceptable in low visibility conditions and potentially very dangerous when the cueing environment is poor. For this reason it is extremely important to ensure that the helicopter is established on the glide-path before the deceleration period is commenced and ideally well before reaching the vessel. The acquisition range of the SGSI beam under these conditions is therefore fundamental to daytime helicopter operations and should always be in excess of the meteorological visibility in order to be operationally safe.

BEAM GEOMETRY (NVG COMPATIBLE VARIANT)

The system offered will comprise a three sector beam with occulting in the upper and lower sectors. The modulation frequencies of the occulting sector are 1.5Hz in the upper sector and 3.9Hz in the lower sector. The difference in these frequency rates has been demonstrated to provide instantaneous sector identification when operating with NVD and colour differentiation is no longer an option. They have also been selected to reduce pilot workload in that the lower flashing rate is fast enough to not leave the pilot anticipating the next signal, but the higher flashing rate does not replicate a strobe effect and cause irritation.

The correct sector remains steady and is 1° wide to provide guidance without being over restrictive. Wider angles allow high rates of decent to be built up by the aircraft within the sector before a corrective signal is received by the pilot.

The beam has been designed to be used with Generation III Class B NVIS devices as defined in MIL-L-8576A, but has been tested and is also in active service with Class A devices.

The system is also compatible with unassisted vision without modification or switching of the beam as the beam contains three distinct sectors.

Full details of the proposed beam geometry are provided in the attached specification PSM 1108.

STABILISATION

The projector must be stabilised to remove the effects of ships pitch and roll motion. Poor stabilisation has the detrimental effect of reducing the width of the command sector. The result is to unnecessarily restrict the descent path of the aircraft and increase pilot workload.

The system offered is dynamically stabilised with respect to the input signal to less than ±2 arc minutes or 1.2m @ 2km. The result is a perfectly stable beam that provides safe and optimal glide path information to the pilot without the need for pilot compensation or increased work load.