you're navigating the ocean waves. Chris Gibbons looks at the latest in satellite navigational technology.

When the cruise ship Song of America negotiates St. George's Town Cut this month, she will be using a new high-precision navigational system that Bermuda boaters will also be able to use.

Since 1987, mariners have been able to use GPS (Global Positioning System), a sophisticated 24-satellite system set up by the US Department of Defence that enables you to pinpoint your geographical position anywhere in the world to within about 100 feet. The drawback has been that the Department routinely alters the satellite signals for defence purposes, resulting in errors of up to 300 feet. Errors can also be caused by the signal travelling through the ionosphere and atmosphere and until recently, only authorised military users could decode the errors with specialised equipment.

But now the US Coastguard and other administrations affiliated to the International Association of Lighthouse Authorities (IALA) have implemented a system called Differential GPS which enables anyone with a suitable receiver to get accurate readings of latitude, longitude and altitude to within less than a metre, depending on the grade of their equipment.

Differential GPS, already in use in the US, is not due to be implemented by the members of the IALA agreement until 1999 but the Royal Caribbean Cruise Line, owners of the Song of America, proposed the use of a private Differential GPS system to meet Bermuda regulations requiring that large ships be fitted with a precision navigation system to help them through the confines of Town Cut.

The cruise ship Horizon, for example, has used Artemis,a microwave positioning system ever since she began visiting Bermuda a few years ago, which similarly enables a captain to detect and quickly correct any course deviation that may occur due to the tricky winds and currents.

But in view of the benefits and wide-ranging applications of such a system for all Bermuda, Marine and Ports has joined with Royal Caribbean to fund a public Differential GPS reference station which will be installed by local company Marine Communications at Gibbs Hill Lighthouse.

So how does Differential GPS work? The Differential Reference Station, established at a fixed, known location, receives GPS signals and then transmits a corrected reading via an existing low frequency radio beacon - in Bermuda's case, the Gibbs Hill Radio Beacon on 295Khz. Any ship or boat equipped with the appropriate receiver can receive the corrections and input them into their GPS receiver and produce position-fixing accuracy down to three to five metres.

Marine Communications manager Peter Waterson says the system installed is from Trimble Navigation of Sunnyvale, California andall future Trimble equipment will have differential capability -much like today's TVs come cable-ready.

Some receivers, like Trimble's NavBeacon, will automatically track down the best signal coming from a beacon transmitting differential corrections.

Says Waterson: "It is the same company which manufactured the GPS survey equipment used by the team of Royal Engineers who recently came to Bermuda to establish new GPS reference points to which the new Ordnance Survey maps (of Bermuda) will be referenced.'' Waterson says Differential GPS has already attracted interest from wreck divers and Dragon, the Government's buoy tender. On land it will enhance GPS for surveyors and could potentially be used for tracking emergency vehicles or for Police use in catching speeding motorists and boat drivers.

Government is currently working on producing a Geographical Information System - a layered digital map of the island showing everything from underground cables to buildings. "Once that comes into place,'' believes Waterson, "more and more people are going to be needing really accurate positioning information.'' How much will this high-tech accuracy cost? Right now, a top of-the-line NavBeacon receiver odem will cost between $750 and $5,000, assuming you already have a Differential-ready GPS. All Trimble's GPS equipment will now be Differential-ready, including the popular hand-held Ensign GPS, which retails for $1,400.

Meanwhile, Motorola is also getting in on the lightweight handheld GPS act with its new Traxar. Retailing for about $1100 through local agents Telecom, it is similar in design to the industry-leading Ensign but has six channels,compared to the Ensign's three. The Traxar's ease of use and sensitivity received top rating in recent tests by Practical Sailor and Powerboat Reports magazines, even though it is not Differential-ready.

Handy: Motorola's top-rated new Traxar GPS Nagigator. But unlike Trimble's Ensign (below), it does not come differential-ready.

RG MAGAZINE MAY 1993