BUST YOUR BONE BUT SAVE THE SEXTANT.
Thus wrote William Willis fifty years back describing his epic voyage all alone across the Pacific on a balsa raft. Willis hadn’t seen modern navigators with accurate GPS position being plotted real-time on electronic charts. The guy would had probably gone into a shock.
The old school of navigation is a dying art. When I started my career in 1979 the sextant was the most important tool in a ship. How many of the current crop of mariners have heard of sounding sextants. It is built specifically to measure horizontal angles. Two such angles taken simultaneously would give an accurate fix plotted with a station pointer.
Those days coastal navigation was an interesting subject. Taking over a watch meant we had to study the chart carefully, pick up the binoculars, go out on the bridge wing and for the next 10 to 15 minutes identify all the important coastal features. We learnt practical geography in a way which no classroom could match. The colour of the sea, the smell in the air all those things had some clues for us. Today, with GPS, the charm of navigation is no more there.
Hyperbolic position fixing systems were developed just after WWII. Decca was the one most commonly used by the navigators.
They laid various chains all over the world but mostly in Europe and Australasia. In India we had the Calcutta chain in the east and the Salaya chain in Gujarat. When the coast dipped out of visual sight the radio receivers helped us to fix our positions till about 300 kms from the stations.
The chains ceased to operate in March 2000 finally bowing out to satellite navigation. Terms such as hyperbolic fixes, lane slips and decca charts disappeared from our vocabulary.
In the early eighties the transit navy navigation satellite system (NNSS) appeared on the scene. We were witnessing a revolution in the making. Satellite navigation would shortly eclipse all other types of navigation.
The NNSS however, was not a threat to terrestrial navigation. We got one Sat fix during a four-hours watch. Acquiring the satellites was a long drawn out process. And it wasn’t always that one could get an acceptable fix. An accuracy of 1 mile was considered to be good. Apart from ocean navigation it was not of much use. Transit NNSS was retired in 1996.
By the nineties the GPS had become an integral part of our bridge. With the deliberately induced Selective Availability (SA) we got a fix accuracy of ± 100 metres. The best part about GPS was that it gave us continuous fixes. With GPS position systems Electronic Charts became a meaningful system.
The midnight of 31st December 1999 was a defining moment as far as celestial navigation was concerned. The techies had somehow managed to create the Y2K scare. As the roll-over to the new millennium approached closer the mariners showed an earnest interest in astro-navigation. I remember we were instructed by our superintendents to practice star sights as ‘GPS might not be available wef 01st Jan 2000.’
That particular night whilst the whole world was merry-making and ushering in the new millennium, I was on the ship hunched over the GPS and other equipment keenly awaiting the chaos predicted to descend on the earth. In the event nothing happened. It was the last time deck officers would seriously consider celestial navigation.
When the SA was discontinued in May 2000 the GPS dramatically improved its accuracy to ± 15 metres. Those who wanted still better accuracy could always install a Differential GPS receiver (± 5 mtrs).
In the quest to continuously improve the accuracy the agencies have come out with the Satellite Based Augmented Systems (SBAS).
Some of the SBAS are the Wide Area Augmentation System (WAAS) operational since 2003 in US, European Geostationary Navigation Overlay Service (EGNOS) which was commissioned a few months back and the MSAS in Japan. India (GAGAN) and China (Beidou) also have plans to provide such systems for both aircraft and marine navigation.
The WAAS has an accuracy of ± 1.5 meters depending on your geographic location.
In SBAS there is no need to carry a separate receiver to receive the satellite corrections. The same GPS receiver is good enough to get the corrections. No additional cost involved here. The other thing is that DGPS works quite close (150 miles) to the land-based reference station. Unlike in SBAS the coverage in DGPS is very limited.
Good news for mariners is that all these SBAS systems – WAAS, EGNOS and MSAS are compatible. One receiver is good enough. In the recent future I expect most ships to have augmented GPSs. With the cost of a receiver less than 50 dollars shipowners can place a number of augmented GPS receivers at strategic points of a big ship. All points of a vessel will be accurately traced in a tight maneuver. It would be very useful when entering docks or navigating in narrow rivers.
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