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What does GPS stand for? |  | | |
In simple terms it means Global Positioning System. It includes 24 satellites that are controlled by ground stations and receivers that can be integrated with a computerised navigation system such as the Yeoman Navigator. The main function of a GPS is to tell you where you are. That's good but you might also want to know how to get to somewhere else. To do this on a boat at sea, where there are few sign posts you need some identifiable marks which confirm you are on the right track. To do this you need Waypoints. If you are on passage you can confirm a significant position on your system for use later. Alternatively you can compile a library of Waypoints to mark your routes to favourite destinations.
How does GPS Work?
Again in simple terms GPS calculates the distances from each of a number of satellites to your boat. If your GPS knows where you are, ie you have it switched on and your unit is working, it will be able to calculate bearings, distances, off course factors and your track. There is a very good website with a more detailed explanation: www.avweb.com.
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As new satellites are launched and old ones are retired the total number will change, but basically they work from an 11,000 mile orbit and at that distance they transmit a very weak signal. That is why you need to ensure that nothing is allowed to get in its way and dilute the signal. Since the identification of your position is determined by time and distance you need a pretty accurate atomic clock, the transmitters (Satellites) have one, and the receivers (GPS unit) are equipped with the accuracy of an inexpensive digital watch. What would Harrison (The search for Longtitude) have made of them? Since light travels at 186,000 miles per second, if the receiver is off by 1/100th of a second your calculated position could be off by 1,860 miles. Accuracy is essential. By adding one more satellite to the calculation the timing can be checked and corrected.
What does your GPS do when it starts up?
It takes a little time, (up to fifteen minutes if the unit is not equipped with current almanac data), when you first start up for your receiver to locate the relevant satellites. Once it has fixed its general position it will be much quicker if you switch on again in roughly the same location. The GPS assumes it is nearby where it was when you last switched off. You can update your almanac data by leaving it on for half an hour every three months to gather the latest data. The receiver tells you when you have a completed fix by the indication bars going solid, not unlike the signal strength indicators on a mobile phone.
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Getting to know the system
Like most new gear it is easier to become familiar by simply using the system. Many products have a simulator mode which allow you to automatically track navigation information. It is a good way to experiment and learn the functions.
Accuracy
GPS is incredibly accurate but if you are in a tight spot, say a narrow channel, it is not a good idea to rely 100% on the device. Get out and look, a good pair of eyes and some cross checks with the chart are essential for safety.
On a boat you want to know the direction you are going, the bearing, how far off course you might be, and the cross track error (how far it is to the waypoint).
To get this easily you need to position the antenna at the optimum point for good reception. Essentially the receiver has to have a clear view of the sky. Materials such as fibre glass and fabric will have little effect on the signal, metal however will.
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Where do you fix the antenna?
You should certainly have an external antenna. Experiment with the orientation it will improve reception. This is a good site for advice on mounting: www.ram-mount.com.
Getting more information
The best way is to visit the manufactures websites:
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| | Discuss this article, 1 of 10 messages, read more: |  stan whincup | |
| Posted: 16/03/06 00:35:51 51 | | I'M NEW TO THIS SO PLEASE BEAR WITH ME...READING YOUR INVITATION TO DISCUSS..( GEAR ) HERE'S MY CONTRIBUTION...HAS ANYONE TRIED MAPTECH..I BOUGHT A CD AT THE BOATSHOW..TO PLAY WITH..AND AM I IMPRESSED..£85 & SO USER FRIENDLY & POWERFUL..I HAVE A NAVCENTRE 600 ABOARD & FIND IT SLOW & NOT CONTEXT SENSITIVE..I INTEND TO REPLACE WITH PC & MAPTECH..WHICH BRINGS ME TO THE POINT..ANYONE KNOW EXACTLY HOW TO INTERFACE THIS WITH ALL MY OTHER INSTRUMENTS..RAYTHEON & WHEELPILOT..CHEERS |
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