It wasn't too many years ago (or maybe it was longer than I'd like to think) that I sailed from Block Island to Watch Hill in what started out to be a beautiful morning. Things started to go downhill rather quickly as I encountered a fog bank that had its origin along the Rhode Island shoreline. Visibility was severely limited. At times I could not even see the bow of the vessel a mere 30' forward of the cockpit! This was my first experience at dead reckoning and it worked thanks to my practice with charts and compass. There were moments that increased my adrenalin levels without a doubt. Hearing the fog whistle from the Block Island ferry passing by like a monster from out of nowhere and the occasional whistle from other vessels as I neared the entrance to Fisher Island Sound. But the fog lifted and I was a more experienced mariner from it all.
Today things are a bit different aboard my current fishing vessel, Osprey. I have the wonders of electronic navigation. Osprey is equipped with CRT radar, a fathometer/fishfinder, an autopilot and the unit I most depend upon, a GPS chartplotter. If I were heading out of Block today under the same conditions I encountered twenty or so years ago I would be a bit more confident. As a result of time on the water I have developed many more skills in navigation and vessel handling. I also have the benefit of knowing where I am at any given time and who is there as well. With the wonders of technology I can now view a chart on a screen that covers an area as large or as small as I desire. I can zoom in and view my vessel's position docked at its slip in the marina or zoom out to see Long Island Sound from the East River to Montauk and more. The capabilities of electronic navigation are numerous and it took me a fair amount of time to be able to reap the benefits of them all. It is not unlike the computer that I am using now to write this article. As I master the various programs available to me I have greater ability to be more productive, accurate and efficient in my work.
By overlaying the location of the vessel on an electronic chart the need to calculate your position from latitude/longitude readings off a loran or GPS unit saves time and reduces the possible error in plotting on the paper chart. You can enter a waypoint and follow a course line to your destination. At all times the position of the vessel is seen relative to the course with cross track error shown as well as the direction to steer to correct. You not only know where your vessel is, you know where it has been . After fishing about fifty miles offshore this past August my return home was accomplished by following the track memory from Montauk made earlier that day. It is also nice to view a productive trolling pattern and be able to follow it again and again.
This is all possible thanks to the Department of Defense who developed and maintains the Global Positioning System. Twenty four satellites orbiting 11,000 miles above the earth are constantly monitored by the Air Force from a base station in Colorado Springs. Correct orbit is one of the parameters that must always be monitored as well as the radio signal or pseudo random code (PRC) that the satellites transmit. Position determination is based upon the time it takes a signal to travel to a receiver on the earth. Remember the speed, time, distance calculations used to determine estimated time of arrival or distance traveled, etc.? It is the same principle. Knowing the time it takes for a signal to travel from a satellite to the earth will enable a GPS receiver to calculate distance. For a satellite directly overhead it would take 0.06 seconds. By using very precise atomic clocks, time differences between satellites and receiver stations can be calculated. For example:
D = ST Then D = 186,000 miles/sec. x 0.09 sec.
D = 16,740 miles
But one signal is not enough, it will merely tell you that you are a certain distance from the satellite to anywhere on the earth! By using three satellites and determining the time differences for each one triangulation is used to calculate a position. The GPS receiver on your vessel actually calculates position by utilizing four different complex pseudo random codes. The fourth signal allows for time discrepancies to be corrected. GPS units do not have atomic clocks for a number of reasons - cost being one!
Remember the speed of light (the radio signal) is 186,000 miles/sec.? That is calculated in a vacuum with no interference. In the real universe there are a number of factors that will impede the signal. The earth's atmosphere, mountains, buildings, etc. will all have an effect. Most vessels at sea do not have to contend with geographic features. They still have to deal with the atmosphere and other factors in space. Satellite positioning and mathematical corrections that allow for fixes that are within a reasonable distance are built in to your receiver by the manufacturer. Standard GPS units do not, however, correct for selective availability (SA).
This system was designed for military use and the Air Force Adithers@ the signal to protect military sites. This can result in discrepancies of up to 30 meters! By using a differential GPS unit corrections are made for this and the fix becomes accurate to within one meter for a fixed receiver.
My experiences with the standard GPS/chartmap have been accurate enough for my use. When passing under the George Washington Bridge the unit showed my vessel directly under the bridge on the chart - that is exactly where it was. Keep in mind, however, that this does not occur all the time. There are just too many factors to allow for exact accuracy without the differential corrections.
Electronic navigation has come a long way but anyone who relies solely on it is in for trouble. Any prudent mariner knows that learning the essentials of basic navigation is important. When all else fails having a compass, a chart and knowing what to do will get you home. As a rule I will plot my courses on the paper chart with true (not magnetic) headings prior to any trip and determine my estimated time of arrival at the destination. While en route I will fix my vessel on the course using the GPS . If my electronics fail I will at least have a fix within a reasonable time and can then continue using traditional navigation.
I often wonder how many fishermen miss out on some great fishing because they feel limited in their navigational skills or, for that matter, venture farther and end up in trouble for the same reason. Being a prudent and knowledgeable mariner with the basics in navigational skills and electronics can certainly expand your fishing horizons.
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Want to learn more? Here are some interesting web sites:
