GPS usage

GPS & Your Boat

WHAT IS GPS ? Global Positioning System is a worldwide Satellite Navigation System. It is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military. It is a US Government run system that uses these "man-made stars" as reference points to calculate positions to interface with your receiver, & by utilizing 3 or more at a time, it can pinpoint your location accurately to a matter of feet. To "triangulate," a GPS receiver measures distance using the travel time of radio signals. It can tell your speed also. If your receiver is properly set it can even tell elevation.

Have you ever wondered how your cellular phone knows when you are in "Roaming"? When it is tuned on, these phones have a built in GPS receiver that relays where you are to the cell phone server. Carrier-phase tracking of GPS signals uses special receivers & has resulted in a revolution in land surveying, where it can pinpoint a section corner to within a quarter of an inch.

GPS uses Latitude & Longitude in Degrees and Minutes to the third decimal place, which can pinpoint where you are. Latitude is numbers that run North & South, while Longitude are East & West. In the early days of GPS, the last set of numbers only read in 2 digits, more recently the receivers are reading in 3 digits. This 3rd digit just pinpoints you that much more precisely, like pennies instead of dollars. As of May 02, 2000 Selective Availability (SA) was discontinued. Prior to that date, the military randomly scrambled some of the signals to help protect our "Homeland Security". You may have been on one time but the next 1/2 hour it could be off by up to 100'. After that date "Selective Availability" was eliminated & now you will be within 10'.

As an example by using the information below, you can calculate your approximate position in relationship to another position

Longitude      ---    using Westport North Jetty 124-11-078 as a reference
1 degree = 60.7 nautical miles at the equator     124    7525.8 NM west of datum line of Grenich Village (England)
1 minute = 1 nautical mile (6071')                        124-11   11 NM west of 124-00
1 second = 60.7'                                                    124-11-0    0 feet west of 124-11
.010 of a second = 6.07'                                        124-11-07   42.49 feet west of 124-11-0
.001 of a second = 7.28"                                       124-11-078    58.24 inches west of 124-11-07

Westport North jetty 124-11-078
Westport Buoy #5     124-12-700 (1 mile & 4127.6'   = approx. 1 5/8 miles west of north jetty

Latitude --- using Westport North Jetty 46-55-540 as a reference, uses the same basic principle as Longitude, but 00 is the Equator

Lowrance 7500c GPS/Plotter unit	Lowrance iFINDER20

This unit is a stationary mount plotter & GPS	Here is a hand-held unit

For instance if we use Westport's buoy #3 as your position (46-55-005, 124-14-820), we see the Latitude as the first set of numbers, with the Longitude as the second set. For Latitude, the numbers get larger as they go farther north, & the Longitude get larger the farther west you go. So from this you can quickly calculate if your friend calls you on the radio & says he is at 47-10-501, 124-44-802, if you compare his numbers to yours, this means he fairly close by & is slightly north & somewhat west of you.

You can enter a GPS numbers from a friend or a chart & set your GPS to "GO TO" those numbers & your unit will direct you by a compass course directly to that location. If you drift off slightly it will re-calculate your go to compass course & help you correct your heading. When you get there a bell will ring usually within a tenth of a mile & another bell when you have arrived. You can make a run in the fog & if coming back in, be precise in returning to your waypoint buoy if you are say returning to the harbor if you follow the "mouse tracks". These "GO TO" of course does not take into account for jetties or other land obstacles in between, unless your unit is also a plotter which shows a map with your position being a blinking dot.

Initially GPS units only showed latitude & longitude numbers to give locations. As things improved most of the units now have incorporated maps that overlay your position on the map. This is usually called a Plotter/GPS. As you move, your "boat" is indicated by a blinking dot, also moves on the chart in the direction you are going. With the GPS & a Plotter, life has gotten a whole lot easier for the mariner.

Many GPS units use "Mouse Tracks" which are simply dots on the screen shown as a trail where you have been. If you catch a fish in one location & circle around to try for another one, you can see the tracks on the screen.

Most units have a function that is called a "Man Overboard" function. This is a button or combination of buttons you can use to mark a spot where you caught a fish, & want to go back to the same location. If you use this feature a lot, you will then have remember which is which & transfer the ones you want to save to your main number section.

You can set up "Routes" where you want to go out the harbor to a certain buoy, then change course to a different buoy, & then on to your fishing grounds. If you set it up right, when you start out to the first one, the unit will automatically change to the next "GO TO" once you are there, until you have arrived at your last location.

One thing that I may be misleading you here is that you do have to steer your boat where the GPS is directing you to go UNLESS you have an "AUTO PILOT". The GPS just gives you a direction as to where to go.

The older GPS units have been improved & are now many new units now incorporate a Plotter into the GPS unit. This meaning that your boat will be a blinking dot overlaid on a marine chart. As you move around, the dot also moves on the chart. By using this system, you can tell where you are at all times & what direction to go if manually going to a location. This is very useful when navigating a small river channel that leads thru a bay where the tide fluxuates & sandbars exist.

For those of you without a lot of dash room on your boat, there are units now that are a combination of GPS/Plotter & Depth Finders. These you can by pushing the right buttons & in the right sequence split the screen so that you can see both these navigation aids at the same time on the same screen.

One thing that is beneficial if you are considering a purchase of these units is to take a second look at the new color units. Sure they cost more, but in my book are worth it. The reason is that if you have a mono-color unit mounted where the sun is shinning on it or at a slightly wrong angle to your helm, the image on the screen can be about impossible to see. These new color units have been improved to the point that the extra bucks are worth it at the time of the initial purchase.

These newer units usually have a built in map system showing water depth footage, sunken ships, buoys, etc. If your needs are greater than this depending on the unit you purchase, you can purchase a small memory card that will allow you to load it into your unit & be able to view MANY more precise details.

Computer Downloadable -- Many of the newer high end units are now compatible with computers. This may mean that you can call up a chart on your computer, plot your course with waypoints & then download it into your GPS boat unit. This is many times easier than punching in the many hundreds of waypoints by hand as before.

Some will even allow you to upload from your GPS unit, the history of your trip so you can then look at where you were fishing & record where you caught the big ones.

Shown in the picture below is a combination GPS & Depth Finder unit. Where space is a constraint, these type of dual unit may be worth considering. Here you could split the screen as shown, displaying both. Or if you did not need the GPS like when on a small lake, you could revert back to a single screen for each if needed with the push of the button.

Lowrance LMS 337C GPS/Plotter-Depth Finder

In the old days before GPS, we used charts of our area, the compass & the depth-finder, we learned to navigate quite well even in the fog. This is at times called "Dead Reckoning" & means that you have to do plotting courses, figuring a current or wind drift, the speed of your boat from point A to point B which relates into time to cover the distance using a compass course. Scary sometimes in the fog, but we managed. The one important thing was we had to be aware of the current & wind drift & where we were at all times. We had to be sure we were either north of the harbor entrance or south of it of the drift or wind was prevalent, because if when we ran back in in fog thinking we were north & ran south, but were really south to start with, we could be way off from where we thought we were by the time we realized something was wrong.

If you get into the habit of relying on your electronics & for whatever reason they malfunction, you can be in a bad situation if you are not prepared.

The above being said, if you are not using this GPS method of navigating either to known buoys, back to the marina or to fishing locations, you are missing a lot & could be putting yourself in danger. However if your method of fishing is in small impoundments or in rivers that satellite guidance is not required, then you can spend your money on other needed boating things.

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Originally Started 05-2005 Last updated 11-22-2007
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