GPS .. Lesson one .. how it works

[Pilar]

Pilar
Ifish Forum Guide
Member # 270

posted 03-27-2002 10:35 AM
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Hi everybody. This is a topic on the 'Salty Dogs' forum but it may have wider interest for the rest of us. I intend to add another installment every day and eventually have some of the specific units covered by their experienced owners. My goal here is to get you to regard the very expensive box in your pocket or on your dashboard as more than a trinket. Ask anyone who knows how to use the 'Magic Box' and they will tell you they cannot live without it.

Have fun!

I probably won't get all of this right but here goes.

GPS - Global Positioning System , it's just a very sophisticated clock.

A constellation of 24 satellites in geosynchronous orbits cover almost the entire earths surface with a synchronized time radio broadcast. All of the satellites are sending the same time in radio waves at the same time. This clock is atomic isotope based and is very stable and accurate. At any point on the globe, except the poles, a receiver can 'see' several satellites and hear their time signal.

This is where the magic comes in. The time signal is extremely accurate. The computer in the GPS compares the value and the source of each time signal. The difference in the time the signals are received is related to the distance from each satellite from the receiver. Since the orbits of the satellites are known at any given moment by the computer we can figure out the position of the receiver in 3 space. Yes! altitude, longitude and lattitude.

The amazing thing is that the time signal is so accurate that the position can be figured to 10 feet! Since radio waves travel at the speed of light the distance traveled by the radio signal for a small bit of time is huge.

To work properly your GPS receiver must see at least 3 satellites. Most modern receivers do at least 12 to prevent momentary drop outs due to bad reception.

More facts ... The satellites are built, launched and operated by the US military. The system is not officially commissioned even though it has operated for over 20 years. Until recently the military scrambled the least significant digits of the time signal to degrade the accuracy for civilian users. Less accurate time means more uncertainty in the position. This keeps maniacs from sending a Cessna full of C-4 and a GPS autopilot into building of choice.

This is where DGPS comes in. A thing of the past now but this was a network of land based transmitters that enhanced the time signal to overcome the SA (selected availability) scramble of the satellites. You got a special receiver and paid a subscription fee to use it. This system made the GPS accurate to 3 ft.

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Now for some basic terms regarding Navigation. I must assume you know nothing as this is true for some of you, so skip it if you already know these terms.

Positions or waypoints are laid out in a north-south, east-west grid. Waypoints are recorded in one of two formats. Degrees, Minutes, Seconds or for GPS Degrees, Minutes, Minutes/1000. All distances mentioned here are in statute miles unless otherwise noted.

As an example, here's my favorite Halibut hole. N44 28.483' W124 43.682'. Note don't try this spot unless you are a Salty Dog, it's 30 miles offshore! If you were to report this position to the CG to request a rescue you would say ... 'My position is North forty four degrees, twenty eight decimal four hundred eighty three minutes ... West one hundred twenty four degrees, forty three decimal six hundred eighty two minutes.' Listen for a while on VHF16 and 22 and follow the Coast Guard example of how to say the numbers.

Lattitude - Location of a waypoint with regards to north and south. The Equator is 0 degrees and the number increases to 90 degrees at the North pole. The lattitudes I just described are North. In Oregon the lattitude is typically N45 or N44.

Longitude - The location of a waypoint with regards to east and west. The Prime Meridian is 0 degrees. This is also known as Greenwich Meridian. A Meridian is a line of Longitude stretching from North to South pole. Greenwich is in England. So heading west from there the Longitude increases to 180 degrees at the date line, somewhere in the Pacific. Typical Longitudes for Oregon are W123, W124, W125.

Degree - A arc unit of measurement. A circle has 360 degrees in one rotation. In navigation it means the same thing but we don't go 360 degrees. How far is one degree in miles? Lets figure this out. The earth is 24,000 miles in circumference at the equator. If there are 360 degrees in the full circle then there are about a thousand miles in 15 degrees. Or about 66 and 2/3 miles in a degree. This is a handy thumbrule to guess distance between Waypoints. These distances are dead on at the equator and less accurate at other points on the globe.

Minute - A minor unit of arc measurement. There are 60 minutes in a degree. Following the math above we get to about 1.11111 miles in a minute. For GPS users there is another unit of arc measurement. What comes after the decimal is thousandths of minutes. 0.001 minutes (One Thousandth of a minute)is equivalent to about 6 ft. GPS does not use seconds but many charts and Loran waypoints do. You should know how to convert from one to the other.

Second - A much smaller arc unit of measurement. There are 60 seconds of arc in a minute of arc. Continue on with dividing up the distance and you get about 100 ft in a second.

OK .. enough for now ... Any questions so far?

BTW there are no dumb questions .. fire away.

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The bend is your friend!

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Posts: 1668 | From: Portland, Or., U.S.A | Registered: Aug 2000 | IP: Logged

husker
Chromer
Member # 2010

posted 03-27-2002 10:49 AM
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very nice.....master!!!
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Posts: 116 | From: hillsboro | Registered: Jan 2002 | IP: Logged

hustlerrjim
Chromer
Member # 455

posted 03-27-2002 10:56 AM
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exellent post, but how do I turn my machine on?????????????????
I have an eagle
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Posts: 175 | From: nehalem oregon usa | Registered: Oct 2000 | IP: Logged

hoochie
Chromer
Member # 1204

posted 03-27-2002 11:52 AM
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Dunno about the rest of you but I'm going to stay tuned to this thread.

Pilar, you are my hero of the day!!!

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Who says "life's too short?" It's the longest thing you'll ever do. Enjoy it, go fishing!

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Posts: 173 | From: Hobart, Washington | Registered: Jun 2001 | IP: Logged

Kaus
Chromer
Member # 921

posted 03-27-2002 12:28 PM
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I have a Lorance 315, after using it for sometime now. I have a number of waypoints set, if I loose my note book I am in trouble. It seems so darn complicated to input names for all of the way points. Is there any easy way to do this? This tool does so much more than I will every need.
I wish the tool was made more beginner user friendly. All I need are waypoints, MOB, trips, and a few of the simple functions. Unless I use it all of the time I forget how it functions.
Ideas ---
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Posts: 170 | From: Vancouver,WA | Registered: Mar 2001 | IP: Logged

Barviewrocks
Fry
Member # 2272

posted 03-27-2002 12:28 PM
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Great tutorial I will look forward to future ones. I can forsee the time when we are able to exchange info via coordinates for example: bouy locations, channel coordinates, and alas - fishing locations or even meeting places. Now for those of us who do not have these units I can forsee a group purchase with significant discount based on the numbers of people from this group. Is this the type of thing we can do - keeping in mind our sponsors of course.
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Posts: 5 | From: Portland | Registered: Mar 2002 | IP: Logged

Pilar
Ifish Forum Guide
Member # 270

posted 03-27-2002 01:07 PM
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Homework assignment .. there will be a test and your life may depend on it.

1) Use your book and turn on the unit.

2) Read about how to save a waypoint. IE: you are somewhere where large fish are dying to get in your boat, flopping around your feet, covering the deck with blood and plugging up the fishbox.

You really want to save this spot!

Most units will do this with just a keypress or two. On the 350A Lowrance it's called 'Quicksave'. Press that button and you save your present coordinates.

3) Save the location you call home and name it before saving it. Yes that is in the book too.

4) Go somewhere else and figure out how far you are from 'Home'.

Yes, tough assignment but there is more.

Any of you more experienced GPS guys get on here and give the answers for your unit so others can do it.

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LMS-350A Lowrance ...\
1) Press the 'On' button for 3 seconds. After initial screen, select backlight on or off. Wait until unit begins tracking and sounds 'I'm OK' sound.

2) Waypoint is saved by pressing the 'Quicksave' key.

3)When you reach the spot you want to save press 'Quicksave'. The waypoint # will display, make a note of it.

Press 'Waypoint' and then the soft key 'Save Waypoint'. Move the cursor to the softkey 'Name', follow the cursor and enter a name for the waypoint 'HOME'. Press the 'Accept Name' and then the 'Save Waypoint' softkeys.

4) Go to some other place. Press the 'Waypoint' key. Press the 'Recall Waypoint' softkey. Select the 'HOME' waypoint number, lets use number 1. Accept this number. Press the 'GoTo Waypoint' softkey.

The display changes to a racetrack display called a 'Steer Screen'. This looks like a highway straight up to the horizon. There is an icon, an arrow or boat on the track. The track represents your course to the destination, the boat is you. The boat will point in the actual direction you are going and the screen will display some important information. To get to your destination point the boat at the waypoint near the center line on the screen. You do this by driving the boat along the desired heading.

DTG - Distance to go. Get your answer here ... My answer is 142 miles, Chicken Ranch to SE PDX.

TTG - Time to go. The time it will take you to get to your waypoint at your present speed.

SOG - Speed on ground. Your speed in KPH or MPH (You can select metric or English) relative to the earth.

XTG - Cross tracking error. The drift off your desired track. This is useful to correct for current or Wind.

HDG - The heading or desired compass number. IE: 242 degrees. This is the number to follow from Newport to the Halibut spot offshore.

The 350A has many other things that can be displayed on this screen.

Good luck and have fun. There will be a test.

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The bend is your friend!

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Posts: 1668 | From: Portland, Or., U.S.A | Registered: Aug 2000 | IP: Logged