Notes

Outline

A Very Brief Tutorial on the Use of GPS on Self-Propelled Outings How Does it Work & What Good is It?

GPS = Global Positioning System Constellation of 24 satellites Orbit about 12,500 miles above the earth Not geo-synchronous Means that there is a constantly changing group of satellites overhead.

GPS is a One-Way Satellite Receiver

In Answer to the Question: Where Am I? I could use a street address

Other Ways to Describe Location

Fundamental Principle Light and radio signals travel at 186,000 miles per second. Signal traveling from satellite 12,500 miles away will take about 67 milliseconds (67/1000) to travel the distance. Knowing position to within 25 feet requires a time resolution of 25 billionths of a second.

Effectively, this is What Happens

Your Point on the Planet is Determined from the Time (Distance) from 4 Satellites

The Bottom Line In most cases, modern hand held GPS units (12 Channel) typically are good to within 30 feet. Exception:  interference from steep walled slopes or under wet leaves during rain.

GPS Can Provide Position Determination in Many Different Coordinate Systems Latitude/Longitude in Degrees, minutes, seconds Latitude/Longitude in Degrees and decimal minutes. British Grid, Irish Grid, German Grid, Indian Grid, etc But the most useful for self propelled outdoors people is UTM, or Universal Transverse Mercator

Why UTM? Quick Question: About how far is 2 minutes 36.25 seconds of longitude at 39° North Latitude? Answer: I don’t know either, and that is my point: There is no inherent relationship between latitude/longitude and distance:  it is dependent on where you are on the planet.

How Do UTM Coordinates Work?

How Do UTM Coordinates Work? Part 2

Where are Those UTM Coordinate Grids on a Map?

What I Do with a GPS Reading? Part 1 Where Am I?

Figure your Position on a Map

Part 2:  How Far is it to Where I Want to be?

Part 3:  Where Have I Been?

Bottom Line GPS can be very useful for assisting in figuring out where you are. You should be using it to confirm where you thought you were already. Without a map to provide context, a GPS will be of minimum utility. Knowing where you are in a tangle of vegetation does not, in itself, help you very much.

Bottom Line II In some terrain, a GPS will be as “useful” as a cell or satellite phone.   (They can be used to pound in tent stakes!) In those cases, you will be using all the map reading skills you have. Don’t forget your compass.