How To Use A GPS: The Basics And Background

As they teach us when training for Search & Rescue, one should never bypass solid map and compass skills and go straight to GPS use. Be sure you know how to read a topographic map and use a compass, so you can double-check your position or route and not rely solely on the GPS unit as you navigate.

Gadgets can fail. You might run out of battery juice and not have any spare. You might drop and break or lose it. Or the signal may not be sufficient for accurate navigation. So don't leave that map and compass at home just because you have a GPS.

 
Basic Map And Compass

I can't say there's just one thing you should never go without when heading into the backcountry -- in fact, I have a whole list of what I consider essential....

Both the United States and Russian governments have GPS systems. Here in the U.S., our system used to be scrambled in order to reduce the accuracy for the Russians, once considered our enemy. This was called "Selective Availability," and it was turned off in 2000.

If you're interested in reading more about the end of Selective Availability, see GPS Signal Degradation Ended from May 1, 2000, a statement by the President regarding the United States' decision to stop scrambling the GPS system.

Though the GPS system was developed for the military, it now obviously has many civilian uses. And now that Selective Availability has been turned off, the system is much more accurate for you and me.

When using a GPS in the field, it's important to remember that, except for the trip odometer function like that in a vehicle, distances are "as the crow flies" -- in a direct line from point A to point B. They don't take into account necessary turns in your route by trail or road or in order to bypass obstacles.

The above image is in the public domain.

NAVSTAR stands for NAVigation Satellite Timing And Ranging, which was conceived in the 1960s, begun in 1978, and went operational in 1995.

NAVSTAR is made up of a Space Segment, a Control Segment and a User Segment. The Space Segment is the satellites. The Control Segment is comprised of the receivers or dishes on the ground. And the User Segment is you, me and anyone else who holds a GPS unit in his or her hand.

The Space Segment of NAVSTAR is composed of 24 satellites and 2 spares, which orbit 12,000 miles above the Earth. There are six orbital paths, with each orbit taking 12 hours. At any one time, any spot on Earth should have a line of sight to a minimum of 6 satellites in the absence of impenetrable obstacles.

The above image is in the public domain.

In order to accurately navigate with a GPS, you need a good line of sight to the satellites so your unit can receive their signals. These signals don't pass through most solid objects, such as trees, canyon walls, bridges or metal roofs. Even your body can block a satellite's signal.

A GPS will work well outdoors in the open and in vehicles, boats and planes as long as there's a good sky view. It should also work well in a tent and under canvas or nylon as long as, again, there's a good sky view on the other side.

A GPS may work okay in a wooden structure on an upper level and near windows, in open forest, a canyon or even in a city between tall buildings, but the accuracy will be compromised.

And the GPS won't work well in thick forest and not at all in metal buildings away from windows, in tunnels, caves, under water, enclosed vehicles or in parking garages, for example.

For the best accuracy, your GPS should receive signals from six satellites but no less than four. Your unit will have a screen that displays the number of satellites it's making a connection with at any time.

The above image is in the public domain.

Your GPS unit communicates with satellites, which send out radio signals. Since the speed of those radio signals is a known factor, the GPS receiver is able to use that speed to calculate its distance--and therefore the distance of the one holding it--from a satellite. This calculation puts you and the GPS in an imaginary sphere.

So imagine yourself holding your GPS and standing in a big circle (See: "A" in the above image) on the ground. This reflects the information from a single satellite.

The GPS will do the same thing with the other satellites with which it can communicate via a clear line-of-sight. So for the second satellite, imagine another circle which intersects with the first. (See: "B" above) This second calculation will put your near the intersection of the two circles (See: "A^B").

The above image is in the public domain.

Then add a third satellite and another intersecting circle. This third calculation will place you in the intersection of the three circles. This is considered a "2D" (or two-dimensional) mode, figuring your location horizontally only. In this case, you can input your elevation, if you know it, for more accuracy with the GPS unit. Keep in mind that 2D navigation can be unreliable.

If your GPS can communicate with 4 or more satellites, your location will be more accurate and you'll be in "3D" mode, which will include an elevation calculation. This is what you're aiming for.

Your unit display will indicate whether you are in 2D or 3D mode. If it's displaying 2D, try moving around and getting into a more open area, free of obstacles like tall trees, to hopefully get to 3D mode if at all possible.

**You should always check your "EPE" or "Estimated Position Error," which will display on your GPS, usually right on the same screen that indicates how many satellites it's communicating with--the satellite page. This will also show you how accurate, or perhaps inaccurate, the position fix is. An EPE of 20 feet or less is good. The lower the EPE the better.

The above image is in the public domain.

Where in the sky the satellites your GPS is communicating with are located relative to where it -- and you -- are located is important when it comes to the accuracy of the information the unit will provide. And your handheld unit will display that configuration on the satellite page. Again, this is page where you can determine how many satellites the GPS is connecting or communicating with and the Estimated Position Error (EPE). So this is yet another piece of information right on one screen.

The satellite screen is the first screen you should use when you fire up your GPS.

The satellite configuration is usually depicted within two concentric circles on the screen, with the satellites represented by dots or numbers. The inner circle on the screen represents the sky more above you than to the sides. The outer circle represents the horizon.

If your GPS is connecting with four or more satellites and is in 3D mode, and your EPE is low, that means your position fix will be good. And a good satellite configuration is even better.

What you want to see, ideally, is that one or two satellites are overhead -- within that inner circle -- with the rest spread out around the sky, somewhere in that outer circle. If all of those satellite "dots" or numbers are in the outer circle, with none over head, try moving around and see if you can improve that configuration.

The above image is in the public domain.

GPS units range from basic to complex, with a wide range of features, capabilities and prices from around $100 into the thousands of dollars, believe it or not. Me, I use fairly basic units, which do everything I need for Search & Rescue work and backcountry travel. I'll show you which units I use below.

But when it comes to choosing a GPS for yourself, think about what you want to use it for, where you'll be using it, and which features you really want and need, not to mention your budget. Features to consider might include:

• Weight and bulk--Where will you being carrying the unit? In a jacket pocket? A radio harness? In or on a backpack?


• Durability--Is it water-resistant or waterproof? It is dust-proof? How well can it handle extreme temperatures?


• Screen and text size--Can you see the text and numbers well enough?


• Data entry--Is it easy to enter information? How are the buttons laid out? Can it be used one-handed? How easy or difficult is it to move between menus and screens? And are those menus and screens set up logically?


• Battery type--What kind of batteries does it take, and do they match your other gadgets? (I prefer a GPS that takes AA batteries, because that's what my headlamp and flashlights take. That way, I don't have to carry multiple types for spares, and the batteries are interchangeable if necessary.)


• Mapping capabilities--Does the GPS take topo maps? Does it have computer download and upload capabilities? How much memory does it have, and is it enough?

You may also want to purchase a GPS case and a strap to attach the unit to a belt or backpack.

Photo Credit: Courtesy of Wikimedia / GNU License

• GPS units can break or the batteries can run out. Be sure to carry extra batteries ... and that map and compass.


• A satellite signal can bounce off of other objects before it reaches your GPS, which changes the timing and therefore makes the location fix inaccurate. This is called "multipathing." Move away from buildings, cliffs and other large objects as much as possible to minimize this problem.


• Using the unit while in 2D mode can be very inaccurate. The GPS will try to use the elevation of the last time it was turned on, which can cause the calculation to be off by many miles. For example, if you last used the GPS at 500 feet in elevation and then travel to 5,000 feet and turn it on again and use it in 2D, the read-out could be more than a HUNDRED miles off. So either input your new, known elevation or wait till you get 3D mode.


• Signals can be blocked by solid obstacles, like trees, buildings and cliffs or canyon walls.


• Extreme temperatures can slow down the GPS or make the screen fade. In very cold weather, keep it in your jacket.


• Because your GPS is calculating a distance across the surface of a datum (to be discussed in another article) like that on a map, GPS distances can be off when calculated for a slope.


• Operator error is the biggest GPS "problem." Too many people become "out of the box users," not practicing with the technology before using it--and relying on it--in the backcountry. Key stroke errors are also quite common, with an error potentially causing a discrepancy of hundreds or thousands of miles.

 
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