GPS for Common Folk Like Us

By Larry Soo

Finally! GPS technology has improved and prices have dropped to the point where useful, consumer-grade GPS units (also known as receivers) are available for the average Joe (or Joette) backwoods explorer. My aim with this article is to give you an idea of what features are desirable and to straighten out some misconceptions about GPS. If you're already bored by my writing style, you can skip to the last paragraph which itemizes the important points. For the rest of you, I'll start by telling you how GPS works.

The Real Dirt

Ok, here's the deal. The US Department of Defense coughed up the dough to establish a constellation of 24 satellites. These satellites constantly broadcast their position and atomic time in an encoded format to earth. If a GPS unit can receive information from any four of these satellites, it can determine its location anywhere on the earth to within 15 metres of accuracy. There's a catch, though. To receive that kind of accuracy, you have to have a military GPS unit capable of decoding the encoded signal. When the US government decided to let civilians use GPS, they did not make the military decoding technology available. Instead, they released a different encoding technology that would be susceptible to something called, Selective Availability (SA). SA was introduced to degrade the signal such that non-military GPS units could only reach a guaranteed accuracy of 100 metres. The reason for this was that they didn't want enemies of the state to buy a few hundred GPS units from Wal-Mart and then use them as super-accurate guidance devices for guided missiles.

As always, there are exceptions to the rule. SA's degradation varies so on some days, accuracy can actually improve to 50 metre accuracy. In some rare cases, the US turns off SA for whatever reason during which time, civilian GPS receivers have the same 15 metre accuracy as the mil-spec units. The most well-known case of this happening is during the Gulf War when the US didn't have enough mil-spec GPS units to issue to its troops. As an emergency measure, they supplied them with civilian GPS units and turned-off SA.

Common Misconceptions

GPS units will not work under all conditions. Mountains, trees, human bodies and damp foliage will all hamper its ability to receive satellite signals. For fourwheelers, metal hard-tops will also degrade signal reception. Plunk one down in the console, under a metal roof, blocked on both sides by wet, meaty bodies, and you'll have a GPS that will not know where it is.
Having GPS does not mean you can throw away your compass and map. It is a tool. A tool which can be quite fragile and is reliant on batteries. You would be a fool if you ventured into the woods relying solely on GPS for navigation.
GPS does not tell you the direction you're facing. Unless the unit has a built-in electronic compass, it can only tell you your heading which means you also have to be moving. The faster you move, the more accurately it can determine your heading.

What to Look For

If you buy a new GPS from the big three manufacturers, Eagle, Garmin and Magellan, they will all have backlighting, AA-battery compatibility, and map views which will trace your route on the screen (this is not the same as having an actual map built into the unit). Everyone has different requirements but I will insist that you buy a unit with this feature: a 12-CHANNEL PARALLEL RECEIVER. Let me say this again, BUY A 12-CHANNEL PARALLEL RECEIVER. 8-channel parallel or 5-channel multiplex don't cut it. The reason I want you to buy a 12-channel is because it will give you the best performance when trying to get satellite signals under tree cover. 8- and 5-channel units pale in comparison and the difference is very, very noticeable. The second reason is that the 12-channel receivers acquire satellite signals in less than a third of the time of the older 8/5 units.

If you're interested in recording a track log of an entire trail, maintaining a continuous lock on satellite signals becomes more important. For that, you'll need these features:

External antenna capability is a good thing. A GPS will have a lot of trouble receiving satellite signals in a vehicle. The best solution is to use an external antenna or if you have a GPS capable of doing so, remove the GPS unit's antenna. Place the antenna in a position where it will get the most unobstructed view of the sky and connect it via a cable to your GPS.
If you have a computer, buy a GPS unit which can upload/download data to your PC. Besides allowing you to save all those track logs and coordinates you've recorded, the data link will also let you use your GPS in conjunction with moving map programs on your PC. For example, I can scan and load map of the area I'll be exploring onto my laptop computer. As I drive, the GPS receiver will relay its coordinates to the laptop which will then place a pointer over the map, indicating our position.
To avoid draining your receiver's batteries, buy a power cable that will let you power the GPS from your car's battery.

That's it for suggested features. From there on in, your own tastes, requirements and budget will dictate which unit to purchase. For those of you who can afford the deluxe toys, look for the latest models that allow you to upload maps from a computer's CD-ROM directly into your GPS unit.

To Make a Long Story Short

GPS units will not work under all conditions.
GPS is not a substitute for a map and compass and knowing how to use them.
GPS is guaranteed accurate to within 100 metres. It is sometimes better than that but don't count on it.
BUY A GPS WITH A 12-CHANNEL RECEIVER. Accept no other.
If you can afford it, I recommend that you also buy a unit that can use an external antenna, can upload/download to a PC, and can be powered by 12V DC.

The Most Important Thing

Just like a map and compass, it's important to practice using your new navigation tool under controlled conditions before relying on it in the woods in a stressful situation.

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