What the heck is a GPS?
GPS is the acronym for Global Positioning System. The United States and other nations have launched constellations of satellites used for locating one’s position of the surface of the earth with decent precision. GPS is the acronym for the the U.S. constellation of 24 satellites. Colloquially, most people are referring to a small portable GPS radio receiver when they talk about their GPSes.
Garmin, the leading U.S. manufacturer of consumer GPS receivers has a nice overview of GPS. Trimble, a well known manufacturer of professional grade products, has a pretty detailed tutorial. Wikipedia has a moderately technical article. I won’t repeat their words. Instead, let’s talk about what happens when you turn on a typical GPS receiver. I’m working from my (not) vast knowledge base–a Lowrance GlobalNav 212, a Garmin Foretrex 201, a Garmin Edge 305, and my current Garmin 60CSx. Most runners are likely to prefer a new style of GPS receivers that you wear like an oversize wrist watch (e.g. the Garmin ForeRunner, Suunto GPS watch, etc.). I use a larger model more suitable for the map-making I do for our trail run series.
When you turn on your GPS, it starts looking for satellites. My 60CSx displays this screen sitting on the hood of my car in bright sunlight at Reid Park in Tucson (Click on the thumbnail to see the full image.)–
The display shows the satellites it can get a signal from and the strength of the signal. Once it gets enough information from the satellites, it shows it’s current location at the top of the screen with an estimate of the accuracy of that location. A GPS needs at least 4 good satellite signals to get a decent positional fix. I can have 8-10 good signals out on the trail. Usually my 60CSx figures out where it is in less than a minute after I turn it on. Then there are a series of screens you can cycle through with a “Page” button. For all the gory details, check out the 60CSx manuals.
Once the GPS knows where it is, it starts tracking. My friend Joe (who’s an actual rocket scientist of sorts), says if it works like the GPS circuits they use in the missile systems he helps design, that the GPS just starts noting it’s position at some regular interval, say once a second. From that information it can calculate which direction you’re moving, as well as how far and how fast. That translates into all sorts of information the GPS can provide. For example, it has it’s own internal maps it displays on it’s little color screen. Most of the time, however, I have the display on this page (again, click to see the full-size image)–
You choose which fields you want to display on this page (there are dozens of choices). These are the ones I find most useful. At the start of the run (Or the bike ride I did last weekend–I didn’t run 85 miles.), you reset all these counters. At the end of your run, these counters will give you an estimate of how far, how fast, how much climb, and other interesting bits of information. Read my page about GPS accuracy before you trust these numbers.
What you hear about a lot are “waypoints”, “routes”, and “track logs”. My GPS has a “Mark” button. When pressed, a screen pops up with your current location, a waypoint number, and a name you can edit. This is useful if you want to mark a point of interest along your route. Routes are a linked series of waypoints you can download to your GPS, say from your mapping program on your PC or Mac. Then you can use the navigation functions of your GPS to lead you along the route. I’ve only used that feature once in all the years I used GPSes, but it worked nicely (I know my home mountains here pretty well…). Track logs are the feature of my GPS I use the most. At intervals the GPS will record it’s location to a log file. Some GPSes decide when they will log an data point, some let you decide. After your event, you can download that log file to a computer and examine it and manipulate it in various ways. Here’s a few entries from a typical track log–
Format: DDD M/D/Y H:M:S -7.00 hrs Datum[115]: WGS 84
ID Date Time Latitude Longitude Altitude
L BEAR CANYON WINTER 2008 FFFF 00C8 2215
T 01/20/2008 07:01:35 32.30929 -110.82206 827.8
T 01/20/2008 07:01:56 32.30930 -110.82205 831.2
T 01/20/2008 07:01:59 32.30930 -110.82206 828.3
T 01/20/2008 07:02:01 32.30930 -110.82206 813.9
T 01/20/2008 07:02:02 32.30930 -110.82207 824.5
T 01/20/2008 07:02:04 32.30930 -110.82207 834.6
This is obviously a pretty scary string of numbers. If you’re so inclined, you can analyze these yourself with spreadsheet programs (I’ll explain that elsewhere.). Most people will let a computer program like Garmin’s MapSource, or 3rd party programs from one of several other mapping software makers. I’ve worked with several mapping programs, and lately I’m tinkering with the spreadsheets, too. I’ll do one or more pages on these topics.
So this is the quick introduction. The best way to learn about this technology is to purchase a reasonably priced model and start using it for your own purposes. Most of you will probably prefer the runner’s models. I’ll borrow some of my friends’, test their accuracy and function, then share what I’ve learned.
