THE SKY'S THE LIMIT: Navigating the Wilderness with GPS
The ARS Sojourner
WHAT IS GPS AND HOW DOES IT WORK
The global positioning system (GPS for short) is a satellite-based navigation system. It consists of a network of 24 orbiting satellites situated 11,000 nautical miles above the earth and transmitting in the neighborhood of 1.575 GHz. Each satellite transmits a telemetry signal which contains an identification code, its current position and a time signal. The orbital paths of these satellites are precisely controlled so that it is possible to receive satellite signals anywhere in the world, any time of day or night. Back on earth, a handheld GPS receiver will simultaneously monitor the signals from as many as a dozen different satellites. The receiver compares the satellite's transmitted time signal to its own internal clock to calculate distance. At the same time, the receiver “triangulates” signals from several satellites to determine location and altitude. By continuously updating its position, a GPS receiver can provide speed and direction of travel information as well. The question of GPS accuracy is always in the forefront of conversation. For various reasons, position errors can be as great as 100 meters or more. On the other hand, accuracy is typically much better than that with errors as little as 60 feet being quite common.
GPS IN ACTION
In early June I spent a 3-day weekend in the Sierra National Forest in central California. I set up camp at 5,800 feet above sea level with a ridge of high mountains at my back and a small stream full of really mean rainbow trout at my feet. My objective (besides catching a few fish) was to use the GPS to guide me to a 6,200 foot high rock pinnacle a short distance from camp. Simple enough, except that between myself and the mountain top was a high ridge followed by a very steep canyon. I began by plotting a possible route using a topographical map as a guide. Then, using latitude and longitude coordinates from the map, I entered a series of “waypoints” into the GPS's memory. Waypoints are simply intermediate places along the proposed route. In my case they were like invisible highway intersections. My first waypoint (beyond my starting point labeled, CAMP) was a point on an old trail fairly close by. I programmed it in and named it, TRAIL. According to the GPS it was only about 800 yards due west from CAMP. Unfortunately there was a 100 foot high cliff blocking my approach so I began hiking south until the terrain moderated. All the while the little direction arrow on the GPS screen continued to point squarely toward TRAIL. As I began my ascent the GPS showed me not only my speed across the ground but the current altitude as well. Just short of 6,000 feet a.s.l. the GPS announced my arrival at the first waypoint. And there it was, a few feet ahead of me was a narrow trail. So far, so good. The trail ran in a northwesterly direction, and because it paralleled the ridge it was fairly level. I had to follow it for almost a mile before turning left to backtrack slightly toward the mountain top. Accordingly, the next waypoint was labeled LEFT.
NOTHING IS PERFECT
Less than 200 yards down the trail I noticed the GPS was no longer updating my speed and direction of travel. Then a message flashed across the screen that read, “Poor GPS Coverage.” I switched to another screen that showed which satellites the receiver was tracking only to discover that it had lost all but two signals--and neither of these were very strong. The problem was that I had walked under a fairly thick canopy of trees. At a frequency of 1.575 GHz, it doesn't take much to block the signals. To compound the problem, on my immediate left was a solid wall of granite a couple hundred feet high. For all intents and purposes the sky was completely blocked from view for r.f. But as I continued to walk along the trail and away from the obstructions the GPS sprang to life again. Two more times the receiver lost sight of the sky, but as before, moving toward a clearer area fixed the problem. As soon as I reached LEFT, I could see my destination. All I had to do was cross a small ravine and climb the remaining 200 feet to the top. According to the GPS, I was less than 0.2 miles away. It turned out that there was no clear, direct route to the top. At the bottom of the ravine was an impenetrable wall of Manzanita that forced me to detour 200-300 yards to one side. Still, it wasn't long before I was savoring the magnificent view and an ice cold Pepsi from atop the rock.
GETTING BACK HOME
From my perch on a massive granite boulder I could see another possible route back to camp. There was a second ridge, parallel to the first, that looked passable. I punched the GOTO button on the front panel of the GPS and watched as a list of waypoints popped onto the screen. I scrolled up to CAMP and pressed ENTER. The receiver immediately gave me the bearing and distance. Away I went. It took less time to get back to camp because the terrain was mostly downhill. I've discovered that at my age, travel downhill is much better than travel uphill. I only lost GPS coverage once on the return trip and that was when I dropped down into a shallow canyon. I could see it coming so wasn't surprised when the “Poor GPS Coverage” message flashed on the screen. I was back home and sloshing down a second Pepsi in no time.
WILL A GPS RECEIVER SOLVE ALL YOUR NAVIGATIONAL PROBLEMS?
No, probably not. As you have seen, there are places where the receiver cannot effectively ‘see the sky.’ And there is always the possibility of your batteries going dead at some crucial point. For those reasons I make it a rule to know where I am (on the map) at any given time, especially when I'm hiking in unfamiliar territory. Despite a couple shortcomings, I have come to appreciate my GPS receiver more and more with each outing. As a consequence, it has earned a permanent place in my backpack. Even on short hikes it has become my primary tool of choice as a navigational aid. For more information on GPS and how it works, log onto Garmin’s web site at http://www.garmin.com. Click on, “All About GPS.” You may find this fascinating technology is just the ticket to help you find the way to your next adventure.
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Bill Jones, KD7S, is an avid QRPer, field operator, outdoorsman and contributing editor to The ARS Sojourner living in Sanger, CA.