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Orientation, Part 1
Notes From The Cone of Silence
(Maps/Navigation #1: September 25, 1998)

    I've been using GPS receivers in the field for more than four years. They're reliable, rugged, just plain fun; but they do have their limitations.
    My first GPS was a Micrologic SuperSport. I got it for about $360 just as the Gulf War was ending. It was as big as a brick and required six AA cells, which ran the unit for about 3-1/2 hours in the field. It would do almost everything but get lighter as you hike. I still like the SuperSport, because it has some great features, but it has become a casualty of the lack of foresight of the GPS designers. See the note, at right.(1)

Magellan Meridian
Magellan Meridian

    About a year later I bought a Magellan Meridian for about $170 because it used only three AA cells and got about six hours life on them. The Meridian is a sweetheart. It's simple, light, big enough to handle, has a large display, and I like its white case—easy to find when I lay it down. It's only limitation is it's memory size. It's my primary hiking GPS now.
    By now I was hooked on GPS, and I bought a third one, a Garmin GP38. It's the smallest of the lot. I got it for about $130. It's a fantastic, very small unit with all the features anyone could ever want. It uses four AA cells and runs about eleven hours on the set.
    I finally had to swear off buying GPS receivers. After all, there were kids in the third world and members of my church who needed a little money more than I needed another GPS. Although...now Magellan and Garmin have come out with a lot of nifty new models that looks very rugged, operate for more than 16 hours with only two AAA cells, and sell for less than a c-note...
    Each of my units came with an owner's manual with a prominent disclaimer to the effect that the user should never rely on GPS as his/her only navigation tool, but as a part of a more complete navigation system. After a little use in the field I found out why.
(1) By the first GPS leap week (week 1025 of the GPS system's life, about a year after I wrote this article) my SuperSport stopped locking on to satellites' signals. This was not the first time this had happened. A couple of years earlier, when the military switched the system from PPS back to SPS, I had to have the software updated.
   Leap week was the GPS equivalent of Y2K for computers. Some units were unaffected, some had to be reprogrammed. My SuperSport was a casualty. By this time Miocrologic was no longer in the GPS business. This great, very capable unit is now what I call "dead iron."

GPS Limitations
(2) The two methods mentioned are methods of applying corrections to the GPS signal your receiver processes. DGPS locates ground stations precisely and transmits a correcting signal; you must have a second receiver to pick up this correction. DGPS stations are scarce. WAAS locates ground stations that serve wide areas. It collects atmospheric data that cause GPS errors and transmits them to an orbiting satellite. GPS receivers equipped to receive WAAS apply these error corrections to your position indications.
The civilian GPS, called SPS (Standard Positioning System), is deliberately corrupted in precision with random errors that limit it's position accuracy to ±100 meters, 95% of the time. It's possible to get much greater accuracy through methods called DGPS (Differential GPS) and WAAS (Wide Area Augmentation System)(2), but locations served by DGPS and WAAS ground stations are rare.
   So what can you expect in the way of accuracy and repeatability? I use this analogy:
  • If you're in a small plane flying above clouds, when your GPS says you've arrived and you drop below the cloud cover, your destination will be in sight below you.
  • If you're in a boat headed for a buoy or a marked dive site, when GPS says "Arrived" you'll see the buoy nearby.
  • If you're on the ground and you put your Rolex down as a marker, with a later return in mind, when you get back, according to your GPS, you'd better be ready to kiss your watch goodbye or spend some time searching.
If GPS has done its job well, in each case you'll be within 100 meters, 95% of the time—OK when relative distances are great and targets are clearly visible, not OK when targets are small and unmarked.

The net result of DGPS and WAAS is increased ground location accuracy. Current estimations for the position accuracy for WAAS is ±20 meters 95% of the time. Still not micro-precise, but very good for normal humans.
Workarounds In the first two analogous situations there was a second navigation system in operation—visual identification. In the third there wasn't. Working with GPS on the ground, I always carry the makings for a second system: a map and compass and a roll of fluorescent engineering tape. When I stash something, water for example, I save the position in the GPS memory and note its location in UTM grid units in my notebook. I also mark the position very visibly with the tape. If I want to hide something, I record compass bearings to several landmarks that are on the map.

Deep Canyons and Tree Cover I've found out on the ground that if your GPS can't see a cone of sky of at least about 45º (as in a deep canyon), the likelihood of obtaining a usable 3D fix is very small. The same goes for heavy tree cover, though it doesn't seem to be as limiting as the cone angle.

Nearby Interference For some reason I can't get a fix if I'm running more than one GPS receiver at the same time. I've tried many times, and it just hasn't worked for me. Could there be some kind of RF field being generated? Beats me.

System Errors Hey! There's no guarantee that the GPS in your area is even working today! I remember one day, driving from Tallahassee, FL, to Rockledge, FL, I was reading a ground speed of more than 600 knots. No way! Not in my Subaru. I found out later that one of the satellites I was using for the track had a bad atomic clock. That'll do it.
   I've also driven several hundred miles (North Florida, Alabama, West Texas, even my own home town) with no satellite reception at all. Similarly, my cousin and I experienced a sudden total lack of reception at the top of a mountain in New Mexico. Too much cloud cover!
   I have to keep reminding myself that this system belongs to the military; I'm just tagging along for the best free ride I can get. I can dig it!
The Lesson Always have at least one backup system available. After all, who says you won't drop your GPS in a lake or something? Some good nav system fixin's to keep in your pack:
  • GPS receiver
  • spare batteries
  • engineering tape
  • compass
  • topo map
  • flashlight (LED preferred!)
  • notebook
  • pencil or pen.
How About All Those Claims? Har de har har! All the hype you've seen about finding your friend's house on a rainy night in a strange town—just hype—unless that city is running a DGPS transmitter and you've subscribed to their service. Otherwise, consider the claims a good joke.

I Hate To Burst The Bubble, But...
    GPS is a good (great even) navigation tool as long as your expectations don't exceed the system's accuracy specs. I always try to remember that GPS is just a machine. Machines can't get lost. People can. If you're trying to navigate in the wilds without a good basic understanding of maps and navigation in general, sooner or later GPS will bite you.

    There are lots of good books you can read about GPS. One of the better ones is GPS Made Easy by Lawrence Letham. You can get it from our Online Bookstore.
    For online information on current GPS products see the GPS-related websites listed in our Library.

    See you next time with another maps/nav topic.

-z-
GPS Made Easy

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