If you're like me, you'll agree with Will
Rodgers who observed "We should be thankful we don't get all the government
we pay for." However, in rare cases, a little chunk of that pile of money
that you and I shovel into that seemingly bottomless pit called the IRS makes a
round trip to come back home and actually do us some good. A high tech military
project called the Global Positioning System (GPS) is one of those too rare
examples. GPS, eh? Never heard of it? Well, it is a system of satellites
orbiting the earth transmitting a continuous signal that handheld receivers down
here on land can tune into and use to calculate your earthly position to within
about 30 ft. night or day, rain or shine, winter or summer. Wow, eh? Think about
the power of that technology for just a moment. Given the entire surface mother
earth, the marvel of GPS has reduced your uncertainty as to exactly where you
are standing to the distance you'd lob a rock at some nuisance varmint. That's
pretty mind-boggling when you really think about it! In the future, or today
with special equipment, that radius of uncertainty will close to within spitting
range. Simply amazing!
Backcountry riders can immediately grasp what this means to them. How many times
have you been in the middle of a long ride with a map, or worse yet no map,
spread over a seat with two or three of you engaged in a spirited "Wherethefugawi!?"
discussion? How about being in thick trees and unable to get a view of any
landmarks and having the same argument? How about having one hand on the bars
and the gas cap in the other while rocking the bike back and forth eyeballing
just how much gas is left in there? Enough to make it back if you make all the
right turns? Maybe? Did it ever come to drawing straws to see who in the group
will get all the gas that is left so they can return with fresh fuel supplies to
get everybody else back? Were they able to find their way back to where you were
cooling your heels? At night? Have you ever been cold, wet, hating life and
staring at a section of trail in the feeble headlight beam trying to remember if
you've ever seen this damn section before?
The worst case is having an injured rider down who cannot be moved or is too
hurt to ride out. How to best get the rescue folks there quickly with a minimum
of delay? I have been in at least one of those situations wishing I had a GPS
but I'm not telling which one it was. It suffices to say that one of my good
riding companions claims "It isn't an adventure unless you're riding back
into camp out of water and food and running low on gas and daylight." True
but, by using GPS, you can be much more precise about just how much adventure
you're going to end up with in any one day.
It can also greatly increase your possibilities for adventure. I really enjoy
riding new places but, not knowing beans about what the trails are like or where
they are, it can be too confusing or intimidating to try on your own if you've
never been there. When was the last time you tried to find a great trail from
directions you got at the parts counter weeks back and without a local rider as
a guide? Would it be different if you had GPS coordinates? You can bet on it.
The confidence that comes with knowing exactly where that hard to find trailhead
is located will make the difference between riding or not riding that trail.
Being able to provide GPS coordinates with other riders can greatly increase the
diversity of rides for all of us and also have more riders enjoying great
trails. Having more riders on your trails can only help to increase the count of
voices on your side when you try to keep the wilderweenies from turning it all
into Wilderness because it is a "roadless area" and they will - sooner
or later.
In all of these situations, GPS can do more for you than a steering damper on
head-shaker. There's nothing magic about it and it won't keep you from being
stupid if you're really determined to be. It isn't a panacea nor is it a
substitute for traditional navigating skills with map and compass. It is an
adjunct to paper maps and it will allow you to plan rides better, make the most
out of a day's ration of light and fuel and will let you share those trails with
other riders with more precision than ever before. With the fine work done by
manufacturers on the menus and interface, it doesn't take an engineering degree
to figure out how to use and operate one either. Anybody can put them to good
use and navigate effectively with just a little bit of experimentation around
the neighborhood before hitting the trails.
The worst part about putting GPS to work for you is figuring out which model is
the right one and how much money to spend. As with most things electronic, like
stereos and computers, unless you want to subscribe to a magazine and spend
weeks reading reviews, it is tough to come to a decision. Relying on the short
little blurbs in a catalog description or trying to get help at most retail
sports stores doesn't quite cut it either. The flunky on the sales floor usually
doesn't know what he's talking about or just slings a bunch of acronyms around
to make it sound like do. Pretty worthless either way. That's why I took on the
task of writing up this series of articles. I'd like to see more riders take
advantage of this wonderful technology and use it to its fullest extent. The
series will be in three or four segments. This first one will brief you on how
it all works including terms and acronyms. Then, in the second article, I'll
cover how you can use this technology to navigate effectively and, perhaps even
more important, what GPS won't do for you. In a third article (and maybe fourth
article - there is a lot of detail there), I'll cover how you can plug a GPS
into a computer to both download and upload data and use mapping software to
plan rides or map out trails you log while riding.
I've been using GPS for about two years now and have successfully used it to
document a favorite area in southeast Utah that wilderweenies are trying to turn
into Wilderness (you can check out what I've done by surfing to my web site at http://www.vlj.com
and boinking on the buttons you'll find at the tops and bottoms of my pages).
There is tremendous power in this technology and more of us need to put it to
work and exploit it to benefit our riding and riding opportunities for others.
It is tedious but not difficult. If you're ready to jump into GPS with both
feet, keep on reading.
How Does It Work?
GPS does its thing by using very fast and accurate clocks. These internal
devices time the propagation delay of the signal from satellite source to
receiver destination. Using that delay in making the space to earth trip, the
distance traveled may be calculated using the speed of light as the measure.
You've seen this phenomenon before when your eyes have seen a flash of lightning
off in the distance and then your ears detect the clap of thunder some seconds
later. Timing the delay between flash and sound and dividing by the speed of
sound in air, you can calculate how far away the lightning hit was. A GPS
receiver does the same kind of thing but measuring signals that travel at the
speed of light rather than the much slower speed of sound. The orbit of the
satellites is at a height of approximately 11,000 miles so it takes only a few
hundredths of a second for the signal to make its trip. That's why you need
those fast and accurate clocks!
Knowing the distance between you and a single satellite isn't enough though, you
need more than one satellite to calculate your earthly position. You actually
need at least three satellites for a two-dimensional fix and four satellites for
a three-dimensional fix. Why four birds for a 3D fix? Three are needed to solve
three simultaneous equations in three-dimensional space in terms of latitude,
longitude and elevation (X, Y and Z). The fourth is used to adjust for timing
errors in the signals in the other three. If you'd like to learn more on all of
the specifics of how all this works, you can point your web browser to Trimble's
web site (http://www.trimble.com/gps/aa_abt.htm)
and step through their illustrated tutorial. In that tutorial, they cover much
more detail than I can here and its done in a very easy to read fashion.
So, if you can "see" four satellites in the sky out of the
theoretically twelve visible in any hemisphere, you can get a three-dimensional
fix to determine your position. In areas with unobstructed views of the sky,
somewhere between 6-12 satellites are usually visible. If you are in areas with
obstructed views of the sky, like among city buildings, forested areas, desert
canyons, mountains, etc., you may only be able to see a few satellites. If the
obstructions are big enough, numerous enough or thick enough, you may not be
able to get a lock on enough satellites to get a fix. Older generation handheld
units employed an eight channel multiplexing receiver (meaning it could only
track one signal at a time and hopped around among eight strongest signals in
round-robin fashion). These were pretty notorious for not being able to get a
fix when and where you could really use one. Newer units are using
twelve-channel parallel receivers that can track up to twelve satellites in
parallel and which allow the receiver to use the optimum four to give you the
best performance. These are much more tenacious about getting and maintaining a
lock on signals in marginal conditions and give you the best possibility of
getting a fix.
Once you have that fix, you can use that information to locate your position on
a map ruled in latitude/longitude or UTM coordinates or relative to waypoints
previously stored in your unit's memory. If you don't have a map, other
waypoints to reference or a tracklog of where you've been/going, your fix will
inform you the exact location of where you're lost. Again, there is nothing
magic about this technology and it isn't much good without surrounding context
when you are navigating. However, when you do have that surrounding context,
your GPS is a tool for accurate and precise navigation.
Well, this is a pretty good start. Some of the terms used above a probably
unfamiliar so below is a glossary of what those terms are and what they mean. In
the next article, I'll jump into the details of how one can use a GPS in
conjunction with hardcopy maps to get from one point to another. I'll also get
into the detail of what the various features and functions of basic handheld
units offer. In the mean time, if you have questions that can't wait or just
want some pointers to get you started on your own, just zap me off some email at
gps@vlj.com and I'll do what I can to answer
back. Otherwise, come back here next month and I'll have a whole bunch of whizzy
graphics and descriptions on how to use all this good technology on the trail
...
Good ridin' to ya,
Victor Johnson
Terminology
Signal - The GPS signal itself is relatively weak and does not
pass through objects very well. Buildings, mountains, trees (especially if the
leaves are wet) will block signals. The are, however, pretty good at getting
through rain and/or snow.
Initialization - When your first turn on a GPS unit, it has to
figure out where it is. Most have an initialization process they must go through
when you first turn them on or have moved more than 500 miles from where they
were last turned on. If you don't have a good view of the sky, you may not be
able to initialize at all. If your unit is turned on and used frequently (and
you don't let the batteries completely discharge) initialization won't be
something you'll need to do very often.
Cold Start - If it has been over a half an hour since you last
turned on your GPS, the stored almanac data stored in memory will have gone
stale. It will need to download fresh almanac data over a period of being on for
about a half an hour to get a fresh almanac restored. This is one of the reasons
it is easier to get a fix in marginal conditions if your unit is left on rather
than turning it on and off each time you want a fix.
Hot Start - If it has been less than about half an hour since
you turned your unit off, the stored almanac data will still be fresh enough for
the unit to be able to
Almanac - A stored timetable of satellite numbers and their
orbits. Satellites being tracked by your GPS are moving across the sky both in
and out of your viewing area. Embedded in the signal you unit is receiving are
the orbital characteristics of all satellites and these are accumulated and
store in your receiver. This way, your GPS knows about satellite's distance and
position even if it isn't currently tracking the signal from that satellite.
Fix - A computed position given a solid lock on at least three
or four satellites. Three satellites will give you a two-dimensional position
and four satellites will give you a three-dimensional position. Once you have
locked on a satellite, it is much easier for you receiver to keep that lock and
track satellites for a continuous fix than it is to turn it on cold and have it
try to acquire a fresh lock.
Bearing - A heading or direction based on 360 degrees in a
circle in a clock-wise direction. A true north is a bearing of 0/360 degrees.
East is a bearing of 90 degrees, south is a bearing of 180 degrees and west is a
bearing of 270 degrees. So, if your bearing is 317 degrees you are headed in a
N-NW direction. Bearings are real useful to pilots and seamen who can travel in
any direction they want. The "as the crow flies" direction isn't
normally as easy for land travel so your bearing is normally a "general
direction" type of thing.
Waypoint - A stored position in your GPS memory. These can be
obtained by storing your position into memory when you're out on the trail,
gotten from someone else and keyed into your receiver manually or entered from
map coordinates. Once you have waypoints in memory, your GPS unit will tell you
how far you are from that waypoint and what bearing you need to get to that
waypoint.
GoTo - A handy navigating feature that lets you specify a
specific way to "go to" and your GPS will track your progress in
getting towards that destination. Some units will display all kinds of handy
statistics on this progress including estimated time of arrival, estimated time
on route, bearing error, etc.
Route - A series of waypoints tied together in segments. Each
segment between waypoints is a "leg" of that route. As you approach
the end of a leg, some units will give you a visual and audio alert and will
then automatically switch over to the next leg when you pass the endpoint.
Routes are limited in how may legs they will allow. 30 is an average number.
Tracklog - This is a "bread crumb" trail. Many GPS
receivers have a record mode where you can just turn them on and they will
sample your position periodically and then store that position into memory. That
sampling can either be a period of time, such as every few seconds, minutes,
etc. or may be some kind of algorithm to decide when to store a point or save
memory for future points. The unit I use has a sophisticated algorithm that
computes your heading and velocity and only stores points into memory if either
of these changes significantly. Using the automatic mode, I can fit about 35
miles of single-track riding into my 1024 point tracklog memory. Some newer and
more expensive units have more memory like 2048 points or more. If you plan to
do mapping of trails, tracklog memory capacity is a very important feature to
consider.
Coordinates - Latitude/Longitude, UTM, etc. These are
coordinates for determining just where your position is and on maps. Most people
are familiar with latitude and longitude. I prefer UTM (Universal Transverse
Mercator) coordinates since they are based on meters and decimal divisions
making it easier for me to interpolate on maps than the typical
degrees/minutes/seconds system used with lat/long coordinates. Most of the maps
ruled for GPS are in lat/long coordinates so I'll use a ruler or an image editor
to lay out my own UTM 1 kilometer (about 0.6 mile) grid on the map.
Datum - The mathematical model representing the curvature of
the earth for your location and map. The earth isn't a perfect sphere and has
warps and puckers that vary across the continents. The most popular modern datum
is WGS-84 and is what most GPS units operate in by default. Most of the USGS
maps of North America use an older datum named NAD27. If your GPS is in WGS84
and your map uses the NAD27 datum, you'll need to shift your point just a tad.
Depending on what scale map you are using, that shift may or may not be
significant. Be sure and check your reference map to know what datum you should
be using.
Multipath - Since GPS receivers use triangulation and
propagation delay to compute your current position, that calculation can be
unduly influence by signals that have bounced off several objects before getting
to your receiver. Since that zigzag path adds propagation delay that wouldn't
otherwise be there if the signal traveled direct, multipath errors can give you
bogus readings. This is particularly noticeable when you are down in deep rocky
canyons. I've seen my GPS give me some pretty wild excursions of up to a mile or
more when it got confused by multipath signal errors.
GDOP - Geometric Dilution of Precision. More triangulation
stuff here. If the satellites you are tracking are bunched up in the sky, the
accuracy of your fix is degraded since that accuracy is proportional to the
spatial distribution of the satellites. The best scenario for a good fix is
tracking satellites that are widely dispersed from one another. That will give
you the best triangulation possible. If they are all clustered in the sky, which
does happen from time to time, your circle of uncertainty gets larger.
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