The new GPS units that include heart rate monitoring and other fitness
features are cool. Something like that is going to be on my want list
sometime soon.
But GPS does have limitations. It will only give you decent accuracy
for speed and distance when it can get good reception. At times it
will not be able to get any reception at all which means that it is
only guessing or not displaying speed or distance at all while it is
out of reception.
Things that affect reception and accuracy are satellite geometry in the
sky (which is of course constantly changing), buildings nearby
(reception in a downtown area with skyscrapers will be more difficult
than in areas with lower buildings or no buildings at all), trees
overhead, how many leaves are on the trees, how much rain is on those
leaves, fog, rain, snow, how the GPS antenna is oriented, you body can
even block the signal. Putting your hand over the antenna can be
enough to block the signal. So there is lots that can affect accuracy
and even whether or not you'll be able to get a lock on the
satellites.
If you're really interested in instantaneous speed readings and
accurate speed readings then a traditional cycle computer that has a
magnet on the wheel will give better results.
If you're looking at GPS units now it is better to go with one of the
new models that includes the newer and more sensitive GPS chips. They
have better signal reception and better battery life. For Garmin units
that new chip technology is by SiRF. Here's 'Garmin's Press Release
about the SiRF chip'
(http://www.garmin.com/pressroom/corporate/083005.html). You'll have
to dig around to find out which of the newer Garmin units are using
that chip.
For accuracy the number that the GPS units report as accuracy (like say
20 feet of accuracy) is just a guess based on satellite geometry in the
sky and not anything to go by. The actual accuracy could be very
different. Again, that number is just a guess and not something that
you should go by.
GPS units don't always do a good job by themselves for altitude or
elevation info. Their first priority is to get the 2D location right.
The 3D info (elevation) is secondary. 2D location is going to be more
accurate than the elevation. So if the unit says you've got 52 feet of
accuracy you can figure that the elevation accuracy is going to be
worse than 52 feet. 2D location requires a lock on at least 3
satellites. 3D location requires a lock on at least 4 satellites. If
you want more accurate elevation readings you should get a unit that
includes a barometric altimeter that supplements the GPS calculated
elevation readings. The barometric altimeter will continue to get
readings even if there are no satellites in view and its accuracy is
also better.
Garmin has a good description of 'GPS technology'
(http://www.garmin.com/aboutGPS/).
WAAS is nice but it is not guaranteed to always make the GPS reading
more accurate. At times it can actually decrease the real (as opposed
to the reported) accuracy. WAAS will only improve the accuracy if the
WAAS satellite is in view. The WAAS satellite sits low on the horizon
so it can be easily blocked by trees, buildings, cliffs, etc. If
you're in an area where you are not likely to be able to pick up the
WAAS satellite it is best to turn that feature off. Out in the ocean
or up in a plane the WAAS works great because it is easy to see the
satellite. In the woods or in the urban jungle, WAAS is less useful.
Knowing what is going on with GPS and how to use it correctly quickly
gets in to technical matters. To use it correctly you need to know how
it works along with where and when it will or will not work.
I have an old Garmin Vista. It frequently loses satellite lock due to
tree cover when I go on muni rides. It also loses the satellites at
times when I go on Coker rides. It uses the older and less sensitive
GPS chip. The new SiRF chip should do better, but I haven't used a GPS
with that new chip yet to compare.
--
john_childs
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