| |
|
| Global Positioning Systems ...continued |
| |
| accuracy improves dramatically. Satellite
geometry also becomes an issue when using a GPS receiver in
a vehicle, near tall buildings, or in mountainous or canyon
areas. When the GPS signals are blocked from several satellites,
the relative position of the remaining satellites will determine
how accurate the GPS position will be (and the number of remaining
satellites will determine if a position can even be determined).
As more and more of the sky is obstructed by buildings or terrain,
it becomes increasingly difficult to determine a position. A
quality GPS receiver indicates not only which satellites are
available for use, but where they are in the sky (azimuth and
elevation) so you may determine if the signal of a given satellite
is being obstructed. |
|
A number of issues come into play
here:
1. What is the intended application? The most important
issue is finding a GPS suitable for your application. You can
quickly narrow your choices down by identifying which models
are available for your application. For example, do you need
GPS mapping or just co-ordinate information?
2. In some cases, you may still have a lot of options
from which to choose. For example, if your intended use is hiking
or fishing, a GPS for outdoor recreation is suitable, but so
is a handheld GPS designed for boating or flying. In this case,
you may have to examine specific features more closely. |
|
 |
Another source of error is multipath.
Simply put, multipath is the result of a radio signal being
reflected off an object. Multipath is what causes ‘ghost’ images
on a television set. With GPS, multipath occurs when the signal
bounces off a building or terrain before reaching the GPS receiver’s
antenna, meaning that the signal takes longer to reach the receiver.
This added time makes the GPS receiver think the satellite is
farther away than it really is, which adds error to the overall
position determination. When they occur, multipath errors typically
add well under 3 metres of error to your overall position.
Yet another source of error is propagation delay due to atmospheric
effects and internal clock errors. In both cases, the GPS receiver
is designed to compensate for these effects and will do so quite
efficiently, but very small errors can still occur.
A typical civilian GPS receiver now provides 1m to 10m accuracy,
depending on the number of satellites available and the geometry
of them. More sophisticated and expensive GPS receivers can
provide accuracies within a centimetre by using more than one
GPS frequency. |
|
3. What is the price range? Once you’ve
narrowed the field, you’ll most likely still have several models
over a range of prices from which to choose. Examine each model
closely to determine what the higher-priced models have that
the lower-priced models don’t, and whether the lower-priced
model is sufficient to do the job.
4. Which model do you like the best? Choosing the right GPS
receiver for you is two parts rational planning and one part
simple preference. If rational planning still leaves you with
two or three models to choose from, try operating each one.
Sometimes the differences in operation are dramatic.
You may find one very easy to use and understand, while another
seems much more complicated and difficult. Choose the GPS receiver
that you LIKE best! Then you are more likely to still be happy
with the decision you made in years to come.
A number of issues come into play here:
What is the intended application? The most important issue is
finding a GPS suitable for your application. You can quickly
narrow your choices down by identifying which models are available
for your application. |
|
| |
Previous Page |
| |
|
|
