Eagle Map Guide, Installation And Operation Instructions Manual

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S/A - What is it?
Another factor that greatly influences the receiver’s ability to determine
position is SA. The United States government intentionally degrades the
satellite’s signal for civilian users. They introduce small errors into these
signals that makes the GPS receiver less accurate. These errors are called
selective availability, or SA. How bad is it? They guarantee that the posi-
tion reported by a GPS receiver that meets their specifications is within
100 meters horizontally and 150 meters vertically 95% of the time. (The
position can be better than that or worse than that the other 5% of the
time.) In other words, the position shown on your receiver is within 100
meters of your actual position, 95% of the time. That’s over 300 feet! Not
exactly pinpoint accuracy, but then few people need positioning accuracy
greater than this. However, if you do want better performance, (and who
doesn’t?) many manufacturers (including Eagle) sell a DGPS receiver
that attaches to your GPS receiver. The DGPS system transmits correc-
tion signals that nullify the effects of SA. The DGPS receiver takes signals
from these land-based transmitters and gives them to the GPS receiver
which then uses them to show a more accurate position. The ironic part is
the federal government implemented SA and is also operating many DGPS
transmitters. (You can use the signals from all of the Coast Guard DGPS
stations for free, by the way.) The downside to this is it requires another
piece of electronic gear (the DGPS receiver) which usually isn’t small
enough to carry with you, but will work nicely on a vehicle. And you have
to be close enough to a station to receive the DGPS signals.
Don’t Get Lost
Generally, you find that using your GPS receiver without DGPS is both
easy and amazingly accurate. It’s easily the most accurate method of
electronic navigation available to the general public today. Remember,
however, that this receiver is only a tool. Always have another method of
navigation available, such as a chart or map and a compass. It’s a good
idea to carry spare batteries with you, especially if you’re venturing into
unknown territory.
Also remember that this unit will always show navigation information in
the shortest line from your present position to a waypoint, regardless of
terrain! It only calculates position, it can’t know what’s between you and
your camp, for example. It’s up to you to safely navigate around obstacles,
no matter how you’re using this product.