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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
satellites signal for civilian users. They introduce small errors into the
satellite’s signals that makes the GPS receiver less accurate. These er-
rors are called selective availability, or SA. How bad is it? They guarantee
that the position reported by a GPS receiver that meets their specifica-
tions 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 Lowrance) sell a DGPS
receiver that attaches to your GPS receiver. The DGPS system transmits
correction 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 operat-
ing 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.
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