Re: GPS Accuracy
Bob Morehead (rmorehead@NLS.NET)
Wed, 22 Jul 1998 10:26:19 -0400
OK, for the record from a former Coast Guardsman:
GPS is a weapon that just happens to have a useful civilian application.
It was created to guide our ships, planes and missiles to an exact
point. Selective availability is an intentional error of 10 - 30 meters
generated by the U.S. Space Command (a unified command of the Department
of Defense responsible for all defense satellites) to keep other
countries from sending missiles at US with pinpoint accuracy. It has
nothing to do with how many satellites civilian GPS receivers are
"allowed" to use. Military receivers automatically filter out the SA
error. Their EXACT accuracy is classified, but our cruise missiles DO
seem to find the mark well ;).
Differential GPS is a Coast Guard administered service in which a
separate DGPS transceiver is used in conjunction with a standard GPS
unit. The DGPS unit takes the GPS data and coordinates it with a beacon
transmitting from a fixed point whose exact coordinates are known. This
greatly, although not completely, corrects for the SA error. Please
don't ask me why the government would go to such lengths to fudge the
accuracy of civilian GPS and then launch a program that corrected it.
No one to whom I ever spoke knew.
As for substituting them for a map and compass, I'll say this.
Professional navigators never use only one source of navigation, but
several, including a good old-fashioned compass azimuth. GPS is only
one, although it is now, by far, the preferred. But, they take the GPS
information and use it to draw a dot on a map (or chart)!
For the "list enforcers": Since handheld GPS units are sweeping the
outdoors, I definitely see a discussion on their operation and
application to be germain to the list.
ASM, Troop 381
Terry Howerton Sakima Group, Inc. SCOUTER Magazine Kansas City