Russell Kent wrote:

The U.S. government (actually U.S. Military) already agreed to discontinue "Selective Availability."  The effective date of
the discontinuance may have moved, but the fact that the SA was going away is old news.

Your GPS receiver will probably *still* have a +/- 25m error.  :-)

What was it before? See the raw data below.

This looks like it was about +/- 0.025 degree in Latitude or about
+/- 150 feet. 25 meters or +/- 75 feet is a vast improvement.

If you are seventy five feet away from the threshold and you can
not see it it is time to go some place e
lse. Heck the runway is fifty
feet wide. If you are flying through a canyon and you are closer than
a 100 feet to the wall on each side you deserve to die :-)

What is your point Russel? What would you like to do with it
that requires more than 75 feet instantaneous resolution?

Oh I get it your joking.  Right? Sorry :-)

I think it will be better than +/- 75 feet  but I will have to take
some raw data to be sure.

Yes, I was joking.  But here's something not-so-funny.  Because of the effects of the physics involved, your GPS has much better horizontal resolution than vertical.  So you might be within +/- 25m of the threshold *horizontally*, but
vertically the error is usually closer to +/-100m [note: without SA] So if your altimeter setting is incorrect, you could be in for a rude landing (short is worse than long).  This is the very big reason why DGPS is such a big win: local
stations provide zeroing of SA, *PLUS* provide fixed-points that can be used to get much better resolutions.  DGPS is typically +/-10m (or less) in both horizontal and vertical errors.  If you cannot see 33 ft, you go elsewhere.  I've
even heard of high-precision DGPS designs (not deployed) that have sub-meter resolution over a several nautical mile circle.

Russell

 
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