Whoops- SA stands for "Selective Availability" which was an intentional
error that was introduced into the clocks that controlled the C/A code,
"Coarse Acquisition" which all civilian GPS units are based (there are some
exceptions for particularly high accuracy differential based set-ups) C/A
was only designed to get an approximate position, which is used as a seed
to aid acquisition of the P(Y) code (precicise(encrypted precise)) The keys
for the Y code are held very tightly by the US government.  C/A code has
a 1MHz clock rate, P(Y) code has a 10MHz clock rate, and two different
frequencies for ionospheric correction.  C/A code is much easier to acquire
since the pattern repeats every ~1mS, the P(Y) code is 267 days long, but a
different encrypted chunk is used each week, so it is much harder to
correlate.

SV stands for "space vehicle" which is the commonly used term for the GPS
satellites.  GPS is a system of precise clocks in orbit with a way of
synchronizing them to your handheld unit.  The handheld unit looks at the
various delays, and using some a-priori knowledge of where the SVs must be,
determines the location of the receiver.  There are at least 24 SVs in the
sky, each transmitting with a different PN (pseudo noise) code.

I think EPE (from Jim's message below) stand for estimated position error,
which, if my logic holds, is related to the GDOP- Geometric Dilution of
Precision.  The more SVs in good position, the better your solution would be
(lower EPE and GDOP).  Horizontal error is usually about half of the
vertical
error, due to the geometry of the position solution.

Is that enough acronyms for ya?

Matt Bennett

-----Original Message-----
From: Quitt, Walter
Sent: Tuesday, May 02, 2000 5:49 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [OT] [AR] FW:GPS S/A To Be Turned Off
Importance: Low


Dah... what's an SA and SV? -W

-----Original Message-----
From: Bennett, Matt [mailto:Matt.Bennett@ANDREW.COM]
Sent: Tuesday, May 02, 2000 3:29 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [OT] [AR] FW:GPS S/A To Be Turned Off


From: James R. Cunningham
> Out of idle curiosity I took my Garmin 92 into Memphis with me this
morning.
> During a 25 mile drive, with decent reception on 8-10 satellites, EPE
ranged
> between 11-14 feet with an occasional excursion to 39 feet.  Among the
> downtown buildings, with 6-8 satellites, EPE was about 22-25 feet with an
> occasional excursion to 41 feet.  I sat on a bench for about 10 minutes
with
> 6 satellites, EPE was 24-25 feet, and Altitude varied by about 10 feet
with
> maximum excursions of 50 feet.   The excursions took about 10-15 seconds
and
> occurred about once every 2-4 minutes.
>
> Jim

The error number you see is not related to the presence or absence of SA,
that
error number is related to the accuracy of your solution from the geometry
of the SVs (the smaller the included angle of all the SVs in your solution,
the larger the error will be).  The excursions you see are probably due to
SVs
being switched in and out of your position solution.

Matt Bennett