More than 12 satellites

Some of the new GPS units like the 338 advertise they can get more
than 12 (18 maybe) satellites at once. I thought that was impossible ,
that only 12 were in view at any one time as a maximum as there are
only 24 satellites. Has that changed. Is it wroth it to get such
units?

Gilbert Baron wrote:

Yes, if the cost is not too much extra and mostly it won't be. If you
are using SBAS (WAAS or EGNOS for instance) then the additional channels
can be used to look at the geostationary satellites without losing any
of the GPS SVs.

--

Brian Morrison

please observe reply-to address

Gilbert Baron wrote:


There are currently 30 active satellites in the GPS constellation,
so, in theory, it is possible to "see" more than 12 satellites
simultaneously from some locations. When (and if) Galileo becomes
operational, there will be another 30 active satellites to compliment
the GPS constellation.


UNCLASSIFIED
GPS OPERATIONAL ADVISORY 318.OA1
SUBJ: GPS STATUS 14 NOV 2005

1. SATELLITES, PLANES, AND CLOCKS (CS=CESIUM RB=RUBIDIUM):
A. BLOCK I : NONE
B. BLOCK II: PRNS 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15
PLANE : SLOT F6, D7, C2, D4, B4, C1, C4, A3, A1, E3, D2, F3, F1, D5
CLOCK : CS, RB, CS, RB, RB, RB, RB, CS, CS, CS, RB, RB, RB, CS
BLOCK II: PRNS 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29
PLANE : SLOT B1, D6, E4, C3, E1, D3, E2, F4, D1, A2, F2, A4, B3, F5
CLOCK : RB, RB, RB, RB, RB, RB, RB, RB, CS, CS, RB, CS, RB, RB
BLOCK II: PRNS 30, 31
PLANE : SLOT B2, C5
CLOCK : RB, CS

-Sam Wormley
http://edu-observatory.org/gps/status.html

Sam Wormley wrote:

It's more important to have a receiver with a good selection algorithm
than one with more than 12 channels.

Tom

:: It's more important to have a receiver with a good selection
:: algorithm than one with more than 12 channels.
::
:: Tom

This is clearly a "looking to the future" feature. And the selection
algorithm isn't all that difficult, as it's been batted around for quite a
number of years.

If possible, 3-4 widely spaced, 20º-35º degrees above the horizon for
horizontal accuracy, throw in a few vertical ones, spaced around 70º+ if
possible for vertical accuracy, check them against each other for clock
accuracy, pick what your research has told you is the optimum number for a
solution based on the number, arrangement and elevation of available sats,
and start working out a position. That is a very simplistic explanation, but
not very far from reality.

Also, if your luck is extremely wonderful and you have more than 12 sats in
view, a GPSR can choose between two or three during aquisition phase and
decide which ones to leave out based on almanac data. If the ones looked at
through available channels are shown to have timing/clock errors, due to
atmospheric or other issues, they can be dropped and one or more of the
others added. This can be explained much better. I'm just giving it the
kindergarten approach. When there are two or three constellations of
satellites up there, look for the firmware for middle-aged receivers to be
changed to allow something like this selection process.

Frank