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J

Joe G \(Home\)

What's the difference between the number of GPS channels?

What is the performace difference

Thanks in Advance

Joe
 
P

Phil Pemberton

Joe said:
What's the difference between the number of GPS channels?

More channels allows the receiver to track more satellites -- you're still at
the mercy of the number of satellites in view and signal quality, though. Also
most receivers may track satellites that are <= 30 degrees above the horizon,
but they won't use them in triangulation as the signal will likely have
suffered enough atmospheric delay and degradation to foul up the position fix.
What is the performace difference

The receiver will be able to track more satellites, so if you've got e.g. a
six-channel receiver and four satellites in view, the receiver will use the
other two channels to scan for satellites that (according to the almanac data)
should be coming into view. Startup time should be reduced significantly too,
as e.g. a twelve-channel receiver will be able to search for more satellites
at a time.

The GPS sats send out a pseudorandom sequence (the C-code or Standard
Positioning Service) that changes at a rate of 1.024MHz. This is different for
each satellite, so you need to know which satellite you're looking for before
you go looking for it.

From a cold-start (no ephemeris, almanac or timing data), most receivers will
start hunting for a single available satellite (or more) to get a timing lock,
then go hunting for the almanac (coarse orbit data). If you've got 12
channels, you pick (say) SVs 1 through 12 for the first search. Chances are,
you'll find at least one satellite in that range that's visible. If there's
something there (carrier lock) then the receiver will attempt to get a "code
lock" (synchronise the PRBS generator against the satellite) and acquire
timing data. If there's physically nothing there, it'll go hunting for a
different satellite on that channel.

Next you need the Almanac data to find other satellites. This takes about 15
minutes to download, but ISTR that some modern receivers will do a download
from multiple satellites -- bad packets in the stream are replaced with ones
from other satellites. I suspect if one sat is sending (say) block 30 of 50
and another is sending block 49, some receivers will grab blocks 30-50 from
one sat, then wait for the sat sending #49 to roll back to #1, and grab the
rest of the data from there. I'm not sure if that's how it works in real life,
or if all the sats send the same part of the Almanac at the same time.

Once the receiver has the Almanac, it goes looking for the Ephemeris (which is
transmitted individually for and by each satellite). This improves the
precision of the Almanac and allows the receiver to calculate a position fix
(assuming at least three satellites are in view and locked).

A better place to ask this question would be the newsgroup alt.satellite.gps...
 
J

Joe G \(Home\)

Phil Pemberton said:
More channels allows the receiver to track more satellites -- you're still
at the mercy of the number of satellites in view and signal quality,
though. Also most receivers may track satellites that are <= 30 degrees
above the horizon, but they won't use them in triangulation as the signal
will likely have suffered enough atmospheric delay and degradation to foul
up the position fix.


The receiver will be able to track more satellites, so if you've got e.g.
a six-channel receiver and four satellites in view, the receiver will use
the other two channels to scan for satellites that (according to the
almanac data) should be coming into view. Startup time should be reduced
significantly too, as e.g. a twelve-channel receiver will be able to
search for more satellites at a time.

The GPS sats send out a pseudorandom sequence (the C-code or Standard
Positioning Service) that changes at a rate of 1.024MHz. This is different
for each satellite, so you need to know which satellite you're looking for
before you go looking for it.

From a cold-start (no ephemeris, almanac or timing data), most receivers
will start hunting for a single available satellite (or more) to get a
timing lock, then go hunting for the almanac (coarse orbit data). If
you've got 12 channels, you pick (say) SVs 1 through 12 for the first
search. Chances are, you'll find at least one satellite in that range
that's visible. If there's something there (carrier lock) then the
receiver will attempt to get a "code lock" (synchronise the PRBS generator
against the satellite) and acquire timing data. If there's physically
nothing there, it'll go hunting for a different satellite on that channel.

Next you need the Almanac data to find other satellites. This takes about
15 minutes to download, but ISTR that some modern receivers will do a
download from multiple satellites -- bad packets in the stream are
replaced with ones from other satellites. I suspect if one sat is sending
(say) block 30 of 50 and another is sending block 49, some receivers will
grab blocks 30-50 from one sat, then wait for the sat sending #49 to roll
back to #1, and grab the rest of the data from there. I'm not sure if
that's how it works in real life, or if all the sats send the same part of
the Almanac at the same time.

Once the receiver has the Almanac, it goes looking for the Ephemeris
(which is transmitted individually for and by each satellite). This
improves the precision of the Almanac and allows the receiver to calculate
a position fix (assuming at least three satellites are in view and
locked).

A better place to ask this question would be the newsgroup
alt.satellite.gps...


Thankd - Joseph
 
J

JosephKK

What's the difference between the number of GPS channels?

What is the performace difference

Thanks in Advance

Joe

There is no possible advantage in going above 12 channel. That is the
maximum number of active satellites that could ever be visible at any
given time. Other than that, 4 to 6 channels looks like the best
range for quality versus cost.
 
J

Joe G \(Home\)

JosephKK said:
There is no possible advantage in going above 12 channel. That is the
maximum number of active satellites that could ever be visible at any
given time. Other than that, 4 to 6 channels looks like the best
range for quality versus cost.

Thanks Joseph,
But why do the GPS makers have 12 - 50 channels? if 12 is the maxium,
what is the performance benifit for more than 12?

Joe
 
J

JosephKK

The GPS reciever can keep _track_ of more then 12 channels, but only
_use_ 12 in their calculations.

I don't think there are currently more then 14 flying at the moment anyway.

donald

The full constellation of 24 satellites and some (5 IIRC) in orbit new
spares are already flying. The issues is, that half or less are
visible from any point on earth because earth itself is in the way.

Due to some design decisions 24 active satellites is all that there
will ever be.
 
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