On 01/02/12 12:35, Azelio Boriani wrote:
In your opinion, is it possible for a GPS receiver to align the PPS pulse
on multiple of the C/A code repetition rate because of (for example) badly
received satellite signals? Maybe this can happen, after the initial
acquisition, on the following updates.
In a typical receiver you do not align the PPS to any particular C/A
code phase or rate. Rather, you have a system clock (typically timed
from a TCXO, sometimes steerable from external source) which produces a
1 kHz sample rate. The channels track the signal continuously producing
integrated samples over the C/A sequence and also phase-state. On each
sample set, the PLL loop is updated and when a symbol (20 samples) is
accumulated a for the full symbol. The phase state is also sampled, and
when on the second marker (or a suitable offset) it is used to build a
raw measurement. Most simple receivers just uses the code phase, i.e.
the phase of the C/A code. It's built up from chip-phase, chip-number,
C/A sequence in symbol, symbol in frame etc. The bias number can be
cracked, but in the end 4 or more observations establishes the X, Y, Z
and T positions of the receiver. If you then bother to crank out a full
solution out of that is another thing. The T offset is then used to
correct the internal clock of the receiver and the PPS output. The rate
of the various C/A code is usally always shifted away as there is
doppler shift as birds comes closer or goes away.
Cheers,
Magnus
On Wed, Feb 1, 2012 at 10:47 AM,<[email protected]> wrote:
On Tue, 31 Jan 2012 16:29:07 -0800
Chris Albertson<[email protected]> wrote:
I'm pretty sure those GPS recievers that send out more frequent data,
at say 2Hz or 5Hz are just interpolating. It is not more accurate.
The GPS sats only send a frame once over 6 seconds.
As Magnus already wrote, once you have a fix, you can use code tracking
to get an updated fix up to rates of 1kHz. If you use codeless P(Y)
code tracking or carrier phase tracking you can get even higher rates.
But, you can only update an already available fix, not calculate
a fresh fix from scratch at that rate. This is because carrier phase
and codeless P(Y) code tracking has an ambiguity of the phase, which
has to be first resolved by a "conventional" fix. Once you have this,
you can use those two techniques to get fixes at high rates.
Attila Kinali
The classic GPS receiver architecture use early and late correlators to
track the correlation peak. You can make fresh single fix solutions as
quick as your hardware can cope. However the correlator tracking loops
have a limited bandwidth - 5 to 25Hz-ish. It is of limited interest to
sample quicker than some 10-40Hz.
--
Björn
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