[time-nuts] Building a GPSDO & trouble using Jupiter-T

Magnus Danielson magnus at rubidium.dyndns.org
Wed Feb 1 20:03:52 UTC 2012


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,<bg at lysator.liu.se>  wrote:
>
>>> On Tue, 31 Jan 2012 16:29:07 -0800
>>> Chris Albertson<albertson.chris at gmail.com>  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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