[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

Magnus Danielson cfmd at bredband.net
Fri Dec 15 01:48:29 UTC 2006


From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
Date: Fri, 15 Dec 2006 12:55:19 +1300
Message-ID: <4581E467.8070706 at xtra.co.nz>

> Björn

Bruce,

> You will need a dual frequency receiver to more accurately correct for the ionospheric delay.
> Currently non approved users do not have access to the codes required to use the L2 frequency signals in an optimum way.
> Various kludges are required to extract some info from the L1 frequency carrier phase if one doesn't have the despreading codes.

HP wrote a paper on that which I seem to have missplaced. Should be on my
laptop but I can't seem to find it. Their trick was to compare the code and
carrier trackings on standard Oncore receivers. There was some sound reasoning
behind it, but it doesn't handle the full error. I do not know if it ever made
it into any commercial product, but it should not supprice me. In effect it
used the fact that the ionospheric dispersion is such that the carrier delay
shift and code delay shift has opposite signs (and I beleive amplitude, but I
am not sure) for L1. They had made measurements to compare between sites and
relative cesiums. The improvement was there. I beleive it was a PTTI paper.

Otherwise you could go codeless to a combined L1 & L2 tracking using say
Z-tracking.

> However this should change as the GPS system is enhanced to provide 2 or more frequencies to civilian users.

With now three GPS IIR-M sats in orbit, a modern receiver capable of L2C should
be able to do it. Good luck finding one on the second hand market. Actually
finding one cheap would be a challenge. I am tempted thought.

> Multipath effects become even more critical as one attempts subnanosecond timing performance.
> One may have to use either a choke ring ground plane antenna (as used in geodetic receivers) or resort to phased array techniques.

L1 choke rings can be found fairly cheaply. L1 and L2 is a bit harder. Also
notice that modern sats have a bigger footprint than they used to. Infact, your
ability to mitigate multipath improves with a higher bandwidth.

> There is at least one commercially available GPS disciplined OCXO system 
> that uses carrier phase measurement techniques to discipline a crystal 
> oscillator.
> A fractional standard deviation of 1E-11 for a measurement time of 
> around 1 second is claimed. When using carrier phase measurement 
> techniques it is advantageous to use the oscillator being disciplined to 
> generate all the receiver local oscillator frequencies as well as the 
> correlator clock frequencies.

This was exactly what Björn and I was looking at. By replaing the 10 MHz TCXO
with a 10 MHz OVCXO and some additional circuit all the mixer frequencies,
sampling frequencies etc. becomes synchronous, the steering of the OCXO has the
goal of making the PPS output synchronous. Unfortunatly there is a little
design-bug in the correlator design which makes the PPS jump about in a 7
second cycle. However, this can be mitigated with external circuit.

BTW. Does anyone have FW for the CRC Allstar? I need it.

Cheers,
Magnus




More information about the Time-nuts_lists.febo.com mailing list