SAIDJACK@aol.com wrote:

Said

There's no particular reason that a GPS carrier phase disciplined OCXO
need be particularly expensive unless of course it incorporates an
Oscilloquartz 8607 with ultra low adev specs.
Suitable receivers seem to be readily available. The only major
complication is the need to phase lock their local oscillator to the
OCXO being disciplined. In some case this is easy as a 10<Hz crystal is
used.

You've misinterpreted the statement 2E-11/Tau actually  means:
2E-11 @ 1 sec
2E-12 @ 10sec
2E-13 @100 sec

Bruce

On Thu, 2008-04-24 at 02:16 -0400, SAIDJACK@aol.com wrote:

Hi Said,

What is affordable for you?

For some $330 you can get a L1 GPS receiver outputting carrier phase
measurements at 5Hz. This receiver also have a 10MHz tcxo driving the
whole receiver. Pseudorange measurement rms of a few dm. Carrierphase
rms below 1cm.

Btw... the Oncore VP did carrierphase measurements.

--

Björn

Björn Gabrielsson wrote:

Even the Rockwell/Connexant Jupiter GPS receivers have carrier phase
measurement capability.
However you will need to replace the 10.95MHz crystal with a 10.95Mhz
source phase locked to the OCXO being disciplined.
The 3.3V version of this module has been available at very low cost from
time to time on ebay.
Whilst not suitable for production purposes they should be more than
adequate for experimental purposes.

Bruce

Which make/model is this?

Well, yes, but are these measurements suitable as part
of a GPSDO project?

/tvb

Tom Van Baak wrote:

Novatel Superstar II?

Tom

Yes, however you have to do all the required corrections for ionospheric
phase delay etc in real time in an external processor.
These particular receivers have been used in real time arrays to monitor
earth deformations of volcanic fields.

Bruce

Bruce,

Can you explain this a bit more? I know you often mention
this point. But it seems there must be something more to it;
otherwise we all would have seen low-cost carrier-phase
GPSDO products on the market over the past 15 years...

Instead there are only a few, they are all very expensive,
and none (?) of them use standard OEM GPS receivers.

I ran a carrier-phase Ashtech Z12T here for a while. It was
my understanding that the reason it performed so well was
a combination of carrier-phase tracking, L1/L2 choke ring
antenna, phase stabilized cables, and dual band receiver.

It also required an external free-running 20 MHz laboratory
reference (I used a 4x multiplier off a cesium or maser).
Further, one used it by collecting raw RINEX data and daily
sent the batch files to be post-processed for two weeks.

It would be interesting to know how much each of these five
pieces contributed to its overall performance. My hunch is
cheap OEM timing receiver carrier-phase measurement
alone is not enough.

/tvb

Tom Van Baak wrote:

Whilst you need to do this to use precise satellite orbit data and
repair cycle slips for the ultimate performance.
However for slightly lower performance, especially when disciplining a
frequency standard as opposed to a time standard this is not necessary.
The very fact that  a commercial carrier phase disciplined standard
using a single frequency L1 receiver is available surely attests to that.
These devices do not seem to require either phase stabilised cables or
use of a choke ring antenna (they appear to use a quadrifilar helix
antenna).
However they do use a local oscillator and mixer to downshift the
carrier frequency before transmission over the antenna cable.
The same local oscillator reference plus a mixer then upshifts the
carrier frequency again.
The local oscillator only needs relatively low short term phase noise
and hig short term stability as long term (>> antenna cable delay) local
oscillator phase errors cancel out.
Whilst this system has advantages in reducing the cable attenuation I'm
not convinced it improves the phase shift stability when the fact that
the local oscillator signal is transmitted up the cable is taken into
account.
A relatively low frequency reference is transmitted up the cable, where
a frequency multiplier or harmonic mixer is used.
The reason given for using a custom single channel GPS receiver (which
uses no custom parts) is to ensure continuity of component supply for
several decades.
This receiver periodically switches from one satellite to another.
When discipling a frequency standard carrier cycle slips arent as
important (provided you can detect them) as when making position or
equivalently time measurements.
With a fixed position receiver one can take advantage of the fact that
the antenna's position is very stable, at least in the short term.
If one samples the carrier phase data at a high enough rate then a
sequence of intervals is available when no carrier cycle slips have
occurred.
The carrier phase differences over these time intervals can be used to
estimate the local oscillator frequency error.
A multichannel receiver tracking several SVs should ensure that time
intervals where cycle slips occur for all tracked SVs are relatively
infrequent.

One could easily test that assertion by replacing the receiver's local
oscillator with a source locked to a high stability source such as a
hydrogen maser, logging the receiver carrier phase data and then later
analysing it.

Bruce