[time-nuts] Software Sawtooth correction prerequisites?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat May 12 02:00:22 EDT 2007


Tom Van Baak wrote:
>> Define cheap.
>> You can already get essentially single chip TICs with a resolution (and 
>> accuracy) better than 100ps for around 100 Euros or so.
>>     
>
> Has anyone in the group tried one of these? I would very
> much like to see the results.
>   

All except Xavier seem to have shied away from obtaining these on cost 
grounds and the need to layout a 4 layer PCB.
Although the vendor does offer an evaluation system (External box plus 
card plugs into PC with fat multiconductor SCSI type (but not SCSI 
signalling)cable connecting the two), too expensive for general use but 
perhaps useful for performance evaluation.
The above price was for an earlier version the later reduced cost higher 
resolution version (range 5 nanosec - 4millisec) should be cheaper.
The range can easily be extended to years if required (using CPLD 
counters (or even a PIC with built in counters - eg PIC18F4550) + 
software+ 74AC164).
>> However the optimal solution doesn't use a TIC at all, just use a good 
>> GPS receiver that makes the carrier phase observables available.
>> Quite a few of the older receivers used to make the carrier phase data 
>> available.
>>     
>
> Many of the Oncore VP's did this, but I don't know of
> anyone, amateur or commercial, that built a GPSDO
> using that feature. That makes me wonder if carrier
> phase observables alone is not enough.
>   
You also need code phase measurements as a sanity check, to determine 
the ionospheric delay, and to initialise the control loop.
To obtain maximum performance you also need to know the GPS antenna 
location accurately.
This may take several days to establish using code phase SV transit data.
>> The catch is that the GPS receiver local oscillator must be phase locked 
>> to the OCXO.
>> Some receivers use a 10MHz crystal oscillator in which case this can be 
>> removed and replaced with an external 10MHz signal derived from the OCXO 
>> being disciplined.
>>     
>
> Right, this is the key. Do you know any OEM receivers that
> allow an external 5/10/20 MHz? I know my Z12T does (but that
> is far from an OEM receiver).
>
>   
The Novatel SuperstarII receivers use an on board Rakon 10MHz TCXO (made 
a little (120km) to the North of me) local oscillator which can be 
disconnected with relatively little surgery and replaced with a piece of 
thin coax and a connector. However the required amplitude is relatively 
small - easily fixed with a pair of resistors - typically the output 
from the onboard TCXO itself has to be attenuated.
Magnus acquired some of these cheaply, however even the new price isn't 
too steep. UNSW has used these receivers in geodetic survey/monitoring 
applications. The code phase field data from some of these exhibits 
encouraging stability (about 5 -10x better than the oscillator specs) 
when averaged over a few seconds even with the standard TCXO.

If you want to roll your own GPS receiver the MITEL/ZARLINK chipsets use 
a 10MHz reference.
>> Unfortunately there are as yet no relatively inexpensive off the shelf 
>> implementations of the GPS carrier phase discipling technique available.
>> The calculations involved for maximum performance are somewhat complex, 
>> however not too much computing horsepower should be required.
>>     
>
> The QL carrier phase GPSDO is unique and probably not
> cost-effective. Sounds like it's a custom single-channel GPS
> receiver designed from the ground up. If there were an easier
> way surely someone would have done it in the past ten years.
>
>   
Yes $30,000 Euros or so is a little steep.
Depends what you mean by easy.
Vested interest in an existing receiver can lead to inertia.
The other players seem wedded to using PPS disciplining, possibly 
because of all the existing equipment using it.
Investigations into using multichannel carrier phase disciplining have 
been somewhat sporadic.
Although single channel receivers employing carrier phase measurement 
have been successfully deployed in geodetic arrays for monitoring 
applications.
>> Magnus hopes to remedy this lack of suitable software and hardware 
>> sometime in the future.
>>     
>
> That would be very nice. Will it be L1 only or L1/L2?
>   
Only L1 as that's all the Superstar receivers provide.
However combining carrier and code phase measurements allows correction 
for the ionospheric delay.
> /tvb
>   
Bruce






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