[time-nuts] Compensating phase differnces in dual frequency GPS receviers?

Jim Lux jimlux at earthlink.net
Fri Dec 2 08:57:35 EST 2011

On 12/2/11 4:59 AM, Attila Kinali wrote:
> On Thu, 1 Dec 2011 19:56:12 -0800
> Peter Monta<pmonta at gmail.com>  wrote:
>>> So, that'd mean there would be an automatic calibration system inside
>>> the device, because i dont have any equipment with which i could
>>> calibrate delays over a temperature range.
>> I suppose they could do that---provide a weak broadband source (say a
>> comb) combined with the antenna signal prior to the channel filters,
>> and provide a reference channel for this calibration signal to compare
>> against on-line.
> Juup, i thought exactly at that. Use a dirac pulse generator with
> a ~100MHz comb. That would generate nice signals in the L1 and L2 bands.
> This could then be measured later... But it's ugly, and you have to deal
> with either switching of the input source, or have to distinquish the
> calibration signal somehow from the normally received signal.

Combs are used all the time for this kind of thing (e.g. calibrating 
Deep Space Network).  There's an old paper about calibrating a 
interferometer radio telescope at Stanford using this kind of thing (by 
Bracewell, as I recall)

It's easy to distinguish the comb from the GPS signal... the GPS is PN 
coded, the comb is not.  If you pick your levels right, depending on 
your digitizing strategy, it might not even jam the GPS, so you could 
leave it on.

However, even that's not a panacea, because generating and distributing 
that idealized comb is non trivial without destroying the phase 
relationship between the comb "teeth".  I guess it really depends on how 
nutty you want to be.  1 nS is pretty easy, I would think. 1pS is a lot 
harder. 1 fS is very hard.

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