[time-nuts] Measuring GPS noise floor (sort of)

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 2 23:17:47 UTC 2012

On 12/02/2012 11:37 PM, Mark Spencer wrote:
> I'm wondering how useful it is to compare two different GPS receivers to a single Rb to get a sense of the GPS noise floor.   I've been collecting some data for the last week or so seem be making some headway with this.
> Not having a cesium or an Hmaser I elected to compare two different GPSDO's to a single Rb and then assuming the data seemed sane, I thought I would then compute the Adev between the two GPSDO's.   My assumption is that by comparing the two GPSDO's to the Rb it should be possible to detect when one GPSDO is mis behaving or possibly when the Rb mis behaves.  So far other than a bit of a glitch at the start of the process things seem to be proceeding as I expected.   I hope to collect another two weeks or so of data if all goes according to plan.
> I'm really only interested in the longer term numbers and I wouldn't put to much trust in the shorter term results.
> Comments would be welcome especially if there is a fundamental flaw with this process.
> Looking at the data gave me something to do this afternoon during some down time on a business trip (:  I reazlize this approach is probably not how the pros would do this.

You do realize that several issues with the GPSDOs will be common mode, 
such as the errors of the ionspheric model and actual ionspheric effect 
and that of multipath. The rubidium will to some degree aid to 
illustrate these shifts, but it will be accounted for on the rubidium 
deviation from the common. If you look at the CRIT paper we discussed 
yesterday, they did exactly what you proposes. For most stuff the 5 
recievers they used didn't show much differences. If you increase the 
resolution you might get out more. You might have use for additional 
local references, in order to separate out other effects. For instance, 
with three rubidiums measured, you could single out which of them has an 
error, and any common mode effects of the GPSDOs could be fairly well 
separated out from the local performance of your rubidiums.


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