[time-nuts] New GNSS chipsets

Bob kb8tq kb8tq at n1k.org
Mon Apr 2 09:35:23 EDT 2018


Hi



> On Apr 1, 2018, at 9:47 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> magnus at rubidium.dyndns.org said:
>> I have advocated for receivers able to handle multiple frequencies and
>> multiple GNSS for some time, sneaking it into documents, so there should be
>> some preparations for this now.
> 
> How well do various GNSS track UTC and/or eachother?

These days Glonass puts out an offset number that should be good to the single 
digit nanoseconds (unless it’s broke). The Europeans have set up to effectively stay
“as close as you can get” to sync. Right now those are the only three that are likely 
candidates for time sources. 

> 
> 
>> The benefit is naturally redundancy, but also higher precision. 
> 
> I've been assuming the cheap GPS jammers will kill the others too.

Maybe not. Glonass is not on quite the same frequencies. The signal formats of
each system are very different. A jammer that nukes one may not have any 
impact on the rest. Indeed a megawatt level broadband DC to light jammer 
would take out a lot of things. It’s also a pretty easy item to track down. 

Straight broadband jamming *should* simply shut a receiver down. That’s why 
GPSDO’s go into holdover (and why you have GPSDO’s). Are all receivers ever 
made perfect in the face of any and all crud … maybe not. 

The bigger issue is a “spoof” signal that deliberately tricks the receiver into thinking
it is locked to legitimate satellites. There are ways to do that. Receivers are not
going to reject that solution and away you go. Doing a working spoof for multiple 
systems …. much harder than a single system. 

Bob


>  Are there 
> any signals far enough away from L1 that they might get through?
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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