[time-nuts] GPS interference and history...

Jim Cotton jim.cotton at wmich.edu
Fri Jan 13 09:53:33 EST 2012


Any large IT organization has multiple "stratum 1" GPS based
timing receivers.

The public key for our internal routing updates is the time.  No time and
the routing would break.  We route ~10+ Tb/hr in the 8am-5pm business
day.  That would be noticed by our users...

On one building on our campus (College of Engineering ~1/4 mile long 
building )
I counted 14 mushroom antennas, I see other patch antennas on windows...

Jim Cotton

On 1/13/12 9:29 AM, Jim Lux wrote:
> On 6/10/11 7:01 PM, Hal Murray wrote:
>>
>> lists at rtty.us said:
>>> There's an enormous amount of gear out there that gets timing off of 
>>> GPS.
>>
>> That's an interesting claim.  Does anybody have any data on the usage 
>> of GPS
>> for timing?
>>
>> I assume there is one in every cell tower and one in every 911 call 
>> center.
>> Are there other large categories of users?
>
>
> GPS is pretty ubiquitous as a time source for data loggers in the 
> field, things like traffic signals, etc.   There's real value in an 
> inexpensive little box that makes sure you don't have to set the 
> clock, even if the clock accuracy requirement is something like 1 minute.
>
>
>
>
>>
>> What would it cost to replace all of it?  If you wanted to do 
>> something like
>> that, what would "it" cover?  How about people like us running old 
>> recycled
>> gear?  (Z3801A, ThunderBolt, ...)
>
> A fortune, quite literally
>
>>
>>
>> I think I saw one last week.  It was on a river level measuring 
>> station on
>> the Sacramento River.  It was a small block building.  There was an 
>> antenna
>> pointing up into the sky.  I assume there is a satellite up there.  
>> There was
>> also a small (~3 inch dia) hemisphere antenna. I assume it was GPS.  
>> (They
>> had power going into the building (no solar panels) so it should have 
>> been
>> simple to get a phone line too.)
>
> Not necessarily.  And it's not cheap.  Don't forget that you can't run 
> power and phone in the same conduit, cable, etc.   So basically you're 
> doubling the physical plant installation costs to bring in phone, just 
> for the labor to bring it from the nearest point of presence. 
> Especially in rural farm kinds of areas, power is more pervaisve than 
> phone (gotta run irrigation pumps, etc.)
>
> Adding a $100-200 GPS receiver (we're not talking GPSDO with OCXO 
> here..) is probably cheaper than running ANY length of phone wires: 
> just for the termination costs.
>
> I suppose one could use some sort of GPRS cellular service and get 
> time, but then you're on the hook for a monthly subscription fee, etc.
>
>
> cheap L1 only GPS is a great solution.  Apply power, wait, you've got 
> accurate time.  No need to have someone visit periodically and check 
> to see if the clock needs to be reset, etc.
>
>
>
>
>>
>> I'm not sure why they need GPS at the recording house.  They know 
>> where it is
>> so timing is the only use I can think of.  But they could also get 
>> that at
>> the receiving end.  Millisecond accuracy isn't helpful.  Second level
>> accuracy might be interesting if something breaks and you want to 
>> know when
>> the wave got to downstream stations.  The risetime is probably over a 
>> second.
>>
>>
> You're right, they don't need milliseconds, nor do they need seconds, 
> probably.
>
> There's really no other convenient way to get time to the nearest 
> minute that is as reliable and cheap as GPS. Think about it... WWVB? 
> WWV? Vertical pointing sun sensor?
>
>
>
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