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

Bob Bownes bownes at gmail.com
Fri Jan 13 10:00:38 EST 2012


When you are thinking about replacing GPS receivers, don't forget about
every police car, ambulance, fire truck and most of the tractor trailer's
in the US...The latter don't need timing down to the second, but the first
three use it to well under a minute.

One of the first things you learn when on an ambulance crew is what lump on
the roof to wrap the aluminum foil over when you are going to park the rig
after that run of 5 middle of the night calls and go to sleep. ;)

Some day I'll get the laptops in the rigs to sync up with the GPS directly
rather than using NTP. ;) We probably should be using it to drive the time
code in the video recorders...Hmmmm.

Bob


On Fri, Jan 13, 2012 at 9:29 AM, Jim Lux <jimlux at earthlink.net> 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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