[time-nuts] Timing Distribution in Mountainous Terrain

Ralph Smith ralph at ralphsmith.org
Thu Sep 9 18:57:00 UTC 2010


On Thu, September 9, 2010 1:55 pm, Matthew Kaufman wrote:
>   On 9/9/2010 10:42 AM, Ralph Smith wrote:
>> On Thu, September 9, 2010 1:10 pm, Matthew Kaufman wrote:
>>>    On 9/9/2010 8:37 AM, Ralph Smith wrote:
>>>> We have a requirement for approximately ten radio sites to be
>>>> synchronized
>>>> to within 30 ns of each other.
>>> 30 ns seems a little closer than most radio site applications need...
>>> what drives this requirement?
>> Aircraft surveillance using multilateration.
> So timing errors just become position errors. How do the sites talk back
> to the display? Can't you null out position errors if enough sites can
> see a single plane, and thus learn the timing error of the drifting
> (relative to other) site?

Sites communicate via landline telco. If there are sufficient mutually
visible networked sites to form a solution on an aircraft visible to
stations not in the timing network that would work, and is one of the
options we are studying.

>>>> Ordinarily you could throw in an
>>>> appropriate GPSDO and be done with it. However, we also have the
>>>> reqirement to be able to operate independent of GPS for up to six
>>>> days.
>>> Wow, ok, and what drives *that* requirement? Can you use any other
>> Paranoia. People making the requirements are concerned with GPS going
>> away
>> due to solar flare or some other reason.
> Once everyone relies on GPS approaches and ADS-B, the planes will be
> grounded long before 6 whole days of GPS outage anyway.

You're making the mistake of applying logic. ;) Actually, aircraft can
continue to fly VFR or navigate using VOR/DME and inertial navigation. The
radios are part of an ADS-B installation.

>>> mutually visible thing, or do we assume all satellites have vanished
>>> from orbit?
>> No satellites.
>>
>>
> Ok then. My best answer is to use the planes themselves as the common
> reference, at least the ones high enough that enough sites can see them.
>
> Also consider that you might be able to find additional mountaintop
> sites to plant fixed squitter-emitter transponders at that can be seen
> by 2 (or more) sites.

Thanks, all of these are various options we are considering, considering
all of the engineering trade-offs.

Ralph



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