[time-nuts] Timing Distribution in Mountainous Terrain
matthew at matthew.at
Thu Sep 9 17:55:00 UTC 2010
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
>>> 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?
>>> 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.
>> 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.
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