[time-nuts] NIST

Gerhard Hoffmann dk4xp at arcor.de
Sat Sep 1 05:31:29 EDT 2018



Am 31.08.2018 um 20:17 schrieb Hal Murray:
> attila at kinali.ch said:
>> I have somewhere a paper (which i cannot find currently, sorry) that used a
>> dish trained at one of the EGNOS satellites and used it as the only source
>> for timing. IIRC the results were promising, but not spectacular. The problem
>> being that all the ionospheric and tropospheric ...
> There is another problem in that area.  How accurately is the location of the
> satellite known?  published?
>
> Geo-sync satellites actually wander around their nominal positions.  How much
> does that effect timing?  I've seen figure-8 pictures of the pattern, but I
> don't remember any data on elevation changes.

Geo-stat sats usually do their navigation via their linear transponders,
the rest is in some ground stations, so the added cost for flight hardware
is essentially nil.

The operators know the position of their sats quite precisely since,
for a phone sat as an example, the ground station transmission timing
must be aligned quite carefully to avoid both overlapping and idle time
of the channel. They switch both between cities and to give the phone
user the illusion of a continuous 2 way connection without too much delay.

The absolute position is less important as long as it is known.

Small countries like Luxembourg have just one geostationary parking lot
but operate several sats. They may have a more pronounced need to
keep the positions precise.

Overly precise position shortens the lifetime of a sat since it eats up 
fuel.

The navigation is simply made by PN streams say > 20 dB below the
MPEG data.

I have made the PN generators,  bit / frame generators /synchronizers,
correlators, de/modulators for some of them.

I don't think that the exact position data is published. It is a closed
system, after all.

But a bunch of hams with enough criminal energy could probably
measure it for themselves. The down link is already there in every
household with a SAT TV.  Oh, no, I do not promote that!

best regards,
Gerhard






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