LF: Sat-TV timebase

[MarkusVester@aol.com]

Hi Alberto and LF group,

the Astra downlink I observed was analog FM (10964MHz horizontal). The satellite signal was an exact, but delayed replica, with apparently no buffering. The only problem is that the path delay has some daily Doppler variation due to the small excentricity and inclination of the not-quite geostationary orbit. During my comparisons in November 2001, this shift amounted up to about 50µs peak-peak over 24h. Furthermore, the time-shift is not easily predictable, as tidal effects and occasional correction maneuvers cause a day-to-day variation in the orbital parameters. - If you can receive terrestrial ZDF, one very crude but enlightening experiment is to simply combine the two video sources via different resistors and watch the weaker signal's sync-bar wander across the screen in the course of a day.

Another stable signal on Astra is 3sat (11347MHz v), which seems to have a precise video clock and does not exhibit the notorious 1µs short-term jitter of ZDF. However it is not available on a terrestrial network. I then tried to measure the orbital delay from the time-difference of the two ZDF signals and apply that as a correction to the 3sat-signal (f = f_3sat + f_zdf_sat - f_zdf_terr). But the scheme failed, simply because ZDF and 3sat are carried on different spacecraft in the cluster (Astra 2C and Astra 1H respectively).

73 and happy experimenting

Markus


In einer eMail vom 27.12.2003 19:17:23 Westeuropäische Normalzeit schreibt dibene@usa.net: 

> Markus 
>  do you (or anybody else) know if the re-broadcasting of ZDF via the Astra satellite keeps
> the timebase accuracy, or some frame buffer is inserted in between, that would of course
> ruin that precison ?   Thanks
>
> 73  Alberto  I2PHD
> P.S. My wife watches ZDF more often than the Italian television :-)