Re: Sun burst

Bob Bob <bob3bob3@xxxxxxxxxxxxxx> wrote in news:7kjom6-m74.ln1
@p400bob.personal.cox.net:

Hi Paul

Just for interest what is the S/N margin with most commercial satellite
systems and how "strong" is the sun relative to those levels? I would
assume the margin for an end use TV viewer would be less than this?

Bob, it is a few years since I did satellite path designs... but relying
on memory...

Most satellite TV here is digital, and the issue becomes a C/N ratio that
delivers acceptable error rates after FEC. The characteristic has a knee,
and error rates degrade rapidly at the knee. Satellite facilities are so
expensive, that operation is usually quite close to the knee... save a
margin for such things as weather, and that margin is usually no more
than necessary for most but not all such variables. You don't need much
Sun noise to ruin performance. (BTW, I think that the satellites used for
this purpose here are bent pipe technology (aka linear transponder), the
uplink is not regenerated on the bird.)

The issues discussed in the thread seem to mix up two distinct effects.
If the earth station sees the sun behind the satellite, C/N may be
degraded sufficiently to disrupt digital services. At equinoxes, the bird
rotates into the earth's shaddow and some times of day, and that means it
has to operate exclusively on battery, and if it uses the Sun for antenna
tracking, it will need to change reference. Using CO2 for a reference
(earth's atmosphere) is not reliable if the Moon appears from behind the
earth, the antennas could track the moon. I am not up to date on the
latest tracking references... but I am sure the Sun is still a good
reference, good contrast, visible most of the time.

Owen
.

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