IONOSPHERIC EFFECTS ON LEO SATELLITE SIGNALS.
"There and back is just as far" ... or is it?.
Due to the inherent behaviour of a low earth orbit satellite in space one
does *NOT* need to be a budding Professor Steven Hawking to understand that the
ionospheric parameters at one particular QTH at any given time are not
necessarily the same as those at a different QTH located a few hundred
kilometers (or more) distant.
We needn't go into planetary motion or quantum mechanics (just yet), it being
sufficient to remind the potential new-beginner to satellite communications that
the signal must pass through the ionosphere twice, transmitted on
the uplink frequency and received on the downlink frequency.. and unlike FM
"parrot" repeaters using the same frequency for the uplink and downlink, in
analogue communications, the uplink and the downlink are on different amateur
bands. So, a quick shake of the grey gunge between your ears ought to get the
cells in enough motion to realise that unlike terrestrial direct link
communications ... (read 2m repeater), where the signal more or less follows the
curvature of the earth and is only mainly susceptible to tropospheric ducting
and the like ... the signal to and from a LEO satellite will and more often than
not does !! suffer from a couple of other quirks of mother nature ... namely ...
scintillation, Faraday rotation and other propagation anomalies.
Scintillation is the effect of the ionosphere that makes the stars "twinkle". To
understand it you have to consider several factors. Solar radiation, the Earth's
magnetic field, the electron density and even the temperature(!).
Faraday rotation is the end result of a linearly polarised signal wave-front
*rotating* as it passes through the ionosphere ... resulting in different forms
of "elliptical" polarisation at the receiving antenna/aerial.
Important note.
Please read the articles about "Circular Polarisation" in "Observations from
Scotland" by GM1SXX
(1) elsewhere on this site. The subject is also mentioned in the
Beginners guide to Amateur Radio Satellites by LA2QAA & GM1SXX. Circular
polarisation is very often misunderstood in the amateur satellite community.
Living on Frei Island 63 degrees north, 7 degrees east, I'm lucky/unlucky enough
to have daily aurora and have experimented with LEO satellites via this
propagation mode everyday for many years.
Practical experience has taught me that The ionosphere *DOES* have a DEFINITE
effect on signals transmitted to and from LEO satellits that operate with
frequencies in the 2m and 70cm amateur band. As the frequency increases, these
anomalies decrease in their severity ... but they still have some effect.
This article won't go into the myriad of details that effect a signal that
traverses the ionosphere TWICE but those wishing to know more can
use The Wikipedia online encyclopaedia for further information ... (LA2QAA is
just too bone idle to add all the links) ... For those without Internet access,
your local library is still a good source of information.
++++++++++++++++++++++++++++++++++++++
Web links with some relevance in no particular order. (added by SXX!)
73 John. la2qaa@amsat.org
Keywords ... LEO satellites, Polarisation, Solar activity, Faraday rotation, Scintillation, The Earth's magnetic field, Density, General ionospheric propagation.