Satgen432  Prepare for SatDX Pt2  by GM4IHJ  (BID SGEN432)  5 July 97

Continuing the saga, of how, when, and where to anticipate
opportunities to extend your HF satellite QSOs , beyond the normal
satellite footprint horizon, mention must be made of Sporadic E (SpE)
For years terrestrial HFers refered to this phenomenon as " Short
Skip", not being sure how it originated. But good work by VHF users,
who regularly use SpE, soon revealed the seasonal nature of this form
of propagation which results from dense patches of ionisation building
up in the E layer of the ionosphere at about 90 kms altitude.

SpE is seasonal over the temperate zones of the earth, and it can
extend far enough north to affect satellites far above the Arctic
circle. The big season is local summer ( May to early September in the
Northern Hemisphere). With a short winter season ( mid December to mid
January in the northern Hemisphere ). It is generally, a local
daylight only phenomenon, but may occasionally extend until some time
after sunset.

In the Tropic regions of the earth , SpE occurs all year round. Again
being usually restricted to daylight hours.

With Low Earth orbit satellites , SpE extends their footprints for up
to about 1600 kms . But be warned, as a typical RS12 orbit leaves the
IHJ footprint, signals are usually lost for a minute or two or, become
very difficult to copy, before resuming good strength and readability
for a few minutes as the satellite proceeds further south.

For some reason, ( some observers credit the high altitude Jet
streams), SpE clouds seem to appear regularly, overhead the same
places on the earth. Eg a patch of SpE has appeared on at least 40
days of the roughly 120 day summer season , over Nantes in France, as
sighted from IHJ at 56N in Scotland, every year for the last 20 years.
Other clouds of SpE appear with about half this regularity over the
German Ruhr , and the Skagerrak as seen from IHJ.  But again caution
is necessary. This view from IHJ is restricted . Other observers
locate clouds of SpE over southern England , which are too close to
IHJ to give good SpE propagation most of the time.

So each station operator needs to locate the most frequent locations
of SpE as seen from their own station. Eg in the 1940s IHJ in Malta
regularly "saw" clouds of SpE to the south, extending VHF coverage
deep into Africa. You can locate Spe centres roughly, by noting which
29MHz, 6MHz and 144MHz beacons appear at your station during outbreaks
of SpE. Then you can plot the overhead locations of the SpE clouds as
that point roughly half way between your station and the station being
received via SpE. In addition these stations once noted in your log
will give you warning in the future, of when you can expect both the
enhancement of signals from distant terestrial stations and the
extensions of LEO satellite footprints.

PS Iridium have launched a further 7 Mobile phone sats, making a total
of 12 now in orbit.