Satgen 475  Satellites and HF DX  by GM4IHJ  (BID SGEN475) 02-05-1998

It was common knowledge amongst American and European HF terrestrial
DX enthusiasts, as far back as the 1970s, that both 10m and 6m
opportunities for communication one continent with the other, often
occurred , when the great circle propagation route was dead. The trick
was to point your antennas west of Africa in the morning and , at the
Caribbean in the European afternoon. Clearly there exists at certain
times, a partial reflector high in the ionosphere which facilitates
this oblique form of propagation. Indeed in 1987 direct measurement of
electron density levels at a range of excitation from 30ev to 13 kev,
as Uosat 2 came up over the Atlantic firmly delineated a step like
function in these measurement at one point in the track. Uosat2
1.P160787 data file refers.

But this test required dedicated use of a busy satellite. So only a
few records were taken. Meanwhile however at GM4IHJ a different
approach had been tried whereby sub horizon tracking of satellite
Cosmos 1686 ( 15m downlink) revealed some very distinct doppler
patterns which indicated the presence of this anomaly quite clearly.
In these 1986 experiments Cosmos 1686 was tracked as it passed down
the Atlantic far to the west of,and sub horizon from GM, with a very
strange doppler pattern such that long after the satellite passed
through its distant point of closest approach it was still being
received with a very high doppler shift similar to that which would
have been expected if the satellite had been coming towards and not as
it was, going away.That the signal was not coming direct to GM was
obvious. It was clear that the satellite was sending a high doppler
signal ahead, to a reflector to its south, and this reflector was
sending the signal back to GM. Confirmation that this was the case was
received when a sudden drop in 1686 beacon strength was accompanied by
a sudden drop to doppler low as the satellite overflew the presumed
reflector and passed to its south side.

Checks of north going runs of 1686 revealed a reverse of this
phenomena whereby, although the satellite was coming towards GM, the
signal was permanently doppler low ie coming off the back of the
satellite to the reflector to its south and thence being reflected
north to GM. These experiments were reported in the RSGB Space Radio
Handbook 1990 and in Amsat Journal Nov 93.

Reports of RSGB correspondence in 1998 indicate that very few UK
amateurs are aware of this phenomena, which has already started to
occur on 15m and 10m as solar flux rises. Indeed past experience
suggests that in these early days a year or two before solar cycle
peak, this oblique method of transatlantic propagation will be present
far more often that the more northerly direct great circle path.
Fortunately we still have two excellent Russian satellites RS12 and
RS15 which are capable of revealing the presence of this reflector via
doppler shift checks of their sub horizon performance. Indeed good RS
HF mode K operating may well be available particularly next 98/99
winter season permitting some excellent sat DX, and it may even be
possible to do experiments over other parts of the earth, to establish
whether this reflector is worlwide, not just Africa/Caribbean