Walter,
Would you send me the attachment direct please?
Many thanks,
73 de
John Rabson G3PAI
----- Original Message -----
From: "Walter Blanchard" <[email protected]>
To: <[email protected]>
Sent: Saturday, November 17, 2001 5:26 PM
Subject: LF: Propagation
I'll try this one again without the attachment. It didn't get through
first
time (too long? or was it the attachment?)
Anybody wants the attachment I'll send it privately.
Flg might be of interest , culled from an old Decca document:
Quote :
Propagation at Decca frequencies. (70-130 kHz)
Signals at DECCA frequencies are reflected by the ionosphere. At medium
latitudes and distances in summer the reflection coefficient is small,
about 2 per cent, and the effective reflection height is about 70 km. At
night the effective reflection height increases, thereby increasing the
attenuation because of the increased propagation path length, but this
effect is not enough to balance the attenuation of the signals in the
lower
parts of the ionosphere in the day time. The sky wave is therefore
stronger
at night than in the day time at average and large distances. The
reflection coefficient is about 25 per cent at 500 km of distance at
night,
and the effective reflection altitude is about 95 km . The range of a
chain
is usually defined with regard to the distance where the wave reflected
from the ionosphere reaches the same level as the ground wave, which is
about 440 km at night and about twice that value in the day time .
Measurements of ground waves have shown that their magnitudes are
approximately Rayleigh distributed. Even if the ground wave and the sky
wave have about the same magnitude at about 800 km distance from the
transmitters at night, there may be strong and rapid fluctuations of the
received signal at shorter ranges, when the two waves have opposite
phases.
This can cause lane slippage even when
a skywave has only half the power of the ground wave. This may happen
about
400 to 500 km from the transmitters. The range where the sky wave and the
ground wave are about equal is about 500 to 1300 km from the transmitters.
After 1300 km the sky wave dominates. The DECCA lines-of-position maps
have
been worked out based on the ground wave alone. Sky wave interference
gives
fluctuating phase errors because of amplitude and phase variations of the
sky wave. This gives rise to complicated error functions in the position
measurement because the slaves are phase-locked to the master. Therefore,
there are three different sky waves with influence at each
line-of-position
determination: master slave, master receiver and slave receiver. The
statistical distribution of the errors can be predicted and, for example,
be expressed as the part of the time the errors are below a certain value.
The sky wave does not give rise to errors in lane counting until it
dominates the ground wave. This happens, as mentioned above no closer to
the transmitters than 400 to 500 km at night (800 to 1000 km in the day
time).
It was mentioned above that line-of-position deviations have turned out to
have a Gaussian distribution. Approximately, this applies also at night
even if the measured density function has turned out to have a slightly
different shape as about 75 per cent of all measurements are found within
one sigma (compared to 68 per cent for the Gaussian distribution) . This
also implies that fewer measurement errors are found far from the average,
compared to what can be expected according to the Gaussian distribution.
Extensive measurements of sky wave effects have been carried out wherever
DECCA chains are established, at different places within the chain
coverage
and at different times. Consequently, it has been possible to map
variations as a function of time of year, atmospheric phenomena,
geographical position and earth conductivity. In order to avoid too much
influence by short time effects, the measurement series have been divided
into time intervals of seven days.
The ionospheric effects are correlated to the sun spot activity which has
a
period of about 11 years. The uncertainty brought about by this activity
is
10 to 20 per cent around the average, which the error predictions are
based
on, and is greatest in winter. The DECCA errors are at a minimum when the
activity is at a maximum, and this is due to the attenuation of the sky
wave under such circumstances. The largest error sources within the
coverage of the chain are irregular propagation and unfavourable aspect
angles between lines of position. Maps have been published for the
different chains showing accuracy contours as a function of time of year
and time of day.
(end of quote)
The maps referred to here were called "onion" maps (sample attached) and
are quite interesting because they give the type of propagation to be
expected hour-by-hour for every month of the year.
Walter G3JKV
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