Message-ID: <005c01c1702c$5275ac40$3d2df7c2@zetnet.co.uk>
From: "John Rabson" <word.factory@zetnet.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <5.0.2.1.2.20011117171924.00a32840@mail.pncl.co.uk>
Subject: LF: Re: Propagation
Date: Sun, 18 Nov 2001 09:06:47 -0000
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Walter,

Would you send me the attachment direct please?

Many thanks,
73 de
John Rabson G3PAI

----- Original Message -----
From: "Walter Blanchard" <blanch@pncl.co.uk>
To: <rsgb_lf_group@blacksheep.org>
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
>
>
>