Reception is also seriously affected by interference from other
radio signals.
This is not intended to be a treatise on the ionosphere, but some of the effects which have a significant impact on digital mode design will be discussed. First, the various frequency ranges and their characteristics will be described, followed by a discussion of each of the more significant effects and how they can be overcome - especially by MFSK 2000 of course!
reception of signals is characterised by slow
diurnal
and seasonal variations of signal strength, and
very stable phase.
The signals follow the line of the earth, so are not to any
great extent affected by ionospheric reflections. At times, late at night for
example, fading (interaction of ground wave and sky wave) can occur.
LF is plagued by man-made noise, especially TV, motor and power noise.
and HF
up to about 5 MHz have short (ground wave) range during the day, as a result of
sky wave absorption in the D layer. This area of low ionisation levels but high ion density
is quickly activated at sunrise, and blankets the arriving sky wave (from the E and F layers).
At night, the D layer fades quickly, allowing the sky waves to penetrate, giving a range
of several thousand km, and sometimes more, as the signal is now able to
bounce off the higher ionised layers. At night signal strengths are very high, but
so too is the strength of the lightning noise, which travels long distances. These frequencies are
a real challenge for digital transmission and reception.
This area is affected by man-made noise, especially during the day, and by lightning
burst noise at night. Because the sky wave propagation modes can be numerous, interaction
between them, and with the ground wave, leads to very strong fading and significant variations
in signal arrival time.
Reception is also seriously affected by interference from other
radio signals.
is best.
This area is strongly affected by ionospheric reflection, so
multiple "bounces" and both short and long paths can be effective. Short path signals
generally follow the obvious "great circle" route between stations. Long path generally
takes place in the evening, travelling on a reciprocal path, through the night-time ionosphere
which is generally more stable.
HF is strongly affected by the activity of the sun, ionising the many reflective layers on a daily basis. This ionisation varies with short term solar activity, and the 11 year solar cycle. When the sun is especially active (during a solar storm) HF propagation can disappear for days. During a solar activity minimum, ionospheric propagation is generally limited to single hop range unless unusual propagation paths occur.
The ionization of the reflecting layers varies with altitude, time of day and latitude. A strong concentration of ions at the poles and the variation in concentration during the day leads to relatively fast changes in refractive index, modulating the phase, frequency and amplitude of signals in both random (noise) and predictable ways (diurnal rising or lowering reflective height).
Lightning noise is a problem at the lower end of the HF range. Very weak signals are a bigger problem at the upper end, and all the HF area is affected by fading and doppler effects. Reception of the more popular HF frequencies is also seriously affected by interference from other radio signals.
and non-existent on UHF. Ducting through tropospheric temperature inversion,
is common, but unlike ionospheric reflection, this is a "lens"
effect, like a light pipe, rather than a reflective effect. VHF and UHF signals
are characterised by stable phase when ground wave communications are used.
Satellite communications suffers from gross (movement induced) doppler frequency shifts. Moon-bounce suffers from "libration fading" which affects the phase and amplitude of signals arriving from the moon. Fading and polarisation rotation effects are noticed on satellite and terrestrial signals. Reflection from other objects causes random and often very deep fades. When the transmitter or receiver is in motion, or a large moving object (like an aircraft) creates a reflective path, this fading is rhythmical at a rate dependent on the speed and the signal wavelength (sometimes called the picket fence effect").
To design a digital mode to address all these problems is a tall order!
It is this last "Rag-Chew" category that we will concentrate on.
| HF "Rag-Chew" Mode Typical Specifications |
|---|
|
systems are seriously affected by such
noise since it causes loss of
synchronism. The simplest systems (such as RTTY) simply pass on the errors caused by
burst noise to the user, and accept the occasional serious loss of synchronism. Burst noise can be
managed very well using error recovery systems, especially if an interleaver is used to
spread the lost data bits across several characters, thus reducing the instantaneous
error correction load.
or dynamic range will be affective agains fading (slow changes in
signal strength, or faster changes due to multi-path cancelling). Systems such as PSK that
are independent of signal amplitude are also very effective.
One of the best counters to multi-path problems is to use a very low symbol rate. MFSK is ideal in this regard. For example, at 10 baud, typical of MFSK systems, 5ms timing error represents only 5% distortion. PSK systems with asynchronous detection (PSK-Hell is the only known example) perform reasonably well, but conventional synchronous demodulation PSK modes such as PSK31 do not cope well with bad multi-path, which is very common on the lower HF bands at night. The following diagram illustrates multi-path and other effects. The ionosphere is not a perfect mirror, and in addition to the different path lengths and therefore time of flight, there is considerable delay at the reflective layer as the refractive index and therefore speed of the radio waves changes.
The best counter to this problem is to employ a very sensitive narrow band system with very slow symbol rate. PSK31 is a modern example of such a mode. Hellschreiber (Feld-Hell and PSK-Hell) is also very effective. MFSK is also very good in this regard, provided the detection of each carrier is independent of the others, or the system can cope with missing symbols (such as by using an error reduction system).
The ionosphere, particularly in the regions near the poles, can be especially disturbed by solar activity, and this random variation of refractive properties, like waves in the ocean, unfortunately causes significant random modulation of the phase, frequency and amplitude of signals. The effect is of course most noticeable on paths across the poles, such as the long-path from New Zealand and Australia to Europe. This is especially a problem with PSK modes, but does effect all modes to some extent.
