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energy in the upper atmosphere is likewise affected as the
energy of radiation is absorbed by molecules of air, water,
and dust. The characteristics of radio wave propagation vary
according to the signal frequency and the design, use, and
limitations of the equipment.
Ground Wave
A ground wave travels across the surface of the Earth. You
can best imagine a ground wave’s path as being in a tunnel
or alley bounded by the surface of the Earth and by the
ionosphere, which keeps the ground wave from going out
into space. Generally, the lower the frequency, the farther
the signal will travel.
Ground waves are usable for navigation purposes because
they travel reliably and predictably along the same route
day after day, and are not influenced by too many outside
factors. The ground wave frequency range is generally from
the lowest frequencies in the radio range (perhaps as low as
100 Hz) up to approximately 1,000 kHz (1 MHz). Although
there is a ground wave component to frequencies above this,
up to 30 MHz, the ground wave at these higher frequencies
loses strength over very short distances.
Sky Wave
The sky wave, at frequencies of 1 to 30 MHz, is good for
long distances because these frequencies are refracted or
“bent” by the ionosphere, causing the signal to be sent back
to Earth from high in the sky and received great distances
away. [Figure 7-1] Used by high frequency (HF) radios in
aircraft, messages can be sent across oceans using only 50
to 100 watts of power. Frequencies that produce a sky wave
are not used for navigation because the pathway of the signal
from transmitter to receiver is highly variable. The wave is
“bounced” off of the ionosphere, which is always changing
due to the varying amount of the sun’s radiation reaching it
(night/day and seasonal variations, sunspot activity, etc.). The
sky wave is, therefore, unreliable for navigation purposes.
For aeronautical communication purposes, the sky wave
(HF) is about 80 to 90 percent reliable. HF is being gradually
replaced by more reliable satellite communication.
Space Wave
When able to pass through the ionosphere, radio waves
of 15 MHz and above (all the way up to many GHz), are
considered space waves. Most navigation systems operate
with signals propagating as space waves. Frequencies above
100 MHz have nearly no ground or sky wave components.
They are space waves, but (except for global positioning
system (GPS)) the navigation signal is used before it reaches
the ionosphere so the effect of the ionosphere, which can
cause some propagation errors, is minimal. GPS errors
caused by passage through the ionosphere are significant
and are corrected for by the GPS receiver system.
Space waves have another characteristic of concern to users.
Space waves reflect off hard objects and may be blocked if
the object is between the transmitter and the receiver. Site
and terrain error, as well as propeller/rotor modulation error
in very high omnidirectional range (VOR) systems is caused
by this bounce. Instrument landing system (ILS) course
distortion is also the result of this phenomenon, which led
to the need for establishment of ILS critical areas.
7-3
Figure 7-2. ADF Indicator Instrument and Receiver.
Generally, space waves are “line of sight” receivable, but
those of lower frequencies will “bend” somewhat over the
horizon. The VOR signal at 108 to 118 MHz is a lower
frequency than distance measuring equipment (DME) at 962
to 1213 MHz. Therefore, when an aircraft is flown “over the
horizon” from a VOR/DME station, the DME will normally
be the first to stop functioning.
Disturbances to Radio Wave Reception
Static distorts the radio wave and interferes with normal
reception of communications and navigation signals. Lowfrequency
airborne equipment such as automatic direction
finder (ADF) and LORAN are particularly subject to static
disturbance. Using very high frequency (VHF) and ultrahigh
frequency (UHF) frequencies avoids many of the
discharge noise effects. Static noise heard on navigation
or communication radio frequencies may be a warning of
interference with navigation instrument displays. Some of
the problems caused by precipitation static (P-static) are:
• Complete loss of VHF communications.
• Erroneous magnetic compass readings.
• Aircraft flying with one wing low while using the
autopilot.
• High-pitched squeal on audio.
• Motorboat sound on audio.
• Loss of all avionics.
• Inoperative very-low frequency (VLF) navigation
　
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