Vector Sensor Reference Manual 139
Radiobeacon Range
The broadcasting range of a 300 kHz beacon is dependent upon a number of factors including
transmission power, free space loss, ionospheric state, surface conductivity, ambient noise, and
atmospheric losses.
The strength of a signal decreases with distance from the transmitting station, due in large part to
spreading loss. This loss is a result of the signal’s power being distributed over an increasing
surface area as the signal radiates away from the transmitting antenna.
The expected range of a broadcast also depends upon the conductivity of the surface over which
it travels. A signal will propagate further over a surface with high conductivity than over a surface
with low conductivity. Lower conductivity surfaces such as dry, infertile soil, absorb the power of
the transmission more than higher conductivity surfaces, such as sea water or arable land.
A radiobeacon transmission has three components: a direct line of sight wave, a ground wave, and
a sky wave. The line of sight wave is not significant beyond visual range of the transmitting tower,
and does not have a substantial impact upon signal reception.
The ground wave portion of the signal propagates along the surface of the earth, losing strength
due to spreading loss, atmospheric refraction and diffraction, and attenuation by the surface over
which it travels (dependent upon conductivity).
The portion of the beacon signal broadcast skywards is known as the sky wave. Depending on its
reflectance, the sky wave may bounce off the ionosphere and back to Earth causing reception of
the ground wave to fade. Fading occurs when the ground and sky waves interfere with each
other. The effect of fading is that reception may fade in and out. However, this problem usually
occurs in the evening when the ionosphere becomes more reflective and usually on the edge of
coverage areas. Fading is not usually an issue with overlapping coverage areas of beacons and their
large overall range.
Atmospheric attenuation plays a minor part in signal transmission range, as it absorbs and scatters
the signal. This type of loss is the least significant of those described.
Radiobeacon Reception
Various sources of noise affect beacon reception, and include:
• Engine noise
• Alternator noise
• Noise from Power lines
• DC to AC inverting equipment
• Electric devices such as CRT’s electric motors, and solenoids
Noise generated by this type of equipment can mask the beacon signal, reducing or impairing
reception. Section 2.4.1 presents an effective procedure to minimize impact of local noise on
beacon reception when using this correction service.
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