Datawell 45100, Руководство пользователя

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A.3 Communication channel
The communication channel between the buoy and its receiver is the atmosphere through which
the buoy’s RF signal propagates wirelessly. This is the least predictable part of the buoy
communication link and determines for a large part the quality of the received signal.
Phenomena which play a role in the communication channel between the buoy and its receiver
are distance, noise, interference, propagation, wave height and the existence of other
transmitters. Those parameters will be described below.
A.3.1 Distance
The distance between the buoy and the receiver determines for the largest part the signal loss
between transmitter and receiver. The larger the distance the larger the signal loss.
If this distance becomes too large, the buoy signals will be so weak that they cannot be
discerned from the always existent natural noise, and data recovery from the RF signal of the
buoy will be impossible.
Receiving a buoy over a large distance will thus make the buoy signal weaker. The weaker the
received buoy signal, the more sensitive the communication link will be to errors induced by
mechanisms such as interference and distant buoys transmitting at the same frequency.
Technical details:
The signal loss due to the distance between the transmitter ranges between 53 dB at 1 km
separation and 112 dB at 60 km separation at 30 MHz over seawater, while over dry city ground
the loss will be between 91 dB at 1 km separation and 169 dB at 60 km separation.
The following table can be used as a guide for estimating losses due to distance:
Ground Loss 1 km
(dB)
Loss 3 km
(dB)
Loss 6 km
(dB)
Loss 10 km
(dB)
Loss 30 km
(dB)
Loss 60 km
(dB)
Sea water 53 63 71 77 95 112
Fresh water 74 87 100 109 130 148
Wet land 83 103 116 125 145 162
Dry land 91 110 123 131 151 169
Fresh water
ice 96 115 127 136 156 174
Numbers in yellow indicate that it is difficult to guarantee reception under all circumstances
(e.g. when the wave height is 20 meters) for a typical setup normal antenna at a normal height of
5 to 10 meters. The numbers in red indicate that reception will be impossible under certain
circumstances for a maximum setup with a three element directional antenna at an height of 20
meters above sea level.