Environmental Engineering Reference
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frequency in the band from 90 to 110 Khz for all stations, which means that wave lengths are
approximately 3,000 m. A chain of transmitting stations consists of one master station and
two to four slave stations. Master and slave stations are separated by distances of 1,000 to
1,300 km to increase the accuracy of triangulation, and the operating range from a vessel to
the stations is in excess of 1,600 km.
Since the objective of a LORAN-C system is to provide over-the-ocean coverage, its
transmitting stations are often located on islands or in coastal regions where, with favorable
wind conditions, wind turbines may also be sited. A wind turbine in the vicinity of a
LORAN-C transmitting station could affect communications by interacting with the
transmitting antenna or by acting as a secondary transmitter, radiating similar LORAN-C
signals delayed in time. Both of these effects have been investigated theoretically
[Sengupta and Senior 1979b]. Since the antenna interaction effect could be corrected at the
transmitting station by using proper impedance matching of the antenna input terminals, it
is unlikely to affect communication system performance. In particular, it has been found
that such effects will be insignificant even for a large-scale HAWT (like the 2.5-MW
Mod-2) located as close as l/12 (approximately 250 m) to the transmitter.
The second effect, that of acting as a secondary signal source, is potentially more
severe. Assuming that the transmitting antenna is linear and oriented vertically above an
ideally conducting flat earth and again modelling a HAWT blade as a rectangular metal
plate oriented so as to direct the maximum scattered signal to the receiver, the modulation
index, m R , assumes its largest value when the transmitter, the receiver and the wind turbine
are collinear. Generally, forward-scattering is the more critical, with the wind turbine much
closer to the transmitter than to the receiver. Under these conditions, the maximum modu-
lation index can be estimated as follows [Sengupta and Senior 1979b]:
(z 2 + z w )/z 2 + z T
exp(0.184p R 2 /h S A P )
m R ยป p R 3
6l 2 z
(9-35)
where
z = distance from the wind turbine to the transmitter (m)
z T
= elevation of the transmitter (m)
Since the rotor radius of even the largest HAWT is very small compared to the 3,000-
m wave length of LORAN-C signals, the signals that could be scattered by a wind turbine
are about 100 dB lower than the direct signal at the receiver, even when the wind turbine
is located as close as 100 m to the transmitter or the receiver. Therefore, it is unlikely that
LORAN-C communications would be degraded by such low levels of interfering signals.
From these results, it appears that a wind turbine located at a distance greater than l/12
(about 250 m) from any transmitter or receiver will not have a significant effect on the
performance of a LORAN-C system.
Radar Systems
Radar is a system that uses electromagnetic waves to identify the range, altitude, di-
rection, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles,
weather formations, and terrain. Wind turbine interference with radar signals is a concern
to both the Federal Aviation Administration (FAA) and the Department of Defense (DOD),
because of potential impacts on air traffic control and military missions. Experience to date
shows that only a small percentage of wind turbine installations (less than 5 percent) produce
interference with radar signals that is significant [Seifert 2005 and 2006, Meyers and Seifert
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