Environmental Engineering Reference
In-Depth Information
Interference with Navigation Communication Systems
VOR and DVOR Systems
The VOR
(
Very High-Frequency Omnidirectional Range
)
and DVOR (
Doppler VOR
)
systems are used for short-range ground-to-air communication which provides navigation
information to flying aircraft [Beck 1971, FAA 1968]. In an ideal situation, the VOR
ground station effectively radiates a VHF carrier signal (in the 108-118 Mhz frequency
range) which contains two synchronous 30 Hz modulation signals. One is known as the
reference signal
and is constant in phase and independent of the aircraft azimuth. The
phase of the other signal, known as the
variable signal
,
varies directly in accordance with
the bearing of the aircraft from the VOR ground station. A phase-measuring device in air-
borne receivers enables pilots to find their bearings by determining the phase difference
between these two modulation signals.
In a conventional VOR system, the variable signal is a 30-Hz amplitude-modulation
(AM) on the carrier, while the reference signal is a 30-Hz frequency modulation (FM) of
a 9.96 KHz sub-carrier. In the DVOR the roles of 30 Hz AM and FM signals are inter-
changed (
i.e.
,
the variable signal is FM while the reference signal is AM). This makes the
DVOR system relatively immune to interference from a potential multipath source in the
vicinity, such as a wind turbine.
Scalloping errors
in the bearing indications of a VOR (DVOR) system that might be
produced by the rotating blades of a wind turbine near a ground station have been
investigated theoretically with the following assumptions: The system is in free space, the
airborne VOR receiver is ideal, the vertical-plane response characteristics of the transmitting
antenna are constant, the antenna is located above a perfectly-conducting plane earth, and
the scattering effects of the wind turbine tower are negligible [Sengupta and Senior 1978].
The rectangular metal plate model of a HAWT blade expressed in Equations (9-17) is used
to obtain the strength of the scattered VOR signals at the airborne receiver for both the
static and rotating blade conditions.
For a rotating blade, a Fourier-series analysis of the modulation function
f
m
obtained
with Equations (9-17) indicates that it contains even harmonics of the rotor speed W,
and
that its spectrum may extend up to a frequency many times larger than W. In fact, the
frequency spectrum of
f
m
may contain the VOR AM modulation frequency of 30 Hz and
its harmonics. However, calculations indicate that the theoretical modulation index,
m
R
,
obtained for a rotating HAWT blade is much smaller than that for a static blade. An
analysis of DVOR performance also leads to the same conclusion [Sengupta and Senior
1978]. The reduced scalloping errors produced by rotating blades may be explained by the
fact that the scattered energy is distributed over a band of frequencies in the time-varying
case, whereas the VOR receiver is sensitive only to 30-Hz modulated signals. It is there-
fore concluded that potential wind turbine interference with VOR or DVOR communications
may be adequately investigated by following the procedures developed by the Federal
Aeronautics Administration for static interference sources [FAA 1968].
LORAN-C System
LORAN-C (
Long-Range Navigation System: Version C
)
is a hyperbolic maritime navi-
gation system that enables users to determine their positions very precisely anywhere within
the designated coverage area, by measuring the arrival times of signals received from at
least three stations [Beck 1971, Kayton and Fried 1969]. The coverage area encompasses
about one-fourth of the earth's surface. LORAN-C is a pulse system with a common carrier
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