15.2 Radio Direction-Finding Goniometric Radio Navigation Systems
307
If oscillation phases of frame and non-directional antenna are coincided (cophased), then rotation direction of frame should be to the right, if in antiphase—to
the left. As HF oscillations of frame are cosinusoidal, and non-directional antenna—
sinusoidal, then in order to compare them (in-phase or antiphase), the oscillation
phase of frame is turned further at 90° using capacitor.
Hence, the ARC principle of action in automatic measuring of HA is based on use
of directional properties of frame antenna and signals phase comparison, received
by frame and non-directional antennas. Because of phase comparison (in-phase of
antiphase), the frame rotation direction and HA indicator needle are determined, and
its turn is carried out until the moment, when a signal at frame output becomes equal
to zero. Since “zero” readings of RV indicator needle correspond to frame position
along longitudinal axis of an aircraft, then its turn from reference position will point
a heading angle (relative bearing).
Nowadays, ARC with inner phase modulation are widespread used since they
are more stable against different interference influence, rather than ARC with inner
amplitude modulation.
The time diagrams illustrating the ARC with inner phase modulation operation
principle are shown in Fig. 15.28. At output of phase switch (Fig. 15.28a), the HF
balanced-modulated oscillations is formed, which have the initial phase switched
through half cycles of low-frequency signal from 90° into 270° or vice versa. After
summation of HF oscillations of non-directional antenna (Fig. 15.28b) and balancedmodulated, the phase-modulated oscillations are formed (Fig. 15.28c).
For time intervals t 1 , t 2 and t 3 (Fig. 15.28d), vector diagrams of combined
signals are shown: U non —signal vector of non-directional antenna; U mBM —vector of
balanced-modulated (BM) signal, the direction of which differs from a direction of a
previous vector at 90° or 270°. Deviation angle of combined signal vector from direction of U non vector corresponds to a phase shift ϕ. As we can see from Fig. 15.28,
a phase shift value depends on amplitude value of BM oscillation, and a sign—from
relation of phases of comparing signals.
As the amplitude U mBM is changed according to a law sin t, where = 2π F,
F—a frequency of low-frequency signal, then ϕ is change according to the same
law.
From Fig. 15.28e, we can see:
tgϕ =
U mBM
U non
=
U mBM max
U non
sin HA sin t,
(15.35)
where U mBM max —maximum value of BM oscillation at HA = 90° or 270°.
Phase modulation index (maximum phase deviation):
ϕ = arctg
U BM max
U non
sin HA sin t
(15.35)
As you can see, phase modulation index depends on HA.
307
If oscillation phases of frame and non-directional antenna are coincided (cophased), then rotation direction of frame should be to the right, if in antiphase—to
the left. As HF oscillations of frame are cosinusoidal, and non-directional antenna—
sinusoidal, then in order to compare them (in-phase or antiphase), the oscillation
phase of frame is turned further at 90° using capacitor.
Hence, the ARC principle of action in automatic measuring of HA is based on use
of directional properties of frame antenna and signals phase comparison, received
by frame and non-directional antennas. Because of phase comparison (in-phase of
antiphase), the frame rotation direction and HA indicator needle are determined, and
its turn is carried out until the moment, when a signal at frame output becomes equal
to zero. Since “zero” readings of RV indicator needle correspond to frame position
along longitudinal axis of an aircraft, then its turn from reference position will point
a heading angle (relative bearing).
Nowadays, ARC with inner phase modulation are widespread used since they
are more stable against different interference influence, rather than ARC with inner
amplitude modulation.
The time diagrams illustrating the ARC with inner phase modulation operation
principle are shown in Fig. 15.28. At output of phase switch (Fig. 15.28a), the HF
balanced-modulated oscillations is formed, which have the initial phase switched
through half cycles of low-frequency signal from 90° into 270° or vice versa. After
summation of HF oscillations of non-directional antenna (Fig. 15.28b) and balancedmodulated, the phase-modulated oscillations are formed (Fig. 15.28c).
For time intervals t 1 , t 2 and t 3 (Fig. 15.28d), vector diagrams of combined
signals are shown: U non —signal vector of non-directional antenna; U mBM —vector of
balanced-modulated (BM) signal, the direction of which differs from a direction of a
previous vector at 90° or 270°. Deviation angle of combined signal vector from direction of U non vector corresponds to a phase shift ϕ. As we can see from Fig. 15.28,
a phase shift value depends on amplitude value of BM oscillation, and a sign—from
relation of phases of comparing signals.
As the amplitude U mBM is changed according to a law sin t, where = 2π F,
F—a frequency of low-frequency signal, then ϕ is change according to the same
law.
From Fig. 15.28e, we can see:
tgϕ =
U mBM
U non
=
U mBM max
U non
sin HA sin t,
(15.35)
where U mBM max —maximum value of BM oscillation at HA = 90° or 270°.
Phase modulation index (maximum phase deviation):
ϕ = arctg
U BM max
U non
sin HA sin t
(15.35)
As you can see, phase modulation index depends on HA.