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Description  |
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BACKGROUND OF THE DISCLOSURE
The field of this invention is radio communications systems, and more
particularly, systems for radio direction finding.
One form of prior art system utilizes a radio frequency (RF) transmitter
and receiver located at opposite ends of a radio communication path, with
a continuously rotating directional antenna located at either the
transmitting or receiving end of the path, and an omni-directional antenna
located at the other end. A reference signal is provided at the receiving
end of the path to identify a particular point (such as North) in the
cycle of the rotating antenna. The rotating antenna causes a periodic
modulation of the RF signal (AM, or FM, or a combination of both), and
this modulation is detected by the receiver. The demodulated output signal
provided by the receiver is periodic at the rotational frequency of the
antenna, with the phase of that output signal relative to that of the
reference signal being a measure of the relative bearing of the receiver
and transmitter locations. In known prior art systems, the rotating
antenna may be physically rotated (as by a motor), or alternatively, with
a multiple element configuration, the elements may remain fixed with the
pattern rotated by an electronic modulating arrangement.
One problem encountered by the prior art radio direction finding systems is
bearing indication error due to phase shift introduced by the receiver.
The usual remedy in such systems is a phase adjustment provided in the
receiver to offset the characteristic phase delay for the particular
receiver. In general, the characteristic phase shift varies with the
individual receivers, and thus each receiver must be separately adjusted.
When the antenna rotational frequency is low, (such as 30 Hz of the
so-called Omnirange systems) relatively easily compensated delays in the
audio circuits are generally the only consideration. However, if the
antenna rotational frequency is high, and approaches the bandwidth of the
receiver, compensation poses a much more difficult problem, particularly
if the transmitter center-frequency deviates somewhat from its nominal
value. Under such conditions, the time delay (caused primarily by
characteristics of the IF filters) suffered by the intelligence-bearing
side bands of the RF signal generally varies with various system
parameters such as receiver tuning. These variations can easily be
substantial compared with the rotational period of the antenna, thereby
leading to correspondingly high error in the bearing indication. Since the
frequency-determining components (e.g. crystals) of both the transmitter
and receiver are necessarily provided with certain tolerances with regard
to center-frequency, even in the most rigidly controlled communication
systems, this source of error may become substantial.
Thus, in the prior art radio direction finding systems, variations in a
number of factors can cause substantial errors due to receiver introduced
phase shift, with such errors not always being easily predicted or
corrected. One such factor is the variation in tuning of either the
transmitter or receiver from the exact "center-frequency" (the variation
may be dependent upon temperature, power supply, or simply arise because
two different transmitters or receivers happen to be tuned within their
specified tolerances but slightly differently). A second factor
contributing such error is variation in the rotational frequency of the
antenna (such as might be due to temperature, or power supply variation).
A third source of error lies in the tuning and adjustment of the receiver
audio filters or other circuits, which may be affected by temperature,
power supply, aging so that frequent re-calibration is required.
It is an object of the present invention to provide a radio direction
finding system which minimizes the effect of receiver-introduced phase
shift.
It is another object of the present invention to provide a radio system for
producing a bearing indication which is substantially free from error due
to receiver-introduced phase shift.
SUMMARY OF THE INVENTION
According to the present invention, a radio direction finding system
includes a transmitter and a receiver located at opposite ends of a radio
communication path. In one form of the invention, the transmitter includes
a rotating directional antenna and the receiver includes an
omni-directional antenna. In an alternative form, the transmitter includes
an omni-directional antenna and the receiver includes a rotating
directional antenna. In both of these forms, the directional antenna
includes a means to periodically reverse the direction of rotation of that
antenna at a rate low compared to its rotation frequency, but high
compared to changes in bearing occurring between the transmitter and
receiver.
A reference signal is provided at the receiving end of the path to identify
a particular point of the rotational cycle of the rotating antenna. The
periodic modulation of the RF signal caused by the rotation of the antenna
is detected by the receiver which generates a demodulated output signal.
This demodulated signal provides a measure of the relative bearing of the
receiver and transmitter locations.
In operation, for each of the two directions of antenna rotation, the
receiver site processing system produces a bearing signal which includes
bearing error, but the error portions from each direction of rotation are
equal in magnitude and have opposite polarities. The present system
averages the bearing indicating signals for the two directions of antenna
rotation, with the result that the bearing error portions cancel, yielding
a bearing-indicating signal which is relatively free from
receiver-introduced phase error.
As a consequence, the configuration of the present invention is relatively
immune to variations in transmitter or receiver tuning, as well as
variations in the rotational frequency of the antenna and in the tuning
and adjustment of receiver filters and other circuits.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects of this invention, the various features
thereof, as well as the invention itself, may be more fully understood
from the following description, when read together with the accompanying
drawings in which:
FIGS. 1 and 2 show, in block diagram form, radio direction finding systems
in accordance with the present invention;
FIG. 3 shows in detailed block diagram form, a receiver station for the
system of FIG. 1; and
FIG. 4 shows waveforms representative of signals in the receiver station of
FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1 and 2 illustrate alternative configurations for the present
invention. In FIG. 1, a radio communication link 10 is established between
a transmitting antenna 14 and associated transmitter 12 and a receiving
antenna 16. The antenna 14 is characterized by substantially
omni-directional radiation pattern, and the antenna 16 is characterized by
a directional pattern (indicated by the dotted line 20). The antenna 16 is
adapted to rotate its characteristic pattern 20 in a manner whereby the
direction of rotation is periodically reversed at a rate relatively low
compared to the angular velocity. The angle .theta. denotes the current
angular position of the pattern 20 relative to a reference direction N
(North). The antenna 16 further includes a generator for producing a
periodic phase reference signal at the frequency of rotation of the
antenna and having a fixed time relationship to the rotational position of
the pattern 20. The form of the phase reference signal may vary for
different embodiments. For example, this signal may be a pulse signal, as
described more fully below in conjunction with the operation of the FIG. 1
configuration. In other embodiments, such as in the FIG. 2 configuration,
the phase reference signal may be sinusoidal to minimize bandwidth
requirements over the communications link between the transmitters and
receiving antennas.
In the FIG. 1 configuration, the antenna 16 also includes a generator for
producing a scan direction signal which is representative of the current
direction of rotation of the antenna 16, i.e. clockwise or
counter-clockwise as shown in the figures. The antenna 16 further provides
an RF signal which corresponds to the RF signal received from the
transmitter 12 by way of antenna 14 and link 10, as modulated by the
rotating of the directional pattern of antenna 16.
The RF signal received at antenna 16 is applied to receiver 22. In the
illustrated embodiment, receiver 22 is an AM receiver which detects the
modulation to provide a phase modulated signal at the frequency (f.sub.a)
of rotation of the antenna 16. The phase modulated signal is applied to a
bandpass filter 24 which is tuned to f.sub.a. The filtered phase modulated
signal is in turn applied to a phase demodulator 26. The phase reference
signal and scan direction signal are applied to a sync generator 30 which
provides the sampling signals for the phase demodulator 26. Demodulator 26
provides a demodulated output signal which is representative of the
relative bearing of antenna 14 with a respective antenna 16. This output
signal in the present embodiment is applied to a display 34. By way of
example, the display may comprise a continuous resolving type display,
either in a two-phase or four-phase form, with an associated integrating,
or averaging, network to filter out signal variations due to the periodic
reversal of antenna rotation direction. Alternative display means may be
used.
The configuration of FIG. 2 is similar to that of FIG. 1 except that a
communication link 40 is established between a rotating directional
antenna 42 (characterized by the directional radiation pattern 44, and
driven by transmitter 46) and a substantially omni-directional antenna 48
at the receiver site. In the FIG. 2 configuration, the direction of
rotation of the pattern 44 of antenna 42 is periodically reversed at a
frequency which is relatively low compared with the rate of rotation of
that pattern, in the same manner as the rotating antenna 16 of the FIG. 1
configuration. The antenna 42 also includes an associated omni-directional
radiating element which is driven by transmitter 46 to provide a phase
reference signal (bearing a fixed time relationship to the antenna
position) and a signal representative of the current direction of rotation
of the rotating element of antenna 42. The current direction signal and
phase reference signal may use the same or a different carrier but are
provided in a conventional manner so that they may be readily separated at
the receiver site. In alternative embodiments, the phase reference and
direction of rotation signals may be transmitted by way of the rotating
element of antenna 42.
The omni-directional antenna 48 receives both the RF signal generated by
the rotating element of antenna 42 and also the current direction and
phase reference signals from the omni-directional element, and applies
these signals to a receiver 50.
In the illustrated embodiment, receiver 50 is coupled to an associated
direction finding (DF) signal separator and FM detector 52 which produces
a phase modulated signal at the frequency (f.sub.a) of rotation of the
rotating element of antenna 42. The phase modulated signal is applied to a
bandpass filter 54 which is tuned to f.sub.a and whose output is in turn
applied to a phase demodulator 56. The receiver 50 also is coupled to a
phase reference and current rotation direction signal separator 60 which
provides output signals representative of the phase reference signal and
current rotation direction signal to a sync generator 64. The sync
generator 64 and phase demodulator 56 operate in a manner similar to the
sync generator 30 and phase demodulator 26 of the FIG. 1 configuration,
and to provide a bearing signal which may be applied to a display 66
(which may function in the same manner as display 34).
In both configurations, with the rotating antenna directed in a
counter-clockwise direction, the output signal from the audio filter is
proportional to cos (wt+.theta.+.alpha.), where w=2.pi.f.sub.a, .theta.
equals the relative bearing of the transmitter with respect to the
reference direction N, and .alpha. corresponds to the phase delay
introduced by the radio receiver and bandpass filter. During the time
period when the rotating antenna is directed in a clockwise rotation, the
bandpass filter output signal is proportional to cos (wt-.theta.+.alpha.).
When the periods of the counter-clockwise and clockwise rotations are
equal, the phase demodulator produces an output signal from the phase
demodulator which may be averaged over an integral number of cycles of
antenna direction switching so that the phase delay .alpha. introduced by
the receiver and bandpass filter exactly cancels. For a resolver type
display, the vertical coil is driven by a signal proportional to cos
(.theta.+.alpha.)+cos (.theta.-.alpha.), or cos.theta.cos.alpha., and the
horizontal coil is driven by a signal proportional to sin
(.theta.+.alpha.)+ sin (.theta.-.alpha.), or sin.theta.cos.alpha., with
the result that the resolver pointer exactly indicates the angle .theta.,
regardless of the phase shift .alpha. introduced by the receiver. The
magnitude of the vertical and horizontal force components on the pointer
(i.e. the forces that hold the pointer in place) is thus proportional to
cos.alpha., which is a maximum at .alpha.=0. Consequently, the system of
the present invention may be adjusted for maximum performance by
introducing a compensating phase delay to the reference signal to match
the expected, or measured, delay .alpha. for the receiver.
FIG. 3 illustrates in detailed block diagram form an alternative form
exemplary receiver site station suitable for use in conjunction with a
transmitter providing an RF signal in the Marine Band 156-163 MHz. By way
of example, receiver 22 may comprise a VHF/FM model no. 655, manufactured
by Hy-Gain Electronics Corporation, Lincoln, Nebraska. The antenna 16
comprises four quarter wave stub antenna elements, equally spaced around
the circumference of a reference circle 68 and mounted on a ground plane
formed by four horizontal reflector elements. The quarter wave stub
elements are denoted in FIG. 3 by the encircled reference designations A,
B, C and D. The four element antenna is adapted to provide a directional
pattern which rotates at 3.5 KHz, by selectively gating the four stub
elements to be active for equal periods in a continuous sequence around
the circle 68. With any one antenna element activated, the other three
elements are relatively transparent to an arriving wave front.
The characteristic pattern for antenna system 16 is effectively rotated by
the electronic switching arrangement shown in FIG. 3 and which comprises a
crystal oscillator 61, divider network 63, decoder-driver logic 65, and
PIN diodes 70, 72, 74 and 76. In the illustrated embodiment, the PIN
diodes are type MPN 3401, manufactured by Motorola Semiconductor Products,
Inc., Phoenix, Arizona. The oscillator 61 provides a 3.58 MHz signal which
is divided down by divider network 63 to form squarewave signals at 7.0
KHz, 3.5 KHz, and 27.3 Hz. The squarewave signals are applied to
decoder-driver logic 65 which provides a sequence of 71 .mu.sec gating
pulses to the switching diodes 70, 72, 74 and 76 such that the stub
elements are successively gated on in the order A, B, C and D for a first
period and then successively turned on in the order D, C, B, A for a
second period. The 27.3 Hz signal defines two periods to be 18.3 msec in
duration.
With this configuration, logic 65 produces the stub element drive waveforms
A, B, C and D illustrated in the correspondingly referenced lines of FIG.
4, wherein the low levels of those signals activate the correspondingly
identified antenna elements. The 27.3 Hz scan direction signal is denoted
by the reference designation X in FIGS. 3 and 4 and controls the periodic
reversal of the direction of scan for antenna 16.
The sync generator 30 is coupled to the decoder-driver logic 65 in a manner
to provide a pulse coincidental with each fall time of the drive signal
for antenna element C. This pulse train is identified by the designation
COS in FIGS. 3 and 4 and provides the phase reference signal for phase
demodulator 26. The COS signal represents the strobe pulse for the phase
demodulator 26 which bears a fixed time relationship to the rotation of
the pattern associated with the antenna 16.
Sync generator 30 also produces a pulse train which includes pulses
coincident with the fall time of the drive signal for element D. This
latter signal is identified by the designation +SIN or -SIN in FIGS. 3 and
4. During the time period when the antenna scan direction is clockwise as
shown (sequence A, B, C, D), each pulse of the SIN signal is ahead of an
associated COS pulse by one quarter of the antenna rotational period,
thereby establishing the +SIN signal strobe pulse for demodulator 26.
During the time period when the antenna scan direction is reversed
(sequence D, C, B, A), each pulse of the SIN signal is behind an
associated COS pulse by one-quarter of the antenna rotational period,
thereby establishing the -SIN signal for strobing demodulator 26.
In the present embodiment, the phase demodulator 26 is a quad-bilateral
switch, such as the CMOS type CD 4016AE, manufactured by RCA, Somerville,
N.J., with the output from audio filter 24 being applied in non-inverted
form to pins 4 and 1 of that integrated circuit, the output from audio
filter 24 being applied in inverted form to pins 8 and 11, the COS signal
being applied to pins 12 and 13 and the .+-.SIN signal being applied to
pins 5 and 6. The output signals from pins 3 and 9 (denoted +S and -S) of
the integrated circuit are complementary representations of the phase
modulation during the counter-clockwise rotation of the antenna pattern,
and the output signals from pins 10 and 2 (denoted +C and -C) are
complementary representations of the phase modulation during the clockwise
rotation of the antenna pattern.
The +S, -S and +C, -C signals are then applied to display 34 where the
respective pairs of signals are integrated (to remove the 27.3 Hz
components of those signals) and applied to the horizontal and vertical
coils of a 360.degree. d.c., continuous resolving-type display, such as
the type 800 C, manufactured by Pilot Instrument Corporation, Waldoboro,
Maine. In alternative embodiments, different type displays may readily be
utilized in keeping with the present invention. By way of example, the
phase modulated output from filter 24 may be applied first to a limiter
and then to a differentiating network to form a pulse at every
positive-going zero crossing. These pulses are then applied to the SET
inputs of a set-reset flip flop. The phase reference signal pulses (i.e.
the COS signal) are applied to the RESET input of that flip flop. The Q or
Q output signals of the flip flop are alternately selected, depending on
the current direction of antenna rotation, and the selected output signal
is then averaged to its d.c. value and applied to either an analog or
digital voltmeter (calibrated 0.degree.-360.degree.) to provide a bearing
indication.
The invention may be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The present
embodiments are therefore to be considered in all respects as illustrative
and not restrictive, the scope of the invention being indicated by the
appended claims rather than by the foregoing description, and all changes
which come within the meaning and range of equivalency of the claims are
therefore intended to be embraced therein.
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