U.S. patent number 3,878,523 [Application Number 05/328,820] was granted by the patent office on 1975-04-15 for generation of scanning radio beams.
This patent grant is currently assigned to Commonwealth Scientific and Industrial Research Organization. Invention is credited to John Paul Wild.
United States Patent |
3,878,523 |
Wild |
April 15, 1975 |
Generation of scanning radio beams
Abstract
A radio aerial is disclosed by which planar beams can be swept
through wide angles in space without the need for moving aerials,
and which may be used in radio location systems using scanning
beams and in radio navigation systems. The aerial consists of a
reflector the surface of which is, or approximates part of, a
surface produced by the rotation of a generating curve of finite
length about an axis, and a plurality of radiating elements
disposed about an arc centred on the axis of rotation which are
excited sequentially. In its simplest form the reflector is part of
a right circular cylinder and the radiating elements are located at
the half radius of the cyliner.
Inventors: |
Wild; John Paul (Strathfield,
New South Wales, AU) |
Assignee: |
Commonwealth Scientific and
Industrial Research Organization (Canberra, AU)
|
Family
ID: |
25629299 |
Appl.
No.: |
05/328,820 |
Filed: |
February 1, 1973 |
Foreign Application Priority Data
Current U.S.
Class: |
342/408; 343/840;
343/779 |
Current CPC
Class: |
H01Q
19/17 (20130101); H01Q 3/245 (20130101) |
Current International
Class: |
H01Q
19/17 (20060101); H01Q 19/10 (20060101); H01Q
3/24 (20060101); G01s 001/16 () |
Field of
Search: |
;343/779,854,18M,840 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Attorney, Agent or Firm: Ladas, Parry, Von Gehr, Goldsmith
& Deschamps
Claims
What I claim is:
1. An aerial for producing a scanning radio beam comprising:
a. a substantially cylindrical reflector having a surface the shape
of which is substantially the surface produced by the partial
rotation of a generating curve of finite length about an axis, the
reflector being so mounted that the said axis is the axis of
rotation of the said beam, and
b. a plurality of radiating elements disposed about an arc centered
on the said axis, and of radius substantially half that of the
cylindrical reflector, each element being arranged to radiate a
beam towards the reflector, which radiation is reflected from the
portion thereof,
said beam being divergent in the direction parallel to the axis to
produce reflected radiation comprising a planar fan beam, with the
plane of the fan parallel to the said axis of rotation, and
including means to excite a group of four adjacent elements
sequentially, the power to each element being modulated with time
in accordance with a cosine-squared function, a detector responsive
to the power of incident radiation being located substantially at
the axis of symmetry of the reflected radiation to monitor the
scanning beam.
2. An aerial as defined in claim 1, in which the output of the
monitoring detector is arranged to control an error detector which
is operable to ensure a substantially constant amplitude of the
transmitted radiation.
3. An aerial as defined in claim 1, in which said means to excite
comprises a single power source connected to each element by
waveguide connections, and a plurality of modulated ferrite or
diode switches to switch power from the source to selected ones of
the elements.
4. An aerial as defined in claim 1, in which a second reflecting
surface is provided to intercept a portion of the reflected
radiation and reflect the intercepted radiation in a direction
extending behind the first reflector.
5. An aerial as defined in claim 4, in which the second reflecting
surface is a plane reflector.
6. An azimuth aerial comprising two aerials according to claim 1
located at the stop end of an aircraft runway on either side
thereof, each of said aerials being arranged to perform a single
scanning cycle and then be inactive while the other aerial performs
a single scanning cycle.
7. An elevation aerial comprising an aerial according to claim 1,
in which the generating curve is a straight line with the central
portion thereof shaped to reduce obstruction of the reflected
radiation by feed elements which are located in the path
thereof.
8. An aerial as defined in claim 7 in which said means to excite
comprise a single power source connected to each element by
waveguide connections, and a plurality of modulated ferrite or
diode switches to switch power from the source to selected ones of
the elements.
9. A ground-station aerial for use in a scanning beam instrument
landing system of an aircraft navigation system comprising
a. A substantially cylindrical reflector having a surface the shape
of which is substantially the surface produced by the partial
rotation of a generating curve of finite length about an axis;
b. A plurality of radiating elements facing said reflector and
disposed about an arc centered on said axis, the center of each
element being spaced from its neighbor by not substantially less
than one half wavelength and being arranged to radiate a beam
towards a corresponding portion of the reflector, the radius of the
arc of radiators being substantially half that of the cylindrical
reflector and also not substantially less than the width of
reflector illuminated by the primary beam in the plane of the arc;
and
c. A single power source connected to each element by branching
waveguide or coaxial connections having diode or ferrite switch
means to modulate and switch the source of power to excite
sequentially a small group of adjacent elements in accordance with
a predetermined modulation pattern,
whereby radiation emitted from said elements is after reflection by
the reflector, formed into a single narrow and precisely-defined
high-quality radio beam which may be continuously and smoothly
scanned by the aerial over any selected range of angles about the
said axis, the beam being accurately planar with its plane parallel
to said axis.
Description
This invention is concerned with the generation of radio beams and,
more particularly, seeks to provide improved means by which planar
beams can be swept through wide angles in space without the need
for moving aerials. The invention therefore has application in
radio location systems using scanning beams and in radio navigation
systems in which a wide ranging pattern of `signals in space` is
established for the guidance of aircraft and ships. Although the
present invention is not confined to air navigation systems (other
applications include radar and communication systems), the
requirements of such systems are most demanding and, for the
purpose of illustration, the invention will be described in
relation thereto.
The Radio Technical Commission for Aeronautics (RTCA) of the United
States has called for the design of a ground base transmitter
system which will provide a pattern of signals in space so that
aircraft approaching a runway from an azimuth angle of up to
60.degree. on either side of the runway center line and an
elevation of up to 20.degree. can be guided by the pattern. Some
azimuth back-cover of 40.degree. on either side of the runway is
also desirable for use by aircraft which abort their landing; in
addition, the renewal of information in the aircraft by the system
should be equivalent to at least five updatings per second. After
examining available systems, the RTCA concluded that, until a
satisfactory electronically scanned system for producing a pattern
of planar beams could be devised, large mechanically rotated
aerials would have to be employed. In particular, a system
including two aerials, up to 20 feet across, mounted back-to-back
and rotated mechanically at 21/2 revs per second (5 revs per second
if a back-to-back configuration is not used) was preferred. Such a
system produces a planar beam but does not have the geometrical
precision required to enable a time coded system (the simplest) to
be used and is therefore restricted to transmitting an azimuth
controlled frequency coded signal which is extravagant in its use
of bandwidth and requires elaborate signal processing in the
aircraft.
One example of a non-mechanical technique for generating signals in
space is the `Doppler` system in which the separate aerial elements
-- typically about 100 -- of a linear array are sequentially
excited to simulate a moving radiator. Each element is excited in
turn and a different frequency is seen at each angle from the
array. However, this system demands the maintenance of a precise RF
phase relationship between individual elements and, since conical
beams are produced, in order to simulate a planar beam system, a
second linear array perpendicular to the first is required and data
from the two arrays must be processed by an airborne computer
carried in each aircraft using the system.
Another beam-generating system which has been proposed for aircraft
navigation involves the use of a phased array. In this system a
linear array of aerial elements is excited by a common source, but
a different and progressive time delay is inserted between the
elements and the source. The time delays can be changed to produce
scanning beams but, again, a very precise RF phasing is required
and the beam is conical rather than planar.
A further system that has been proposed for the generation of
scanned narrow beams of radiation has been effected by irradiating
a mirror having the form of part of a sphere (which is sufficiently
similar in shape to a paraboloid over a limited aperture to produce
effective focusing), the beam being scanned by physically moving
the focal radiating element around a focal are centered on the
centre of the circle. An example of this technique for generating
plane wavefront, scanned beams is found in the paper by J. Ashmead
and A. B. Pippard in J. Inst. Elec. Eng. 93, part 3A, pages
627-632, 1946.
The present invention similarly depends on the focusing power of a
mirror of circular cross section but scanning is achieved by
simulating the physical movement of the radiating element by the
use of many closely spaced radiating elements around an arc which
are progressively excited. Also instead of using a spherical
mirror, which produces a narrow pencil beam, the present invention
employs a cylindrical or nearly cylindrical mirror so as to produce
a narrow fan (planar) beam.
One object of the present invention is to provide an aerial for the
generation of scanning planar radio beams, and also to provide a
method for the generation of such radio beams. In addition, it is
an object of a development of the present invention to provide
systems, such as aircraft navigation systems (including instrument
landing systems), which incorporate scanned planar radio beams. In
a particular example, the present invention provides an instrument
landing system which complies with the minimum requirements
proposed by the RTCA and which can utilise a time coded arrangement
for angle measurement.
According to the present invention, an aerial for producing a
scanning beam comprises:
a. a reflector having a surface the shape of which is or
approximates part of the surface produced by the rotation of a
generating curve of finite length about an axis, the reflector
being so mounted that the said axis is the axis of rotation of the
said beam, and
b. a plurality of radiating elements disposed around an arc centred
on the axis of rotation, each element being arranged to radiate
towards the reflector, which radiation is reflected from a portion
thereof, whereby, by exciting the elements sequentially, a beam of
radiation transmitted from the aerial scans through a range of
angles.
Depending on whether the angular width of the primary radiation
from the radiating elements is small or relatively large, the
secondary (reflected) radiation will comprise a pencil beam or a
planar fan beam. The curve of the reflector may be a parabola or
circle, or any other shape. In its simplest form the generating
curve is a straight line, i.e., the reflector is a right
cylindrical surface. In many instances, however, particularly with
horizontally (azimuth) scanning beams where it is desirable to
shape the beam in the vertical plane, it is necessary to shape the
curve to distribute the energy of the reflected beam in a desired
angular distribution. For example, enhanced sensitivity in low
elevations, yet avoiding ground reflections, can be achieved if the
reflector is shaped so that the power polar diagram of the
secondary beam approximates to a cosecant-squared pattern. (Some
relevant results on the geometry of such mirrors have been reported
by L. J. Dolan, in report No. RADC-TR-59-231 of the Radiation
Engineering Laboratory, dated December 1, 1959.)
In a preferred form of elevation signal generation aerial, the
reflector is essentially a right circular cylindrical shape, but
with a modified, for example, parabolic, central portion.
If the curve which is rotated about an axis to produce the
reflector surface shape is substantially a straight line, the feed
elements are preferably located a distance from the reflector which
is approximately equal to the half radius of the circle of rotation
of the line.
A step-scanned planar beam is produced by actuating each feed
element in turn. In a preferred form of the invention, a
quasi-continuous scanning by the planar beam is obtained by
concurrently exciting several, e.g. four, adjacent feed elements in
a particular modulation. It has been found that modulating the
excitation of each feed element so that its transmitted power
varies with time according to a class of functions such as
cosine-squared, will produce quasi-continuous scanning.
The present invention also includes a method for generating and
scanning a radio beam through space about a given axis, which
method comprises sequentially exciting a series of fixed aerial
feed elements arranged about the said axis so as to initiate a
series of primary beams, and reflecting said primary beams from
respective portions of a common reflector aerial element having a
shape which is part of the surface defined by the rotation of a
curve about the said axis.
Insofar as the aerial and method of the present invention may be
used in radar systems, communication systems and the like, the feed
elements may be replaced in part or entirely by receiving
elements.
The number of radiation feed elements is preferably greater than
10; for a system in which the beamwidth is 1.degree. there are
typically 64 elements per 45.degree. angle (i.e., the angular
separation of feeds is about 0.degree..7 ). The feeds will
generally be located on an arc of a circle (for special purpose
aerials they could be otherwise located). In an azimuth signal
generating system they are conveniently off-set to be out of the
path of the radiation from the reflector. In an elevation
generating system it may not be possible to employ off-set feeds
and it may be desirable to compensate for the blocking of the
radiation by the feed by procedures to be described below.
In an elevation generating system the horizontal angle of coverage
of the fan-like beam can be increased by increasing the horizontal
length of the cylinder. To reduce this length for a given
horizontal angular coverage, the reflector may be convexly curved
(as viewed from the focal side) though this procedure may introduce
some degradation of the beamwidth. Alternatively a secondary
reflector or lens may be used in the path of the radiation from the
primary reflector to expand the fan angle of the planar beam.
The frequency of radiation may be any suitable value, but C-band or
K.sub.u -band frequencies are thought to be most appropriate in the
case of instrument landing systems. Switching of feed elements may
be performed by known electronic means, for example diode switching
or switching using ferrite devices. The beam may be identified by a
timed system or any other suitable code. With a time identified
arrangement, stepped or continuous scanning may be used to effect
beam rotation, and in either case, one-way or two-way scanning can
be utilised.
A plane mirror may be used with the azimuth aerial to provide
overshoot information for an instrument landing system.
The present invention also encompasses instrument landing systems
and other aircraft navigation systems, tracking systems, radar and
communication systems which include the scanning aerial and method
of this invention. In particular the present invention provides an
instrument landing system which, in a simple example has a first
aerial of the form described above which is adapted to produce a
planar beam which scans in azimuth, and a second aerial of the form
described above which is adapted to produce a second planar beam
which scans vertical angular elevations, the generation of the
planar beams being such that they may be utilised by aircraft for
navigation and/or landing purposes.
A description of embodiments of the present invention will now be
given with reference to the accompanying drawings, in which:
FIGS. 1a and 1b are, schematically, a plan and elevation,
respectively, of an azimuth signal generating aerial of a type
which can be used in an instrument landing system,
FIGS. 2a and 2b are a schematic plan and elevation, respectively,
of a modified azimuth signal generating aerial which incorporates a
back-reflecting arrangement,
FIGS. 3a and 3b are, respectively, a schematic plan view and
elevation of an elevation signal generating aerial,
FIG. 4 illustrates how power may be supplied to excited feeds at
sequential time intervals to obtain quasi-continuous scanning,
FIG. 5 depicts one form of modulator configuration that may be used
for feed power modulation, and
FIG. 6 shows an example of cycle of time-shared functional
operations in a landing system suitable also for an area navigation
system.
Referring to FIG. 1a, a reflector 3, shown in section, has an axis
of symmetry C. Radiation feed elements R(1), R(2), . . . R(n) . . .
R(N) are located at the half radius of the circle of generation of
the reflector and are adapted to beam radiation to a segment of the
reflector 3. Because the geometry of the circle and parabola are
essentially the same over small angles, a well-collimated parallel
beam of radiation is reflected from the surface of reflector 3. The
angular dispersion of the beam in a vertical direction will depend
upon the shape of the reflector 3 in the vertical plane. In FIG.
1b, this shape is a straight line, but as indicated above, and as
shown for example in FIG. 2, it may be any desired shape. By
switching each radiator feed element R(1) . . . R(n) . . . R(N) on
in turn, a beam is set up which scans from one extreme position to
another, i.e., from beam B(1) to beam B(N) in FIG. 1a. For example,
in an instrument landing system providing azimuth beams which are
1.degree. wide and scanned over 120.degree., about 200 feed
elements R(n) will be required for satisfactory performance.
As shown in FIGS. 1b and 2b, the feed elements are preferably
offset below the reflector so that they do not obstruct the
beam.
In the embodiment of FIGS. 2a and 2b the reflector 3 has an
optional additional reflector portion 3A formed atop it (it could,
in other embodiments, be located below it), so shaped that
radiation incident on it from a feed element R(n) is reflected
above the aircraft approach elevation (generally about 20.degree.)
on to a mirror 4, typically a plane mirror, so positioned above the
main beam that it reflects its incident radiation backwards at an
angle extending from the horizontal to an elevation determined by
the shape of portion 3A. Such azimuth information is required for
aircraft which overshoot the landing position and cannot land. This
embodiment, however, can only be used when the combined height of
the primary reflector 3 and the back reflector 4 is such that the
maximum obstacle height for the airport is not exceeded. In
general, the preferred arrangement for providing overshoot
information is to have two azimuth transmitting aerials, one at
each end of the runway, each directed along the runway.
A separate aerial using substantially the same cylindrical optics
arrangement and plurality of feeds as the azimuth aerial described
above, can be used to generate planar beams which scan vertically.
In the RTCA required instrument landing systems, a beam which is
horizontally planar and has an angular width of 120.degree. has to
be scanned 20.degree. vertically. One form of elevation scanning
aerial is shown in FIGS. 3a and 3b. A plurality of wide-angle feed
elements 31 are located on the central half-radius of a cylindrical
reflector 30. If the feed elements 31 cannot generate a wide-angle
(e.g. 60.degree.) fan-like beam, the reflected beam can be expanded
by inserting a lens in the reflected beam. A conventional slatted
lens is suitable for this purpose. Alternative possible
arrangements for expanding the width of the fan-like beam include
shaping the reflector 30 to present a convex surface to the beam
from elements 31.
One problem with using a cylindrical reflector is that the feed
elements, if within the path of the reflected beam, obstruct the
reflected beam and can give rise to diminished intensity of the
secondary radiation in certain directions. Such a situation is
undesirable in aircraft navigation systems. It can, however, be
avoided by suitable shaping of the reflector 30 at its central
station as shown at 32, so that the resultant reflected beam has a
substantially plane wavefront at its central portion, which
experiences minimal obstruction by the feed elements.
To obtain optimum accuracy in angle measurements with instrument
landing systems, it is important to maintain equal amplitude of the
signals in space radiated in different directions. A feature of the
aerials of the present invention is that, in use, the centres of
the beams produced by the individual feed elements pass through a
single point which, in the case of the illustrated arrangements of
FIGS. 1a to 3b inclusive, is located on the axis of symmetry, C, of
the reflector. A single detector may therefore be installed at this
point and coupled to a conventional error corrector to maintain the
constant amplitude of the beam radiation.
In use in an instrument landing system, the beams of radiation may
be distinguished from each other either by (a) exciting beams at
different times in a known sequence, particularly a simple
progressive sequence causing the beam to rotate about the axis, or
(b) by applying a code to each beam by modulation or frequency
change. In the former case, which here is called a time coded
system, continuous or quasi-continuous scanning may be achieved by
a progressive excitation of a group of feeds with appropriate
modulated intensity. Where continuous scanning is not needed, the
simpler stepped scanning may be utilised, with each feed element
excited sequentially.
It has been found in practice that continuous or quasi-continuous
scanning can be satisfactorily achieved by simultaneously
transmitting power from four feed elements, the power being
modulated as a suitable function of time (for example,
cosine-squared) as shown in FIG. 4. The quasi-continuous nature of
the scanning is caused by the continuous movement of the excitation
pattern along the feed system.
The switching system for effecting modulation of the feed element
outputs may be any suitable arrangement. Ferrite switches have been
found satisfactory but diode switching arrangements may also be
used. One layout for modulators for an aerial havng 32 feeds is
shown in FIG. 5. In this layout, all the waveguide interconnections
are in the same plane and no right angle bends are necessary.
Aperture blocking problems are minimised, but a form of continuous
modulation is needed to achieve beam scanning. The power output of
the individual feeds is determined by the current waveforms applied
to the modulators.
With a time-coded system, a one-way or two-way system of beam
rotation can be applied. In a typical one-way system, an
omnidirectional reference pulse is emitted at the start of, or a
known time before, the scan. The N feed elements (or groups
thereof) are then excited from R(1) to R(N). At the end of this
sequence, a new reference pulse is emitted and the sequence is
repeated.
With a two-way system, the elements are excited from R(l) to R(N)
and then the excitation is reversed from R(N) to R(l). No
omnidirectional pulse is required as part of the angle-measurement
signal, the ambiguity as to which direction a beam is being scanned
when a pulse is received by a receiver in the space swept by the
beam being avoided by the use of a function identification signal,
such as a combination of three FM tones on the scanning beam with a
different tone combination for each function, or, in the case where
no function identification is required, and the delay between
transmissions exceeds the scanning period, by utilising the
shortest time between pulses. A preferred signal format is the
"to-to-fro" pulsed system, in which the scan is from element R(1)
to R(N), then R(1) to R(N) followed immediately by R(N) to R(1).
Function identification can be achieved by variation of the lag
between the first "to" scan and the second "to" scan. Further
information may be encoded by varying this lag from scan to
scan.
The azimuth component of an area navigation system for aircraft may
be set up using the present invention. Typically three aerials
would be required to cover 360.degree. azimuth.
FIG. 6 shows one way in which a cycle of time-shared functional
operations for a compound landing system may be constructed. This
system provides two azimuth scans of 180.degree. (the second a
missed approach, back azimuth signal) followed by three elevation
scans, the last of which is a missed approach elevation signal. The
updating rate is 20 repetitions per second with a scanning rate of
40 microseconds per degree of scan.
With a one-way timed system, a separate omnidirectional aerial is
required for each function to transmit a synchronising pulse as
well as reference data. With a two-way system, an omnidirectional
aerial is not required except possibly for reference data (in which
case, timing is not critical and one omnidirectional aerial could
be used for all functions of the entire landing or navigation
system). If a rear mirror arrangement is to be used to provide
overshoot information, the omnidirectional aerials must be split
into two semi-omnidirectional aerials.
Thus it will be seen that one form of the present invention
comprises an instrument landing system or area navigation system
for aircraft, the system having at least one azimuth and one
elevation aerial, each having axial symmetry and each fed by a
plurality of feeds to produce planar beams, as described above, and
each aerial incorporating a signal monitor for intercepting its
associated transmitted beam and producing an output controlling an
error detector which is operational to ensure a constant amplitude
of the transmitted radiation. Additionally the system may include
an omnidirectional transmitter for transmitting identification and
auxiliary data.
A feature that may be included in airport arrangements is the
division of the azimuth signal generator into two aerials, one at
either side of the stop end of the runway and operated on alternate
scans or on different frequencies. This configuration permits the
azimuth part of the proposed landing system to be operated at the
same time as an existing Instrument Landing System (ILS) localiser
and also provides added integrity because azimuth information is
lost only when both aerials fail. It has the further advantage that
the aerials may be mounted on towers without violating operational
height restrictions and so ease the problem of obtaining adequate
signal strength near the touch-down zone at the approach end of the
runway. Small aircraft can take the average of the two signals and
fly along the centre-line, in the same manner as existing localiser
systems are used.
The advantages of systems using the basic aerial of the present
invention and the planar (fan-like) beams generated thereby -- such
as rapid updating and the absence of critical phase adjustment or
rotating parts -- are also of value in other applications. For
example, the present invention can be applied to rapid scanning
radar and communications systems using a single azimuth beam (or
group of beams) whereby different messages can be almost
simultaneously passed to different destinations without risk of
confusion. The basic invention described in this specification is,
therefore, a tool of considerable value.
* * * * *