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Claims  |
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What is claimed is:
1. A radar system, comprising:
controllable signal generating means including a pulse control input port,
for generating pulses of radio-frequency energy in response to pulse
control signals applied to said pulse control input port;
controllable antenna array means including (a) a plurality of antenna
elements, (b) a like plurality of solid state amplifier means, each of
which amplifier means includes an input and an output port, and is
associated with one of said antenna elements, and (c) a like plurality of
phase shifting means, each of which phase shifting means includes (i) a
radio frequency input port coupled to said controllable signal generating
means for receiving said pulses of radio-frequency energy therefrom, (ii)
a beam direction control input port, and (iii) a radio frequency output
port coupled to said input port of one of said amplifier means, for
generating at said output port of said phase shifting means pulses of
radio-frequency energy phase-shifted by an amount determined by beam
direction control signals applied to said beam direction control input
port, each of said antenna elements being coupled to said output port of
the associated one of said amplifier means by an RF coupling path, said
antenna elements being distributed in said array in a fashion which
provides a greater density of said antenna elements near the center of
said array than near the ends of said array to thereby inherently provide
a tapered amplitude distribution when each of said antenna elements
receives the same amount of power from its associated amplifier means,
said controllable array means being adapted for responding to said pulses
of radio-frequency energy of a given power by transmitting a single pencil
beam of higher power centered in a direction established by the phase
shifts of said phase shifting means in response to said beam direction
control signals applied to said beam direction control input ports of said
phase shifting means;
beam direction control means coupled to said beam direction control input
ports of said phase shifting means for, from time to time, generating beam
direction control signals, including elevation component control signals,
for application to said beam direction control input ports, for
determining the beam direction, including the elevation component, of said
beam direction;
beam multiplex and PRF control means coupled to said controllable signal
generating means, to said controllable array antenna means, and to said
beam direction control means, for generating said pulse control signals
for applying said pulse control signals to said pulse control input port
of said signal generating means for controlling the pulse recurrence
frequency of said pulses of radio-frequency energy to be responsive to
said elevation component of said beam direction control signals, and for
causing said signal generating means to produce a single pulse of said
radio-frequency energy during the transmit portion of each of recurrent
first and second transmit/receive intervals, and for causing said beam
direction control means to apply first and second azimuth beam direction
control signals to said beam direction control input ports of said phase
shifting means during said first and second transmit/receive intervals,
respectively, said first and second azimuth beam direction control signals
causing said antenna beam to be directed in first and second mutually
different azimuth directions, respectively; and
receiving means coupled to said antenna array means for responding to
returns from targets, said receiving means including pulse separation
means for separating returns received during said recurrent first
intervals from returns received during said recurrent second intervals.
2. A system according to claim 1 wherein each of said RF coupling paths for
coupling said output port of each of said amplifier means to the
associated antenna element is free of a signal attenuator.
3. A system according to claim 1 wherein said distribution of said antenna
elements in said array is a Taylor distribution.
4. A system according to claim 1 wherein said beam direction control means
includes means for generating said elevation component control signals for
causing said elevation component of said beam direction to, in a first
mode of operation, assume one of thirteen discrete elevation angles,
ranging from near zero degrees elevation to near sixty degrees elevation
angle.
5. A system according to claim 1 wherein said beam direction control means
includes means for generating said elevation component control signals for
causing said elevation component of said beam direction control signals
to, in a second mode of operation, assume one of three discrete elevation
values ranging from about zero degrees to about fifteen degrees.
6. A system according to claim 1 wherein said beam multiplex and PRF
control means including means for generating said pulse control signals
for controlling said pulse recurrence frequency of said pulses of
radio-frequency energy to be responsive to said elevation component of
said beam direction control signals in a manner which generally increases
said PRF with increasing value of said elevation component of said beam
direction.
7. A system according to claim 1, wherein said antenna array means
comprises space feed means.
8. A method for detecting targets by radar, comprising the steps of:
generating pulses of radio frequency energy;
applying said pulses of radio-frequency energy to a controllable active
antenna array means for forming a single pencil beam which may be steered
in selected azimuth and elevation directions;
tapering the aperture distribution of said antenna array means for causing
said beam to have relatively low sidelobe levels;
from time to time, controlling said active antenna array means for
directing said pencil beam at selected elevation angles;
controlling the pulse recurrence frequency of said generation of pulses of
radio-frequency energy in response to the elevation angle of said pencil
beam;
controlling said generation of radio-frequency pulses for producing a
single pulse during each of recurrent first and second transmit/receive
intervals;
controlling said antenna array means for causing said beam to be directed
at a first azimuth angle during said first transmit/receive intervals and
at a second azimuth angle different from said first azimuth angle during
said second transmit/receive intervals; and
receiving echo signals during said recurrent first and second
transmit/receive intervals; and
segregating said echo signals received during said first transmit/receive
intervals from those received during said second transmit/receive
intervals.
9. A method according to claim 8, wherein said step of generating pulses
includes the step of generating pulses of relatively low-power
radio-frequency energy; and
said applying step includes the step of amplifying said pulses of
relatively low-power radio-frequency energy in an array of solid-state
radio-frequency amplifiers.
10. A method according to claim 8, wherein said step of controlling said
active antenna array means includes the step of controlling an array of
controllable radio-frequency phase shifters.
11. A method according to claim 8, wherein, in at least one mode of
operation, said step of controlling the pulse recurrence frequency
includes the step of causing said pulse recurrence frequency to be
relatively high in response to relatively high elevation angles and
causing said pulse recurrence frequency to be relatively low at relatively
low elevation angles.
12. A method according to claim 8, further comprising the step of:
in at least one mode of operation, controlling the duration of said pulses
of radio-frequency energy to be relatively longer when said pencil beams
are directed at relatively lower elevation angles and to be relatively
shorter when said pencil beams are directed at relatively higher elevation
angles.
13. A method according to claim 8, wherein said step of segregating said
echo signals comprises the steps of:
storing said echo signals in storage means in the order in which said echo
signals are received; and
retrieving said echo signals from said storage means in an order different
from the order in which they were stored.
14. A method according to claim 13, wherein said retrieving step includes
the recurrent steps of: retrieving from said storage means those echo
signals received during said first transmit/receive intervals;
followed by the steps of:
retrieving from said storage means those signals received during said
second transmit/receive intervals, whereby signals representing echoes
received when said beam is directed in said first and second mutually
different azimuth directions are grouped into separate sequences.
15. A method according to claim 8 wherein said step of controlling said
generation of radio-frequency pulses includes the step of producing a
single pulse during each of recurrent first, second and third
transmit/receive intervals. |
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Claims |
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Description |
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This invention relates to radar systems, and especially to radar systems
intended for operation with targets which are known to be below a
predetermined altitude.
The volume of air transportation is placing increasing demands on air
traffic control systems. Air traffic control systems may utilize
surveillance radar systems for detection of aircraft approaching and
within a controlled region, beacon systems for activating transponders on
aircraft equipped therewith, communications between air traffic
controllers and aircraft, wind shear detectors, weather radar, terminal
approach systems, terminal approach systems for use with parallel runways,
wake vortex monitoring, and possibly other functions. The various
equipments required at each airport are individually expensive, their
independent siting requires extensive installation and large land area,
and also requires extensive communications lines and facilities for
interconnection of the equipments with a control center. The independent
sites must each be provided with security and maintenance, which increases
costs. Present air traffic control primary surveillance radars such as the
ASR-9 are mechanically scanned fan-beam systems.
Mechanically scanned reflector antennas for surveillance use generally use
a "cosecant squared" fan-beam radiation pattern to provide coverage in
elevation while scanning in azimuth. Mechanically scanned systems cannot
advantageously be adapted for common use for tracking and either
final-approach control or atmospheric-disturbance monitoring, because the
reflector antenna has substantial inertia, and cannot be moved quickly
from one position to another. In radar, any condition generating a
reflection, such as an aircraft or a localized weather phenomenon, is
termed a "target". For aircraft final approach control, the delay from one
rotation of the reflector antenna to the next is so long that proper
aircraft control may not be possible under all circumstances, especially
with high-speed targets such as aircraft, and atmospheric disturbance
targets may change or move significantly during a rotation. Long pulse
repetition intervals (PRI) are required to provide unambiguous coverage
over long distances using pulse Doppler waveforms. The long PRI requires
the rotating-reflector antenna to dwell for a relatively long time at each
incremental azimuth position, so the antenna rotational speed cannot be
increased without reducing its maximum unambiguous range. For an
instrumented range (maximum range for which the equipment is designed and
optimized) of 60nautical miles (nm), the ASR-9 completes a 360.degree.
scan in about 5 seconds. One nautical mile equals 1852 meters or 1.1508
statute miles.
The long-range requirement also requires the use of relatively high
transmitted power to reliably detect small targets. High transmitted power
implies a relatively higher peak transmitter power or a longer duration
transmitter pulse (also known as a "wider" pulse). Assuming a maximum
available peak power, longer range implies a longer duration transmitted
pulse. A longer duration pulse tends to reduce range resolution, which is
the ability to distinguish among targets which are at similar ranges.
Pulse compression techniques can be used to improve range resolution in
spite of the longer pulse duration, thus eliminating the need for high
peak power short pulses, but the minimum range at which a target can be
detected increases with the transmitted pulse length. Thus, if the
transmitter pulse duration is 100 microseconds (.mu.s), the minimum
distance at which a target may be detected is about 8 nautical miles (nm).
Clearly, a surveillance radar using pulses of such a duration cannot be
used to detect aircraft which are landing or taking off from an airport.
An additional problem associated with pulse compression is the appearance
of range sidelobes (as distinguished from antenna sidelobes) in addition
to the main range lobe. The time position, or range, of the main lobe is
the position that is tested for the presence of a target and for
estimating the parameters of that target (reflected energy or power,
closing speed, fluctuations in echo power and closing speed, etc.). The
presence of range sidelobes on the compressed pulse results in interfering
echoes which originate at ranges other than the range of the main lobe.
This interference, known as "flooding" can cause erroneous estimates of
the echo characteristics in the range cell (i.e., range increment) covered
by the main lobe. Prior art techniques for suppressing range sidelobes
include the "zero-Doppler" technique, in which the range sidelobe
suppression scheme is based in part upon the assumption that the
interfering echoes, as well as the desired echo, have a closing velocity
that has no significant Doppler phase change or shift over the duration of
the uncompressed pulse. The Doppler phase shift .phi..sub.Dv across the
uncompressed pulse is 2.pi. times the product of the Doppler frequency
shift and the uncompressed pulse duration (i.e. .PHI..sub.DV =2.pi.
f.sub.d T.sub.O radians). When this Doppler phase shift is actually zero
or very small, moderate sidelobe suppression is achievable with the zero
Doppler design. However, the zero Doppler design is very sensitive to
small Doppler frequency shifts, making deep sidelobe suppression
impossible for applications in which very deep sidelobe suppression is
desired, as in weather mapping, clear air turbulence detection, and
microburst detection.
Electronically scanned array antennas are inertialess, and may be capable
of rapid scanning. The rapid scanning ability gives rise to the
possibility that various air traffic control and atmospheric monitoring
uses could be multiplexed with the surveillance. An array antenna using a
centralized power transmitter and a "corporate" feed has lossy
transmission-line components, including power splitters, between the
transmitter and the element of the array antenna. Such losses may make it
difficult to achieve the desired power gain with antennas of reasonable
size, low-power phase shifters, and moderate-power transmitters.
An active phased-array radar may provide improved reliability over a
single-transmitter radar by virtue of its many transmitter modules. Also,
it may provide high power gain by virtue of its many transmitter modules,
and because power losses occur at low power levels before final
amplification, which results in low power losses between the transmitters
and their antennas. The active antenna architecture also provides reduced
system noise during reception because the majority of the receiver losses
follow low-noise amplification. Because of the inertialess scanning, it
provides the possibility of integration of functions other than
surveillance, thereby providing an overall cost reduction.
SUMMARY OF THE INVENTION
A radar apparatus for deteotion of targets includes a controllable signal
generator with a pulse recurrence frequency (PRF) control input port, for
generating pulses of radio frequency signals at a recurrence frequency
which is controlled by PRF control signals applied to the control input
port. According to an aspect of the invention, a controllable active array
antenna is coupled to the signal generator. The antenna has a thinned
aperture. The antenna includes a control input port, and is adapted for
responding to the radio frequency (RF) signals by transmitting at least
one pencil beam in a direction established by control signals applied to
the control input port of the antenna. An elevation determining
arrangement is coupled to the control input port of the antenna for
generating elevation angle control signals for directing the beam of the
antenna in a predetermined direction. According to an aspect of the
invention, the signal generator is coupled to the elevation determining
arrangement for control of the recurrence frequency in response to the
elevation control signals. In a particular embodiment of the invention,
the PRF control signals are generated at a relatively high rate (as high
as 15 or 16 KHz) when the elevation control signals direct the beam to a
relatively high elevation angle (near 60.degree.), and the PRF signals are
generated at a relatively low rate (near 1 KHz) when the elevation control
signals direct the beam at a relatively low elevation angle (near
0.degree.). According to another aspect of the invention, the volumetric
scan is speeded by a beam multiplex mode of operation, in which the pencil
beam alternates between spaced-apart positions during sequential
transmit/receive intervals, so that a portion of the interpulse time
otherwise used only for range ambiguity reduction is used to derive
additional useful information. In a particular embodiment of the
invention, the pencil beam alternates in azimuth angle about positions
spaced apart in azimuth by at least 12.degree.. According to another
aspect of the invention, the volumetric scan is speeded by using
relatively short transmitter pulses at high elevation angles and
relatively long pulses at low elevation angles, whereupon only the low
elevation angles need to be scanned again with short pulses to fill in the
short-range coverage. In a particular embodiment of the invention, pulses
of 100 .mu.S duration are transmitted at elevation angles below about
10.degree., and pulses of 1 .mu.S duration are transmitted at elevation
angles above about 15.degree.. According to another aspect of the
invention, the loss of gain or signal-to-noise margin occasioned by
scanning the pencil beam off-axis is compensated by relatively increasing
the number of pulses transmitted on each beam compared with the relatively
smaller number which is transmitted on axis, which increases the total
power transmitted in directions in which antenna gain is lower. According
to another embodiment of the invention, Doppler processing is used to
separate returns into frequency bins representative of radial speed, and
interference from scatterers at other ranges is reduced by range sidelobe
suppression applied to the signals in each frequency bin. Other ancillary
aspects of the invention are described below.
DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective or isometric view of a shelter or building adapted
for supporting phased-array antennas;
FIG. 2a is a simplified functional block diagram illustrating a method for
establishing energy distribution between a feed point and one of the
arrays of FIG. 1, and also illustrating some details of the array, and
FIG. 2b is a simplified functional block diagram of a transmit-receive
(TR) module which may be used with the arrangement of FIG. 2a; FIGS. 2a
and 2b are together referred to as FIG. 2;
FIG. 3a is a simplified block diagram of a radar system according to the
invention, FIG. 3b is a simplified schematic diagram of a portion of FIG.
3a for implementing PRI and beam multiplex control, FIG. 3c is a
simplified block diagram of a portion of FIG. 3a a for implementing off
axis beam integration control, and FIG. 3d is a simplified flow chart
illustrating the operation of the arrangement of FIG. 3c. FIGS. 3a, 3b, 3c
and 3d are referred to together as FIG. 3;
FIGS. 4a through 4d illustrate the thinning of the aperture of the array
antenna of FIG. 2;
FIG. 5a illustrates the elevation radiation pattern of a fully populated,
uniformly illuminated aperture as a reference, and FIG. 5b illustrates the
elevation radiation pattern of the thinned aperture of FIG. 4, with
uniform power applied to the elements of the array;
FIGS. 6a and 6c together illustrate all the beams generated in one octant
by the thinned array of FIG. 4, and FIG. 6b is a detail thereof; FIGS. 6a,
6b and 6c are jointly referred to as FIG. 6;
FIG. 7 illustrates in superposed form the elevation radiation patterns of
several pencil beams sequentially produced by the antenna of FIG. 4,
showing how complete coverage is obtained to a specific altitude and
range; FIGS. 8a and 8b are elevation angle representations of the beams of
FIGS. 6a and 6c, showing slant range coverage;
FIG. 9 illustrates a time line;
FIG. 10 tabulates summarized parameters of an embodiment of the radar as a
function of angle; and
FIGS. 11a-11p tabulate details of the number of pulses per beam as a
function of azimuth angle for each elevation angle of the beam structure
of FIG. 6, and also tabulates elapsed time per beam; and
FIGS. 12a-12g are partial flow charts, together representing the logic for
control of a radar according to the invention;
FIG. 13 is a simplified block diagram illustrating a prior-art processor
for pulse compression, range sidelobe reduction and Doppler filtering;
FIG. 14a is a simplified block diagram of a corresponding processor
according to an embodiment of the invention, and FIG. 14b is a simplified
block diagram of a portion of the arrangement of FIG. 14a;
FIG. 15a is a simplified block diagram of another processor for performing
the same processing as in FIG. 14a, FIG. 15b is a simplified block diagram
of a portion of the arrangement of FIG. 15a, FIG. 15c is an alternative to
FIG. 15b, and FIG. 15d is another embodiment of the invention; and
FIG. 16 is an amplitude-frequency representation of the effect of
processing in accordance with FIGS. 14 and 15.
DESCRIPTION OF THE INVENTION
FIG. 1 is a perspective or isometric view of a building or structure.
Structure 10 is in the form of a truncated quadrilateral pyramid including
faces or sides 12 and 14. Structure 10 sits atop a base or foundation 16.
Each face 12, 14 of structure 10 bears a planar array antenna 18. Array
antenna 18a is associated with face 12, array antenna 18b is associated
with face 14, and two other array antennas are associated with the two
hidden faces of structure 10. Those skilled in the art of array antennas
know that array antennas such as 18 may be two-dimensional arrays of
hundreds or thousands of antenna elements, which may be used with either a
space feed or a constrained "corporate" feed, and with phase-shifters for
scanning along one or two axes. One conventional axis is azimuth angle
.phi., measured in the x-y plane relative to the .phi.=0.degree. axis,
illustrated in FIG. 1. Another angle which is commonly used is the zenith
angle, measured from the zenith or z axis. An alternative to the zenith
angle is the elevation angle .theta., measured from the horizontal x-y
plane.
A portion of antenna 18b is illustrated in simplified functional form in
FIG. 2a. Antenna 18b includes a face portion designated generally as 19
together with a feed portion designated generally as 30. In FIG. 2a, face
portion 19 is illustrated in cross-section, and its outer, visible "front"
face is illustrated as a dash-line 20. A plurality of antenna elements
22a, 22b, 22c . . . 22n are illustrated as being associated with front
face 20. Line 20 may be considered to be the edge of a plane which is the
locus of the phase centers of antenna element 22. A dash-line 24, which is
orthogonal to front face 20, represents the broadside direction relative
to the array. Reference azimuth .phi.=0.degree. is the projection of
broadside line 24 onto a horizontal plane. Front face 20 of array antenna
18b is tilted relative to the horizontal so that broadside direction line
24 makes an elevation tilt angle .theta..sub.T with the horizontal. In a
particular embodiment of the invention, .theta..sub.T is selected to be
15.degree.. Thus, broadside direction 24 of the antenna array is tilted at
an elevation angle of 15.degree. above horizontal azimuth reference line
.phi.=0.degree..
Each elemental antenna 22a, 22b . . . 22n of FIG. 2a is associated with a
bidirectional transmitreceive (TR) processor or module illustrated as a
block 26. Thus, elemental antenna 22a is associated with a TR module 26a,
elemental antenna 22b is associated with TR module 26b, and elemental
antenna 22n is associated with a TR module 26n. As described below, each
TR module may include a power amplifier, one or more phase shifters, a low
noise amplifier, and multiplexing or diplexing arrangements. A bus
conductor line 42 carries operating power and control signals for the
operating mode, for the phase shifters, and the like, to TR modules 26.
In accordance with an aspect of the invention, the signal source driving
each of the TR modules in a transmit mode is a further elemental antenna
28, and the load on each of the TR modules in the receive mode is the same
elemental antenna 28. As illustrated in FIG. 2a, this further set of
elemental antennas, termed inner antenna elements, is illustrated as
28a-28n. Thus, inner antenna element 28a is coupled to a port of TR module
26a, inner antenna element 28b is coupled to TR module 26b, and inner
antenna element 28n is coupled to TR module 26n. A plane 34 represents the
locus of the phase centers of inner antenna elements 28.
A central monopulse space feed arrangement is illustrated generally as 30
in FIG. 2a, and includes a monopulse horn antenna 32 located near the
projection of boresight 24, and spaced away from plane 34. Horn 32 is fed
with radio-frequency (RF) signal from a circulator 36, which in turn
receives the signal from a transmitter (not illustrated in FIG. 2a) by way
of a transmission line 38. The term radio frequency for this purpose
includes microwave and millimeter-wave frequencies. Horn 36 generates sum
and difference monopulse signals from the signals which it receives from
antennas 28, and the sum and difference signals are coupled by separate
sum and difference channels (not separately illustrated) from horn 32 to a
receiver (not illustrated in FIG. 2a) by way of circulator 36 and a
further transmission line 40.
FIG. 2b is a simplified block diagram of a TR module which may be used in
the system of FIG. 2a. For definiteness, FIG. 2b illustrates
representative module 26b of FIG. 2a. In FIG. 2b, signals entering inner
antenna 28 are routed by a circulator 208 to a controllable phase shifter
210, which controls the phase shift in accordance with commands received
over a portion of data path 42. The commands select the phase shift to
define the characteristics of the transmitted antenna beam. Phase shifter
210 may have an attenuation characteristic which changes in response to
the commanded phase shift. A variable attenuator illustrated as 212 may be
cascaded with phase shifter 210 and controlled in a manner which
compensates for the attenuation of phase shifter 210. The
constant-amplitude, phase shifted signals are coupled from the output of
attenuator 212 to the input of a power amplifier (PA) 214. PA 214
amplifies the signal to produce a signal to be transmitted, which is
coupled by way of a circulator 216 to antenna 22b for radiation into
space. Returning signals reflected from targets are received by antenna
22b, and are coupled to circulator 216. Circulator 216 circulates the
received signal to a low noise amplifier (LNA) 218, which amplifies the
signal to maintain the signal-to-noise (S/N) ratio during further
processing. The amplified received signal is coupled from LNA 218 to a
phase shifter 220, which is controlled by signals received over a portion
of data path 220 in a manner selected to define the receive antenna beam.
The phase-shifted, received signals are circulated by circulator 208 to
inner antenna 28b for radiation back to antenna 32 of FIG. 2a. Those
skilled in the art know of many modifications which may be made to the
general structure of FIG. 2a. In particular, the attenuation or loss of
circulators 208 and 216 may be reduced by substituting controlled switches
therefor, and the switches may be controlled by way of bus 42 during
prescribed transmission and reception intervals.
Those skilled in the art know that antennas are passive reciprocal devices
which operate in the same manner in both transmitting and receiving modes.
Ordinarily, explanations of antenna operation are couched in terms of only
transmission or reception, the other mode of operation being understood
therefrom. Elemental antennas 22 and 28, and horn antenna 32 of FIG. 2
operate in both transmission and reception modes depending upon the mode
of operation of the radar system. Thus, the description herein, while
referring to transmission and reception, as appropriate, should not be
interpreted to exclude the other operation. One central space feed 30 is
associated with each array antenna 18. An antenna array having a transmit
amplifier associated with each antenna element is known as an "active"
array. Active array antenna 18a has a central feed 30 independent of the
corresponding feed for array antenna 18b.
In operation, horn 32 of FIG. 2 is fed with low-level transmitter pulses,
which are radiated as an electromagnetic field toward antennas 28.
Antennas 28 receive the pulses, and couple the resulting low-level signal
pulses to TR modules 26. Each TR module phase-shifts its signal by an
amount determined by appropriate beam direction control signals applied
over bus 42, amplifies the resulting phase-shifted signal, and applies the
amplified signal to an antenna element 22 by a path which includes no
discrete attenuator (although all paths include inherent attenuation).
While each solid-state TR module 26 can produce only a relatively low
power, the cumulative result of this process performed over the entire
aperture of antenna 18b is the generation of a pulse of high-power
radiation transmitted in the desired direction. After each pulse is
transmitted, as described below, the system reverts to a receive mode, by
which signal received at each elemental antenna 22 is coupled to its TR
module to be amplified by its low-noise amplifier 218, and the amplified
received signal is passed through the controlled phase shifter 220, to be
radiated by the corresponding elemental inner antenna 28. The cumulative
effect of radiation by all such inner antennas is to radiate a beam of
amplified received signal back toward monopulse horn 32. Horn 32, in turn,
separates the received signal into sum and difference signals, and couples
the cumulated received sum and difference signals through circulator 36 to
the system receivers as described below. During reception, the beam may be
pointed in the direction of the preceding transmission, or in another
direction, also as described below.
FIG. 3a is a simplified block diagram of a system in accordance with the
invention. Elements of FIG. 3a corresponding to those of FIGS. 1 and 2 are
designated by the same reference numerals. Active antenna 18b is at the
right of FIG. 3a. The beam direction of antenna 18b is controlled by
beamsteering logic (BSL) illustrated as a block 48, which receives timing
and control signals over a data bus 42 from a timing and control signal
unit (TCU) 58. It should be emphasized that BSL 48 may be an external unit
which feeds command data in common to all the TR modules, or each TR
module may contain its own portion of the BSL for reducing the amount of
data which must be routed to each TR module. Central radio-frequency feed
30 of antenna 18b is coupled by transmission lines 38 and 40 to a
transmit-receive (TR) multiplex (MPX) arrangement illustrated as a block
50. In a receive mode, multiplexer 50 receives low-amplitude or low-level
signals from RF feed 30, as described above, and couples the RF signals
from central RF feed 30 to a receiver Analog Signal Processor (RCVR/ASP)
illustrated as a block 52. A radio frequency waveform generator (WFG)
illustrated as a block 54 provides low-level reference local oscillator
(LO) signals to receiver 52 in a receive mode, and also provides low-level
transmitter waveforms by way of a transmission line 56 to multiplexer 50.
Multiplexer 50 also receives timing and control signals by way of a bus 59
from a timing and control unit (TCU) 58 for controlling its operation to
couple low level transmitter waveforms to RF feed 30 in a transmit mode,
and for thereafter providing a path by which received sum-and-difference
signals may be coupled to receiver 52.
Received signals, in sum and difference channels, if appropriate, are
downconverted and low-noise amplified in block 52 of FIG. 3a, and the
resulting downconverted or baseband signals are coupled by way of
transmission lines illustrated as 60 to analog-to-digital convertes (ADC)
illstrated as part of a block 62. Block 62 also includes a buffer for
storing digitized received signals as described below, all under the
control of timing and control signals received from TCU 58 by way of a
data path 64. The analog-to-digital conversion is performed at a "range"
clock rate, which defines the smallest discernible range increment.
Digitized sum in-phase and quadrature signals, and digitized difference
in-phase and quadrature signals, together representing the target returns
to antenna 18b, are coupled from ADC and buffer 62 of FIG. 3 over a data
path 66 to a Digital Signal Processor (DSP) illustrated as a block 68.
DSP block 68 of FIG. 3a performs the functions of (a) pulse-to-pulse
Doppler filtering by means of a Fast Fourier Transform (FFT) algorithm,
with data weighting to control signal leakage from neighboring Doppler
shifts (frequency leakage); (b) digital pulse compression; (c) range
sidelobe suppression; and (d) further signal processing including CFAR
(constant false alarm rate) processing, thresholding for target detection,
spectral processing for weather mapping, etc. Items (a), (b), and (d) are
performed in ways well understood in the art, and form no part of the
invention. The range sidelobe suppression (c) is advantageously Doppler
tolerant as described below in conjunction with FIGS. 13-16. The results
of the processing done in block 68 may include (a) target detection
reports (aircraft); (b) radar track detection reports; (c) weather
components for each resolvable volume of space, including (c1) echo
intensity; (c2) echo closing speed, and (c3) spectral spread of the echo,
and these components of information may be includes in Digitized Radar
Detection Reports (DRDR). The DRDR reports may also include data relating
to the angular coordinates of the antenna beam in which the detection
occurred, the range of the detection, the monopulse sum and difference
values extracted from the digitized received signals, and the PRF of the
dwell in which the detection occurred. The target ID may also be included
if the detection occurs in a tracking beam. The DRDR reports are applied
over a data path 70 to Detection Processor (DP) block 72.
In generating the DRDR reports, DSP block 68 of FIG. 3a performs pulse
Doppler and moving target indicator (MTI) or moving target detector (MTD)
filter processing. A person skilled in the art of pulse compression will
known that the radar pulse must be coded in some manner that allows DSP
block 68 to correlate received signals with the known transmitted pulse
code. The correlation process simultaneously improves the signal to noise
ratio and the range resolution of target echoes. A person skilled in the
art knows that a variety of satisfactory pulse coding techniques are
available in the prior art. Such techniques include the well known Barker
Codes, pseudorandom noise codes, and linear FM coding techniques. DSP
block 68 therefore also performs digital pulse compression on the received
signals.
The processed amplitude output of the sum channel is also compared to a
detection threshold level in DSP block 68 of FIG. 3a, and if the amplitude
exceeds the threshold a detection is declared, and the above-mentioned
range, sum and difference values and other data are formatted into the
Digitized Radar Detection Reports and communicated to DP block 72 via data
path 70. A pserson skilled in the art of radar detection will known how to
set the threshold level according to the radar characteristics and the
desired probability of detection (Pd) and probability of false alarm
(PFA), and he will known how to design the threshold detector to use a
smoothed estimate of interference and outputs from the moving target
detector (MTD) to yield a detection process that has the characteristic of
a Constant False Alarm Rate (CFAR) detector. DSP block 68 can be
implemented in a variety of embodiments, including 1) specially designed
hardware which performs only the specific processes required for the DSP;
2) a high speed general purpose computer which is programmed to perform
the specific processes required for the DSP; 3) a high speed general
purpose array processor which is programmed to perform the specific
processes required for the DSP; and 4) combinations of the above.
Detection Processor block 72 receives the DRDR reports, including track
reports (defined below), from DSP block 68 by way of data path 70 and
processes the digitized sum and difference values to estimate monopulse
corrections, and adds the corrections to the beam angular coordinates to
calculate the angular position of the detected target. Detection Processor
block 72 also calculates the range and range rate of the detected target.
Detection processor 72 appends the processed range, angles and range rate
to the digitized detection report and sends the resulting DRDR reports and
track reports to Radar Control Computer (RCC) block 78 by way of data path
76, and to other external users over a data path 74. RCC block 78 uses the
detection and track reports to identify new targets, to identify
maneuvering targets, to identify dedicated tracks, and to update track
files, and also uses the results to construct new sets of control
parameters according to the Radar Scheduling Control Program (RSCP)
illustrated as block 80 in FIG. 3 and further described below in
conjunction with FIGS. 12a-g. The Radar Scheduling Control progam actually
resides in Radar Control Computer block 78.
FIG. 4 illustrates the thinning of the aperture of the array of antenna
18b. Thinning of the aperture is an aspect of the invention which may
advantageously be used in conjunction with other aspects of the invention.
As illustrated in FIG. 4, the rectangular aperture includes 55 columns in
which an antenna element may appear, and 59 rows, for a total of 3245
locations or "slots". In a fully populated array, the row spacing is such
that elements are required in every other row to implement a triangular
element lattice. Column 1 of a fully populated aperture contains elements
in odd numbered slots 1 to 59 for a total of 30 elements, column 2
contains elements in even numbered slots 2 to 58 for a total of 29
elements. Therefore, every other slot is filled, and the fully filled or
populated aperture (a non-thinned aperture) contains 1623 elements located
in 3245 slots. The existence of an antenna element (and its corresponding
transmit-receive module and inner antenna) in a slot (a member of the
population) of the thinned array is represented in FIG. 4 as a numeral "1"
located at the intersection of the corresponding row and column. The
absence of a numeral "1" indicates that the thinned array includes no
antenna element at that location. Column 57 of FIG. 4 lists a numeral in
each row, which represents the number of antenna elements of the thinned
distribution in the row, and column 58 shows the number 829, which
represents the total number of elements. Row 62 similarly includes
numerals representing the total number of elements in each column, and the
total of those numbers appears in row 65 as a check.
The thinning of the array is in accordance with probability based upon a
Taylor distribution. This type of thinning is described in the July, 1964
issue of IEEE Transactions on Antenna and Propagation, at page 408 in an
article by Skolnik et al. Naturally, other thinning distributions may
provide satisfactory performance for some purposes. The thinning reduces
the antenna element density near the edges of the aperture compared with
the density near the center of the aperture. Those skilled in the antenna
arts know that an element distribution of this sort generates an antenna
pattern with relatively low sidelobes compared with a uniform distribution
of the elements. Thus, thinning illustrated in FIG. 4 avoids the need for
modulating the power output of each TR module 26 of FIG. 2. That is, the
power amplifier 214 of each TR module 26 can operate at the same output
power, and the effective amplitude distribution across the aperture of the
antenna array is such as to yield desirable sidelobe levels and beam
shapes. | | |
