TECHNICAL FIELD
The present disclosure relates to scanned array of radar systems. More particularly,
embodiments of the disclosure relate to evaluating performance of active phased array radar
systems under all operating modes and ensuring performance specification are met.
BACKGROUND
Presently, development of sophisticated radar systems require advanced simulation
and measurement techniques, which evaluates performance under all operating modes and
ensure performance specification are met and fully characterized subjective check out of the
end to end system; response to clutter, multipath, jamming and noise; algorithm performance;
performance under multiple target Doppler and RCS.
Radar signal simulation is a radar target simulation, which includes effects of
environment disturbances or clutter and manmade electronic countermeasure (ECM). Signal
injection allows the radar to be stressed in an electronic environment similar to what will be
encounter in the field. This provides high degree of repeatability and flexibility to inject the
signal at various points along the signal path to characterize individual sections of the radar
or to isolate problem areas. Actual radar signals are used to evaluate and characterize the
radar from its RF front end through digital signal processing and radar data processor. This is
particularly useful when verifying and stressing signal processing and tracking loop
algorithms.
In order to tune the radar, one need clutter and target. It can take a while before live
traffic flies across the clutter to evaluate the settings. Furthermore radar cross section of live
target is not known. So, target has been generated in the site passing through the radar
antenna injected from active array calibration network. On site injection allows to inject
multiple targets in various directions.
In conventional known radar systems, a composite radar target return is used to test
radar transmit and receive functions. Fiber optic test radar test target provides the capability
to generate radar target return signal that ideally allows the radar to independently test itself.
The independent phase means that no other radar signals are needed for testing other than the
radar transmit signal.
3
Active phased array antenna architecture typically includes a calibration network,
whose purpose is to provide injection of a predetermined calibration signal to each antenna
and to the T/R module connected to it.
Presently known in the art is, that a calibration device for an antenna array or an
improved antenna array that can be viewed as a set of RF couplers (one coupler per antenna
element) interconnected and drive by a passive network having a common feed point. The
passive network splits the drive signal in a predetermined manner so that the signal fed to
each of antenna element is known in advance and the phase and gain offset are known and
predetermined.
Phased array antennas are found in a verity of application primarily because of their
ability to produce a radiation pattern of specified characteristic which may be steered
electronically to any desired angle within certain coverage limits. The phased array antenna
application of particular interest herein is in the multi-beam active phased array radar, but it is
to be understood invention may be used in conjunction with phased array antenna in other
application.
Conventional target simulators are typically very hardware intensive. In order to
simulate a single target a conventional target simulators requires RF (radio frequency)
hardware to generate or receive a radar transmit waveform, mix the transmit signal with an
appropriate Doppler frequency (dependent on the simulated target’s velocity relative to
radar), and add as appropriate delay (dependent on the simulated target’s range). When
simulating multiple targets each target is generated separately and thus each added target
need additional RF hardware. The system can therefore become very large and expensive
when several targets need to be simulated.
Also known in the art is a moving target simulator is provided for testing a wide
variety of radars without the need for a direct connection between the simulator and the radar
under test. The simulator accurately replicates the pulse width and amplitude of transmitted
radar signals and provides a delay feature that permits simulated target scenarios to be
presented to radar under test. The simulator incorporates control features that allow Doppler
frequency changes to be accounted for and that permit signal level changes to be made as in
accordance with inverse 4th law distance dependent target return power variation.
4
A radar target generator is disclosed which is not merely a simulator, generating video
information from viewing by an operator, but is designed to be used in conjunction with an
operating radar system, actually producing high fidelity radar targets, rather than targets that
merely look like radar targets. Thus it may be used for training of radar operators as well as
for calibrating, maintaining, and evaluating multiple types of radar system. The system
includes a first tap for sampling a portion of the radar signal traveling through a transmission
line and redirecting that sample into the sampled radar signal portion, following which a
second tap returns the modified radar signal portion to a return radar signal traveling through
a second transmission line.
A phased array antenna including a number of waveguide radiators in a power supply
system. A calibration network provides attest pulse. By directly connecting the calibration
network to waveguide radiators, a more accurate calibration is obtained due to the processing
of any phase and amplitude error arising in the waveguide radiators in the calibration
algorithm. The calibration network can be designed as a system of waveguide.
A system is disclosed for generating simulated radar targets that eliminates the
necessity for large outdoor test ranges and is relatively low in cost. The simulated radar target
generating system provides complex targets of given simulated dimensions at given
simulated distances when simulated by signals emitted by the radar sensor in the sensor’s
operational frequency. The dimensions are simulated by the use of multi tap delay device
while the distances (or, range) are simulated by routing the signal, in the form of light,
through a fiber optic delay of a desire length. This system, which costs less than fifty
thousand dollars, can be located as close as eight feet to the sensor under test.
A radar target identification apparatus utilizing a pseudorandom digital code to
modulate a target’s return signal to include a two dimensional range and cross range Doppler
coded return with a target’s skin return and thereby identify friendly targets to the
interrogating radar unit.
A phased array antenna arrangement and a method for estimating the calibration ratio
of an active phased array antenna having a plurality of phased array antenna elements are
described. The phased antenna arrangement includes a plurality of antenna elements, a
plurality of receiving channels, an injection unit for injection of calibration signals into the
receiving channels, appoint RF source, located in a far field zone, a distance measurement
5
unit, amplitude and phase measurement unit and a data processing unit. Method comprises
injecting an internal calibrating signal having a known amplitude and phase to each antenna
element. An external calibration signal from a stationary RF source is sequentially injected to
the entire phased array antenna element so that different phases of the external calibration
signal arrive at each of the antenna elements. The difference in phase of external calibration
signal reaching the antenna elements are compensated so as compute effective signal
amplitude that would reach the entire antenna element at zero phase difference. Calibration
ratio is calculated is calculated as the ratio between the amplitude of the internal calibrating
signal to the effective signal amplitude of the external calibration signal.
A method for determining target echo detection efficacy of a signal processing
algorithm of a radar system involves generating a simulated noise complex envelope
sequence, generating a simulated radar target echo signal complex envelope pulse sequence
and adding the simulated noise complex envelope sequence to the simulated radar target echo
signal complex envelope pulse sequence, thereby producing simulated noisy radar target echo
signal complex envelope sequence in inputted into the signal processing algorithm and the
output of the signal processing algorithm is analyzed to determine target echo detection
efficacy of the signal processing algorithm.
Hence, there is a need of a solution which can provide an inbuilt test for evaluating
performance of active phased array radar systems under all operating modes.
SUMMARY
The shortcomings of the prior art are overcome and additional advantages are
provided through the provision of method of the present disclosure.
Additional features and advantages are realized through the techniques of the present
disclosure. Other embodiments and aspects of the disclosure are described in detail herein
and are considered a part of the claimed disclosure.
In one embodiment, the present disclosure provides a system for injecting in active
phased array radar target using a calibration network. The system comprises an exciter to
generate a predefined radar target waveform. The exciter consists of a processor to generate
digital data stream corresponding to a radar transmit waveform; a digital to analog converter
to convert the digital data stream is converted into analog signal. The analog signal is a low
6
frequency signal. An up-converter converts the low frequency signal in to radar target
waveform. Also, the system comprises a calibration distribution network communicatively
connected to the exciter to receive the radar target waveform. The calibration distribution
network is connected to each of antenna elements of the active phase array radar through
transmit/ receive (T/R) blocks to inject the radar target waveform. The calibration distribution
network connected to each antenna element of the active phase array radar corrects a phase
value to a predefined value using an adjustable phase shifter. The calibration distribution
network provides a single calibrated path to each of T/R blocks to the measurement input to
the calibration error sensing circuit at the receiver.
In one embodiment, the present disclosure provides a method for generating and
injecting radar target in an active phased array radar using a calibration network. The method
comprises generating a predefined radar target waveform using an exciter, where the
predefined target waveform is generated upon generating a digital data stream corresponding
to a radar target waveform, converting the digital data stream is converted into low frequency
signal analog signal and an up converter to convert the low frequency signal in to radar target
waveform. Also, the method comprises injecting the generated predefined target waveform to
each of antenna elements of the active phase array radar through transmit/ receive (T/R)
blocks by a calibration distribution network. Further, the method comprises measuring
injected signal using array receiver and comparing with predefined calibration data to
generate an error in target waveform received by each of the T/R blocks; and calibrating the
T/R blocks based on the generated error in target waveform at each of the T/R module. The
calibration distribution network connected to each antenna element of the active phase array
radar corrects a phase value to a predefined value using an adjustable phase shifter.
In one embodiment, the present disclosure provides an active phase array radar
system. The system comprising plurality of transmit/ receive (T/R) antennas to perform one
of receiving a radar target signal or transmit a predetermined radio frequency signal, each
T/R antenna comprises a radiating element amplitude and phase control for beam steering
and transmit receive calibration error compensation. Also, the system comprises a beam
forming network connected by each of the plurality of T/R blocks to perform one of receiving
and transmitting a radar beam. Further, a signal processing unit to process the receive radar
beam and obtain predetermined parameters, said obtained predetermined parameters are
displayed on a display. Further the system comprises a target injection system using a
7
calibration network comprising an exciter to generate a predefined radar target waveform.
The exciter consists of a processor to generate digital data stream corresponding to a radar
transmit waveform; a digital to analog converter to convert the digital data stream is
converted into analog signal. The analog signal is a low frequency signal. An up-converter
converts the low frequency signal in to radar target waveform. Also, the target injection
system comprises a calibration distribution network communicatively connected to the
exciter to receive the radar target waveform. The calibration distribution network is
connected to each of antenna elements of the active phase array radar through transmit/
receive (T/R) blocks to inject the radar target waveform. The calibration distribution network
connected to each antenna element of the active phase array radar corrects a phase value to a
predefined value using an adjustable phase shifter. The calibration distribution network
provides a single calibrated path to each of T/R blocks to the measurement input to the
calibration error sensing circuit at the receiver.
The foregoing summary is illustrative only and is not intended to be in any way
limiting. In addition to the illustrative aspects, embodiments, and features described above,
further aspects, embodiments, and features will become apparent by reference to the drawings
and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features and characteristic of the disclosure are set forth in the appended
claims. The embodiments of the disclosure itself, however, as well as a preferred mode of
use, further objectives and advantages thereof, will best be understood by reference to the
following detailed description of an illustrative embodiment when read in conjunction with
the accompanying drawings. One or more embodiments are now described, by way of
example only, with reference to the accompanying drawings.
Fig. 1 illustrates an active phased array radar system with calibration network, in
accordance with an embodiment of the present disclosure;
Fig. 2 shows a calibration network in active Phased Array Antenna, in accordance
with an embodiment of the present disclosure;
Fig. 3 illustrates a radar target generation using waveform generation using DDS, in
accordance with an embodiment of the present disclosure;
Fig. 4 shows a radar target injection to calibration network, in accordance with the
present disclosure;
8
Fig. 5 shows a flowchart of radar echo planar wave front calibration algorithm, in
accordance with the present disclosure; and
Fig. 6 shows a plot tracking of an injected target, in accordance with the present
disclosure.
The figures depict embodiments of the disclosure for purposes of illustration only. One
skilled in the art will readily recognize from the following description that alternative
embodiments of the structures and methods illustrated herein may be employed without
departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
The foregoing has outlined rather broadly the features and technical advantages of the
present disclosure in order that the detailed description of the disclosure that follows may be
better understood. Additional features and advantages of the disclosure will be described
hereinafter which form the subject of the claims of the disclosure. The novel features which
are believed to be characteristic of the disclosure, both as to its organization and method of
operation, together with further objects and advantages will be better understood from the
following description when considered in connection with the accompanying figures. It is to
be expressly understood, however, that each of the figures is provided for the purpose of
illustration and description only and is not intended as a definition of the limits of the present
disclosure.
The present disclosure provides a method and system for target injection using a
calibration network in active phased array radar.
An exemplary embodiment of the present disclosure provides a system for injecting a
target in active phased array radar using a calibration network. The system comprises an
exciter to generate a predefined radar target waveform. The exciter consists of a processor to
generate digital data stream corresponding to a radar transmit waveform; a digital to analog
converter to convert the digital data stream is converted into analog signal. The analog signal
is a low frequency signal. An up-converter converts the low frequency signal in to radar
target waveform. Also, the system comprises a calibration distribution network
communicatively connected to the exciter to receive the radar target waveform. The
calibration distribution network is connected to each of antenna elements of the active phase
array radar through transmit/ receive (T/R) blocks to inject the radar target waveform. The
9
calibration distribution network connected to each antenna element of the active phase array
radar corrects a phase value to a predefined value using an adjustable phase shifter. The
calibration distribution network provides a single calibrated path to each of T/R blocks to the
measurement input to the calibration error sensing circuit at the receiver.
In one embodiment, the present disclosure provides a method for generating and
injecting a radar target in active phased array radar using a calibration network. The method
comprises generating a predefined radar target waveform using an exciter, where the
predefined target waveform is generated upon generating a digital data stream corresponding
to a radar target waveform, converting the digital data stream is converted into low frequency
signal analog signal and an up converter to convert the low frequency signal in to radar target
waveform. Also, the method comprises injecting the generated predefined target waveform to
each of antenna elements of the active phase array radar through transmit/ receive (T/R)
blocks by a calibration distribution network. Further, the method comprises measuring
injected signal using array receiver and comparing with predefined calibration data to
generate an error in target waveform received by each of the T/R blocks; and calibrating the
T/R blocks based on the generated error in target waveform at each of the T/R module. The
calibration distribution network connected to each antenna element of the active phase array
radar corrects a phase value to a predefined value using an adjustable phase shifter.
In one embodiment, the present disclosure provides an active phase array radar
system. The system comprising plurality of transmit/ receive (T/R) antennas to perform one
of receiving a radar target signal or transmit a predetermined radio frequency signal, each
T/R antenna comprises a radiating element amplitude and phase control for beam steering
and transmit receive calibration error compensation. Also, the system comprises a beam
forming network connected by each of the plurality of T/R blocks to perform one of receiving
and transmitting a radar beam. Further, a signal processing unit to process the receive radar
beam and obtain predetermined parameters, said obtained predetermined parameters are
displayed on a display. Further the system comprises a target injection system using a
calibration network comprising an exciter to generate a predefined radar target waveform.
The exciter consists of a processor to generate digital data stream corresponding to a radar
transmit waveform; a digital to analog converter to convert the digital data stream is
converted into analog signal. The analog signal is a low frequency signal. An up-converter
converts the low frequency signal in to radar target waveform. Also, the target injection
10
system comprises a calibration distribution network communicatively connected to the
exciter to receive the radar target waveform. The calibration distribution network is
connected to each of antenna elements of the active phase array radar through transmit/
receive (T/R) blocks to inject the radar target waveform. The calibration distribution network
connected to each antenna element of the active phase array radar corrects a phase value to a
predefined value using an adjustable phase shifter. The calibration distribution network
provides a single calibrated path to each of T/R blocks to the measurement input to the
calibration error sensing circuit at the receiver.
The present disclosure relates to electronically scanned array development of today’s
sophisticated radar systems required advanced simulation and measurement techniques in
order to evaluate performance under all operating modes and ensure performance
specification are met and fully characterized i.e. subjective check out of the end to end
system; response to clutter, multipath, jamming and noise; Algorithm performance;
performance under multiple targets Doppler and RCS.
Radar signal simulation generally implies radar target simulation, including effects of
environment disturbances (clutter) and manmade electronic countermeasure (ECM). Signal
injection allows the radar to be stressed in an electronic environment similar to what will be
encounter in the field. This provides high degree of repeatability and flexibility to inject the
signal at radiating elements along the signal path for characterization of radar. Actual radar
signals are used to evaluate and characterize the radar from its RF front end through digital
signal processing and radar data processor. This is particularly useful when verifying and
stressing signal processing and tracking loop algorithms. Simulating the receive signals
requires that the input to each Tx/Rx module be phase coherent each other with a known
phase relationship. They must be independently adjustable in time, phase, and amplitude.
Radar return signal simulations are capable of simulating a maneuvering target by varying the
relative phase I and Q channel baseband input signals, the speed of the target by adjusting the
baseband input frequency and size and / or distance of the target by adjusting the signal
strength.
The objective of the present disclosure is designing and realization of improved
phased array radar system having improved means of performance evaluation under all
operating modes and ensuring performance specification are met and fully characterized.
11
That is the system is subjective check out of the end to end system; response to clutter,
multipath, jamming and noise; algorithm performance; performance under multiple targets
Doppler and RCS. For instance the system is not fully instrumented when fielded and often
the data needed for proper evaluation is not available. Also, testing becomes situational
dependent, making it difficult to reproduce a given scenario for system characterization.
There are alternatives for generation of actual radar test signals for performance testing of
radar system like flight testing and custom simulator becomes very expensive and time
consuming. The most affordable and suitable approach versatile and realistic signal is inbuilt
calibration network.
In one embodiment, development of radar receivers for real world target detection
involves modeling of return pulse trains that vary in terms of range, Doppler and aspect
angle. When a radar target consists of many reflecting surfaces offset from each other, the
amplitude and phase of the return pulse are dependent on the orientation of the target. If the
target has motion with respect to the radar receiver like yaw, pitch or tumble the radar cross
section will vary as a function of time. These fluctuations are characterized as either slow i.e.
change from scan to scan, or fast i.e. change from pulse to pulse.
Fig. 3 shows radar target generation using waveform generation using DDS, in
accordance with an embodiment of present disclosure. A point target I/Q echo signal is
produced using simulation echo based on FIFO data, based on delayed frequency modulated
radar waveform generation, with built-in programmable Doppler phase shift in the waveform,
corresponding to the designated speed of the target, and converted to analog I/Q echo signal
using D/A converter, I/Q baseband signal is modulated to be intermediate frequency signal by
I/Q modulator and generated low frequency analog signal which is up converted; RF signal is
generated which is taken as signal source in the radar BITE (calibration network) for target
injection.
Fig. 4 shows RF signal is generated which is taken as signal source in the radar BITE
(calibration network) for target injection. This injected signal is coupled to the plurality of
radiating elements from calibration network which are at the equi-phase surface of the planar
wave front of the coupled injected target signal which is amplified by LNA and down
converted in array receiver and get processed in signal processor, tracked and displayed.
12
Fig. 5 shows a Radar echo planar wave front calibration algorithm is implemented in
calibration processer unit. Planar wave front is generated at the singular port of calibration
network in the active array antenna by utilizing target calibration S matrix for the plurality of
radiating elements and is performed for the individual radiating elements in the calibration
processor unit. Radar target planar wave front calibration error is uploaded into the T/R
modules of plurality of radiating elements.
Fig. 6 shows Simulation radar target signal in injected in calibration distribution
network to simulate reflections of a target objects moving at particular azimuth, elevation,
distance and with different velocities. Injected target maneuvering can be simulated in
combination with the phase shifter at the radiating elements by setting phase gradient at the
antenna aperture one of injected target is shown in figure. Simulated target is used for
verifying and stressing signal processing and tracking loop algorithms without actual
transmission of the radar system in the field.
In conventional phased array radar system, power generation is accomplished through
the use of a power tube or amplifier and then distributed to the individual radiating elements
through a transmission line network or space feeding using a horn. Care must be taken to
insure that all the individual transmission paths are of the same or known length to
accomplish beam steering and control over the desired frequency bandwidths. In solid state
radar a low power exciter usually generates the carrier of the transmitted signal. The exciter
output is often modulated in amplitude and phase including pulsing to generate radar signal
of low power. These low power signals are then distributed to an array of power amplifying
modules each arranged to drive an antenna element of the phase array.
A phased array antenna is composed of lots of radiating elements each with the ability
to shift the phase of its transmitted /received signal. Beams are formed by shifting the phase
of the signal emitted/ received from/ at each Tx/Rx module, to provide constructive and
destructive interferences in order to steer the beams in a particular direction. A calibration
loop is completed for transmit operating path calibration by switching the exciter output into
a measurement path consisting initially for transmit operating path, secondly a calibration
path, and ending at the receiver measurement port. The order is reversed for receive operating
path calibration. The exciter output is switched in to a measurement path initially of a
calibrating path, thereafter receive operating path and ending at the receiver measurement
port. The antenna system provides an improved self-monitoring/ calibrating phase array radar
13
system in which T/R subassemblies may be used interchangeably anywhere in the active
array system. Thus, a tight phase tolerances are automatically established for the antenna
aperture in order to establish low side lobe for the phased array antenna.
Also, a loosely coupled constant amplitude, linear phase travelling micro strip line,
which is a built in test equipment(BITE) calibration network system is attached to each row
of radiating elements of electronically phased array antenna to distribute significant amount
of RF energy to each radiating element with constant phase differential between all adjacent
elements from a test generator. The complex voltage signals received at the antenna input
port are recorded for a predetermined no of electronic scan angles.
Fig. 1 shows an active phased array radar system, in accordance with one embodiment
of the present disclosure. The system comprises a plurality of radiating elements with the one
to one plurality of T/R modules with individual radiating element amplitude and phase
control for beam steering and transmit receive calibration error compensation. Each of the
radiation elements has the ability to shift the phase of its excitation. Beams are formed by
shifting the phase of the signal received/emitted from each Tx/Rx module, to provide
constructive and destructive interferences in order to steer the beams in a particular direction.
A calibration network is another embodiment of active phased array antenna system with
improved self-monitoring/ calibrating of active phase array radar system in transmit and
receive. The calibration network is radar target injection in the active array antenna which is
disclosed in this invention. This is useful when verifying and stressing signal processing and
tracking loop algorithms. In this embodiment of active phased array antenna T/R module
subassemblies may be used interchangeably anywhere in the active array system.
In an exemplary embodiment of the present disclosure, the active phased array
antenna system comprises a regular, periodically spaced grid, contains approximately 1000
T/R for more than 1000 antenna elements which has been shown in Fig. 1. The T/R
subassemblies are interchangeable and thus represent the lowest replaceable unit in the field,
are conceived to have long life, with the T/R modules which are the principle source of
failure. The active phased array antenna system is arranged to operate in stationary position
and to from an electronically scan beam. In an example embodiment, each of the T/R
modules are suitable for scanning azimuth electronically of ± 60o with the separation more
than 0.5λmin in discrete radiating elements, and in elevation, the array has electronically
scanning of ± 50o, with the separation of more than 0.5λmin in discrete radiating elements.
14
This is without any visible grating lobe in the electronically zone of scan, using the plurality
of isotopic radiating element taking in to account. Scanning of transmitted or received beam
is achieved by shifting the phase from antenna element to antenna element in nearly equal
increments as one progress across the aperture along a vertical or horizontal coordinate line.
This causes the beam to be scanned in radar coverage volume from bore sight. The scanning
is proportion to the phase increment between elements.
The alignment the radiating elements of the antenna system is obtained by conducting
phase and amplitude measurement at the multiple antennas, receive ports with a calibration
network. Predefined computations are conducted generating phase and amplitude correction
for each element in the antenna array, which are recorded or stored. The record information is
then subjected to a complex variable matrix inversion, which produces phase and amplitude
indication for each radiating element the negative of the phase values thus established
constituting the desired correction factor to be supplied to the beam steering computer. The
correction factor is applied to the antenna through a beam steering computer which results in
a low side-lobe radiation pattern.
The calibration network microstrip line or BITE coupler system simulates far field
signal reception without the aid of an antenna range or near field probe in front of the
aperture. The angle of view may be varies for a particular set of uniformly incremented
phaser settings, which can be computed. The phasers are effective for conducting an
electronic scan through for N scan angles of the BITE coupler injected signal. During this
electronic scan through N angles of view, information with respect to amplitude and phases
of the simulated target at θ degrees can be feed into radiating aperture.
One embodiment of the present disclosure is a calibration network. Phase delay in a
unique path through the beam former is accordingly a fixed quantity which combines with the
other phase delays present in the path from exciter to antenna element. Thus, the phase delay
in the full path from exciter to particular antenna element is a fixed and accessible quantity,
which may be readily corrected to a desire value by installing logically controlled adjustable
phase shifter in each antenna element path using calibration network. For the calibration
process to work, the calibration network must provide a single calibrated path from the micro
strip calibration network to each of T/R subassemblies to the measurement input to the
calibration error sensing circuit at the receiver. Determining a linear dynamic range and
15
signal to noise ratio (SNR) for a receive element in the array for making phase and amplitude
measurements within a given accuracy range.
Fig. 2 shows a calibration network in accordance with an embodiment of present
disclosure, according to an embodiment of the present disclosure. As shown in fig. 2, a RF
signal is generated from the exciter unit which is routed to calibration distribution network
for n row of loosely coupled micro strip line, in calibration network of active phased array to
distribute significant amount of RF energy to each radiating element. The complex voltage
signals received at the antenna port with the perturbation of passive calibration network Smatrix.
This received signal is amplified by LNA in TRM and travels through corporate beam
forming network to processing unit, where it get processed.
A strip line/ micro strip line network are of high reliability and low phase error, while
the active electronics including the phase shifters themselves are less reliable and exhibit
significant phase error for each transmission /receive path. The accuracy of calibration
network utilized to calibrate the paths to individual antenna elements must be executed with
care in the equality of path lengths and is preferably a corporate feed, which may be branched
once, or twice, or perhaps N times without significant loss in accuracy for one, two or N
numbers of T/R modules.
One embodiment of the present disclosure is a method of measurement of a passive
calibration network S-matrix parameter for each antenna element. The phased array antenna
is integrated and mounted in near field test range with a scanner probe position to measure
the calibration network of each radiating element. The calibration network measurement for
each path is achieved by coupling the exciter output as it reaches each antenna element back
by active path. The calibration network S-matrix parameter table is created through several
iterations of this measurement as the antenna element. The calibration network S- parameter
table is then used in the antenna control algorithm for receive and transmit calibration of
active phased array antenna system.
A passive calibration network S-matrix receives contribution from RF power dividers
and calibration lines, relative variation of coupling between radiating element and the
coupling line and cables used for interconnections. The passive calibration network S- matrix
is defined for plurality of all radiating element connected with the matched individual T/R
modules. RF signal is connected to the port of calibration network in antenna array from the
16
exciter; the RF signal will be coupled through micro strip calibration line network to active
elements, which can be represented by mathematical equations as below. When measuring
the passive calibration network S-matrix of an n-port, all n ports must be terminated by a
matched load (radiating elements), including the port connected to the exciter. The relation
between ai and bi (i= l...n) can be written as a system of n linear equations (ai being the
independent variable, bi the dependent variable). The waves going towards the n-port are a =
(a1, a2, ..., an), the waves travelling away from the n-port are b = (b1, b2 , ..., bn).
     


     


     


     


  
  
  

    


    


n n n n nm nm m
m m
m m
n a
a
a
S S S S S S
S S S S S S
S S S S S S
b
b
b
....
....
................. .................
................. ................. ................. ................. .................
................. ................. ................. ................. .................
................. .................
................. .................
....
....
2
1
1 1 2 2
21 21 22 22 2 2
11 11 12 12 1 1
1
1
The passive calibration network S- matrix is defined as:
 
     


     


  
  
  

n n n n nm nm
m m
m m
S S S S S S
S S S S S S
S S S S S S
S
................ ................
................ ................ ................ ................ ................
................ ................ ................ ................ ................
................ ................
................ ................
1 1 2 2
21 21 22 22 2 2
11 11 12 12 1 1
The calibration network comprises a row/column feed network both corporate in
nature including distribution network, micro strip calibration line network to active elements.
The term corporate means that each path from a singular to a plural port is measured to
higher degree of accuracy; which is stored in a computer memory and used to remove any
error in transmit / receive calibration process.
On embodiment of the present disclosure is an active phased array calibration. The
radio frequency characteristic of active modules are subjected to variation due to
manufacturing tolerances, their impedance, gain, insertion loss, and phase may vary from one
module to another. In this embodiment of the present disclosure, a phase shifter common to
transmit and receive path is adjustable in increments under active logical control for
calibration, while power and gain performance in monitored and actively controlled by
common attenuator. Often typical calibration and maintenance after operational deployment
is only performed when necessary to compensate for defective elements, compensation for
17
change in element performance over time, temperature or other influencing factors to
maintain desired radiation pattern characteristics of active array to maintain overall peak
performance.
On embodiment of the present disclosure is a transmit path calibration. During
transmit path calibration exciter signal is amplified in plurality of T/R subassemblies and
coupled to micro strip corporate feed calibration network assembly having its singular port.
The calibration loop is completed for transmit operating path calibration by switching the
exciter output into a measurement path consisting initially of transmit operating path,
secondly a calibration path, and ending at the receiver measurement port. The correction of
phase error in m rows x n columns transmit operating paths requires one phase shifter per
path. The phase shifter must be capable of being reset to a value correcting for any departure
from the desired reference value during calibration with the ability to continue through 3600.
When all the paths are calibrated a vital bore sight condition is produced. During transmit
path calibration one antenna element is powered at a time under control chip control and no
true beam is formed. During transmission the phase correction must be retained as a true zero
phase setting to which phase increments are added in steering the beam to a desired offset
from bore sight condition. During transmission all antenna elements are powered and a beam
is formed.
One embodiment of the present disclosure is a receive path calibration in which, the
calibration network is the same as the calibration network used during transmit path
calibration, and the paths associated with each T/R subassembly are used for both calibration
although the direction of the measurement signal through the network are reversed. The
receive path calibration differs principally from the transmit path calibration in the reversal of
the direction of signal flow through the calibration and operating signal paths. In receive path
calibration exciter supplies a signal directly to the calibration network assembly which get
coupled from singular port of micro strip to singular radiating element of jth port of T/R
subassemblies.
In receive operating path, the measurement signal proceeds from the selected antenna
element to the associated T/R module containing phase shifter and transmit/receive
electronics. As in transmit path calibration, the individual modules are subject to enable
control during receive path calibration, which turn off each modules except the one being
calibrated. A control signal is provided for adjusting the digital phase shifter in the path being
18
calibrated to the appropriate true zero value. The effect on the antenna pattern of the digital
nature of the phase shifter and the need for a significant random error, in the individual phase
shifters and for randomness in the disposition of the phase shifters over the antenna is also
present for reception.
One embodiment of the present disclosure is verifying calibration method through a
final antenna measurement. The near field range is used to take a scan a far field plot is
calculated. A good calibration will yield a good antenna pattern with symmetric main beam
and low side-lobes. Pattern discrepancies can be used as indications of an undesirable
calibration. Phased array antenna systems according to the invention are by no means
restricted to the above mentioned embodiments. Features from the above embodiment can be
applied to any combination.
One embodiment of the present disclosure is Radar Target Echo Generation. Radar
signal simulation generally implies radar target simulation, including, but not limited to
effects of environment disturbances or clutter and manmade electronic countermeasure
(ECM). A signal injection allows the radar to be stressed in an electronic environment similar
to the real time/ world scenario encountered in a field, which provides a high degree of
repeatability and flexibility to inject signal at various points along the signal path to
characterize individual sections of the radar or to isolate problem areas. Actual radar signals
are used to evaluate and characterize the radar from its RF front end through digital signal
processing and radar data processor. This is particularly useful when verifying and stressing
signal processing and tracking loop algorithms.
The waveform can be a computer generated digital data stream that is converted to a
low frequency analog signal and then coherently up converted to the radar received signal
frequency. An electronic device such as, but not limited to computer hardware and FPGA can
be used to generate data stream which acts as the real time target. The target signal generation
or simulation is performed by generating the same data stream as the radar transmit
waveform, but with different amplitude and at a time delay with respect to the transmitted
pulse, corresponding to the range of the simulated target. The waveform characteristics of the
target generated will be same as the transmitted pulse, in terms of pulse width, number of
pulses and pulse to pulse interval. This method of generating target allows for providing a
large number and variety of complex RF targets, and radar waveforms where the number of
radar return signals generated is not function of the amount of hardware. This method is
19
opposed to the current technique where each target is generated with individual RF hardware.
The term ‘target’ is intended to include any signal that might be received by a radar system
such as, but not limited to fixed targets and moving targets. Radar return signal simulations
are capable of generating or simulating a manoeuvring target by varying the relative phase I
and Q channel baseband input signals, the speed of the target by adjusting the baseband input
frequency and size and/or distance of the target by adjusting the signal strength and time
delay.
In one embodiment of the present disclosure, a target model can take on many different
forms depending on the level of intricacy necessitated by the simulation or analysis. As an
example, a simplest representation utilized is a single point scatterer. A target is represented
as a discrete point in space, which is subject to its own trajectory and is the simplest
representation of any target.
Transmitter signal is LFM signal, which can be written as
where f0 is the carrier frequency, u (t) is the envelope of St and
, stationary point target’s echo signal has the form
Where K is related to but not limited to a system model, antenna pattern, environment
model and target model, where ; is Doppler frequency shifted signal.
By ignoring the fixed phase depending on , K and  equation (2) is approximated as
Where
Corresponding time discrete quadrature components are
20
On embodiment of the present disclosure provides the following simulation models
for radar echo signal generation;
Signal Models:-
Clutter Models:-
Noise Models:-
A simulation model which is written as;
Based on the above models, point target echo signal model may be described by
where, radar signal may have the transmitted form
21
where and φ (t) can be expressed by
After radar signal is modulated using equations (7) and (8), output echo signal may be
written as
where and are initial phase value, they are constant substituting equation (7)
and (8) into equation (10) yields
The above implementation can produce point target echo signal in real time, which
becomes more convenient to test radar’s performance.
One embodiment of the present disclosure is radar target echo injection to calibration
network. As an example, let the system be not fully instrumented on the field and often the
data needed for proper evaluation is not available, testing becomes situational dependent,
making it difficult to reproduce a given scenario for system characterization. The most
affordable and suitable approach which is versatile and realistic signal is inbuilt calibration
network for target injection into active antenna aperture to evaluate and characterize the radar
from its RF front end through digital signal processing and radar data processor. Also, each
radiating element in the array is being controllable by parameters such as, but not limited to
amplitude, Doppler effects and phase, a reflection off one or more target objects may be
accurately simulated. By differentially applying phase control to the radiating antennas within
the array, constructive and destructive interference can be created and utilized to steer the
beam to a particular angle. If a vertically rising target were to be simulated, for example, the
phase across the horizontal direction may be same, but phase up and down in the vertical
direction may be modified to a linear gradient. Differential amplitude control can be used to
better simulate multiple targets object in different range cell within an environment. Signal of
22
varying amplitude throughout the array to simulate reflections off of multiple target objects
within the same range cell. Doppler modulation may be used to simulate the Doppler effects
that a moving target object would have on a reflected signal. By applying the differential
Doppler effects, the simulation device can generate signal of varying Doppler effect to
simulate reflections off of multiple target objects moving at different velocities such as, but
not limited to use of phase shifter, attenuators, Doppler modulators, differential effects can be
used for obtaining better simulation target objects.
By combining the simulated target generation computer to a real time manual control, to
change the target heading, speed, etc., combined with the processing power of beam steering
computers, it is even possible to generate a real time multiple target scenario, which can be
controlled by an operator online. The application for which simulated target generation and
injection can be used are such as, but not limited to testing air/ ground targets, artillery,
mortar, aircraft, unmanned aerial vehicles (UAV), and other moving objects.
In one embodiment, radar systems use modulated waveforms and directive antennas
to transmit electromagnetic energy into a specific volume in space to search for
targets. Objects within search volume reflect portions of this energy back to the Radar,
these echoes are then processed by the radar subsystems to extract target information
such as range, velocity, angular position, and the other target identifying characteristics.
Targets are the desired objects required to be detected by the radar system returns are the
copies of the transmitted waveform delayed in time due to the range of target from
radar, changed in carrier frequency due to Doppler shift of moving target and distorted by
system noise.
Thus for, evaluation and optimization of radar system performance of radars radar
target can be simulated electronically and radar system performance can be measured without
expensive field trips to harsh regions. For this purpose a simulated radar target is designed
to generate a radar illuminated environment in the actual radar system in the field using
calibration network. These simulated radar environments are used to develop, optimize and
test the radar receiver design and signal and data processer detection and tracking algorithm
during development phase of the radar system.
For this purpose to inject a simulated radar target; a planar wave front is generated at
the singular port of calibration network in the active array antenna by utilizing target
23
calibration S matrix for the plurality of radiating elements and is performed for the individual
radiating elements in the calibration processor unit. In the active array plurality of the
attenuator and phase shifter corresponding to each radiating element is reset to absolute zero
value, calibration network is excited by calibration signal which is generated using exciter
unit. Calibration signal injected to calibration network is distributed and coupled to the
plurality of radiating elements with the strip line. Calibration signal from the plurality of
radiating element is amplified by LNA and perturbed by attenuator and phase shifter settings
then get measured by array receiver.
Amplitude and phase error in the target planar wave front is calculated and verified
that need to be within resolution of phase shifter and attenuator of the T/R module in the
active array. To achieve target planar wave front required calibration amplitude and phase
errors are set into the plurality of attenuator and phase shifter in the T/R module. The target
plane wave front calibration at the singular port of calibration network is completed by
switching the exciter output into calibration network at singular injection port and ending at
the receiver measurement port.
Radar target planar wave front matrix is generated using calibration processor unit
which is defined as below;
 
     


     


        
        
        

n en n en n en n en nm enm nm enm
e e e e m e m m e m
e e e e m e m m e m
S a S S a S S a S
S a S S a S S a S
S a S S a S S a S
S
  
  
  
1/( ) 1/( ) ................ ................ 1/( )
................ ................ ................ ................ ................
................ ................ ................ ................ ................
1/( ) 1/( ) ................ ................ 1/( )
1/( ) 1/( ) ................ ................ 1/( )
1 1 1 1 2 2 2 2
21 21 21 21 22 22 22 122 2 2 2 2
11 11 11 11 12 12 12 12 1 1 1 1
Thus the position of simulated target is obtained by phase delay in programmable
digital phase shifters for receive path along with the calibration error for planar wave front at
the singular port of calibration network in plurality of each antenna element. Scanning of
radar target in the simulator is achieved by shifting the phase from, antenna element to
antenna element in nearly equal increments as one progress across the aperture along a
vertical or/and horizontal coordinate line. This causes the beam to be scanned in any direction
from bore sight. The scanning is proportion to the tracking of simulated radar target in any
direction within radar coverage.
24
Finally, the language used in the specification has been principally selected for
readability and instructional purposes, and it may not have been selected to delineate or
circumscribe the inventive subject matter. It is therefore intended that the scope of the
invention be limited not by this detailed description, but rather by any claims that issue on an
application based here on. Accordingly, the disclosure of the embodiments of the invention is
intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in
the following claims.
With respect to the use of substantially any plural and/or singular terms herein, those
having skill in the art can translate from the plural to the singular and/or from the singular to
the plural as is appropriate to the context and/or application. The various singular/plural
permutations may be expressly set forth herein for sake of clarity.
In addition, where features or aspects of the disclosure are described in terms of
Markush groups, those skilled in the art will recognize that the disclosure is also thereby
described in terms of any individual member or subgroup of members of the Markush group.
While various aspects and embodiments have been disclosed herein, other aspects and
embodiments will be apparent to those skilled in the art. The various aspects and
embodiments disclosed herein are for purposes of illustration and are not intended to be
limiting, with the true scope and spirit being indicated by the following claims.
25
We claim:
1. A system for injecting target in an active phased array radar using a calibration network,
said system comprising:
an exciter to generate a predefined radar target waveform, said exciter comprising a
processor to generate digital data stream corresponding to a radar transmit waveform; a
digital to analog converter to convert the digital data stream is converted into analog
signal, said analog signal is a low frequency signal; and an up converter to convert the
low frequency signal in to radar target waveform; and
a calibration distribution network communicatively connected to the exciter to receive
the radar target waveform, said calibration distribution network is connected to each of
antenna elements of the active phase array radar through transmit/ receive (T/R) blocks to
inject the radar target waveform;
wherein the calibration distribution network connected to each antenna element of the
active phase array radar corrects a phase value to a predefined value using an adjustable
phase shifter, the calibration distribution network provides a single calibrated path to each
of T/R blocks to the measurement input to the calibration error sensing circuit at the
receiver.
2. The system as claimed in claim 1, wherein the calibration distribution network strip line
to micro strip line divider network and transition are unaffected upon replacement of
failed T/R module without change of electrical performance of calibration network.
3. The system as claimed in claim 1, wherein switching the exciter output for transmit
operating path measurement in to a loop comprises a transmit operating path to antenna
element, micro strip to strip line calibration network of calibration path and finally
returning to the receiver measurement port.
4. The system as claimed in claim 1, wherein a calibration reflecting phase error in each
transmit operating path and receive operating path of T/R blocks using stored phase error
data to remove said phase error thereby reducing undesired variation in the phase of the
operating path.
5. The system as claimed in claim 1, wherein the injected radar target waveform is
generated by varying at least one of phase and amplitude across plurality of radiating
elements with phase shifters at antenna aperture of each T/R block.
26
6. The system as claimed in claim 1, wherein selectively switching the exciter output from
transmit or calibration for transmit operating path measurement in to a loop consisting
initially of the transmit operating path to antenna element, micro strip to strip line
calibration network of calibration path and finally returning to the receiver measurement
port.
7. The system as claimed in claim 1, wherein the calibration reflecting phase error in
simulated target injection mode in calibration network means to adjust the setting of said
phase shifters by utilizing radar echo planar wave front calibration algorithm, to remove
phase error to achieve simulated far field planar wave front at singular port of calibration
network.
8. The system as claimed in claim 1, wherein the exciter for generating target object for one
or more targets with parameters corresponding to at least one of the azimuth, elevation
and distance, with reflected target echo characteristics in real time.
9. A method for generating and injecting a radar target in an active phased array radar using
a calibration network, said method comprising:
generating a predefined radar target waveform using an exciter, said predefined target
waveform is generated upon generating a digital data stream corresponding to a radar
target waveform, converting the digital data stream is converted into low frequency signal
analog signal; and an up converter to convert the low frequency signal in to radar target
waveform;
injecting the generated predefined target waveform to each of antenna elements of the
active phase array radar through transmit/ receive (T/R) blocks by a calibration
distribution network;
measuring injected signal using array receiver and comparing with predefined
calibration data to generate an error in target waveform received by each of the T/R
blocks; and
calibrating the T/R blocks based on the generated error in target waveform at each of
the T/R module.
wherein the calibration distribution network connected to each antenna element of the
active phase array radar corrects a phase value to a predefined value using an adjustable
phase shifter.
27
10. An active phase array radar system comprising:
plurality of transmit/ receive (T/R) antennas to perform one of receiving a radar target
signal or transmit a predetermined radio frequency signal, each T/R antenna comprises a
radiating element amplitude and phase control for beam steering and transmit receive
calibration error compensation;
a beam forming network connected by each of the plurality of T/R blocks to perform
one of receiving and transmitting a radar beam;
a signal processing unit to process the receive radar beam and obtain predetermined
parameters, said obtained predetermined parameters are displayed on a display; and
a target generation and injection system injection using a calibration network as
claimed in claim 1 comprising:
an exciter to generate a predefined radar target waveform, said exciter comprising
a processor to generate digital data stream corresponding to a radar transmit
waveform; a digital to analog converter to convert the digital data stream is converted
into analog signal, said analog signal is a low frequency signal; and an up converter to
convert the low frequency signal in to radar target waveform; and
a calibration distribution network communicatively connected to the exciter to
receive the radar target waveform, said calibration distribution network is connected
to each of antenna elements of the active phase array radar through transmit/ receive
(T/R) blocks to inject the radar target waveform;
wherein the calibration distribution network connected to each antenna element of
the active phase array radar corrects a phase value to a predefined value using an
adjustable phase shifter, the calibration distribution network provides a single
calibrated path to each of T/R blocks to the measurement input to the calibration error
sensing circuit at the receiver.