Design Considerations of IRST System

13 Pages·1997·0.655 MB·English

by  Venkateswarlu, Ronda

PROCEEDINGS OF SPIE
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Design considerations of IRST
system
Venkateswarlu, Ronda, Er, Meng, Tam, Siu Chung, Chan,
Choong, Choo, Lay
Ronda Venkateswarlu, Meng Hwa Er, Siu Chung Tam, Choong Wah Chan,
Lay Cheng Choo, "Design considerations of IRST system," Proc. SPIE 3061,
Infrared Technology and Applications XXIII, (13 August 1997); doi:
10.1117/12.280379
Event: AeroSense '97, 1997, Orlando, FL, United States
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Design considerations of IRST system.
R.Venkateswarlu, M. H. Er, S.C.Tam, C. W. Chan, L.C.Choo.
Nanyang Technological University, School of EEE, Singapore 639798, Singapore
ABSTRACT
InfraRed Search and Track (IRST) system is a wide field of view surveillance system, meant for autonomous search,
detection, acquisition, and cue of potential targets. The first and second generation IRSTs utilized detectors with multiple
elements followed by discrete preamplifiers for signal read-out. They have many performance limitations. With the advent of
infrared focal plane array (IRFPA) sensors, the present trend is to build IRSTs based on line FPA sensors to achieve higher
sensitivity and resolution. However, due to system limitations of line IRFPA sensors, scanning mode of IRST cannot be
stopped at any desired position to scan a small sector of interest. They also suffer from more false alarms in target detection.
In future, it may be desirable to reduce false alarms, and also to use an IRST system for closed-loop-tracking of a potential
target, in addition to its surveillance mode. IRST based on area array sensors may be a better option for this purpose, but it
may pose some problems when used in a surveillance mode. This paper addresses this issue. Design considerations of all
sub-systems of an IRST based on line/area array sensors, such as scanner assembly, interface electronics with the sensor,
nonuniformity correction, signal processor, and the display methodology to cover 3600 are also discussed.
Keywords: IRST, scanner assembly, nonuniformity, display, interface electronics, closed-loop-tracking.
1. INTRODUCTION
InfraRed Search and Track(IRST) system is similar to a surveillance radar with a wide field-of view(FOV) but operates in a
passive mode. Being passive, it would not give away its position to the enemy and hence it is resistant to jamming. The
technologies involved are similar to those used in thermal imagers(TI) and forward looking infrared (FUR) systems but with
a difference. TIs are meant for obtaining high resolution imagery within specified FOVs in order to detect, recognize, and
track the target at maximum possible range with human intervention. It uses line FPAs (LFPA) with mirrors to scan in
azimuth and elevation. FUR is also an imaging system ,whoseoutput is usually an JR image of the forward scene. It is used
as a navigation and landing aid. The present FURs are also being used for target cueing. JRST, strictly speaking, particularly
first generation IRST, is not an imaging system, characterized by radar like scan patterns, covering a wide FOV. With the
advent of high resolution line sensors ,thecurrent IRSTs are able to form images and cover 3600.
Thefirst and second generation JRSTs utilized detectors with multiple elements followed by discrete preamplifiers for signal
read-out. They have many performance limitations. With the advent of infrared focal plane array sensors, the present trend is
to build IRSTs based on LFPA sensors to achieve higher sensitivity and resolution. However, due to system limitations of
LFPA sensors, scanning mode of IRST cannot be stopped at any desired position to scan a small sector of interest. In future,
it may be desirable to use an 1RST systemfor closed-loop-tracking ofa potential target, in addition to its surveillance mode.
IRST based on area FPA (AFPA) may a better option for this purpose, but it may pose some problems when used in a
surveillance mode. This paper addresses this issue, including design considerations of all sub-systems of an IRST based on
line/area array sensors, such as scanner assembly, interface electronics with the sensor, nonuniformity correction, signal
processor, and the display methodology to cover 360°. JRST comprises of (i) scanner assembly with detector, optics and
necessary electro-mechanical assembly, (ii) interface electronics to supply the necessary bias voltages and different clock
signals, (iii) 2-D signal processing to read-out the signals from JRFPA in real time, apply nonuniformity compensation, and
reformat the signal data, (iv) signal processor for target cueing, (v) display system to display 360° image in a user friendly
format. The block schematic of a typical third generation IRST is shown in figure 1.
Further author information -
R.V.:Email: [email protected]
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If we forget the sensitivity aspect for a while, AFPAs can offer lot advantages in respect of target detectabilty with less false
alarm rates, variable scan rate, and closed-loop tracking of the target. These aspects are discussed in section 5.
Figure 1. IRST system block diagram
The following sections briefly discuss various subsystems of IRST. Criteria for selecting an IRFPA sensor is discussed in
section 2. A brief account of optical system, particularly to cover 360° is given in section 3. The interface electronics and
nonuniformity compensation are mentioned in section 4. The advantages of using AFPA sensors are discussed in section 5.
Thedisplay methodology is given in section 6. The conclusions and future scope of work are given in section 7.
2. SELECTION OF AN INFRARED SENSOR FOR IRST
The focal plane array sensor is the key component in IRST, and it must provide adequate sensitivity, and should support
compact optical configuration. There are three generic detector types ,namely, (a) LFPA, (b) LFPA with TDI, and (c) fully
staring area FPA (AFPA). These are illustrated in figure 2.
2
11 SCAN L SCAN
LINEARRA/Y -, .) ,- WLINITEII ID!/ A(SRTEAAR IANRGR)AY
ARP.AY
Figure 2. Three generic types of detectors suitable to IRST
The applicability of these detectors to IRST has been extensively studied by various users, particularly by Pilkington
Optronics. In terms of better sensitivity and high resolution, the LFPA with TDI shown in figure 2(b) is the best choice for
IRST. So far no one has used area arrays for IRST application due to some problems which are addressed in this paper. The
advantages and disadvantages of line and area array FPAs are given in table 1.
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Table 1. Applicability of line /area array IRFPAs to IRST
SINo. parameter linear array IRFPA Area array IRFPA
1 . sensitivity about 1 51 W1 Hz 1/2 cm about 1 .5x 101 1 W1Hz"2cm
@0.25 sr -
2. size 288x4 & 480x4 available 256x256 & 320x 240 in 3-5 jim
256 x 256 in 8-12 jim with frame rates of 400
frames/sec
3. resolution high resolution possible currently not possible
4. spectral region 3-5 & 8-12 jim 3-5 pm ok, 8-12 yet to be field worthy
5. TDI mode established only possible with futuristic architectures with
sweep_and_read-out rates_are_matched
6. applicability excellent may be feasible with innovative methods
7. target detection and target cueing possible may be possible with new techniques
tracking
8. FOV wide FOV feasible limited to array size. WFOV, can be obtained
by mechanically scanning/panning -butresults
in severe image smear
9. track record systems available no system using area arrays for IRST
10. variable scan rate very difficult possible
If we forget the sensitivity aspect for a while, area array FPAs can offer lot advantages in respect of target delectability with
less false alarm rates, variable scan rate, and closed-loop tracking of the target. These aspects are discussed in section 5.
3. SCANNER-ASSEMBLY DESIGN CONSIDERATIONS
dome
,
dewar and adjustable supports
Slip-ring
direct drive motor
Figure 3. Sketch of experimental IRST prototype
The main issue in this paper is to explore the possibilities as to how to use area FPAs for IRST to achieve some advantages
mentioned above. Hence, a prototype IRST(experimental platform) is under development using 3-5 jim sensors, to try out
some new concepts for signal processing. The basic configuration of IRST with a lot of flexibility to align optics and FPA in
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all three axes, change of sensors, optics, etc. is shown in figure 3. A steering mirror provides the elevation control. The size of
the mirror decides the instantaneous vertical FOV. Scanning the mirror in azimuth, one may get a maximum of 600 coverage1.
One way to achieve 3600 coverage is to move the whole assembly to a new position and get another 600 and so on. This
demands a derotation optics and slip-rings, in addition to mechanical scanning in azimuth. For experimental prototypes, it
may be better to opt for only mechanical scanning, thus doing away with derotation requirement. However this scheme
requires a slip-ring to take out the signals from FPA which is on-board rotating platform.
3.1 Optics design considerations
The design of the 3 -5 imoptics is fundamentally not very different from the design of visible optics. The major design
considerations are F/number, field-of-view focal length, first-order layout, and the availability of optical materials. In
addition, however, the optical designer has to cater for the stringent requirements of the cold shield efficiency, narcissus
effects, and methods of athermalization2 .The designer may also have to decide on the use of refractive or reflective optics,
scanners3 ,aspherics4,and/ordiffractive optics.
3.2 Optical materials
There are many types of optical materials5 that could be used for the MWIR waveband. These include semiconductor
materials, chalcognide glasses, alkali halides, and dielectrics. Of these materials, four types are most common, viz. silicon
(Si), germanium (Ge), zinc selenide (ZnSe), and zinc suiphide. This issue of selecting an optical material for a particular
application are well discussed6. In general, the desired properties for field applications are: high refractive index, low
dispersion, low absorption, compatibility with anti-reflection coatings, low thermal coefficient of refractive index, high
surface hardness, high mechanical strength, and insolubility in water.
3.3 Scanmirror
Depending on the search field of view in azimuth and elevation, this mirror has to be scanned with required speed6 .This can
be static if the complete assembly is rotated in as shown in figure 3.
3.4 Optical derotator
Image rotation occurs when a beam is steered in azimuth by a 45°mirror.This means that the vertical lines in object space do
not remain in a constant angular relationship to the detector. Generally it is compensated by means of optical derotator, which
is difficult to realise and implement. However, for the purpose of experimental platform, it is avoided by rotating the
complete assembly.
3.5Narcissuseffect
The narcissus effect is a phenomenon in which the cooled detector senses its own cold surface as a result of internal retro-
reflections from the refractive surfaces of the system. It arises at any time during a scan, when part of the cold detector focal
plane is reflected by a refracting surface within the system, such that this reflected image is focused back onto the detector
array. This cold image will be superimposed onto the signal field and part of the warm background, resulting in systematic
pattern noise. The shape of narcissus depends on the shape of the cold focal plane area and the extent to which it is focused.
Design methods with which narcissus may be reduced include: (i) reduce the effective radiating cold area of the focal plane
by warm baffling, (ii) reduce the surface reflections of lenses using high-efficiency antireflection coatings, (iii) defocus the
cold return by designing the optical system so that no confocal surfaces are present, and (iv) slant all flat windows at an angle
to increase the angle of incidence. Practical measures to cut down narcissus can be effected through the use of a thermal
source or by electronic video signal compensation7.
3.6 Athermalization
Thermal effects are inherent in infrared imaging systems as most of the useful infrared materials, especially germanium, have
a significant value of nJT, or thermal coefficient of refractive index. In addition, most infrared imaging systems are used in
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environments where extreme operating temperatures of —20°C to +40°C are often encountered. Hence, the effects on the
imaging systems due to changes in refractive index with temperature must be considered. The thermal defocus caused to a
single thin lens in air, Af, is given by8
f
where a is the coefficient of thermal expansion of the refracting medium, f is the focal length, cc,1, is the coefficient of thermal
expansion of the mount, L is the overall length of the system, n is the refractive index of the lens, and AT is the temperature
change. However, the thermal effects of multiple lens groups are more complicated in optical designs that are more complex,
such as the telephoto design, the Petzval design, and the inverse telephoto design.
If Af is greater than the depth of focus of the system, compensation schemes must be implemented. Active and passive
athermalization may be attempted. Some possible solutions are: (i) To provide a manual focus adjustment, (ii) To provide an
automatic electromechanical focus adjustment, (iii) To athermalize the lens by a combination of lens materials having an
effective an/T of zero, (iv) To athermalize the lens using lens mountings of different coefficients of thermal expansion so
that the optics will move passively to compensate the defocus, and (v) To use diffractive optics to attain passive
athermalization.
3.7 Optical system
Dome
1.0
.9
.8
Objective
Lens UI0-- .7
.6
I-
h
C3l)
.3
.2
.0
-1. Relay
jLens
FPA —4 SPATiAL FREQUENCY IN CYCLES PERMIWMETER
Figure 4.Optical lay-out for an Figure 5.PolychromaticModulation Transfer Function
F/1.6 MWJR lens for optical system shown in fig.4
An optical system was designed around a line array sensor 288x4 from Sofradir, with a focal length of 92.35mm,F-number
of 1.6, and cold efficiency of 100%. The optical layout is shown in figure 4. A dome is used for the protection of the lens
from the field environment. A steering mirror provides the elevation control. The objective is made of a standard triplet
having near-diffraction-limited performance. A relay lens transfers the intermediate image onto the MWIR focal plane array.
Cold shield efficiency is 100%. The MTF plots are given in figure 5. Atotal of three aspherical surfaces are used for
aberrational control and the spherical surfaces are test-plate fitted to the vendor's stock. To minimize costs, only Si and Ge
are chosen for the refractive elements. Diffractive optics were not used because of resource constraints. In an earlier design,
diffractive optics were designed for aberrational control as well as a means for athermalization9 ,but not opted for this
application.
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Narcissus may be controlled in the optical design through the use of the paraxial quantity, yni, which is a product of the
marginal ray height (y), refractive index (n), and the incidence angle at the refracting surface (i). The absolute value of this
quantity measures the severity of narcissus signal. The smaller this quantity is, the greater the narcissus effect will be.
4. INTERFACE ELECTRONICS AND NONUNIFORMITY COMPENSATION
As per the commercial data sheets available from the vendors, line array FPAs with TDI architecture require a good number
of well regulated bias voltages, clock signals, parallel channels to read-out signals, and complicated address sequence. In
contrast, the interface signals required for area array FPAs are very few and less complicated. Signal processing is straight
forward.
4.1 Nonuniformity compensation
FPAs offer higher sensitivity, amenability to signal processing and mechanical simplicity. However these sensors contain
large detector-to-detector dark current (offset) and responsivity (gain) variations. These variations result in nonuniformity in
the detector array.
A typical solution for NUC of IRFPA sensors is to calibrate the sensor by presenting uniform sources of constant intensity
over the detector field of view. The outputs are used to calculate the correction coefficients i.e. gain and offset terms. Low
and high temperature references have to be set around the mean temperature of the scene. If the individual pixel outputs are
perfectly linear within the calibration range and stable in time, then it is possible to completely correct the pixel
nonuniformities. However, 1/f noise and system instabilities create the need for recalibration .Thiscalls for precise variable
temperature references to be generated on board the system. This in turn calls for a mechanical/electro-optical shutter to bring
the references into the detector' s field of view for periodic re-calibration. This detracts from the mechanical simplicity of
IRFPAs. It is better if nonuniformity compensation can be implemented by making use of the moving IR scene statistics
which dispenses with the temperature references. The SBNUC concept makes use of the scene statistics for calibration and
update of compensation parameters without masking the field of view. This scheme assumes a moving scene across the
IRFPA sensor over a long period of time. It also assumes that each pixel in the array sees the same scene statistics.
It may not be possible to incorporate temperature references on-board IRST if it has to cover 3600. in view of this, two-point
as well as scene-based nonuniformity compensation algorithms were studied and analysed10. It was found that scene -based
algorithm was also performing well, but resulting in artifacts. It has been decided to develop a common hardware to
implement either of the algorithm.
5. ADVANTAGES OF USING AFPAs
Let the specifications of IRST are (i) cover 36O in azimuth and 5° in elevation, (ii) automatic target detection with low false
alarm rate, (iii) if necessary observe for more time in the desired direction, and (iv) if possible track the target in autonomous
mode. Just to understand the concept, let us as consider 256x4 line array with TDI, and 256x256 area array for this
application. The general scan pattern in both cases are shown in figure 6.
LFPA covers the 360° in continuous scanning mode. The scan rate has to be synchronised with integration time, signal read-
out time for obvious reasons. To vary these parameters in real-time may be very difficult and therefore it may not be feasible
to vary the scan rate from the designed specification.
AFPA covers the scene in discrete steps as shown in the figure 6(b). If it covers 3600 in a continuous mode ,there will be
serious objectionable smear in the image. This is the main reason why AFPAs are not used. Assuming 36000 pixels in 3600,
256 pixels correspond to 2.56°, say 2.5°. That is AFPA needs about 140 steps to cover 360° without any overlap. If 0.5°
overlap is allowed as shown in figure 6(c), it may need 1 80 discrete steps. In case of AFPA, in order to see a contiguous
picture it is necessary to register adjacent overlapping areas. It is reported that more than 400 frames per second can be
expected ,i.e. it needs about 0.45 seconds to cover 360°. LFPAs ,due to their complicated read-out mechanism, it may take
about two seconds to cover 360°. That means, LFPA takes 10 msecs to cover 2.5°, where as AFPA can stay at the same
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frame during that time getting four frames of data. Assuming overheads, the worst case can be two frames. If we can realize
this, it is of much use to detect targets with low false alarm rate.
3600
2.5° 2.5° 2.5°
Linear array
9 ___(a)
___________
Areaarray 2.5° 2.5° 2.5°
(b)
___
i
25
2•5° 2.5°
Areaarray 4
(c)
H ..I.II... .
overlapping image
Figure 6. Scan pattern with linear array and area array
5.1 Target detection with low false alarm rate
48km
Figure7. Sketch representing relationship and object space and image plane
Let us assume an aircraft target, with physical dimensions of 3mX3m, crossing across at a range of 8 km. Considering an
instantaneous sector of 2.5°,as shown in figure 7, the sector at 8 km covers an area of 350m.That is 350meterscorresponds
to 256pixelsin the image plane. Each pixel corresponds to about 1 .5metersin space at 8 km. 3x3 meter target may occupy
about 4 pixels in the image plane. If we consider that the target is moving across with a speed of one Mach i.e. approximately
300m/s, the target may move by about 1 .5meters(corresponds to one pixel) during 5msecs, i.e. two frame times of AFPA. In
such cases it is easy to detect the target without false alarm.
The above concept is tried out on some images as shown in the figures 8 and 9. Ideally when the two frames are identical
except for target movement, the difference picture should give out the moving target. The frames 8(a) and (b) are taken with
AFPA sensor. There was no sensor movement, but the target was moving. The difference of these two frames (figure 8c)
results in detection of the target. The black and bright spots correspond to the previous and the present position of the target.
Based on one frame, which should be the case in LFPA scanning, it is very difficult to detect point targets. The same exercise
is repeated on other images as shown in figure 8 (d), (e) & (f). However, due to intensity fluctuations, moving tree leaves, and
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