Electronic Scanned Array Design John S. Williams

128 Pages·2016·5.89 MB·English

Electronic Scanned Array Design
SCF01
John S. Williams
The Aerospace Corporation (retired)
[email protected]
Slide 1
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Course Objjectives
• Provide a basic understanding of ESA design principles,
history and applications
– PPrreesseennttaattiioonn wwiillll ffooccuuss oonn aanntteennnnaa hhaarrddwwaarree
– Radar antennas are the focus of this presentation
• Communications and receive antennas differ onlyy in details
– ESA functionality enables or enhances radar modes but radar
modes will not be addressed in any detail
Slide 2
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Abstract
Design Principles and Approaches
GGeneral design principles off aperture antennas are applied to the speciffic case off ESSA
design. System applications set the framework for requirements allocation and flowdown.
Antenna Architectures and Functional Partitioning
The advantages and disadvantages of ESA and reflector antennas as well as ESA feeds for
reflectors are compared and contrasted. Common ESA design issues are described, including
array partitioning and subarrays, lattice tradeoffs, feed design, causes and mitigation of
ssiiddeelloobbeess, bbeeaamm sstteeeerriinngg aapppprrooaacchheess aanndd tteecchhnniiqquueess ffoorr bbeeaamm sshhaappiinngg. NNuummeerriiccaall eexxaammpplleess
using Matlab illustrate performance of specific designs.
Practical Design Considerations
EESSAA ppeerrffoorrmmaannccee iiss ccoonnssttrraaiinneedd bbyy tthhee sseelleeccttiioonn aanndd lliimmiittaattiioonnss ooff ssppeecciiffiicc ccoommppoonneennttss.
Objectives of size, weight, power, thermal dissipation, performance and cost drive tradeoffs
among radiating elements, T/R modules, monolithic microwave integrated circuits (MMICs),
microwave distribution and packaging.
Proposed and Operational Examples
Recent radar satellite designs will be assessed to illustrate actual performance and design
tradeoffs. Current L-band syystem ppropposals contrast different desiggn apppproaches.
Slide 3
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Antennas
•• OOnnee ooff tthhee mmoosstt iimmppoorrttaanntt ddeetteerrmmiinnaannttss ooff mmiiccrroowwaavvee ssyysstteemm
(radar, communications, other) performance
• Requirements are determined by system performance allocation and
ffllooww-ddoowwnn
• Attributes include:
– Beam width, shape and sidelobes
• UUnniiffoorrmm iilllluummiinnaattiioonn ssiiddeelloobbeess-1133 ddBB ((rreeccttaanngguullaarr aappeerrttuurree)) oorr -1177 ddBB ((cciirrccuullaarr
aperture) are too high for most purposes
– Instantaneous and tunable bandwidth
– Size
•• SSAARR ((ssqquuaarree)) vvss GGMMTTII ((rreeccttaanngguullaarr)) AAssppeecctt RRaattiiooss
• Deployment
– Thermal Dissipation
– Weigght
– Cost
• Thermal dissipation and power consumption will restrict system duty
factor
Slide 4
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Electronicallyy Scanned Arrayy ((ESA))
• An ESA combines multiple elements with phase or time
delays to form a beam in a specified direction
– IInn ccoonnttrraasstt ttoo aa mmeecchhaanniiccaallllyy sstteeeerreedd aanntteennnnaa pphhyyssiiccaallllyy rroottaatteess
an antenna to point a beam in a specified direction
• Phase or time delayy is reqquired to scan the beam
• Gain control is required for beam shaping
• EESSAA’ss ccoommmmoonnllyy iinncclluuddee aammpplliiffiieerrss
– overcome distribution and control loss
– Replace transmitter power amplifier (TWTA)
Slide 5
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Reflector Antenna Radar Block Diagram
Exciter Transmitter
al
b
FFrequency r mm a
Data request Control &R eT fi em r ei n n g c e Processor uplexe Gi ntenn
D A
RRaaddaarr ddaattaa SSiiggnnaall PPrroocceessssoorr RReecceeiivveerr
Receiver
Protection
ESA incorporates functions
Power Supply
shown in dashed box
Slide 6
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Electronically Scanned Array Radar Block
DDiiaaggrraamm
TRM
Exciter n)
os
iuti(d( TTRRMM
bol TRM
trinif
s TRM
DiMaM
Data request Control &FF r Te iq m u ie n n g c y Processor B gic ng TTTRRRMMM
Reference e Lomi
r TRM
&o
aa
r mff TRM
m wea
e TRM
o
B
P
TRM
RRaaddaarr ddaattaa SSiiggnnaall PPrroocceessssoorr RReecceeiivveerr((ss))
TRM
ESA incorporates functions
Power Supply
shown in dashed box
Slide 7
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ESA Benefits
• Multiple beams
• Instantaneous beam steering (agile beam)
– Reduces slew and settle time
• Mainlobe shaping, sidelobe control and nulling for clutter and interference
mitigation
• Multiple phase centers for MTI & multi-channel SAR
– EEnnaabblleess aannggllee ooff aarrrriivvaall mmeeaassuurreemmeenntt
– Additional degrees of freedom for clutter and interference mitigation
• Multiple concurrent radar modes.
• Lower loss between amplifiers and free space
• Inherent redundancy (multiple elements)
– Graceful degradation
• Electronic Attack (EA) with very high Effective Radiated Power (ERP)
•• SStteeaalltthh
– Better match to free space –much less reflection/reradiation
• Antenna surface deformation (deliberate or accidental) may be compensated
• Space combining (low loss) of solid state power amplifiers
Slide 8
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ESA Performance Impprovement

26 -3 dB • Multiple Azimuth Beam
24 – Improved SAR resolution
2222 • MMullttiiplle EEllevattiion BBeam
20 – Improved stripmap area
)
m
raattee
k 1188
((
e ← Boresight
g – SCORE (SCan On
n 16
a Receive)
R
14
12
Sensor altitude is 10.0 km
10
Range to horizon is 357.3 km
Boresight range is 20.0 km
8 Grazing angle = 30.0°

-10 -5 0 5 10
Cross Rangge ((km))
Slide 9
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Technologgyy Environment
• ESAs have recently become very prevalent for the sole
reason that they have become much more affordable
((tthheeyy wweerree aallwwaayyss kknnoowwnn ttoo ooffffeerr ssiiggnniiffiiccaanntt bbeenneeffiittss bbuutt
were unaffordable)
• TT//RR mmoodduulleess aarree aa ssmmaallll ffrraaccttiioonn ooff rraaddaarr ssyysstteemm ccoosstt
and a very small fraction of system cost
Slide 10
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Aperture Design
Slide 11
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Antenna Function
• Antenna objective is to create a current/voltage
distribution which creates a specified beam pattern or
vv//vv.
– Omni directional radio signals of little use (except for
broadcasting)
• Difficult to arrange in general
– Arrays permit a sampled representation of current/voltage
permiittiing allmost any ddesiiredd arrangement
• Two design approaches – analysis and synthesis
Slide 12
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Basic Apperture Shappes
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
• Sqquare apperture
• RRoouunndd aappeerrttuurree
– 4 by 8 wavelengths – 3 wavelengths radius
– First sidelobe is -13.2 dB – First sidelobe at -17.8 dB
– 3 dB beamwidth = ±0.866 λ/D – 3 dB beamwidth = ±1.03 λ/D
– first null at ±λ/D – first null at ±1.22 λ/D
From Balanis
“AAnntteennnnaa TThheeoorryy”
Chapter 11
Slide 13
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Analysis Regions
((eexxaacctt ttoo aapppprrooxxiimmaattee))
Fresnel or Fraunhofer
Near Field Transition or Far Field
Region Region Region
a
n
n
e
ntn
A
Nominal
Beamwidth
FFoorrλλ== 33ccmm aanndd 00 1D62λ D4λ2 D2λ2 Dλ2 2Dλ2
D = 1 meter 2m 8m 17m 33m 67m
D = 10 meter 208m 1,667m 3,333m 6,667m
833m
Illustration from Lynch (© SciTech PublishinSgli,d Inec 1),4
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Reggions
EEvanescentt NNear FFiielldd FFar FFiielldd
Fresnel Fraunhofer
NNeeaarr lliimmiitt 00 33λλ 22DD²//λλ
Far limit 3λ 2D²/λ ∞
Power decay R-n 1 R-1
E and H No Yes Yes
orthogonal
ZZ == 337777 ΩΩ NNoo YYeess YYeess
0
•• LLaasseerr PPooiinntteerr
• (cid:540) = 630 nm, D = 1 mm => farfield at 3 meters
Slide 15
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Another Visualization
λ
4
Slide 16
λ λ
S3CF01 Electronic Scanned Array Design 2D²/
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General Conceppts
• LLiineariitty andd superposiittiion
• Reciprocity (Lorenz)
– System behavior is independent of direction of energy transfer, ie antenna
pattern iis thhe same ffor transmiit andd receiive
• Antenna pattern is the Fourier transform of aperture illumination
– Discrete (sampled) vs continuous
– The sample interval is the element spacing
– λ/2 element spacing assures no grating lobes
(Nyquist-Shannon sampling theorem)
– RResolluttiion lliimiitt ((RRaylleiighh criitteriia))
– Round vs square
• Projected aperture (cosine θ dependence)
– Wheeler - Pozar
• Polarization and principal planes
• Raaddaar Raanggee Eqquuaattioon
Slide 17
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Resolution
• RRange measurementt iis ddiirecttlly rellattedd tto bbanddwiiddtthh
– Wide bandwidth waveform (eg chirp) required
• AAnnggllee mmeeaassuurreemmeenntt iiss ddiirreeccttllyy rreellaatteedd ttoo aanntteennnnaa ((aappeerrttuurree))
size
– Can generate “synthetic” apertures larger than physical antenna
ssiizzee bbyy eexxppllooiittiinngg oowwnn ppllaattffoorrmm mmoottiioonn
• Angular resolution (Rayleigh criterion)
– Coherent or non-coherent
– Deconvolution of PSF allows higher (super) resolution subject to
S/N
– Consider two point sources (sinx/x) separated by small distance, fit
siinx’’//x’’ andd ttakke ddiifffference, llookk att PPdd//PPffa
– Elements spaced closer than (cid:540)/2 potentially provide better
resolution
Slide 18
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Projjected Apperture
• Projected aperture is the apparent angular extent of the
aperture as viewed from a specified direction
• AAnttenna gaiin iis proporttiionall tto projjecttedd apertture
• Harold A. Wheeler derived this relationship in an early
ppaappeerr
Broadside
θ=30 θ=60 θ=90 Slide 19
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Radar Rangge Eqquation
• Radar range determined by antenna size (area), transmit
power, receive noise figure and bandwidth
P G262<
t
SNR =
((44::))3kkTT BBFFLLRR4
ee
P = transmit power
t
G = antenna gain
λλ= wavellengtthh
σ= target cross section
k = Boltzmann's constant
TT = ssyysstteemm tteemmppeerraattuurree
B = system bandwidth
F = system noise figure
L = system losses
R = range to target
Slide 20
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