CTD MTI Radar Filter with Charge-Transfer Inefficiency Compensation

--a) a) :3 4Ji 0 _-0.3 0.5 0.7 1.0 0.3 0.5 0.7 1.0

Roll -off factor : k Roll-off factor: k

(A) (B)

Fig. 3. (A) Full cosine roll-off. (B) Half cosine roll-off. Quadrupler outputcarrier and noise power; noise power is evaluated in thevicinity of thecenterfrequencywithfrequencyslot of 1/256THz,whereT=

symbol length.

I. INTRODUCTION

[1] Gardner,F.M.(1980) Self-noise insynchronizers.

IEEE TransactionsonCommunications, 1980, COM-28,

1159-1163.

[21 INTELSAT Document(1980) TDMA/DSIsystemspecification. BG-42-65E, June 1980.

[31 Davenport, W.B., and Root, W.L.(1958) RandomSignalsand Noise.

New York:McGraw-Hill, 1958. [41 Bennett,W.R., andDavey, J.R.(1965)

Data Transmission.

New York:McGraw-Hill, 1965.

CTD MTI RADAR FILTERWITH

CHARGE-TRANSFER INEFFICIENCY

COMPENSATION

The design and implementation ofa second-order nonrecursive

moving target indication (MTI) radar filter using commercially availablecharge-transfer devicesasdelay linesaredescribed. Asimple

techniqueis included tocompensateforthedevicecharge-transfer

in-efficiency and its sensitivityisanalyzed.

Experimental laboratory tests and results in an operating radar system are reported showing the good performance of the realized

MTI radar filter.

Manuscriptreceived May21, 1981 revised December 1, 1981.

0018-9251/82/0900-0704$00.75 1982IEEE

The operation of present-day tactical and

surveillance radars in heavy clutter environments

re-quires efficient means to extract moving targets from

the clutter background. The criterion of discrimination

between thetwo typesofradar returnsignalsuses their

associate Doppler frequency shifts. Amoving target

in-dication (MTI) filter is employed to suppress the low

frequencies of the clutter echoes. The MTI filter

operates on a pulse train modulated by the received

Dopplerfrequencies obtained after demodulation ofthe

echo signals to the video range and is able to discern

moving targets even in the presence of clutter echo

signals many orders of magnitudegreater.

Inparticular, digital recursive andnonrecursiveMTI

filters have been proposed for clutter suppression in

operatingradar systems [1-3] for theirinherent stability

and flexibility. The performance of digital MTI filters

can be very satisfactory if digital words of sufficient

length are employed, especially when a large dynamic

range is required. However, the presence of

analog-to-digitalconversion andthenecessarydigital circuitry give

riseto a solution forthe MTI problem that insome

ap-plicationsmayhaveanexcessivecost orwhose technical

realizationmaybeprecludedbecause ofphysical

dimen-sions, weight, power consumption, or pulse-repetition

frequency (PRF)value.

Morerecentlythe advances incharge-transfer device

(CTD) technology have shown the technical feasibility

IEEE TRANSACTIONSON AEROSPACEAND ELECTRONIC SYSTEMS VOL. AES-18, NO. 5 SEPTEMBER 1982

REFERENCES

ofan analogdiscrete-timesolution to the MTI problem

[4,

51,

which may provide satisfactory performance

while maintaining the characteristics of flexibility and

good performance at a low cost. CTDs can be used as

the delay lines required in first- and higher order MTI filters. However, they are not ideal delay lines and one

of the major contributions totheir nonideal behavior comes

from the charge-transfer inefficiency (CTI), i.e., from

the fraction e of the stored signal sample that remains

behind at any transfer step [6].

The design and realization of a second-order MTI

filter implemented by commercially available CTD

delay lines is described here. In Section 1I the problem

of theCTIisconsidered inparticular.Asimple and

effi-cient technique for compensating the CTI effects is il-lustrated and its sensitivity is analyzed. Section III

shows the performance of the CTI-compensated

second-orderMTI filter obtained in laboratory tests and

in connection with an operating radar system.

II. CTDMTI FILTER

in

_|~~CTD

DELAYLINE

-T

EAYLN

out

Fig. 1.Structureof second-ordernonrecursiveMTIfilterusingCTDs.

which corresponds to an actual impulse response

d'(t)

=

(1

- NE) (t -

NT)

+ N£ d(t - NT-

7)

A. CTI Compensation Technique

Inaradarsystem atthe inputof thedigitalordiscrete

MTI filter the whole-range echo signal is

sam-pled at an appropriate rate IIT, in order to prevent

un-acceptable loss indetectability [1]. Hencethe MTI filter

must delay the echo signal samples by multiples ofNT,

where N = TIT isthe total number ofecho signal

sam-ples contained inoneradarinterpulse period

T,

= /PRF.

Inparticular it is known [I] thatthe operation ofa

non-recursive second-order (three-pulse) MTI filter is

char-acterized by a z-transfer function

G(z) = 1 - 2z-N + z-2

instead of the desired one

d(t) = d(t -

NT)

(5)

where d(t) is the Dirac delta function.

Asa consequence of the attenuation 1 - NE of the

firstpulseandthepresenceof thesecondpulsein the

ac-tual response(4), clutter residueswillremainattheMTI

filter output of Fig. 1.

The CTI effects on the CTD delay line can be

eliminatedby cascadingasuitableequalizerfilterE(z)to

the transfer function D'(z). The equalizer E(z) has to

satisfy the relation (1)

where z1 is the delay operator associated with the

sampling frequency 1/T.

The filter (1) can be implemented by using two

N-stage CTD delay lines as shown in Fig. 1. However

theirCTIEcausesthe actualtransfer functiontodeviate

from itsideal valueZ-N.Assumingtheproduct NE< I (a

condition almost always satisfied in practical

applica-tions), it was shown in [7] that an N-length transversal

filter, designed to havea desired transfer function H(z),

when realized using CTDdeviceshaving aCTI value £,

givesrise to an actual transfer function H'(z) expressed

by

H'(z) = H(z) + E(z -

1)[dH(z)/dz].

(2)

Hence for anideal delaylineD(z) = zN, the actual

transfer function D'(z) becomes

D'(z) = (1 - NE) Z-N + NFZ-(N+1) (3)

D'(z) E(z) = zN.

From(3) it follows for E(z)

E(z) = 1/[(1 - NE) + NE

z1

Z

(6)

(7)

which can be easily realized as a first-order recursive

discrete filter. The resulting structure of the equalized

CTDdelay line is shown inFig. 2. The inputandoutput

waveforms ofaCDTdelayline with andwithout the CTI

compensation accomplished bytheequalizer (7)areshown

in Fig. 3. In Fig. 3(A) the output signal distortion due

to the device CTI is very clear, while in Fig. 3(B) the

outputsignal appearstohave been wellequalized. In this

test T was equal to 2 ,s and the measured product NE

was 1/12, with Nequal to 256.

0018-9251/82/0900-0705 $00.75 1982 IEEE

CORRESPONDENCE

(4)

I N STAG

- l 1 L NP

Di

Let US SLppose tnat in the CTD delay line of Fi! 2.

etqalized for a spectfic CTI value t, hechargec

Ltransf-se

loss changes to a nce x,aleI ¢ i- An. The overall

transfer function A4(z; of tthe systenm oft Fig. 2 is the

cascade

of (3),

exalatetaad

for the new

value}

, and of i

4s 4E%I 11(z) (RI 1VE ) Nit z

- I1

whose inveise tranristorTn is [8)

n(4

t) [(I )/ )j

d6(1

NT)C

'\,lr) "

>2 \c'l/)--eI 6K(; NI '-k

Thie

assumption Vt « i. and of course also NE7t

m1pisee that ()id\ thic t'Irs' aXv U rn. ire tt practcal a

tereNt Hence )9c n-)

approx

mated

ats

'n(t) oJ(1 \5 A)-: a4t;s -NT

NA A (t! ---- NJi -- II

B. Sensitivit, Anaki.Y'SI

tie

cqujatw

. -. irtnersate, otaf.,xed va'tue

t-4 lt"ii w-oduct \Nir HoVQ""lC..chanPges 'in te

g&x1Ing

to

temper.-atu.reo .t, A ariatiocrs, -art p

pr~pra~el a(":~'-lf. ,rCroua,ec by mewasuring at n1ierkalsthe

product

\`E and

dhangintg-

the feedback oc-op weight and,

ruea'nliticr ata. _a e-}rcbngix

u2A

alt akitomnalIL"'

exter-fiall> ,xtnim.otieu

v

In tihe

fo~-llowling.

sensitivity analvs;ls ot hthe

loin-I~~~~

nIEl1

'iue: "lIMls

pensatro-

eco O tito\4h ro n

'imormitria (a'to ia Value is gi-V-'

j -4 _0 W*th i,u' H- '19 IFsIUt

i-(It))0)

v%hich is ot the samie ''r o

iwounequalized

d C P'

delay line imptlse 1esp.-nse (4), ;xrt rcplacede by A

The restult showin in (10) max be exploited o find a

bounid

for the desiation

Ar0

of

tthe

device ('Ti from

v.s

nomrinal salie 1:used ir the

eqiualizer,

according

to the

acceptable levtl of'clutter residuets at the TI titter oun,"

put

HI1. IMPLEMENTATION AND PERFORMANCE

OF THE CTD MTII[EUER

'The complete

snie,C_ture

o-f' e-ie second-orde-r

ClTL)

MWrl filter including

tilc,

CI

Ic-ompensation

is illustrated

in Fig 4 and its

perfa-rnmance w.was

verified by laboratorS

cts)ts arnd

in an operating

radar environmenr.

It can be

observed that

th-ie

final adder of the filterof

Fig

4works

or sampled

signals,

differenttly

fi-om other reported ap

proaches [4, 51 that add signals after their reconstrutIc

I-oni)

continruouis

fo-Drm-r The

solutior of' Fig, 4 was

preferred because it leads to an accurate time quantiza

tion and

alignmnenit

of the sigrals at the

tihree

adder

if-puts without

requitniag

thle

presence of equal

transmrsi,_

sion networks tor the reconstruction ofthe continuous

signals along

thie

three ditferent paths

The actual

implemenltation

of the equalizer E(z) as

the

first-order

recuLiSiVe filte-r of Fig, 2 i's

straightfo-ward. The

operationl c`--t

does not require any

additional

delay element like a CTD cell but is

appropriately

realized by means of a sample and-hold circuit at the

differential

atmplifier

output

One also observes that ;n response to asynchronous

interference

pulses

theonlveffect

ol+f

the recursive

struc-Si 'iMRtN 9v f( I I\TE) -+ ;NT.- II 1 I -,. llcla, 1..,.-, 12 t .-,.I t. Al C. .-' c.ir,rcnsatscl-..-C' II1-1 7, I U " 1'I" 1, \- !-.) -11 -. A I I" % ., "N " I ,. . . ,. ., !; "). --.1,_". F-w() N,I '- ..-,, -,,"i N'!`..1

Input

.5

out

Fig.4. StructureofCTI-compensatedsecond-orcier nonrecursive MTI filter.

() 7.8125

Fig. 5. Frequencs response ofMTI filter with I/PRF

- l_ | lOutput

, | | _

i2 is~

Fig. 6. MTI filter iniput and output signals, simulating simultaneous presence oftarget and clutter echoes.

Kb' 0.512 ms.

ture of the equalizer is to possibly introduce unwanted

signals into distance-contiguous range bins, in case of

nonperfect delay lineequalization, leaving unaltered the

transversal MTI filter performancewith respect toradar

echoes from the same range cell at subsequent sweeps.

However, according to the results of the compensation

technique sensitivity analysis, these unwanted effects

can be made negligible.

A Reticon SAD 1024 device was used for the two

CTD delay lines of the filter of

Fig.

4, for which N

256. This CTD delay line is not the best choice for radar

applicationsdue to its inherent low sampling frequency.

However, its relatively high transfer inefficiency and its

immediate availability allowed us to test the proposed

method of CTI compensation. Fig. 5 shows the

measured frequency response of the realized MTI filter

with a sampling

period

T = 2 ps, and Fig. 6 shows an

example of thefilter input and output signals. The input

waveform simulates a moving target echo with a

Dop-pler frequency equal to half the PRF embedded in a

clutter return signal extended to several range bins. The

output waveform clearly retains only the moving target

signal component. The achieved MTI filter cancellation

ratio was about 56 dB.

Tests in an operating environment were carried out

including this filter in a radar system (SMA APS-705)of

the noncoherent type. Figs. 7 and 8 show the

plan-position indicator (PPI) displays without (Figs. 7(A)

(A)

(B)

Fig. 7. PPIdisplay ofarea around Florencewithmaximum rangeof 20nm. (A) without MTI filter. (B) with MTI filter.

and 8(A)) and with (Figs. 7(B) and 8(B)) the

second-order CTD MTI filterfortwodifferent maximum ranges.

The straight lines in Fig. 7(B) are derived from echoes

coming from

adjacent

radars that wereoperatingon slightly

different carrier

frequencies

at the moment the test was

performed. The straight lines are not present in Fig.

0018 9251 82090t)0707 $00.75 (eC 1982 IEEE Co)RRESP()ND NCN

7i-i

plex zeros can he synthlesized irstead of two real coinci

denit onies at z-z Therefore a wider clUtter rejectiona

region can be obtained by positioning Womplex zeros

around the point Z I

Experimental tests carried out in both a

laboiator-and ani operating raciar evironment hlave shown the

geod perfor-ma'nIce achiteved bK thec (TI-compensated

CTD second-order MT1 filter. F;'rther specific features

of this filter ire a low power consurnptioni, simall

[)h]N sicail

diilerit-t

ionsi

wctuhc.h

- md

(ht

ctsu c h

ane

Ing} the

sampimpng

rate. hmt,amh io the filtet to he used

iM3 radars einphI{n tiocerid PpRho

-KNOWIlEDGMIE:NI

The authors t-hank

Florence, for his helpful

f'B1

1 g 8 PPI displayofarea paodf to Coce vit rxironm .i's i '

$(B) because the test wNas performed at a differer' tine,

In these tests the

samphlng

intersal was f I 2 us. At

this ciock rat-

rhxaiue

the ofthe product

iN

was L C Even

Lhough this value is c'rnparable with mily the caipen sated MTul filter attained a good cluutter rejectwnA.m as

demonstrated by

FigsA

and 8,

'where

the retaiied

echoesc-orrespoad to

b-;De

mnoving

clotids of a windsvdayx

IV.- CONCLUSIONS

A second-order transversal

MITI

filter-l1

based

a,

tXe

useof

comnmrercially

asvalable CT[) delay

linses

has been-i

described, The filter includes

s,pecfic

compensation

net-w 'rks for elirntingn3 X,he cit A c.1devilc 1

which

oftherwvise

souhd

great]'

educe

;he

Nil] filter

-performnance. it$

>'mcmiIa F

n.rt'peii.aF1on

(

allowsKsLCcorid orde M1T

c{-anticeler

to be

rel

ahzedK byi

thcecompaCt suruCtu t.Fi1g-2tnsteadC. twe.a-ascaded

wiltht TI UtifA'%J'i3bit() be unfiatsi5l hei.a<iuse

of, its creatermn liTD CT m

An attractixeortN wsfor str utre Ci I-ic ;5

tohat, by chan^1rigine e weghtring coeffIcients in (1 om0 M

Dr. MN. Bernabo of

SMA,

cooperatiorn.

STIEFANO BOZZI

MBER1T() (.A XilATT11 \ 110 ('.APPELI,IN1 ENRI(O() DEL RE

IARR(X() NXIARGHFRI

:titO!ti di I}LtttrO]iiv1

vtiz\/ ItIi'J ii'i-!Iic

a.[ S. NMarita

IX AN SAiJLIN i55t.1 tr,i,rj'

REFEREN(IS

Linder, RA and kiaii. (1967)

Dlgitra' ot!iVg N tiidLa

Supplement to-'E JianScwron on Aerospace and lee ironic Sr.tisers, :Ne it I96 if S-3

E1] Roecke. A,V (I9271

The- applrtn- ofregital filters thr mnovingtargetl indica tion

IEEI-c'EJascr' AIdic and Electroacusts, Ma. I 92 s--1I9

h1] MarL W alidm Wood, 1A- (19'2-t

A recursive dlgla± M1 I radar fitrer. Pro(ceeding.s-< thetIEEEL,7 Junic 972, 60. [41 Lotbenstvin, 13. and 1Ludinrgton, 1) 11975)

\ enarge .rafe UiSnpiemC11ATI) rneniation).

irn r3roceeditngs oK:) the lIE i'nternataonai' Radar Con-ferer.e VsWashingtroir, V i. >91975

i51 Btieler J eLigght e'-I odherg H.S_ Pckeuttc, CIN

and tobenstelin H_ (19 6)

Chargeransfe t 410 ieniorts for radar and EGNI

svste'tA

IEEE Journal fN Sitl-State " rcunds, laeb, 1976, SC

'-?6] Gersho, A ¼979)

Clbarge-iarin,ser tltlerngn.

Proceeding.s tI-tie EEE Feb, 1979, 67.

lt)i Re, F (1980)

Anatais a pe-sdai m on . CITi ffects an C(&C trarnisversali f [tier aepotme

[EEI' Tra'nor n)cousac, Speeckh and .SIgnal Pro cessing, Dec. 198(1'1,ASSP28.

(8h jury, .L (1964)

The'r. an AIpplication * --Trant/erra Vfefirt,ho

Nesk Yrmok Wilel's 964

'.'f t ' I ) t) 1 Fy"Ii fli R t)'