Introduction to ISAR imging systems SAR_RANGE 1
Rdr imging systems More in generl rdr imges re obtined by exploiting the reltive motion between the rdr ntenn nd the scene to be imged. SAR: KNOWN reltive motion ISAR: UNKNOWN reltive motion SAR used to imge extended scenes & ISAR used to imge mn-mde moving trgets. SAR_RANGE 2
Airborne SAR (Synthetic Aperture Rdr) Courtesy of Selex Glileo S.p.A. (Miln, Itly) SAR_RANGE 3
Airborne SAR (Synthetic Aperture Rdr) SAR_RANGE 4
Spceborne SAR Exmple: spceborne SAR Look ngle Incidence ngle Dt provided by the Itlin Spce Agency (ASI). SAR_RANGE 5
Spceborne SAR P. Lombrdo Dip. INFOCOM, Univ. di Rom L Spienz ISAR_MAGING_INTRO SAR_RANGE 6
ISAR (Inverse SAR) Rdr Rdr Opticl Opticl Nvl Reserch Lbortory From High-bndwidth 220 GHz experimentl rdr, Electronics Letters, Vol. 43, No. 20, Sept 2007 SAR_RANGE 7
ISAR (Inverse SAR) 8 SAR_RANGE 8
SAR & ISAR modes High resolution imging modes (both SAR & ISAR) cn be embedded in surveillnce rdr systems; An exmple: GABBIANO rdr fmily (SELEX ES) cost effective, lightweight X-Bnd rdr solution for dvnced surveillnce. effective in severl surveillnce missions over se, lnd, in the ir nd long costlines in ll-wether conditions; Cpbilities for trgets detection, locliztion, trcking nd identifiction, ground high resolution imging nd nvigtion id. From http://www.selex-es.com/-/gbbino-1, Gbbino fct sheet SAR_RANGE 9
SAR & ISAR modes From http://www.selex-es.com/-/gbbino-1, Gbbino fct sheet More exmples vilble t http://infocom.unirom1.it/rrsn/wiki/min/elborzionedelleimmginirdr Slides from the tlk «Selex-ES Airborne Rdr High Resolution Imging» SAR_RANGE 10
High resolution rdr Low resolution rdr High rnge resolution rdr Imging rdr: high rnge nd cross rnge resolution High rnge resolution is chievble by trnsmitting widebnd signls: c c R c 2 2B High cross-rnge resolution is chievble by mens of ntenns with wide pertures: the cross-rnge resolution on the scene/trget depends on its distnce from the rdr CR R R d d Imges of scenes/trgets t long distnces re formed with poor resolution unless very lrge ntennns re employed SAR_RANGE 11
Doppler frequency Trnsmitted signl: pulse with length p nd crrier frequency f c s tx t rect t cos f t 2 p c Echo bck-scttered from trget in cse of reltive motion between the rdr ntenn nd the trget (not considering the scle fctor tking into ccount the ttenution between tx nd rx) s rx t s rx 2R 2R 2R t rect t cos2 f t p c c c c 2R 2R 0 Rt rect t cos 2f t rect p c c c 2R t c cos2 f 2R t R0 0 0 4 c p R 0 : rdr-trget distnce t time t=0; R : trget rdil velocity. The echo bck-scttered from the trget is shift in frequency with respect to the trnsmitted frequency of quntity 2R 2vr fd DOPPLER Frequency SAR- Synthetic Aperture Rdr: zimuth resolution is obtined by exploiting the Doppler frequency induced by the reltive motion. SAR_RANGE 12
Frequency pproch to SAR (1/3) A B V C A C v ra P v rc v ra V sin A vrmax vrc V sin C vrmin Position A ntenn is pproching mximum rdil velocity v rb 0 Position B ntenn t the closest pproch null rdil velocity Position C ntenn is leving minimum rdil velocity SAR_RANGE 13
Frequency pproch to SAR (2/3) Synthetic Antenn Doppler frequency Synthetic Aperture f d t 2V sin 2V f d, MAX t t f d, MIN Doppler chirp 2V Vt R 0 B d AZ 2V time R V f d, MAX f d, MIN 2V sin 2V sin 2V A A 2V C C Doppler frequency bndwidth B d AZ f d, MAX f d, MIN 2V 2V A C SAR_RANGE 14
Frequency pproch to SAR (3/3) Signl s function of the zimuth time (slow-time) with frequency bndwidth: B d AZ 2V After pulse compression the resolution is zimuth time is given by: t d AZ 2V The reltionship between zimuth spce nd zimuth time is given by the pltform velocity: Azimuth sptil resolution fter zimuth chirp compression: x 1 B x V V t SAR_RANGE 15 t V B d AZ 2 In generl zimuth resolution depends on the overll observtion ngle
ISAR imge formtion (1/2) GOAL: formtion of high resolution 2D imges of mn-mde trgets (ground, ir nd ship trgets, usully non-coopertive) to feed ATR (Automtic Trget Recognition) procedures. ISAR techniques wide bndwidth is trnsmitted to chieve fine rnge resolution; coherently processing the echoes returned from the trget t different spect ngles gives fine cross-rnge resolution. Trget model: rigid body with dominnt sctterers with constnt complex reflectivity; Trget motion: decomposed s the trnsltion of reference point nd the rottion of the rigid body round the reference 1. Trnsltion to be compensted: Motion compenstion techniques for ISAR; 2. Rottion to be exploited: ISAR imge formtion techniques. MAIN PROBLEM: trget motion is unknown nd needs to be estimted directly from the cquired rdr signl. SAR_RANGE 16
ISAR imge formtion (2/2) Z Rdr ntenn R 0 (t) r n (t) R 0 (t) r n (t) Z Trget sctterer Y Y T/2 Trnsmitting t frequency f 0 =c/ the rdr echo from the n-th sctterer (0-th sctterer s the reference point) A n e j 2 f 0 2 R t 0 t c r n ( t ) In the hypothesis distnce rdr-trget much lrger thn trget size, i.e. R 0 (t) >> r n (t) t t t t t R r R r r 0 n 0 0 n X T/2 X 1: term tking into ccount the j 2 f t 0 crrier frequency: common to n ll the sctterers. 2: phse of the reference point tking into ccount the trnsltionl motion: common to ll the sctterers to be compensted. 1 2 4 3 4 j R t j ro t rn t 0 A e e e 3: term tking into ccount the rottionl motion: different for every sctterer to be exploited. SAR_RANGE 17
Frequency pproch to ISAR (1/2) Point sctterer P rottes round the fulcrum O with ngulr velocity The distnce of the rdr from the fulcrum in much higher thn the distnce of the point P from the fulcrum R t r t R t rˆ t r t R r t P 0 0 P 0 0 0 sin T Point sctterer P is interested by Doppler frequency given by: f d 2 d dt 2 t R r sin t r cos t x t 0 0 0 18 SAR_RANGE 18 2
Frequency pproch to ISAR (2/2) In the observtion time T centered round t=0 the sctterer Doppler frequency cn be expressed s: f d t x t r cos r sin t... x y t... Depending on trget size (i.e. r) nd processed time the sctterer Doppler frequency cn be ssumed constnt during time perture (i.e. first order term negligible s unfocused SAR) or linerly vrying with time t (i.e. first order term not negligible s focused SAR); Two sctterers P 1 -P 2 locted t x 01 -x 02 will show Doppler frequencies 1 2 1 2 2 2 fd x0 & fd x0 1 Filter bnk (FFT) with resolution f d to obtin the cross rnge resolution T x 2 2 fd 2 2T 2 Achievble resolution depends on trget motion nd observtion time; Imge scling (Hz m) requires the knowledge of the trget rottion rte. 19 SAR_RANGE 19 2 2 0 0 0 0 2
Frequency pproch to ISAR (2/2) For exmple by nlyzing the Doppler frequency of signls received in one rnge gte, it is possible to mesure the height of the trget reflectors reltive to the center of rottion. Two possibilities: Rottion speed known: the height mesurement is n bsolute mesurement Rottion speed is unknown: the mesurement is reltive. The scle fctor long the height xis remins undetermined in the cse of observtion of n uncoopertive trget, nd must be estimted from the dt. SAR_RANGE 20
ISAR: Imge Projection Plne (1/4) Trget with complex motion: the ctul rottion vector ccounts for the yw, pitch nd roll motion 21 SAR_RANGE 21
ISAR: Imge Projection Plne (2/4) The effective rottion vector E is the projection of orthogonl to the Line of Sight (LOS) RADAR The imge plne is the slnt rnge/doppler frequency plne, contining the LOS nd orthogonl to E Imge Projection Plne (IPP) SAR_RANGE 22
ISAR: Imge Projection Plne (3/4) Projection Plne in Reltion to the Rottion Axis of the Trget nd the Direction of Observtion Projection Plne: Generl Cse Different ISAR projections with the sme cquisition geometry chrcteristics highly suitble for clssifiction & recognition purposes SAR_RANGE 23
ISAR: Imge Projection Plne (4/4) Horizontl rottion component h Verticl rottion component v Horizontl&verticl rottion components Side-view imge Top-view imge Mixed (top&side) imge SAR_RANGE 24
Rdr imge focusing (SAR&ISAR) The vrition of the distnce rdr ntenn-imged sctterer with slow-time t (i.e. trnsmitted pulse) ffects dely of the echo: R(t )-R 0 comprble with rnge resolution Rnge Cell Migrtion (RCM): undesired effect to be compensted before zimuth processing; phse of the echo: R(t )-R 0 comprble with wvelength Doppler (Azimuth) Chirp: desired effect to be exploited to chieve fine zimuth resolution Focusing scheme of principle Rw signl RANGE COMPRESSION (MATCHED FILTER to TRASMITTED WAVEFORM) RANGE CELL MIGRATION COMPENSATION Observtion: focusing requires the knowledge of the cquisition geometry (reltive motion between ntenn nd imged scene: known for SAR nd priori unknown for ISAR) AZIMUTH COMPRESSION (MATCHED FILTER to DOPPLER CHIRP) Focused Rdr Imge SAR_RANGE 25