MULTI-CHROMATIC AALYSIS OF ISAR DATA: VALIDATIO AD POTETIAL Fabo Bovenga (), Vto Martno Gacovazzo (), Alberto Refce (), cola Venezan (), Raffaele Vtull () () CR ISSIA, Bar, Va Amendola, /d - 706 Bar (Italy), tel/fax +39 080 59945 / 60 E-mal: bovenga@ba.ssa.cnr.t () ESA - ESTEC TEC/EDP, oordwjk, The etherlands, E-mal: Raffaele.Vtull@esa.nt ABSTRACT The resent aer resents the results of the alcaton of Mult Chromatc Analyss (MCA) for heght retreval by rocessng both AES- arborne data and satellte TerraSAR-X data. In artcular, a test of the robustness of the MCA technque wth resect to total rocessed bandwdth has been erformed through comarson of results from datasets wth bandwdths sannng form 00 to 400 MHz. A frst valdaton of the mentoned technque has been carred out by comarng the retreved heghts w.r.t. ground elevaton from external SRTM DEM, as well as by verfyng the relablty of the frnge classfcatons based on the nteger number of hase cycles comuted through MCA. Results are resented and commented by addressng otental and lmtaton of the technque.. ITRODUCTIO The Mult-Chromatc Analyss, as ntroduced n [], uses nterferometrc ars of SAR mages rocessed at range sub-bands and exlores the hase trend of each xel as a functon of the dfferent central carrer frequences. The hase of sutable frequency-coherent scatterers evolves lnearly wth the sub-band central wavelength, the sloe beng roortonal to the absolute otcal ath dfference. Unlke the conventonal monochromatc InSAR aroach, ths new multchromatc technque allows erformng satally ndeendent and absolute toograhc measurements, f the attenton s focused on sngle targets exhbtng stable hase behavour across the frequency doman. These can be used to retreve unambguous heght nformaton, wthout the need for a satal hase unwrang rocess. Alyng the same sub-band slttng to a sngle SAR mage, the hase wll also vary lnearly wth wavelength, the sloe beng roortonal to the whole absolute otcal ath for the scatterng center under concern. Potental alcatons for the study of frequency-stable targets nclude toograhc measurements, atmosherc research, and urban montorng. Prevous work on the subject has started demonstratng the ractcal feasblty of the technque by usng smulatons [] as well as a set of SAR data collected by the arborne AES- radar nterferometer, oeratng at X- band by mult-channel electroncs, whch rovdes a total radar bandwdth of 400 MHz []. The technque aears otmally suted for the new generaton of satellte sensors, whch oerate wth larger bandwdths than those of revously avalable nstruments, generally lmted to about 0 MHz. In the followng, we brefly revew the MCA mathematcal formulaton as well as some mlementaton asects. A arametrc analyss rovdes a frst evaluaton of the mact of the MCA rocessng arameters on the heght estmaton erformances. Then, the alcaton of the technque to Level TerraSAR-X data s descrbed, as well as to AES- arborne data acqured over an area of gently rollng toograhy, sted n the west-central art of Swtzerland close to the vllage of Langnau. Results of the valdaton exerment are resented and commented.. MCA FORMULATIO To erform MCA n InSAR confguraton, sub-look mages,{i M,, I S, } wth =,..., are generated for both master and slave vews wth the same arameters. Then, a set of nterferometrc hase felds can be generated by cross-multlyng the master and slave sub-look mages relatve to the same central frequency f, wth hases: 4π ( = π τ f = R( f () c where R = R M ( R S ( and ( are the azmuth and range xel coordnates, resectvely. Snce only wraed hase values are measured, the followng relatons hold: W ( ( k( = + π W 4π ( = π k( R( f = () c = C0( + C( f Ths means that by exlorng the trend of the wraed nterferometrc hase values along frequences, the range dfference between master and slave can be nferred, whch n turn s related to the elevaton of the xel: c R( = C( (3) 4π Moreover, the coeffcent, C 0 (, gves the nteger number of π cycles that has to be added to the wraed nterferometrc hase of the xel ( n order to nfer ts absolute value: Proc. Frnge 009 Worksho, Frascat, Italy, 30 ovember 4 December 009 (ESA SP-677, March 00)
C0 ( k( =. (4) π Ths arameter allows to classfy the frnges and to assgn to each class the roer value to comute the absolute hase. Therefore, once the set of nterferometrc hase felds has been generated, the followng rocessng ste s a ont-wse estmaton of C 0 and C through lnear regresson. An a osteror estmaton of the mult-frequency hase error s gven by the root mean square dfference between the nterferometrc hases measured at the dfferent frequences f, and the corresondng values comuted by the lnear model: σ =. (5) f = [ ( C0 ( C( f ] As n the monochromatc aroach, where the coherence consttutes the qualty factor for the measures, n the same way the quantty n (5), evaluated on a xel-by-xel bass, defnes an nherent nter-band coherence for the MCA and can be assumed as a qualty ndex of the measurements: the smaller σ, the better s the achevable accuracy for terran heghts. Hence the last rocessng ste conssts n selectng those xels (targets) whose nter-band hase STD s below a relablty threshold: PS fd = {( σ ( σ th }. The statstcal dstrbuton of the k( values can also be exloted, n order to decrease the error of the ontwse estmates. To valdate the MCA, the absolute heght values measured through relaton (3) on selected xels (PS fd ) can be erformed based on an external DEM. Ths ste ams at assessng the accuracy of the mult-chromatc toograhc hase and evaluatng the mortance of atmosherc delays, f resent. 3. IMPLEMETAITIO ASPECTS To erform MCA n InSAR confguraton, for each selected frequency, a ar of master and slave sub-vew mages have to be generated and coregstered to a reference geometry: {SLC M,, SLC S, } wth =,...,. The frst rocessng stes are band-slttng and coregstraton. Prevous works and actvtes relatve to MCA started demonstratng the ractcal feasblty of the technque by focusng raw SAR data to generate the sub-look mages. However, the technque aears otmally suted for the new generaton of satellte sensors, whch oerate wth larger bandwdths than those of revously avalable nstruments, generally lmted to about 0 MHz. SAR sensors such as those mounted on TerraSAR-X or COSMO-SkyMed sacecraft, all ose great exectatons on the otental use of multchromatc methods, but data olces of these mssons do not foresee the delvery of raw data thus mosng to erform MCA startng from SLC mages. In [] the equvalence between MCA erformed startng from Level 0 and Level data has been roved. The standard rocessors for Level data roducton are consdered as lnear and hase reservng and all nformaton n the frequency sace s theoretcally reserved excet for Hammng flterng at the end of the focusng rocess. In order to avod asymmetry n the sectrum, before the ass-band flterng a de-hammng flter s requred along range (see [] for more detals). Concernng coregstraton, n order to save rocessng tme, t can be erformed once between full-band mages before sub-looks generaton nstead of -tmes after band-slttng. In ths confguraton, the sub-band flterng erformed after coregstraton nduces a further hase term related to the shft of range xel, sh r, aled to the slave mage n order to be resamled onto the master geometry. Accordng to ths effect the nterferometrc hase becomes: 4π ( ) = R( ) R ( ) f (6) ( sh ) c where R sh = dr sh r and dr s the xel sacng. Thus, n order to nfer R through (3), t s necessary to take nto account ths term related to the coregstraton range shft. Ths hase comensaton s a trval and not comutatonally exensve task comared wth that requred to erform coregstraton for every sub-look. The range shft matrx sh r () s already avalable as sde roduct of the coregstraton ste. It s worthwhle to ont out that R sh () reresents a coarse estmaton of the ath dfference between master and slave based on the amltude cross-correlaton, smlar to what s done through radargrammetrc technques by usng very large arallax values [3]. Once C s avalable through MCA, the heght H() w.r.t. to a reference ellsod can be comuted as (from [4], eqs. 3 and 6): ref c RM, sn( ) H ( ) C( ) r( ) 4 ϑ = δ (7) π B, ref where δr = R sh () R ref (), R M, and θ are resectvely the range dstance and the look angle relatve to the master vew comuted w.r.t. the reference ellsod heght at the xel, B s s the geometrcal dstance (baselne) between the two acqustons, B, s the comonent of the baselne orthogonal to the slant drecton of the master comuted at xel, the angle α defnes the orentaton of the baselne and, R ref () s the absolute ath dfference comuted w.r.t. the heght of the reference ellsod at the xel. All these geometrcal arameters vary on a xel-by-xel bass accordng to the sensor coordnates, and are known from InSAR re-rocessng. To erform valdaton we used a Dgtal Elevaton Model (DEM) rovded by the SRTM msson, whch has been back-rojected onto SAR geometry. Fnally, we can observe that the matrx δr rovdes a very rough estmaton of the heght w.r.t. the reference ellsod.
Therefore, n Eq. (7) MCA contrbutes, through the C coeffcent, to refne ths coarse heght model based on amltude correlaton. 4. PARAMETRIC AALYSIS A arametrc analyss has been carred out wth the am of evaluatng the mact of the MCA rocessng arameters on heght estmaton erformance. By alyng error roagaton theory to the LMS lnear regresson, and assumng the same hase varance for all the sub-looks nterferograms, σ = σ =,,, the STD for the coeffcents C 0 and C s: σ C = f ( f ) f = σ, σ = C0 f ( ) ( ) f f f f f f = = = = The arameters nvolved n ths analyss are n artcular the number of sub-looks, the sub-looks bandwdth B, and the full bandwdth B. Once fxed B and B, the maxmum frequency range sanned by MCA s B e = B B. The set of frequences {f } deends of and df, whch s the sectral dstance between adjacent sub-looks. In order to model the σ deendency on B and to evaluate the behavor of the above relatonshs, we consder the favorable case of a scatterer whch s an deal metallc lanar reflector, thus showng hgh Sgnal to Clutter Rato (SRC). In ths hyothess the followng relatonshs hold [5], [6]: 4π A σ =, SCR = λ (8) SCR σ 0A clutter where σ 0 s the normalzed radar cross secton, whch we set to, A s the area of the metallc lanar reflector that we assume to have a squared shae wth sde of 0.5 m, and A clutter s the area of the resoluton cell whch deends on B. In fg. we sketch the trend of the roagated errors on the heght dfference, σ R (to) and that on the cycle number, σ k (bottom) comuted from (3)-(4) as a functon of, for B rangng from 00 to 400 MHz and B = 40 MHz. Both fgures decrease as ncreases whth a trend saturaton whch deends on B. The estmaton of both R and k are exected to mrove as B, and consequently B e, ncrease. Moreover, n Fgs. and 3 we show the trends of the same arameters for dfferent B values and for B = 00 MHz and B = 400 MHz, whch are the bandwdth values of the real dataset avalable for the resent exerment. As exected, the estmaton s mroved by usng wder B and n turn the maxmum allowable B value ncreases as B ncreases. Of course ths estmaton s erformed n favourable settng, thus results from real data are exected show worst erformances. σ Ths analyss onts out the mortance of bandwdth n MCA. In artcular, a value of 00 MHz seems to be nadequate for a relable alcaton of MCA for absolute dfference ath retreval ( R) as well as for frnge classfcaton though k values estmaton, even when usng a large number of sub-looks. For nstance, the condton for roer k value determnaton s σ k <<, whch s rarely satsfed by usng B = 00 MHz, even n the smulated favourable scenaro of a very coherent scatterer. Ths analyss s confrmed by erformng MCA on a real SAR dataset, as shown n the followng secton. 5. SAR DATASETS Two SAR datasets were avalable for the resent study: one acqured from arlane, hereafter referenced to as AES- dataset, the other acqured from satellte, hereafter ndcated as TSX dataset. The AES- dataset was acqured by bstatc antennas mounted on an arlane and oeratng at X band wth a total radar bandwdth of 4 00 MHz resultng from four non-overlang channels each 00 MHz wde, whose central frequences are 9.4, 9.5, 9.6, and 9.7 GHz, resectvely. The samlng frequency f s s 00 MHz, corresondng to a slant range sacng of 0.75 m. The syntheszed Doler bandwdth s 50 Hz wth a resultng azmuth resoluton of.5 m. The area covered by the AES- dataset (yellow borders n fg. 4) s a scarcely urbanzed area located n Swtzerland close to the town of Langnau, wth an extenson of about.5 km. The TSX dataset conssts of a ar of InSAR mages obtaned from the TerraSAR-X archve and acqured on 07/09/008 and 0/0/008, resectvely, whch nclude the same area around Langnau as the AES- dataset (see red borders n fg. 4). Table reorts some relevant arameters of the acqustons. Data are acqured n strma mode, along descendng orbts, wth 00 MHz range bandwdth and ~3 degrees look angle. The Setember acquston has been selected as master mage for the InSAR rocessng. In order to mnmse both temoral and geometrcal decorrelaton, the mages have been selected by mnmsng the temoral and satal dstance: B t = 33 days, B = 06.5 m. 6. RESULTS FROM TERRASAR-X DATASET In ths secton we resent the results of a frst alcaton of MCA to SAR Level- data by usng the TSX dataset resented n the revous secton. Ths choce was drven by the ossblty to obtan the data needed for valdaton by usng a standard InSAR re-rocessng. Although the arborne AES- dataset aears more romsng n term of both bandwdth and coherence, t resents some fluctuatons related to both geometrc and radometrc arameters as well as some
roblems n erformng standard InSAR rocessng desgned for satellte acqustons. Acquston Mode Beam TSX (Langnau) Look Angle Incd. Angle STRIPMAP Str 009 3.0 35. Range Bandwdth (MHz) Carrer Freq. (GHz) POL Range Res. Look/Pass Drecton Rght/ Descendng Range Sacng. 00 9.65 HH.49.36 Acqus. Date yyyy/mm/dd 008/09/07 B t (days) B B // H a - - - - 008/0/0 33-06.5-3.3 5.7 Table. Parameters of the TSX acqustons. In order to maxmze the nterferometrc coherence the area of nterest (AOI) selected for the exerment encloses manly the urban area of the vllage of Langnau (see red borders n the nset of fg. 4). Hereafter, fgures and results wll refer to ths AOI whch corresonds, n SAR geometry, to a matrx of 04 (range) 5 (azmuth) xels. The SAR mages has been rocessed n order to generate the data requred n nut to the MCA tool as well as for valdaton, through eq. (7). In fg. 5 (to) we show the exected heght rofle, H DEM (, over the AOI obtaned by resamlng the SRTM DEM onto the master geometry for drect comarson wth the MCA results: the ground elevaton ranges from 70 to 840 m. The δr( y) matr on the bottom of the same fgure, shows a clear correlaton wth the ground elevaton, as commented n secton 3. The basc set of arameters exloted n exerment s: B = 50 MHz, df = 48 MHz, =. Ths confguraton has been set to avod hase alasng along frequences through the followng constrant: B, 4 max df < c H ( ) δr( ), sn( ) RM ϑ ref A useful analyss for valdate the outut of the MCA rocessng s the nsecton of the dstrbuton of the xels n PS fd suermosed on both the SAR amltude and an otcal mage of the scene. Ths dstrbuton deends of course on the value of σ th, whch should guarantee the relablty of the xels selecton. In fg. 6 the dstrbuton of PS fd relatve to σ th = 0.0 rad s shown on an otcal mage from GoogleEarth as well as on the SAR amltude. It can be notced that the targets labeled as PS fd are located manly on xels whch show hgh amltude values, whch can be seen from the otcal mage to corresond to man-made objects such as buldngs. In fg. 7 we show the trends of the nterferometrc hase along frequences for a hghly coherent xel (σ = 0.0038 rad) and the related erodogram whose eak oston deends on the C coeffcent. Fg. 8 shows the man roducts of MCA rocessng: σ ( (to-left), K( (to-rght) and, the estmated heghts H( (bottom-left). It s ossble to arecate as xels showng σ < 0.5 rad cover the bult-u area of the town. On the bottom-rght of the same fgure, t s sketched the hstogram of the dfference between the heght estmated and that rovded by the SRTM DEM: ε H = H H DEM. As exected from the revous analyss, ths error n the heght estmaton shows a wde dstrbuton wth a eak around 00 m. Of course, by choosng very coherent xels (as those reresented n fg. 6) the estmaton aears mroved. However some comments are n order. Frst, the number of coherent xels s very lmted. Second, also for these xels unaccetable estmaton errors ε H can occur, thus meanng that the selecton rule s not always effectve and could be mroved. Thrd ont, a bas exsts that requres further nvestgatons. In artcular, the resduals can be modelled as the sum of dfferent contrbutons: ε H = ε APS + ε nose + ε roc + ε DEM where ε APS deends on the mact that the atmoshere has on the sgnal roagaton; ε nose s related to the decorrelaton affectng the two acqustons; ε roc s due to the recson of sloe estmaton n MCA rocessng whch n turn s related to the avalable bandwdth; ε DEM reresents the error n the actual ground elevaton. The SRTM DEM has a coarse satal resoluton (90 90 m ) comared wth the TSX data (also for the sub-look mages) whose xels can refer to sngle structure. In the resent case, snce the SAR mages were acqured at a dstance of about one month, we can argue the resence of both temoral decorrelaton and a ath delay dfference nduced by the atmoshere. The frst contrbutes to ncrease the nose level n the measurement, whle the second ntroduces a bas n the estmaton of absolute heght. Consderng that MCA deals wth absolute range measurements erformed on a xel by xel bass wthout satal dfferences, the resence of an addtonal ath delay roortonal to λ/ causes a heght error roortonal to the heght of ambguty H a whch n our case s about 50 m (see tab. ). After a rough estmaton and subtracton of the bas value (related to the atmoshere), for very coherent xels, t s ossble to rovde a heght estmaton close to the exected value. However, the dffculty of searatng dfferent sources of hase sgnals wthout the hel of ancllary data or further temoral analyss remans. Moreover, as exected from the analyss erformed n secton 3, the bandwdth value reresents a lmtng
factor n the MCA erformace. In the next secton we try to evaluate the mact of ths arameter on heght retreval through MCA rocessng. 7. RESULTS FROM AIRBORE AES- DATA- SET Usng the AES- dataset rovdes the ossblty to exlore dfferent values of bandwdth from 00 to 400 MHz. For ths dataset the normal baselne B s 0.75 m, thus ensurng a very lmted geometrcal decorrelaton effect. Moreover, snce the two antennas acqure smultaneously, temoral decorrelaton and atmosherc contrbutons to the nterferometrc hase are avoded (ε APS = ε nose 0). Ths confguraton thus aears very romsng for MCA testng n artcular for what concerns the mact of the bandwdth value on ε H. However, arborne data resent some fluctuatons n some geometrcal arameters, and n the resent case t was not ossble to rovde unvocal values needed to evaluate eq. (7) for drect heght comarson and valdaton. Moreover, nsecton of the metadata onted out the resence of a delay along the hardware acquston lne of one of the two antenna. Snce the nsecton dd not allow to defne the actual value of ths delay, t resulted mossble to erform absolute nterferometrc hase estmaton. For these reasons, the exerment wth AES- data was erformed by analysng the data n slant range geometry wthout a drect comarson wth the SRTM heght rofle. Anyway, ths lmtaton dd not revent to verfy how the MCA erformances deend on the avalable bandwdth. The area selected for the test s sketched n yellow n the nset fgure of fg.. Fg. 9 shows the reflectvty ma (left) and InSAR hase feld (rght) for a sngle sub-look acquston at 00 MHz. Fg. 0 shows the k values dstrbuton comuted through MCA analyss by usng dfferent bandwdths (400, 300, 00 and 00 MHz). The k values are related to the nterferometrc frnges attern n fg. 9 (rght): each frnge corresonds to a certan k value. It can be noted that the k values ma obtaned by rocessng a bandwdth of 400 MHz reroduces roerly the frnge attern. To rovde a numercal roof, we selected the range lne sketched on the nterferometrc hase felds n fg. 9 and, on the k values ma at 400 MHz n fg. 0 (to-left): along ths range lne, nterferometrc frnges can be counted, and corresondngly k value classes. It s worthwhle to ont out that the k values ma s comuted on a xel by xel bass wthout any satal ntegraton of the nformaton. Ths means that the number of k value classes can be nferred by the dfference between the k values comuted on two xels PS fd located at near and far range resectvely. Ths s not true for the InSAR hase whch nvolves satal dfferences between neghbourng xels. The mages n fg. 0 show clearly the erformance of the measure as a functon of the sanned bandwdth: as the bandwdth becomes narrower, the frnge classfcaton aears noser. A bandwdth of least 300 MHz seems to be requred to rovde relable results. In artcular, t results evdent that for B = 00 MHz the correlaton wth the frnge attern dsaears. Therefore, a bandwdth of 00 MHz aears nadequate for relable heght nference. It s worthwhle to remember that the AES- dataset t s not affected nether by atmosherc dstorton, nor by temoral decorrelaton, and also the geometrcal decorrelaton s extremely lmted. Therefore, ths exerment confrms that, n the case of TerraSAR-X dataset, where the hase nformaton s corruted by temoral and geometrcal decorrelaton and by the resence of atmosherc sgnal, a bandwdth of 00 MHz leads to unrelable heght estmaton as reorted n the revous secton. The mas n fg. show the k values only for xels n PS fd. The threshold σ th whch defnes PS fd has dfferent values n order to guarantee the same relablty n k estmaton. Indeed, as the bandwdth becomes narrower, the number of samles along frequences decreases and the ft along frequences becomes less relable. The number of xels n PS fd n the case of B = 00 MHz (ma at the bottom-rght) results extremely low. σ R σ K (π unt) 0.035 0.03 0.05 0.0 0.05 0.0 0.005 0 0 0 0 30 40 50 60 fo = 9.65GHz, B = 40 MHz, σ = 0.057548.5 B = 00, Be = 60 MHz B = 00, Be = 60 MHz B = 300, Be = 60 MHz B = 400, Be = 360 MHz.5 0.5 fo = 9.65GHz, B = 40 MHz, σ = 0.057548 B = 00, Be = 60 MHz B = 00, Be = 60 MHz B = 300, Be = 60 MHz B = 400, Be = 360 MHz 0 0 0 0 30 40 50 60 Fgure. σ R (to), and σ k (bottom) trends as a functon of comuted for B rangng from 00 to 400 MHz and B = 40 MHz.
0.07 0.06 fo = 9.65GHz, B = 00 B = 0, Be = 40 MHz, σ = 0.084 B = 40, Be = 40 MHz, σ = 0.0575 B = 60, Be = 40 MHz, σ = 0.047 0.05 σ R 0.04 0.03 0.0 0.0 0 5 0 5 0 5 30 35 40 0.04 fo = 9.65GHz, B = 400 B = 0, Be = 00 MHz, σ = 0.084 σ R 0.0 0.0 0.008 0.006 B = 80, Be = 00 MHz, σ = 0.0407 B = 40, Be = 00 MHz, σ = 0.0308 B = 00, Be = 00 MHz, σ = 0.057 Fgure 4 Red and yellow borders enclose the areas covered by the AES- and TSX SAR data resectvely. The samle areas selected for the rocessng are sketched n the nset fgure. 0.004 0.00 0 0 5 0 5 0 5 30 35 40 Fgure. σ R trends as functon of, for B rangng from 0 to 00 MHz and B = 00 MHz (to) and B = 400 MHz (bottom). 4.5 4 3.5 fo = 9.65GHz, B = 00 B = 0, Be = 40 MHz, σ = 0.084 B = 40, Be = 40 MHz, σ = 0.0575 B = 60, Be = 40 MHz, σ = 0.047 3 σ K (π unt).5.5 0.5 0 5 0 5 0 5 30 35 40 0.9 fo = 9.65GHz, B = 400 B = 0, Be = 00 MHz, σ = 0.084 0.8 B = 80, Be = 00 MHz, σ = 0.0407 σ K (π unt) 0.7 0.6 0.5 0.4 B = 40, Be = 00 MHz, σ = 0.0308 B = 00, Be = 00 MHz, σ = 0.057 Fgure 5. SRTM heghts (to) and δr matrx (bottom) for the TSX AOI n meters. 0.3 0. 0. 0 5 0 5 0 5 30 35 40 Fgure 3. σ k trends as a functon of, for B rangng from 0 to 00 MHz and B = 00 MHz (to) and B=400 MHz (bottom).
Fgure 9. Reflectvty ma (left) and InSAR hase feld (rght) for the AES- dataset. The data refer to a sngle sub-band acquston at 00 MHz. Fgure 6. Dstrbuton of PS fd relatve to a σ th = 0.0 rad shown on an otcal mage rovded by GoogleEarth (to) as well as on the SAR amltude (bottom). Fgure 0. K values mas comuted through MCA analyss by usng dfferent bandwdth (400, 300, 00 and 00 MHz, resectvely). Fgure 7. Interferometrc hase trend and related erodogram of a coherent xel (σ = 0.0038 rad). Fgure 8. Man roducts of MCA rocessng: σ ( (to-left), K( (to-rght) and, the estmated heghts H( (bottom-left). The hstogram of ε H = H H DEM s also sketched on the bottom-rght. Fgure. K values mas on xels selected accordng to σ th and comuted through MCA analyss by usng dfferent bandwdth (400, 300, 00 and 00 MHz resectvely).
8. COCLUSIOS Ths aer has ntroduced the MCA and rovded some detals relatve to ts mlementaton and valdaton. Through a arametrc analyss, the lmts of alcablty of the technque accordng to the rocessng arameter and the avalable bandwdth have been evaluated. Then, a frst valdaton exerment has been erformed by usng both Level TerraSAR-X data and AES- arborne data. It has been also demonstrated that, n the case of AES- dataset, whch s not affected by nether atmosherc dstortons nor temoral and geometrcal decorrelaton, a bandwdth of least 300 MHz seems to be requred to rovde relable results. Ths result confrms the evaluaton obtaned by the arametrc analyss desgned under favourable condtons. In the case of TerraSAR-X dataset, where the hase nformaton s corruted by temoral and geometrcal decorrelaton and by the resence of atmosherc sgnal, a bandwdth of 00 MHz leads to unrelable heght estmaton. Thus, the alcaton of MCA for heght retreval by usng satellte data requres wder bandwdth and needs further nvestgatons. However, ncreasng bandwdths s not the only acton ossble. For examle, benefts could derve by ntegratng an ndeendent method to measure the ntegral atmosherc delay durng the data takes, based on a combnaton of wde area mang (ossbly suorted by an atmosherc model) and calbraton coherent scatterers on the test ste. The data analyss onted out also that the crteron for target selecton based on a threshold on σ may result occasonally unrelable, thus requrng further develoments to ncrease the robustness of the selecton. The technque has shown ts otental n measurng ground elevaton, when roer bandwdth s avalable and rovded the atmosherc sgnal can be evaluated and removed. Moreover, the aroach can also be used to suort standard hase unwrang as well as to nvestgate the scatterng roertes of the targets [7]. Future work wll be devoted to rocess TerraSAR-X sotlght data wth a bandwdth of 300 MHz and to add temoral analyss for atmosherc sgnal flterng. 0. REFERECES []. Venezan, F. Bovenga, and A. Refce, A wdeband aroach to absolute hase retreval n SAR nterferometry. Multdmensonal Systems and Sgnal Processng, vol. 4, no. -, 003. [] F. Bovenga, V. M. Gacovazzo, A. Refce,. Venezan, R. Vtull, Frst valdaton exerment for a mult-chromatc analyss (MCA) of SAR data startng from SLC mages, Proc. IGARSS 009, Cae Town, South Afrca, July 3-7, 009. [3] T. Toutn, L. Gray, State-of-the-art of elevaton extracton from satellte SAR data, ISPRS Journal of Photogrammetry & Remote Sensng, vol. 55,. 3 33, 000. [4] Hanssen, R. F., Radar Interferometry: Data Interretaton and Error Analyss, Kluwer Academc Publshers, Dordrecht (00). [5] G. Ketelaar, P. Marnkovc, and R. Hanssen, Valdaton of ont scatterer hase statstcs n mult-ass InSAR, n Proc. CEOS SAR Worksho, Ulm, Germany, May 7 8, 004. [6] A. Ferrett, G. Savo, R. Barzagh, A. Borgh, S. Musazz, F. oval, C. Prat, and Fabo Rocca, Submllmeter Accuracy of InSAR Tme Seres: Exermental Valdaton, IEEE TGRS, vol. 45, n. 5, May 007. [7] Gacovazzo, A. Refce, F. Bovenga,. Venezan, Identfcaton of coherent scatterers: sectral correlaton vs. Mult-Chromatc Phase Analyss, n Proc. of IGARSS 008, Boston, Massachusetts, USA, July 6-, 008. 9. ACKOWLEDGEMETS Work suorted by ESA ESTEC Contr.. 39/07/L/HE. Samle AES- data courtesy of SARMAP, whle TerraSAR-X mages are rovded by DLR under a Scentfc Use Lcense for roosal MTH0397. The authors thank Ing. Davde tt from Poltecnco d Bar for hs hel n re-rocessng TSX data.