SEA-SURFACE POLARIZATION RATIO FROM ENVISAT ASAR AP DATA AND COMPARISON WITH MODELS

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1 SEA-SURFACE POLARIZATION RATIO FROM ENVISAT ASAR AP DATA AND COMPARISON WITH MODELS Harald Jhnsen (1), Geir Engen (1), Gilles Guittn (1) (1) Nrut IT AS, PP 6463 Frskningsarken, N-9294 Trmsø, Nrway, ABSTRACT Plarizatin rati (VV/HH) ver cean surface is rcessed frm Envisat ASAR AP data and cmared with existing EM scattering mdels. The rcessing f the AP data is based n Level 0 rduct which enables us t recisely cmute the cntributin frm additive nise searately frm each burst. This is f imrtance in rder t estimate a crrect larizatin rati, esecially at higher incidence angles. A recently ublished EM scattering mdel (GCM), is extended t als incrrate scattering frm breaking waves. The erfrmance f this mdel with and withut breaking waves is cmared with AP data, and with ther mdels fr the larizatin rati. The inclusin f wave breaking in the GCM mdel reduces the larizatin rati RMS errr with 17% and the bias with 87%. The achieved RMS errr fr the larizatin rati is 0.47 and the bias is 0.04, while the best semi-emhirical mdel frm litterature gives 0.54 and 0.37 fr the RMS and bias, resectively. 1. EM SCATTERING MODELS Varius EM scattering mdels, emhirical [1], semiemhirical [2], as well as theretical [3], exist in literature. In the aer by Engen et. al [4], a generalized curvature mdel (GCM) was develed fr describing analytically the EM scattering frm cean surface. The GCM is a cnsistent frmalism fr describing EM scattering frm an elevated cnducting curved surface. Exressins fr the surface current and the crresnding scattered fields were derived using the Strattn-Chu frmalism and a generalized curvature exansin f the fields at the surface. This new frmalism allws us t analytically describe the scattering f EM waves frm a regular curved surface, withut searating int different scales. The aer rvide exlicit exressin fr the surface EM fields, including bth the rjectin and the self-induced fields, u t first rder in surface curvature. Analytic clsed frm exressins fr the scattered fields were derived frm the surface field slutin. Fr nnshadwing imaging gemetries, the resulting mdel is exact t first rder in surface curvature. Als, the mdel beys the fundamental laws f recircity and tilt invariance, and it rerduces the SPM results f Valenzuela [5] in the lw frequency limit as well as the Kirchff [6] arximatin in the high frequency limit. The frmalism has been alied t the secial case f backscattering, rviding a general exressin fr the σ nrmalized radar crss-sectin, fr different larizatins,. In this aer we have evaluated the larizatin rati, σ σ as redicted by scattering mdels frm litterature [1], [7], [8], in additin t the GCM mdel [4], recently extended with wave breaking effects as frmulated in [2]. 1.1 Extensin f GCM with wave breaking effect The mtivatin fr the extensin f the GCM mdel with wave breaking effects was triggered by the wrk f Kudryavtsev et. al. [2],[9]. The exansin f the GCM is dne by including a wave breaking term t the nrmalized radar crss sectin. In [2] Kudryavtsev et al. intrduce a nrmalized radar crss sectin (NRCS) taking int accunt scattering frm regular (i.e nn breaking) surfaces and scattering frm breaking waves, because the cmmn mdels based n the Bragg thery are nt able t crrectly rerduce the NRCS in all the cnfiguratins and wind waves cnditins. EM scattering frm the sea surface is described frm three mechanisms : Bragg scattering ( σ br ) reflectin ( s ), secular σ frm regular surfaces, and nn-bragg scattering frm breaking waves ( σwb ). Since σ s and σ wb are statistically indeendent f Bragg scattering, the ttal NRCS can be exressed as a sum given by: (1) σ = σbr + σs + σwb dentes the larizatin state. Accrding t radar bservatins, it is suggested that σ wb as well as σs are indeendent f larizatin. In the mdel used by Kudryavtsev, the sea surface is reresented as a regular surface cmbined with a number f breaking znes, where the scattering is surted by quasi-secular reflectin and the verall cntributin is rrtinal t the fractin f sea surface cvered by breaking waves. Using this arach, Kudryavtsev writes fr the NRCS: where { HH, VV} br (2) σ = ( σ + σ )( q) + σ q s 1 wb Prc. Envisat Symsium 2007, Mntreux, Switzerland Aril 2007 (ESA SP-636, July 2007)

2 where q is the fractin f the sea surface area cvered by breaking waves. Adting the frmalism f eq. (2), the NRCS using the GCM mdel extended with wave breaking effects, can be written as: gcm 1 wb (3) σ = σ ( q) + σ q σ gcm where is the NRCS frm the generalized curvature mdel [4] incrrating bth secular and Bragg scattering. The nn Bragg scattering art frm breaking waves ( σ wb and q ) is described by Kudryavtsev [2] by an extensin f the arach develed by Phillis [13], in where the cntributin f breaking waves is related t the wave breaking frnts. The wave breaking frnts are linked with a statistical descritin f the sea surface. In the wrk by Kudryavtsev [2], the descritin f the sea surface is based n the shrt wind wave sectrum [10] and wave breaking statistics rsed by Phillis [12]. 2. ESTIMATION OF POLARIZATION RATIO FROM ENVISAT ASAR AP DATA The sea-surface larizatin rati is cmuted frm data acquired by the Envisat ASAR instrument in Alternating Plarizatin (AP) mde. The data set and the estimatin rcedure are described in the fllwing. 2.1 Data Set A large number (49) f ASAR AP Level0 rducts are cllected and cllcated with in-situ measurements f cean wind field. The in-situ data cnsists f wind field frm NOAA buys, Nrwegian il latfrms, and QuickScat. The list f ASAR AP data and crresnding in-situ data is given in Table 1. Table 1. List f ASAR AP and in-situ data used in the analysis. θinc = incidence angle, φsat = satellite track angle, Hs =significant waveheight, U10 =wind seed, ϕwind =wind directin, r =cllcatin radius θinc φsat H s U 10 ϕ wind r lat ln [deg] [m] [m/s] [m] filename [deg] l. statin [deg] ASA_APC_ T16: 28,4-89,7 22,9-168,1 HHVV NOAA ,47 3, ,9 ASA_APC_ T10: 61,5 1,4 33,8-167,7 HHVV trll_a 1,2 2, ,3 ASA_APC_ T21: 55,9 3,4 28,8-12,6 HHVV ekfisk 0,6 2, ,1 ASA_APC_ T21: 58,1 2,5 28,8-12,9 HHVV sleiner 0,4 4, ,2 ASA_APC_ T21: 55,8 2,7 28,8-12,6 HHVV ekfisk 0, ,6 ASA_APC_ T10: 57 3,1 28,8-167,3 HHVV ekfisk 1,2 8, ,6 ASA_APC_ T10: 56,9 3 28,8-167,3 HHVV ekfisk 1,2 8, ,9 ASA_APC_ T14: 40,4-70,7 33,8-169,1 HHVV NOAA ,85 3, ,7 ASA_APC_ T14: 40,4-70,7 33,8-169,1 HHVV NOAA ,85 3, ASA_APC_ T10: 61,7 2,2 33,8-167,7 HHVV trll_a 0,8 3, ,8 ASA_APC_ T21: 55,7 3,5 28,8-12,5 HHVV ekfisk 1,1 9, ,5 ASA_APC_ T14: 40,5-70,7 37,7-169,6 HHVV NOAA ,58 5, ,1 ASA_APC_ T14: 40,2-70,5 28,8-168,5 HHVV NOAA ,92 3, ASA_APC_ T02: 39,9-69,8 33,8-10,9 HHVV NOAA ,85 5, ,5 ASA_APC_ T14: 40,3-70,7 33,8-169,1 HHVV NOAA ,55 3, ,4 ASA_APC_ T03: 24,9-88,7 28,7-11,5 HHVV NOAA , ,8 ASA_APC_ T10: ,8-166,6 HHVV nrne 1,4 4, ,6 ASA_APC_ T20: 66,2 8,7 33,8-13,5 HHVV nrne **** 6, ASA_APC_ T20: 66,2 7,3 33,8-13,5 HHVV nrne **** 9, ,2 ASA_APC_ T10: 66 8,7 33,8-166,6 HHVV nrne 1,4 6, ,7 ASA_APC_ T10: 66 7,3 33,8-166,6 HHVV nrne 1,9 4, ASA_APC_ T20: 66,2 8 33,8-13,4 HHVV nrne 6, ,3 ASA_APC_ T10: ,8-166,6 HHVV heidrun 1,7 8, ,2 ASA_APC_ T09: 66,3 7, ,4 HHVV nrne 4,5 13, ,6 ASA_APC_ T21: 51 5,1 28,8-12 HHVV QuickScat 0 327, ,2 ASA_APC_ T03: 27,2-86,4 18,8-12,2 HHVV NOAA ,15 12, ASA_APC_ T04: 26,2-93,2 33,8-11,1 HHVV NOAA ,58 8, ,7 ASA_APC_ T03: 27,2-87,2 22,8-11,9 HHVV NOAA ,1 10, ASA_APC_ T03: 27,2-88,7 22,8-11,9 HHVV NOAA ,35 3, ,5 ASA_APC_ T04: 26,2-94, ,3 HHVV NOAA ,62 5, ,6 ASA_APC_ T10: 66 9,1 28,7-165 HHVV QuickScat 0 9,23 162, ,9 ASA_APC_ T10: 65,7 8,9 28,8-165,1 HHVV QuickScat 0 9,17 165, ,3 ASA_APC_ T20: 66 8,6 22,9-16,8 HHVV QuickScat 0 13,15 237, ,6 ASA_APC_ T10: 66,2 6,9 22,8-163,1 HHVV QuickScat 0 14,12 357, ,3 ASA_APC_ T10: 65,9 6,7 22,8-163,2 HHVV QuickScat 0 14,09 358, ,9 ASA_APC_ T20: 65,9 8,5 28,8-15 HHVV QuickScat 0 5,81 275, ,9 ASA_APC_ T20: 65,9 8,9 33,8-13,3 HHVV QuickScat 0 12,82 130, ,5 ASA_APC_ T21: 65,8 7,6 37,7-11,9 HHVV QuickScat 0 10,14 205, ,6 ASA_APC_ T21: 65,8 7,6 37,7-12 HHVV QuickScat 0 10,14 205, ,3 ASA_APC_ T21: 65,9 7,5 37,7-12 HHVV QuickScat 0 10,14 205, ,9 ASA_APC_ T10: 66,1 7,7 28,7-164,9 HHVV QuickScat 0 13,08 335, ,3 ASA_APC_ T10: 65,8 7,5 28,8-165 HHVV QuickScat 0 12,83 330, ,5 ASA_APC_ T10: 66,1 8,3 22,8-163,1 HHVV QuickScat 0 9,54 61, ,9 ASA_APC_ T10: 65,8 8 22,8-163,3 HHVV QuickScat 0 8,82 47, ,9 ASA_APC_ T21: 65,7 6,7 44-9,5 HHVV QuickScat 0 13,1 212, ,1 ASA_APC_ T09: 65,7 8,6 41,1-169,3 HHVV QuickScat 0 12,37 233, ASA_APC_ T21: 65,8 7,4 44-9,5 HHVV QuickScat 0 10,93 316, ,1 2.2 Estimatin f larizatin rati hh The VV/HH larizatin rati ( σ ) is vv σ estimated frm ASAR AP Level0 rduct, which allws us t recisely cmute the cntributin frm additive nise searately frm each cmlex burst. The cntributin frm nise is f majr imrtance, esecially at high incidence angles (IS4-IS7) r at lw backscatter (lw wind situatins). The rcessing stes are as fllws: rcess the level0 data int individual fcused cmlex bursts at full reslutin extract azimuth intensity and nise rfiles, and cmute the nise crrected intensity rfiles frm each burst (see Figure 1), at different range lcatins estimate mean intensity frm all the crrected rfiles f same larizatin that cvers exactly the same area n grund cmute the larizatin rati by dividing the mean VV intensity with the mean HH intensity frm same area n grund The arach described abve is used t cmute 6 10 (range azimuth) indeendent estimates f the larizatin rati frm each ASAR AP image. The mst rnunced effect f additive nise is at high incidence angles. Examle f nise crrectin f AP mde intensity data is shwn in Figure 1 fr data frm IS2 and IS4. Figure 1 shws that the relative cntributin frm additive nise is significant, esecially at IS4. Since the additive nise is exected t be relatively mre rnunced at higher incidence angles, a saturatin f the larizatin rati is exected at higher incidence angles rvided n nise crrectin is erfrmed. This is als what is bserved as shwn in Figure 2, where we shw the larizatin rati as functin f incidence angle with and withut additive nise crrectin.

3 The full lines in Figure 2 shw the best fit lynmial t the data with and withut nise crrectin. Figure 2 shws that saturatin f the larizatin rati indeed ccurs at higher incidence angles if n crrectin fr additive nise is erfrmed (blue line). Hwever, after the crrectin the larizatin rati nicely increases with increasing incidence angle, as exected frm thery within the range f incidence angle cnsidered here. a) 3. RESULTS In the fllwing the sea-surface larizatin rati ( σ σ ) as estimated frm ASAR AP data is cmared against varius EM scattering mdels. First we evaluate the imact f breaking waves in the GCM mdel. In Figure 3 we shw the larizatin rati as functin f incidence angles frm the GCM mdel (with and withut wave breaking included) ver ltted the ASAR AP measurements. b) Figure 1. Azimuth intensity rfiles (blue) f AP data ver ltted the crresnding nise rfile (red) and the nise crrected intensity rfile (green) fr tw different swaths: a) IS2, b)i S4 Figure 3. Plarizatin rati frm ASAR AP data as functin f incidence angle ver ltted rati curves frm the GCM mdel with and withut including wave breaking fr a range wind seed f 10m/s and an azimuth wind f 10 m/s. Figure 2. Plarizatin rati frm ASAR AP data as functin f incidence angle withut (blue) and with (green) crrectin f the HH and VV intensities fr additive nise. Full lines are best fit lynmial t the data ints. As exected Figure 3 shws that taking int accunt wave breaking effects in the GCM mdel make the rati decrease, because nn-bragg scattering frm breaking waves is assumed t be indeendent f larizatin. Cmared t measurements, the extended GCM erfrms better. This is further investigated and quantified in Figure 4, where scatterlts f simulated versus measured larizatin rati are shwn fr a cllcatin radius f 50km (figure a) and 100km (figure b).

4 the cllcatin radius t 50km further reduces the RMS errr and bias t 0.47 and 0.04, resectively. Finally, in Figure 5 we cmare the larizatin rati frm varius EM scattering mdels in literature against ASAR AP measurements as well as against the extended GCM mdel. a) Figure 5. Scatterlt f simulated larizatin rati frm varius EM scattering mdels versus estimated larizatin rati frm ASAR AP data. The simulatins are dne using cllcated wind field frm buy, latfrm r QuickScat. The cllcatin radius is set t r = 100km. b) Figure 4. Scatterlts f simulated larizatin rati frm GCM mdel with (blue) and withut (red) wave breaking term versus estimated larizatin rati frm ASAR AP data. The simulatins are dne using cllcated wind field frm buy, latfrm r QuickScat. Figure a is fr a cllcatin radius f r = 50km and figure b is fr a cllcatin radius f r = 100km. Figure 4 shws that extending the NRCS f the GCM mdel with wave breaking effects reduces bth the larizatin rati RMS errr and the bias, resectively frm 0.59 t 0.49 and frm 0.58 t Reducing Figure 5 shws that the GCM mdel with the inclusin f wave breaking term has bth the lwest RMS errr and bias fr the σ σ rati. Esecially, we nte that the bias is a factr f 4 lwer than the best emirically based mdel [1]. In Table 2 we have summarized the results f the validatin f the varius EM scattering mdel and ASAR AP fr the larizatin rati. Table 2. Summary f GCM versus ASAR AP larizatin rati cmarisn. Mdel Cllcatin Radius 50km Cllcatin Radius 100km Std. Bias Std. Bias GCM GCM+BW Muche (md 1) Muche(md 2) Thmsn Elfuhailly

5 The deendency n wind seed and wind directin relative t range has als been simulated using the extended GCM mdel, but the number f ASAR AP bservatins is t lw t state any cnclusin. 4. CONCLUSION Envisat ASAR AP data can be used t study the larizati rati as fuctin f imaging gemetry and sea-state, rvided a recise crrectin f additve nise is dne. Plarizatin rati frm ASAR AP and theretical EM scattering mdel agrees well, and imrves significantly by including effects f breaking waves in the nrmalized radar crss sectin f the mdel. ACKNOWLEDGEMENT: The wrk was c-funded by ESA/ESRIN Cntract N /03/I-OL, and Nrges frskningsråd Cntract N We acknwledge the use f the ESA ASAR data, and the cllcated data frm CERSAT, at IFREMER, Pluzané (France) and met.n (Nrway). REFERENCES 1. Muche, A., D. Hauser, JF Dalze, C. Guerin (2005), Dual-Plarizatin measurements at C-band ver the cean: results frm airbrne radar bservatins and cmarisn with Envisat ASAR data, IEEE Trans. Gesci. and Remte Sensing, 43, Kudryavtsev, V., D. Hauser, G. Caudal, and B. Charn (2003), A semi-emirical mdel f the nrmalized radar crss-sectin f the sea surface: 1. The backgrund mdel, J. Gehys. Res., 108(C3), 8054, di: /2001jc Elfuhaily, T., Guerin C-A. (2004), A critical survey f arximate scattering wave theries frm randm rugh surfaces, Waves in Randm Media, 14, 4, di: / /14/4/R01 4. Engen, G., I.F. Pedersen, H. Jhnsen, and T. Elfuhaily (2006), Curvature effects in cean surface scattering, IEEE Trans. Antennas Prag., 54(5), Valenzuela G. R. (1967), Delarizatin f EM waves by slightly rugh surfaces, IEEE Trans. Antennas Prag., vl.ap-15, n.4, Stgryn A. (1967), Electrmagnetic scattering frm rugh finitely cnductin surfaces, Radi Sci., vl2, n.4, Elfuhaily, T. (1996), Mdele cule vent/vagues et sn alicatin a la teledetectin ar micr-nde de la surface de la mer, Thesis frm University Paris 7, Nv Thmsn D., Elfuhaily T., Charn B. (1998), Plarizatin rati fr micrwave backscattering frm cean surface at lw t mderate incidence angles, Prc. f IGARSS 98, Seattle, WA, July Kudryavtsev, V., and J. Jhannessen (2004), On effect f wave breaking n shrt wind waves, Gehys. Res. Lett., 31, L20310, di: /2004gl Kudryavtsev, V., V. Makin, and B. Charn (1999), Culed sea surface atmshere mdel : 2. Sectrum f shrt wind waves, J. Gehys. Res., 104, Phillis, O. M. (1977), The Dynamics f the Uer Ocean, Cambridge Univ., Press, New Yrk. 12. Phillis, O. M. (1985), Sectral and statistical rerties f the equilibrium range in the wind generated gravity waves, J. Fluid Mech., 156, Phillis, O. M. (1988), Radar returns frm the sea surface Bragg scattering and breaking waves, J. Phys. Oceangr., 18,