Modelling the GNSS Reflectometry Signal over Land: sensitivity to soil moisture and biomass N. Pierdicca1, L. Guerriero2 , E. Santi3, A. Egido4 1 2 3 4 DIET - Sapienza Univ. of Rome, Rome, Italy DISP - University of Tor Vergata, Rome, Italy CNR/IFAC, Sesto Fiorentino. Italy Starlab, Barcelona, Spain Introduction: GNSS-R GNSS Reflectometry performs bistatic measurements, with most of the signal coming from around the specular direction Specular reflection and diffuse scattering from the Earth surface are combined This is similar to the radar altimeter, which however works in monostatic configuration AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 2 Introduction ESA has funded two projects aiming at evaluating the potential of GNSS signals for remote sensing of land bio-geophysical parameters (soil moisture and vegetation biomass), through ground based (LEIMON, 2009) and airborne (GRASS, 2011; in progress) experimental campaigns developing a simulator to theoretically explain experimental data and predict the capability of airborne and spaceborne GNSS-R systems for moisture and vegetation monitoring This presentation resumes some issues addressed and some experience gained on land GNSS-R signal modeling AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 3 Content The projects The problem Simulator description Validation results and sensitivity Conclusions AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 4 LeiMon project The crane The GNSS receiver antenna The monitored fields (West and East side) AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 5 The flight track and monitored fields GRASS project The airplane The antenna AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 6 L e i m o n v O v e r v i e w AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 7 Coherent vs. incoherent power Incoherent scattering diffused in any direction Eincoh=E-<E> Wrq (incoh) Wt p 2 4 3 D tp ( i , i ) Dqr ( s , s ) rt2 rr2 TX TX opqdA pq Coherent reflection along specular direction Ecoh=<E> RX °(i,s) Wrq ( coh) pq 2 Dtp DqrWt p 4 2 rt rr 2 Different dependence on ranges (rt and rr) and resolution (dA) makes the relative magnitude of incoherent and coherent components varies with receiver height, besides dependence on surface roughness, vegetation. AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 Signal correlation time Incoherent component fluctuates (speckle) with correlation time Tc dependent on system configuration In LeiMon ground based steady receiver → TC15 s In GRASS airborne receiver → TC7-8 ms Long coherent integration (TI>>TC) can reduce incoherent, zero mean, component when possible Alternatively, long incoherent integration is required to mitigate fading, still preserving the incoherent signal power direct ≈TC reflected Confirmed by the slow fluctuation of LeiMon signal (red line) AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 Incoherent component model Absent or homogeneous vegetation cover. RTE solution considerind attenuation and multiple scattering by a discrete medium (Tor Vergata model) Indefinite mean surface plane with roughness at wavelength scale. Bistatic scattering of locally incident plane waves by AIEM (Fung, Chen) Contributions from single independent surface elements, whose dimension can be assimilated to the roughness correlation length (order of the wavelength) , add incoherently. Then, the incident wave can be assumed locally plane AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 Coherent component model Scattering of spherical wave by Kirchoff approxi. (Eom & Fung, 1988) • • • • This a canonical problem in electromagnetism (antenna above a dissipative plane, Sommerfeld equality, Exact Image Theory, and so on) Signal is determined by a large portion of the mean surface, at least the first Fresnel zone, from few meters (ground) to tens of meters (airborne), to kms (spaceborne) Kirchoff approximation can be useful. The incident wave must be assumed to be spherical (as in Fung & Eom, 1988) We have removed some constraints of Fung & Eom, i.e., consideration of identical transmitting and receiving antennas at the same distance from the surface, and restriction to backscattering and specular scattering cases. AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 The Bistatic Radar Equation The mean power of received signal vs. delay t and frequency f is modeled by integral Bistatic Radar Equation which includes time delay domain response 2t’-t and Doppler domain response S2 (f’-f ) of the system (Zavorotny and Voronovich, 2000). Y (tˆ, fˆ ) |Y |2 PT GT , GR RR, RT Ti 2 S2 dA 2 Ti 2 PT 2 GT GR 2 (t '-t ) S 2 ( f '- f ) 0 dA 3 2 2 (4 ) RR RT Processed signal power at the receiver vs. delay t and frequency f. The transmitted power of the GPS satellite. The antenna gains of the transmitting and the receiving instrument. The distance from target on the surface to receiving and transmitting antennas. The coherent integration time used in signal processing. Bistatic scattering coefficient provided by the electromagnetic model The GPS correlation (triangle) function The attenuation sinc function due to Doppler misalignment. The longer the time Ti the narrower the filter in Doppler space Differential area within scattering surface area A (the glistening zone). AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 12 Model validation: LeiMon angular trend Reflected (down antenna) over direct (up antenna) April 8th SMC=30% August 26th SMC=10% • • • • East field Z=3cm West field Z=1.75cm East field Z=0.6 cm West field Z=1cm • Wetter and smoother fields exhibits higher down/up • Simulator reproduces quite well LR signal versus at incidence angles ≤45°. • Higher angle may suffer from poor antenna characterization (pattern, polarization mismatch) AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 13 LeiMon coherent & incoherent: soil April 8th SMC=30% August 26th SMC=10% • • East Z=3cm East Z=0.6 cm total coherent Comparison of LR theoretical simulations and data shows that incoherent component strongly contributes to total signal when soil is rough. AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 14 Model vs LeiMon data: bare soil LR Overall LR LeiMon model vs data comparison over bare soil 10%<SMC<30% 0.6<z<3cm Slight model overestimation of the DOWN/UP ratio but good correlation LR -8 -12 -14 -16 LR -8 15 West -18 15 East 25East 25West -20 -20 -18 -16 -14 -12 Leimon data DN/UP [dB] -1035East -8 35West Without considering incoherent component lower values (rough surfaces) would be strongly underestimated Simulated DN/UP (only coherent) [dB] SImulated DN/UP [dB] -10 -10 -12 -14 15 West -16 15 East 25East 25West -18 35East 35West -20 -16 -14 -12 -10 Serie1 Leimon DN/UP data [dB] AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 15 -8 Model vs GRASS data: overall LR RMSE2dB Bias 1dB Overall LR GRASS model vs data (DOWN/UP) comparison over bare soil and forest Good performance over forest and slight model underestimation over bare soil (but still good correlation) Note that soil underestimation can be easily reduce by changing correlation length AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 16 Model vs data: RR component LeiMon experiment GRAA experiment The Simulator underestimates the signal at RR polarization especially when it is expected to be low (e.g. rough soil) Besides the possible model errors (underestimation of crosspolarized incoherent scattering) e limit due to instrumental noise seems to saturate observations below -18 dB (LeiMon) and -22 dB (GRASS) AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 Sensitivity to SMC Reflected power at 35° incidence in Left Right (LR) polarization exhibits a good sensitivity÷correlation (0.3 dB/%÷0.76) with SMC of the West field (the one covered by sunflower), whilst the correlation with SMC is poor on the East field (which was always bare, but with change in roughness). By using the ratio RR/LR, we observed a significant negative sensitivity÷correlation (0.2 dB/%÷0.84) to the SMC on both fields AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 18 LeiMon LR data bivariate fitting Best fitting of LR versus z and mv, for the East field data, by equation: LR a0 a1 a2 mv a3mv • Data (blue diamonds) and fitting surface (gray) • Slopes of the surface measures sensitivity Sensitivity to z: -4 to -2 dB/cm Sensitivity to mv : 1 to 3 dB/10% AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 LeiMon: LR sensitivity to crop biomass At 35°, the coherent component is attenuated by about 1 dB each 1kg/m2, which would predict a large sensitivity to PWC (Ulaby et al., 1983; Jackson et al., 1982; O’Neill,1983) t e -20.1PWCsec The Simulator predicts a quite large incoherent component according to the short coherent integration time (1 msec), which explains the saturation effect with PWC in the data. The model reproduces the measured (low) sensitivity (0.3 dB/kg·m-2 , about 2 dB for the whole PWC range) thanks to the consideration of both component AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 20 GRASS: LR sensitivity to forest biomass • Coherent component is dominant. • Incoherent component is reduced by the longest coherent integration time (20 msec) which filter a narrower Doppler band • Data and simulations agree in showing a fairly good sensitivity to biomass (1dB every 100 m3/ha) • The observed Highest Tree Volume corresponds to a Dry Biomass of about 110 t/ha (beyond the SAR saturation limit ) • Sensitivity to even highest biomass to be verified AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 Conclusions A simulator has been developed which provides DDM’s, waveforms and peak power (reflected/direct) of a GNSS-R system looking at bare or vegetated soils (LHCP and RHCP real antenna polarization) It singles out coherent and incoherent signal components. Validated using data over controlled experimental sites (bare soil, sunflower, forest) (LeiMon and GRASS). Simulator results and experimental data show a fair agreement at LR polarization and angles <45° (the antenna beamwidth) The incoherent component may be high in the ground based LeiMon configuration, whereas was reduced by coherent integration in airborne GRASS Sensitivity to SMC is significant and well reproduced by the simulator Sensitivity to vegetation is reproduced and it is quite low when the incoherent contribution cannot be reduced. AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 22 AIT-CeTeM – Telerilevamento a Microonde, Bari 4,5 dicembre 2012 23

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