Koreksi Atmosfer: Lalu Muhamad Jaelani, Ph.d

Koreksi Atmosfer Lalu Muhamad Jaelani, Ph.D LMJaelani.com

Mengapa Koreksi Atmosfer diperlukan?

Sun

ρtoa

Data yg diperoleh

Thermosphere

≈

Altitude (km)

No atmosphere 100 90 80 70 60 50 40 30 20 10

sensor

Mesosphere

ρr

Stratosphere O3, N2, (NH4)2SO4

Troposphere O2, H2O

0

Water

+ ρa + ρ

w

≠

Data yg diinginkan 2

Apa yang dimaksud dengan Koreksi Atmosfer? = menghilangkan efek atmosfer dari data yang direkam oleh sensor

0.05

Data yang diperoleh (direkam oleh sensor) 0.04

0.03

0.02

Efek Atmosfer

Remote Sensing Reflectance (sr-1)

Lake Kasumigaura

Data yang diinginkan

0.01

0 413

443

490

510

560

620

Wavelength (nm)

665

681

709

754 3

Teori Dasar TOA reflectance

Aerosol scattering

Water leaving reflectance

toa ( )  r ( )   A ( )  t ( )  w ( ) Rayleigh scattering

Dari data satellite

transmittance

Dapat dihitung dari model 2 unknowns

4

Koreksi Atmosfer di Air Jernih Dapat diasumsikan bahwa ρw pada kanal NIR (779 nm and 865 nm) = 0.0

Air Jernih ρw(NIR) ≈ 0.0

NASA Ocean Biology Processing Group

Near Infra Red (NIR) 5

Mendapatkan Aerosol Scattering untuk kanal NIR di air jernih

toa ( NIR)  r ( NIR)   A ( NIR)  t ( NIR)  w ( NIR)

toa ( NIR)   r ( NIR)   A ( NIR)

zero

One unknown

NIR bands = 779 and 865 nm 6

Proses Koreksi Atmosfer di Air Jernih ρA(779) ρA(865) Multiple scattering

LUT

ρa(779) ρa(865)

 a (779)   (779,865)  a (865)

Single scattering

Aerosol type/model

at other wavelengths

 w(  ) 

ρa(λ)

 ( ,865)

at other wavelengths

Extent epsilon to other wavelengths

LUT

ρA(λ)

toa(  )   r (  )   A(  ) t(  )

Atmospheric corrected reflectance

Remote Sensing Reflectance (sr-1)

Koreksi Atmosfer di Air Keruh (Turbid Water) 0.02

Lake Kasumigaura The spectral value at near infrared wavelength≠ 0

0.015

0.01

two unknowns

0.005

0 412 443 490 510 560 620 665 681 709 754 762 779 865 885 900

Wavelength (nm)

Near Infra Red (NIR)

toa ( )  r ( )   A ( )  t ( )  w ( ) 8

Masalah Koreksi Atmosfer di Air Keruh

toa ( )  r ( )   A ( )  t ( )  w ( ) One equation has two unknowns. How to solve this problem ?

Several approaches to solve the above problem toa ( )   r ( )   A ( )  t ( )  w ( ) 1. Directly predict the aerosol reflectance different assumption Twousing unknowns [Ruddick et. al. (2000); Hu et. al. (2000); Wang and Shi (2007); Guanter et. al. (2007, 2010)] 2. Estimate water leaving reflectance firstly, and then estimate aerosol reflectance [Stumpf et. al. (2003); Bailey et. al (2010); Wang et. al. (2012)] 3. Estimate water leaving reflectance and aerosol reflectance simultaneously [Schroeder et. al. (2007); Doerffer & Schiller (2007,2008); Kuchinke et al. (2009a, 2009b)] 10

Koreksi Atmosfer Sederhana • Metode DOS (Dark Object Substraction) • Metode Radiative Transfer 6SV (Second Simulation of a Satellite Signal in the Solar Spectrum – Vector)

DOS • Data dalam format Reflektan-Sensor (ρtoa) • Nilai Pixel Minimum (NPM) dari citra harusnya adalah NOL • Cari NPM minimum (> nol) • Semua Pixel dikurangi NPM • Hasil akhir berupa reflektan-permukaan (ρboa)

6SV • Data dalam format radian, • Citra dikoreksi dengan menggunakan rumus:

• acrλ=yλ/(1.+xcλ*yλ) • yλ=xaλ*( Lλ)-xbλ;

• acrλ adalah reflektan-permukaan, Lλ adalah radian. • Parameter koreksi diperoleh dengan menjalankan perangkat lunak 6SV berbasis web yang ada di http://6s.ltdri.org/. • Untuk mendifinisikan konsentrasi dari aerosol, digunakan parameter meteorologi berupa horizontal visibility yang dapat dimasukkan secara langsung dalam 6SV.

6S input for ALOS-VNIR2 • Geometrical conditions

6S input for ALOS-VNIR2 • Geometrical conditions – Month =9 (year=2010) – Day=1 – Solar Zenith Angle =90-Solar Elevation Angle=9061.88=28.12 – Solar Azimuth Angle=57.37 – Sensor Zenith Angle = 0 (Img_PointingAngle) – Sensor Azimuth Angle =12 (Img_SceneCenterOrientation)

Visibility • http://www.wunderground.com/history/airpo rt/WARR/2010/9/1/DailyHistory.html?req_city =Surabaya&req_state=&req_statename=Indon esia&reqdb.zip=00000&reqdb.magic=9&reqd b.wmo=96933 • Total second 9660 > jam 9.40 • 5.0 km

Band 1 • • • • • • • • • •

atmospheric correction result * * ----------------------------* * input apparent reflectance : 0.100 * * measured radiance [w/m2/sr/mic] : 52.386 * * atmospherically corrected reflectance * * Lambertian case : -0.00674 * * BRDF case : -0.00674 * * coefficients xa xb xc : 0.00603 0.32266 0.14326 * * y=xa*(measured radiance)-xb; acr=y/(1.+xc*y)

Band 1 • • • • • • • • • •

atmospheric correction result * * ----------------------------* * input apparent reflectance : 0.100 * * measured radiance [w/m2/sr/mic] : 52.386 * * atmospherically corrected reflectance * * Lambertian case : -0.00674 * * BRDF case : -0.00674 * * coefficients xa xb xc : 0.00603 0.32266 0.14326 * * y=xa*(measured radiance)-xb; acr=y/(1.+xc*y)