Distance of Earth-Sun

Ok. The issue is resolved:
d(t) = 1/ ((1-0.01673cos(0.0172(t-2)))^2) = 1/d^2
d(t) – this is value from metadata!
Distance of Earth-Sun: d = SQRT(1/d(t)).

Thank you.


Hi @Igor @Jan

Can you please explain how to get the values to calculate the TOA or BOA reflectance ?
L = (rToa * e0__SOLAR_IRRADIANCE_For_band * cos(Z__Sun_Angles_Grid_Zenith_Values)) / (PI * U__earth_sun_distance_correction_factor) (Equation 1)
I know that :
rToa is the band we need to calculate its reflectance
e0__SOLAR_IRRADIANCE_For_band can be obtained from the metadata as below for Band 4 for example :

U__earth_sun_distance_correction_factor from the metadata as below :

But how to obtain:
1- cos(Z__Sun_Angles_Grid_Zenith_Values))
2- PI ( what is it ?)

I have calculated the d(t) according to the equation :

As t= 0.98358 (for the day for day 358, the earth-sun distance is 1.00992).
d(t) = 1.03431890515045328251315
But do not know where to insert d(t) in the (Equation 1)

Another question regarding :
(Equation 2)

Another question, why using the Equation 1 and not Equation 2 for calculating the reflectance? Also would be more appreciate if you could give an example of how to fine the parameters in Equation 2 from Sentinel 2 metadata.

Thanks a lot for your help and time,

Hi, Daniel

Data Sentinel-2A L1C MSI already have TOA Reflectance. According to the documentation, You must divide the pixel value by the value of QUANTIFICATION_VALUE:


Possible also some deviation in DN from the desired range. For example:

For BOA Reflectance – use, for example, SEN2COR package:

L – this is the TOA spectral radiance – the formula is the reverse formula for finding TOA spectral reflectance. For example:


d = SQRT(1/d(t)) or d =SQRT(1/U)
Theta = Z__Sun_Angles_Grid_Zenith_Values – the interpolated value from the matrix from the metadata:
GRANULE \ … granule.xml, tags: “Tile_Angles Sun_Angles_Grid Zenith… Values_List…”

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L = (rToa * e0__SOLAR_IRRADIANCE_For_band * cos(Z__Sun_Angles_Grid_Zenith_Values)) / (PI * U__earth_sun_distance_correction_factor)

Please note that earth_sun_distance_correction_factor – this value U.

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@Igor thanks a lot for the reply and for the explanation.
Just still on question: what is PI in L equation ?

L = (rToa * e0__SOLAR_IRRADIANCE_For_band * cos(Z__Sun_Angles_Grid_Zenith_Values)) /** (PI** * U__earth_sun_distance_correction_factor)

What is PI in L equation ?
PI ~3.14159


Thanks @Igor

The correct expression for the radiance should be the following:
L = (rToa * e0__SOLAR_IRRADIANCE_For_band * cos(Z__Sun_Angles_Grid_Zenith_Values) * U__earth_sun_distance_correction_factor) / PI
because U=1/d^2

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It’s the same formula!

rToa = (PILd^2)/(ESUNcos(theta));
L = (rToa
d^2 = 1/U;
L = (rToaESUNcos(theta))/(PI/U);
L = (UrToaESUN*cos(theta))/PI;

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Now it is correct. In previous posts you’ve used the following expression:
L = (rToaESUNcos(theta))/(PI*U)

see also in this topic “Radiometric convertion of sentinel 2 images”

I was misled by help to sen2three.
Now the link is unavailable:

But in the program sen2tree has the following lines:


def refl2rad(self, indataArr):
‘’’ Converts the reflectance to radiance.
:param indataArray: the digital numbers representing TOA reflectance.
:type indataArray: a 2 dimensional numpy array (row x column) of type unsigned int 16.
:return: the pixel data converted to radiance.
:rtype: a 2 dimensional numpy array (row x column) of type unsigned int 16, representing radiance.

        Additional inputs from L1 user Product_Image_Characteristics metadata:
        * QUANTIFICATION_VALUE: the scaling factor for converting DN to reflectance.
        * U: the earth sun distance correction factor.
        * SOLAR_IRRADIANCE: the mean solar exoatmospheric irradiances for each band.
        Additional inputs from L1 tile Geometric_Info metadata:
        * Sun_Angles_Grid.Zenith.Values: the interpolated zenith angles grid.
    # This converts TOA reflectance to radiance:
    nrows = self.config.nrows
    ncols = self.config.ncols
    # The digital number (DN) as float:         
    DN = indataArr.astype(float32)
    xp = L3_XmlParser(self.config, 'UP1C')
    pic = xp.getTree('General_Info', 'Product_Image_Characteristics')
    c0 = 0

    # The quantification value for the DN from metadata:      
    c1 =  float32(qv.text)

    # TOA reflectance:        
    rtoa = float32(c0 + DN / c1)

    rc = pic.Reflectance_Conversion

    # The earth sun distance correction factor,
    # apparently already squared:
    u2 =  float32(rc.U.text)

    # The solar irradiance:        
    si = rc.Solar_Irradiance_List.SOLAR_IRRADIANCE
    e0 = float32(si[self._bandIndex].text)

    # The solar zenith array:
    x = arange(nrows, dtype=float32) / (nrows-1) * self.config.solze_arr.shape[0]
    y = arange(ncols, dtype=float32) / (ncols-1) * self.config.solze_arr.shape[1]
    szi = rectBivariateSpline(x,y,self.config.solze_arr)
    rad_szi = radians(szi)
    sza = float32(cos(rad_szi))
    rtoa_e0_sza = float32(rtoa * sza * e0)

???–>> pi_u2 = float32(pi * u2 )

    # Finally, calculate the radiance and return array as unsigned int, this is multiplied by 100,
    # to keep resolution - glymur only allows integer integer values for storage.        
    L = (rtoa_e0_sza / pi_u2 ) * 100.0
    return (L + 0.5).astype(uint16)



It seems to me that this formula is correct:
L = (UrToaESUN*cos(theta))/PI;
And in their program - error.

L = (UrToaESUN*cos(theta))/PI is correct expression, but you’ve written in other posts L = (rToa * e0__SOLAR_IRRADIANCE_For_band * cos(Z__Sun_Angles_Grid_Zenith_Values)) / (PI * U__earth_sun_distance_correction_factor)

perhaps it would be appropriate to correct

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In sen2cor formula is correct!
Their review:
. . .

adding again the earth sun correction d2 = 1.0/U:

. . .

The final formaula is:

rad = rho * cos(radians(sza)) * Es * sc / (pi * d2)

where: d2 = 1.0 / U

scale: 1 / (0.001 * 1000) = 1 (default)

. . .
They use d^2:
L = (rToaESUNcos(theta))/(PI*d^2);


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I saw that you corrected in Radiometric conversion of sentinel 2 images

now it’s okay

How do you calculate theta?

Hello, fifauzan!

How do you calculate theta?
To get there:

Or get from file S2A_PRODUCT_NAME.SAFE/GRANULE/TILE_NAME/MTD_TL.xml (tag: Sun_Angles_Grid).

Hello, How do i write a band math expression to convert TOA Reflectance to TOA Radiance in SNAP.
Thank you. I am using this expression

1512.9 * 1.028880315 * B3 * cos(sun_zenith) / PI

u = 1.028880315
ESUN = 1512.9

ERROR: Referenced raster must all be of the same size.


There is already an operator doing the conversion from reflectance to radiance, for Sentinel-2 products. You can find it under the menu Optical > Thematic Land Processing > Reflectance to Radiance.

The earth-sun distance in astronomical units excel spreadsheet file link is down, i find excel file at :

Download link is below: