MODIS L1B Offsets

[rs937]

Hello,

The Equation 5.10 of MODIS Level 1 User’s Guide (MODIS Level 1B Product User's Guide) implies that the radiance_offset parameters are by definition all negative. However, the parameters I obtain by using either HDFView or using MATLAB’s hdfread, for many different sets of 1 km L1B data, are all positive. Furthermore, Eq. 5.8 indicates that these offsets are subtracted from SI, meaning that effectively positive values should be added to the SI (before multiplying by the scale factor) to obtain the radiance. My question is that, since I am seeing positive values for the offsets, whereas the documentation says they should be negative, should I be subtracting or adding these positive values to the counts?

I should add that Equations 5.8 through 5.10 collectively imply that SI is 0 at minimum radiance and 32767 at maximum radiance, so that all the bits are in principle efficiently used. The reason I am confused about the sign of the offsets is that I have noticed that in some data sets the minimum count starts above 0 or the maximum count is less than 32767.

Thanks,

[LAADS_UserServices_M]

The Lmin values for the 1km thermal emissive bands are negative values, therefore the offsets as derived in equation 5.10 come out to be positive as you correctly noted in your query below. To retrieve the radiance, equation 5.8 from the document should be used with the scales and offsets as read in from the files.

[rs937]

Thanks very much for your response. I should have mentioned that the positive offset values I am seeing are both for Emissive and RefSB bands. If Lmin can be negative also for RefSB, then it would make sense that I am seeing all radiance offsets to be positive.

[rs937]

I apologize if I may be a bit confused, but isn't L the calibrated TOA spectral radiance? I am not sure I understand how Lmin can be negative.

[LAADS_UserServices_M]

Please consult Section 5 (specifically section 5.4) of MODIS Level 1B Product User’s Guide at:
https://mcst.gsfc.nasa.gov/sites/default/files/file_attachments/M1054E_PUG_2022_1005_V6.2.2_Terra_V6.2.3_Aqua.pdf
Should you have specific question, post it at:
https://mcst.gsfc.nasa.gov/contact-us