Earthdata Forum

Hello,
I know I have asked many l2gen questions in the past week, so thank you in advance for your patience.

I am currently trying to run l2gen on L1A VIIRS-SNPP data (scene: SNPP_VIIRS.20230712T180000.L1A.nc). I am starting by running calibrate_viirs on the L1A scene to generate L1B. To do this, I did which installed folders viirsj1 and viirsn files to /ocssw/var, viirs to /ocssw/share, and viirs_l1_benchmark to /ocssw. It also output the following:

However, I get this error when running calibrate_viirs: By the way, polar_wander.ascii does exist in my directory /ocssw/var/viirsn and has a size of 2.2MB.

Thank you!
Best,
Sami

Are you setting the correct environment?
export OCSSWROOT=path_to_your_ocssw_directory
export OCVARROOT=$OCSSWROOT/var

We stage the operational GEO files so you don't have to make them.
Replace "L1A" with "GEO" in the filename.

Tommy

Hi Tommy,
Thanks. So if I am understanding correctly, I can download the GEO file and use that as a parameter for l2gen? Do I still need to generate L1B with calibrate_viirs then? Can I just generate an l2 directly from l1a with a command like this: Is there a need to use getanc or calibrate_viirs then?

Best,
Sami

Yes, to make an L2 file you should provide the ancillary data. We make an L1B file from the L1A and GEO and then pass the L1B to l2gen.
I think l2gen has the code to make L2 directly from L1A and GEO but I will defer to one of our SeaDAS experts on running that...

Tommy

Yes, l2gen can take a VIIRS L1A file as input (performing the calibration on-the-fly), but the VIIRS geolocation and calibration programs share code, so if you are unable to process the geoocation, you will likely have issues calibrating - either with calibrate_viirs or l2gen.

Sean

Thanks all for the support.
So the process to go from L1A to L2 includes two steps, each with ifile, ofile, and geofile:

• 1. calibrate_viirs create L1B: ifile=[VIIRSN.L1A.nc] l1bfile_mod = [VIIRSN.L1B.nc] (ofile and geofile are not valid params https://seadas.gsfc.nasa.gov/help-8.1.0/processors/ProcessCalibrate_viirs.html)
  2. l2gen create L2: ifile=[VIIRSN.L1B.nc] ofile = [VIIRSN.L2.nc] geofile=[VIIRSN.GEO.nc]

Is this correct? I'm not sure where getanc goes in this process.

Yes, that is the process. The getanc call goes before l2gen to identify (and download) the ancillary inputs to the l2gen process. BTW, calibrate_viirs can output three different files, thus in place of ofile it uses l1bfile_[img,mod,dnb].

BTW, the calibrate_viirs code includes the bits of the geolocation code necessary to do the calibration, so a geofile is not required as input...which is why I said in my previous post that if geolocate_viirs fails due to an incorrect configuration or missing LUTs, calibrate_viirs is also likely to fail.

Sean