Subarray 1 String short circuit DC current

Hello,

I would like to know how is SAM calculating "Subarray String short circuit DC current"?

Does SAM uses ISC (short-circuit current) equations from the Sandia “Photovoltaic array performance model” to calculate short-circuit current? IF so, I am seeing different results. I took hourly irradiance and cell temperature output from SAM and calculated the hourly ISC. My calculation for the maximum current value came as 17.65 Amps whereas in SAM I am seeing 17.9 Amps. 
could you please share the helpful resource if the calculations are different from the PV performance model?

Also, since we are using a Bifacial module, is there a way we get rear side or total irradiance before IAM losses? I don’t see this option available in the Lifetime hourly data tables.
 “Subarray 1POA rear side total irradiance (W/m^2)”  before reflection (IAM)
“Subarray 1POA total irradiance (W/m^2)”  before reflection (IAM)

Thanks
NG