[GRASSLIST:7422] Convert DN's to reflection

[rchopra at uchicago.edu]

Hi all,

I was wondering if I could get some assistance with respect to
the conversion of digital numbers to reflectance values for
Landsat 5 TM.

I first converted DN's to radiance for a band using:
 
 r.mapcalc “bandn_rad = 1.0 * (bandn * GAINn) + BIASn))” 

This gave me for the band :

Range of data:    min =  -1.520000 max = 151.497576

I now want to convert  radiance values are at-the-top-of-the
atmosphere to measures of reflectance using

rhop = (pi * L * d2)/(ESUN * cos(thetas))
where L is the measured TOA radiance, d is the earth-sun
distance in astronomical units, and thetas is the solar zenith
angle

I was unsure of what L should be in this case. Also could
someone help me see the light with the concept of Julian Days,
so that I can get the value of d. The data is from November
22, 1988.

Any help would be supremely appreciated.

Thanks
Rahul