When I bring up GOES Imager or Himawari-8 visible or near-IR data via ADDE, I am given at least 3 choices for units, raw count, brightness, and albedo. I understand the first two but not the albedo parameter. How exactly is this parameter defined, i.e. the equation?
I see that "albedo" is not a calibrated reflectance which involves using the raw count, a calibration gain and offset (as a function of day since satellite launch), space count, earth sun distance factor, and cosine(solar zenith angle). Ultimately I (and many others in the research community) desire calibrated reflectance for our work but we would need McIDAS-V to supply the solar zenith angle and other parameters. Or allow the user to supply the gain and offset, and have Mc-V derive everything else. There has been discussion in other forums on having Mc-V derive angles (viewing, solar zenith, relative azimuth) but I'm not sure where this is at in the development priorities.
definition of "albedo" parameter and calibrated reflectance
Re: definition of "albedo" parameter and calibrated reflectance
Hi Kris -
We passed your questions around to some programmers, and here are their responses:
McIDAS-X Inquiry 11947 should have all of the necessary information about the GVAR values. Here is some information from the Action of that inquiry:
Our Himawari programmer had this to add for Himawari:
For Himawari8, Albedo is the percentage of reflected brightness. Values have a range 0<->120.
As for extracting satellite and solar angle information from data, this is written up as McIDAS-V Inquiry 1501. The inquiry is set as being resolved by a programmer. We've pinged the programmer to see if there are any updates, and we are waiting to hear back.
Thanks -
Bob Carp
We passed your questions around to some programmers, and here are their responses:
McIDAS-X Inquiry 11947 should have all of the necessary information about the GVAR values. Here is some information from the Action of that inquiry:
alb = (m * DN + b)* k
where m = .5515, b = -15.32, k = 1.92979e-3, DN = digital number [raw count]
The values for m & b above are approximate. I did an average, out to about 4 places, of the factory coefficients. We take the values out of the GVAR signal [which are the same as the fac. coeff.], but this is going to change in the future to account for sensor degradation.
An example, McIDAS outputs a raw count of 4512 with an albedo 12.2%. If you run this through the equation above, you will get 12% [dividing 4512 by 32 first].
Our Himawari programmer had this to add for Himawari:
For Himawari8, Albedo is the percentage of reflected brightness. Values have a range 0<->120.
As for extracting satellite and solar angle information from data, this is written up as McIDAS-V Inquiry 1501. The inquiry is set as being resolved by a programmer. We've pinged the programmer to see if there are any updates, and we are waiting to hear back.
Thanks -
Bob Carp