Scale Factors ASTER L2 Surface Radiance TIR V004

[mirteriemens]

Hi,

I am using the ASTER L2 Surface Radiance TIR V004 to run my own TES algorithm as I want to see what happens in the different modules.
However, the final emissivity I get are not as similar as I would expect it to be to the ASTER emissivity. I believe this might be due to the scaling of the bands.

For all five bands there is are sir and sra tif rasters available.
I take the sra to be the surface leaving radiance, and found the corresponding scale factors for these on https://www.earthdata.nasa.gov/data/catalog/lpcloud-ast-09t-004#variables to be 0.006882, 0.006780, 0.006590, 0.005693, 0.005225 for the five different bands.

However, I cannot find believe the scale factors for the sir tif rasters, which I assumed to be the downwelling sky radiance. Could it be that the scale factor for the sir values are not the same? If so, what are / where can I find the correct values?

I tried to look for the scale factors in the metadata, but I cannot seem to find them in here. I am accessing the metadata from https://search.earthdata.nasa.gov/ , where I also downloaded the rest of the data from.

Thanks in advance

[LP DAAC - dgolon]

Hello @mirteriemens We're checking with our science team and will report back when we have any answer. Thanks --Danielle

[LP DAAC - jwilson]

@mirteriemens Our science team sent the following below on your inqury.
Hello,

Thanks for sharing the details of your workflow.

From your description, it looks like your handling of SRA (surface leaving radiance) is correct — the scale factors you found are appropriate for those bands. Your observation that the final emissivity differs from the LP DAAC ASTER product is understandable; these differences often arise from subtle differences in how inputs are interpreted or processed.

One point to clarify based on your workflow: the SIR bands are downwelling sky irradiance (units: W·m⁻²·µm⁻¹), not radiance like SRA. Because of this, they cannot be used in exactly the same way as SRA, and applying the same scaling approach or directly feeding them into TES as if they were radiance can lead to differences in emissivity. If you want to include SIR in your TES calculations, you would need to account for the unit difference (for example, converting irradiance to radiance under appropriate assumptions).

It’s also worth noting that ASTER’s official TES products incorporate internally derived atmospheric corrections and radiative transfer modeling via MODTRAN. So even with correctly scaled inputs, reproducing the exact emissivity results can be challenging.

The main takeaway is that the SIR values are in irradiance units, so differences in handling them are likely the source of the discrepancies you’re seeing.

If you have any other questions, please don't hesitate to reach out.

[mirteriemens]

Hello,

Thank you for the reply, it was very useful. I do still have one question:

To convert the SIR values I assumed a Lambertian sky and divided them by π. However, the values seem way too big (between 80 and 200 W m^−2 sr^−1 μm^−1) and shape of the curve also feels a bit unexpected to me. I attached an image of this sky downwelling radiance (SIR / pi) for the relevant pixels over the ASTER bands.

As such, this still feels like a scaling issue to me. Is there any scaling of the SIR bands that I am still missing?

Thanks for your help,
Mirte

[LP DAAC - jwilson]

@mirteriemens I forwarded you question to our science team. Once we have more information, I will let you know.
Janice