Earthdata Forum

Hi, I'm using the VNP46A3 data. My task is to downscale the near-nadir (NN) and off-nadir (ON) products and measure the impact of the viewing angle (VA) on the blooming effect (or PSF) from a geometric perspective. Let me explain.

If I project a viewing cone from nadir the footprint (support) is circular and has minimum area (given fixed viewing height).

If I project the same viewing cone at an angle, off-nadir, then the support is an ellipse and it has a larger area. The greater the angle off-nadir, the more pronounced the ellipse and the larger the area.

The areal increase can be calculated from geometry (my problem).

I am interested to test if the predicted increase in the PSF is equal to this known geometric effect.

What I have done so far is the following:

Assuming that the PSF width (measured in pixels) of the NN product is 1 pixel and the PSF of the ON product is 1.3 pixels. These two values are what I predicted during the downscaling.

To test the predicted value of the ON product, I did:

1/cos(40) ~1.34 which is close to what I predicted. The number 40 is the viewing angle of the sensor (in degrees) and 1 is the PSF width of the NN product.

Here are my questions:

1) is this geometric solution valid? Does it make sense?
2) where can I find the actual degrees of the viewing angle in each NTL image?

Many thanks.

The monthly composite product (VNP46A3) is created using observations from varying view angles collected throughout the month. As a result, a single view angle cannot be isolated for the off-nadir composite. Pixel-level view angle information is only available in the at-sensor top-of-atmosphere product (VNP46A1) under the layer “Sensor_Zenith.” For more details, please refer to the user guide.
https://viirsland.gsfc.nasa.gov/PDF/BlackMarbleUserGuide_v1.2_20220916.pdf

Thank you for your response. Let's say I use one daily image and not the monthly composite, is the equation I posted correct?

d/cos(θ)

where cos(θ) is the sensor's VA and d is the PSF at the (near) nadir.

The spatial resolution of DNB remains stable across varying view angle since each pixel is aggregated from several detectors. The following document might be helpful:

(https://ladsweb.modaps.eosdis.nasa.gov/api/v2/content/archives/Document%20Archive/Science%20Data%20Product%20Documentation/Product%20Generation%20Algorithms/NASARevisedVIIRSGeolocationATBD2014.pdf).