Stray Light in GOES Imager data
GOES-13 0.63 µm visible channel images (click image to play animation)
Each year, about every 6 months, the Earth-Sun-Satellite geometry is such that the GOES Imager can look right at the Sun. In the past, there were ‘keep-out zones’ in which the satellites did not image because it was known to be looking at the Sun during those times. The imagery above, from GOES-13, shows visible light in the night-time imagery. (Click here for a similar GOES-15 animation). Stray light values typically peak around 0500 UTC for GOES-East and around 0900 UTC for GOES-West.
In addition, imagery was not possible during the so-called ‘eclipse season’ because the satellites lacked sufficient batteries to power the instruments as they passed through Earth’s shadow. Now, an improved battery system on the current generation GOES-13/14/15 satellites allows for imaging to proceed while the satellite is in the Earth’s shadow.
This new scheduling, however, introduces issues. The GOES Imager is calibrated by periodic looks into deep space, regions from which only very small amounts of radiation (at 3.9, 6.5, 10.7 and 13.3 µm) are being emitted. These ‘space looks’ are on either side of the full-disk GOES Image. During the ‘eclipse season’, that space look can include part of the solar energy, meaning the very small amount of radiation that the satellite is designed to detect is actually potentially significant. Thus, the calibration of the image can be affected. NOAA NESDIS does operationally correct images with ‘stray light’, but this correction does not consider the impact of a corrupted space view. The GOES-13 stray light corrections were implemented in 2012, as discussed here on this blog.
In addition to the calibration images, solar radiation can also be scattered off clouds towards the imager. So, instead of detecting only emitted radiation at night, the GOES Imager is detecting emitted terrestrial radiation in addition to scattered/reflected solar radiation. This solar radiation contaminates the signal, and results in ‘too much’ radiance being detected, resulting in warmer-than-actual inferred blackbody/brightness temperatures.
GOES-13 imagery from infrared channels (click image to enlarge)
When Stray Light issues occur, the most noticeable effects are in the 3.9 µm channel (Above loop, bottom left) and in products that use the 3.9 µm channel, such as the brightness temperature difference (Above loop, top left). In other words, this calibration issue can affect derived products that use 3.9 µm data at night. The image below shows how the 3.9 µm imagery can change when Stray Light is an issue. Compare the 0415 UTC image, on the left, when Stray Light did not contaminate the space look, with the 0502 UTC image on the right, when Stray Light was an issue.
GOES-13 3.9 µm imagery
NESDIS is considering methods of mitigating the stray light issues that occasionally occur in the GOES Imager.