NASA Uses Moonlight to Improve Satellite Accuracy - Technology Org:
NASA's airborne Lunar Spectral Irradiance, or air-LUSI, flew aboard NASA's ER-2 aircraft from March 12 to 16 to accurately measure the amount of light reflected off the Moon. Reflected moonlight is a steady source of light that researchers are taking advantage of to improve the accuracy and consistency of measurements among Earth-observing satellites.
"The Moon is extremely stable and not influenced by factors on Earth like climate to any large degree. It becomes a very good calibration reference, an independent benchmark, by which we can set our instruments and see what's happening with our planet," said air-LUSI's principal investigator, Kevin Turpie, a research professor at the University of Maryland, College Park.
The air-LUSI flights are part of NASA's comprehensive satellite calibration and validation efforts. The results will compliment ground-based sites such as Railroad Valley Playa in Nevada, and together will provide orbiting satellites with a robust calibration dataset.
NASA has more than 20 Earth-observing satellites that give researchers a global perspective on the interconnected Earth system. Many of them measure light waves reflected, scattered, absorbed, or emitted by Earth's surface, water and atmosphere. This light includes visible light, which humans see, as well as invisible ultraviolet and infrared wavelengths, and everything in between.
[...] The ER-2 is a high-altitude aircraft that flew at 70,000 feet, above 95% of the atmosphere, which can scatter or absorb the reflected sunlight. This allowed air-LUSI to collect very accurate, NIST traceable measurements that are analogous to those a satellite would make from orbit. In order to improve the accuracy of lunar reflectance models, air-LUSI measurements are accurate with less than 1% uncertainty. During the March flights, air-LUSI measured the Moon for four nights just before a full Moon.
(Score: 2) by Runaway1956 on Wednesday April 06 2022, @08:32PM (3 children)
https://www.youtube.com/watch?v=MUakXak9-O0 [youtube.com]
Abortion is the number one killed of children in the United States.
(Score: -1, Offtopic) by Anonymous Coward on Wednesday April 06 2022, @09:24PM (1 child)
You know the movie Iron Sky was satire right? The moon nazis are not coming to help your plans for white supremacy.
(Score: -1, Offtopic) by Anonymous Coward on Wednesday April 06 2022, @11:54PM
The guy pushing race based socialism has been moderated into obscurity. No more moonlighting! [wikipedia.org]
(Score: 1, Interesting) by Anonymous Coward on Wednesday April 06 2022, @10:41PM
RIP Major Tom
(Score: 3, Interesting) by Anonymous Coward on Thursday April 07 2022, @02:57AM
Radiometry is measuring energy flow, and here it is measuring the amount of power that lands on your detector. If you've ever seen raw imagery off of a sensor [wp.com], even your cell phone, the picture looks pretty crappy. There's random noise, fixed pattern noise, nonuniform response such as getting darker at the edges, all sorts of stuff, and there are a number of things you can do to fix it up. You can subtract off a lot of the fixed noise, and you can average out the random noise. You can correct for nonuniformities, such as if the lens you're using gets darker at the edges, you can multiply those pixels up a bit to get them level with the rest of the sensor. Your can think of all of this stuff, loosely, as relative radiometry. Your cell phone camera does all of this before you see the final result. Absolute radiometry is much harder. Here you're interesting in not making a pretty picture, but you actually want to count the number of photons that is hitting your sensor to determine the amount of Watts on it. This is important if you are making absolute measurements of something, like determining the temperature of something with an IR camera. This is a much harder problem.
To do absolute radiometry you need to be able to periodically calibrate your sensor, and to do that, you want to point it at some sort of calibration source, which is something that you know very well how much power is radiated from it at all the wavelengths you care about. At NIST they have a special lightbulb where they measure the snot out of it and it is a radiometric standard. Then you can buy the same kind of light bulb that has been compared to the master bulb allowing you to calibrate your bulb to the master bulb. This is what they mean by NIST-tracable calibration. Now you know that if you put a certain amount of current through it, you can calculate how many photons at all wavelengths are coming off of it, and you can use that to calibrate your sensor. Unfortunately, it isn't practical to put these bulbs in space to calibrate your sensor, so you need to find another way. What they typically do is periodically point at a ground calibration site, like a desert playa where the reflectance of the ground is measured very well as well as the amount of sunlight hitting it, and then you run some models to calculate how much reflected light makes it to the top of the atmosphere and to your instrument. This is not a very precise calculation because you have to account for all of that messy atmosphere in the way. However, there is a pretty good mathematical model for how much light is radiated off of the moon, called the ROLO model, and it takes into account the reflectance of the lunar surface, the phase of the moon, the distance, etc., but the model is only as good as the measurements you make to validate it. That's what this instrument is all about. They have this very very well calibrated sensor to measure the light from the moon from the top of the atmosphere so that they can improve the ROLO model, and then if you have a satellite you need to calibrate, instead of pointing it at the ground and getting 10 to 20 percent radiometric error, you can point it at the Moon and compare what you measure to what the ROLO model tells you and (in principle) do much better than that..