Landsat looks to the Moon (w/ Video)

July 12, 2014 by Kate Ramsayer

Every full moon, Landsat 8 turns its back on Earth. As the satellite's orbit takes it to the nighttime side of the planet, Landsat 8 pivots to point at the moon. It scans the distant lunar surface multiple times, then flips back around to continue its task of collecting land-cover information of the sunny side of Earth below.

These monthly lunar scans are key to ensuring the land-imaging instrument aboard Landsat 8 is detecting light consistently. For this, engineers need a consistent source of light to measure. And while there are some spots on Earth – like the Sahara Desert or other arid sites – that reflect a relatively stable amount of light, nothing on our planet beats the moon, which lacks an atmosphere and has an unchanging surface, barring the odd meteorite.

We really wanted something we could trust for Landsat 8," said Brian Markham, leader of the calibration team for Landsat 8, which was built and launched by NASA and is now operated by the U.S. Geological Survey. "We do have Earth sites we look at for calibration. But the precision with which you can track things by using the Earth, because of the atmosphere, is not as good as the moon."

Landsat 8's Operational Land Imager, or OLI, collects information on the visible, near infrared and shortwave-infrared light reflecting off Earth's surface. Each wavelength of light provides information about the ground surface below. OLI has 14 detector modules, each of which contains hundreds of individual that record different spectral bands. The calibration team at Goddard and the U.S. Geological Survey's EROS facility in South Dakota is tasked with making sure each of those detectors register light consistently over time.

Aboard the spacecraft, lamps provide light to calibrate OLl's detectors, but the lamps aren't perfect. On the Landsat 7 satellite, the lamps started to fade before the detectors did. Another option, solar diffusers, which use indirect sunlight, can darken as well.

This video is not supported by your browser at this time.
Every full moon, Landsat 8 turns its back on Earth. As the satellite's orbit takes it to the nighttime side of the planet, Landsat 8 pivots to point at the moon. It scans the distant lunar surface multiple times, then flips back around to continue its task of collecting information on Earth. Credit: NASA's Goddard Space Flight Center

"Everything else we've tried to use to monitor the stability of our instruments has often not been as good as the instruments themselves," Markham said. But the moon is a steady, not-too-bright light in the sky. "As long as we know what its illumination conditions are, we can trend our instrument performance to it because we trust its stability."

So Landsat 8 planners designed this latest satellite to image the moon as a baseline calibration. If, during these lunar tests, the OLI detectors indicate that the moon is getting slightly duller or brighter, then the Goddard scientists will know the instrument –not the moon – is off. With that data, they can adjust the algorithms that calculate land cover information during Landsat's regular Earth-observation orbits.

It's a fairly complicated operation to scan the moon each month, said Susan Good, a engineer at Goddard who works with Landsat 8.

"There are 14 detector modules," Good said, "each of these has to scan the same path along the moon, so that you collect exactly the same data on each sensor."

The flight dynamics software determines precisely where the spacecraft will need to point during a lunar calibration. The timing is set for just after the moon is completely full. Then, as Landsat 8 passes over Antarctica and heads north in Earth's shadow, the spacecraft maneuvers to the precise location to start the first scan across the .

It executes tiny and precise scans to take seven or eight passes across the moon – each one angled so that a different detector is centered on the moon. This takes about 18 minutes, by which time the spacecraft has almost reached the Arctic. So it maneuvers back to point at Earth, and complete its day-lit imaging. Then, Landsat 8 pivots to face the moon again, completing additional passes to test the remaining detectors. After two orbits, the lunar calibration is complete.

In Landsat 8's first year, the lunar calibration tests show that the detectors are stable, Markham said, within a fraction of a percent. If the lunar calibrations and other tests show the detectors are off, the scientists can adjust the calculations that turn the raw Landsat data into information on land cover brightness, maintaining their accuracy.

Since the regular checks on Landsat 8's performance, Good jokes that she will never look at the full moon the same. "I think oh, we're having a lunar ," she said. "I always know what Landsat' 8's doing when the is full."

Explore further: Critical milestone reached for 2012 Landsat Mission

Related Stories

Critical milestone reached for 2012 Landsat Mission

August 9, 2011

The Operational Land Imager (OLI), built by Ball Aerospace & Technologies Corp., Boulder, Colo., has been approved by NASA Goddard Space Flight Center for shipment to Orbital Sciences Corporation, Gilbert, Ariz. for integration ...

New NASA satellite takes the Salton Sea's temperature

April 22, 2013

(Phys.org) —An image from an instrument aboard NASA's Landsat Data Continuity Mission or LDCM satellite may look like a typical black-and-white image of a dramatic landscape, but it tells a story of temperature. The dark ...

Landsat 8 satellite begins watch

May 30, 2013

NASA transferred operational control Thursday of the Landsat 8 satellite to the U.S. Geological Survey (USGS) in a ceremony in Sioux Falls, S.D.

Goddard helps set two Guinness World Records

June 21, 2013

Setting two world records in two consecutive months, NASA's Goddard Space Flight Center in Greenbelt, Md., helped share some of NASA's amazing accomplishments. The awards highlight the tremendous amount of work by many of ...

Image: Landsat 8's first year

February 12, 2014

On Feb. 11, 2013, the Landsat 8 satellite rocketed into a sunny California morning onboard a powerful Atlas V and began its life in orbit. In the year since launch, scientists have been working to understand the information ...

Recommended for you

Born-again planetary nebula

July 28, 2015

Beneath the vivid hues of this eye-shaped cloud, named Abell 78, a tale of stellar life and death is unfolding. At the centre of the nebula, a dying star – not unlike our Sun – which shed its outer layers on its way to ...

'Bathtub rings' suggest Titan's dynamic seas

July 28, 2015

Saturn's moon, Titan, is the only object in the Solar System other than Earth known to have liquid on its surface. While most of the lakes are found around the poles, the dry regions near the equator contain signs of evaporated ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.