New direction for sensor calibration

Scientists from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the US scientists are using the moon to check and calibrate sensors on board weather satellites.

The ‘moon tuned’ sensors are expected to provide a wealth of improved information about climate change and air pollution.

‘The moon is the perfect object to point the satellite sensors at in order to check them,’ said Dr Ian Grant from CSIRO Atmospheric Research. ‘The moon’s surface is bare and unchanging and there is no air between it and the satellite.’

Formally, scientists have calibrated satellite sensors by checking their views of uniform targets on Earth, such as deserts or clouds. However, the atmosphere gets in the way and vegetation and rainfall can alter the appearance of deserts.

‘We are doing lunar calibrations with the imager on the Japanese Geostationary Meteorological Satellite. This is the imager that gives us the pictures of clouds over Australia that we see on TV weather reports,’ said Dr Grant.

Although the moon’s surface is unchanging, the brightness of any point on the moon changes with the direction of sunlight and the direction from which the satellite looks.

The US Geological Survey is said to accurately map the moon’s brightness from its lunar observatory in Arizona. The satellite imager is calibrated at times when its view of the moon matches the view angle of instruments at the observatory.

‘In future, we’ll be able to check the imager from a range of viewing angles,’ added Dr Grant.

Additionally, the newly calibrated satellite sensor will be able to more accurately monitor smoke plumes from large fires that are prevalent in the tropics.

Climate scientists are especially interested in the way in which such smoke can affect global warming.

There are currently eight geostationary meteorological satellites in orbit, and the lunar technique could be used in future to calibrate all of them.