Abstract
PLEIADES is a dual Earth observation system composed of two satellites, PLEIADES-1A and PLEIADES-1B, respectively launched at the end of 2011 and 2012. This imagery system, led by CNES, has four spectral bands, blue, green, red and near infrared, with a spatial resolution of 2.8 m and a panchromatic band with a resolution of 0.7 m in vertical viewing. Its swath is about 20 km. In the framework of the PLEIADES radiometric calibration, studies took place in order to determine the calibration precision that could be reached from the acquisitions realized on the Moon. Indeed, the precisions reached from observations of calibration sites on Earth (African deserts, Antarctica, clouds, instrumented sites) are about 2-3% for most of the spectral bands in the visible and the near infrared spectra. It is very difficult to further improve this precision down to 1% because each method has its own limitations, generally due to atmospheric disturbances. In this context, the Moon seems to be an ideal calibration site: there is no atmosphere and its surface properties – thus its optical properties - are perfectly stable. Taking advantage of the high level of agility of PLEIADES, we performed an intensive observation campaign of the Moon in addition to the nominal acquisitions – when the Moon phase angle is about 40°. This intensive observation of the Moon, named POLO for Pleiades Orbital Lunar Observations, consists of a thousand acquisitions covering the phase angle range ±115 deg. The Moon was acquired as frequently as once every orbit, which represents acquisitions every 100 minutes. This paper provides an overview of these lunar experiments and an assessment of the variation of the irradiance of the Moon with phase angle. This paper also discusses a way to improve the phase angle dependence of existing lunar models.
