EarthCARE will measure the 3D structure of cloud and aerosols, together with collocated observations of solar (shortwave) and terrestrial (thermal) radiation. For this purpose, the satellite carries four scientific instruments, two active instruments – a lidar and a radar – to measure the vertical distribution of clouds and aerosols, one passive instrument – an imager – to collect cloud and aerosol information across the satellite ground track and a broadband radiometer for measuring outgoing reflected solar and emitted thermal at the top of the atmosphere.
The lidar and radar are referred to as active instruments, as they actively emit light targeted at the atmosphere. The targets in the atmosphere scatter this light back to the satellite where it is received and analysed. From the received backscattered signals, atmospheric constituents are estimated.
The lidar emits a linearly polarised pulse at a wavelength of 355 nm. Rayleigh scattering of atmospheric molecules causes the backscattered signal to be broadened in wavelength compared to the (cloud and aerosol) particle backscatter. The use of a high spectral resolution filter in the lidar receiver distinguishes molecular and particle backscatter. Furthermore, the polarisation change of the backscattered signal is measured, which provides information of the type of scattering cloud or aerosol particle. The lidar is sensitive to aerosol and thin clouds, but attenuated in thicker clouds.
The radar emits microwave pulses at 94 GHz. This frequency is sensitive to clouds and precipitation and penetrates through clouds where the lidar signal is attenuated. The combination of lidar and radar allows for the observation of the full cloud profile. Furthermore, the radar utilises the Doppler Effect providing information on convective motions as well as ice and rain fall speed, leading to improved drizzle, rainfall, and snowfall rates.
While the lidar and radar collect vertical profiles below the satellite track, additional information of the cloud and aerosol scene is required in the across-track direction. This is provided by an imager, with a swath width of 150 km. The primary objective of the imager is to provide cloud scene context. Through synergistic processing of lidar, radar and imager observation, however, 3D cloud and aerosol scenes will be constructed over a swath width of about 30 km. Using radiative transfer modelling, these scenes will be used to calculate atmospheric heating rates as well as the reflected solar and emitted thermal radiation corresponding to these scenes.
These calculations will be compared to the measurements of the fourth EarthCARE instrument, the broadband radiometer. This instrument has three fixed viewing directions: nadir, forward and backward. In each direction, the reflected solar and emitted thermal radiation is measured. Matching up the calculated and observed solar and thermal radiation allows the impact of the observed clouds and aerosols on solar and thermal radiation to be linked and verified.