The BBR (Broad-Band Radiometer) on board EarthCARE provides an estimate of the outgoing solar reflected and Earth emitted thermal fluxes for a 10 km scene.

Three identical, fixed mirror, dedicated telescopes image each of the fore, nadir and aft views onto linear array detectors. The pixel responses are given a near triangular shape by introducing a deliberate defocus of each telescope. This is in order to give uniform system-point spread function (PSF), achieved by summing of pixels, and to minimise the effects of diffraction. Less defocus is applied to the oblique view telescopes, in order to avoid loss of spectral resolution at the increased ranges. The detectors used are linear, 30 pixel, micro bolometer arrays from the 'Canadian Institut National d'Optique (INO), to which a special gold-black coating has been applied to the 0.1 x 0.1 mm pixels in order to increase their sensitivity to Thermal Infrared (TIR). BBR's radiator is located approximately in the middle of the Optics Unit. The telescopes are mounted onto a baseplate that is parallel to that radiator.

A chopper drum with four apertures rotates continuously around the telescopes, chopping the signal onto the detectors through SW/drum skin/TW/drum skin, etc.

The nominal duration of each measurement view, 19 ms, is not long enough for the detector to reach a stable voltage and the digitised waveform of the measured view is processed to predict the asymptote of the exponential rise in the detected signal.

Around the Chopper Drum sits the Calibration Target Drum, which houses hot and cold blackbodies, a diffusor, three fold mirrors to view the diffusor from each telescope, and the telescope baffles.

The BBR will perform sampling such that the views from each of the three observation angles are spatially coincident, slightly separated in time. This requires the instrument to accommodate the residual shifts in scene position of the three fields of view that arise from Earth's rotation between measurements after the implementation of yaw steering. For each telescope view a 10 x 10 km PFS is synthesised by summation of individual pixel PSF responses along and across track. Pixels are selected to form scenes at 1 km intervals along track. To reduce the effect of overlapping radiances (from pixels outside of the nominal footprint) the PSF of edge pixels are weighted accordingly.

The match of the scenes, for the two channels and the three view directions, is controlled by the Integrated Energy requirements.