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Proceedings Paper

The EarthCARE mission BBR instrument: ground testing of radiometric performance
Author(s): Martin E. Caldwell; David Spilling; William Grainger; E. Theocharous; Martin Whalley; Nigel Wright; Anthony K. Ward; Edward Jones; Joseph Hampton; David Parker; John Delderfield; Alan Pearce; Tony Richards; Grant Munro; Oliver Poynz Wright; Matthew Hampson; David Forster
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Paper Abstract

In the EarthCARE mission the BBR (Broad Band Radiometer) has the role of measuring the net earth radiance (i.e. total reflected-solar and thermally-emitted radiances), from the same earth scene as viewed by the other instruments (aerosol lidar, cloud radar and spectral imager). It does this measurement at 10km scene size and in 3 view angles. It is an imaging radiometer in that it uses micro-bolometer linear-array detector (pushbroom orientation), to over-sample these required scenes, with the samples being binned on-ground to produce the 10km radiance data. For the measurements of total earth radiance, the BBR is based on the heritage of Earth Radiation Budget (ERB) instruments. The ground calibration methods of this type of sensor is technically very similar to other EO instruments that measure in the thermalIR, but with added challenges: (1) The thermal-IR measurement has to have a much wider spectral range than normal thermal-IR channels to cover the whole earth-emission spectrum i.e. ~4 to >50microns; (2) The 2nd channel (reflected solar radiance) must also have a broad response to cover almost the whole solar spectrum, i.e. ~0.3 to 4microns. And this solar channel must be measured on the same radiometric calibration as the thermal channel, which in practice is best done by using the same radiometer for both channels. The radiometer is designed to be very broad-band i.e. 0.3 to 50microns (i.e. more than two decades), to cover both ranges, and a switchable spectral filter (short-pass cutoff at 4μm) is used to separate the channels. The on-ground measurements which are required to link the calibration of these channels will be described. A calibration of absolute responsivity in each of the two bands is needed; in the thermal-IR channel this is by the normal method of using a calibrated blackbody test source, and in the solar channel it is by means of a narrow-band (laser) and a reference radiometer (from NPL). A calibration of relative spectral response is also needed, across this wide range, for the purpose of linking the two channels, and for converting the narrow-band solar channel measurement to broad-band.

Paper Details

Date Published: 5 September 2017
PDF: 14 pages
Proc. SPIE 10402, Earth Observing Systems XXII, 1040204 (5 September 2017); doi: 10.1117/12.2273148
Show Author Affiliations
Martin E. Caldwell, STFC Rutherford Appleton Lab. (United Kingdom)
David Spilling, Thales Alenia Space UK Ltd. (United Kingdom)
William Grainger, STFC Rutherford Appleton Lab. (United Kingdom)
E. Theocharous, National Physical Lab. (United Kingdom)
Martin Whalley, STFC Rutherford Appleton Lab. (United Kingdom)
Nigel Wright, Thales Alenia Space UK Ltd. (United Kingdom)
Anthony K. Ward, STFC Rutherford Appleton Lab. (United Kingdom)
Edward Jones, STFC Rutherford Appleton Lab. (United Kingdom)
Joseph Hampton, STFC Rutherford Appleton Lab. (United Kingdom)
David Parker, STFC Rutherford Appleton Lab. (United Kingdom)
John Delderfield, STFC Rutherford Appleton Lab. (United Kingdom)
Alan Pearce, STFC Rutherford Appleton Lab. (United Kingdom)
Tony Richards, STFC Rutherford Appleton Lab. (United Kingdom)
Grant Munro, ESR Technology Ltd. (United Kingdom)
Oliver Poynz Wright, ESR Technology Ltd. (United Kingdom)
Matthew Hampson, ESR Technology Ltd. (United Kingdom)
David Forster, ESR Technology Ltd. (United Kingdom)

Published in SPIE Proceedings Vol. 10402:
Earth Observing Systems XXII
James J. Butler; Xiaoxiong (Jack) Xiong; Xingfa Gu, Editor(s)

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