Design and Analysis of a Next-Generation Wide Field-of-View Earth Radiation Budget Radiometer

Abstract

Climate on Earth is determined by the Earth Radiation Budget (ERB), which quantifies the incoming and outgoing radiative energy fluxes. The ERB can be monitored by non-scanning wide field-of-view radiometers, or by scanning narrow field-of-view radiometers. We propose an enhanced design for the wide field-of-view radiometer, with as key features the use of a near-spherical cavity to obtain a uniform angular sensitivity and the integration of the shuttered electrical substitution principle, eliminating long term drifts of the radiometer and improving its time response. The target absolute accuracy is 1 W/m 2 and the target stability is 0.1 W/m 2 per decade for the measurement of the total outgoing Earth’s radiation. In order to increase the spatial resolution and to separate the total outgoing radiation into reflected Solar and emitted thermal radiation, we propose the joint use of the radiometer with wide field-of-view Shortwave (400–900 nm) and Longwave (8–14 μm) cameras. This paper presents the concept and design of the novel wide field-of-view radiometer, including simulations and analyses of its expected performance. We focus on mechanical design and the measurement characteristics based on optical and thermal analyses. In combination with the cameras, we obtain an estimated accuracy of 0.44 W/m 2 .

Optical System Design of a Wide Field-of-View Camera for the Characterization of Earth’s Reflected Solar Radiation

“…From this figure, we conclude that the broadband albedo error is lower than 3%. This is better than our target, which was a maximal error of 5% [10].…”

“…A broader view can be obtained when considering the polychromatic diffraction MTF, which is better to assess the image quality. The performed radiative transfer simulations indicate that, when using the SW camera on a stand-alone basis, the broadband albedo can be estimated with a random error lower than 3% across all simulated scene types and all solar-zenith angles, which is better than the design target of 5% that was put forward in [10]. The next step will be the realization of a prototype of this SW camera.…”

“…(1) The Earth should be seen from limb to limb, from a nominal altitude of 700 km. (2) The camera should enable scene identification, (3) while measuring SW radiation, allowing to reconstruct the RSR on a stand-alone basis with an accuracy of minimum 5% [10]. (4) The camera should have a resolution at nadir of maximum 5 km, in order to allow discriminating between cloudy and clear-sky scenes.…”

“…A new concept to measure the radiative fluxes at the TOA is proposed in [10], using a combination of a WFOV radiometer with WFOV camera systems. Instead of using a single instrument combining high accuracy and high spatial resolution, separate instruments are used in order to focus on both objectives separately.…”

“…The radiometer is supplemented with high-resolution shortwave (SW, [400-1100] nm), and longwave (LW, [8][9][10][11][12][13][14] µm) WFOV cameras. These cameras not only provide for a better radiometer accuracy, but also enable scene identification, while increasing the spatial resolution and enabling the spectral separation between Reflected Solar Radiation (RSR) and Outgoing Longwave Radiation (OLR).…”