EARTH RADIATION BUDGET
Building upon four decades of Earth radiation budget experience
Why It Matters
Climate is determined by the Earth’s radiation budget (ERB), which describes the amount and distribution of absorbed solar and outgoing longwave radiation within the Earth-atmosphere system. The Clouds and the Earth’s Radiant Energy System (CERES) is the only dedicated global ERB record available that spans over 26 years. Currently, five CERES instruments on four satellites (Terra, Aqua, S-NPP, and NOAA-20) are tracking changes in ERB. All are well beyond their original 5-year design lifetimes. By the end of 2027, CERES instruments on Terra, Aqua and S-NPP will end science data collection, leaving only CERES Flight Model 6/NOAA-20 until Libera/JPSS-4 launches in September 2027. There currently are no plans to launch another ERB instrument after Libera to extend this critical climate data record (CDR).
Without overlapping missions, a gap could occur in the ERB CDR. Stitching together data across a gap is extremely difficult and compromises the integrity of the record, reducing the ability and fidelity to track trends in ERB.
DEMETER is designed to extend the ERB record begun by CERES, continuing the same climate-critical measurements on a smaller, lower-cost satellite platform made possible by technical innovations in the instrument design.
Instrument Description
DEMETER is a non-scanning, wide-field-of-view broadband radiometer designed to measure top-of-atmosphere (TOA) radiation. It uses four identical wide-field-of-view tiled telescopes, each coupled to a 2-D detector array, to provide limb-to-limb (±60°) coverage of the ground as the satellite orbits Earth.
The instrument measures three spectral bands as CERES instruments: shortwave (0.3–5.0 µm), longwave (5–>50 µm), and total (0.2–>50 µm).
While DEMETER is flexible enough to fly on any suitable host satellite, the DEMETER team is currently exploring the NovaWurks SLEGO architecture, a modular satellite bus structure that provides pointing control, thermal and power management, radiation tolerance, propulsion, and onboard processing, forming a complete autonomous small satellite.
Measurement Approach
Like CERES and Libera, DEMETER samples Earth’s radiation in the cross-track direction, but instead of scanning the radiometer, it uses a “push-broom” approach: successive readouts of the detector array represent consecutive limb-to-limb swaths, providing the spatial and angular coverage needed to observe TOA radiation fields during nominal operations.
This eliminates the complex scanning mechanisms required by CERES and Libera, which use narrow field-of-view Cassegrain telescopes to build up spatial coverage. DEMETER’s broadband instantaneous field of view is about 12 times smaller in area than CERES/Libera (7 km vs. 24 km ground footprint diameter at 825 km altitude), improving sampling of cloud-free regions for aerosol-radiation and cloud feedback research.
CERES vs. DEMETER Non-Scanner measurement geometry
Comparative Technology
DEMETER is the next generation of ERB observations, providing an innovation, cost-effective small-satellite approach to seamlessly extend the global ERB CDR begun by CERES in 2000.
Instrument Design
The DEMETER instrument core consists of a Broadband Optical Module (BOM) and a Broadband Calibration Module (BCM).
The BOM comprises four identical tiled Sensor Assemblies (SA), each consisting of a wide FOV optical telescope based on a single freeform mirror, integrated with a Focal Plane Module (FPM). The FPM contains a 3×65 thermopile detector array, with the three rows representing the broadband channels to measure the Earth-reflected and Earth-emitted radiances, along with readout electronics. The three channels are identical to the CERES broadband channels—broadband shortwave (SW) (0.3 – 5 μm), broadband longwave (LW) (5 – >50 μm) and the broadband total (TOT) (0.2- >50 μm). Optical filters are installed on a diamond substrate to filter the radiation into these bands. The 65 pixels in each detector row provide spatial sampling of the the radiation fields in the cross-track direction. Each SA has a 32° FOV. Four tiled SAs provide simultaneous limb-to-limb observations of Earth from LEO, enabling daily global coverage.
Sensor Assembly CAD model
Optical ray trace of DEMETER’s wide-field lens system
Optical Design
Each Sensor Assembly uses a single freeform mirror telescope paired with a metering structure and an optical filter assembly. The design achieves a full field of view of 4.57° × 32° at f/1.7, with a 38 mm focal length and 22.1 mm pupil diameter.
Ground testing confirms strong optical performance: measured 90% Encircled Point Spread Function (EPSF) solid angles of 0.173–0.222 msr, well within the 0.278 msr requirement, and distortion matching nominal pixel locations within 0.1°.
Detector Array
The Focal Plane Module contains a two-dimensional, uncooled thermopile detector array with 3 × 65 pixels (spectral × spatial), 180 µm pixel size, with a gold black absorber sensitive to wavelengths from 0.2 to >50 µm. It is derived from flight-qualified hardware developed under JPL’s PREFIRE project.
Current development work is focused on improving pixel yield, responsivity, and noise performance by lengthening support beam length, screening readout integrated circuits (ROICs) before assembly, and adding a low-capacitance interlayer dielectric to the detector chip.
Full detector array assembly CAD model
The Broadband Calibration Module (BCM) comprises the Shortwave Onboard Calibration Source (SOCS) and the Longwave Calibration Source (LOCS). The SOCS is an integrating sphere illuminated by multiple LEDs, a broadband Quartz Tungsten Halogen lamp, and contains a photodiode reference detector. The LEDs and lamp were selected to cover the reflected solar radiance of Earth observations at controllable illumination levels.
The Longwave Calibration Source (LOCS) consists of a thermally controlled, variable-temperature blackbody that presents radiometrically traceable, representative of Earth outgoing longwave radiances over a sufficient dynamic range to the aperture of the SA.
The BCM enables periodic on-orbit calibration of each detector in the FPM, allowing the instrument to account for and correct any detector drifts over time. DEMETER’s BCM improves upon the CERES heritage design and provides enhanced on-orbit calibration capabilities to ensure long-term radiometric stability and compliance with the measurement requirements recommended in the ERB instrument Earth Venture Continuity Report.
DEMETER instrument during vacuum chamber testing
Testing & Calibration
DEMETER’s Sensor Assembly has undergone thermal vacuum (TVAC) testing to verify point spread function, field of view, distortion, linearity, polarization sensitivity, and noise performance with results meeting or exceeding requirements across most parameters.
The current effort will conduct a system-level TVAC test integrating the Sensor Assembly with three onboard calibration systems (shortwave, longwave, and solar). A rotary stage will mimic the satellite’s scene-select carousel, rotating the sensor to sequentially view each calibration target plus a cold-space reference, tracking noise, linearity, spectral response, and conversion gain over time.
