FUTURE EARTH RADIATION BUDGET
Why It Matters
Six CERES instruments currently fly on four satellites: Terra, Aqua, S-NPP, and NOAA-20, all well beyond their original 5-year design lifetimes. In 2026, CERES instruments on Terra and Aqua will end science data collection, and S-NPP will likely end operations before Libera, the planned CERES follow-on, launches on JPSS-4 in 2028.
Without overlapping missions, a gap could occur in the Earth Radiation Budget Climate Data Record (ERB-CDR), the same continuous record CERES has produced since 2000. Stitching together data across a gap is extremely difficult and compromises the integrity of the record, reducing the ability to track Earth’s Energy Imbalance, which has already doubled since 2000.
DEMETER offers a low cost, small satellite path to help close that gap. It is designed to overlap with Libera and reliably carry the ERB record forward for decades to come.
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: shortwave (0.3–5.0 µm), longwave (5–>50 µm), and total (0.2–>50 µm).
DEMETER integrates with 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 a scanning mirror, 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 coverage. DEMETER’s broadband instantaneous field of view is about 20 times smaller in area than CERES/Libera (5.5 km vs. 25 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
CERES and Libera class instruments have not evolved in the last 30 years. DEMETER moves the observational paradigm forward with a smaller, more affordable, small satellite approach.
Instrument Design
DEMETER’s flight configuration uses four identical Sensor Assemblies to achieve full limb-to-limb (±60°) coverage. The current technology maturation effort is repackaging the Sensor Assembly to reduce its external mechanical dimensions while preserving the internal optical prescription to reduce mass, improve tiling efficiency, and simplify interface requirements with the host satellite’s scene select carousel.
The prototype build tests a single Sensor Assembly with a reduced ±16° field of view in the swath direction, preserving ±2.3° in the ground-track direction, as a technology demonstration ahead of the full four-assembly flight design.
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 and not a multi-element lens system 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 confirmed 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
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) at Space Dynamics Laboratory. 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.
