Radiometric Uncertainty Analysis of the CHISI Instrument for a Constellation of LEO Infrared Sounding Satellites

The Compact Hyperspectral Infrared Sounding Interferometer (CHISI) has emerged as a promising instrument for acquiring moisture and temperature profiles vital to Numerical Weather Prediction (NWP) forecasting. Operating as a pushbroom, field-widened interferometer with a 20 mm optical aperture, CHISI offers a cost-effective alternative to conventional infrared sounders. CHISI's adaptability extends to deployment within a constellation of small satellites, enabling a global revisit time of under one hour.

A pivotal aspect of CHISI's performance lies in achieving the required radiometric uncertainty (RU). The instrument calibration requirement is a RU of less than 0.15% (k=1) of the radiance from a 287 K blackbody in the LWIR (660 to 965 cm^-1) and MWIR (1210 to 1610 cm^-1) bands. This uncertainty requirement is set below the CHISI Noise Equivalent Spectral Radiance (NESR) level to enable interchangeable field of view (FOV) responses within a constellation of satellite sensors.

This publication describes the analysis used to estimate RU, as well as the results of a proof-of-concept hardware prototype used to validate parameters and assumptions in the analysis. Finally, the next steps to validate the remaining parameters are explored.

To assess the impact of factors affecting RU, a RU tree was developed. The tree, shown diagrammatically in the attached figure, uses a weighted root-sum-square to combine the uncertainty of radiance coming from hot and cold calibration sources and the uncertainty in the measured radiance due to interferometer gain and offset. These k=1 uncertainty values are compared to radiance from an ideal blackbody reference, and the specified goal is met when the uncertainty in the measured radiance is less than 0.15% of the blackbody reference radiance across the spectral band.

Instrument calibration uses views of hot and cold internal calibration targets (ICTs). Uncertainties in the interferometer gain and offset come from instrument drift between views of the calibration targets, and frequent target views keep these terms small. Uncertainty in the radiance from these targets make up the sub-branches in the RU tree. Relevant factors and values used in the analysis are listed in the table below.

Initially, the analysis used reasonable values for each of the table parameters to assess the feasibility of the RU specification. Values related to the hot and cold internal calibration targets were based on a custom design under development. To validate the analysis, a proof-of-concept ICT was constructed and evaluated. The ICT design features a moving paddle system, where hot or cold targets flip into the instrument view at regular intervals. The targets themselves are flat aluminum pucks with a high emissivity carbon nanotube coating called Vantablack, from Surrey NanoSystems. NIST evaluated the coated pucks for total hemispherical reflectance and BRDF, and these measured values are used in the current edition of the RU tree. The NIST-measured nanotube emissivity was lower than expected, with the spectral emissivity in the 660 cm^-1 – 1610 cm^-1 band dipping as low as 0.990, compared to the datasheet value of 0.995. However, the effect on RU of this lower-than-expected material emissivity was minimal. Previously, analysis had shown that the RU was highly sensitive to uncertainty in the target emissivity knowledge, and less sensitive to the specific value of the puck emissivity. The effective emissivity of the ICT is enhanced by a cavity baffle and by algorithms that correct for stray light reflection effects. Therefore, using these enhanced techniques, an ICT effective emissivity near 0.999 can be achieved with a carbon nanotube flat disk emissivity as low as 0.990. More critical to expected performance is the uncertainty of the reflectance measurement, which was sufficiently small in the NIST data to achieve the desired 0.15% RU.

The fully-integrated ICT prototype was tested under vacuum to assess ICT thermal gradients. The lateral temperature gradient across the puck surface, as assessed by a thermal camera, was much higher than predicted. Subsequent exploration revealed that the puck resistive heater arrangement needed further optimization to reduce these gradients. A future iteration will incorporate a custom heater that minimizes these effects.

Finally, a raytrace analysis of the puck’s view factors showed that the puck would have less than a 4% view of the warm instrument, with a cold baffle filling the remaining view. Analysis confirms that this small view of the warm environment has negligible impact on the RU.

This study describes the analytical basis for the estimated RU of CHISI. A proof-of-concept hardware prototype ICT successfully validated numerous parameters and assumptions incorporated into the analysis. Furthermore, the investigation delved into the remaining parameters, paving the way for a more complete validation process.