Accurate, long-term solar spectral irradiance (SSI) measurements are vital for interpreting how solar variability impacts Earth’s climate and for validating climate model sensitivities to spectrally varying solar forcing. The Compact Spectral Irradiance Monitor (CSIM) 6U CubeSat successfully launched on Dec. 3rd, 2018 as part of the SpaceX SSO-A: SmallSat Express Mission ultimately achieving a sun-synchronous 575 km orbit. CSIM brings new, emerging technology advancements to maturation by demonstrating the unique capabilities of a complete SSI mission with inherent low mass and compact design. The instrument is a compact, two-channel prism spectral radiometer incorporating Si, InGaAs, and extended InGaAs focal plane photodiodes to record the solar spectrum daily across a continuous wavelength region spanning 200 – 2800 nm (>97% of the total solar irradiance). A new, novel electrical substitution radiometer (ESR) using vertically aligned carbon-nanotube (VACNT) bolometers serves as an absolute detector for periodic on-orbit spectral calibration corrections. Pre-launch component level performance characterizations and final instrument end-to-end absolute calibration achieved low combined standard uncertainty (uc<0.5%) in irradiance. These calibrations were performed in the LASP Spectral Radiometer Facility (SRF), a comprehensive spectral irradiance calibration facility utilizing a tunable laser system tied to an SI-traceable cryogenic radiometer. On-orbit, optical degradation corrections to better than 0.05% / year uncertainty are achieved by comparing periodic, simultaneous solar measurements of the two CSIM channels operating with significantly different solar exposure duty cycles. Operational overlap of CSIM with existing SSI measurements validate concepts for maintaining critical long-term solar data records.
The long-term balance between Earth’s absorption of solar energy and emission of radiation to space is a fundamental climate measurement. Total solar irradiance (TSI) has been measured from space, uninterrupted, for the past 40 years via a series of instruments. The Compact Total Irradiance Monitor (CTIM) is a CubeSat instrument that will demonstrate next-generation technology for monitoring total solar irradiance. It includes novel silicon-substrate room temperature vertically aligned carbon nanotube (VACNT) bolometers. The CTIM, an eight-channel 6U CubeSat instrument, is being built for a target launch date in late 2020. The basic design is similar to the SORCE, TCTE and TSIS Total Irradiance Monitors (TIM). Like TSIS TIM, it will measure the total irradiance of the Sun with an uncertainty of 0.0097% and a stability of <0.001%/year. The underlying technology, including the silicon substrate VACNT bolometers, has been demonstrated at the prototype-level. During 2019 we will build and test an engineering model of the detector subsystem. Following the testing of the engineering detector subsystem, we will build a flight detector unit and integrate it with a 6U CubeSat bus during late 2019 and 2020, in preparation for an on-orbit demonstration in 2021.
Currently at NIST, there is an effort to develop a black array of broadband absolute radiometers (BABAR) for far infrared sensing. The linear array of radiometer elements is based on uncooled vanadium oxide (VOx) microbolometer pixel technology but with the addition of two elements: vertically aligned carbon nanotubes (VACNTs) and an electrical substitution heater. Traditional microbolometer pixels use a thermistor film as an absorber, which is placed a quarter wavelength above a reflector, typically limiting absorption to a narrow band from 8 μm to 15 μm. To extend the sensing range of the imaging array into the far infrared (20 μm to 100 μm), we are replacing the cavity with a single absorber of VACNTs. In addition, each pixel has an electrical substitution heater which can be used to determine equivalent incident optical power when the device is non-illuminated. This device forms the basis of an absolute radiometer eliminating the need for an external reference (e.g. blackbody source).