Measuring Earth's energy budget from space is an essential ingredient for understanding and predicting Earth's climate. We have demonstrated the use of vertically aligned carbon nanotubes (VACNTs) as photon absorbers in broadband radiometers own on the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) 3U CubeSat. VACNT forests are some of the blackest materials known and have an extremely at spectral response over a wide wavelength range. The radiation measurements are made at both shortwave, solar-reflected wavelengths and in the thermal infrared. RAVAN also includes two gallium phase-change cells that are used to monitor the stability of RAVAN's radiometer sensors. RAVAN was launched November 11, 2016, into a nearly 600-km sun-synchronous orbit and collected data over the course of 20 months, successfully demonstrating its two key technologies. A 3-axis controlled CubeSat bus allows for routine solar and deep-space attitude maneuvers, which are essential for calibrating Earth irradiance measurements. Funded by the NASA Earth Science Technology Office, RAVAN is a pathfinder to demonstrate technologies for the measurement of Earth's radiation budget that have the potential to lower costs and enable new measurement concepts. In this paper we report specifically on the VACNT growth, post-growth modification, and pre-launch testing. We also describe the novel door mechanism that houses the gallium black bodies.
Two thematic drivers are motivating the science community towards constellations of small satellites, the revelation that many next generation system science questions are uniquely addressed with sufficient numbers of simultaneous space based measurements, and the realization that space is historically expensive, and in an environment of constrained costs, we must innovate to ―do more with less‖. We present analysis that answers many of the key questions surrounding constellations of scientific satellites, including research that resulted from the GEOScan community based effort originally intended as hosted payloads on Iridium NEXT. We present analysis that answers the question how many satellites does global system science require? Perhaps serendipitously, the analyses show that many of the key science questions independently converge towards similar results, i.e. that approximately 60+ satellites are needed for transformative, as opposed to incremental capability in system science. The current challenge is how to effectively transition products from design to mass production for space based instruments and vehicles. Ideally, the lesson learned from past designs and builds of various space products should pave the way toward a better manufacturing plan that utilizes just a fraction of the prototype‘s cost. Using the commercial products industry implementations of mass customization as an example, we will discuss about the benefits of standardization in design requirements for space instruments and vehicles. For example, the instruments (payloads) are designed to have standardized elements, components, or modules that interchangeably work together within a linkage system. We conclude with a discussion on implementation plans and the new paradigms for community and international cooperation enabled by small satellite constellations.
An infrared transfer radiometer has been recently developed at the Low-Background Infrared Calibration (LBIR) facility at the National Institute of Standards and Technology (NIST) for the Ballistic Missile Defense Organization (BMDO) program. The BMDO Transfer Radiometer (BXR) is designed to measure the irradiance of a collimated source of infrared light having an angular divergence of less than 1 mrad. It is capable of measuring irradiance levels as low as 10-15 W/cm2 over the spectral range from 2 micrometer to 30 micrometer. The radiometer uses an arsenic-doped silicon blocked impurity band (BIB) detector operated at temperatures below 12 K. Spectral resolution is provided by narrow bandpass interference filters and long-wavelength blocking filters. All the components of the radiometer, which include a mechanical shutter, an internal calibration source and detector, a long baffle section, a spatial filter, two filter wheels and a two- axis detector stage are cooled with an active flow of liquid helium to maintain temperatures below 20 K. A cryogenic vacuum chamber has been built to house the radiometer and to provide mechanical tilt alignment to the source. The radiometer is easily transported to a user site along with its support equipment.
The capability of the Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology to spectrally calibrate infrared detectors was demonstrated with the spectral calibration of arsenic doped silicon blocked impurity band (BIB) detectors. The BIB detectors were calibrated over the 2 micrometer to 30 micrometer range, using light from a monochromator with a nominal 2% bandwidth. Photon fluxes used for the calibration ranged from 1013 photons/s/cm2 to 1014 photons/s/cm2. The large area detectors (10 mm2) calibrated in this paper were very linear up to 2.5 X 1014 photons/s/cm2, where they showed a 1% drop in signal from linearity. The calibrations contained less than 6 1% standard component of random noise uncertainty, and there was about a 6 5% standard component of uncertainty arising from systemic effects that will be discussed in detail. The calibrations were performed in ultra- high vacuum in a 20 K background environment by making direct intercomparisons between the power measured by an absolute cryogenic radiometer and the response measured by a detector irradiated by the same beam. A detailed description of measurement methodology and system apparatus is given. Detector linearity and uniformity are also discussed. The LBIR facility can now provide calibrated BIB detectors as transfer standards as well as evaluate and calibrate customer's large area detectors and detector arrays provided the detectors stay within certain physical limitations.
Collimated infrared sources covering the 2 micrometer to 30 micrometer range of wavelengths are necessary to simulate infrared radiation from distant objects. This is important because on-orbit servo and tracking systems make extensive use of infrared radiation for remote sensing. Collimators are used to calibrate infrared detectors in terms of absolute power within a given spectral range. The National Institute of Standards and Technology (NIST) operates and maintains the Low Background Infrared Calibration (LBIR) facility, which uses a 2 K electrical substitution radiometer, the Absolute Cryogenic Radiometer (ACR), that is the primary national standard for broadband and infrared spectral measurements. At this facility, users can calibrate blackbody sources with at most 1% uncertainty. However, users must then rely on optical systems at their own facility to collimate the radiation from the blackbody. The effect of the optics on the output of the beam must then be calculated from models. For this reason, NIST is developing a portable transfer radiometer (BXR) that can be taken onsite to directly measure the spectral output, thus eliminating intermediate steps in the calibration chain. NIST is also developing a source having 1 cm diameter collimated beam, for a preliminary calibration of the BXR at the LBIR facility from 2 micrometer to 8 micrometer. The source must fit into a volume of about 0.03 m3 (1 cubic foot), have an angular divergence of less than 700 (mu) rad, a power output greater than 10 nW, and demonstrate 1% repeatability or better. The development and characterization of this source is the main topic of this paper.
A radiometric calibration station (RCS) is being assembled at the Los Alamos National Laboratory (LANL) which will allow for calibration of sensors with detector arrays having spectral capability from about 0.4-15 micrometers. The configuration of the LANL RCS is shown. Two blackbody sources have been designed to cover the spectral range from about 3-15 micrometers, operating at temperatures ranging from about 180-350 K within a vacuum environment. The sources are designed to present a uniform spectral radiance over a large area to the sensor unit under test. THe thermal uniformity requirement of the blackbody cavities has been one of the key factors of the design, requiring less than 50 mK variation over the entire blackbody surface to attain effective emissivity values of about 0.999. Once the two units are built and verified to the level of about 100 mK at LANL, they will be sent to the National Institute of Standards and Technology (NIST), where at least a factor of two improvements will be calibrated into the blackbody control system. The physical size of these assemblies will require modifications of the existing NIST Low Background Infrared (LBIR) Facility. LANL has constructed a bolt-on addition to the LBIR facility that will allow calibration of our large aperture sources. Methodology for attaining the two blackbody sources at calibration levels of performance equivalent to present state of the art will be explained in the paper.