Labsphere has created automated vicarious calibration sites using convex mirror technology in the new FLARE (Field Line-of-sight Automated Radiance Exposure) Network. FLARE has been operational for over two years, with network expansion and performance validation against industry standards and common methods for calibration and validation (cal/val) of 350-2500nm optical Earth Observation Systems (EOS). The FLARE point sources provide absolute and traceable data, creating a new tool in harmonization of satellites with ground sampling distances (GSD) of 0.3m to 60m. This paper provides an overview of the FLARE system and presents findings and improvements in operational hardware and software performance. Once commissioned, all FLARE nodes have been repeatedly targeting Landsat 8, Landsat 9 (starting 2022), and Sentinel 2A/B. This has produced a multi-year archive of radiometric and spatial calibration imagery. Landsat and Sentinel are the premier reference programs for Earth Observation performance and utilize both on-board calibration equipment and on-ground reference sites such as RadCalNet and PICS. This work compares the results of the FLARE technique to current official radiometric coefficients and spatial performance metrics for these satellites. Discussion will center on new insights gleaned from the archive analysis and FLARE’s contribution to the community’s capability for data fusion, instrument harmonization, and the potential to support the concept of Analysis Ready Data (ARD) for easier data use and information extraction. Finally, the future progression of FLARE sites, capabilities, and activities will be outlined.
The P4001 standard defines characteristics for hyperspectral camera performance. The standard will also include guidelines for measurement of these characteristics. The ambition to give a complete set of performance characteristics tends to require an extensive set of tests. An important aspect of the work is therefore to devise test protocols that are time efficient and have moderate requirements on the test equipment. Work is underway to define tests for radiometric performance, co-registration, spatial and spectral resolution, as well as stray light. The complete set of tests can be carried out using four test setups. These test methods, according to the current draft, will be outlined.
KEYWORDS: Reflectivity, Data conversion, Sensors, Calibration, Spectroscopy, Remote sensing, Near infrared, Bidirectional reflectance transmission function
In remote sensing, the conversion of sensor recorded radiance for each pixel in a scene to surface reflectance is a first step in many diagnostic analysis tasks. This process, known as reflectance conversion, is vital in the production of accurate information for a variety of applications, one of which is precision agriculture. Several calibration methods have been explored in previous research and are widely used today, with two of these being the empirical line method (ELM) and the at-altitude radiance ratio (AARR). The AARR approach is attractive to remote sensing practitioners as it allows reflectance conversion to be carried out in real-time throughout data collection, accounting for changes in illumination conditions, which can substantially reduce collection setup and subsequent data analysis time/effort. Illumination changes during a collection greatly influence the recorded scene radiance, which can confuse subsequent analysis results. While ELM has been demonstrated to report lower error when compared to AARR, the error introduced is often times acceptable depending on the application requirements and natural variation in the reflectance of the targets of interest. An onboard, downwelling irradiance spectrometer integrated onto a small unmanned aircraft system as part of this research is utilized to characterize the expected error in generated reflectance at varying aircraft altitudes and is cross compared to the ELM approach. Although the error introduced by AARR is larger when compared to ELM, this study allows the reader to determine if the ease of collection afforded by this real-time reflectance conversion methodology produces sufficiently accurate data for their particular remote sensing application.
Labsphere has created automated vicarious calibration sites using the SPecular Array Radiometric Calibration (SPARC) mirror technology in the new Field Line-of-sight Automated Radiance Exposure (FLARE) network. A short introduction to FLARE and SPARC will be given showing how arduous field ground calibrations can now be done remotely through FLARE nodes via an internet portal. Preliminary results of the performance of the system’s absolute radiometric and spatial calibration capability were published in 2020, demonstrating validation and uncertainty against current methods of remote calibration and spatial and geometric performance against edge and line targets. This paper will describe FLARE’s impact to ongoing evaluation and maturation of automated analysis processes for all data processing levels for space satellite and UAV imagers.
The SPecular Array Radiometric Calibration (SPARC) methodology uses convex mirrors to relay an image of the sun to a satellite, airborne sensor, or other Earth Observation platform. The signal created by SPARC can be used to derive absolute, traceable calibration coefficients of Earth remote sensing systems in the solar reflective spectrum. This technology has been incorporated into an automated, on-demand commercial calibration network called FLARE (Field Line-of-site Automated Radiance Exposure). The first station, or node, has been successfully commissioned and tested with several government and commercial satellites. Radiometric performance is being validated against existing calibration factors for Sentinel 2A and diffuse target methodologies. A radiometric uncertainty budget indicates conservative 1-sigma uncertainties that are comparable to or below existing vicarious cal/val methods for the VIS-NIR wavelengths. In addition to radiometric performance, SPARC and FLARE can be utilized for characterization of a sensor’s spatial performance. Line and Point Spread Functions, and resulting Modulation Transfer Functions, derived with SPARC mirrors are virtually identical to those measured with traditional diffuse edge targets. Ongoing development of the FLARE network includes improved radiometric calibration, web portal scheduling and data access, and planned expansion of the network to Railroad Valley Playa and Mauna Loa, Hawaii.
Two key areas of emphasis in contemporary experimental exoplanet science are the detailed characterization of transiting terrestrial planets and the search for Earth analog planets to be targeted by future imaging missions. Both of these pursuits are dependent on an order-of-magnitude improvement in the measurement of stellar radial velocities (RV), setting a requirement on single-measurement instrumental uncertainty of order 10 cm / s. Achieving such extraordinary precision on a high-resolution spectrometer requires thermomechanically stabilizing the instrument to unprecedented levels. We describe the environment control system (ECS) of the NEID spectrometer, which will be commissioned on the 3.5-m WIYN Telescope at Kitt Peak National Observatory in 2019, and has a performance specification of on-sky RV precision <50 cm / s. Because NEID’s optical table and mounts are made from aluminum, which has a high coefficient of thermal expansion, sub-milliKelvin temperature control is especially critical. NEID inherits its ECS from that of the Habitable-Zone Planet Finder (HPF), but with modifications for improved performance and operation near room temperature. Our full-system stability test shows the NEID system exceeds the already impressive performance of HPF, maintaining vacuum pressures below 10 − 6 Torr and a root mean square (RMS) temperature stability better than 0.4 mK over 30 days. Our ECS design is fully open-source; the design of our temperature-controlled vacuum chamber has already been made public, and here we release the electrical schematics for our custom temperature monitoring and control system.
The Habitable-Zone Planet Finder (HPF) is a stabilized, fiber-fed, NIR spectrometer recently commissioned at the 10m Hobby-Eberly telescope (HET). HPF has been designed and built from the ground up to be capable of discovering low mass planets around mid-late M dwarfs using the Doppler radial velocity technique. Novel apects of the instrument design include mili-kelvin temperature control, careful attending to fiber scrambling, and optics, mounting and detector readout schemes designed to minimize drifts and maximize the radial velocity precision. The optical design of the HPF is an asymmetric white pupil spectrograph layout in a vacuum cryostat cooled to 180 K. The spectrograph uses gold-coated mirrors, a mosaic echelle grating, and a single Teledyne Hawaii-2RG (H2RG) NIR detector with a 1.7-micron cutoff covering parts of the information-rich z, Y and J NIR bands at a spectral resolution of R~55,000. The use of 1.7 micron H2RG enables HPF to operate warmer than most other cryogenic instruments- with the instrument operating at 180K (allowing normal glasses to be used in the camera) and the detector at 120K. We summarize the engineering and commissioning tests on the telescope and the current radial velocity performance of HPF. With data in hand we revisit some of the design trades that went into the instrument design to explore the remaining tall poles in precision RV measurements in the near-infrared. HPF seeks to extend the precision radial velocity technique from the optical to the near-infrared, and in this presentation, we seek to share with the community our experience in this relatively new regime.
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