A necessary condition for accumulating fundamental climate data records is the use of observation instruments whose stability and accuracy are sufficiently high for climate monitoring purposes; the number of instruments and their distribution in space should be sufficient for measurements with no spatial or temporal gaps. The continuous acquirement of data over time intervals of several decades can only be possible under the condition of simultaneous application of instruments produced by different manufacturers and installed on different platforms belonging to one or several countries. The design of standard sources for pre-flight calibrations and in-flight monitoring of instruments has to meet the most stringent requirements for the accuracy of absolute radiometric measurements and stability of all instruments. This means that the radiometric scales should be stable, accurate, and uniform. Current technologies cannot ensure the high requirements for stability and compatibility of radiometric scales: 0.1% per decade within the 0.3 - 3 µm band and 0.01 K per decade within the 3 - 25 µm band. It is suggested that these tasks can be aided through the use of the pure metals or eutectic alloy phase transition phenomenon that always occur under the same temperature. Such devices can be used for pre-flight calibrations and for on-board monitoring of the stability of radiometric instruments. Results of previous studies of blackbody models based upon the phase transition phenomenon are quite promising. A study of the phase transition of some materials in small cells was conducted for future application in onboard monitoring devices and its results are positive and allow us to begin preparations for similar experiments in space.
The full potential of current remote sensor technology is limited by the inability to correct biases once an exo-atmospheric remote sensor becomes operational. Even when the calibration is traced to the International System of Units, SI, and the instrument is performing within the operational envelope wherein it is calibrated, the problem exists and a Space Metrology Program is a potential solution to the problem. This paper discusses such a program, suggests a feasibility study to address the issues and recommends a plan of action.
Any operational instrument has a bias and reducing the magnitude of the bias can only be accomplished when an adequately accurate standard is accessible by the instrument while the instrument is in its operational environment. Currently the radiometric flux from the sun, the moon and the stars is inadequately accurate SI to provide a standard that is consistent with the remote sensor state-of-the-art technology. The result is data that is less accurate than it could be often leading to confusing and conflicting conclusions drawn from that data. Planned remote sensors such as those required to meet future program needs (e.g. the United States National Polar-Orbiting Operational Environmental Satellite System (NPOESS) and the proposed international Global Earth Observation Program) are going to need the higher accuracy radiometric standards to maintain their accuracy once they become operational. To resolve the problem, a set of standard radiometers on the International Space Station is suggested against which other exo-atmospheric radiometric instruments can be calibrated. A feasibility study for this program is planned.
The large variety of high-precision unique blackbody sources: those operating at fixed temperatures provided by phase transitions of metals and metal-carbon eutectics, and variable-temperature ones had been designed in VNIIOFI for high-precision radiometry, radiation thermometry and spaceborne remote sensing within a 100 to 3500K temperature range. Paper reviews the blackbodies (BBs) ranged to low, middle and high temperatures, and describes spectral radiance and irradiance calibration facilities on the base of these BBs in IR and V-UV spectral ranges.
The latest investigations of high-temperature fix-points based on metal-carbon eutectics Re-C (2748K) demonstrated an excellent reproducibility of freezing plateau (up to 0.01% in terms of radiation temperature) between series of measurements/crucibles, and about 0.003% within a sample measurement session, i.e. better than 100mK. Further Re-C (spectral irradiance measurements) and TiC-C (3057° C) eutectics are being investigated for use as high-stable radiance/irradiance sources above the conventionally assigned values of temperatures of ITS-90.
The calibration of IR sensors, thermal imagers, instrumentation for remote earth sensing, signature recognition, and low background spacebome radiom-eters requires development of low- and near-ambient-temperature reference standard sources capable of operating in vacuum at low or medium background conditions. The paper contains the characteristics of blackbodies (BB), developed at the All-Russian Research Institute for Optical and Physical Measurements (VNIIOFI). A novel Medium Background Facility (MBF) intended for calibrating IR sources has been developed at VNIIOFI. The MBF used for the calibration under vacuum conditions (10-3 Pa) and medium background temperatures includes a Ga Fixed-Point Blackbody (29.76 °C), Variable Temperature Blackbody (-60 °C ~ +80 °C) and Filter Radiometer. A brief description of the operating principles and specifications of the MBF is presented. The results of a comparison of the Variable Temperature Blackbody and Ga Fixed Point Blackbody in the Medium Background Facility are given. The measurement uncertainties radiance temperature associated with spectral bandwidth are discussed.
The paper contains the state-of-the-art of the precision radiometry principles based on using high temperature blackbody sources, as well as the characteristics of blackbodies (BB), developed at the All-Russian Research Institute for Optical and Physical Measurements (VNIIOFI). A novel Medium Background Facility (MBF) intended for calibrating both IR sources and sensors has been developed at VNIIOFI. The MBF used for the calibration under vacuum conditions (10-3 Pa) and medium background temperatures includes a Ga Fixed-Point Blackbody (29.76 degree(s)C), Variable Temperature Blackbody (-60 degree(s)C ~ +80 degree(s)C) and Filter Radiometer. Both blackbody spectral radiometric calibration and effective emissivity measurement are performed in the spectral range from 2.5 micrometers to 14 micrometers for a blackbody temperature from -60 degree(s)C up to +80 degree(s)C. A brief description of the operating principles and specifications of the MBF is presented.
This work summarizes the progress in the development and testing of the telescope for a space imaging spectrometer 'OMEGA' of a new stereo-spectral-imaging system ARGUS developed in the frame of the International project for MARS 94/96 missions. The results of the telescope aberration calculation are given. Brief description of the facility for the main optical characteristics measurements are described. The telescope was tested at a special vacuum chamber at the temperature range from 180K to 300K. The results of the telescope investigation are given.
A new low-cost absolute cryogenic radiometer of the electrical substitution (ES) type optimized for performing black-body models calibration is described. Nitrogen is used as a cryogenic liquid for cooling of a radiometer receiving cavity up to temperature 80 K. This absolute ESR has been developed for measuring the irradiance in the range of 10-3 divided by 10-6 W/cm2 with the uncertainty of 0.1% for the upper level of irradiance range. The receiving cavity having 16 mm aperture, is fabricated from copper foil with thickness of 30 micrometers . A substitution winding of the receiving cavity is made of manganin wires. Measured heat conduction of the receiving cavity is 1.1 X 10-3 W/K. A single time constant of the receiving cavity is 80 sec. Brief description of the design, operating principles, and measurements results of the new cryogenic radiometer at nitrogen temperature are given.
A diffuse target illuminated by the sun radiation is used for on-board calibration of a new stereo-spectral-imaging system ARGUS developed in the frame of the International project for MARS 94/96 missions. The target manufacturing technique is described. The experimental procedure used for getting the spectral and angular responses of target reflectivity is presented. The diffuse target was tested for the spectral properties at angle illumination -70 degree(s) (relative to the normal of the target) and angle of viewing +20 degree(s). A brief description of the setup for measurements of the spectral and angular responses of the target reflectivity is given. Results of the diffuse target calibration in the spectral range from 320 nm to 5200 nm are presented.
The method for on board calibration of the space imaging instruments using a diffusing target illuminated by the sun radiation is gaining acceptance. This raises a problem of measurements of the spectral and angular response of the target reflectivity in illumination conditions similar to the actual one on-board. This work summarizes the progress in the development and testing of the target for on-board radiometric calibration of the space video-spectrometers in stereo- spectral imaging system ARGUS. The spectral and angular response of the target reflectivity and the experimental procedure used for getting these characteristics are described. The target manufacturing technique is described. This target was tested for the spectral properties from 0.3 micrometers to 5.2 micrometers at angle of illumination -70 degree(s) (relative to the normal of the target) and angle of viewing +20 degree(s). The angular response was measured with three incident angles 70 degree(s), 75 degree(s) and 80 degree(s) (relative to the normal) for angles of viewing varying between 0 degree(s) and 60 degree(s) (relative to the normal). Brief description of the setup for measurements of the spectral and angular response of the target reflectivity are presented. The values of angular and spectral response of the reflectivity have been determined with the accuracy of 2 - 5% depending on the wavelength.
The European Union and DLR are funding a new 79-channel airborne imaging spectrometer: DAIS-7915, which is built by GER Corporation. Based on the requirements for ground calibration of the DAIS-7915, a Universal Calibration Facility (UCF) for VIS-TIR wide-angel videospectrometric airborne sensors has been developed at the DLR-Institute of Optoelectronic. The spectral coverage of the UCF is 0.4 - 14.5 micrometers . The UCF consists of the spectrometric-geometric calibration part (SCP), the relative diffuse radiometric source (RDRS), the thermal absolute calibration part (TACP) and the absolute radiometric calibration part (ARCP). The SCP, RDRS, and TACP can be used for laboratory calibration as well as for hangar calibration of the sensor installed in the aircraft. The ARCP consists of an integrating sphere with 165 cm diameter and an opening of 40 X 55 cm2. The sphere is intercalibrated by means of an absolute diffuse source (ADS) and a spectro- radiometer. The ADS has been recognized and admitted for application as a reference instrument for measuring the spectral radiance in the wavelength region of 0.4 - 2.5 micrometers .
A radiometric calibration of the space imaging spectrometers especially with a wide field of view is performed with a help on an integrating sphere as a radiance source. Spectral radiance of the integrating sphere is defined by a comparison with the absolute diffuse source. As a diffuse source for a calibration of the integrating sphere in units of radiance may be used a halon target irradiated by a halogen lamp. The absolute diffuse source based on the halon target/four halogen lamps configuration with a good radiance homogeneity is described. The diffuse screen with a diameter 180 mm, which is made of polytetrafluoroethylene (PTFE), is irradiated by 4 symmetrically located halogen lamps. The type of the lamp is KGM 24-150. The angle of incidence on the diffuse screen is 34°. Viewing is carried out through the output window along the direction, which is normal in relation to the diffuse screen. The diameter of the output window is 140 mm. A facility for relative radiance graduation and calibration of this diffuse source against a standard lamp traceable to All-Russian Research Institute of Optical and Physical Measurements (VNIIOFI) and the results of the absolute diffuse source radiometric calibration in the spectral range from 400 nm till 2500 nm are described. A spatial inhomogeneity not more than 2.5% along the whole output aperture. A level of radiance at 900 rim is 1.6.108W/m3sr. Described absolute diffuse source was created in according to a contract with the Space Research Institute of the Russian Academy of Sciences for the radiometric calibration of imaging cameras and spectrometer in stereo-spectralimaging system ARGUS for Mars 94/96 missions.
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