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The Blackbodies for Radiometeric Calibration

GBB Concept

Since GLORIA is exposed to the hostile environment of the UTLS with mutable low temperature and pressure, the in-flight blackbody calibration system has to be carefully designed to cope with those adverse circumstances.

The GLORIA in-flight calibration system (see Fig. 1) consists of two identical high-precision blackbody radiation sources (GBB-C and GBB-H), which are independently controlled at two different temperatures and emit radiation in the range of the atmospheric infrared radiance emissions. In order to operate GBB-C at 10 K below and GBB-H at 30 K above ambient temperature respectively, Thermo-Electric Coolers (TECs) are used offering the advantage of avoiding cryogens and mechanical coolers. An additional benefit of TECs is the dual utilization for cooling and heating by just switching the direction of the electrical current.

structural design configuration including electronic box on top
a) structural design b) configuration including electronics box on top
Figure 1. Design of a GLORIA BlackBody (GBB)

The Atmospheric Physics group at the University of Wuppertal designed, built and tested the GLORIA in-flight blackbody calibration sources for its deployment on board different research aircraft. The design of the GLORIA in-flight blackbody calibration system fulfils all technical specifications as given in Table 1; it is weight-optimized and ensures minimal power consumption. The GBBs have been comprehensively characterized for its spatially (full aperture) and spectrally (7 µm to 13 µm) resolved radiation properties in terms of radiation temperature traceable to the international temperature scale (ITS-90) at the national metrology institute of Germany (PTB).

 Optical surface  126 mm x 126 mm
 GBB-C temperature  = ambient temperature - 10 K
 GBB-H temperature  = ambient temperature + 30 K
 Temperature uncertainty  < 0.1 K
 Emissivity  < 0.997
 Spatial temperature inhomogeneity  < 0.150 K
 Long-term temperature stability  < 0.025 K/min
Table 1. Technical specifications for the GLORIA in-flight blackbody calibration system

GBB Design

The optical surface (OS) of a GBB (see Fig. 2) consists of an array with 49 individual pyramids made of aluminum. Since surfaces perpendicular to the line of sight should be avoided as direct reflections diminish the emissivity of a blackbody, the pyramids have different square bases on different levels (see Fig. 4). The base of type A is 18 mm x 18 mm whereas type B and type C have a slightly bigger base (20 mm x 20 mm). The NEXTEL-Velvet Coating 811-21 is used as surface finish for the pyramids as well as for the inner walls.

a) Array of 49 individual pyramids
b) varnished with NEXTEL-Velvet Coating
Figure 2. Design of the optical surface

The optical surface of a GBB is temperature-controlled by a system consisting of an assembly of four two-stage Thermo-Electric Coolers (TECs). The aluminum casing which surrounds the GBB-OS is partly thermally decoupled, while the front part serves as a stray light baffle which is also temperature-controlled by TECs (see Fig. 1a). The stray light baffle is operated at a slightly lower temperature in order to act as a water vapor trap to inhibit condensation on the black optical surface.

Temperature is measured at five specific pyramids using 10 PRTs (platinum resistance thermometers); for each of these five pyramids one PRT is located at the apex and one at the base of the pyramid (see Figure 2a). One pyramid with temperature sensors is located in the center of the optical surface; the four others are used for temperature control in the four sectors of the array.

The GBBs are suspended by GFRP (Glass-Fibre-Reinforced-Plastic) tubes for thermal decoupling. In order to reduce the adverse influence of the thermal environment, the GBBs are covered with a 50 mm-insulation made of extruded polystyrene foam (XPS) sheets. The outermost layer of the cover is composed of aluminum foils for EMC (Electro-Magnetic Compatibility) reasons. The overall weight of one GBB is 9.5 kg. An electronics box, which houses the temperature control unit as well as the power supply, is part of the GLORIA in-flight calibration system and was provided by the institute for Data Processing and Electronics @ KIT (see Fig. 1b).

Emissivity of a GBB

The NEXTEL-Velvet Coating that is used as surface finish has a measured emissivity of greater 0.967 in the spectral range from 5 µm to 12 µm. Due to the steep angles of the pyramid apexes (83° - 79°) most of the incoming radiation, which is not absorbed, is reflected into the back, and thus the effective emissivity is enhanced. Since the bases of type B and type C pyramids are elevated, light traps are formed which prevent direct back reflection. The NEXTEL surface coating has a nearly Lambertian radiation characteristic. Geometry factors for the pyramid array in a medium-size box (L = 182 mm) have been calculated for the radiative exchange between each area element of the pyramids and the aperture leading to a total calculated emissivity of 0.9996. Measurements at PTB, the national metrology institute of Germany, confirm the results of the emissivity calculation. With respect to the uncertainties of these measurements neither significant deviations from 1 are visible nor are local effects apparent for both blackbody calibration sources, which corroborates the assumption about the emissivity and verifies the absence of spectral features.