The active cavity radiometer (ACR)-type detector is used in the unshuttered mode of operation for the earth viewing radiometric channels of the Earth Radiation Budget Experiment (ERBE) nonscanner instruments. The offset, or bias, levels of ACR detectors may change over time unless specific curing techniques are used following fabrication. Previous applications of ACR detectors have primarily been in the solar measurement area using a narrow field-of-view design which allows redetermination of detector offset by viewing a shutter (known radiometric source) every few minutes. Because the offset changes in improperly cured detectors occur very slowly relative to the shutter operation cycle, there is no impact on radiometric measurements made in the shuttered mode of operation. In the unshuttered mode of operation, however, a small change in offset can have a significant impact on the measurement. This phenomenon is the result of the design, fabrication technique, and physical characteristics of the materials used in temperature sensing for this detector. Temperature sensitive resistance wire is wound around a cylindrical thermal impedance and serves as two legs of a bridge circuit in the standard ACR detector design. The winding of the resistance wire in tight loops around the cylinder induces strain in the wire and changes its molecular structure causing a change in its resistivity. During the curing process, the wire returns to its natural state. As this takes place, the bridge resistance values change causing the detector offset to change. If this curing process is not complete prior to ground calibration, certain calibration coefficients may be affected. The origin and physics of this phenomenon are discussed along with the techniques developed during the ERBE program to insure detector offset stability. Ground test and flight data from the ERBE program are presented, and alternate solutions to the problem are discussed.
Michael R. Luther, Thomas A. Evert, Raymond J. Hesser, William D. Potter, and Andres Castineiras, "Performance characteristics of active cavity radiometers in unshuttered applications," Appl. Opt. 25, 3705-3709 (1986)