The CIRIMS design goal is to provide ocean skin temperature data with an accuracy of +/- 0.1 °C from a system that has the ability to run autonomously at sea for extended periods with no involvement from the vessel crew.
The CIRIMS design incorporates two Heitronics infrared KT11 radiometers with a spectral bandwidth in the 9.6-11.5 μm range. One radiometer is housed within the unit itself and measures sea surface radiance. The second radiometer is housed externally in an enclosure and measures sky radiance. Reliable calibration of the internal radiometer is achieved by two-point calibration using a modified Hart Scientific microbath. A custom designed, copper cylindro-cone blackbody is immersed in a water/ethylene glycol solution within the temperature-controlled microbath. Two calibration points are set a few degrees above and below the scene temperature allowing for dynamic calibration over a wide range of scene temperatures.
The temperature-controlled housing provides a stable, dry environment for the internal radiometer and the blackbody. The insulated housing is heated and cooled by a thermoelectric heater/cooler unit, which maintains the internal case temperature to within +/- 0.5 °C of the set point, generally 35 °C.
Protection of the radiometer and blackbody is arguably the most challenging and debated aspect of a practical design. We have chosen to use an IR transparent window to provide complete protection under all conditions. This approach relies on our ability to correct for the effect of the window. The external housing contains the IR transparent window and an external heated blackbody. Since the window is not perfectly transparent, the effect of the transmission, emission, and reflectance of the window on the measured radiance must be determined. In order to quantify the effect of the window, a two-point hot blackbody has been mounted on the back of the door of the external housing. The door of the housing is closed, protecting the internal components, and measurements are made of the heated blackbody with and without the window. In this way we are able to correct for the window effect. For further information about the window correction, see Jessup et al. [2002]. For alternative approaches, see ISAR designed by Craig Donlon of the Eurpoean Commission Joint Research Center or the SISTeR designed by Tim Nightingale of the Rutherford Appleton Laboratory.