The Calibration and Intercalibration of Sea-Going Infrared Radiometer Systems Using a Low Cost Blackbody Cavity

Abstract

There are many infrared radiometer systems available for the measurement of in situ sea surface skin temperature (SSST). Unfortunately, the marine environment is extremely hostile to optical components, and to ensure the accuracy of SSST measurements, an absolute calibration of instrumentation using an independent calibration reference is required both before and after any sea deployment. During extended deployments it is prudent to have additional regular calibration data to monitor instrument performance characteristics. This paper presents a design for an ambient temperature (278-325 K), wide aperture (100 mm), reference blackbody unit that may be used to calibrate a variety of seagoing infrared radiometer systems both in the laboratory and in the field. The blackbody consists of a spun copper cavity coated with well-characterized high emissivity paint (Mankiewicz Nextel Velvet Coating 811-21) immersed in a water bath that is continuously mixed using a strong water pump. The radiant temperature of the blackbody cavity is determined from the measured water bath temperature. Results derived from validation and intercomparison experiments show this blackbody design to be an accurate and reliable reference blackbody source. However, in order to ensure that the best possible calibration data are obtained, extreme care must be taken to ensure the accurate measurement of the water bath temperature, proper positioning of a radiometer in front of the cavity itself, and prevention of condensation on the cavity surface. Four blackbody units have been specifically built for the European Union combined action for the study of the ocean thermal skin (CASOTS) program. Using these units as reference radiance sources, the authors describe the strategy adopted and present results obtained from the CASOTS radiometer intercalibration experiment. These results highlight the need to obtain independent calibration data both before and after seagoing radiometer deployments and the need to standardize field radiometer calibration protocols.

An Infrared Sea Surface Temperature Autonomous Radiometer (ISAR) for Deployment aboard Volunteer Observing Ships (VOS)

“…In geographical areas characterized by persistent low wind speed and diurnal variability, the use of infrared radiometers to measure the SSTskin from ships is the only way to provide suitable in situ measurements for validating infrared satellite SST measurements (Donlon et al 2002). Recent years have seen the development of several ship-based radiometer designs capable of target measurements with accuracy better than 0.075 K. Most notable are the scanning infrared sea surface temperature radiometer (SISTeR; see Donlon et al 1999a), the marine atmospheric emitted radiance interferometer (M-AERI; Minnett et al 2001), the DAR011 ( Barton et al 2004), and the Calibrated Infrared In situ Measurement System (CIRIMS) radiometer. CIRIMS was specifically developed to provide autonomous SSTskin observations (Jessup et al 2002) and has been deployed for several years in an autonomous configuration on research vessels (see http://cirims.apl.washington.edu/ CruiseMaps.html).…”

“…Despite these developments, the regular collection of a globally comprehensive in situ SSTskin dataset has remained an elusive goal for the agencies and scientists tasked with the sustained validation of satellite SST data. The available number of SSTskin validation data is limited in number, spatial, and temporal extent (e.g., Thomas et al 1995;Donlon et al 1999a;Kent et al 1996;Donlon and Robinson 1997;Noyes et al 2006) and there is a clear need for in situ infrared radiometers capable of sustained long-term operation (i.e., without operator intervention) to provide adequate satellite SST validation measurements.…”

The Hillarys Transect sea surface temperature measurement and validation program

“…In order to calibrate the radiometer we use a water-filled blackbody calibration unit similar to the CASOTS blackbody [7], [8]. However, our blackbody can not be heated and cooled at will, which limits its effectiveness.…”

“…Offsets in the measured radiometric temperature occur due to changes induced in the optics of the radiometer by changes in its operating temperature. These can only be effectively accounted for in real time by ongoing two-point calibration [7], [8].…”

“…la and b. Here, the raw and calibrated data are plotted against corresponding bulk sea surface temperature (BSST) measurements acquired from a bucket sample of ocean water using a mercury-in-glass thermometer [2]. Although there is an offset between BSST and SSST, due to the radiative cooling of the skin of the order of 0.5K (a difference of 0.25K has been measured in the tropical Indian Ocean by [8]), changes in BSST should also be reflected in the corresponding measurement of SSST. The BSST are well known (iO.…”