AIRBORNE INSTRUMENT FOR PRECISION ME ASUREMENT 

 OF SEA SURFACE TEMPERATURE USING INFRARED 

 RADIATION EMITTED BY THE SEA 



R. PELOQUIN 

 U. S. Navy Oceanographic Office 

 Suitland, Maryland 



M. WEISS 

 Barnes Engineering Company 

 Stamford, Connecticut 



INTRODUCTION 



The combination of a precision radiation col- 

 lecting and detecting system with highly stable 

 electronics has resulted in an infrared radio- 

 meter capable of measuring absolutely the tempera- 

 ture of the sea surface to an accuracy of io.2°C. 

 The instrument uses an in-line black body refer- 

 ence radiation cavity temperature controlled to 

 better than -0.05°C. Integral with the cavity 

 are the detector, germanium optics and chopper 

 system, comprising a highly stable optical unit 

 independent of temperature. The electronics 

 system is largely transistorized and uses pre- 

 cision components to provide the overall stabil- 

 ity to maintain the 0.2 C performance . 



Operation of the instrument has been reduced 

 to the essential steps, requiring only selection 

 of temperature range and selection of the mode 

 of operation. No other operating adjustments are 

 required. The output is a direct indication of 

 sea surface temperature and it is presented in 

 degrees Centigrade on a meter and on a strip chart 

 recorder. 



A second highly compact instrument, known as 

 the infrared thermometer, of a less complex design 

 has also been developed. It is capable of a 

 measuring accuracy of 0.5 to 1.0°C. Its output 

 is displayed directly in degrees C on a panel 

 meter. 



Such a surface emits maximum radiation at a wave- 

 length of 9-6 microns. The radiation emitted 

 for the extremes of ocean surface temperature are 

 shown in Fig. 1 as radiation emission curves for 

 black bodies at +35°C and -2°C. Examination of 

 these curves shows that the major portions of 

 both peaks are included in the region between 

 6 and 20 microns. 



Atmospheric Attenuation 



Since the instrument is to be used aboard air- 

 craft it is important that atmospheric attenua- 

 tion effects be considered. Fig. 1 also shows a 

 curve of the spectral attenuation through 1,000 

 feet of atmosphere in the 6 to 20 micron region. 

 The curve shows that there is a good transmission 

 window between 7 . 5 and 12 . 5 microns and that the 

 infrared radiation outside of this window will be 

 highly attenuated by the atmosphere. The effects 

 of atmospheric attenuation, therefore, can be 

 largely eliminated by use of a 7-5 "to 12.5 micron 

 band-pass optical filter. Approximately 29$> of 

 the radiation emitted by the surface of the 

 ocean falls within this band-pass and would be 

 available for detection. 



In this application an optical filter con- 

 sisting of arsenic trisulfide glass and a thin 

 coated slab of indium antimonide was used. The 

 transmission characteristic is shown in Fig. 2. 



THEORY OF OPERATION 



Radiation Characteristics of the Ocean Surface 



Over different parts of the earth the tempera- 

 ture of the ocean surface ranges from a minimum 

 of -2°C (271°K) to a maximum of +35°C (308°K) . 

 Only a thin layer of water is required to absorb 

 infrared radiation completely. In the infrared 

 region of from k to 12.5 microns the emissivity 

 of the ocean surface is O.98 for radiation normal 

 to the surface. The reflection for normal radia- 

 tion is 2$ and increases to k^o for radiation 

 incident at 60° from the normal. 



The ocean surface has essentially the radia- 

 tion characteristics of a black body at 300°K. 



There is still some attenuation present within 

 the pass band. It is possible to account for 

 this effect and correct the temperature measure- 

 ment by introducing a quantity known as "optical 

 thickness" which is dependent on specific humidity, 

 pressure and layer structure of the atmosphere. 



Another approach to eliminating atmospheric 

 effects is to use a narrower window, 9-2 to 

 10.9 microns. Fig. 1 shows the relatively com- 

 plete transparency of the atmosphere in this 

 region. 



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