winds) has been found to have a significant 

 effect on the accuracy of the ART record. This 

 fact has prohibited the effective use of the ART 

 during roughly 40% of the reconnaissance mis- 

 sions flown by the Ice Patrol. 



The Center for Cold Ocean Resources Engi- 

 neering (C-CORE) at Memorial University of 

 Newfoundland in St. John's has recently tested 

 two techniques for applying correction factors 

 to account for atmospheric attenuation. Al- 

 though not presently using either of these tech- 

 niques, IIP is considering their use for improving 

 the absolute accuracy of the ART surveys. In 

 these methods, an emissivity value of almost 

 unity is assumed for water; thus, no emissivity 

 correction is contained in either procedure. 



Pickett Method 



This technique uses an empirically derived 

 correction equation that uses multiple regression 

 (EFROYMSON, 1964). Environmental vari- 

 ables considered in the derivation were altitude, 

 altitude squared, square root of altitude, air tem- 

 perature squared, square root of air temperature, 

 the difference between air temperature, and ship 

 bucket temperature (PICKETT, 1966). Pickett 

 used these variables because they could be easily 

 and accurately measured. He did not take into 

 consideration humidity effects. Using correla- 

 tion coefficients between the ART error and the 

 environmental variables, Pickett determined that 

 altitude and air temperature were the two most 

 important variables. From his results the fol- 

 lowing empirical environmental correction equa- 

 tion was determined: 



C = 1.54 + 0.00046A - 0.043T, 

 where, C = environmental correction to be added 

 to the ART value (°C) 



A = altitude in feet, and 



T = air temperature at 1,000 feet (°C). 



Pickett devised a chart for quick determination 

 of the correction for the radiation temperature 

 that compares that altitude (feet) versus air 

 temperature at flight level. 



Atmospheric Environment Service Method 



This method evolved from a computer proce- 

 dure that was used to correct Richards' (1966) 

 data (SHAW, 1966). Shaw and Irbe (1972) 

 and Irbe (1972) have described a graphical 

 method that required knowledge of the vertical 

 distribution of temperature and humidity in the 



vicinity of the aircraft. They felt that correc- 

 tions for the air column above 2,000 feet were 

 unnecessary, and that the correction using the 

 graphical means was comparable to the measure- 

 ment error of the recording instrument (±0.5C°) 

 specifications. They found that an overcast cloud 

 layer increased the ART reading by 0.5C° above 

 the values for clear sky. Irbe (1969) found that 

 the atmospheric correction was of utmost im- 

 portance for reducing data if unusual surface 

 water temperature patterns were to be discerned 

 and Shaw and Irbe (1972) felt that the instru- 

 ment could be extremely useful in monitoring 

 surface water temperatures near freezing. 



The correction technique involves the determi- 

 nation of instrument drift over the flight period 

 using inflight calibration; the plotting of an en- 

 vironmental correction graph which is a plot of 

 the ART temperature with drift corrections 

 versus the measured surface water temperature; 

 and the application of a correction factor for 

 errors due to the water vapour mass under the 

 aircraft. The water vapour data were recorded 

 from independent information available from the 

 nearest upper air meteorological station. Irbe 

 (1972) contains the required graphs for carrying 

 out the corrections. This technique replaced the 

 computer method of Shaw (1966), and has proven 

 satisfactory for the AES program. 



Of the two correction methods, the AES cor- 

 rection is preferred because it attempts to account 

 for changes in temperature and humidity of the 

 air column under the aircraft. This correction 

 is also sensitive to changes from clear to overcast 

 skies. The Pickett method is very insensitive to 

 altitude changes and outside air temperature and 

 makes no allowances for humidity. The Pickett 

 method was normally 2°C for most of the Ice 

 Patrol Grand Banks surveys in 1976, regardless 

 of atmospheric conditions. 



To implement the AES method, accurate out- 

 side air temperature and humidity at altitude 

 should be collected at the same time as ART 

 information. This requires an accurate outside 

 air temperature sensor and an airborne hygrom- 

 eter. Accurate navigational information is avail- 

 able from Ice Patrol aircraft's inertial navigation 

 system, as are altitude readings. Cloud cover 

 could be monitored by the ice observer during 

 the flight. 



78 



