An In Situ Technique for Intercalibrating Temperature or Conductivity Sensors on Towed Arrays

Abstract

Vertical arrays of temperature and conductivity sensors are towed from research vessels to obtain cross sections of the ocean's internal structure that result from the dynamic finescale and microscale processes. To estimate local water displacements and other quantities of interest from these sensors, it is required that they be accurately calibrated. However, drifts in sensor electronics and sensor aging result in offsets from these calibrations. These offsets generally cannot be accounted for during laboratory calibration, so techniques must be used either at sea or later to intercalibrate the array of sensors. In this paper, a method is presented for intercalibrating the sensors by cycling the array in depth while the array is towed horizontally to determine the relative offsets and gains between the sensors. This method eliminates the ambiguity of other methods which require some assumed shape of the vertical temperature or conductivity profile and which are especially problematic in arrays having a small vertical aperture or a small number of sensors. In addition, theoretical estimates of the amount of data required to obtain the desired intercalibration accuracy are given, and an example using a towed array that is not well calibrated is provided. Offset errors of up to 0.5°C and gain errors of up to 5% were calculated from the towyo data. Approximately 2 km of towyo data are required to attain 0.5 millidegree Celsius intercalibration accuracy.