Calibration Procedures and Instrumental Accuracy Estimates of TAO Temperature, Relative Humidity and Radiation Measurements

H. Paul Freitag, Yue Feng, Linda J. Mangum, Michael J. McPhaden, LT Julia Neander, and Linda D. Stratton

INTRODUCTION

Fig. 1. Map of the tropical Pacific Ocean with location of ATLAS and PROTEUS moorings within the TAO array shown as of December 1994.

TAO moorings are designed, tested, calibrated and constructed at NOAA's Pacific Marine Environmental Laboratory (PMEL). TAO began in 1985 as regional-scale meridional arrays spanning the equator along 110W and 165E and has steadily expanded to its present size of approximately 70 moorings. Moorings are typically separated by 2 - 3 degrees of latitude and by 10 - 15 degrees of longitude.

The majority of TAO moorings are ATLAS moorings (Hayes et al., 1991) which measure surface wind, air-temperature (AT), relative humidity (RH), sea-surface temperature (SST), subsurface temperature (SBT) and pressure (P) (Fig. 2a). ATLAS moorings are designed for a nominal 1-year deployment.

Fig. 2a. Schematic drawing of typical ATLAS mooring.

At a few sites PROTEUS moorings (McPhaden et al., 1990) (Fig. 2b) are deployed, which measure and transmit the same surface parameters as well as current profiles from Acoustic Doppler Current Profilers (ADCPs). Recently shortwave radiation (SWR) has been added to the real-time PROTEUS meteorological measurement suite. Surface meteorological measurements on PROTEUS moorings are made by an AMP (Argos Meteorological Package) which is similar in design to the ATLAS. In addition, internally recording temperature sensors (MTRs), temperature-conductivity sensors (Seacats; western-Pacific sites only), and mechanical current meters (MCMs) measure temperature at up to 17 depths, conductivity at up to 10 depths, and current velocity at up to 7 depths, depending on the particular mooring site. Temperature and current data from the above internally recording instruments are not available in real time. PROTEUS moorings are designed with a nominal 6-month deployment.

Fig. 2b. Schematic drawing of typical PROTEUS mooring.

This report covers the calibration techniques and estimated accuracies of AT, RH, SST, SWR and SBT measurements as made on presently deployed moorings. (Other measured parameters, e.g., wind speed, will be addressed in future reports.) The sensors used to make these measurements were purchased from commercial vendors. The manufacturer, model number, and manufacturer's specifications for the sensors used are listed in Table 1. The electronics hardware and software packages which digitize and record the sensor output and pass it to the Argos transmitter were designed by PMEL's Engineering Development Division (EDD) and constructed by TAO Project technicians.

Table 1. Manufacturer, model, and specifications for temperature, humidity, and shortwave radiation sensors used on TOGA-TAO moorings.

Measurements were typically a two stage process (Fig. 3). The environment was sampled by the sensor and output as an analog signal (voltage, V, or resistance, R). The sensor output was converted and stored in digital memory by input/output (I/O) boards employing either analog to digital (A/D) or voltage to frequency (V/F) converters. Most calibrations were performed separately upon the sensor and electronics portions of the measuring systems. An exception was the SBT cable for which sensors and electronics were constructed as one and calibrated as a whole. Sensors and electronics were calibrated at PMEL with the exception of SWR sensors which were calibrated by the manufacturer.

Fig. 3. Flow diagram of ATLAS/AMP operation. Boxes on left contain calibration equations relating environmental parameter (T = temperature; RH = relative humidity; SWR = shortwave radiation) to engineering units (V = voltage; R = resistance) output by sensors. Boxes in center contain calibration equations relating engineering units to number (N) stored in memory by electronic I/O boards. Calibration coefficients are denoted by a, b and c.

The ATLAS and PROTEUS mooring projects were originally headed by separate principle investigators with separate support teams of technicians and programmers. Although the projects have since merged, calibrations are still performed independently and calibration data bases remain separate. (For example, ATLAS I/O board calibration coefficients are routinely scaled to be of order zero (for bias) and 1 (for gain), but have not been normalized in this report to simplify comparison with PROTEUS I/O boards.) Because of this separation and because of the different mooring design lifetimes, this report will make a distinction between like sensors (AT, SST, RH) which are used on both types of mooring. More unified and standardized procedures for calibrating ATLAS and PROTEUS moorings are presently being instituted, however, to insure a uniform quality to all TAO measurements.

This report has been organized into two major sections with each section subdivided by sensor or I/O board type. The first section defines calibration equations and describes calibration methods for each sensor or I/O board type. For each calibration the maximum residual (the largest difference between the known input and the sensor or I/O board output as calculated using the calibration coefficients) is computed. Focus is on the root-mean-square (RMS) of maximum residuals from the calibration equation computed over all calibrations of a sensor or I/O board type, which may be interpreted as an estimate of the initial sensor or board accuracy as they were deployed. The second section deals with sensor or board drift as indicated by the difference between multiple calibrations of the same sensor or board as it was used and reused on multiple deployments.