Retrieval of accurate surface meteorological data is an integral part of the GAUS sounding. Surface meteorological instruments are used to capture a data point which anchors the balloon sounding data to the surface. The surface pressure is used as the starting point for sonde pressure data and altitude calculation. The surface temperature, humidity and wind data are also used as starting points in the sounding data. The surface data are collected with independent surface meteorological instrumentation. These instruments are connected to a Campbell CR10 datalogger which processes the inputs into real numbers and outputs one-minute average data. These data are transferred to the GAUS personal computer, via RS-232, where they are used as the first point in a sounding.

A continuous record of surface data processed through the Campbell datalogger can also be logged to a floppy disk for a complete surface record at the site. During the flight, the surface data are buffered for recovery after the sounding is completed.

Surface Pressure Measurement

The surface pressure is measured with a Vaisala PTA427 or PTA427A pressure sensor. The PTA427 pressure range is 800 to 1060mb while the PTA427A pressure range is 600 to 1060mb. These sensors have an accuracy of +/- 0.5mb and +/- 0.8mb respectively.  Both are silicon capacitive pressure sensors patented by Vaisala. Both are temperature-compensated and produce a linear voltage output over the full operating range. In order to interface with the Campbell datalogger a 2:1 voltage divider was incorporated into the cable from the pressure sensor.

Surface Temperature and Humidity Measurement

The temperature and humidity sensors are contained in a Vaisala HMP35C instrument probe. The actual sensors are a Fenwal Electronics UUT5J1 thermistor and a Vaisala Humicap capacitive relative humidity sensor. The temperature sensor accuracy is +/- 0.4 degree C over the range -33 to + 48 degrees C. The accuracy of the humidity sensor against field references is approximately +/- 2% with a long term stability of better than 1% RH per year. The HMP35C sensor probe is protected and vented by an R.M. Young aspirated radiation shield, model number 43-408.

Surface Wind Measurement

Wind speed and direction are measured with an R.M. Young 05103 Wind Monitor. The monitor is a propeller wind vane with a 0.9 m/s threshold for wind speed and a 60 m/s maximum. Wind direction is measured using a 360-degree mechanical precision conductive potentiometer. The wind direction measurement has a threshold of 1.0 m/s at a 10 degree displacement and a threshold of 1.5 m/s at a 5 degree displacement. The potentiometer is 10 K-ohm, with a life expectancy of 50 million revolutions, and has a 0.25% linearity through the entire range.

Radiation Measurements

 If needed for a given project, a  radiation sensor (shortwave incoming only) can be added to the data collection system. Measurement specifications will depend on the sensor model used.