Radiosonde Sensor Specifications

Radiosonde Specifications

Manufacturer - type	Vaisala RS92-SGP
Mass	550 grams (with activated wet battery)
Dimensions	220 x 80 x 75 mm
Ascent Rate	4 m/s avg
Transmitter Frequency	403.5 MHz
Transmitter Power	60 mW min
Pressure Sensor	BAROCAP Capacitive aneroid
Temperature Sensor	THERMOCAP Capacitive wire
Humidity Sensor	HUMICAP thin film capacitor

Pressure Measurement

The pressure sensor is an encapsulated steel aneroid sensor. It utilizes a capacitive transducer with a vacuum inside the capsule. The entire unit is precision welded requiring no mechanical adjustment. The unit is friction free and continuously variable.

Manufacturer	Vaisala
Sensor	Capacitive aneroid
Range	3 to 1060 mb
Accuracy	0.5 mb
Data System Resolution	0.1 mb
Sensor Resolution	0.1 mb

Temperature Measurement

The temperature sensor is a capacitive bead in GAUS encapsulation. The temperature sensors are calibrated at the factory.

The manufacturer's specification for the time constant is less than 1 second at 100 mb. The time constant of the thermistor, combined with the ascent rate of the sonde produce a slight lag in temperature measurement through the sounding. However, with typical atmospheric lapse rates the resultant smoothing of the temperature profile is less than the accuracy of the thermistor. The smoothing resulting from the lag time becomes more significant when the sonde crosses frontal boundaries or goes through strong inversions.

Experience has shown that if the sonde sensor arm is not protected or properly ventilated prior to launch, it can be adversely affected by solar heating. This results in a temperature reading that is too high, producing a false near-surface super-adiabatic lapse rate. Due to the small thermal mass of the temperature sensor and its supporting structure this effect is not long-lived. The thermal time constant of the sensor arm is 13 seconds and thus the problem goes away soon after launch, once the sensor is adequately ventilated.

Manufacturer	Vaisala
Sensor	Capacitive wire
Range	-90 C to 60 C
Accuracy	0.25 C
Data System Resolution	0.1 C
Sensor Resolution	0.1 C

Relative Humidity Measurement

The humidity sensor is a thin-film capacitive type sensor. The Vaisala type H radiosonde utilizes the type H sensor, which uses a humidity algorithm incorporating temperature compensation. This new sensor has improved humidity measurements over previous sensors, particularly in the high end of the humidity range (95% to 100%).

Solar heating of the sonde temperature/humidity sensor arm prior to launch can produce an error in the low level humidity measurement (and hence dew point).The humidity sensor gives a reading of the humidity relative to the temperature of the sensor surface itself. In a situation where the sensor surface is warmer than the surroundings, the humidity reading will be lower than ambient (vapor pressure remains unchanged, "sensed" saturation vapor pressure value goes up). Due to the thermal time constant of the sensor arm (13 seconds), the initial heating of the sensor arm affects the humidity data for roughly the first 40 seconds of the flight. (In a shaded, well ventilated situation, in which the sensor surface is in thermal equilibrium with its surroundings, an accurate ambient humidity measurement at the surface can be obtained.)

The effect of the heated sensor arm persists for a longer time in the humidity measurement than it does in the temperature measurement. The portion of the sensor arm where the temperature sensor is mounted is an isolated small cylinder which quickly comes to a thermal equilibrium with its surroundings whereas that portion of the sensor arm on which the humicap is mounted is much larger and thus takes more time to come to a thermal equilibrium with its environment.

Manufacturer	Vaisala
Sensor	HUMICAP thin film capacitor
Range	0 to 100% Relative Humidity
Accuracy	1.5% Relative Humidity
Data System Resolution	0.1% Relative Humidity
Time Constant	0.5 second @ 6m/s flow, 1000mb, 20 C

Wind and Position Measurement

Winds derived from GPS have a constant accuracy once the minimum number of satellites (typically four) are received.

Manufacturer / Model #	Vaisala
Wind Accuracy	0.5 m/s
Averaging Time	0.5 seconds
Data System Resolution	0.1 meter; 0.1 m/s

Surface Instrumentation Specifications

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.