GAUS is a balloon-borne rawinsonde system. Through GPS technology, the GAUS measures high vertical resolution of temperature, humidity and winds at locations around the world. Each system includes a meteorological observing station to record thermodynamic data at the surface and the infrastructure to provide local data processing, display and communications. Data from GAUS can be transferred by phone, Internet or the Global Telecommunication System (GTS) to scientists at universities, other research institutions and operational weather centers. Mobile GAUS (MGAUS) is used by scientists to investigate rapidly moving or targeted phenomenon.
GAUS Facility Description
The GAUS rawinsonde system, typically housed in a towable trailer (see Mobile GAUS below), includes equipment to conduct atmospheric soundings and make supporting surface meteorological observations. For GAUS windfinding, a Vaisala RS 92G radiosonde is used to obtain thermodynamic measurements and GPS winds. Sounding thermodynamic data measured by the radiosonde are processed on site by the NCAR-designed ground station. Surface data are collected from surface instruments connected to a Campbell CR-10 datalogger.
The standard GAUS data system consists of a power supply, a USB to RS-232 port expander, a 403 Mhz receiver, and a personal computer. Information is transferred to and from the GAUS personal computer through a USB connection. The port expander allows data access from the essential components of the GAUS system, i.e. receiver, surface met, and... to process and display the atmospheric soundings.
The GAUS personal computer operates in Windows XP.
System software is based on the development system LabVIEW from National Insruments. This software system acquires, processes and displays sounding data in both graphic and text formats. Data is stored in raw form for post processing and archiving.
The latest version of the NCAR sounding post-processing and display software, Aspen, can process GAUS sounding data. Here are examples of Aspen display output from a GAUS (GPS) sounding during CASES-99: Winds Plot and Skew-T Plot.
GAUS Radiosonde Deployment
Before a sonde is launched, it is run through a pre-flight procedure in which the telemetry is verified and radionavigation signals are acquired. Thermodynamic data - pressure, temperature, and humidity - are checked and as soon as a sufficient number of GPS satellites are acquired, the sonde can be launched. It is important to maintain clear sky view and vertical orientation to ensure the sonde GPS receiver detects the maximum number of GPS satellites available in that area at that time.
There are two launch configurations possible at any GAUS site. The balloon can be launched (1) from a "bag" launcher, a heavy vinyl tarp that contains and protects the balloon prior to launch (used in shipboard deployments); or (2) by hand when wind and other conditions allow.
Balloons of varying weight can be used with the radiosondes. Typically, a 200-gram balloon is filled with roughly 40 cubic feet of helium, which will take a Vaisala sonde to 50 or 60 mb before the balloon bursts. The ascent rate obtained with this amount of helium and a Vaisala sonde is about 4 m/s.
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 |
Radiosonde Sensor Specifications (specs)
Pressure Measurement (specs)
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.
RADIOSONDE:
Pressure Sensor Specifications |
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 (specs)
The temperature sensor is a capacitive bead in GAUS encapsulation. The temperature sensors are calibrated at the factory.
RADIOSONDE:
Temperature Sensor Specifications |
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 |
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.
Relative Humidity Measurement (specs)
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%).
RADIOSONDE:
Humidity Sensor Specifications |
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 |
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.
RADIOSONDE:
Wind and Position Measurement Specifications |
Manufacturer / Model # |
Vaisala |
Wind Accuracy |
0.5 m/s |
Averaging Time |
0.5 seconds |
Data System Resolution |
0.1 meter; 0.1 m/s |
Wind and Position Measurement (specs)
Winds derived from GPS have a constant accuracy once the minimum number of satellites (typically four) are received.
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.
Mobile GAUS
The Mobile GAUS system components are the same as the standard GAUS components. It is completely self-contained and is can be installed in a minivan, full sized van or pickup truck. The mobility gives the project planner the option to deploy to a specific site make a sounding and if required move to another site for the next sounding. The first sounding can be active and in the air while the vehicle is relocating, although this does affect sounding quality. Sounding site station elevation values are typically taken from a topographic map. If that is not available or if the location is not absolutely certain, a GPS altitude solution is used.
Data communications from the truck to a central operations center can be managed in one of three ways. 1) Data can be transmitted using a cellular phone, 2) it can be sent using a packet radio communication system, or 3) it can be sent using a satellite internet system.
Mobile GAUS Surface Instrumentation Specifications
The Mobile GAUS uses the standard GAUS surface instrument package. The configuration of the Mobile GAUS surface instrument package is designed to minimize effects of truck itself on those measurements. Those aspects of the instrumentation which are unique to the Mobile GAUS facility are described in the following sections.
Mobile GAUS Surface Temperature and Humidity Measurement
The entire temperature and humidity sensor, attached to the end of a cross-arm, is mounted on a lightweight moveable tripod. The tripod can be placed up to 15 meters from the truck to remove the measurement from any influence of the truck itself. The cable is deployed as the tripod is positioned for operation.
Mobile GAUS Surface Wind Measurement
The propeller windvane (R.M. Young 05103 Wind Monitor) is mounted on a telescoping pole on the surface met tripod mentioned above. In its fully raised position, the windvane is approximately 6 meters above the ground.
The tripod is positioned using a compass for accruate orientation.
Radiation Measurements
Although rarely required for Mobile GAUS operations, radiation measurements can be made. The data collection system has space for a radiation sensor (shortwave incoming only) and can be configured like fixed GAUS to include radiation measurements.
Mobile GAUS Radiosonde Deployment
The Mobile GAUS has everything required for a successful radiosonde launch. Helium for balloon filling is stored in the vehicle and includes a pressure regulator that easily attaches to a helium tank for balloon inflation. The vehicle holds a maximum of three bottles of helium, which is enough for about 15 releases. Sondes and balloons are stored inside the vehicle.
A radiosonde is released from Mobile GAUS in one of two methods. If the winds are calm a balloon can be inflated at the back of the vehicle and tied off with little chance of damage. If the winds get a little stronger the "bare" balloon technique can still be used if the operator fills the balloon just before release and then uses his body to protect the balloon before release. When the wind and weather get too dramatic a bag launcher is used. In this mode, the balloon is inflated while protected and held secure by a heavy vinyl material (the bag) before release. |