In Loran-C based windfinding, a Vaisala RS 80-15 L radiosonde is used with an Advanced Navigations Inc. (ANI) Model 7000 Loran-C navigator. In Omega based windfinding, a Vaisala RS 80-15N radiosonde is used with a Trimble Mini Omega (TMO) Model 8402 Omega navigator. Each of these navigators have modifications built in to NCAR's specifications. Sounding thermodynamic data measured by the radiosonde are processed using a Vaisala PP-11 or an NCAR RS-80 "Met" processor. Surface data are collected from surface instruments which are connected to a Campbell CR-10 datalogger.
The Vaisala radiosonde contains either a Loran or an Omega receiver, a 403 MHz band transmitter, and pressure, temperature, and relative humidity sensors multiplexed to an oscillator which generates a tone that is transmitted to the surface receiver. The thermodynamic data are transmitted from the sonde roughly every 1.5 seconds. Loran or Omega radionavigation signals are received by the sonde and re-transmitted to the surface station. (The sonde does not perform any signal processing on the navaid signals.) Both signals, the thermodynamic data and the radionavigation signals, are transmitted to the surface on separate subcarriers of the 403 MHz band transmitter. Upon reception, the two signals are separated and directed to their respective processors. The thermodynamic data are transferred to the NCAR RS-80 "Met" processor while the radionavigation signals are sent to the appropriate navigator.
The standard CLASS surface installation consists of a power supply, an RS-232 Multiplexer, a rack controller, a 403 Mhz receiver, a navigation data processor, a meteorological data processor, and a CLASS system personal computer. Information is transferred to and from the CLASS system personal computer through RS-232 connections using the MUX mentioned above. The MUX switches between the navigator, the "Met" processor and the Campbell datalogger to gather the data required to process and display the atmospheric soundings.
The CLASS system personal computer operating system is DOS with the software written in HTBasic. The Basic interpreter is written by TransERA Corp. and is licensed by NCAR. The CLASS system personal computer provides sounding data displays in real time. The displays are both graphical and text based. The data are stored to the hard disk in binary files which contain the information required to "recreate" the flight. These data files are saved and used in subsequent post-processing.
There are differing launch configurations possible at any CLASS site. There can be an enclosed air conditioned launcher or a "bag" launcher can be used. A "bag" launcher is a heavy vinyl tarp that contains and protects the balloon prior to launch. It is typically used on shipboard CLASS installations. In addition, a site can function without a launcher, in which case, the balloon and sonde are tied outside the CLASS trailer and then released.
Varying weight balloons are used with the radiosondes. A 200 gram balloon filled with roughly 40 cubic feet of helium 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 on the order of 4.5 m/s.
The sonde specifications are tabulated below.
TABLE 7 Radiosonde Specifications
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Manufacturer - type Vaisala RS 80-15 L or N Navaid Sonde
Mass 300 grams (with wet battery)
Dimensions 6cm X 9cm X 15cm
Ascent Rate 5 m/s avg
Transmitter Frequency 403.5 MHz
Transmitter Power 300 mW
Pressure Sensor Capacitive aneroid
Temperature Sensor Capacitive bead
Humidity Sensor HUMICAP thin film capacitor
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TABLE 8 Radiosonde Pressure Sensor Specifications
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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
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TABLE 9 Radiosonde Temperature Sensor Specifications
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Manufacturer Vaisala
Sensor Capacitive bead
Range -90 C to 60 C
Accuracy 0.2 C
Data System Resolution 0.1 C
Sensor Resolution 0.1 C
Time Constant 2.5 seconds @ 6m/s flow and 1000 mb
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The manufacturer's specification for the time constant is 2.5 seconds. 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 can result in a temperature reading that is too high. This might produce a false near-surface super-adiabatic lapse rate. Due to the small thermal mass of the temperature sensor and its supportive structure this effect is not long lived. The thermal time constant of the sensor structure is on the order of 2.5 seconds and thus the problem goes away within the first ten seconds after launch (adequate sensor ventilation).
TABLE 10 Radiosonde Humidity Sensor Specifications
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Manufacturer Vaisala
Sensor HUMICAP thin film capacitor
Range 0 to 100% Relative Humidity
Accuracy 2.0% Relative Humidity
Data System Resolution 0.1% Relative Humidity
Time Constant 1.0 second @ 6m/s flow, 1000mb, 20 C
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Heating of the sonde temperature/humidity sensor arm prior to launch (during sunny daytime launches) 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 (convective cooling) of about 10 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.
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TABLE 11 Wind and Position Measurement Specifications ---------------------------------------------------------------------------------- Manufacturer / Model # Advanced Navigation Inc.In Loran-C windfinding (Vaisala RS 80-15 L radiosonde), the ANI 700 navigator processes available Loran-C signals relayed from the sonde, automatically acquiring the stations to be tracked for a given geographic location. Reception of at least three Loran stations is required for position and wind calculation. The ANI 7000 outputs an ASCII status message containing field strength, signal to noise ratio, and signal time of arrival (TOA) information for each of up to eight stations being tracked.
Trimble Mini Omega (TMO)
Model #7000 (ANI 7000) Model #8402 Wind Accuracy 1.0 m/s 2.0 m/s Averaging Time 60 seconds 240 seconds Data System Resolution 0.1 meter; 0.1 m/s 0.1 meter; 0.1 m/s Absolute Position Accuracy 200 meters 2000 meters Differential Position Accuracy 20 meters 200 meters ----------------------------------------------------------------------------------
In Omega windfinding (Vaisala RS 80-15 N radiosonde), the TMO Model 8402 navigator processes available Omega signals relayed from the sonde to the surface. The Trimble navigator selects the strongest Omega signal as the "Master Station". It then detects and outputs phase differences between that "Master Station" and each of the other Omega signals received. (Note that phase data output for the "Master Station" are always zero.) A minimum of three stations is required for wind and position determination. If there are not enough stations for position and wind determination, no data are output.
It is important to note that the winds obtained, in real time and in the final data product, are those that would be obtained if the navigator at the surface were connected directly to the radiosonde (it is connected, via the 403 MHz band telemetry link).
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 CLASS system 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.
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 calibrated aircraft pressure altimeter can be used.
The Mobile CLASS system components are the same as the standard CLASS components. As with the CLASS system, the facility can process windfinding based on either Loran-C or Omega navigation signals. Because the van is used mainly in the contiguous United States, the majority of soundings are made using Loran-C windfinding. In this configuration, a Vaisala RS 80-15 L radiosonde is used with an Advanced Navigations Inc. (ANI) Model 7000 Loran-C navigator. Sounding thermodynamic data measured by the radiosonde are processed using either a Vaisala PP-11 or an NCAR RS-80 "Met" processor. Surface data are collected from surface instruments which are connected to a Campbell CR-10 datalogger.
Data communications from the van to a central operations center can be managed in one of two ways. Data can be transmitted using a cellular phone or it can be sent using a packet radio communication system. Partial messages or pseudo real time data can be sent during the sounding using a second computer and the cellular phone.
A radiosonde is released from the van in one of two methods. If the winds are calm a balloon can be inflated at the back of the van 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.
Ned Chamberlain
<chamber@ucar.edu>
Last modified: Wed Sep 11 15:01:16 1996