2.0 Cross-chain Loran Atmospheric Sounding System (CLASS)

2.1 CLASS Sounding System Description

The CLASS system is a sounding system which includes equipment to make supporting surface meteorological observations. The CLASS facility, although named for its use with cross-chain Loran-C navigation signals, can utilize either Loran-C or Omega navigation information for windfinding.

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.

2.2 CLASS Radiosonde Deployment

Before a sonde is launched, it is run through a pre-flight procedure. At this point, the radiosonde telemetry is checked and radionavigation signals are acquired. Thermodynamic data - pressure, temperature, and humidity - are checked and as soon as a sufficient number of Loran or Omega stations are acquired, the sonde can be launched. During the pre-flight phase the navaid signals are received through an independent whip antenna attached to the sonde. The sonde "Nav" antenna is wrapped around a reel-out device and cannot be used for signal reception prior to launch. At launch, a switch is thrown on the equipment rack which changes the Loran-C source from the independent whip antenna to the sonde antenna as it reels out. This assures the best possible Loran-C reception for windfinding near the ground.

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
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           

2.3 Radiosonde Measurement Specifications

2.3.1 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.

TABLE  8  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              

2.3.2 Temperature Measurement

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

TABLE  9  Radiosonde Temperature Sensor Specifications
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  
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).

2.3.3 Relative Humidity Measurement

The humidity sensor is a thin film capacitive type sensor. It has the registered trademark HUMICAP. The new Vaisala type "H" radiosonde utilizes the new type "H" "humicap" and a new humidity algorithm incorporating temperature compensation. This new sensor and algorithm have resulted in improved humidity measurement, particularly in the high end of the humidity range (95% to 100%). Table 10 summarizes the humidity sensor specifications.

TABLE  10  Radiosonde Humidity Sensor Specifications
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  
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.

2.3.4 Wind and Position Measurement

The wind accuracy obtained from the Navaid is dependent on a number of factors which in general relate to the quality of coverage for a given area. The number of stations received (three is minimum), the strength of signal, and the geometry of the receiver with respect to the stations are all important factors. Specifications for Loran and Omega wind and position measurements are presented in Table 11.


TABLE  11  Wind and Position Measurement Specifications
Manufacturer / Model #          Advanced Navigation Inc.
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 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.

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).

2.4 CLASS Surface Measurement Instrumentation Specifications

Retrieval of accurate surface meteorological data is an integral part of the CLASS 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 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.

2.4.1 CLASS 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. They are both 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.

2.4.2 CLASS 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 RM Young aspirated radiation shield model number 43-408.

2.4.3 CLASS 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.

2.5 Mobile CLASS

2.5.1 General Description

The NCAR Mobile CLASS facility is a CLASS system completely self-contained in a van. The basic system is mounted in a rack inside a passenger van with all the hardware required to make atmospheric soundings, including equipment to make supporting surface meteorological observations. The van 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 still be active and in the air while the van is relocated.

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.

2.5.2 Mobile CLASS Surface Measurement Instrumentation Specifications

The Mobile CLASS uses the standard CLASS surface instrument package. The configuration of the Mobile CLASS surface instrument package is designed to minimize effects of van itself on those measurements. Those aspects which are unique to the Mobile CLASS facility are described in the following sections. Mobile CLASS 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 van to remove the measurement from any influence of the van itself. The sensors are wired through a 17 meter cable that is wound on a spring loaded reel for cable retraction. The cable is pulled out as the tripod is positioned for operation. The cable is easily retracted by pulling on the cable which allows the spring loaded reel to wind it in. Mobile CLASS Surface Wind Measurement
The propeller windvane (R.M. Young 05103 Wind Monitor) is mounted on a telescoping pole on the roof of the van. The pole is raised from inside the van and locked into position with a key pin. In its fully raised position the windvane is approximately 6 meters above the ground. The direction is fixed as if north were the front of the van. A "North Seeker", a magnetically driven potentiometer, is included in the system which reports magnetic north in relation to the front of the van. With magnetic declination included the true wind direction can be calculated regardless of van position. Radiation Measurements
Although rarely required for Mobile CLASS operations, radiation measurements can be made. The data collection system has space for radiation sensors and can be configured like fixed CLASS to include radiation measurements. Mobile CLASS Radiosonde Deployment
The Mobile CLASS van has everything required for a successful radiosonde launch. Helium for balloon filling is stored under a platform at the back of the van. A pressure regulator stored in the van easily attaches to a helium tank for balloon inflation. The van holds three bottles of helium which is enough for about 15 releases. Sondes and balloons are stored inside the van.

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.

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Ned Chamberlain <chamber@ucar.edu>
Last modified: Wed Sep 11 15:01:16 1996