RADIOSONDES

W F Dabberdt and R Shellhorn, Vaisala Inc., Boulder,CO, USA

H Cole, National Center for Atmospheric Research,Boulder, CO, USA

A Paukkunen, J Horhammer and V Antikainen,Vaisala Oyj, Helsinki, Finland

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Dabberdt, W FVaisala Inc.,Boulder, CO, USA

Shellhorn, RVaisala Inc.,Boulder, CO, USA

Cole, HNational Center for Atmospheric Research,Boulder, CO, USA

Paukkunen, AVaisala Oyj,Helsinki, Finland

Horhammer, JVaisala Oyj,Helsinki, Finland

Antikainen, VVaisala Oyj,Helsinki, Finland

Introduction

The radiosonde is an expendable, balloon-bornedevice that measures the vertical profile of meteoro-

logical variables and transmits the data to a ground-based receiving and processing station. These profilesare typically obtained twice each day and are the coreof the global weather observing system that providesinputs to numerical forecast models. The sensorpackage routinely measures the variation with altitudeof temperature, humidity, and pressure as the balloonascends from the land or ocean surface to heights up toabout 30 km (a pressure altitude of about 11 hecto-pascals, hPa). When the device also measures winds, itis more properly called a rawinsonde, although theterm radiosonde is commonly applied to both. Theheight profile of these meteorological variables con-stitutes an upper-air sounding that is known as aradiosonde observation or RAOB. In some cases, aballoon without a radiosonde is tracked by eitheroptical or radar techniques in order to measure onlywinds. This type of balloon is known as a pilot balloonor simply a pibal, but it is not a radiosonde.

In 1999, there were 100 operational or synopticradiosonde stations in the United States; they made adaily average of 182 soundings. In the continental US,an average distance of 315 km separates radiosondestations. In 1999, there were 992 radiosonde stationsworldwide (Figure 1) that made an average of 1209soundings each day in support of weather forecastactivities, while an additional 65 pibal stations made576 wind soundings daily. Additional soundings aremade for specialized purposes of which defenseapplications are the most significant. The globalnumbers of RAOB and pibal soundings are downconsiderably from their peak daily values in 1988 of1660 and 964, respectively. The approximately half amillion radiosondes used annually are manufactured

80

60

40

20

0

− 20

− 40

− 60

− 80

− 150 − 100 − 50 100 150 500

0344-F0001 Figure 1 Global radiosonde station network.

0344-P0005

0344-P0010

RADIOSONDES 1

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

by less than 10 companies worldwide. Of these, theVaisala company headquartered in Helsinki manufac-tures about 70% of the global supply of radiosondes.Vaisala was founded in 1936 by Professor VilhoVaisala, who in 1931 invented one of the world’s firstradiosondes (see Appendix).

Since 1957 all stations have made their soundings atthe same times, 00.00 and 12.00 UTC, although manystations outside the US and Europe have reducedsoundings to one per day because of budgetaryconstraints. Countries launching operational radio-sondes are members of the World MeteorologicalOrganization’s World Weather Watch program; assuch, they freely share their sounding data with eachother. Shortly after an operational upper-air soundingis completed, a standard data message is prepared andmade available to all nations using the GlobalTelecommunications System. These TEMP messagesare transmitted in a universal format that reportsmeteorological conditions at various standard or so-called mandatory (pressure) levels as well as atsignificant levels, which represent levels where pre-scribed changes in meteorological conditions occur.

There are two primary purposes of upper-airsoundings: to analyze and describe current weatherpatterns, and to provide inputs to short- and medium-range computer-based weather forecast models. Onevery important, specialized use of atmospheric sound-ings is in support of forecasting hurricane movement.Special radiosondes called dropwindsondes arelaunched from weather reconnaissance aircraft toobserve atmospheric structure in the core of thehurricane as well as in the area downwind of thestorm itself. These dropwindsonde measurementswere the single most important factor in a 20%increase in hurricane forecast accuracy over thedecade of the 1990s. Other uses of radiosonde datainclude climate studies, air pollution investigations,aviation operations, and defense applications. Theradiosonde continues to be the backbone of an eclecticsuite of measurement technologies (measurementsboth remote and in situ that are made from ground-based, airborne, and satellite platforms) used toprovide data for input to numerical weather forecastmodels.

Radiosonde Operations

The radiosonde is carried aloft by a balloon as part of aflight train (Figure 2). The balloon itself may be madeof either natural rubber (latex) or synthetic rubber(neoprene). The mass of the flight train, the desiredascent rate, the type of gas used, and the maximumheight of the sounding determine the size of the

balloon. Operational radiosonde systems typically useballoons that weigh anywhere from 300 to 1200 g;they are filled to ensure an ascent rate of 300 m min�1.Hydrogen is the gas most commonly used to inflate theballoon and provide its lifting capacity, althoughhelium and natural gas are sometimes used for specialapplications. The flight train consists of five compo-nents: (1) the balloon; (2) a parachute to bring theradiosonde safely back to Earth after the balloonbursts; (3) 20–60 m of nylon separation line thatisolates the radiosonde’s sensors from water vapor andthermal contamination by the balloon; (4) a dereeler tolet out the nylon line after launch; and (5) theradiosonde itself. A few countries such as the US andSwitzerland actively seek to recover and then reusetheir radiosondes. In the US, it is estimated that about18% are reused after extensive refurbishment, while in

Balloon

Parachute

Hanger board

Unwinder

Radiosonde

60 m string

0344-F0002Figure 2 Typical radiosonde flight train, including balloon,

parachute and hanger board, unwinder mechanism, separation

line, and radiosonde.

0344-P0015

0344-P0020

0344-P0025

2 RADIOSONDES

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

Switzerland, more than 60% are recovered andreused.

Components of the ModernRadiosonde

The radiosonde is an electronics unit that comprisesthree major sections: a suite of sophisticated meteor-ological sensors; signal-processing electronics; and aradio transmitter to relay the measurements back to areceiver at the radiosonde launch station. The mete-orological measurements are made at intervals thatvary from 1 to 6 s, depending on the type andmanufacturer of the radiosonde. The meteorologicalcommunity has been assigned two radio frequencybands for use in transmitting meteorological data:400–406 MHz and 1675–1700 MHz. These bands areunder continuing pressure from the telecommunica-tions industry, which seeks to use them for commercialpurposes. All of the world’s radiosondes are requiredto meet certain performance standards that have beenestablished by the WMO (see Table 1). Figure 3illustrates four different radiosondes currently in usearound the world.

Overview of Thermodynamic Sensors

Thermodynamic sensor types vary widely amongradiosondes currently in use throughout the world.Temperature sensors are of four designs: capacitancesensors, thermistors, resistance wires, and bimetallicelements. The two common humidity elements arecarbon hygristors and planar thin-film capacitancesensors, although gold-beater’s skin is still used inRussia and China. Pressure measurements are typi-cally made with either an aneroid cell or a piezore-sistance element. There are about a dozen different

radiosonde designs presently in use. As radiosondeshave become more advanced, their changes have alsocreated special challenges to climatologists seeking topiece together a consistent and homogeneous multi-decadal global database to analyze and understandclimate change. As a result, climate researchers mustaccount for biases in the historical records due tochanges in instrumentation and observing methods,many of which have poor or no documentation. In theUnited States alone, these changes have been variedand significant. Four distinctly different humiditysensors have been in use since 1943. Temperaturemeasurements have undergone major changes, includ-ing sensor type, size, and coating, exposure to the airstream, and corrections to account for radiationbiases. At present, the US National Weather Serviceuses radiosondes from two different manufacturers,each having its own distinct set of pressure, temper-ature, and humidity sensors. The Vaisala companyproduces about 70% of the world’s radiosondes, andadded emphasis is given below to aspects of the designof its radiosondes and sensors.

Thermodynamic Sensors

Sensors used with Vaisala radiosondes are all of thecapacitance type. Changes in pressure, temperature,and humidity result in changes in the capacitanceinformation from each sensor, which in turn ischanged to a frequency signal by using sensor trans-ducer electronics. Sensor frequency measurements arecompared with the frequencies of reference capaci-tance transducers, and these in turn are converted tophysical measurements based on factory calibrationmeasurements. In the case of pressure, the distancebetween capacitance plates changes as atmosphericpressure changes, causing a change in the measuredcapacitance. Older pressure sensors use an aneroid or

0344-T0001 Table 1 Accuracy requirements (expressed as standard error) for upper-air measurements for synoptic meteorology

Variable Range Accuracy requirement

Pressure Surface to 5 hPa 71 hPa

Temperature Surface to 100 hPa 70.5 K

100 to 5 hPa 71 K

Relative humidity Troposphere 75% (RH)

Wind direction Surface to 100 hPa 751 for wind speed o15 m/s

72.51 for wind speed 415 m/s

100 to 5 hPa 751

Wind speed Surface to 100 hPa 71 m/s

100 to 5 hPa 72 m/s

Geopotential height

of significant

levels

Surface to 100 hPa 71% near the surface decreasing to

70.5% at 100 hPa

Source: World Meteorological Organization (1996) Guide to Meteorological Instrumentsand Methodsof Observation, 6th edn. Publication

No. 8. Geneva: WMO.

0344-P0030

0344-P0035

0344-P0040

RADIOSONDES 3

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

bellows-type sensor that responds mechanically topressure changes. Modern pressure transducers arevery small silicon, micromechanical sensors. Pressuresensors also have a temperature dependence that iscompensated by factory calibration of the sensor. Thetemperature change of capacitive sensors is measuredby the change in the dielectric constant of the sensor.Older capacitive sensors consisted of a sensor that washermetically packed inside glass. Newer capacitivetemperature sensors are extremely small and fast,owing to a special twin-wire construction. Essentialaspects of modern temperature and humidity sensors(and their supporting members, i.e. the sensor boom asseen in Figure 4) are their different coatings andtreatments to minimize solar heating and improvewater repellency. The approach used to measurehumidity in all Vaisala radiosondes is also based onchanges in the dielectric constant. The humidity-sensing technology is based on so-called thin films. Thedielectric material is a very thin layer of a specialproprietary polymer that has an optimum combina-tion of measurement properties, including stability,repeatability, hysteresis, response time, and tempera-ture dependence. Thin-film humidity sensors arecalibrated to provide output in terms of percentrelative humidity with respect to water; the tempera-ture dependence is compensated by use of tempera-ture-dependent calibration coefficients determinedfrom factory calibration tests. Some Vaisala humidityprobes incorporate two sensor elements that includeheating of the sensor elements to minimize affects ofwater condensing on the sensors as the radiosonde

moves from warm to cold layers during its ascent. Thetwo sensors are alternately heated in sequence, and themeasurement is taken from the passive sensor. Thesensors are very small and designed for fast response.

The accuracy of radiosonde data is a combination ofmultiple factors: sensor performance; related trans-ducer electronics; mechanical construction of thesonde and sensor housing; sensor and sensor-boomcoatings and treatments; calibration technology; andcalibration and correction algorithms. In addition to

RRS100RRadiosonde((China)

Mesei RS-01GRadiosonde(Japan)

Vaisala RS80Radiosonde(Finland)

Sippican Mark IIMicrosonde(USA)

0344-F0003 Figure 3 Examples of Radiosondes in Current Use Around the World.

150 mm

50 mm

90 mm

0344-F0004Figure 4 Vaisala RS90 Radiosonde with sensor boom.

0344-P0045

4 RADIOSONDES

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

issues of radiosonde performance, the uncertainty ofupper-air measurements includes sampling considera-tions, such as the density of the observation network,time interval between observations, and the homoge-neity of the atmosphere. Together, these instrumentaland environmental factors govern the accuracy andrepresentativity of the observations.

Specialized Radiosonde Sensors

Some radiosonde manufacturers offer optional sen-sors to make supplemental environmental measure-ments. Additional electronics are used to interface thesupplemental sensors to the radiosonde. Measure-ments of ozone concentration and radioactivity are thetwo most common supplemental measurements. Ra-diosonde measurements of ozone are made world-wide, although at fewer stations and typically onlyonce per day or less often. The most commonradiosonde ozone sensor is the electrochemical type,while radioactivity is typically measured with Geiger–Muller tubes. Other supplemental measurements inuse today include dew point, optical backscattering byfine particles, electric field, and video imaging ofparticles and hydrometeors. Most advanced radio-sonde ground systems effectively support both synop-tic and research users, and offer options for post-ascent data calculation and analysis of supplementalmeasurements.

The Vaisala ozonesonde consists of an electrochem-ical ozone sensor connected to an interface unit and amodified radiosonde. Consequently, humidity, pres-sure, temperature, and geopotential height can bemeasured simultaneously with ozone sampling. Up-per-air winds are also measured. This lightweight,balloon-borne instrument is capable of measuring thevertical distribution of atmospheric ozone up to 3 hPa.The uncertainty of the ozone measurement is of order5–10% of the local values. The electrochemicalconcentration cell (ECC) ozone sensor detects ozoneon the basis of an iodine–iodide oxidation-reductionor redox electrode reaction in neutral buffered solu-tion. The sensor consists of an electrochemical con-centration cell that contains two platinum electrodesimmersed in separate potassium iodide solutions ofdifferent concentrations, which are separate anodeand cathode chambers. The chambers are linked withan ion bridge. As air containing ozone flows into thecathode solution, a chemical reaction occurs and theplatinum electrodes carry electrons between the cellsof the sensor. An electrical current is generated inproportion to the rate at which ozone enters the cell.The ozone concentration is determined from theelectric current measurement using an equation thatconsiders the airflow rate, air pressure, and pump

temperature. The interface can also be used with othersensor types, such as the Brewer–Mast sensor. TheBrewer–Mast sensor uses similar ozone detectionreaction, but instead of a reference chamber, thedriving potential for the measurement circuit is anelectrical circuit. The ECC-type sensor is more accu-rate and is more widely used.

The Vaisala radioactivity sonde is a combination ofa radioactivity sensor and a modified radiosonde. Theradioactivity sonde can measure the vertical profile ofradioactivity in the troposphere and in the lowerstratosphere, up to altitudes of 40 km. The radioac-tivity sensor measures radiation with two Geiger–Muller detectors – ionization chambers filled withspecial gas mixtures. One detector is sensitive only togamma radiation, while the other measures bothgamma and beta radiation. This way it is possible tomake measurements of both gamma and beta radia-tion. The detectors have pulse outputs; the count rateis proportional to the radiation intensity and is read atfixed time intervals. The measurement accuracy isabout 710%.

Overview of Windfinding

There are several techniques for measuring winds withonly a balloon or with a combination balloon andradiosonde. When a radiosonde measures winds it iscalled a radio-wind-sonde or rawinsonde. Raw-insonde windfinding methods vary widely. In all cases,the winds are determined by observing the drift of theballoon. One class of wind measurement techniquestracks the balloon externally using one of threemethods: (1) optical systems use a theodolite tovisually track the balloon’s azimuth and elevation;(2) radio theodolites track a radio signal sent from atransmitter on the radiosonde, again to obtain azi-muth and elevation information; and (3) radar systemstrack a radar retroreflector suspended from theballoon to obtain slant range, azimuth, and elevation.The second class of wind measurement techniques usesvarious navigation systems. Two such systems cur-rently in use employ the LORAN-C navigation systemand various VLF systems, such as the Russian ALPHAsystem and the US Navy’s VLF system. A newnavigation-based windfinding technique is now com-ing into widespread usage. A receiver inside theradiosonde accurately measures the horizontal andvertical Doppler velocity of the radiosonde withrespect to those Global Positioning System (GPS)satellites that can be observed at any given time(typically, four to eight satellites). Other types of GPSreceivers also observe the latitude, longitude, andaltitude of the radiosonde. In both cases, the GPS

0344-P0050

0344-P0055

0344-P0060

0344-P0065

RADIOSONDES 5

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

receiver measures directly the drift velocity of theballoon and hence the wind. Two major advantages ofthe GPS-based techniques are the high accuracy andprecision of the wind measurements, and the world-wide coverage of GPS.

Tracking Techniques

Optical tracking methods One of the earliest meth-ods for determining the winds aloft was to visually oroptically track small balloons, called pilot balloons(pibals). This method was developed in the mid-1870susing a small expendable balloon tracked with a smalltelescope. The small optical device, similar to asurveyor’s transit, is called a theodolite and canaccurately measure elevation and azimuth angles. Ifthe balloon’s height can be determined then itsposition can be found by trigonometry. There arebasically two pilot balloon techniques still in use: (1)single-theodolite and (2) double-theodolite. In theformer, the elevation and azimuth angles of theballoon are measured at regular intervals (typicallyonce per minute). Balloon altitude is determined byassuming a constant ascent rate that is determinedfrom the size and free lift of the balloon. Balloonposition is then calculated from the height and theazimuth and elevation measurements. Tracking theballoon during a nighttime observation is accom-plished by attaching a light stick or small battery-powered light. In the double-theodolite technique twotheodolites are located a known distance apart (thebaseline) and simultaneous observations taken of theballoon at given time intervals. By measuring theazimuth and elevation angles to the balloon from thetwo known positions, the three-dimensional balloonposition can be determined by the law of sines. Thedouble-theodolite method enables accurate measure-ments of the balloon position without assuming aconstant rate of ascent for the balloon, which can be asource of error. In this method the baseline distanceneeds to be accurately measured and should be at leastone-fifth of the maximum range to the balloon. Thebaseline should also be perpendicular to the prevailingwinds. The method is not routinely used because ofbaseline restrictions and the cost and difficulty ofcoordinating two sets of observers.

Radiotheodolite and radar methods Another track-ing technique used for determining winds is calledradio direction finding or RDF. During World War IIthe US Army Signal Corps developed the first RDFsystem, called the SCR-658. This system operated at400 MHz and used two separate operators to steer alarge antenna array to determine the direction of theradiosonde transmitter. A more modern radio direc-

tion finding antenna automatically tracks the1680 MHz telemetry signal transmitted from theradiosonde. The antenna azimuth and elevation dataare sent to a computer at the ground station along withthe pressure height data from the radiosonde todetermine the change in radiosonde position (winds)during flight. The RDF technique (Figure 5) is theradio frequency equivalent of the optical theodolitemethod, and the tracking system is called a radiothe-odolite. There are different types of RDF antennas,including 2–3 m diameter dish antennas and phased-array flat-plate antennas. RDF systems can resolve theazimuth and elevation angles to within 0.051. If theupper-level winds are high then the radiosonde will bea long distance away, resulting in the antenna eleva-tion angle being near the horizon. At stations thatexperience high winds, the radiosondes can beequipped with a transponder to measure slant rangeor distance to the radiosonde. Winds can then bedetermined using azimuth, elevation, slant range, andheight of the radiosonde. A similar method fortracking the radiosonde uses a radar reflector on theballoon flight train so that it can be tracked bywindfinding radar. Slant range to the radiosonde ismeasured by the radar as well as azimuth and elevationangles. Radar windfinding is a common method usedin many countries around the world; in 1998 about45% of the stations used radar, as tallied in Oakley(1998).

In the Russian and Chinese upper-air networks acombination RDF-transponder method is used, calledsecondary radar. Some 200 such systems are deployedworldwide. The parabolic or array-type RDF antennatransmits a short pulse that is received by the radio-sonde. The radiosonde then ‘wakes up’ and retrans-

Free-floatingsensor

Vertical

North

Azimuth

Slant rangeHeight

Elevation

Radar,radiotheodolite, etc.

0344-F0005Figure 5 Angle-dependent tracking system.

0344-P0070

0344-P0075

0344-P0080

6 RADIOSONDES

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

mits the pulse by transmitting the temperature andhumidity data, which are received by the ground-based RDF antenna. The RDF antenna azimuth andelevation angles are measured and the slant range isdetermined from the travel time of the pulse. Second-ary radar systems use radiosondes that do not have apressure sensor; pressure is calculated from thehydrostatic equation.

Navigation aids (NAVAIDS) The use of navigationaids for obtaining upper-air winds from radiosondesbegan in the early 1960s. The US Weather Bureau(now the National Weather Service of the NationalOceanic and Atmospheric Administration, an agencyof the US Department of Commerce) sought to find away to measure winds at sea for the Ships ofOpportunity Program. At that time the only way tomeasure winds aloft at sea was with a radar or RDFsystem; both systems were costly and required amechanical stabilization system for the trackingantenna. In 1964, the bureau awarded a contract toBeuker’s Laboratory Inc. (BLI) of New York todevelop a windfinding system using retransmittedLoran-C navigation signals to track the radiosonde.The technique proved successful. Owing to the limitedcoverage of Loran-C, two years later the worldwideOmega navigation system was proposed as an alter-native for windfinding. Radiosondes that use theseNAVAID signals to determine winds contain a small,inexpensive radio receiver to receive the navigationsignals from fixed ground stations. The radiosondethen retransmits (Figure 6, usually at 400 MHz) thesignals to the data processing system at the groundstation. There are at present three types of NAVAIDsignals in use: (1) Loran-C, (2) very low-frequency

(VLF) systems, and the (3) Global Positioning System(GPS).

Loran-C coverage has increased since 1964, butbecause its primary use is for coastal navigation it doesnot provide worldwide coverage. Loran-C stationstransmit a unique series of pulses at 100 kHz thatidentify each station. If the radiosonde receives andretransmits signals from at least three stations, thenthe data processing system at the ground station candetermine the time of arrival of those signals at theradiosonde and its distance from each ground station.Winds are determined from the change in position ofthe radiosonde.

The other class of ground-transmitting navigationsystems is the VLF systems. These operate in the 10–30 kHz frequency range and their long-wavelengthsignals are characterized by low attenuation and theability to propagate long distances; this allows world-wide coverage with a minimum number of groundtransmitters. The Omega navigation system was themost widely used VLF system for both navigation andwindfinding until it was closed down on 30 September1997 because of cost considerations and the emer-gence of more accurate GPS windfinding systems.However, other VLF stations operated by US andRussian defense agencies continue to operate. VLFwindfinding is similar to Loran-C, except for thedifference in radio frequency and the correspondingdecreased windfinding accuracy of VLF.

The third type of NAVAID windfinding system usessignals from the so-called Global Positioning System(GPS) satellites. GPS was conceived in the early 1970sfor the US Department of Defense (DOD), and isoperated by the US Air Force. The GPS system becamefully operational in late 1995. There are 24 satellites insix orbital planes spaced 60 degrees apart. Thesatellites are in a 20 200 km circular orbit, with aninclination angle of 551 and a periodicity of 12 hours.At any time or place in the world, there are 6 to 11 GPSsatellites 51 or more above the horizon and henceusable for GPS windfinding. There are two primaryGPS techniques for determining winds from radio-sondes. The GPS signals cannot be retransmitted fromthe radiosonde back to the ground because thebandwidth of the 1575 MHz (called the L1 band)GPS carrier signal is too wide (B2.0 MHz). Theworldwide civilian use of GPS has become so great thatmany manufacturers produce inexpensive, small GPSreceivers each the size of a credit card that can decodethe navigation message every second and produceaccurate three-dimensional position coordinates, aswell as speed and heading. A second, less expensivemethod uses a codeless receiver in the radiosonde thatmeasures only the Doppler shift of the carrier fre-quency. The Doppler shift has two components: (1) the

Free-floatingsensor

NAVAIDgrid

Receivingequipment

Radio link

0344-F0006 Figure 6 NAVAID retransmission system.

0344-P0085

0344-P0090

0344-P0095

0344-P0100

RADIOSONDES 7

rwas.2003.0344 23/9/02 17:18 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

Doppler shift due to the satellite motion (i.e., thelargest component), and (2) the Doppler shift due toradiosonde movement. The radiosonde receiver sendsthe Doppler information back to the ground datasystem. The ground data system must have a local GPSreceiver that can decode the GPS message and inde-pendently measure the Doppler shift from each satel-lite. The satellite Doppler shift is subtracted from theradiosonde Doppler shift and the difference yields theradiosonde motion.

Specialized Types of RadiosondeSystems

Dropsonde

The dropsonde is the airborne counterpart to theconventional radiosonde (sometimes also called anupsonde). Dropsondes are ejected from researchaircraft and float to earth on a special balloon-likeparachute. Current state-of-the-art dropsonde sensorsinclude capacitance fine-wire sensors to measuretemperature, capacitance silicon pressure sensors,and GPS receivers to measure winds. Humidity ismeasured with a pair of thin-film capacitance sensorsthat are heated alternately to avoid condensation ondescent from colder to warmer air. All measurementsare made twice every second, while the 400 g drops-onde falls at an initial rate of about 25 m s� 1 at 15 kmaltitude, decreasing to about 10 m s� 1 at sea level.Dropsonde data are transmitted by radio from thesonde to a data system in the aircraft. Atmosphericsoundings from dropsondes provide the ability tomeasure conditions over remote areas such as theoceans, polar regions, and sparsely inhabited land-

masses; they also provide a means to obtain soundingsin and around severe weather systems, such ashurricanes. Atmospheric soundings obtained fromdropsondes during hurricane reconnaissance flightshave improved the accuracy of forecasts of hurricanelandfall by about 20% over the decade of the 1990s.

Dropsondes were first developed in the 1960s forthe US Navy and Air Force for hurricane reconnais-sance and were an adaptation of radiosonde technol-ogy. These early dropsondes were heavy – about 2.5 kg– and did not have inherent windfinding capability;windfinding at that time still used only radar or RDF.With the development of Omega NAVAID windfind-ing technique for the radiosonde it became possible toincorporate that technology into the dropsonde. Thisoccurred in 1974 when the National Center forAtmospheric Research (NCAR) developed an OmegaDropwinsonde (ODW) for use in the Global Atmos-pheric Research Program’s Atlantic Tropical Experi-ment. In 1982, the Air Force adopted the ODW systemfor hurricane reconnaissance and this system was useduntil the early 1990s.

In 1985 NCAR began development of a smart (i.e.microprocessor-based), lightweight digital dropsondethat incorporated Loran and Omega windfinding. TheOmega version of this dropwinsonde was adopted bythe US Air Force in the early 1990s for its hurricanereconnaissance mission (Figure 7). The next majorimprovement in dropsonde technology occurred in1995 when NCAR completed development and test-ing of a new GPS dropsonde with codeless GPSwindfinding capability and an advanced aircraft datasystem (AVAPS). In 1996 NCAR licensed Vaisala Inc.of Woburn, Massachusetts, to commercialize produc-tion and sales of the GPS dropsonde (Figure 8) and

0344-F0007 Figure 7 USAF C-130 Hurricane Hunter launching a GPS dropsonde.

0344-P0105

0344-P0110

0344-P0115

8 RADIOSONDES

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

AVAPS. In the relatively short time the GPS dropsondehas been in use it has found research applications in thedetermination of hurricane structure and motion, thestudy of clear-air turbulence associated with upper-

level jet stream structure, and observing strategies formidlatitude weather forecasting. Current adaptationsof the GPS dropsonde technology are focusing onlaunches at higher altitudes – including the lowerstratosphere – as well as autonomous launches thateliminate the need for operators to launch the sondeand record the data, and offer the promise that it willbe possible one day to obtain operational drop-winsonde profiles from commercial aircraft. Figure 9shows a test dropsonde launcher mounted on theunderside of an ER-2 high-altitude weather researchaircraft.

Driftsonde System

Improvements in short- and medium-range synoptic-scale weather forecasts will depend on improvedupper-air soundings over the data-sparse regions ofthe Northern and Southern Hemispheres. Progresstowards this objective will require the optimal use ofexisting data sources, creative new observing meth-ods, and improved numerical methods for dataassimilation. The driftsonde system is being developedas a cost-effective sounding system that could fill thesecritical gaps in data coverage over oceanic and remotearctic and continental regions. The driftsonde conceptseeks to obtain a large number of high-vertical-resolution GPS dropsonde profiles through the lowerstratosphere and the entire troposphere by autono-mous launching of dropsondes from specially designedballoon platforms. The driftsonde system includes apolyethylene carrier balloon with an attached gondola(Figure 10) that carries a payload of up to 24 GPSdropsondes. The carrier balloon ascends to between50 and 100 hPa (20 and 16 km) and then drifts in theprevailing stratospheric westerlies for up to five days,

0344-F0008 Figure 8 GPS dropsonde descending on its parachute.

0344-F0009 Figure 9 Remote-controlled GPS dropsonde launcher system installed on the NASA ER-2 high-altitude weather research aircraft.

0344-P0120

RADIOSONDES 9

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

deploying dropsondes at prescribed and special timesover data-sparse regions of interest. The first applica-tion of the driftsonde system will be in support of TheHemispheric Observing System Research and Predict-ability Experiment (THORPEX), a five-to-ten-yearinternational program of atmospheric observing sys-tem research and development, and experimentationwith numerical forecast systems that will be conductedin the 2002–2010 time frame.

Automated Shipboard Aerological Program (ASAP)

The Automated Shipboard Aerological Program(ASAP) is a multinational effort initiated by Canadain 1982 to obtain upper-air soundings over the oceans.Omega NAVAID radiosondes are launched fromcommercial ships of opportunity using a speciallydesigned launch system (Figure 11) that permits flighttrains to be launched in high-wind conditions. Theupper-air sounding data from the radiosonde are sentback to the shipboard ASAP system where the data areprocessed in near real time to create a TEMP SHIPmessage. This message is the ocean equivalent of theTEMP message generated for land-based RAOBsystems. The ASAP system sends the message to aGOES geostationary satellite that relays the informa-tion to the Global Telecommunications System (GTS),which then transmits it to the numerical weatherprediction centers around the world.

The ASAP program had its beginning in June of1981, when Canada decided to discontinue its weathership program owing to the high costs of operating andmaintaining the ocean weather ship PAPA located inthe Gulf of Alaska at 501N, 1451W. The originalintent was to replace the weather ship data with

satellite observations; however, persistent cloudinessin areas such as the Gulf of Alaska and the NorthAtlantic, coupled with the lack of surface weatherdata, made this goal impossible to attain. To remedythis problem, the Atmospheric Environment Service(AES) of Environment Canada, the National WeatherService (NWS) of NOAA, and NCAR established ajoint ASAP project to develop a modular, mobile,moderately priced, upper-air sounding system. Thissystem, when placed on commercial vessels (ships-of-opportunity) routinely crossing the Pacific and Atlan-tic oceans, provides real-time upper-air soundings thatcomplement those of the global land-based upper-airnetwork.

The ASAP program operated by AES Canadastarted in the spring of 1982 with one commercialship (a Japanese automobile carrier) that operatedfrom Vancouver, British Columbia, to Japan. By 2001,it had evolved into an international program with 11countries operating 22 ASAP units – see Table 2. Since1994, the ASAP program has made about 5300 upper-air soundings per year. In 2001 a lengthy SouthernHemisphere route was inaugurated with port calls inGermany, the UK, South Africa, Australia (both eastand west coasts), New Zealand, and South America.

Rocketsonde

The rocketsonde is similar to a dropsonde except that arocket is used to carry the sonde to the desireddeployment altitude where the sonde is ejected andfloats to Earth on a small parachute. Two types ofrocketsondes are in use today, and are classifiedaccording to their maximum altitude. High-altituderocketsondes are used primarily by the military and

ATLANTIC OCEANEUROPE

NORTHAMERICA

~58,000 ft50−100 mb

OrbCommLEO satellite

Zero-pressureballoon

Gondola(24 sonde capacity)

6 hoursbetweendrops

0344-F0010 Figure 10 The driftsonde system concept.

0344-P0125

0344-P0130

0344-P0135

0344-P0140

10 RADIOSONDES

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

use a large rocket to carry the sensor package toaltitudes in excess of 70 km. The Super Loki solid-fuelrocket motor is typically the launch vehicle for high-altitude rocketsonde deployments. Two meters long, itaccelerates to 1500 m per second, and delivers itsmeteorological payload above the stratosphere intothe mesosphere. The typical payload package, called adart, is approximately 1.1 m long with an inside tubediameter of less than 5 cm, and contains the meteor-ological sonde. After the rocket motor burns out, thedart continues to coast to an altitude ranging from 70to 110 km. At apogee, a timed detonation of a smallexplosive charge located in the tail of the dart ejects themeteorological payload, which then begins its para-

chute-aided descent. The payload consists of either ameteorological sensor package – the rocketsonde – oran inflatable sphere. The high-altitude rocketsondesoften contain a transponder, a miniature receiver–transmitter that can be tracked by a radio directionfinding and ranging system to determine winds andaltitude. The inflatable sphere provides atmosphericdensity data, obtained from its fall velocity as deter-mined by a precision tracking radar.

The second type of rocketsonde is smaller and lessexpensive, and is used to measure only thermodynam-ic variables in the lower 1–3 km of the atmosphereabove earth. The Vaisala RK91 low-altitude rockets-onde (Figure 12) is primarily designed for naval

0344-F0011 Figure 11 The ASAP system is housed in either a standard 3 m (shown above) or 6 m sea container with a specially designed hatch to

enable routine radiosonde launches in sustained winds up to 25 m s� 1 (gusts up to 35 m s� 1). (Bottom part) Worldwide ASAP radio

soundings for November 2001.

0344-P0145

RADIOSONDES 11

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

shipboard operations that require observations of therefractive index profile near the ocean surface, but canalso be used over land where only thermodynamicdata are required. The RK91 can be prepared forlaunch in less than 10 min; it reaches apogee in lessthan 20 s, and provides a detailed thermodynamicprofile with 1 s resolution. After ejection of the sondepayload, the sonde drifts on a parachute to the surfacefrom an altitude of 1 km in less than 6 min. Verticalresolution is dependent on the rate of descent (typi-cally 3 m s�1), rate of data transmission (1 Hz) andsensor response time. At temperatures above freezing,vertical resolution is about 3 m.

Appendix

Historical milestones leading to the development ofthe modern meteorological radiosonde

Year Milestone

1643 Evangelista Torricelli invents the barome-ter in Florence, Italy.

1648 French mathematician Blaise Pascal ob-serves the decrease of atmospheric pressurewith altitude.

1749 Alexander Wilson, Glasgow, Scotland,uses kites to study the variation of temper-ature with altitude.

1783 The French Montgolfier brothers, Joseph-Michel, and Jacques-Etienne invent thehot-air balloon.

1783 Jacques Alexandre Cesare Charles, Paris,France, uses a manned balloon to make thefirst measurements of variations of pres-sure and temperature with altitude.

1784 Englishman John Jeffries, London, andFrenchman Jean-Pierre Blanchard beginthe systematic study of the atmosphereusing manned balloons.

1804 French physicists Louis Gay-Lussac andJean Baptiste Biot ascend to 7 km in aballoon and discover that water vapordecreases with altitude.

1822 Englishmen Sir Edward Parry and the Rev.George Fisher use kites with recordingthermometers to study the Arctic atmos-phere.

18 in8 45.7 cm7

227 in688.6 cm

67 cm

0344-F0012 Figure 12 Low-altitude rocketsonde unit with detached sonde descending on parachute (insert).

0344-T0002 Table 2 Number of ASAP units operating in 1989 and 2001

Country Number of ASAP units

Year 1989 2001

Australia/UK/USA (Southern

Hemisphere)

1

Canada 5a

Denmark 2 2

EUMETNET 2

Finland 1

France 4 4

Germany 4 3

Iceland/Sweden 1

Japan 5

Russia 1

Spain 1 1

United Kingdom 2 1

United States of America 5a 1

aJointly supported by Canada and US.

0344-P0150

12 RADIOSONDES

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

1847 William Radcliff Birt is the first to measurewinds aloft (and temperature) with a kiteflown from Kew Observatory, London.

1892 Frenchmen H. Hermite and G. Besanconlaunch the first free-flying weather balloonwith mechanical recording system (the‘meteorograph’).

1893 Lawrence Hargrave, Sydney, Australia,invents the box kite; by end of decade,many major observatories are using boxkites routinely to measure the atmosphere;they include: Blue Hill (near Boston, Mas-sachusetts), the Central Physical Observa-tory (Moscow), Trappes (near Paris), Kew(London), Lindenberg (Germany), andIlmala (Helsinki).

B1900 British scientist W. H. Dines invents themechanical meteorograph design that iswidely used until 1939.

1901 Richard Assmann, Germany, is first to use‘extensible’ rubber balloons for free-flyingsoundings with meteorographs.

1917 Germans F. Herath and M. Robizsch usethe ‘telemeteorograph’ to transmit mete-orological data from a kite using the steelkite cable as the signal cable.

1920 US Weather Bureau and Army Air Corpsestablish a program of daily upper-airsoundings using airplanes at 20 locationsnationwide.

1921 US Weather Bureau establishes a kitenetwork for routine upper-air observa-tions; this remains in operation until 1933.

1927 M. R. Bureau and M. Idrac (France) inventthe ‘shortwave’ (RF) tube-type transmitterand publish a paper describing the flight oftheir first balloon-borne sonde (although itis unclear whether any meteorologicalvariables were actualy measured). Theirpaper is the first documented use of theterm ‘radiosonde,’ which they attribute toH. Hergesell (president of the internationalaerological commission).

1929 January 17: M. Idrac and M. R. Bureau testthe first free-flying radiosonde, called the‘Thermoradio,’ with a bi-metallic temper-ature sensing element to transmit temper-ature data to a ground station.

1930 January 30: P. A. Moltchanov (Russia) usesa radiosonde to measure temperature andpressure to a height of 10,000 m fromSlutzk. From 1930 to 1936 several thou-sand soundings were made in the USSRwith the Moltchanov radiosonde.May 8: M.R. Bureau lanuches a radio-

sonde measuring temperature and pressurefrom Trappes, France, reaching an altitudeof 14,400 m.May 22: P. Duckert (Germany) flies the firstradiosonde measuring pressure, tempera-ture and humidity to a height of 15,000 mfrom the Aerological Observatory at Lin-denberg.

1931 December 30: Prof. Vilho Vaisala (Finland)flies a radiosonde from Helsinki telemeter-ing temperature to the ground up to aheight of 7 km; like Duckert, Vaisala usedthe measuring elements to control thecapacitance of the radio oscillator circuit.

1936 July 30: Prof. Vaisala establishes theVaisala Company and delivers the firstcommercial order for 20 radiosondes,delivered to Prof. Carl Gustav Rossby atthe Massachusetts Institute of Technology.

1974 The National Center for AtmosphericResearch (Boulder, Colorado) developsthe dropsonde, a special radiosonde thatis launched from research aircraft andmeasures winds, pressure, temperature,and humidity while descending on a par-achute.

1976 The Vaisala Oy company (Helsinki) intro-duces the first computer-controlled upper-air sounding systems.

1982 The US National Oceanographic and At-mospheric Administration begins routineuse of dropsondes for hurricane research;one year later, the US Air Force initiates itshurricane reconnaissance program.

1995 The first commercial radiosonde systemsusing the satellite Global Positioning Sys-tem to measure winds are introduced bythe Atmospheric Instrumentation Re-search company (Boulder, Colorado) andthe Vaisala Oy company (Helsinki).

See also

Observation Platforms: Kites (0257); Rockets (0258).Observations for Chemistry (Remote Sensing): Mi-crowave (0271). Satellite Remote Sensing: GPS Mete-orology (0350).

Further Reading

Beelitz P (1954) Radiosonden, VEB Verlag Technik, Berlin,Germany.

RADIOSONDES 13

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH

Federal Meteorological Handbook No. 3 (1997) Raw-insonde and Pibal Observations, FCM-H3-1997. Wash-ington, DC: Office of the Federal Coordinator ofMeteorology.

Hock TR and Franklin JL (1999) The NCAR GPS drop-windsonde. Bulletin of the American MeteorologicalAssociation 80(3): 407–420.

Oakley T (1998) Instruments and Observing Methods.World Meteorological Organization Report No. 72.Geneva: WMO.

Shea DJ, Worley SJ, Stern IR and Hoar TJ (1994) AnIntroduction to Atmospheric and Oceanographic Data.Report TN-40411A, Boulder, CO: National Center forAtmospheric Research.

World Meteorological Organization (1996) Guide to Mete-orological Instruments and Methods of Observation, 6thedn. Publication No. 8. Geneva: WMO.

14 RADIOSONDES

rwas.2003.0344 23/9/02 17:19 Ed:: M. SHANKAR No. of pages: 14 Pgn:: SURESH