ISO 8932-3:2026

Meteorology — Radiosonde — Part 3: Laboratory test method for solar radiation error of temperature sensor in radiosonde

This document specifies a test method for estimating the magnitude of radiosonde temperature sensor warming, induced by direct solar radiation, based on variations in air pressure, temperature, ventilation speed, tilt angle of its supporting sensor boom, and light illumination angle on the boom through a laboratory evaluation. This document provides the following: technical requirements for a laboratory setup to measure the effect of direct solar radiation on radiosonde temperature measurement under simulated sounding conditions; a test procedure for estimating radiosonde temperature measurement errors due to direct solar radiation in the air pressure range of 3 hPa to 1 000 hPa, temperature range1) of −70 °C to 50 °C, ventilation speed range of 3 m∙s−1 to 7 m∙s−1 at a specified irradiance (e.g. 1 000 or higher), sensor boom tilt range2) from 0° to 45° with respect to the air ventilation direction and the range of light illumination3) angle from 0° to 90° with respect to the sensor boom plane; a method to evaluate uncertainty in the results under the test conditions.

Météorologie — Radiosonde — Partie 3: Méthode d'essai en laboratoire pour les erreurs liées au rayonnement solaire du capteur de température dans la radiosonde

General Information

• Status: Published
• Publication Date: 14-May-2026
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: ISO/TC 146/SC 5 - Meteorology
• Drafting Committee: ISO/TC 146/SC 5 - Meteorology
• Current Stage: 6060 - International Standard published
• Start Date: 15-May-2026
• Due Date: 22-Jan-2026
• Completion Date: 15-May-2026

Overview

ISO 8932-3:2026 defines a standardized laboratory test method for measuring and estimating solar radiation error in radiosonde temperature sensors. Radiosondes are instrumental in meteorology, providing upper-atmosphere temperature data essential for weather forecasting and climate modeling. However, the accuracy of these sensors can be significantly impacted by direct solar radiation, which causes sensor warming during flight. To address this, the standard specifies a comprehensive procedure to assess and quantify solar-induced errors under controlled laboratory conditions, simulating a wide range of environmental and geometric factors.

Key Topics

• Laboratory Simulation of Solar Radiation:
  • Outlines technical requirements for creating simulated sounding conditions, including control over temperature, pressure, ventilation speed, sensor tilt angle, and incident light angles.
• Measurement Conditions and Ranges:
  • Applicable to pressures from 3 hPa to 1,000 hPa, temperatures from −70 °C to 50 °C, and ventilation speeds from 3 m·s⁻¹ to 7 m·s⁻¹, at irradiance levels typical of direct sunlight.
• Experimental Setup:
  • Details the required laboratory setup, including the use of climate and test chambers, wind tunnels, solar simulators, sensors, and measurement instruments.
• Error Quantification and Uncertainty Evaluation:
  • Provides methodologies for determining the warming error caused by solar radiation and evaluating the uncertainty of results according to internationally recognized measurement standards.
• Data Collection and Reporting:
  • Establishes guidelines on how to collect, process, and report test results in a consistent and traceable manner.

Applications

The methods outlined in ISO 8932-3:2026 have several practical uses in meteorological science and instrumentation:

• Radiosonde Manufacturers:

  • Ensure compliance with international standards by verifying the accuracy of temperature measurements under simulated upper-air conditions.
  • Optimize sensor design to minimize solar radiation errors and improve reliability in operational radiosoundings.

• Meteorological Institutes and Research Bodies:

  • Validate and calibrate radiosonde equipment to meet traceable measurement requirements.
  • Investigate the influence of environmental variables-such as sunlight angle, pressure, and ventilation-on sensor performance to refine correction algorithms.

• Quality Assurance for Weather Data:

  • Support the correction of radiosonde temperature data to align with the International System of Units (SI), reducing uncertainties in atmospheric profiles.
  • Facilitate international data comparability through the use of consensus-based laboratory practices.

Related Standards

• ISO/IEC Guide 98-1 and ISO/IEC Guide 98-3:
  Guides to the expression and evaluation of measurement uncertainty.
• ISO/IEC Guide 99:2007:
  International vocabulary of metrology, providing terminology used in this document.
• IEC 60050-713:
  Vocabulary for radiocommunications transmitters, receivers, and networks.
• IEC 60068-3-6 and IEC 60068-3-11:
  Supporting documentation and guidance for environmental and climatic testing.
• WMO No.182:
  International Meteorological Vocabulary, supporting consistency in terminology for meteorological instrumentation.

Practical Value

Adopting ISO 8932-3:2026 enables organizations to:

• Conduct reliable and reproducible laboratory tests for radiosonde temperature sensors, ensuring measurement accuracy.
• Assess and minimize the impact of direct solar radiation on measurement errors, contributing to more accurate meteorological data.
• Align with international best practices in meteorological instrumentation and data correction, enhancing global data comparability and confidence in upper-air observations.

By adhering to the methods set out in this standard, stakeholders can improve the quality of temperature measurements from radiosondes, supporting the broader goals of reliable weather prediction and climate monitoring.

Standards Content (Sample)

International
Standard
ISO 8932-3
First edition
Meteorology — Radiosonde —
2026-05
Part 3:
Laboratory test method for solar
radiation error of temperature
sensor in radiosonde
Météorologie — Radiosonde —
Partie 3: Méthode d'essai en laboratoire pour les erreurs liées
au rayonnement solaire du capteur de température dans la
radiosonde
Reference number
© ISO 2026
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or ISO’s member body in the country of the requester.
ISO copyright office
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Published in Switzerland
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and subscripts . 6
4.1 Symbols .6
4.2 Subscripts . .6
5 Technical requirements for the laboratory setup . 6
5.1 General .6
5.2 Open suction-type wind tunnel setup .6
5.2.1 General .6
5.2.2 Climate chamber . .7
5.2.3 Dry air generator .9
5.2.4 Liquid bath .9
5.2.5 Test cell .9
5.2.6 Pressure gauges .9
5.2.7 Laser Doppler anemometry (LDA) .10
5.2.8 Solar simulator .10
5.2.9 Vacuum pump .10
5.2.10 Sonic nozzles .10
5.3 Closed-type wind tunnel setup . 12
5.3.1 General . 12
5.3.2 Climate chamber . . 12
5.3.3 Dry air generator . 12
5.3.4 Liquid bath . 12
5.3.5 Test cell . 12
5.3.6 Pressure gauges . 12
5.3.7 Laser Doppler or sonic anemometry . 12
5.3.8 Solar simulator . 12
5.3.9 Vacuum pump . 12
5.3.10 Fan . 12
6 Test preparation .13
6.1 Environmental conditions . 13
6.2 Preparation of radiosondes . 13
6.3 Examination of the laboratory setup . 13
6.4 Operation of a solar simulator . 13
6.5 Installation of radiosondes .14
6.6 Test conditions . 15
6.6.1 General . 15
6.6.2 Sensor boom tilt angle . 15
6.6.3 Light illumination angle . 15
6.6.4 Temperature . 15
6.6.5 Pressure and air ventilation speed .16
6.6.6 Solar irradiance .16
6.7 Testing sequence .16
6.8 Data collection .16
6.9 Test finalization .17
7 Data processing . 17
7.1 Determining radiation error from the raw temperature .17
7.2 Mathematical measurement model . . .18
8 Evaluation of measurement uncertainty . 19

iii
8.1 General .19
8.2 Equation for combined standard uncertainty .19
8.3 Calculation of expanded uncertainty . 20
9 Method for reporting test results .20
Annex A (informative) Analytical functions of radiation error data set .21
Annex B (informative) Evaluation of uncertainty of environmental parameters .22
Annex C (informative) Application of radiation correction to radiosoundings .24
Bibliography .28

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
A list of all parts in the ISO 8932 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
Temperature and water vapour (i.e. humidity) are two of the basic atmospheric state variables and are
important for developing weather and climate prediction models. To measure the temperature and humidity
in upper-air, radiosondes are generally used. Radiosonde is an instrument intended to be carried by a balloon
up through the atmosphere, equipped with devices to measure one or more meteorological variables such
as pressure, temperature, humidity, and is provided with a radio transmitter for sending this information
[1]
to an observing station. Radiosonde observations are often used in concert with other measurement
techniques such as remote sensing using satellites to provide comparative data. For radiosonde-derived data
to be useful, the measurement accuracy of radiosoundings is needed. From a metrological perspective, this
measurement accuracy should be expressed in terms of uncertainty that is traceable to the International
System of Units (SI).
Correction for solar radiation effects is critical for precisely measuring the actual temperature in the upper
air. Since the temperature sensor is exposed to the outside air, the temperature measured by the sensor will
be different from the air temperature due to solar radiation impacting the sensor. During daytime soundings,
solar radiation heats the sensor. Therefore, all radiosonde temperature measurements show positive value
radiation-induced errors in daytime. This is one of the main errors decreasing the accuracy of temperature
measurements; thus, the measured temperature in soundings requires correction based on an algorithm or
look-up table supplied by manufacturers.
There are many parameters that affect the radiation correction in soundings such as radiation flux, air
temperature, air pressure, ventilation speed, solar elevation angle, and radiosonde orientation and motion
with respect to the sun. The radiative heating of sensors increases with a decrease in temperature (T),
pressure (P) and ventilation speed (v), and, in general, the radiation correction increases as the radiosonde
ascends. The decrease in these parameters (T, P, and v) results in a reduced heat transfer from the sensor to
the air. With respect to the impact of ambient temperature on sensors, the reduction in thermal conductivity
of air at low temperatures contributes to lowering the heat transfer coefficient, thereby resulting in an
[2]
increased radiation correction at such temperatures. The thermal conductivity of air plays a crucial role
in influencing heat transfer at the boundary between the air and the sensor.
In radiosoundings, cloud cover and distribution as well as surface albedo also affect solar radiative heating.
These conditions change in real time and are difficult to model in manufacturer’s correction algorithms,
thus introducing uncertainty. The radiation correction is a complicated process which depends on multiple
factors and parameters as well as different radiation correction algorithm features as applied by various
manufactures which are proprietary and rarely disclosed. This document provides a testing method for
measuring the radiation error for integrated radiosonde temperature sensors, using a limited number of
samples from a batch of mass produced sensors.
The Standing Committee on Measurements, Instrumentation and Traceability (SC-MINT), one of the
Technical Commissions and Research Board at the World Meteorological Organization, offers advice,
recommendations, and promotes studies on the effective and sustainable use of instruments, such as
radiosondes as well as offering methods for upper air weather observations. The SC-MINT Guide recommends
that for temperature correction, solar radiation correction should be applied using software during data
[1]
processing. Consideration of additional heating sources, the temperature sensor and supporting hardware
are designed such that solar heating does not vary significantly as the radiosonde rotates in flight relative to
the sun.
Despite recognising the importance of correcting sensor based meteorological temperature measurement,
there is limited information in the SC-MINT Guide or other technical documents on test methods, procedures
and related instrument details. Therefore, there is a need to publish a consensus procedure for evaluating
radiosonde temperature error induced by solar radiation in an experimentally controlled way.
The procedure presented in this document provides laboratory setup technical requirements for conducting
solar radiation test, test procedure for determining solar radiation error on radiosondes with various
environmental and geometrical parameters, and a method for assessing uncertainties within test results.
Temperature, pressure, ventilation and radiation flux, as well as geometric parameters including sensor
boom tilt angle and light illumination angle are evaluated over ranges that are varied to mimic in-flight
conditions as experienced by radiosondes.

vi
Note that when considering uncertainty in soundings, other factors such as temperature spikes due to
patches of warm air coming off the sensor housing and the balloon, time-lag, and albedo should also be
considered, as summarized in Table 2 of Reference [3]. While all uncertainty terms affecting the results
should be considered, this procedure specifically focuses on testing environmental and geometrical effects
on the radiation correction.
The core procedure discussed in this document involves the SI-traceable generation of air ventilation speed
in the test environment to simulate the ascendance speed of radiosondes. In addition, the solar irradiance,
temperature, and pressure in the test cell should closely replicate those observed in upper air. To attain the
required range variability of atmospheric parameters in the test setup, this testing method employs both an
open suction-type wind tunnel and a closed-type wind tunnel as illustrative examples. These methods are
chosen because of their traceability to the SI and validation by metrological and meteorological experts in
[2,4]
testing radiosonde temperature sensors.
Note that while the open suction-type wind tunnel using a combination of sonic nozzles and vacuum pump is
[5,6]
based on Korean and US patents, the patent holder has granted a license, free of charge, to an unlimited
number of applicants globally, without discrimination, and under reasonable terms and conditions. This
license allows for the creation, use, and sale of implementations based on this ISO document. However, it's
crucial to clarify that this ISO document does not detail the patented technique and is not an endorsement
for use of patented material. Alternative equivalent procedures, employing different types of wind tunnel
systems, can also be used to determine air ventilation speed at varying temperatures and pressures if they
are SI-traceable and meet the data quality objectives of the application.
In this document:
— the requirements for a climate chamber, wind-tunnel, a test cell, thermometers, pressure and vacuum
gauges, a laser anemometer and a solar simulator are proposed;
— the test preparation, the procedure for installing a radiosonde in the test cell, the operation of the
laboratory setup, the experimental range and sequence, and data processing are presented;
— a method to evaluate and report uncertainty of the determined radiation errors on the temperature
using the uncertainty propagation law, based on a mathematical model, is proposed.
NOTE 1 Since the test method is limited by the use of ground-based facilities, radiative cooling effect of radiosonde
temperature sensors observed in stratosphere cannot be reproduced and represented in the test result.
NOTE 2 The light source spectrum can be limited in the infrared (IR) region compared to that of the visible light
[1]
solar spectrum. As recommended by the WMO guide by SC-MINT , the heat exchange in infrared radiation (IR)
needs to be avoided by using sensor coatings that have low emissivity in the IR. Otherwise, the effect of IR can be
underestimated in the test.
NOTE 3 Due to potential limitations in the number of test setups or laboratories capable of conducting this test,
peer-reviewed reports or papers published online or offline resulting from research activities conducted by academia
or meteorological institutes can be utilized as a test report when following this test procedure.

vii
International Standard ISO 8932-3:2026(en)
Meteorology — Radiosonde —
Part 3:
Laboratory test method for solar radiation error of
temperature sensor in radiosonde
1 Scope
This document specifies a test method for estimating the magnitude of radiosonde temperature sensor
warming, induced by direct solar radiation, based on variations in air pressure, temperature, ventilation
speed, tilt angle of its supporting sensor boom, and light illumination angle on the boom through a laboratory
evaluation. This document provides the following:
a) technical requirements for a laboratory setup to measure the effect of direct solar radiation on
radiosonde temperature measurement under simulated sounding conditions;
b) a test procedure for estimating radiosonde temperature measurement errors due to direct solar
1)
radiation in the air pressure range of 3 hPa to 1 000 hPa, temperature range of −70 °C to 50 °C,
2
−1 −1
ventilation speed range of 3 m∙s to 7 m∙s at a specified irradiance (e.g. 1 000 Wm or higher),
2)
sensor boom tilt range from 0° to 45° with respect to the air ventilation direction and the range of
3)
light illumination angle from 0° to 90° with respect to the sensor boom plane;
c) a method to evaluate uncertainty in the results under the test conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/IEC Guide 98-1, Guide to the expression of uncertainty in measurement — Part 1: Introduction
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
ISO/IEC Guide 99:2007, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
IEC 60050-713:2021, International Electrotechnical Vocabulary (IEV) - Part 713: Radiocommunications:
transmitters, receivers, networks and operation
IEC 60068-3-6:2018, Environmental testing - Part 3-6: Supporting documentation and guidance - Confirmation
of the performance of temperature/humidity chambers
1) Currently, the lowest possible temperature of commercially-available climate chambers is approximately −75 °C. The
temperature range can be adjusted base on the capability of the climate chamber used.
2) The tilt angle of the sensor boom can be adjusted depending on the space of the test cell. If tilting is not possible, a
default angle of 0° can be used.
3) The light illumination angle can be adjusted depending on the ability of moving the light source or tilting the
waveguide. If changing the illumination angle is not possible, a default angle of 90° can be used.

IEC 60068-3-11:2007, Environmental testing - Part 3-11: Supporting documentation and guidance - Calculation
of uncertainty of conditions in climatic test chambers
WMO No.182, 1992, International Meteorological Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC Guide 99:2007,
IEC 60050-713:2021, IEC 60068-3-6:2018, IEC 60068-3-11:2007, and WMO No.182 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
radiosonde
instrument intended to be carried by a balloon through the atmosphere, equipped with devices to measure
one or several meteorological variables (such as pressure, temperature, humidity), and provided with a
radio transmitter for sending this information to the observing station
Note 1 to entry: A series of battery-powered telemetry devices that are suspended high in the atmosphere using
a weather balloon, which measure atmospheric parameters and transmit data to a ground system using radio
frequencies.
Note 2 to entry: To transfer data to a GTS (Global Telecommunication system) or WIS (WMO Information System), the
WMO code (BUFR code 0-02-011) must be obtained. This WMO code shall be unique for each radiosonde, and a single
code shall not be used for more than one type of radiosonde.
3.2
radiosonde body
housing of a radiosonde (3.1) comprising a circuit board with measurement chips and a data transmission
section with antennae and batteries
3.3
radiosonde sensor boom
arm connected to the radiosonde body (3.2) to which the temperature and humidity sensors are attached
3.4
radiosonde installation chamber
installation chamber
chamber in which the body of a radiosonde (3.1) can be installed
3.5
test cell
chamber in which a radiosonde sensor boom (3.3) with temperature sensors can be installed
Note 1 to entry: The test cell has one or more transparent windows allowing the transmission of simulated solar
irradiance (3.20).
3.6
climate chamber
enclosed space where the internal temperature or temperature and humidity can be controlled within
specified limits
Note 1 to entry: The climate chamber can be used to control the temperature of the radiosonde (3.1) in a test cell (3.5).
Note 2 to entry: For huge setups, the laboratory itself, appropriately controlled at room temperature, can play the role
of a climate chamber.
3.7
dry air generator
device that can produce a gas with negligible water vapour content
Note 1 to entry: Dry air is used as a medium of ventilation in a test cell (3.5).
3.8
heat exchanger
device that efficiently transfers heat from a high-temperature object to a low-temperature one
Note 1 to entry: A tubing or pipe with heat exchange pins can be used to adjust the dry air temperature to the target
temperature for a test. The heat exchanger can be installed upstream of the test cell (3.5) in the climate chamber (3.6).
3.9
liquid bath
equipment in which liquid is present in a container, with a specific volume, to maintain a constant
temperature
Note 1 to entry: Liquid whose freezing point is lower than the lowest temperature of the testing can be used for the
bath. In general, ethanol, silicon oil or a halocarbon is used.
Note 2 to entry: When the test pressure and ventilation speed are high, the temperature of dry air can be preadjusted
before entering the climate chamber (3.6). An additional heat exchanger (3.8) in the liquid bath can be used to stabilise
the temperature of the test cell (3.5) in the climate chamber.
3.10
reference thermometer
device used to measure the temperature in the test cell (3.5)
Note 1 to entry: Generally, calibrated platinum resistance thermometers are used with the resistance read by a
calibrated digital multimeter.
3.11
reference pressure gauge
instrument used to measure the pressure in the test cell (3.5)
Note 1 to entry: Generally, a capacitance diaphragm gauge (CDG) (3.12) for which standard retroactivity has been
maintained is used. The reference pressure gauge is used to measure air pressure in the test cell.
3.12
capacitance diaphragm gauge
CDG
pressure or vacuum sensor that measures gas pressure by directly measuring the force applied on the
surface of a thin diaphragm in the sensor
Note 1 to entry: The mechanical deflection of the elastic sensor diaphragm is a function of the applied pressure. The
CDG can be used to measure air pressure in various areas of the system.
3.13
air-tight wind tunnel
pathway through which air ventilation is generated by a wind generator based on an electrical fan or a
vacuum pump (3.15)
Note 1 to entry: The test cell (3.5) is located in the air flow generated by the wind tunnel.
Note 2 to entry: For simulating the ascent speed in radiosoundings, the air ventilation speed and pressure are
controlled simultaneously. For an open suction-type wind tunnel, this can be achieved by using a vacuum pump and
sonic nozzles (3.16). For the continuous circulation of air flow, an electrical fan can be used in an air-tight closed-loop
wind tunnel (schematic diagram presented in Figures 1 and 2).
3.14
flow straightener
device used to reduce disturbances in air flow within the test cell (3.5)

3.15
vacuum pump
device that draws gas, such as air, into the test cell (3.5) to control the pressure
3.16
sonic nozzle
critical flow Venturi
device that can be used as a calibration standard for gas flow meters and used to achieve a specific maximum
constant flow when the ratio of the downstream pressure to the upstream pressure is smaller than a certain
critical point as specified in ISO 9300
Note 1 to entry: The critical point in a sonic nozzle occurs when the gas velocity reaches the speed of sound within
the nozzle. The pressure ratio at this critical condition is approximately 0,528 (= P /P ), where P is the downstream
t o t
pressure and P is the upstream pressure, respectively.
o
Note 2 to entry: The test cell (3.5) lies in the downstream region of the sonic nozzles, wherein the pressure is lowered
using a vacuum pump (3.15) to attain the critical condition.
3.17
electric fan
electrically powered control device used to create air flow
3.18
solar simulator
device that generates illumination with a spectrum like that of sunlight by using Xenon (Xe) arc lamps to
simulate solar radiation effect on radiosondes (3.1)
Note 1 to entry: The range of wavelength emitted from Xe arc lamps is 200 nm to 2 500 nm with a relatively smooth
emission curve in the UV to visible spectra.
3.19
reference pyranometer
device normally used to measure global sunlight irradiance (3.20) on a horizontal plane
Note 1 to entry: Typically, a pyranometer of which traceability to the International System of units (SI) has been
established is used.
Note 2 to entry: A pyranometer can also be used at an angle to measure the total sunlight irradiance on an inclined
plane. In this case, this includes an element caused by radiation reflected from the foreground.
3.20
irradiance
radiant power incident upon a unit area of a surface
−2
Note 1 to entry: Irradiance is expressed in watts per square metre (W·m ).
3.21
irradiance uniformity tester
experimental apparatus comprising an X-Y translation stage using a photodiode or a thermoelectric
pyranometer with either a current or volt meter to evaluate the two-dimensional (2D) uniformity of the
illumination field generated by the solar simulator (3.18)
Note 1 to entry: Pyranometers are more suitable for comprehensive light measurements across a wide range of
wavelengths whereas photodiodes operate mostly within ranges from 400 nm to 1 100 nm.
3.22
quartz window
flat transparent plate that separates the radiosonde sensors in the test cell (3.5) from the outside
environment while allowing light transmission through a solar simulator (3.18) located outside the test cell
Note 1 to entry: One or two quartz plates are necessary in the test cell while one is needed for the climate chamber
(3.6) when the solar simulator is outside the climate chamber.

3.23
light-absorbing plate
plate made of black materials that eliminates light reflectance
Note 1 to entry: The black plate can be inclined at an angle of 45° to prevent potential light reflection back to the test
cell (3.5). If it is not possible, the reflected flux should be measured.
3.24
correction
compensation for an estimated systematic effect
Note 1 to entry: The compensation can take different forms, such as an addend or a factor, or can be deduced from a
table.
3.25
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
Note 1 to entry: Measurement uncertainty includes components arising from systematic effects, such as components
associated with corrections (3.24) and the assigned quantity values of measurement standards, as well as the
definitional uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated
measurement uncertainty components are incorporated.
Note 2 to entry: The parameter may be, for example, a standard deviation called standard measurement uncertainty
(or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.
Note 3 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be evaluated
by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series
of measurements and can be characterized by standard deviations. The other components, which may be evaluated
by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from
probability density functions based on experience or other information.
Note 4 to entry: In general, for a given set of information, it is understood that the measurement uncertainty is
associated with a stated quantity value attributed to the measurand. A modification of this value results in a
modification of the associated uncertainty.
3.26
standard uncertainty
measurement uncertainty (3.25) expressed as a standard deviation
3.27
coverage factor
k
number larger than one by which a combined standard measurement uncertainty (3.25) is multiplied to
obtain an expanded measurement
Note 1 to entry: A coverage factor is usually symbolized.
3.28
expanded uncertainty
product of a combined standard measurement uncertainty (3.25) and a factor larger than the number one
Note 1 to entry: The factor depends upon the type of probability distribution of the output quantity in a measurement
model and on the selected coverage probability.
Note 2 to entry: The term “factor” in this definition refers to a coverage factor (3.27).

4 Symbols and subscripts
4.1 Symbols
For the purposes of this document, the following symbols apply.
P
pressure
direct solar irradiance
S
dir
effective solar irradiance on the sensor
S
eff
simulated irradiance with normal illumination on the sensor boom
S
T
temperature
∆T radiation error with S
rad 0
∆T radiation error with S
rade_ ff eff
U
expanded uncertainty
u
standard uncertainty
v
ventilation speed
4.2 Subscripts
In this document, the following subscripts are used.
cal calibration
DMM digital multimeter
grad gradient
ref reference
rep repeatability
res resolution
sta stability
5 Technical requirements for the laboratory setup
5.1 General
The laboratory setup can be described as the mechanism for generating air ventilation in the test cell of air-
tight wind tunnels such as an open suction-type or a closed-type wind tunnel.
5.2 Open suction-type wind tunnel setup
5.2.1 General
Figure 1 shows the schematic diagram of an open suction-type setup, comprising sonic nozzles and a vacuum
[2]
pump, to induce air ventilation at low pressures.

5.2.2 Climate chamber
A thermo-hygrostat chamber or space is needed to control and stabilise the temperature of the test chamber
during the test. In general, the temperature range of commercially available climate chambers is −70 °C to
50 °C.
The heat exchanger, test cell and installation chamber should be contained within the climate chamber.

Key
1 air flow
2 black surface
3 compressed air
4 climate chamber
5 dry air lines
6 dry air generator
7 flow straightener
8 heat exchanger
9 heat exchanger in liquid bath
10 installation chamber
11 pressure regulator
12 quartz window
13 radiosonde body
14 rotating jig
15 reference pressure gauge
16 reference thermometer
17 sensor boom
18 solar beam
19 sonic nozzles
20 solar simulator
21 test cell
22 temperature sensor
23 valve
24 vacuum pump
25 laser or sonic doppler anemometry
26 measurement light
27 scattered light
Figure 1 — Schematic diagram of the laboratory setup comprising an open suction-type wind tunnel
with a vacuum pump and sonic nozzles

5.2.3 Dry air generator
Dry air should be used to test the radiation error of a radiosonde temperature sensor, especially at cold
temperatures, to prevent frost or dew from forming on the inside of the windows. The outside of the
windows should be blown using dry air.
The frost-point or dew-point of dry air generated from a dry air generator should be lower than the test
temperature to prevent frost or dew formation.
5.2.4 Liquid bath
The heat exchangers should allow for fast thermal exchange; hence, the dry air temperature is adjusted to
the target temperature of the test cell. The dry air temperature can be preadjusted by using a heat exchanger
in a liquid bath before the air enters the climate chamber. In general, the temperature range of commercially
available liquid baths ranges from −80 °C to 60 °C.
5.2.5 Test cell
The test cell should be installed inside a climate chamber or liquid bath maintained at a constant temperature
to control and stabilise the temperature. A climate chamber is more desirable over a bath for the handling of
the test cell with radiosondes.
The test cell should allow light irradiation on the sensor through high light transmittance window(s) such as
quartz.
The test cell should be located between the sonic nozzles and the vacuum pump to control the air pressure
and ventilation speed.
The transmitted light beam should not be reflected back through the test cell. This can be achieved by
placing a light-absorbing plate at the back end of the test cell.
The test cell should allow for the installation of at least one radiosonde sensor boom with temperature
sensors.
If the test cell is big enough to test multiple sensors at the same time, the temperature gradient and irradiance
inside the test cell should be evaluated and compensated for.
The temperature of the test cell can be varied within the range of −70 °C to 50 °C by using a commercially
available climate chamber.
The temperature of the test cell should be measured around the radiosonde temperature sensor by using a
[8]
calibrated thermometer to maintain traceability as per the International Temperature Scale of 1990.
The temperature fluctuation of the test cell should be within ±0,05 °C over 10 min to enable measurement of
a radiation error of 0,1 °C.
The operations of rotating and tilting the sensor boom are optional and can be performed using stepper
motors.
5.2.6 Pressure gauges
The test cell should be capable of mimicking the environmental conditions of air pressure encountered
during a typical radiosonde ascent sounding as closely as possible. To fulfil this criterion, the pressure range
of the gauge should be between 3 Pa and 1 000 hPa.
To mimic realistic air pressure, the entire system, including the sonic nozzles, heat exchangers, test cell,
reference thermometer, pressure gauges, and other ancillary devices, should be airtight.
The pressure in the test cell is measured using a calibrated pressure gauge to maintain SI traceability. The
pressure in the test cell should stay within the larger of these two limits: ±5 % of the target pressure or
±0,6 hPa, for at least 10 min.

5.2.7 Laser Doppler anemometry (LDA)
The ventilation speed in the test cell should mimic a typical radiosonde ascent speed. Specifically, the range
−1 −1
of a
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