Space product assurance
Measurements of thermo-optical properties of thermal control materials
Foreword
This Standard is one of the series of ECSS Standards intended to be applied together for the management, engineering and product assurance in space projects and applications. ECSS is a cooperative effort of the European Space Agency, national space agencies and European industry associations for the purpose of developing and maintaining common standards. Requirements in this Standard are defined in terms of what shall be accomplished, rather than in terms of how to organize and perform the necessary work. This allows existing organizational structures and methods to be applied where they are effective, and for the structures and methods to evolve as necessary without rewriting the standards.
This Standard has been prepared by the ECSS Executive Secretariat, endorsed by the Document and Discipline Focal points, and approved by the ECSS Technical Authority.
Disclaimer
ECSS does not provide any warranty whatsoever, whether expressed, implied, or statutory, including, but not limited to, any warranty of merchantability or fitness for a particular purpose or any warranty that the contents of the item are error-free. In no respect shall ECSS incur any liability for any damages, including, but not limited to, direct, indirect, special, or consequential damages arising out of, resulting from, or in any way connected to the use of this Standard, whether or not based upon warranty, business agreement, tort, or otherwise; whether or not injury was sustained by persons or property or otherwise; and whether or not loss was sustained from, or arose out of, the results of, the item, or any services that may be provided by ECSS.
Published by: ESA Requirements and Standards Division
ESTEC, ,
2200 AG Noordwijk
The
Copyright: 2008 © by the European Space Agency for the members of ECSS
Change log
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ECSS-Q-70-09A
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First issue
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ECSS-Q-70-09B
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Never issued
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ECSS-Q-ST-70-09C
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Second issue
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Introduction
The thermooptical properties of materials are of importance to enable the calculation of the thermal housekeeping and radiative heat transfer.
This Standard describes the methodology, instruments, equipment and samples, used to calculate the thermooptical properties of thermalcontrol materials, i.e. solar absorptance [s or p] and the infrared emittance [h or n].
In general this procedure has been written in connection with instruments and equipment available at ONERA, INTESPACE and ESTEC; however, any supplier is encouraged to built up his own instrument or equipment provided the accuracy of the results is equivalent to the one specified herein.
In this Standard, the supplier is identified as the entity that performs the test.
Scope
This Standard describes the methodology, instruments, equipment and samples, used to calculate the thermooptical properties of thermalcontrol materials.
The following test methods are detailed in this Standard including the configuration of samples and calculations:
Solar absorptance using spectrometer (s) - (see Annex C.2).
Comparative test method (p) - (see Annex C.3).
Infrared emittance using thermal test method (h) - (see Annex C.4).
Infrared emittance using IR spectrometer (h) - (see annex C.5).
Infrared emittance using portable equipment (n) - (see Annex C.6).
This standard may be tailored for the specific characteristics and constraints of a space project in conformance with ECSS-S-ST-00.
Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this ECSS Standard. For dated references, subsequent amendments to, or revisions of any of these publications do not apply. However, parties to agreements based on this ECSS Standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references the latest edition of the publication referred to applies.
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ECSS-S-ST-00-01
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ECSS system – Glossary of terms
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ECSS-Q-ST-10
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Space product assurance – Product assurance management
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ECSS-Q-ST-10-09
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Space product assurance – Nonconformance control system
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ECSS-Q-ST-20-07
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Space product assurance – Quality assurance for test centres
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ECSS-Q-ST-70-02
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Space product assurance – Thermal vacuum outgassing test for the screening of space materials
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Terms, definitions and abbreviated terms
Terms defined in other standards
For the purpose of this Standard, the terms and definitions from ECSSSST0001 apply.
Terms specific to the present standard
absorptance
ratio of the intensity of the incident light to the transmitted or reflected light
emittance ()
ratio of the radiant intensity of the specimen to that emitted by a black body radiator at the same temperature and under the same geometric and wavelength conditions
For example:
- Hemispherical emittance (h) - conditions for incident or viewing of flux over a hemispherical region.
- Normal emittance (n) - conditions for incidence or viewing through a solid angle normal to the specimen.
solar absorptance (
ratio of the solar radiant flux absorbed by a material (or body) to that incident upon it
Differentiation is made between two methods:
- Spectroscopic method using a photospectrometer covering the range from 0,25 m up to 2,5 m for the determination of s.
- Portable equipment using a xenon flash for relative measurements (p).
Abbreviated terms
The abbreviated terms defined in ECSS-S-ST-00-01 apply.
Requirements
Preparatory conditions
Hazards, health and safety precautions
For “Safety and security” ECSSQST2007C shall apply.
For example, for hazard and health (safety) and for access control (security).
Preparation of samples
Configuration
The supplier shall prepare the material samples according to the process specification or manufacturer’s data.
The material shall be representative of batch variance.
For example: the application procedure for paint can result in different thermooptical properties, depending on the painter and the type of spray gun used. This is the reason why the samples are coated or made at the same time as the workpiece.
Cleaning
Cleaning method and other treatment of the sample shall be the same (and not more) as for the flight hardware.
In particular, solar absorptance properties are very sensitive to contamination and if the sample or the flight hardware is contaminated (even by hand grease), the test results are completely erroneous.
Handling and storage
The supplier shall handle the samples with clean nylon or lintfree gloves.
The supplier shall store the samples in a cleanlinesscontrolled area, with a room temperature of (20 3) C and relative humidity of (55 10) %.
The supplier shall shield coated surfaces from contact by using polyethylene or polypropylene bags or sheets.
The supplier shall avoid mechanical damage of the sample by packing the polyethylene or polypropylenewrapped workpieces in clean, dust and lintfree material.
The supplier shall label limitedlife materials with their relative shelf lives and dates of manufacture.
Identification
The supplier shall use a completed “Material identification card” in conformance with DRD in ECSS-Q-ST-70-02 for samples submitted for testing.
The supplier shall verify the existence of a safety data sheet for hazardous samples
The safety data sheet is a set of information provided by the supplier of the chemical substance.
Facilities
Cleanliness
The supplier shall keep the work area clean and free of dust.
The supplier shall use filters for the air used for ventilation.
This is to prevent contamination of the sample.
Environmental conditions
The ambient conditions for the process and work areas shall be (22 3) C with a relative humidity of (55 10) % unless otherwise stated.
Equipment
The supplier shall define the equipment in the test procedure.
Selection of test methods
The supplier shall select one or more of the following test methods depending on the type and size of the samples:
- Solar absorptance using spectrometer (s),
For details and conditions for the samples see Annex C.2.
- Comparative test method (p),
For details and conditions for the samples see Annex C.3.
- Infrared emittance using thermal test methods (h),
For details and conditions for the samples see Annex C.4.
- Infrared emittance using IR spectrometer (h),
For details and conditions for the samples see Annex C.5.
- Infrared emittance using portable equipment (n).
For details and conditions for the samples see Annex C.6.
Quality assurance
Data
The supplier shall retain the quality records for ten years or in accordance with project contract requirements.
Example of quality records are log sheets.
An evaluation report shall be produced in conformance with Annex A and is a quality record
Calibration
The supplier shall calibrate any measuring equipment to traceable reference standards.
The supplier shall record any suspected or actual equipment failure as a project nonconformance report in conformance with NCR DRD in ECSSQST10-09.
This is to ensure that previous results can be examined to ascertain whether or not reinspection and retesting is necessary.
Audit of measurement equipment
General
The customer shall perform the standard audit according to ECSSQST10.
- 1 The main purpose of a standard audit is to ensure the validity of test results by comparison of the test data on identical materials by different test houses.
- 2 The thermooptical property data from test houses for the projects of the customer, obtained in the manner laid down in this Standard, are only accepted for the projects of the customer if the test house is certified to perform the relevant procedure in this Standard.
Audit of the system (acceptance)
The customer’s product assurance department shall audit the system after it has been built or purchased.
The audit is necessary before the system can be accepted for running qualification or quality control tests on materials for use in customer projects.
The customer shall establish an audit report in conformance with Annex B.
The customer shall issue the certificate of conformance after a successful audit or renew it every three years after a successful audit.
Annual regular review (maintenance) of the system
The supplier shall establish an annual regular review report in conformance with Annex B.
For each nonconformance the supplier shall perform the following actions:
- determine the reasons for the nonconformance, and
- perform a further test in accordance with clause 4.4.2.
These actions are necessary before a certificate of conformance is renewed.
The supplier shall deliver the review report to all customers within six weeks after the end of the regular review or evaluation testing.
Special review
The supplier shall report all modifications of the apparatus or associated equipment.
The customer shall audit the modifications, if deemed necessary, before utilization of the modified system for the customer’s project.
For major modifications the supplier shall retest the apparatus as described in clause 4.4.2.
ANNEX(normative)Evaluation report of the measurement of thermo-optical properties of thermal control materials– DRD
DRD identification
Requirement identification and source document
This DRD is called from ECSS-Q-ST-70-09, requirement 4.3.1b.
Purpose and objective
The purpose of the document is to describe the contents of the evaluation report of the measurement of thermo-optical properties of thermal control materials.
Expected response
Scope and content
The report shall contain the trade names and batch numbers of the materials under test.
The report shall contain the name of the supplier through whom the purchase was made.
The report shall contain the summary of the preparation and conditioning schedule.
E.g. mixing proportions, coating thickness, cure time and temperature, postcure, cleaning procedure.
The report shall contain the test method description and calibration.
The report shall contain any noticeable incident observed during the measurement.
The report shall contain the deduced results.
Special remaks
None.
ANNEX(normative)Audit / review report for the measurement equipment of thermo-optical properties of thermal control materials - DRD
DRD identification
Requirement identification and source document
This DRD is called from ECSS-Q-ST-70-09, requirements 4.4.2b and 4.4.3a
Purpose and objective
The purpose of the document is to describe the contents of the audit and review report for the measurement equipment of thermo-optical properties of thermal control materials.
Expected response
Scope and contents
The audit / review report shall describe the inspection of the apparatus and associated equipment.
The audit report shall describe the test on a defined set of materials.
The review report shall describe the mutual comparability evaluation (testing).
The audit / review report shall contain the nonconformances.
The audit / review report shall contain the audit findings.
Special remarks
None.
ANNEX(informative)Test methods
Format
The format of each clause of the present annex is as follows:
C.x.1 General
C.x.2 Configuration of samples
C.x.3 Test apparatus and setting up
C.x.4 Test process and measurement
C.x.5 Calculations
Solar absorptance using spectrometer (s)
General
Solar absorptance is calculated using the absorption spectrum of the material over the region from 0,25 m to 2,5 m and this spectrum is then multiplied with the solar spectrum. The absorption spectrum should be measured using an integrating sphere. For absolute measurements a sphere with central sample mounting should be used.
A sphere with a sample holder on the sidewall can also be used. In this case the reflectivity is compared to a known standard (e.g. calibrated Almirror or calibrated Spectralon standard).
Configuration of samples
Typical dimensions of the sample are 15 mm 15 mm to 25 mm 25 mm. Depending on the method and equipment used, these dimensions can vary.
Flexible samples should be mounted on a rigid surface. Measurements are only valid on flat samples. However, it is possible to perform measurements on spherical curved samples provided the radius of curvature exceeds 300 mm.
Test apparatus and setting up
The test apparatus consists of a spectrometer, covering the range from 0,25 m to 2,5 m. The following are recommendations for the test apparatus and setting up:
The wavelength resolution of the spectrometer should be compatible with the resolution used for the solar spectrum.
The signal to noise ratio should be better than:
1 % full scale in the region between 250 nm and 2 000 nm;
5 % full scale in the region between 2 000 nm and 2 500 nm.
The associated sphere should have a maximum port to total surface ratio of 5 %.
- 1 If the test apparatus is used in a central sample mode, i.e. an “Edwards”type integrating sphere, the measurement is called “absolute”.
- 2 If the sample is mounted on the sidewall, the measurement is done towards a calibrated standard that can be specular (e.g. Almirror) or diffuse (e.g. Spectralon).
The responsible test officer should make the choice of a standard based on the visual aspect of the sample.
For materials having, in the visible region, a large specular component, a standard mirror should be used.
When the diffuse component is predominant in the visible region, a diffuse Spectralon sample should be used.
The standard used for the measurement should be indicated in the report.
Test process and measurement
Before starting a measurement sequence the 100 % and 0 % baseline should be taken (using the standard reference as applicable). The baseline should be measured at least once a day when equipment is switched on.
Calculation of absorptance
The spectrum is taken between 250 nm and 2 500 nm, and covers 96 % of the total energy.
where:
R() is the spectral reflectance after 100 % reference correction;
S() is the spectral solar irradiance ( see ASTM E 490);
d is typically 1 nm;
1 is 0,25 m;
2 is 2,5 m.
For transparent test pieces it is possible to calculate the absorptance by the same method, because:
It is also possible to calculate absorptance for a spectrum other than the solar spectrum, e.g. for solar simulators.
Comparative test method (p)
General
This method is based on comparing the reflection of a Xenon flash by a known appropriate reference material to the reflection of an unknown sample. The nature of the reference material (chemical composition and surface morphology) should be representative for the unknown. The solar absorption of the reference surface should be measured using the method described in C.2.
This method has limitations due to the fact that the flasher spectrum is not identical to the solar spectrum.
Special precautions should be taken when using portable reflectometer equipment.
This equipment should only be used for comparative measurements. It should be used in conjunction with known reference or calibrated materials, identical to or at least having similar optical behaviour to the material to be measured. If such an approach is followed, the equipment should give a direct and correct result within the linearity limitation. If the reference used is not identical to the material under test, the result of the reading not only depends on the linearity of the equipment but also on the spectral reflectivity of the material. Any reporting of results should include the detailed test conditions.
Configuration of samples
The minimal sample dimensions are dictated by the diameter of the aperture on the portable equipment. This diameter is typically between 15 mm and 20 mm.
Measurements are only valid on flat samples. However, it is possible to perform measurements on spherical curved samples, provided the radius of curvature exceeds 300 mm.
Test apparatus and setting up
The equipment consists of a flasher, able to produce a flash with a reproducible spectrum. The total intensity and the reflected intensity of the flasher are measured both with the reference surface and the test surfaces.
Test process and measurement
Before any measurement is performed the equipment should be stabilized following the instructions given by the manufacturer.
The equipment should be calibrated following the instructions given by the manufacturer.
The calibration should, as a minimum, include a “zero” or baseline measurement as well as the measurement of the reference material.
After calibration with the appropriate reference material, the unknown sample should be measured.
If several materials have to be measured, calibration should be repeated at regular time intervals, depending on the known stability of the equipment.
For statistical reasons each measurement should be repeated at least 5 times; the average and standard deviation of the measurements should be calculated and reported.
Some equipment makes these calculations automatically through integrated software. The standard deviation between measurements should be 0,02 or better.
Calculations
Reflectivity reference surface (Rr):
where:
Ir is the intensity of flash reflected on reference surface;
Itr is the total intensity of flash during reference flashing.
Reflectivity sample surface (Rs):
where:
Is is the intensity of flash reflected on sample surface;
Its is the total intensity of flash during sample flashing.
where:
Rr is the measured reference reflectivity (using method C.2);
R is the calculated sample reflectivity.
Infrared emittance using thermal test method (h)
General
By means of the dynamic thermal method, one can determine the total hemispherical emittance from the decrease in temperature of a test item with welldefined thermal characteristics.
Configuration of samples
A “standard sample substrate” as detailed in Figure C- 1 should be used in favour of any other geometrical shape.
In particular, the sample should be made by machining and not by cutting out with shears since this flattens the edges and does not produce a very accurate square shape.
Test apparatus and setting up
For the dynamic thermal method, it is assumed that the specific heat of the test piece is known. A lightweight test piece (i.e. one with a low heat capacity) is bonded by means of doublesided adhesive tape to a goldplated substrate (standard sample substrate) the heat capacity of which is high with respect to that of the test piece.
Initially, the goldplated substrate’s emittance is measured. A copperConstantan thermocouple is fixed in the centre of the goldplated substrate. Four nylon threads (e.g. = 0,16 mm) secure the substrate and test piece to the centre of a sample holder which is fixed on the axis.
This is then lowered to the centre of the cryogenic shroud, which is coated in black and cooled with liquid nitrogen.
The dimensions are such that the shroud area is more than 100 times the total area of the test piece.
A window made of Suprasil quartz (working diameter: 90 mm) enables this test piece to be illuminated by an external heat source.
Test process and measurement
After a sufficient vacuum is attained (< 10-6 hPa), and the temperatures of the shroud and sample holder are stabilized, the test piece is heated up to 30 C. The decreasing temperature is then recorded down to 20 C. The total hemispherical temperature is then calculated for a temperature of 25 C. It is possible to do this for any temperature between -50 C and +75 C by using a similar method.
The errors of measurement depend mainly on the following quantities:
- specific heat of the test piece: Ce
- temperature: T
- time: t
The first quantity, Ce, can be measured from the test piece by means of thermal analyses such as with a differential scanning calorimeter, or is taken from the literature for clearly defined materials.
Figure C- 1: Standard sample substrate
Calculations of total hemispherical emittance
Calculation of the emittance is performed using the following formulas:
where:
(t1, T1) and (t2, T2) are two points on a cooling curve at times t1 and t2 for which the corresponding temperatures are T1 and T2;
(MC) is the total heat capacity of a test piece with goldplated substrate.
If the variation of specific heat is assumed to be parabolic with the temperature, then:
and
where:
is the total hemispherical emittance of test piece plus substrate;
r is the total hemispherical emittance of the goldplated substrate;
e is the total hemispherical emittance of the test piece;
(MrCr)0 is the thermal mass of the substrate at 0 C (JK-1);
Mr is the weight of goldplated piece (kg);
Cr is the specific heat of goldplated piece (J kg K-1);
Me is the weight of the test piece (kg);
Ce is the specific heat of the test piece (J kg K-1);
Sr is the goldplated surface area (m2);
Se is the surface area of the test piece (m2);
S is the total emitting surface area, i.e. S = Se+ Sr (m2);
T0 is the temperature of the cryogenic shroud (K);
t is time (s);
is the StefanBoltzmann constant = 5,7 10-8 W m-2 K-4.
Infrared emittance using IR spectrometer (h)
General
This method is based on optical measurements of absorptance of materials in the infrared range from 3 m to 21 m. This absorptance is determined measuring total hemispherical reflectance of these materials using an integrating sphere coupled with an infrared spectrometer.
As for solar absorptance measurements, one can make absolute measurements with a central sample holder, but another solution is to measure the reflectivity of the sample on the wall comparing the reflectivity of the sample with a calibrated standard (mirror or Infragold). The spectrum obtained is weighted by blackbody spectrum and integrated.
Configuration of samples
The size of the samples depends on the configuration adopted (either central or tangential) and the sizes of the sphere, of the beam, of the sample holder and of the measurement port. The sample should be large enough to receive the complete incident beam but small enough to disturb, at the minimum, the sphere integrity.
A classical size is 20 mm 20 mm or 20 mm diameter for an integrating sphere of 150 mm in diameter with a central sample mounting, with a maximum thickness of 5 mm.
The samples should be rigid.
Test apparatus and setting up
The test apparatus consists of a IR reflectometer covering the range from 3 m to 21 m equipped with an integrating sphere. The range of measurement is limited by the material used for the sphere walls (Infragold).
The spectrometer should be purged with a permanent dry air flux in order to eliminate CO2 and H2O absorption bands.
The signal to noise ratio over the whole interval from 2,5 m to 20 m should be better than 1 % full scale.
Test process and measurement
The measurement on the sample is made after a baseline measurement is obtained either on the standard sample for a tangential sample, or on the wall sphere if the sample is centrally mounted.
The baseline and the sample measurement should be made in the same conditions of the purge of the spectrometer.
Calculation of emittance
The spectrum obtained by the above test method is then weighted and integrating following the formula:
where:
A() is the spectral absorptance of the sample after 100 % reference correction (A() = 1 – R() or A() = 1 – (R() + T()) if the sample is transparent);
E() is the blackbody emittance spectrum at 300 K and can be calculated with the Planck law;
Equation ;
h = 6,626 × 10-34 Js and k = 1,381 × 10-23 JK-1.
The emittance at other temperatures can also be determined with measurements in another wavelength range. In this case, the blackbody spectrum should be calculated at this new temperature.
Infrared emittance using portable equipment (n)
General
This method is used to cover determination of the total normal emittance of opaque surfaces when using portable reflectometer instruments.
This test method is suitable for measuring over large surfaces where a nondestructive test is desired.
Depending on the equipment, the signal obtained is integrated over a defined spectral range (e.g. the “Gier Dunkle” DB100 equipment is an infrared reflectometer and has an integration from 5 m to 25 m).
Configuration of samples
The minimal sample dimensions are dictated by the diameter of the aperture on the portable equipment. This diameter is typically between 15 mm and 20 mm.
Measurements are only valid on flat samples. However, it is possible to perform measurements on spherical curved samples, provided the radius of curvature exceeds 300 mm.
Test apparatus and setting up
The surface to be measured is placed against an aperture on the portable sensing component. The specimen is alternately irradiated with infrared radiation from two heat sources, one at near ambient and the other at a slightly elevated temperature.
The detector receives both the radiation emitted from the test specimen and a constant radiation of all other surfaces inside the optical path. Only the reflected energy from the test specimen varies with the alternating irradiation and the detection amplifying system is made to respond only to this modulated signal.
The instrument should be calibrated with standards of known emittance.
Test process and measurement
The measurement on the sample is made after calibrating the instrument with known reflectance standards.
The calibration should be performed on at least two reference samples having low (black) and high (gold) reflectance properties and verified periodically by remeasuring the standards.
For semitransparent samples, a correction should be made for transmittance losses. Another possibility is to cover the semitransparent sample with an opaque material from behind. In this case, the reflectance of this combination can be measured.
Calculation of the normal emittance
The normal emittance for opaque material is defined as:
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(C-1)
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(C-2)
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where:
N is the normal emittance;
RS is the sample reflectance;
is the StefanBoltzmann constant;
TIR is the infrared source temperature;
ID is the energy seen by the detector.
By keeping the infrared source at a fixed temperature, the terms and TIR become constant:
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(C-3)
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with (C-2) and (C-3):
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(C-4)
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The sample reflectance RS is proportional to the measured signal ID. The normal emittance can be obtained by subtracting the measured reflectance from unity.
Bibliography
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ECSS-S-ST-00
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ECSS system – Description, implementation and general requirements
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ASTM E 490
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Standard Solar Constant and Zero Air Mass Solar Spectral Irradiance Tables
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