Introduction
An accurate knowledge of solar spectral irradiance (SSI) remains central
to the study of the climate on Earth. The variability in the ultraviolet (UV)
part of the spectrum and its influence on climate via the mechanisms of solar–terrestrial interactions, simulated by chemistry–climate models
, constitutes most of the research in SSI
measurements. Despite its extremely low variability, <0.05% over a
solar cycle , the near-infrared (NIR) part of the
spectrum plays a major role in the Earth's radiative budget due to its
quasi-total absorption by water vapour . The determination
of its absolute level remains challenging : the measurement
of the top-of-atmosphere (TOA) SSI started nearly 50 years ago and evolved
both with ground-based and space-borne instruments, and a consensus on the
absolute level in the NIR part is still to be achieved
.
Aircraft-borne instrumentation at an altitude of 12 km provided the first TOA
SSI measurement dataset in 1969 with an on-board standard
of spectral irradiance.
Several ground-based measurement campaigns in the UV, visible and NIR have
been conducted from the top two mountain-top reference sites since then:
At Izaña Atmospheric Observatory (IZO), the IRSPERAD dataset was obtained with a NIR (0.6–2.3 µm)
spectroradiometer and the QASUMEFTS instrument, providing a high-resolution UV spectrum;
both were calibrated against the Physikalisch-Technische Bundesanstalt (PTB) BB3200pg black body
.
At Mauna Loa Observatory (MLO), conducted a campaign with a 10-channel (UV, visible and NIR) filter
radiometer and with a double Brewer spectrophotometer measuring in the range
300–355 nm.
provided TOA SSI in the range 350 to 2500 nm, measured with a spectroradiometer. All of these measurement
campaigns used different types of 1000 W lamps, traceable to National Institute of Standards and Technology (NIST) standards as calibration sources.
Finally, the CAVIAR and CAVIAR2 spectra were obtained with an infrared Fourier spectrometer (FTIR) calibrated against
National Physical Laboratory (NPL) standards at the UK Met Office observation site in Camborne, in the range 1–2.5 µm.
TOA SSI values from all the above-mentioned ground-based campaigns were obtained
using the Langley plot technique that permits extrapolation to the TOA
irradiance in atmospheric windows chosen according to criteria detailed in Sect. . The monitoring of the absolute spectral
calibrations is secured through comparisons with relative stable secondary
standards. The reliability of the traceability to primary irradiance
standards is an advantage for ground-based measurement. Performing these
measurements based on world reference sites for the determination of TOA
physical quantities, such as IZO and MLO, on days with often pristine conditions, ensures a high accuracy of the TOA extrapolations
.
On the other hand, space-borne SSI measurements covering the NIR range
started in the 1990s, though these were limited to wavelengths shorter than 2.4 µm. From the
SOLSPEC instrument family, the instrument SOSP (SOlar SPectrum) on board EURECA that pioneered the
space-borne NIR absolute solar spectroscopy released the ATLAS3 reference
spectrum . An upgraded version of the SOLSPEC instrument,
SOLAR/SOLSPEC, including a fully refurbished NIR channel, readout electronics
and extended wavelength range up to 3 µm of SOLSPEC, flew from 2008 to 2017
on board the International Space Station (ISS) ,
releasing the SOLAR2 and SOLAR-ISS(IR)
; SOLSPEC is the space-borne instrument that measured SSI farther in the NIR. The instrument providing the longest time series
of SSI measurements in the NIR is the SIM (Spectral Irradiance Monitor) prism
spectrometer on SORCE (Solar Radiation and Climate Experiment) launched in
2003 and still on orbit but with infrequent operational time, due to the end of battery life. Another instrument
contributing to NIR SSI measurements is SCIAMACHY (Scanning Imaging
Absorption Spectrometer for Atmospheric Chartography)
, a remote sensing spectrometer adapted to
measure SSI. The latest data release is SCIAMACHY V9 .
All above-mentioned NIR datasets reasonably agree up to 1.3 µm. When
comparing SORCE and ATLAS3, the difference between both does not exceed 2%
in the NIR range, which is a consequence of SORCE being scaled up to ATLAS3,
due to incompatibilities of fractional TSI (total solar irradiance) between
both datasets .
At 1.6 µm, corresponding to the minimum opacity value of the solar
photosphere, differences up to 8% (reaching 10% at 2.2µm) were
observed between ATLAS3 and SOLAR2 . This
bias motivated the development of new ground-based instrumentation measuring
the SSI NIR: CAVIAR and IRSPERAD . The data of
both experiments confirmed this bias, both showing a level closer to that of
SOLAR2. Posteriorly, SOLSPEC and SCIAMACHY data reprocessing processes tend to
intermediate values between ATLAS3 and SOLAR2 .
In this paper we present a rerun of the IRSPERAD experiment, named
PYR-ILIOS, carried out in July 2016. While still using the Langley plot
technique and calibration against the PTB black body, this new experiment
differs from IRSPERAD in three aspects: first, the observation site is MLO
instead of IZO; second, possible sources of systematic uncertainties have
been identified and fixed (see Sect. ); third, the traceability
of the calibration to the primary standard was improved (see
Sect. ). A detailed estimation of the uncertainty budget will
be presented in Sect. , followed by the presentation of the
obtained spectrum and its comparison with space-borne and ground-based
spectra described in this section, along with a discussion on the status of
the NIR SSI measurement.
Methods
Instrumentation
The core of the direct Sun measurement instrumentation is a
Bentham NIR spectrometer: it consists of a double monochromator placed inside
a thermally stabilized container, with light detection by a PbS cell. An optical fiber guides the sunlight between the entrance slit of the spectrometer and
the diffusor of a 7.2∘ field-of-view (FOV) sunlight-collecting optics
(telescope). The telescope is connected to an EKO Sun tracker that provides a
tracking
accuracy
(https://eko-eu.com/products/solar-energy/sun-trackers/str-22g-sun-trackers, last access: 12 December 2018)
of 0.01∘. The working wavelength range is from 0.6 to 2.3 µm, with a nominal 10 nm bandpass.
The instrument characteristics are given
in depth in and have remained unchanged since. No
modifications have been made either to the telescope or to the
spectrometer. Nevertheless, a factory defect in the assemblage of the
components was detected and rectified: the lens focusing the light collected
in the optic fiber into the spectrometer entrance slit was properly fixed
into its barrel support for the PYR-ILIOS campaign, which was not previously
the case for the IRSPERAD campaign at IZO. Another change relative to the
IRSPERAD campaign was that the thermally stabilized spectrometer container
was placed indoors in a thermally stabilized environment, which reduced
thermal stress due to outdoor exposure and improved the stability of the
spectrometer's response.
Langley plot method
The wavelength-dependent direct transmitted solar
irradiance in the atmosphere is described by the Beer–Bouguer–Lambert (BBL)
law. For spectral regions where molecular absorption is negligible and only
Rayleigh and aerosol scattering are present, the BBL law is written in the following form:
Eλ=E0λD-2exp-mRθτRλ-mAθτAλ,
where E0 is the irradiance at the top of the atmosphere (TOA), m is the air
mass factor (AMF) as a function of the solar zenith angle (SZA) θ and
τ is the optical depth that depends on λ. D is the ratio
between the Earth–Sun distance at the moment of the measurement and the mean
Earth–Sun distance; subscripts R and A stand for Rayleigh and aerosol,
respectively. Because the aerosol vertical profile over the measurement site
at the moment of the measurement is unknown, aerosol AMF is approximated to
Rayleigh AMF ; considering mA≈mR≈m,
defining τ=τR+τA and taking the logarithm of
Eq. (), it can be rewritten as
logEλ=logE0λD-2]-mθτλ.
Provided that τλ remains constant for a series of
measurements of E0λ taken over a given range of
mθ (spreading over a half day), the TOA value of
E0λ is thus the intercept at the origin (m=0) of
the least-squares regression to the data series Eλ as a
function of m(θ).
Solar zenith angles (SZAs) are calculated with the NOAA Solar Position
Calculator (https://www.esrl.noaa.gov/gmd/grad/solcalc/index.html, last access: 12 December 2018)
that implements algorithms and are subsequently corrected for
atmospheric refraction effects according to . AMFs are
calculated using the Kasten and Young algorithm .
Atmospheric windows
The wavelength domains for which the Langley plot
method described in Sect. is valid, i.e. atmospheric windows,
were determined through the model using a procedure developed
in and also used in : using a TOA
reference spectrum as input, the MODTRAN (MODerate resolution atmospheric
TRANsmission) RTM (radiative transfer model) was used to
simulate irradiances measured at the ground, as a function of the measurement
site parameters, for a series of AMFs. The Langley plot method was applied to
these simulated irradiances, and the wavelengths for which the synthetic E0
recreated the input TOA within 0.5% were kept as valuable wavelengths for
the Langley plot; these set of wavelengths were grouped in contiguous windows
called atmospheric windows.
Absolute calibration
The absolute calibration was performed against a primary
standard of spectral irradiance, the BB3200pg black body of the PTB. It has
been extensively described in
and . The spectral irradiance equation
describing the black body emission is calculated using Planck's law:
EBB(λ)=εBBABBdBB2c1n2.λ51expc2nλ.λ.TBB-1,
where εBB and ABB stand, respectively,
for the effective emissivity and the aperture of the BB3200pg,
dBB for the distance between the black body aperture and the
optic centre of the telescope and n for the refractive index of air; c1 and
c2 are the first and second radiation constants.
The fundamental parameter, the temperature of the cavity TBB, is known
with a standard uncertainty of 0.5 K (∼0.02% for a nominal
temperature of 3000 K) with a drift lower than 0.5 Kh-1
. The uncertainties on
εBB and ABB are
1×10-4(0.01%) and 0.04 mm (0.03%), respectively
. The distance between the black body aperture and the
telescope optical active surface, the diffuser dBB, is the sum of two
distances: dBB=ds+dT, where dS is the distance between the
black body and the first optical surface of the telescope, the quartz plate,
and dT is the distance between the quartz plate and the diffuser. The
uncertainties on ds and dT are 0.05 mm and 0.5 mm, respectively; the combined uncertainty on
dBB is 0.5 mm, 0.04%
at the nominal distance of 1384.05 mm.
The absolute calibration coefficient R, that converts the spectrometer
signal into irradiance, is given by Eq. ():
R(λ)=EBB(λ,T)SBB(λ),
with SBB being the signal recorded by the spectrometer and
EBB, the emission of the black body, given by Eq. ().
During the calibration campaign at PTB, two different temperature set
points, 3016.5 and 2847.6 K, were used to build the response curve,
RBB. The distance dBB was kept fixed at 1384.05 mm so that the black body aperture was seen by the entrance optics with an
angular extension of 0.5∘.
Radiometric characterization
The spectrometer was characterized at the laboratory of the Belgian Institute
for Space Aeronomy (BIRA-IASB) for the uncertainty on the measured signal,
the detector sensitivity to temperature and for the wavelength scale. The
flat field of the detector was measured during the ground-based campaign at
MLO and the linearity was verified during the calibrations at the PTB laboratory:
The flat field of the entrance optics was measured during the ground-based campaign. The telescope was
angularly displaced from the normal Sun direction thanks to an angular fine-tuning mechanism, for a series of
angular positions for two orthogonal directions. The agreement between both directions' data curves allows
an insensitivity of the signal to solar depointing better than 0.05∘ to be estimated, although a finer angular
sampling would be necessary to accurately determine the angular limits of this insensitivity.
Given the 0.01∘ pointing accuracy of the Sun tracker, the response of the instrument is considered
to be insensitive to pointing during the campaign.
The temperature sensitivity of the spectrometer was determined in the laboratory . During
the campaign, the spectrometer box was placed indoors with its temperature being constant within 0.1 ∘C,
equivalent to the resolution of the temperature probe readout; no temperature correction on the signal was thus applied.
For the verification of the linearity of the detector, the telescope was placed at several different
distances from a stable 200 W lamp. The measured signal as a function of distance was successfully fitted to an
inverse square law function, demonstrating the detector linearity within a 2-decade dynamic range.
Relative calibration
A set of six FEL lamps (F102, F104, F417, F418, F545, F546)
were used as relative calibration standards, to monitor a possible change of
response of the spectrometer during the measurement campaign. Taking as
reference the lamps' signal measured at the PTB (27 April 2016),
SFjPTB(λ), four additional relative calibrations were
performed:
Immediately before the start of the measurement campaign on 29 June (i=1), the signal of the six lamps, SFjMLO1,
was measured on site. This first MLO relative calibration was valuable to monitor the spectrometers' response change between the
calibration at PTB and the beginning of the field measurements. During this 2-month period that included the transportation of
the equipment, a decrease of response varying between 1% and 3% in the 1000 to 2200 nm range was detected.
During the 20-day measurement campaign, three relative calibrations were performed: on 7 July (i=2), 14 July (i=3) and 19 July (i=4).
The cumulated loss of response between 29 June and July varied from 1.5% to 0.5% in the 800 nm to 1.8µm domain.
The corresponding correction factor for each relative calibration is
Ki(λ)=1N∑jNSjMLOi(λ)SjPTB(λ),
where N stands for the total number of lamps, j for the lamp number and
i for the calibration day index. K(λ) was obtained by linear
interpolation for all days of the campaign.
Ground-based campaign
The PYR-ILIOS campaign took place during the first 20
days of July 2016 at the Mauna Loa Observatory (MLO) on the island of Hawaii.
The MLO (19.53∘ N, 155.58∘ W) is situated at 3397 m a.s.l.; it is the leading long-term atmospheric monitoring facility on Earth,
a primary calibration site for the AErosol Robotic NETwork
(AERONET; https://aeronet.gsfc.nasa.gov/, last access: 12 December 2018), a global station
for the Global Atmosphere Watch (GAW) of the World Meteorological
Organization (WMO) and the premier
site
(https://www.esrl.noaa.gov/gmd/obop/mlo/programs/esrl/co2/co2.html, last access: 12 December 2018)
for the measurement of the concentration of atmospheric carbon dioxide. It is
considered a world reference site to accurately determine extraterrestrial
constants via the Langley plot method .
Data selection and analysis
From the 20-day campaign, 12 high-quality half-days, all during morning time,
were kept for analysis. The selection criteria were verification of
cloudless clear skies and a Langley plot correlation coefficient R2>0.9.
The morning data of the days 2, 3, 5, 7, 8, 9, 10, 11, 13, 14, 16 and 17 July 2016 were kept for
analysis; a subset of these selected Langley plots is shown in Fig. ,
for four different wavelengths.
Measured irradiance and respective Langley plot fits for the four AERONET wavelengths, 870, 1020 and
1640 and 2065 nm, shown for the morning data of 2, 9, 10 and 13 July 2016.
Uncertainty budget
Uncertainty on the spectrometer signal
The raw uncertainty of a spectrometer measured signal,
Sxraw, regardless of its source, either solar (SS), black body
(SBB) or lamp SFjPTB,SFjMLO
signal, is a function of the intrinsic noise of the measured physical signal
convolved by the spectrometer's transmission and detector's response. The
uncertainty on a measured signal u(Sxraw) was determined in the laboratory
by calculating the standard deviation for a sample of measured signals at
several intensities from a 1000 W stable lamp . This
uncertainty is shown in Fig. .
Individual uncertainties contributing to the combined uncertainty in the TOA SSI, u(E0).
Black-body-associated quantities (u(EBB), u(SBB) and u(R)) and lamp-associated quantities (u(K)) are plotted
for the full wavelength working range, while solar-measurement-associated quantities (u(Ss), u(E)
and u(E0)) are plotted in the atmospheric windows' wavelengths.
Additionally, all measured signals, Sxraw(λ), are affected by an
uncertainty term due to the finite bandpass of the instrument, u(CΔλ), and the uncertainty on the determination of the true wavelength
scale, u(Cλ) .
u(Sx(λ))2=u(Sxraw(λ))2+uCλ(Sxraw(λ),δ(λ))2+uCΔλ(Sxraw(λ),BW)2,
where δ(λ) stands for the maximum deviation in the determination
of the real wavelength scale of the spectrometer. δ(λ) was
determined in the laboratory by measuring the deviation between the measured and
the corresponding nominal peak values of a series of well-known emission rays
of Xe, Ar and Kr lamps as well as of lasers and Pen-Ray lamps,
δ(λ)<0.2 nm for the working wavelength range. BW stands for
the spectrometer bandpass of 10.63 nm, measured in the laboratory.
List of relative uncertainties terms expressed as percentages. The coverage factor is k=1 for
all terms. A and B stand respectively for type A and type B uncertainties, while C stands for combined
uncertainty according to . u(Cλ) and u(CΔλ) are calculated for a
solar signal. The prefix u, for uncertainty, is omitted for each term of the first row, for the sake of clarity.
AMF
TBB
ABB
ϵBB
dBB
EBB
SBB
Cλ
CΔλ
SS
K
R
E
E0LP
E0AOD
E0
Type
A
B
B
B
C
C
A
B
B
A
C
C
C
A
A
C
AMF
λ (nm)
2
0.04
870
0.11
1.29
0.18
0.62
0.52
0.43
1.29
1.46
0.66
0.06
0.67
4
0.19
1020
0.02
0.04
0.01
0.04
0.10
0.33
0.04
0.07
0.14
0.14
0.34
0.41
0.17
0.11
0.20
8
0.79
1640
0.09
0.17
0.06
0.06
0.13
0.08
0.19
0.26
0.12
0.06
0.14
Langley plot sensitivity to aerosol daily variation
Ratio of ground-based and space-borne spectra relative to SOLSPEC-ISS(IR).
Uncertainty at ±1σ is represented by the shaded areas.
Uncertainty on a calibrated direct Sun measurement
The expression for a calibrated solar measurement, E(λ) is
E(λ)=SS(λ).R(λ).K(λ),
with SS(λ), R(λ) and K(λ) being expressed by
Eqs. (), () and (), respectively.
The uncertainties associated with the factors in Eq. () were calculated using the law of propagation of uncertainties
(LPU) and are represented in Fig. . The similarity of shapes of the curves of the individual uncertainties reflects the
convolution of the measured signals by the spectrometer's response. The largest contribution to the calibrated solar signal comes
from the uncertainty on the absolute calibration which is dominated by the uncertainty u(SBB) of the measured signal, SBB,
of the black body, whereas the uncertainty on the emission of the black body, u(EBB), is known within 0.2% for the totality of
the wavelength range.
Uncertainty on the determination of the TOA irradiance
The uncertainty in the determination of the TOA irradiance via
the Langley plot method, u(E0LP), corresponds to the uncertainty on the
determination of the intercept at origin, P0, when applying a linear
regression on Eq. (). The uncertainty on the measured E
in the Langley plot method logarithmic space, u(log(E)), and the uncertainty
in the u(E0LP) irradiance value are given by
u2(log(E))=∂log(E)∂E2.u2(E)=u(E)E2u2E0LP=∂exp(P0)∂P02.u2(P)=exp(P0)2.u2(P0),
where E0LP=exp(P0) gives the irradiance TOA value. The uncertainty
in P0 was estimated using two independent methods.
A Monte Carlo method was employed. Given a measured Langley plot dataset consisting of (mi,log(Ei)) points, a new
synthetic dataset (mi*,log(Ei*))) is created, where
each log(Ei*) is affected by a random normal distributed quantity, with a standard uncertainty given
by Eq. (),
and each mi is affected by an uncertainty defined in Sect. . The standard deviation in the distribution of
the N>>1 retrieved P0 values corresponds to u(P0), with u(E0LP) given by Eq. ().
The weighted total least-squares algorithm developed by was used. It computes the uncertainty
in the determination for both linear regression parameters using the uncertainties on the measured quantities as inputs,
i.e. the uncertainties on E (Sect. ) and AMF (Sect. ).
The uncertainty on the determination of the TOA irradiance, u(E0), matches
perfectly for both methods; it is below 1% for the central wavelength
range of 0.9 to 2.2 µm. Figure shows the
contribution of all the uncertainty terms detailed in Sect. .
In Table a list of the uncertainty types and values at key
wavelengths is presented.
Quantification of the circumsolar radiation
An ideal sunlight-collecting optic device should ideally have an acceptance angle equal to that of the solar disk seen on Earth,
∼0.5∘. In practice the FOV is much larger than 0.5∘ such that Sun- and sky-scattered radiation enters the FOV
of the sunlight-collecting optics, affecting the direct normal Sun measurement. Circumsolar radiation is strongly dependent on
aerosols' size and their abundance, increasing with AMF and decreasing with wavelength due to Rayleigh scattering .
The estimation of circumsolar radiation was done with the aid of the
LibRadtran RTM. LibRadtran computes the radiance field of
the Sun- and sky-scattered radiation. The integral of this radiance field over the
solid angle of the acceptance cone of the entrance optics is the amount of
circumsolar irradiance (CSI) measured by the spectrometer in excess of the
normal direct Sun irradiance (DNI) . For standard
clear-sky atmospheric conditions observed at MLO and for typical aerosol
charges values measured during the mission, the quantification of CSI is
shown in Fig. . Given the uncertainty budget, the impact of the
circumsolar radiation can be considered negligible.
Estimation of air mass factors' uncertainty
As referred to in Sect. , the absence of knowledge
of the vertical profile of the relevant species, namely aerosols, is a
limiting factor for accurately calculating the AMF. The uncertainty in the
AMF calculation is based on the approach of , who considered that
mA, due to the presence of stratospheric aerosols, could take the form
mA=k1.mR+k2.mO3, with k1+k2=1 and mO3 standing
for the ozone air mass. Assuming a rectangular distribution of mA
delimited by k1=1 and a k1=0.2, the standard deviation of mA can be
calculated as u(mA)=∣mA(k1=1)-mA(k1=0.2)∣.123, to be used as input for the determination of the Langley
plot parameters' uncertainty (Sect. ).
The possible bias introduced at the Langley plot's intercept at origin by a
realistic non-constant aerosol concentration during the measurement was
estimated considering a measured aerosol optical depth (AOD) profile. For a given measured Langley
plot consisting of (mi,log(Ei)) and regression parameters E0LP and
τ, a synthetic Langley plot (mi,log(Ei*)) is determined. The
synthetic Ei* values are calculated with the expression Ei*=E0LP.exp(-miτi*), where τi*=τi*(λ,ti)=τAOD(λ,ti)+τR(λ,ti); τAOD(λ,ti)
stands for the real diurnal aerosol optical depth profile measured with
AERONET (available at λ=870, 1020, 1640 nm) and
τR(λ,ti) the Rayleigh optical depth calculated according
to . This bias at the intercept at origin, expressed as
a ratio, E0LP/E0*, averaged over the selected days is -0.2%,
+0.4% and +0.1% for 870, 1020 and 1640 nm, respectively. The
signal of the bias replicates the signal of the AOD morning trend measured at
MLO, and the larger negative bias at 1020 nm relative to 1640 nm is due to the
more pronounced AOD negative trend at 1640 nm. Assuming that the interval
∣E0LP-E0*∣ comprises the true value of the intercept at
origin, E0, within a rectangular distribution, the corresponding
uncertainty u(E0AOD)=∣E0LP-E0*∣23
amounts to 0.06% at 870 and 1640 nm and 0.1% at 1020 nm, which is
added quadratically to the uncertainty on E0LP (Sect. ) to
determine the uncertainty on E0. u(E0AOD) is interpolated linearly
to the working wavelength range.
Discussion
The difference observed between IRSPERAD and PYR-ILIOS is not
explained by the uncertainties of both datasets. An atmospheric bias is not
considered because MLO and IZO are world reference sites for the
determination of extraterrestrial constants
and the atmospheric perturbations
in ground-based SSI measurements are negligible
. By carrying out the new PYR-ILIOS
experiment, we unveiled a defect of fixation of the focusing lens. Due to the
fact that the instrument was moved between the IRSPERAD pre-campaign relative
calibration (31 May 2011) and the start of the Sun measurement campaign
(1 June 2011 onwards), the effect of the lens' eventual movement was not considered
and therefore not monitored; this defect likely biased the SSI obtained
during the IRSPERAD campaign in a non-reproducible way. This defect was
detected and corrected for the PYR-ILIOS campaign and the relative
calibration strategy adapted to identify possible similar issues: the
instrument was installed and powered on and the lamps were measured; the solar
measurements began immediately afterwards, without displacing or powering
off the instrument. The PYR-ILIOS relative calibration procedure highlights
the importance of monitoring ground-based pre-campaign instruments' response with secondary standards. Additionally it justifies the choice of PYR-ILIOS
as a more reliable measurement than IRSPERAD, due to the higher confidence
in the traceability of the instrument's calibration to the black body primary
standard.
In the higher disagreement region around 1.6µm, the most recent data
versions of SOLAR/SOLSPEC and SCIAMACHY instruments, SOLSPEC-ISS and
SCIAMACHY V9, respectively, as well as PYR-ILIOS converge to an intermediate
level between SOLAR2 and ATLAS3. This convergence is also observed for
longer wavelengths: in the 2µm region PYR-ILIOS and Kindel et al. are in
reasonable agreement, while the level of the two SCIAMACHY V9 adjacent bands
(1.9–2.05 and 2.2–2.4µm) suggests that it is also in agreement with the two ground-based datasets; on the other hand, in
this region, both data versions of the SOLSPEC/SOLAR still retain the 8%
difference to ATLAS3 and SORCE.
A rerun of the measurement campaign at IZO would be crucial to understand the
observed discrepancy between PYR-ILIOS and IRSPERAD datasets. Data from the SORCE
successor, TSIS, which has been on board ISS since December 2017, are expected to further
increase the understanding of SSI in the NIR.