LYRA, Solar UV radiometer on the technology demonstration platform PROBA-2
LYRA, SOLAR UV RADIOMETER ON THE TECHNOLOGY DEMONSTRATION
Y. Stockmana , J.-F. Hochedezb , W. Schmutzc , A. BenMoussab , J.-M. Defisea , F. Denisa , M. D'Olieslaeger d,g,
M. Dominiqueb , K. Haenend,g , J.-P Halaina, S. Kollerc , S. Koizumih ,V. Mortetd,g , P. Rochusa , U. Schühlee ,
A. Soltanif , A. Theissen b.
(a) Centre Spatial de Liège (CSL), University of Liège, Avenue du Pré Aily, B-4031 Angleur, Belgium.
(b) Royal Observatory of Belgium (ROB), Circular Avenue 3, B-1180 Brussels, Belgium
(c) Physikalisch-Meteorologisches Observatorium Davos and World Radiation
Center (PMOD), Dorfstrasse 33, CH-7260 Davos Dorf, Switzerland
(d) Institute for Materials Research(IMO), Hasselt University,
Wetenschapspark 1, B-3590 Diepenbeek, Belgium
(e) Max-Planck-Institut für Sonnensystemforschung (MPS), D-37191 Katlenburg-Lindau, Germany
(f) Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN)
F-59652 Villeneuve d'Ascq, France.
(g) Division IMOMEC, IMEC vzw, Wetenschapspark 1, B3590 Diepenbeek, Belgium.
(h) Optical Sensor Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
LYRA is a solar radiometer part of the PROBA 2
micro satellite payload. LYRA will monitor the solar
irradiance in four soft X-Ray - VUV passbands. They
have been chosen for their relevance to Solar Physics,
Aeronomy and SpaceWeather: 1/ Lyman Alpha
channel, 2/ Herzberg continuum range, 3/ Aluminium
filter channel (including He II at 30.4 nm) and 4/
Zirconium filter channel. The radiometric calibration
is traceable to synchrotron source standards. The
stability will be monitored by on-board calibration
sources (LEDs), which allow us to distinguish between
potential degradations of the detectors and filters.
Additionally, a redundancy strategy maximizes the
accuracy and the stability of the measurements. LYRA
will benefit from wide bandgap detectors based on
diamond: it will be the first space assessment of
revolutionary UV detectors. Diamond sensors make the
instruments radiation-hard and solar-blind (insensitive
to visible light) and therefore, make dispensable visible
light blocking filters. To correlate the data of this new
detector technology, well known technology, such as
Si detectors are also embarked. The SWAP EUV
imaging telescope will operate next to LYRA on
PROBA-2. Together, they will provide a high
performance solar monitor for operational space
weather nowcasting and research.
LYRA demonstrates technologies important for future
missions such as the ESA Solar Orbiter.
Key words: UV radiometer, diamond detectors.
The knowledge of the solar spectral irradiance is of
large interest to Solar Physics, Aeronomy and to other
fields of heliospheric and planetary research. The solar
ultraviolet (UV) irradiance below 300 nm is the main
source of the energy converted in the Earth's
atmosphere, controlling its thermal structure .
Absolute measurements of the UV Sun irradiance are
recognized to be difficult. They always require a space-
born instrument. LYRA will be complementary to
existing radiometers such as UARS, SEE/TIMED,
SORCE, by monitoring the solar flux in four carefully
selected UV passbands.
This paper describes LYRA (the LYman alpha
RAdiometer), a solar EUV - VUV radiometer and the
activities performed to deliver the Proto Flight Model
(PFM). One purpose of the instrument is to
demonstrate several technologies able to enhance
vacuum ultraviolet measurements by increasing the UV
detection efficiency and the ability to maintain
calibration. LYRA benefits from diamond detectors: it
will be the first space assessment of new UV detectors.
Diamond is a wide bandgap material, that makes the
sensors radiation-hard and “solar-blind”. This latter
property allows suppressing the usual filters, that block
the unwanted visible, but attenuate seriously the
desired UV radiation.
While spectral diagnostics are beyond its scope, LYRA
has the advantage of quasi-continuous monitoring with
very high cadence observations, up to 100 Hz, of
interest for the study of solar flares and for the limb
occultation technique. Continuous long-term time
series of the EUV solar irradiance can bring insights
into fundamental questions such as coronal heating, but
here too, the higher the sampling rate, the lesser the
bias of the statistics.
2. INSTRUMENT DESCRIPTION
LYRA is part of the scientific payload of Proba2
(project for on-board autonomy) micro-satellite, a
space mission of the European Space Agency (ESA)
that aims to:
perform in-flight demonstration of a series of
new spacecraft technologies
support scientific mission of a set of selected
PROBA-2 is a technology evolution of the successful
PROBA-1 in orbit since October 2001 . The
spacecraft has a size of less than a cubic metre with a
weight of 100 kg.
PROBA-2 includes major Belgian contributions. It is
developed under an ESA General Support Technology
Program (GSTP) contract by a consortium led by
Verhaert Design & Development (Belgium). It will be
launched in 2007 as a piggy back payload, to reach a
helio-synchronous polar dawn-dusk orbit for a nominal
Near LYRA, SWAP, the Sun Watcher using Active
Pixel Sensor and on-board image processing will
provide an image of the solar corona at 17.4 nm .
The LYRA and SWAP solar payloads both developed
under the management of the Centre Spatial de Liège
(CSL, Belgium) will provide an overall survey of the
solar corona during the PROBA-2 mission.
2.1 Radiometer design
LYRA is a compact solar VUV radiometer ,
designed, manufactured and calibrated by a Belgian -
Swiss - German consortium
international collaborations. It has a set of three
redundant units (see Fig. 1), each including four
spectral channels: 1- 20 nm (zirconium filter), 17-
70 nm (aluminum filter), 115-125 nm (Lyman alpha)
and 200-220 nm (Herzberg). Each channel includes a
pinhole collimator and a head with a precision
aperture, a spectral filter, a detector and two LED light
sources (See Fig. 2).
The design of the head takes into account opening
angle, cleanliness, thermal and mechanical issues.
The Physikalisch Meteorologisches Observatorium
Davos (PMOD, Switzerland) provides the optical,
electronical and mechanical design which is similar to
the VIRGO photometer on board of SOHO and the
SOVIM radiometer. The solar-blind diamond detectors
have been designed and fabricated at IMOMEC,
Belgium, in collaboration with the National Institute
for Materials Science (NIMS), Tsukuba, Japan. The
LYRA development takes into account cleanliness and
thermal issues usually necessary for the EUV spectral
Fig. 1: View of the 3 heads in the FM, each with 4
channels are aligned behind an aperture door
Fig. 2: Exploded view of one of the three identical
LYRA units. Each unit contains 4 spectral channels.
The collimator, the filters, the LEDs and the head can
be seen. The detectors are hidden by the head. The
incoming radiation is coming from the left side
2.2 LYRA Structure and electronics design
The dimensions of LYRA are 315 mm * 92 mm *
222 mm with a mass of 3.533 kg. Given the geometry
of the collimator, view-limiting apertures of 8 mm
diameter, precision apertures of 3 mm diameter and
detector sensitive area of 4 mm, the FOV is 2.09° and
the unobstructed FOV is 7°. An alignment cube on the
top of the box is coaligned with the collimator axes.
This alignment cube allows the co alignment between
SWAP and LYRA on the platform.
Great care is taken on the electronics part since signals
in the range of 100 pA are expected. The electronics is
composed of 5 subsystems, a first PCB pre-amplifier
fixed to the detector, an amplifier, a MUX/VCF board,
to convert the signal current to frequency, a digital
board with the ASIC and the data interface, and finally
a power converter board.
2.3 Detector, filters and radiometric model
The detector technologies developed for LYRA are a
photo-resistive device (metal-semiconductor-metal,
MSM) under 5V bias and a n-i-p photodiode (PiN).
Note that the PIN photodiode is operated in a
photovoltaic mode (unbiased). Classical Si diode
detectors are used for comparison with a well known
technology. A radiometric model based on the solar
spectral irradiance, transmittance of the LYRA filters
and detector responsivity (PiN, MSM and Si) is used to
determine the anticipated photocurrents and their
spectral purity. The anticipated photocurrent was also
required to define the feedback resistor of the
amplifiers. A generic detector was used until all flight
model devices were calibrated across the full necessary
range. Updating the radiometric model with the most
reliable data was and is a permanent process within the
LYRA project. For most of the channels, the
radiometric model fits well with the calibration data of
the overall instrument. Only one channel indicates a
large discrepancy with respect to the radiometric model
computed value. If the accuracy of the current
calculations are well determined, the observed
discrepancy is linked to this problematic channel
configuration. Indeed, this channel has two Lyman
alpha filters (one N and one XN) to get enough spectral
purity, because it is using a Si detector. Each filter
transmission / rejection was measured separately, but it
was not possible to measure the rejection once the
filters are put together. The rejection is about 10–8, well
below the limits of the measurement setup. As an
example, the results of one filter transmittance
measurement are given in Fig. 3.
Fig. 3: Measured transmission and radiometric
extrapolation for the N122 filter
There is no special difficulty with the Herzberg
channel thanks to the large expected signal (>nA) and
the used of PiN detectors. However this must be
moderated by its small variability (<2%) and hence, by
the need for high precision. For Lyman alpha, Al and
Zr channels, the results display the same signal as for
AXUV detectors (from IRD) with two filters. The
superiority of diamond detectors over Si detectors is
clear from this channel, where despite the two filters
the spectral purity and S/N ratio is worse.
Additionally, as a wide bandgap material, diamond
makes the sensors radiation-hard and “solar blind”.
3.1 Traceability to radiometric standards
It is a scientific goal of LYRA to improve the absolute
accuracy of the measurement (goal 5%). This implies
the need for sub-systems and system calibrations, on-
ground and in-flight. The radiometric responsivity of
each LYRA channel has to be determined over a
wavelength range that is extremely large: from the soft
X-Rays (1 nm) to the near infrared. First, subsystems
(filters, MSM detectors,
characterized for their UV responsivity, visible light
blocking, background noise, dark current, linearity,
temporal stability within different wavelength ranges.
Secondly, the LYRA instrument was calibrated, each
channel separately. The measurements were carried out
by several teams at radiometric calibration facilities:
CSL, PTB/BESSY, NIST, and IMOMEC. To cover
such a large spectral range, synchrotron beamline
facilities are required. They also provide the
traceability to a primary source standard. The
calibration results obtained with the different detector
types and filters are reported in dedicated publications.
A first global calibration of the LYRA instrument was
performed in July 2005 with two monochromatic
beamlines (Normal Incidence (NI) and Grazing
Incidence (GI) of the
Bundesanstalt (PTB) at the electron storage ring
BESSY II in Berlin.
Started in November 2003, the pre-delivery tests and
calibration activities were finished in 2005. The LYRA
calibration plan consists of the following calibration
First detector campaign at IMOMEC and
PTB/BESSY. This campaign was used to characterize
and compare the spectral responsivity of MSM and PiN
structures over the spectral range from 1 nm to
1000 nm  (Fig. 4). Based on the various data sets
gathered during the calibration campaigns (Fig. 5), the
PiN diodes show good response in the Herzberg
channel but are insensitive at the Lyman alpha
The MSM structures show higher responses with a
solar blindness of typically 4 decades in magnitude
between 200 and 400 nm. Their time response is of the
order of several ms.
The MSM are selected for the Lyman alpha and Soft
X-rays channel, where a signal of 100 pA is expected
PIN detectors) are
and is manageable by the LYRA electronic. Alternative
detectors are considered to provide a spare or a
calibration backup in this channel.
Fig. 4: MSM11 detector responsivity combining PTB
(Gi and NI) and IMOMEC measurements.
Fig. 5: Typical absolute spectral responsivities (A/W)
measured for PiN and MSM diamond detectors.
(2) Precision aperture area measurements at PMOD.
Only the precision apertures are critical and therefore
calibrated. The open aperture diameter is 3.0 mm. The
manufacturing tolerance of the precision apertures is
H7 or +10/-0 µm in diameter. After manufacturing, the
apertures were sent to the Swiss Federal Office of
Metrology and Accreditation (METAS) for calibration.
The inspection of roundness allows area calibration by
diameter. The overall accuracy must be considered as
not better than 1.2 µm in diameter, that is 8x10-4 in
(3) LEDs calibration campaign at IMOMEC. The
following tests were carried out:
- Operational point stability and drift,
- Measurement and comparison of the operational
point of different photodiodes (Fig. 6),
- Switching behavior after prolonged stress,
- Temperature dependence of the emission and the
emission stability at various temperatures,
- Calibration with respect to diamond detector.
This latter calibration indicates that it is necessary to
add pinholes in front of the LED to avoid saturation of
The LED temperatures are monitored during flight so
that a stability of a few percents is expected.
Emission Vio 23 LED
I fwd (mA
Fig. 6: 380 nm LED emission spectra versus forward
(4) Filters calibration campaign at CSL, MPS, ROB
and PTB/BESSY. Measurements
transmission are performed. The flight filters are
documented with a complete package of certifications.
After these tests, correlation with the manufacturer data
is carried out. Example of comparison between the
manufacturer data ARC (Acton Research Corp. USA)
and the measured one for the Herzberg filters is
presented in Fig. 7. Same as been completed with the
Aluminum and Zirconium filters measured data with
the LUXEL data.
Finally, the filters were mounted on the Flight Model
(FM) instrument for vibration, thermal and solar blind
(5) Second detector campaign at IMOMEC, CSL, MPS,
NIST and PTB/BESSY. This calibration program of FM
detectors is designed to know their XUV-to-VIS
response and the stability of their performance with
time . Part of this campaign addresses other
characterizations (linearity, flat field).
(6) Global instrument calibration campaign at
PTB/BESSY. This program
radiometric performance of the whole LYRA in order
to provide the most accurate knowledge of its spectral
response with the flight electronics configuration
(Fig. 8), time response (Fig. 9), flat field (raster scan)
of the light
has assessed the
Fig. 7: Herzberg filter transmission combining CSL,
BISA-ROB measurements and compared to ARC data
and extrapolated radiometric model.
Fig. 8: Absolute spectral responsivity (in A/W) of the
Zirconium filter channel between 1 nm and 30 nm. For
comparison, the dotted line represents the model used
in the LYRA radiometric model (detector R x Filter T).
(7) Second instrument calibration campaign at
PTB/BESSY. This campaign had the same goals as the
previous one, but was needed since some channels
have been modified with respect to the first calibration
campaign. Additional tests were performed to
- verify the ageing effect after 6 months storage and the
complete set of environmental campaign,
- verify the temperature environment impact,
- evaluate long drift effects (Fig. 9),
- calibrate at temperatures close to the expected orbit
- measure LED signal after EUV illumination.
(8) Heliostat tests. These tests were performed to verify
the correct rejection of the channels. From ground
looking to the Sun, no signal should be recorded by
LYRA. This test was very useful to detect a defect
filter that occurred during integration in the double
After delivery and during flight operations, further
ground calibrations will be performed on the spare unit
of the detector head.
3.2 In-flight calibration monitoring
The redundancy strategy requires that all three units are
made comparable, although they are not identical. One
is used continuously, the second (probably the one with
the IRD-AXUV silicon detectors) on a weekly basis,
while the last remains closed most of the time and is
only used a few times during the mission. In this way,
the radiometric evolution of the sensors and filters will
be assessed. Furthermore, the LEDs will help to
disentangle filter and detector ageing. In-flight flat
field campaigns will look for possible burn-in effects.
Fig. 9: Absolute signal (in A) of the Herzberg
channels as a function of time at 210 nm for a short
period (top) and for a long period (bottom).
4. LYRA DEVELOPMENT
LYRA development is based on a PFM approach. An
EM was build mainly to characterise and check the
electronic behaviour of the instrument. When a good
understanding of the electronic and check of the
interface with the S/C was performed, the PFM has
been manufactured. The major issue during the
instrument preparation was the availability of good
diamond detectors. An additional year in the
development was entirely dedicated to the detector
Once, it was decided to go ahead with the existing
detectors, LYRA underwent a classical ECSS PFM
(European Cooperation for Space Standardization
Proto Flight Model) approach. The functional tests
show good operation of the instrument and comply
with the strict requirements for the power consumption
from the platform.
The nominal power consumption is 2.85 W in nominal
mode, to compare to the 5 W requirements. A thermal
balance test was used to correlate the thermal model.
Vibration test indicates that the eigen frequencies are
far well above the 140 Hz required. The first measured
eigen frequencies are 555 Hz along X, 410 Hz along Y
and 522 Hz along Z. These values are somehow far
from the predicted one, but the discrepancies are well
Finally, thermal vacuum test was performed and
demonstrated good operation at the maximum and
minimum operating temperatures (+ 60 to – 40 °C)
and the survival (none operational) temperature (+70 to
– 40°C). Additionally to these classical tests, some
additional tests were carried out, as a test robustness of
the foil filters. Indeed, as mentioned previously, these
are only 150 nm thick for Al (300 nm for Zr), and
during some preliminary calibration activities one of
them broke. Some modifications were performed on
the initial design to add some venting paths between
the two cavities of the filters. Then, the instrument
successfully underwent an equivalent depressurisation
as the one going through during launch.
The design, the scientific objectives and the
development of the LYRA instrument have been
reviewed. Presently, LYRA is ready to be integrated on
the PROBA-2 spacecraft and is stored its nitrogen
purged container. LYRA will provide valuable inputs
for space weather forecasting, data for new scientific
research and also a continuation to the solar survey of
the ageing SOHO mission.
LYRA is a project of the Centre Spatial de Liège, the
Physikalisch-Meteorologisches Observatorium Davos
and the Royal Observatory of Belgium funded by the
Belgian Federal Science Policy Office (BELSPO) and
by the Swiss Bundesamt
Wissenschaft. LYRA receives development support
für Bildung und
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