Degradation assessment of LYRA after 5 years on orbit - Technology Demonstration -
We present a long-term assessment of the radiometric calibration and degradation of the Large Yield Radiometer (LYRA), which has been on orbit since 2009. LYRA is an ultraviolet (UV) solar radiometer and is the first space experiment using aboard a pioneering diamond detector technology. We show that LYRA has degraded after the commissioning phase but is still exploitable scientifically after almost 5 years on orbit thanks to its redundancy design and calibration strategy correcting for instrument degradation. We focus on the inflight detector’s calibration and show that diamond photodetectors have not degraded while silicon reference photodiodes that are even less exposed to the Sun show an increase of their dark current and a decrease of their photoresponse.
KeywordsUV solar radiometer Onboard calibration Degradation Diamond detector
The detectors are either silicon photodiodes (Si-AXUV, type AXUV20 from former International Radiation Detectors, now OptoDiode Corp.) or diamond detectors, the latter having been specifically designed for LYRA . A detailed description of LYRA performances can be found in .
In this paper, an assessment of the LYRA instrument’s performance and degradation is presented based on our onboard calibration strategy which includes −1- detector dark current (DC) measurements with LYRA covers closed, −2- onboard Light Emitting Diodes (LEDs) signal acquisition to assess the detector stability and to disentangle aging of the detectors from the optical filters, −3- parallel data acquisition sequence (redundancy concept) where data from two LYRA units can be acquired in parallel to inter-calibrate their spectral channels and −4- inter-calibration planning which combines LYRA observations with currently operating UV solar instruments.
2 Onboard detector calibration
The radiometric calibration stability over years of mission time is a goal for a solar instrument such as LYRA. The bi-weekly onboard detector calibration sequence consists of 40 min of dark current (DC) measurement with the covers closed, followed by 100 min with the visible (VIS) LED emitting at around 470 nm, 100 min with (UV) LED emitting at around 365 nm and again 40 min DC acquisition (see in Fig. 8). The sequence is first applied to the nominal unit 2 and to the first backup unit 3, after which this is repeated with unit 1. For LYRA, two types of diamond detectors had been developed by IMOMEC (University of Limburg, Belgium): metal–semiconductor–metal (MSM) photoconductors  and PIN (with the n-doped layer on top) photodiodes . Si-AXUV photodiodes were selected as reference detectors  for comparison with the newly developed diamond devices. All LYRA detectors had been measured regarding their quantum efficiency, stability, response uniformity and linearity showing good radiometric characteristics under laboratory conditions .
2.1 Dark current measurements
Relative variation of the dark current (ΔDC) measured between semester S1 (2010) and S9 (2014). FWHM: Full Width at Half Maximum of the filter transmission range. It should be mentioned that Al and Zr filters used in ch 3 and ch 4, respectively show high transmittance below 3.3 and 1.5 nm wavelength, respectively
Unit - channel
Bandwidth FWHM / nm
ΔDC (47 °C)/ %
Cumul. solar exposure (days)
+4.6 ± 5
+ 0.2 ± 0.2
+4.0 ± 2
+2.0 ± 2
−32 ± 5
+0.2 ± 0.2
−10 ± 2
−26 ± 5
+14 ± 2
+0.2 ± 0.2
+13.5 ± 2
+14 ± 2
The DC decrease in unit 2 is not fully understood. However, a possible explanation could be related to desorption of surface contaminants (mainly water) which increase conductivity. Due to the hydrogen-terminated MSM diamond surface  and the proximity of its interdigitated finger electrodes (15 μm), diamond MSM detectors are very sensitive to the ambient environment and to surface contamination. Special precautions were taken to keep the diamond MSM detectors in a dry inert environment by purging LYRA continuously with nitrogen and keeping covers always closed during on-ground activities but this was perhaps not sufficient.
As shown in Table 1, diamond MSMs of unit 1, that are less exposed to the Sun, show a non-significant increase of their DC during the same period. The relative DC variation (ΔDC) is approximately +4.0 % at 47 °C within the measurement uncertainty. It should be mentioned that the large error bar used, i.e., ±2 to ±5 %, is mainly related to the time needed for each MSM detector to reach a stable signal. Indeed although diamond MSM detectors have negligibly small DC at room temperature (typically 1 to 2 pA at 5 V bias voltage for a 5-mm diameter active area), they show a slow but persistent decrease of their DC, which varies with the cumulated duration and the flux of previous UV radiation exposures  but mainly with the electric field applied between the metal fingers electrodes. Solutions and progress have been already reported: new diamond MSM device architecture with optimized metal layers and with 5 μm (instead of 15) spacing between the interdigitated finger contacts have been investigated and showed excellent results in terms of stability and homogeneity .
Different mechanisms could contribute to the DC increase of Si-AXUV photodiodes however it is well known that UV radiation as well as ionizing particles induce ionization damages in the Si–SiO2 interface, resulting in the formation of oxide-trapped charges and interface states that affect the performance of the device [15, 16].
2.2 LED measurements
It is important to mention that a stable sensitivity to the onboard LEDs does not exclude the presence of a UV radiation-induced thin surface contamination layer on the detector, which is transparent in the near UV and visible ranges but very absorptive in the EUV-VUV spectral range. Ideally, the responsivity change should be monitored over the spectral range in which the detector is used.
3 Exploitation of the redundancy concept
It should be added that in order to reduce the uncertainty of the integrated signal, LYRA uses three reference calibration voltages (0, 2.5 and 5 V) to calibrate its VFCs (AD652). After almost 5 years in orbit, the 8 VFCs show good stability with time (relative variation of 0.029 % at 5 V) and also as a function of the temperature (1.45 10−3 %/K at 5 V).
Long-term UV solar irradiance measurement can only be achieved by combining observations of several space-based instruments. Since January 2010 LYRA has been continuously observing the Sun and, after correction, it could be calibrated with the help of a combined solar spectrum by the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) onboard the Solar Radiation and Climate Experience (SORCE) and the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission. In addition, first results indicated that the spatial resolution of SWAP on PROBA2 complements well the high temporal resolution of the EUV LYRA channels . SWAP—the Sun Watcher using APS and image Processing—is an EUV solar telescope centered at 17.4 nm and the first on orbit that benefits from CMOS Active Pixel Sensor technology [24, 25]. The long-term variations of the EUV irradiance measured by LYRA are also compared with the EUV Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO) satellite . It has been shown that the LYRA channel 2–4 data correlate well with the EVE/ESP (EUV Spectro-Photometer) level-1 data between 2010 and 2013 . LYRA channel 2–4 is also in good agreement with the EVE/MEGS (Multiple EUV Grating Spectrograph) channels . However, LYRA channel 2–3 has shown a much smaller variability due to the degradation of the longer wavelengths included in its passband i.e., above approximately 19 nm. A new inter calibration campaign with EVE/MEGS is under study with improved degradation correction of LYRA channels and will be the subject of a future publication.
The calibrated data (in physical units), corrected from the temperature effects DC, and degradation-trends are available on the LYRA website (proba2.sidc.be/data/LYRA).
Since 2010, LYRA has been exposed to a severe environment and it has consequently experienced significant degradation. The observed degradation is caused mainly by the presence of contaminant species (organics, siloxanes and water) on the optical filter surface, which reduce the UV light transmission. Currently, there is no possibility of mitigating contamination in space once molecules have settled irreversibly after UV-polymerization, which is a classical concern for UV solar instruments. However, methods to largely recover the LYRA calibration were established and successfully implemented. In-flight detector monitoring shows that diamond detectors (both MSM and PIN) exhibit insignificant degradation after almost 5 years on orbit. By its nature, diamond has a much lower thermally induced DC and is a radiation-hard semiconductor, which significantly extend the stability of the LYRA radiometric calibration. Conversely, and despite a much lower duty cycle, the Si-AXUV photodiodes used as reference detectors show an increase of their dark current and a higher decrease of their photoresponse, as demonstrated by the onboard UV LEDs for the EUV LYRA channels, suggesting damage in the oxide passivation layer. Exposure to radiation is often the main reason for detector degradation, and its impacts (ionization and displacement-damage) are frequently underestimated. LYRA profits additionally from a redundancy design concept for tracking instrument degradation. Complemented by other solar inter-calibration instruments, LYRA started its new real-time space weather services. Finally, the successful PROBA2 technological mission offers a unique opportunity to validate diamond detector technology from which lessons are derived for future solar UV radiometers.
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 für Bildung und Wissenschaft. This work was made possible thanks to the Solar-Terrestrial Centre of Excellence (STCE), a collaborative framework funded by BELSPO.
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