On-Orbit Verification of Space Solar Cells on the CubeSat MOVE-II
Martin Rutzinger1,2, Lucas Krempel1, Manuel Salzberger1,2, Mario Buchner2, Alexander Höhn1,
Maximilian Kellner1, Katja Janzer1, Claus G. Zimmermann2, Martin Langer1
1 Technical University of Munich, Germany
2 Airbus DS GmbH, Munich, Germany
Abstract — Several promising multi-junction solar cell
concepts for space applications are currently under development
worldwide. On-Orbit Verification on CubeSats is a cost-efficient
method to gain data on critical hardware early in the design
validation process. The MOVE-II CubeSat will be used for the
verification of novel 4-6 junction solar cells. With a footprint of
10x10 cm², the payload consists of one full size solar cell (8x4 cm²)
and up to 7 positions (each 2x2 cm²) for corresponding isotype
solar cells. The measurement electronics is based on commercial
off-the-shelf hardware. MOVE-II is planned to launch in early
2018 into a 500-550 km sun-synchronous orbit.
Index Terms — CubeSat, on orbit verification, multi-junction
solar cell, degradation.
The majority of today's satellites is powered by photovoltaic
generators. New multi-junction solar cells (MJSC) for space
application are currently under development. Amongst the
most promising concepts are 4-6 junction MJSC’s -. In
addition to terrestrial and laboratory-based characterization
methods ,, this highly dynamic technology development
requires accurate, flexible and low cost on-orbit verification
(OOV) methods. This paper introduces the design of such an
OOV mission using the CubeSat MOVE-II.
Since their introduction in 1999, CubeSats  have evolved
from purely educational missions to spacecraft with a broad
variety of scientific and commercial applications. OOV of
critical hardware has been the objective of multiple CubeSat
missions in the past –. Thereby, the good cost-
efficiency of CubeSats allows the OOV at an early stage in
product development and at a relatively low Technology
Readiness Level (TRL) . The Munich Orbital Verification
Experiment (MOVE) satellite program of the Technical
University of Munich was initiated in 2006 with the ambition
of building a single-unit CubeSat verification platform, called
First-MOVE. The main goal of the program since then has
been the hands-on education of undergraduate and graduate
students. First-MOVE was launched in late 2013 and operated
in space for a month . Currently, the second satellite of the
program, called MOVE-II, is under development .
MOVE-II is planned to launch in early 2018 into a 500-
550 km sun-synchronous orbit (SSO).
Besides the educational goals of the MOVE-II program, the
technical mission objective is to measure the current vs.
voltage (I-V) characteristics of novel multi-junction solar cells
(MJSC) in orbit. The main reason for the OOV of novel
MJSC's is to gain flight experience for these cells. MOVE-II
will provide complimentary data to earth bound , and
high altitude balloon or aircraft based  experiments. The
main advantage of OOV is long time exposure to space
environment, especially the solar air mass zero (AM-0)
spectrum. This spectrum cannot be exactly reproduced with
earth bound sun simulators and even experimental data
obtained from high altitude experiments has to be corrected
for residual atmospheric absorption .
The MOVE-II platform is designed for maximum
flexibility, so that the exact type of MJSC, either with 4, 5, or
6 junctions, will only have to be fixed in late 2016, based on
the technological availability at that time. In addition to one
MJSC, also 4-6 corresponding component (isotype) solar cells
which have the same optical properties as the MJSC but only
one activated p-n junction will be characterized. This provides
additional information about each individual subcell.
As depicted in Fig. 1, the top plate (red area) of the
CubeSat (10x10 cm²) will be utilized for those payload cells.
Fig. 1 Rendering of the MOVE-II CubeSat: the payload area for
novel MJSC's is highlighted by the red frame on the Zenith deck.
II. MISSION AND PAYLOAD DESIGN
A. CubeSat Platform MOVE-II
MOVE-II will be designed as a 1 Unit CubeSat
(10x10x10 cm³), with a total mass of 1.3 kg. The satellite will
incorporate a UHF/VHF transceiver for telemetry and low-to-
mid data rate transmission. Furthermore, an experimental S-
Band transceiver will enhance the capabilities of the satellite
towards higher data rates. The achievable data rate on the S-
Band link is expected to be more than 1 MBit/s. It is planned
to use the downlink capability in both bands for payload data
The attitude determination & control system (ADCS) is
based on magnetic actuation and will allow a pointing of the
satellite with accuracy better than ±10°. In its nominal mode,
the satellite will operate in a position where the payload is
pointing in zenith direction (Fig. 1).
B. Payload Architecture
The payload assembly on the Zenith deck consists of one
full size MJSC (8x4 cm²) and 5 to 7 open positions (each
2x2 cm²) for corresponding isotype (component) solar cells
and additional experimental cells (Fig. 2). The solar cells are
mounted on the outboard side of a printed circuit board (PCB).
The inboard side of the PCB contains all necessary
C. Measurement Electronics
Each solar cell is contacted with a 4-point connection
(Kelvin-connection) with separate contacts for the current path
and the voltage measurement. The solar cell voltage UC is
measured directly between positive and negative contact of the
solar cell. The current is measured as voltage drop US over a
precision shunt resistor with low temperature
The current vs. voltage (I-V) curve sweep between ISC and
VOC is performed by varying the gate voltage of a MOSFET
which serves as a variable electronic load. The voltage sweep
is controlled through a Digital Analog Converter (DAC). Both
voltage signals, cell voltage and shunt voltage, are digitalized
with 24-bit Analog Digital Converters (ADC’s). The selected
COTS parts and their tested radiation hardness are
summarized in Table 1.
The circuit diagram for one payload solar cell is shown in
Fig. 3.The temperature of each solar cell is measured with a
digital sensor which is in direct thermal contact with the back
surface of the solar cell.
The I-V curve measurement accuracy is determined mainly
by the temperature stability of the current sensing shunt
resistor and by the temperature- and radiation-stability of
A prototype with all selected components has been built and
tested with 4J-UMM solar cells and corresponding component
cells J1-J4 from AZUR SPACE Solar Power GmbH . Fig. 4
shows a comparison of the I-V curves of a 4x8 cm² 4J-UMM
solar cell and 2x2 cm² component (isotype) cells measured
with a calibrated source–measurement unit (SMU) and with
the MOVE-II prototype. All I-V curves were measured under
an uncalibrated AM-0 spectrum at room temperature. The
results obtained from this comparison show that the prototype
can yield accuracy better than 0.5% in voltage and current
measurement. A sufficient amount of data points is collected
to allow exact fitting of diode parameters.
Fig. 2 Layout and solar cell positions on the MOVE-II payload in
the configuration with one 4x8 cm² (L1) and 2x2 cm² (S1 – S7) solar
Fig. 3 Circuit diagram for one payload cell of MOVE-II.
SELECTED COTS PARTS FOR THE MEASUREMENT
TI DAC-7512E 
30 krad 
AD 7714 
Fairchild FDS 8870 
Vishay CSM2512 
Complimentary data, such as sun angle and earth visibility,
are collected by the ADCS subsystem and also included in the
transmitted data. This input will be used for on-ground post-
processing and for the investigation of the effects of sun
incidence angle and earth albedo on the solar cell
performance. The seasonal sun-earth distance has to be
corrected by Eq. 1 
where r is the sun-earth distance in astronomical units (AU).
If I-V curves can be obtained at different temperatures it
will be possible to extract the temperature coefficients
according to Eq. 2.
The quantity X represents
III. MISSION SIMULATION
Preliminary simulation results show the feasibility of the
OOV-experiment on MOVE-II.
A. Lifetime Analysis
The maximum lifetime of MOVE-II was calculated using
the STELA tool, version 2.6.1, from CNES . The 500 km
SSO orbit resulted in an average lifetime of 3.6 years, versus
7.6 years for the 550 km orbit height (worst case = minimum
drag area of 10x10 cm²). Therefore, both the feasibility of an
OOV of solar cells due to degradation effects over time and
compliance to the space debris mitigation guidelines  are
B. Radiation Degradation Analysis
We calculated the expected electron and proton radiation
damage on triple junction solar cells using the software
SPENVIS 4.6.7  for a circular 550 km SSO with 97.5°
inclination and 7.6 years lifetime. With a 100 µm thick cover
glass, a 1 MeV End-of-Life (EOL) equivalent electron fluence
can be calculated. For well-
studied triple junction solar cells, this fluence typically leads
to current, voltage and power remaining factors > 99% .
Therefore, no useful degradation analysis can be expected in
the given orbit for solar cells with cover glass.
For a purely experimental study of degradation effects, one
additional 2x2 cm² MJSC will be mounted as bare cell, i.e.
without a cover glass. In this case the calculated equivalent
fluence is increased to cm-2. This
high dose mainly arises due to the lack of proton shielding,
especially the low energy part of the proton spectrum which
normally is completely absorbed in the cover glass. In triple
junction cells, such a dose would result in significant
degradation with a power-remaining factor of
. In this orbit, though, the particle spectrum is dominated
by low energy protons and most of them would be absorbed in
the top junction. Therefore the bare MJSC degradation
behavior is expected to be dominated by the degradation of the
top subcell. Degradation due to low energetic protons has been
studied with mono-energetic accelerator protons under normal
incidence  and for omnidirectional incidence .
However, no on-orbit verification of these ground based
experiments is available up to now.
C. Sun Pointing Mode
A continuous sun-pointing mode of a CubeSat can be
achieved with a magnetically actuated spacecraft . Thus,
although MOVE-II will operate nominally nadir-pointing, it is
planned to also maneuver in sun-pointing mode as a stretch
goal for the mission. The relevance of the technical data
collected in sun-pointing mode and thus, peak power can be
improved under the expected mission operation scenarios.
Fig. 4 (a) and (b) Comparison of I-V curves of a 4J-UMM solar cell
and corresponding component cells J1-J4 from AZUR SPACE Solar
Power GmbH . The I-V curves were measured with a source–
measurement unit (solid lines) and the MOVE-II prototype (open
squares) under an uncalibrated AM-0 sun simulator.
D. Thermal Simulation
To ensure the compliance with the operational limits of the
solar cells and the measurement electronics, thermal
simulation and experiments have been carried out. Thermal
modeling has to deal with various uncertain parameters, such
as unknown conductance through screws or adhesive contacts.
Thus, to achieve reliable thermal simulations, correlation with
actual experiments is important.
For the MJSC experiment, a thermal prototype of an
aluminum CubeSat frame, equipped with two inner dummy
PCBs, and an experimental PCB with two 30.2 cm² cells, was
built. This setup was equipped with ten Pt-100 thermocouples,
observing temperatures directly on the solar cells, the
electronics side, the aluminum structure, and an inner dummy
A thermal vacuum experiment was performed, insolating
the prototype with 1.05 × AM-0 solar intensity for 38 minutes,
and tracking the temperature development until an
approximate steady state was achieved. The chamber walls
were not cooled during testing, and heated up to +42 °C
background temperature. Due to lack of convection, panel
temperatures rose to +147 °C during insolation. Heat
conduction through the Zenith board pushed electronic
temperatures up to +130 °C, while inner PCBs of the satellite
kept more moderate temperatures of max. +77 °C. Although
these values significantly exceed the maximum operation
temperature for solar cell electronics on the Zenith board
(+105 °C), the experimental setup represents an unrealistic
absolute hot case. In space, background temperatures of
°C will prevent such overheating scenarios, due to more
effective radiative heat exchange.
The thermal vacuum experiment provided important data
for correlating a numerical thermal simulation, performed with
ESATAN . The numerical model represents all important
structural features and screw joints of the experimental
prototype. Contact conductance and aluminum surface
properties were varied, until modelled temperatures
sufficiently met experiment observations. The correlated
model predicted interior temperatures with ± 5 °C accuracy,
and solar cell temperatures with larger negative deviation.
The correlated model was used to predict temperatures on a
550 km, 11 h Local Time of Descending Node (LTDN) SSO
orbit. As the test prototype lacked side panels and wings for
additional solar cells, these elements have been added to
create a realistic MOVE-II model in space. Representative
values for contact conductance from the prototype model have
been transferred to all additional screws and material contacts.
Fig. 5 depicts the temperature distribution of the MOVE-II
model during orbital flight. In a Zenith pointing configuration,
temperatures are much lower than in the thermal vacuum
experiment, due to lower background temperatures and shorter
orthogonal insolation of the panels. Peak temperatures of
MJSC’s experiment electronic reach only +63 °C, well within
the designed temperature window. Lowest temperatures on the
electronics board during eclipse reach –3 °C. Thus, the
preliminary results of the thermal simulation conclude safe
operational conditions for the solar cell electronics.
The CubeSat MOVE-II will carry a novel 4-6 junction solar
cell and corresponding component cells intended for future
space application with the purpose of measuring their
performance and degradation behavior in low earth orbit. The
developed measurement electronics was shown to be capable
of measuring current vs. voltage curves with high precision.
The launch of MOVE-II is planned for early 2018.
The authors would like to thank AZUR SPACE for their
support and close collaboration.
The authors acknowledge the funding of MOVE-II by the
Federal Ministry of Economics and Energy, following a
decision of the German Bundestag, via the German Aerospace
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