CLIMB - A 3U CubeSat to Van Allen Belt

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3rd Symposium on Space Educational Activities, September 16-18, 2019, Leicester, United Kingdom
CLIMB – A 3U CubeSat to Van Allen belt
Emmeric Vitztum, Kaarel Repän, Alexander Nemetz,
Martin Eizinger, Ekin Ecem Baspinar,
& Alexandros Sousanis
Department, Aerospace Engineering
University of Applied Sciences Wiener Neustadt
Wiener Neustadt, Austria
Carsten Scharlemann
Head of Department, Aerospace Engineering
University of Applied Sciences Wiener Neustadt
Wiener Neustadt, Austria
carsten.scharlemann@fhwn.ac.at
Abstract — CLIMB is a 3U CubeSat whose goal is to reach the
Van Allen belt with the help of an advanced propulsion system.
Usually, the Van Allen radiation belt is avoided by spacecraft due to
its high radiation levels. CLIMB aims exactly for this region to conduct
various measurements. The spacecraft’s subsystems need to be
designed in a way to cope with this rough environment and to ensure
that even in case of failure or loss of the spacecraft, it still shall deorbit
within 25 years, as required by the Austrian Space Law. Some of the
systems of CLIMB have already flown and been qualified during the
previous Austrian mission PEGASUS.
Keywords — Clean Space; CLIMB; CubeSat; electric
propulsion; FEEP; IFM; magnetic field; orbital debris mitigation;
radiation; re-entry; Van Allen belt
I. INTRODUCTION
The magnetic field of Earth acts as a shield against incoming
radiation from the Sun, as well as from interstellar space: many
charged particles that reach Earth are trapped in the magnetic
field outside of the atmosphere in the form of rings [1, p. 25],
and do not reach the ground. While this shielding effect is
beneficial for life on ground, it is detrimental for life in space [1,
p. 25] due to the higher absorbed dose [1, p. 44] that accumulates
through exposure to the locally increased flux of charged
particles [2, p. 214]. The ISS - and with it the main residence for
humans in outer space - has an average altitude of approximately
400 kilometres [3]. At this altitude, it is below both of Earth’s
high-radiation-density rings, normally called the Van Allen
radiation belts [1, p. 25].
However, many other spacecraft operate in or at least pass
through these regions [4]. As a consequence, details about this
environment are relevant for spacecraft design in terms of life
time and radiation hardness in general.
Efforts have been made to quantify the radiation levels and
size of the Van Allen belts [5]. The shapes of the belts follow
the field lines of Earth’s nearly dipolar magnetic field (see
Fig. 1), forming toroidal regions of high fluxes of protons,
electrons, and ionised atoms (most notably helium, nitrogen, and
oxygen [1, p. 25]. These fluxes decrease exponentially towards
the boundaries of the belts. They peak at approximately 108
electrons and 103 protons per square centimetre and second in
the inner and 106 electrons per square centimetre and second in
1 These values consider only electrons with more than 500 kiloelectronvolts
and protons with more than 100 megaelectronvolts of energy.
the outer belt [2, pp. 214-215].1 These peaks appear at around
2,000 and 31,000 kilometres respectively, but high charged-
particle densities associated with the Van Allen belts have been
predicted at altitudes as low as 500 kilometres near the equator
and even lower in the South Atlantic Anomaly (SAA) - a region
where Earth’s magnetic field is unusually weak [5] [1, p. 25].
The AE-8 model predicts an upper limit of the outer belt at
approximately 50,000 kilometres altitude [1, p. 28], but exact
values vary from source to source (e.g. [6]). This uncertainty
may be due to the temporal variations of the belts.
Fig. 1. Schematic visualisation of the Van Allen radiation belts. Their shapes
result from the magnetic field lines (white), because magnetic interaction is the
dominant force acting on the charged particles that constitute these belts. The
image also shows the temporary third radiation belt.2
Credits: NASA's Goddard Space Flight Center/Johns Hopkins University,
Applied Physics Laboratory
The academic satellite mission CLIMB aims to probe this
environment using a design based on the CubeSat Design
Specification. The mission combines commercial components
with in-house designed solutions, while building on the
experience of the mission PEGASUS (launched by the
University of Applied Sciences Wiener Neustadt (FHWN) in
2 The third radiation belt - a highly transient belt of electrons strongly dependent
on solar activity - was first measured by the Van Allen Probes [10].
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2017). The CubeSat Design Specification describes the volume
of a spacecraft in terms of units, where one unit U is a cube with
a side length of 100 millimetres.
Despite existing measurements, the time-variability3 of the
composition of the belts provides a permanent motivation for
up-to-date data. The use of CubeSat components is one
peculiarity of this mission compared to previous efforts (most
notably NASA’s Van Allen probes).
II. MISSIO N OBJECTIVES
As indicated above, the main scientific goal of the mission is
to investigate the environment in the Van Allen belts in terms
of electromagnetic radiation and charged particles. Another
important objective is to prove that a CubeSat can survive such
a challenging radiation environment without relying on
extremely expensive radiation hardened components. Such
solutions have already been successfully implemented in the
former CubeSat PEGASUS resulting in an extremely low
number of reboots or other radiation based anomalies [7].
In addition to that, a recently commercialised electric
propulsion (EP) module - a crucial subcomponent - is
demonstrated. The mission relies heavily on this component,
which is used for reaching the target orbit as well as for active
de-orbiting.
As a student project, education of participants as well as
academic advance are noteworthy goals of CLIMB. In this
context, it should be noted that the available resources only
allow visiting the lowest regions of the inner Van Allen belt.
III. MISSION DESCRIPTION
The mission CLIMB uses a Field Emission Electric
Propulsion (FEEP) system in order to reach the Van Allen belt
and, upon arrival, conduct various measurements such as
measurements of the magnetic field as well as of the radiation
levels. Reaching the inner Van Allen belt by means of FEEP is
considered a core part of the mission. As a consequence, the
spacecraft operates in different orbits over the course of the
mission. Fig. 2 shows the various orbits where the satellite
operates, along with their associated mission phases.
3 Especially the heavy ion densities vary with solar and geomagnetic activity
[1, p. 25]
Fig. 2. Schematic depiction of the various phases in the CLIMB mission (orbits
are not to scale)
A. Launch and early operation (LEOP)
Following the launch, the early operation phase is conducted.
Most of the activities in this phase are pre-programmed
including the deployment of the antennas and initiation of the
beacon, automatic alignment of the satellite and basic functions
of the thermal system.
B. Commissioning phase
During the commissioning phase a complete check of all
systems shall assess their functionality. The outputs of all
sensors shall be assessed and, if needed, calibrated. In case of
anomalies, these shall be investigated to assess the probability
and severity of their impact on the mission. Based on this
assessment the decision is made whether to proceed with the
mission or not.
C. Orbit raising phase
With the help of the propulsion system, the satellite increases
its apogee while, in the early stage of this phase, leaving the
perigee constant. In frequent steps, a health assessment of all
relevant systems (propulsion, power, communication, etc.) is
conducted. Only if all operational parameters are within the
nominal range, the mission is allowed to proceed. When the
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apogee reaches a certain value, the satellite initiates a periodic
decrease of the perigee while still increasing the apogee. Doing
so ensures that the satellite would at any point naturally deorbit
within 25 years, so as not to exceed the maximum allowable
time in orbit. The final goal of the mission is an apogee with
1,000 kilometres altitude [8].
D. Science phase
The data that are relevant to the scientific objective of the
mission (i.e. measurement of the magnetic field and radiation
dose) are collected from the very beginning of the in-space
operation. When CLIMB reaches the altitude of the Van Allen
belt, the official science mission starts and the mission focuses
on data collection rather than changing the orbit.
E. De-orbiting phase
In consideration of space situational awareness and the Austrian
Space Law, specifically its requirements on orbital debris
mitigation, many features have been implemented into the
mission concept as well as in the technical design of CLIMB in
order to ensure that the total time in orbit is well below 25 years
- even in case of a loss of the spacecraft at any time during the
mission. The Austrian Space Law requires that every space
operator (coming from Austria) has to deorbit or remove4 their
satellite at its EOL (end-of-life) from LEO (Low Earth Orbit).
For non-manoeuvrable objects their post-mission lifetime shall
not exceed 25 years.
Following the achievement of the scientific objectives, the
satellite starts to actively deorbit. This is done in two ways:
 At the apogee, the propulsion system fires in retrograde
direction to lower the spacecraft's perigee into the denser
areas of the atmosphere.
 In the vicinity of the perigee the satellite aligns itself such
that its maximum area (solar array) is perpendicular to
the flight direction.
The mission ends with the satellite breaking up and burning up
completely in the atmosphere.
IV. TECHNOLOGY
Initially CLIMB was intended to be built upon the novel
structure of the previous 2U CubeSat PEGASUS. Though, as
both the dimensional and mass limitations were already
exceeded early in the design phase of CLIMB, the decision was
made to move on to a 3U CubeSat as seen in Fig. 3.
Nevertheless, the structure is in essence identical to the one in
PEGASUS. The main loads are carried by the separate rails in
each corner of the CubeSat. The rails are connected by
aluminium top and bottom panels and by the PCB (Printed
Circuit Board) side panels. The internal PCBs are connected to
the side panels only via electrical connectors, which therefore
carry the mechanical loads. Some of the heavier subsystems
4 Putting the satellite into the graveyard orbit is allowed only if, for example
the satellite is orbiting at GEO (Geostationary Earth Orbit).
(e.g. Attitude Determination and Control System, battery unit)
also have additional aluminium attachment interfaces to the
rails. The PCB side panels house the circuitry, which provides
electrical interface between internal PCBs and other
subsystems. The side panels also serve as a mechanical
interface for the solar cells.
Fig. 3. General view of the CubeSat CLIMB without one side panel.
The main difference between the PEGASUS and CLIMB
structures are the length of the rails and the side panels. Also,
as the power requirement for CLIMB CubeSat is vastly
increased - mainly because of the propulsion system - CLIMB
incorporates deployable solar arrays. The thruster requires a
maximum of 40 watts of electrical power whilst propelling and
up to 5 watts during stand-by to keep the indium propellant in
the liquid phase [9]. Also, other subsystems need at least a few
watts of additional power. To provide this high amount of
power, two double-folded deployable solar arrays are used in
addition to the solar cells on the side panels: facing the Sun
there are two sets of 7 solar cells on two side panels of CLIMB
(nominally at 45 degree angle towards the Sun) and 2 solar
arrays, each with 2 solar array elements (plates), each with 7
solar cells, totalling 28 deployable solar cells. The total power
generated at peak is approximately 35 watts. For the hinges of
the solar array, the usage of commercial fastenings is not
possible because of the tight space requirements on the sides of
the CubeSat - two packed solar array elements may occupy no
more than 5 millimetres of space. The hinges of the solar arrays
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use the combination of laser welding of aluminium and
soldering between aluminium hinge parts and PCB solar arrays.
The biggest subsystem is the Indium FEEP Multiemitter
(IFM) Nano, provided by ENPULSION GmbH (shown in Fig.
4). The thruster has a wet mass of 900 grams and almost fills
one CubeSat unit of volume. By default, the system comes with
230 grams of propellant, which is sufficient for the mission by
a large margin. The propulsion system provides a nominal
thrust of 350 micronewtons at a specific impulse of 4,000
seconds, with the ability to operate with less power at the cost
of thrust or specific impulse. The second largest subsystem for
CLIMB is expected to be the Attitude Determination and
Control System (ADCS) as MAI-400 (from Adcole Maryland
Aerospace, LLC) with a volume of 0.5U and a mass of 700
grams. It features three reaction wheels, a 3-axis magnetometer,
two Infrared Earth Horizon Sensors (IREHS), three
electromagnets and an ADCS computer for a stand-alone, plug-
and-play attitude control system.
Fig. 4. Indium FEEP Multiemitter by ENPULSION GmbH [9]
Precise determination of the orbit is crucial for this type of
mission. In order to support the ADCS, a laser ranging system
is used. A laser on a ground station sends a high intensity laser
pulse to the spacecraft. CLIMB is equipped with reflectors, to
send back the laser pulse to the ground station. With the
reflected signal, the position of the spacecraft and its orbit can
be determined precisely. This helps to plan thrusting times and
attitude corrections in a more efficient and reliable way
compared to relying on the ADCS alone.
The following parts are inherited from the previous mission
PEGASUS with minimal to no modifications or changes: in-
house developed OBC (On-Board Computer), PSU (Power
Supply Unit) and STACIE-D (Space Telemetry And Command
Interface - Delta), commercial off-the-shelf UHF (Ultra High
Frequency) antennas, batteries and solar cells.
Two sets of battery units, each containing four 18650 type
lithium ion rechargeable batteries, act as a buffer for electrical
power (e.g. during eclipse times) and as a backup in case there
are anomalies with the solar power generation. The already
existing PSU needs to be adapted to the higher power
requirements and the increased number of batteries. The outputs
are 5 volts and 3.3 volts. Additional power converters located
on the battery units are used to output 12 volts for the thruster.
Two communication systems are used: the main one operating
in the UHF- (STACIE-D) and the secondary in the S-band
(STACIE-S).
All the individual thermal requirements for different
subsystems must be satisfied. For example, if the temperature
would drop below 0 degrees Celsius, the batteries would
experience reduced capacity. As a countermeasure, the battery
units are placed near the thruster to avoid the need for additional
battery heaters. The waste heat from the thruster is directed
towards the batteries. However, the batteries also exhibit a
shortened lifetime with higher temperatures. If possible, the
thermal design would keep the temperature of the batteries
between 0 and 20 degrees Celsius.
As the power usage and thus power dissipation is relatively high
for such a spacecraft size, a lot of effort is put into the thermal
design. Various thermal elements are considered, including
heaters, phase change materials, thermal straps, heat pipes,
louvres, radiators, multi-layer insulation and different surface
coatings. The highest power dissipation is expected to occur on
the thruster and the battery unit (which includes converters for
the thruster). Both of these subsystems are located at the bottom
part of CLIMB. Another high power system, the S-band
transceiver, is expected to dissipate about 8 watts of power.
Though, as this subsystem is located at the top of the satellite,
it might be less of a concern for the thermal configuration. At
most, CLIMB is expected to dissipate about 26 watts of thermal
power while thrusting.
The temperature ranges are highly dependent on the orbit. In
the worst hot case with dawn-to-dusk orbit and calculated from
the internal power dissipation, the preliminary estimate for the
temperature is around 65 degrees Celsius with the highest
temperatures occurring at the thruster.
During its journey and at its final destination in the Van
Allen belt, CLIMB performs magnetic field measurements. For
this purpose a deployable magnetometer boom is attached to the
side of the spacecraft. The 25 centimetre long boom features
analogue magnetic sensors at the tip and halfway to the tip.
Close to the magnetic field sensors, temperature sensors are
placed to measure the temperature and also to estimate thermal
drift of the magnetic field sensors.
A series of in-house ground tests are performed. The available
equipment includes a solar simulator, a Helmholtz coil, and a
thermal vacuum chamber. Vibration tests are procured at
FOTEC (Forschungs- und Technologietransfer GmbH), the
scientific R&D (Research & Development) company owned by
the University of Applied Sciences Wiener Neustadt.
Presently the CLIMB team focuses on the finalization of the
satellite configuration. Firstly, for the solar arrays the
preliminary design phase has been recently completed and
mechanical (vibration) tests for the 2U-configuration of the
solar arrays have been carried out. Next step is to test the 3U-
configuration. Furthermore, more detailed numerical and
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experimental work is done with regard to the thermal design of
CLIMB. These points are under current consideration and
efforts are made to allow a launch date in early 2021.
V. CONCLUSION
The fulfilment of the scientific goal - i.e. obtaining
measurement data of the space environment in the Van Allen
belts - first requires the fulfilment of a technological goal:
actively changing the orbit of the spacecraft. Orbital
manoeuvres of the intended extent come with the well-known
challenges of high specific impulse or a high propellant mass
fraction. Achieving either of these on a CubeSat platform poses
even greater difficulty. In that sense, CLIMB is pushing the
boundaries of the state-of-the-art in nano-satellite technologies.
Frequent design iterations have led to a 3U design that can
accommodate even the larger and heavier subsystems like the
MAI-400 ADCS and the IFM Nano propulsion system.
Deployable solar arrays increase the irradiated area nearly
fivefold in order to harvest enough power to supply the highly
efficient EP system.
The CLIMB team has so far investigated various options for the
configuration, achieved a detailed design, and specified all
major subsystems. Several open points in the design still exist
but will be tackled in the upcoming months.
ACKNOWLEDGMENT
This project is funded by the Lower Austrian Government
and the University of Applied Sciences Wiener Neustadt. Very
much appreciated is also the support of the CEO and scientific
and technical staff of FOTEC. The CLIMB team also expresses
its gratitude to ENPULSION GmbH for providing the
propulsion system and sponsoring the trip to the symposium.
Furthermore, the team of the University of Applied Sciences
Wiener Neustadt acknowledges the work of contributors, in
particular the Space Tech Group Austria.
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