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Software-Defined Communication on the Nanosatellite MOVE-II

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In this paper, we will report on the results and lessons learned of the development, test and operation of two software-defined transceivers on the CubeSat mission MOVE-II (Munich Orbital Verification Experiment II). MOVE-II, a single-unit CubeSat, is the second satellite of the CubeSat program MOVE of the Technical University of Munich (TUM). The main goals of the mission are verification of a novel satellite bus for demanding payloads, verification of a novel type of solar cells as well as education of students. The MOVE-II satellite bus features two independent communication systems. The system for telemetry and telecommand of the satellite is a software defined radio (SDR) based full duplex UHF/VHF system. All signal processing and protocol handling is done in a XILINX Spartan 6 field programmable gate array (FPGA). It has a fixed baud rate and supports different coding and modulation schemes allowing slightly higher data rates depending on the link margin. For high-speed data transfer, the S-Band system provides additional bandwidth. It is an SDR based half duplex system supporting the same channel coding and modulation schemes at a significantly higher baud rate. The downlink data rate of this system is 3 MBit/s. MOVE-II uses a novel data link layer protocol called Nanolink. We tailored this protocol specifically for the needs of a CubeSat and features virtual channels and optional automatic repeat request (ARQ) while maintaining its low overhead. Thus, it enables an efficient use of the available bandwidth. We verified the functionality and our simulations with different measurements in laboratory environments as well as a BEXUS stratosphere balloon mission. Currently we are performing final preparations and trainings for the launch and early operation phase. The launch of the satellite is scheduled for November 2018. We will report on the development process of the communication architecture within the resource-constraint environment of a university, and focus on the verification of the transceivers' functionality.
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69th International Astronautical Congress (IAC), Bremen, Germany, 1-5 October 2018.
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IAC-18,B2,2,2,x47727
Software-Defined Communication on the Nanosatellite MOVE-II
Sebastian Rückerla*, Nicolas Appela, Rolf-Dieter Kleinb, Martin Langera
a Institute of Astronautics, Technical University of Munich, Boltzmannstraße 15, 85748 Garching bei München,
Germany, s.rueckerl@tum.de
b Multimedia Studio Rolf-Dieter Klein, Straßbergerstraße 34, 80809 München, rdklein@rdklein.de
* Corresponding Author
Abstract
In this paper, we will report on the results and lessons learned of the development, test and operation of two
software-defined transceivers on the CubeSat mission MOVE-II (Munich Orbital Verification Experiment II).
MOVE-II, a single-unit CubeSat, is the second satellite of the CubeSat program MOVE of the Technical University
of Munich (TUM). The main goals of the mission are verification of a novel satellite bus for demanding payloads,
verification of a novel type of solar cells as well as education of students. The MOVE-II satellite bus features two
independent communication systems. The system for telemetry and telecommand of the satellite is a software
defined radio (SDR) based full duplex UHF/VHF system. All signal processing and protocol handling is done in a
XILINX Spartan 6 field programmable gate array (FPGA). It has a fixed baud rate and supports different coding and
modulation schemes allowing slightly higher data rates depending on the link margin. For high-speed data transfer,
the S-Band system provides additional bandwidth. It is an SDR based half duplex system supporting the same
channel coding and modulation schemes at a significantly higher baud rate. The downlink data rate of this system is
3 MBit/s. MOVE-II uses a novel data link layer protocol called Nanolink. We tailored this protocol specifically for
the needs of a CubeSat and features virtual channels and optional automatic repeat request (ARQ) while maintaining
its low overhead. Thus, it enables an efficient use of the available bandwidth. We verified the functionality and our
simulations with different measurements in laboratory environments as well as a BEXUS stratosphere balloon
mission. Currently we are performing final preparations and trainings for the launch and early operation phase. The
launch of the satellite is scheduled for November 2018. We will report on the development process of the
communication architecture within the resource-constraint environment of a university, and focus on the verification
of the transceivers’ functionality.
Keywords: CubeSat, MOVE, software defined radio, satellite communication, Nanolink
1. Introduction
The development of CubeSats at the Technical
University of Munich (TUM) began in 2006 with its
first satellite First-MOVE [1]. Members of the Institute
of Astronautics (LRT) built most parts of this satellite.
The radio transceivers and the power supply module
were the only commercial of-the-shelf (COTS) products
used for First-MOVE. The satellite was launched into
low earth orbit (LEO) in November 2013 from Yasny,
Russia.
MOVE-II [2], the successor mission of First-MOVE,
was under development since April 2015. An
interdisciplinary team of students was developing,
building, and testing this one-unit (1U) CubeSat under
supervision of members of the LRT. The lessons
learned from First-MOVE [3] had a strong influence on
the design of MOVE-II. As a result, strong requirements
for testability, recoverability, reliability and
performance were introduced [4], [5]. This also
motivated the development of an improved
communication system for the mission.
In 2014, we developed the Nanolink [6] data-link
layer protocol. It is designed to meet the requirements of
small satellites while increasing the performance and
robustness over pre-existing solutions. Back then, the
commercially available transceivers were hardware- or
vendor-locked to specific physical and data-link layer
implementations, mainly using inefficient amateur radio
modulations and protocols. Therefore, we decided not to
use COTS transceivers on MOVE-II and instead
developed advanced transceivers based on the software-
defined radio (SDR) approach, which could meet the
requirements of the mission. We use these transceivers
for controlling and monitoring the satellite and
transmitting payload data. The telemetry and
telecommand transceiver operates in the UHF/VHF
band and supports a relatively low data rate. Table 1
shows the basic parameters of the UHF/VHF transceiver.
The high-data rate S-Band transceiver is intended for
payload data downlink but can be used for arbitrary data
transfers. Table 2 shows the basic parameters of the S-
Band transceiver. Figure 1 shows pictures of the
MOVE-II flight model transceivers.
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Table 1. UHF/VHF Transceiver Parameters
Property
Value
Downlink Frequency
145.95 MHz
Downlink Bandwidth
25 kHz
Downlink RF Power
0-30 dBm
Downlink Modulation
BPSK, QPSK, OQPSK
Downlink Symbol Rate
12.5 kBaud
Uplink Frequency
437.8 MHz
Uplink Bandwidth
25 kHz
Uplink Modulation
DBPSK
Uplink Symbol Rate
12.5 kBaud
Table 2. S-Band Transceiver Parameters
Property
Value
Downlink Frequency
2275.9 MHz
Downlink Bandwidth
2 MHz
Downlink RF Power
0-30 dBm
Downlink Modulation
BPSK, QPSK, OQPSK
Downlink Symbol Rate
1.5 MBaud
Uplink Frequency
2097.7 MHz
Uplink Bandwidth
200 kHz
Uplink Modulation
DBPSK
Uplink Symbol Rate
150 kBaud
2. Hardware Design
The high demand on the performance of the
transceivers combined with the physical dimension and
power consumption constraints require the use of
integrated circuits (ICs). ICs reduce the size and power
consumption of the transceivers while maintaining a
high performance level. A direct conversion architecture
of the radio frequency (RF) frontend is used. Combined
with the use of digital signal processing within an
FPGA this reduces the required footprint to a reasonable
size.
2.1 RF Frontend
The UHF/VHF transceivers’ RF frontend uses
discrete ICs for digital to analogue (D/A) conversion,
analogue to digital (A/D) conversion, frequency
synthesis, mixing and amplification. All transmitter
components can be entirely switched off to reduce
power consumption of the system. The UHF receiver
does not have this capability as the system should be
ready to receive commands at all times.
The S-Band transceivers’ RF frontend uses a single
AD9361 IC combining the required D/A and A/D
conversion, frequency synthesis, filtering and up-/down-
conversion. This less conservative design is more
flexible and enables the use of high data rate
modulations and a wide frequency band. Switching off
individual components of this chip is not required, as for
power saving reasons we simply switch off the whole
transceiver. Only the amplifiers required to receive or
transmit a signal are switched on when needed. This
avoids additional noise on the low noise amplifier while
receiving a signal and protects the low noise amplifier
from the output power of the transmitters power
amplifier.
Figure 1. UHF/VHF Transceiver Flight Model (top) and
S-Band Transceiver Flight Model (bottom).
2.2 FPGA
We utilize a Xilinx Spartan 6 LX45 FPGA for
digital signal processing. The Spartan FPGAs offer the
required capabilities and are sufficiently radiation
tolerant. We chose the LX45 as it is programmable with
the free development toolkit and has sufficient resources
for our application.
We use magnetoresistive RAM (MRAM) for the
configuration memory of the FPGA. This memory is
inherently radiation tolerant and especially immune to
single event upsets. Additionally, MRAM can tolerate a
total ionizing dose of up to 75 kRad [6]. This renders
MRAM a good long-time storage.
An additional automated reset mechanism recovers
the FPGA from bit flips that occur during runtime. It
performs a hard reset of the device every 36 hours, fully
reprogramming the FPGA from the MRAM
configuration memory.
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3. Physical Layer
The physical layer is designed for performance. It
uses a digital phase-shift keying (PSK) modulation with
channel coding to increase link capacity.
3.1. Modulation
PSK is the used modulation scheme for both up- and
downlink. It is a digital modulation type with high
robustness to amplitude variation and high spectral
efficiency, which has rendered it the preferred
modulation in space communication.
Offset quadrature phase shift keying (OQPSK) is the
preferred downlink modulation, as it has a relatively
constant signal envelope, thus the linearity of the power
amplifier is less important. Besides OQPSK the
transceivers can be configured to use binary phase shift
keying (BPSK) with or without differential encoding, as
well as regular quadrature phase shift keying (QPSK).
This gives the operator more control over the radio link
and change the data rate if needed.
The uplink uses differential encoded BPSK (DBPSK)
only. DBPSK significantly reduces the complexity of
the receiver at the cost of a slightly increased bit error
rate (BER). Improved ground station equipment can
easily mitigate this disadvantage.
3.2. Channel Coding
The downlink uses CCSDS AR4JA low density
parity check (LDPC) codes [8]. These codes provide
very high performance at the cost of increased receiver
complexity. Therefore, we use these codes only for the
downlink. For the uplink, the missing codes and
therefore the missing coding gain is compensated by a
suitable ground station setup.
The transceivers implement two different coding
options: In case power is the limiting factor, a rate ½
code is used. This is the default option, which is used
for telemetry beacons and in the beginning of an
overpass. If bandwidth is the limiting factor and power
is not an issue, a rate code is available. The higher
code rate increases the user data rate. To mitigate the
disadvantage of higher code rates the code length is
increased. These codes share a similar structure,
allowing us to reuse the encoder design.
Figure 2 shows the performance of the two LDPC
codes compared to convolutional codes in an additive
white Gaussian noise (AWGN) channel simulated with
Matlab. Both LDPC codes provide a system gain of
more than 6dB.
4. Data Link Layer
Nanolink is a reliable, connection oriented, packet
based data-link layer protocol for CubeSats and other
spaceborne assets with similar bandwidth and hardware
resources. It is designed for asymmetric links with small
bandwidth-delay product. The protocol operates
efficiently and reliably in moderate signal quality due to
a type-I hybrid selective acknowledge automatic repeat
request (ARQ) scheme. Extensible header structures
reduce the required overhead, especially regarding the
ARQ return channel [6].
Another important aspect of the protocol is the
availability of virtual channels. Virtual channels allow
the protocol to multiplex up to eight individual data
streams into one physical data stream. This is used to
discern different types of upper level protocols avoid
head of line blocking, and to introduce quality of service
with traffic classes. This feature also guarantees the
transmission of telemetry beacons even while the link is
extensively used for arbitrary task throughout the
mission.
5. Software Design
The transceivers SDR based design implements
many functions traditionally performed in hardware
towards the software domain. Traditionally the software
only performs the I/O, controlling and data processing
functionality. This is extended by the digital signal
processing to a point where complex samples are
exchanged with the RF frontend as described in chapter
2.1. To handle the increased complexity, the
architecture of the FPGA software is divided into three
functional blocks: the I/O interface to the onboard
computer (CDH), a software microprocessor core
(Softcore) handling the protocol logic and controlling of
the transceiver, and the signal processing part. Figure 3
illustrates the functional blocks of the software.
Figure 2. BER of CCSDS LDPC codes with code rate
½, , and a convolutional code (CC) (rate ½; length 7)
in comparison to unencoded BPSK.
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5.1. CDH Interface
The CDH Interface is responsible to exchange data
and control information with the satellites on-board
computer. Directly implementing the required protocol
on the FPGA substantially reduces the load on the
Softcore. The implementation uses a parallel interface to
exchange control information, such as a parameter
update from CDH or updated status. This guarantees
quick updates and the possibility for signalling between
the Softcore and the CDH Interface. Direct memory
access and a shared dual port memory block provide a
suitable interface for user data exchange. CDH Interface
and the Softcore (via Wishbone) can directly modify
this memory region. A full documentation of this
interface is presented in [9].
5.2. Softcore Software
The Softcore offers the possibility to execute
compiled C code. We wrote and tested the software on a
regular PC before deploying it to the transceivers. The
software running on the Softcore is responsible for
controlling the transceiver hardware and signal
processing chains, and to take care of the Nanolink
handling. The logic associated with this task is both
critical to the mission and rather complex. It is therefore
not reasonable to have this code running directly on the
FPGA, since this limits the debugging possibilities and
flexibility severely. As testing is crucial to guarantee a
fully operational system, the Nanolink handling was
verified in a simulated environment. The Softcore
exchanges status information with the signal processing
component via Wishbone, a parallel bus. Nanolink
frames are exchanged via a small I/O buffer based on a
similar mechanism as used to connect the CDH
Interface with the Softcore.
5.3. Digital Signal Processing
The digital signal processing consists of two
independent chains of processing blocks. Each of these
blocks represents the signal processing for one direction,
receiving or transmitting of the signal. All parts of these
chains are implemented in VHDL and directly utilize
the resources of the FPGA. For communication between
different blocks of the chain, the AMBA AXI4 stream
protocol [10] is used.
The transmitting chain serializes Nanolink frames
out of a memory block shared with the Softcore,
performs the LDPC encoding and scrambling of the data,
baseband modulation, pulse shape and up-sample
filtering, and interfacing the RF frontends’ D/A
converter. The receiver chain performs carrier and clock
recovery, DBPSK demodulation, byte alignment and
verification of Nanolink frames, and stores them to a
memory shared with the Softcore.
Both chains operate on burst transmissions by design.
This is necessary to enable half-duplex operation for the
S-Band transceiver.
6. Evaluation
We performed several test campaigns to reach the
necessary confidence in the communication system and
demonstrate a certain technology readiness level. These
tests include laboratory test such as evaluation of the
modulation performance and testing of the critical
receiver parameters as well as testing of the hardware in
a relevant environment and under similar conditions as
we expect them in orbit.
6.1. Modulation Performance
The transceiver quality is best assessed using its raw
amplified and modulated signal. A key figure is the
error vector magnitude (EVM) of the modulation. The
EVM describes the difference of the measured
constellation points from a theoretically perfect signal.
The measurements were performed with a RSA306B
spectrum analyser. The device provides a direct
calculation of the EVM value. The results for this
measurement are given in Table 3. For comparison, a
reference measurement with a VSG25A Vector Signal
Generator was performed. To further evaluate the
performance of the devices, the EVM is compared to the
requirements of common mobile communication
standards (cf. Table 4).
The results reveal small but significant differences in
the EVM between the reference and the transceivers.
Both peak and RMS values for the S-Band are about
10 dB lower than the reference. With a margin of about
9 dB, the S-Band transmitter is well below the limit of
most of the communication standards. The VHF
surpasses the S-Band, but there is still a margin to the
Figure 3. Functional blocks of transceivers’ FPGA software
Parameter
VSG25A
S-Band
VHF
EVM (RMS)
- 35.6 dB
- 24.4 dB
- 29.2 dB
EVM (Peak)
- 27.4 dB
- 17.2 dB
- 23.1 dB
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reference signal. The VHF signal fulfils the
requirements of all listed communication standards with
a wide margin.
In conclusion, the tests demonstrated an overall
good quality of the transmitters, which can fulfil the
requirements of commercial standards.
6.2. Receiver Tests
The maximum possible data rate depends on the
received signal strength and receiver sensitivity.
The measurement of the receiver’s sensitivity was
performed by transmitting Nanolink frames of a certain
size using an SDR, attenuating the signal and receiving
it with the UHF receiver. The computer connected to the
receiver measured and averaged the user data rate over
5 minutes per point. Figure 4 shows the performance of
the receiver over a span of 6 dB. The x-axis is scaled so
that 0 dB correspond to the maximum attenuation
(-104.5 dBm). The graphs in the figure correspond to a
user data size of 120, 500 and 950 B. The green line
marks the maximum achievable user data rate of
1545.5 Bs-1. Measurements were performed in intervals
of 0.25 dB for smaller signal levels and 0.5 dB for
higher levels. The graph shows that smaller frame sizes
increase the receiver’s ability to extract valid frames
from the signal. This is a result of the lower probability
of frame errors for smaller frames. For the smallest
measured frame size, the first frames were received at
0.25 dB. From this level, the graph for frames of 120 B
shows a monotonic increase. For frame sizes of 500 B
the first data was received at 1.5 dB, and at 2 dB for
950 B. All curves exhibit a very flat slope between
2.0 dB and 2.25 dB. This seems to be a result of an
inaccuracy of the variable attenuator. This phenomenon
can also be observed between 2.5 dB and 2.75 dB for
frame sizes of 950 B and 500 B but not for 120 B. This
suggests that the final attenuation is 0.25 to 0.5 dB
higher than indicated.
Above a certain level, the slope of the graphs begins
to decrease, and no significant increase in user data rate
can be achieved with increased signal power levels. For
120 B frames, this point is reached at 2.25 dB. For
500 B and 950 B frames, the turning point is at about
3 dB. At 3.75 dB the peak data rate of 1.33×103 Bs-1 for
120 B frames is reached. At 4.25 dB the peak data rate
of 1.51×103 Bs-1 is reached for 500 B frames. The data
rate peaks at 5.75 dB and 1.54×103 Bs-1 for 950 B
frames. The maximum theoretical data rate for 120 B
frames is 1460 Bs-1. For 500 B frames the maximum
theoretical rate is 1533 Bs-1 and 1545 Bs-1 for 950 B,
respectively. The graphs show that the difference
between the theoretical and practical maximum data rate
decreases with increasing frame length. Transmissions
with shorter frames are not able to reach their maximum
data rate. The cause of this is subject to future
investigation.
Figure 4. Data rate of the UHF receiver depending on
the received signal power for different payload sizes.
The signal level is relative to -104.5 dBm.
The receiver exhibits a sensitivity of -100 dBm for
near error-free reception under laboratory conditions.
This is sufficient as high noise levels are to be expected
in orbit. The MOVE-II ground station has a power
amplifier with an output power of up to 1 kW which can
produce signal levels of -91 dBm or more at the receiver,
once the satellite is in orbit.
An additional test was done with calibrated
measurement equipment and a reduced frame size of
only 2 B. During this test the minimal signal power
necessary to receive any data at all was -116 dB. Again,
we found the same kind of sharp transition and were
able to receive all data frames at a signal power of
-106 dB at the input of the receiver.
6.3. BEXUS
The BEXUS mission showed that both transceivers
operate reliably under near-space conditions. Data rate
measurements of the VHF link revealed a stable link
throughout the flight, with only minor outages due to an
obscured link resulting from a suboptimal position of
the ground station. The measurement furthermore shows
a high sensitivity of the uncoded signal to higher bit
error rates, which causes the signal to deteriorate
quickly. From the VHF and S-Band measurements,
fading margins for the MOVE-II mission were derived
and all previous measurements and analysis were
verified with actual data. Measurements from the VHF
transceiver showed a thermally stable device. The
Table 4. Minimum EVM requirements for various mobile communication standards
Standard
IEEE802.11 [11]
LTE [12]
UMTS [13]
GSM [14]
Maximum EVM
- 10 dB
- 15 dB
- 15 dB
- 21 dB
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temperature values of the transceiver stayed in a
nominal range during the whole flight. The full results
of this verification campaign can be found in [15].
7. Conclusion
We developed the transceivers after an analysis of
the commercial market showed a gap in the availability
of CubeSat transceivers for non-amateur radio
applications.
The design of the transceivers aims to improve over
these commercial transceivers with three central
concepts: Higher spectral efficiency, higher bit rates and
robustness through channel coding, and generally
improved efficiency by utilizing the Nanolink protocol.
To fulfil the requirements of these features, both
hardware and software were designed and implemented
specifically for this purpose.
We performed tests under practical conditions on a
BEXUS stratosphere balloon. The atmospheric
conditions in the stratosphere provide an environment
very similar to space. Both transceivers operated
continuously without any problems during the test.
Further tests of the transmitter validated the good
quality of the transmitters, which also validates the
developed software signal processing chain. When
compared with the performance requirements of
common mobile communication standards, the
performance of the transmitters was satisfactory and
met all requirements.
Tests of the receiver showed sufficient sensitivity
and good performance of the receiver. The practically
achievable data rate was found to rise quickly with
increasing SNR. The maximum data rate was achieved
with medium to large Nanolink frames.
The transceivers developed for MOVE-II are
specialized, yet flexible state-of-the-art devices. We are
certain they will operate reliably on the upcoming
MOVE-II launch and will be used on future satellite and
balloon missions at the LRT.
Acknowledgements
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 Center (DLR) with funding grant
number 50 RM 1509.
The REXUS/BEXUS program is realized under a
bilateral agency agreement between the German
Aerospace Center (DLR) and the Swedish National
Space Board (SNSB). The Swedish share of the payload
has been made available to students from other
European countries through a collaboration with the
European Space Agency (ESA).
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... Extensible header structures reduce the required overhead, especially with regard to the ARQ return channel. A more detailed description of the COM Subsystems can be found in [9]. ...
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MOVE-II (Munich Orbital Verification Experiment) is the second satellite of the Technical University of Munich's educational CubeSat program. On December 3, 2018, the satellite was launched on the SSO-A SmallSat Express from the Vandenberg Air Force Base. The following paper shows on-orbit results of the first eight months of operations. It includes analyses based on our own data as well as the open-source ground station network SatNOGS. Lessons learned from mission operations and recommendations for future educational missions are provided. The technical goals of the mission are verifying the satellite's bus and the qualification of a novel type of quadro-junction solar cells. Over 200 students have been developing and testing all components of the satellite since the beginning of the project in April 2015. During the course of the project, the students designed all necessary technology for a CubeSat bus, with the exception of the electrical power system and the on-board computer's hardware. Furthermore, the students developed ground station software as well as an operations interface from scratch. The technological achievements of the mission range from a linux-based onboard computer software over a magnetorquer-based attitude determination and control system to two novel transceivers for UHF/VHF and S-Band. A reusable mechanism, based on shape-memory-alloys, deployed the four solar panels, providing the necessary power. Only hours after the deployment, we received the first signals of the satellite. The commissioning of the ground station and the effects of an insufficient power budget of the tumbling satellite preoccupied us during the first month, as well as frequent watchdog resets. During the commissioning of the Attitude Determination and Control System (ADCS), a spin rate of 200 °/s was observed, although the actuators were not activated yet. Detailed analysis with the help of recordings provided by our own ground station as well as the SatNOGS ground station network revealed a slow increase of the spin rate since the launch. In the following weeks the spin rate further increased to over 500 °/s. Afterwards we were able to modify our ADCS actuation in a way to reduce the spin rate again. Currently MOVE-II is detumbled and we are moving towards regular scientific operation. After a presentation of the results, lessons learned from our mission operations are discussed. The paper discusses the measured values and analyzes the reasons for the observed behaviour. Also the changes made on MOVE-IIb, a slightly improved copy of MOVE-II, will be explained. The paper concludes with recommendations for designers of upcoming educational satellite missions, especially regarding resilience against negative power budgets.
Conference Paper
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MOVE-II (Munich Orbital Verification Experiment II) is a 1 Unit CubeSat currently under development at the Technical University of Munich (TUM). This paper reports on the technical as well as the organizational advancements of the project. With overall more than 130 students involved so far, the project is currently in Phase D, with the launch of the satellite scheduled for early 2018. For communication purposes, MOVE-II will utilize a novel robust and efficient radio protocol for small satellite radio links, called Nanolink, both on an UHF/VHF transceiver and an S-Band transceiver. The usual power restrictions of the 1U envelope are overcome by four deployable solar panels, which are held down and released by a reusable shape memory mechanism. This allows repeated tests of the mechanism and true test-as-your-fly philosophy. As its scientific goal, the MOVE-II CubeSat will be used for the verification of novel 4-junction solar cells. With a footprint of 10x10 cm, the payload consists of one full size solar cell (8x4 cm) and five positions (each 2x2 cm) for the corresponding isotype solar cells. As opposed to its predecessor mission, MOVE-II will be the first CubeSat of TUM utilizing a magnetorquer based, active attitude determination and control system (ADCS). The system consists of five Printed-Circuit-Boards with directly integrated magnetic coils, forming the outer shell of the spacecraft, and one so-called ADCS Mainboard, located in the board stack of the satellite. Each Sidepanel has its own microcontroller and is connected to the ADCS Mainboard with one of two redundant SPI buses. From an organizational point of view, we tried to increase the reliability of MOVE-II by fast prototyping and releases as well as enhanced hardware-in-the loop tests. We will present the application of agile software development in the project as well as methods that we applied to assure reliability on system level. For that purpose a Reliability Growth Model, based on our CubeSat Failure Database, was adapted for the project.
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In this paper we investigate the data on 178 launched CubeSats and conduct a nonparametric and parametric analysis, where the dead-on-arrival (DOA) cases as well as the subsystem contribution to failure are specifically addressed. Using Maximum Likelihood Estimation, a Single Weibull and a 2-Weibull mixture parametric model are fitted to the non-parametric data. Furthermore, by combining developers' beliefs on several reliability aspects from a survey conducted in late 2014 with data from past missions, we make a first attempt to correlate space engineering " best guesses " and intuition to actual data. Finally, the probabilistic CubeSat reliability estimation tool is introduced as a method to reduce the infant mortality of CubeSats: CubeSat developers should be able to estimate their required functional testing time on subsystem and system level at an early project stage, while targeting a desired reliability goal on their CubeSat.
Conference Paper
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We present Nanolink, a data link layer protocol for CubeSats and other spaceborne assets with similar bandwidth and hardware resources. The protocol is designed to operate with high efficiency and high reliability over links with a small bandwidth-delay product and moderate to weak signal quality. A type-I hybrid automatic repeat request (ARQ) scheme and an extensible header structure reduce the overhead added by unused protocol features, thus minimizing the overhead added on the return channel by the ARQ. Simulations show a good performance of the protocol, despite high bit error rate on the channel. Furthermore, the return channel bandwith efficiency of the protocol allows its use on asymmetric links.
Conference Paper
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This paper presents the on-orbit results and the lessons learned from First-MOVE (Munich Orbital Verification Experiment), the first CubeSat mission of the Institute of Astronautics (LRT) at the Technische Universität München (TUM). The development of the satellite started as a student project in 2006. First-MOVE was launched on November 21st 2013. The student-designed and built satellite was operated for almost a month. On December 19th 2013 a major malfunction occurred, presumably due to an anomaly in the on-board data handling system (OBDH), which left the satellite in a mode where it is only transmitting continuous wave (CW) beacons. Although the short mission duration prevented several mission objectives from being achieved, the overall program can be considered a success, as it permitted more than 70 students hands-on experience and led to major in-house technology and spaceflight processing developments. The main aspects of a university-led satellite development, the results of the mission and both technical as well as educational lessons learned are described, including the management and planning of student projects as well as motivational and system engineering aspects. These aspects include planning the project around student's schedules rather than in a traditional, linear fashion, the careful selection and distribution of team members to subsystem teams and the deviation from traditional systems engineering process flows in order to retain student motivation. The importance of large milestone reviews and kick-off events as short term goals and as a means to recruit new team members are highlighted. Academic outreach programs included a one week summer school held in 2011 to recruit and train new students in a time-efficient setting in relevant technical aspects. The paper explains in more detail the technical lessons learned from the major satellite subsystems, both self-developed and purchased. The self-developed systems include, among others, the design of the solar-panel release mechanism, the unique CMOS latch-up protection system, the hard-command unit and the OBDH system. Although we can report a flawless function of all the purchased subsystems in-orbit, the detailed in-house system-level testing of these components is a major lessons learned of First-MOVE from the prospective of student education and system knowledge. Despite the existence of documentation, the time and knowledge needed for designing a test bed for the electric power system (EPS) and the subsequent testing was underestimated. Furthermore, from a testing prospective, the importance of integrated system-level testing and the need for longer, continuous operations test of the satellite are emphasized. On-orbit flight data, as well as educational lessons learned for efficient student involvement during mission operations are highlighted. An outlook to MOVE II, the follow-up satellite project of LRT, outlines how the lessons learned of the last generation students can be carried over and how they will influence future (student) satellite developments and spaceflight development processes at LRT.
A Reliability Estimation Tool for Reducing Infant Mortality in CubeSat Missions
  • M Langer
  • M Weisgerber
  • J Bouwmeester
  • A Hoehn
M. Langer, M. Weisgerber, J. Bouwmeester, A. Hoehn, "A Reliability Estimation Tool for Reducing Infant Mortality in CubeSat Missions", submitted for IEEE Aerospace Conference, Mar 4-11, 2017, Big Sky, Montana, USA. DOI: 10.1109/AERO.2017.7943598
Nanolink: A Robust and Efficient Protocol for Small Satellite Radio Links
  • N M E Appel
  • S Rückerl
  • M Langer
N. M. E. Appel, S. Rückerl, M. Langer, Nanolink: A Robust and Efficient Protocol for Small Satellite Radio Links, Small Satellite Systems and Services Symposium-4S, Valetta, Malta, 2016, 30. May3. June.
AMBA AXI4-Stream Protocol Specification v1.0, Specification
ARM, AMBA AXI4-Stream Protocol Specification v1.0, Specification, 2010.
Practical Manufacturing Testing of 802.11 OFDM Wireless Devices, online, Whitepaper
  • Litepoint
LitePoint, Practical Manufacturing Testing of 802.11 OFDM Wireless Devices, online, Whitepaper, URL: http://www.litepoint.com/wpcontent/uploads/2014/02/Testing-802.11-OFDM-Wireless-Devices_WhitePaper-1.pdf.