Evaluation of radiation tolerant satellite communication modem Venkata Srikanth Bhuma Santosh Kumar Balsu

Evaluation of radiation tolerant satellite communication modem Venkata Srikanth Bhuma Santosh Kumar Balsu
Master Thesis
Electrical Engineering
June 2012
Evaluation of radiation tolerant satellite
communication modem
Venkata Srikanth Bhuma
Santosh Kumar Balsu
School of Computing
Blekinge Institute of Technology
371 79 Karlskrona
Sweden
This thesis is submitted to the School of Computing at Blekinge Institute of Technology in
partial fulfillment of the requirements for the degree of Master of Science in Electrical
Engineering. The thesis is equivalent to 20 weeks of full time studies.
Contact Information:
Author(s):
Venkata Srikanth Bhuma
E-mail: [email protected]
Santosh Kumar Balsu
E-mail: [email protected]
External advisor(s):
Jan Schulte
ÅAC Microtec AB
Dag Hammarskjölds väg 54,
SE-751 83 Uppsala
Sweden
Phone: +46700910429
University advisor(s):
Craig A. Lindley
School of Computing
University Examiner(s):
Patrik Arlos
School of Computing
School of Computing
Blekinge Institute of Technology
371 79 Karlskrona
Sweden
Internet
Phone
Fax
: www.bth.se/com
: +46 455 38 50 00
: +46 455 38 50 57
ii
ABSTRACT
The design specification of CubeSat standards by
California Polytechnique University has become a
major milestone in development and deployment of
nano satellites. The number of CubeSats that are being
deployed into the orbit has increased in recent years.
The design and development have been transformed
from traditional hardware devices to software defined
radios. However there is a lack of clear understanding
and concrete findings on developing proper
communication transceivers. The lack of knowledge of
proper guidelines to be followed to design and develop
communication subsystems prompts and acts as a base
for us in understanding the communication subsystem
design standards followed by CubeSat projects in
various universities. Particular attention is given to
those CubeSats that are currently in the orbit and are
under development. The main aim of the thesis is to
identify the challenges faced by the CubeSat developers
and provide guidelines for future developers in order to
overcome those challenges to reduce the development
time and costs which are major constraints in CubeSat
developments. A traditional literature review process
was followed in order to identify the potential issues
and in parallel a modem was designed and implemented
in order to know some more challenges while
developing a CubeSat modem. From the literature
review method, eight potential issues were identified
and an additional two more challenges were
experienced while implementing a modem. Potential
advantages and disadvantages of using nanoRTU have
also been identified as part of the work.
Keywords: CubeSat, communication subsystem,
software defined radio modem, challenges.
ACKNOWLEDGEMENT
We would like to express our immense gratitude to our supervisor Prof. Craig A.
Lindley for his valuable guidance, support and motivating us throughout the research. We
appreciate his vast knowledge and skill in many areas, which have made us to improve our
areas of research.
We would like to thank Dr. Fredrik Bruhn and Mr. Jan Schulte for their support
and supervision throughout this work. Very special thanks to ÅAC Microtec AB for
providing us with all the necessary equipment.
We are greatly thankful to our beloved parents for their relentless support that
helped us to reach our goals.
Finally, we offer our sincere thanks to our friends and colleagues for their valuable
contributions and making this work, a success.
Regards,
Santosh and Srikanth.
ii
CONTENTS
ABSTRACT ...........................................................................................................................................I
ACKNOWLEDGEMENT .................................................................................................................. II
CONTENTS ....................................................................................................................................... III
LIST OF FIGURES .......................................................................................................................... IIV
LIST OF TABLES ............................................................................................................................... V
LIST OF ABBREVIATIONS ............................................................................................................ VI
1
INTRODUCTION ....................................................................................................................... 1
1.1
1.2
1.3
1.4
2
MOTIVATION ......................................................................................................................... 2
AIM ........................................................................................................................................ 2
RESEARCH QUESTIONS .......................................................................................................... 2
OUTLINE ................................................................................................................................ 3
BACKGROUND .......................................................................................................................... 4
2.1
KEY CONCEPTS ...................................................................................................................... 4
2.1.1 CubeSat ............................................................................................................................. 4
2.1.2 Communication subsystem ................................................................................................ 5
2.1.3 Modem .............................................................................................................................. 6
2.1.4 Modulation and Demodulation techniques ....................................................................... 6
2.1.5 Communication Protocol .................................................................................................. 8
2.1.6 NanoRTU .......................................................................................................................... 8
2.1.7 Radiation .......................................................................................................................... 8
2.1.8 FPGA and VHDL .............................................................................................................. 9
2.1.9 FPGA vs. DSP .................................................................................................................. 9
2.1.10
Software Defined Radio (SDR) ..................................................................................... 9
2.1.11
Plug and Play ............................................................................................................. 10
2.2
SURVEY OF RELATED WORK ................................................................................................. 10
3
RESEARCH METHODOLOGY AND IMPLEMENTATION ............................................. 12
3.1
RESEARCH METHODOLOGY ................................................................................................. 12
3.2
IMPLEMENTATION ................................................................................................................ 13
3.2.1 Literature Review ............................................................................................................ 14
3.2.2 Design and implementation of modem ............................................................................ 16
4
RESULTS & ANALYSIS .......................................................................................................... 22
4.1
STATE OF ART...................................................................................................................... 22
4.1.1 Successful CubeSat launches .......................................................................................... 22
4.1.2 CubeSats in development ................................................................................................ 26
4.2
CRITERIA FOR MODEM IMPLEMENTATION ............................................................................ 32
4.2.1 Challenges faced by CubeSat developers – Results after literature review .................... 32
4.2.2 Challenges encountered during experimentation ........................................................... 34
4.3
ADVANTAGES AND DISADVANTAGES NANORTU W.R.T CRITERIA ....................................... 35
5
CONCLUSION & FUTURE WORK ....................................................................................... 36
5.1
5.2
CONCLUSION ....................................................................................................................... 36
FUTURE WORK ..................................................................................................................... 37
6
BIBLIOGRAPHY ...................................................................................................................... 38
7
APPENDIX A ............................................................................................................................. 43
iii
LIST OF FIGURES
Figure 2.1 The prototype of the CubeSat with the mother board ............................................. 4
Figure 2.2 A general overview of the communication subsystem ............................................ 6
Figure 2.3 AX.25 protocol stack............................................................................................... 8
Figure 3.1 Constructive research flow .................................................................................... 12
Figure 3.2 Research flow ........................................................................................................ 14
Figure 3.3 Illustration of the digital satellite communication modem .................................... 16
Figure 3.4 ÅAC nanoRTU 211............................................................................................... 17
Figure 3.5 Modem intercommunication ................................................................................. 17
Figure 3.6 Process flow .......................................................................................................... 18
Figure 3.7 AFSK Demodulation algorithm [55] ..................................................................... 19
Figure 3.8 BPSK Modulation algorithm ................................................................................. 19
Figure 3.9 AFSK demodulation simulation ............................................................................ 20
Figure 3.10 BPSK modulation simulation .............................................................................. 21
Figure 3.11 Programming FPGA ............................................................................................ 21
Figure 7.1 Hardware consumption for the AFSK demodulator algorithm (code 1) ............... 43
Figure 7.2 Hardware consumption for the AFSK demodulator algorithm (code 2) ............... 44
iv
LIST OF TABLES
Table 4.1 CubeSat communication subsystems from 2011 to till date ................................... 28
Table 4.2 CubeSat communication subsystems from 2003-2011 [60] ................................... 31
Table 4.3 Verification of nanoRTU with the stated problems in LR...................................... 35
v
LIST OF ABBREVIATIONS
ADC Analog to Digital Converter
ADCS Attitude Determination and Control System
AFSK Audio Frequency Shift Keying
ASK Amplitude Shift Keying
BPSK Binary Phase Shift Keying
BPS Bits per Second
COTS Commercial Off-The-Shelf
DAC Digital to Analog Converter
DSP Digital Signal Processor
DTMF Dual Tone Multi-Frequency
EDAC Error Detection and Correction
EPS Electrical Power Systems
FPGA Field-Programmable Gate Array
FSK Frequency Shift Keying
GMSK Gaussian Minimum Shift Keying
HDL Hardware Description Language
JPEG Joint Photographic Experts Group
LR literature Review
OBDH On-Board Data Handling
PnP Plug and Play
PSK Phase Shift Keying
QAM Quadrature Amplitude Modulation
RTU Remote Terminal Unit
SDR Software Defined Radio
SPA Space Plug and Play Avionics
TC Telecommand
TM Telemetry
TNC Terminal Node Controller
TT&C Tracking Telemetry and Command
VHDL Very high speed integrated circuit Hardware Description Language
vi
1
INTRODUCTION
Technological advancements have led to minimizing the size of innovative
devices. Mobile phones, computers, processors etc. are getting smaller as a result of
new and improved technology and the same is true in the space industry. With the
rapid development of space technology, researchers are oriented towards designing
and building smaller satellites for various commercial, scientific and educational
purposes. CubeSats are a result of this major revolution in the space industry. Time
consuming processes, bulk sizes and costs involved in building traditional satellites for
educational purposes have led the scientific community to develop low cost and light
nano-satellites. One of the major factors differentiating traditional satellites from
CubeSats is the scope for innovation in latest space technology. Traditional satellites
do not allow for much scope in renovating and innovating newer space standards but
CubeSats expand the opportunities for experimenting and working with the latest
technology. The CubeSat standard was developed in 1999 by the California
polytechnic state university and Stanford University. Since then there have been 250
CubeSats deployed in orbit or in development state [18].
CubeSats, or nano-satellites, are very small satellites with specified dimensions of
10x10x10 cm. The primary goal of developing CubeSats is to provide better
educational opportunities for students and researchers in space technology and to test
the latest technology in the space industry. Although there are a number of advantages
to building and operating CubeSats, there are a few limitations such as payload
restrictions, slower communication links, low solar power generated from the surface
of CubeSats and radiation, which can be traded off against the huge benefits of using
such satellites. A CubeSat comprises of different subsystems. Each subsystem has a
specific and important role in effective functioning of the satellite. The communication
subsystem is one of the most important subsystems and is responsible for
communication between the satellite and the ground station.
Communication subsystems of different CubeSats vary and depend on the use and
application of the CubeSat being developed. These subsystems are designed according
to the specific requirements of the CubeSat. To make the development process more
flexible, cheaper, easy and fast, most university CubeSat projects use commercial offthe-shelf (COTS) components as they are cost effective and easy to integrate with the
onboard systems. Most CubeSats were designed using these COTS components.
Further development of the communication subsystem is a good choice of area for a
thesis project in order to improve the communication link between the On-Board Data
Handling (OBDH) system and ground stations, keeping the cost and size of the
CubeSat down.
The terminal node controller along with the modem is the heart of the
communication subsystem. A proper design of the CubeSat modem can establish a
good communication link between the OBDH and the ground station. The two most
important constraining factors during the design of a modem are the limited transmitter
power available for use and the size of the subsystem [14]. In this thesis work, an
attempt is made to design a communication subsystem by replacing the terminal node
controller with a Plug and Play device (PnP) called nano Remote Terminal Unit
(nanoRTU) on which a modem is integrated taking into consideration the power and
size constraints. The modem in this subsystem is designed on a Field-programmable
gate array (FPGA) using software defined radio technology.
1
The work established in this thesis aims to study the various design processes of
different communication subsystems to gain a thorough understanding of the concepts
involved during the development and the complications encountered. A specific
purpose of this thesis is to develop criteria for developing a CubeSat modem which
can act as a guideline for future developers. The criteria developed are based on our
attempt to design and develop a modem based on software defined radio technology
integrated on the nanoRTU.
In order to achieve these research objectives, a literature review was conducted to
identify the potential problems that evolved in designing a modem and a software
based modem was designed and implemented to identify the challenges faced in real
time.
1.1
Motivation
Extensive research has been carried out in developing communication subsystems
for CubeSats which are used for different applications. Researchers so far, have
stressed and concentrated on the development process but very few of them
extensively analyzed the problems faced during the development and launch of these
systems on board the CubeSat. Measures to be taken to avoid such problems are also
left unanswered. Also, no specific evaluation criteria for modems have been proposed
to allow future work in the area to be more agile and easily comprehensible. These
drawbacks in current research have motivated us to carry out the work in our thesis. In
this thesis, a literature review of all possible cases involved in developing a
communication subsystem for a CubeSat was performed to analyze and specifically
point out the problems that may occur during the process of design and development.
The state of the art survey carried out not only suggests complications faced while
developing a communication subsystem but this also led to identification of the
important measures to be taken during subsystem development, providing evaluation
criteria for modems for design in the future. To enrich the thesis in a more practical
way, problems faced during the experiment by using a nanoRTU in the subsystem are
mentioned to avoid any future obstacles in the development process.
1.2
Aim
The main aim of the thesis is to establish a set of criteria to evaluate modem
designs for future CubeSat projects through an extensive state of art literature review
of current CubeSat communication subsystem designs and practical implementation on
a nanoRTU. In addition, evaluating advantages and disadvantages of the nanoRTU
with respect to the established criteria forms the secondary aim.
1.3
Research Questions
The research questions driving the thesis project are:
1. What is the state-of-art in the current research and development of
CubeSat communication subsystems?
2. What are the criteria for evaluating alternative modem implementations
(software/hardware) for CubeSat spacecraft systems?
2
3. What are the respective advantages and disadvantages of the nanoRTU, in
relation to other systems and (how) can this be quantified?
1.4
Outline
The remaining parts of this document are organized
introduction of the concepts used for this thesis and survey
presented in Chapter 2. The methodology and implementation
discussed in Chapter 3. The results and analysis are presented
conclusion and future work are presented in Chapter 5.
as follows. A brief
of related works are
of the experiment are
in Chapter 4. Finally,
3
2
BACKGROUND
This chapter presents the key concepts and the Survey of Related works.
2.1
Key concepts
This section briefly presents the necessary concepts to understand this thesis.
2.1.1
CubeSat
A CubeSat is Nano satellite designed for Low Earth orbit. It is 10x10x10 cm cube
which weighs about 1Kg [2]. The CubeSat project was initially started by California
Polytechnic State University (Cal Poly) and Stanford University in the year 1999. The
aim of the project was to develop a low cost Nano satellite for Low earth orbit in
support of education programs in space engineering [1]. A standard design
specification was developed for CubeSat projects to reduce the cost and development
time for future developers of educational satellites.
Figure 2.1 The prototype of the CubeSat with the mother board
The fig 2.1 shows the prototype of the CubeSat. The available amount of space is
very limited. The low surface area of a CubeSat restricts the amount of solar power
that may be generated, restricting power available for computation, communications,
and payloads. Restrictions on space, power and cost can result in problems like slow
communication links, limited payloads, and minimal information processing
capabilities. In order to incorporate all subsystems and for effective operation, the
4
designing of the CubeSat has become very complex. The major departments of the
CubeSat are onboard data handler, communication sub-system, the electrical power
system, attitude determination and control system and the payloads. Each department
has a specific task of its own [3].
Onboard data handler (OBDH)
OBDH is the most important subsystem for the overall working of the satellite.
This subsystem acts as a main module which controls and manages the remaining
subsystems.
Attitude determination and control system (ADCS)
This system is very crucial in order to make a mission successful and effective i.e.
directing the space craft towards a target.
Pay loads
The overall CubeSat mission is planned because of the payload. The payloads are
some sensors or scientific experiments which have a particular task to perform.
Electrical power systems (EPS)
The main functions of the EPS are distribution and control of the power to all
other power subsystems based on their electrical power consumption. The length of the
mission can be determined on the ability of the power system.
Communication subsystem
This subsystem is responsible for the communication between the satellite and the
ground station. In this thesis, we focus on the communication sub-systems.
2.1.2
Communication subsystem
A communication sub-system is designed to handle three functions (1) to transmit
a tracking signal, (2) download telemetry to a ground station and (3) to receive
commands from a ground station. A satellite communication system is often referred
to as a TT&C (Tracking, Telemetry and Command) system after these functions. The
communication between the satellite and the ground station is known as data link, is
two way communication channels where the uplink is the data commands transmitted
from the ground station to the satellite and the downlink is the telemetry data or the
beacon transmitted from the satellite to the ground station [4].
The fig 2.2 shows the block diagram of the communication subsystem and its
communication with the ground station. Here the OBDH is the heart of satellite or the
main control module. The modem or the Terminal Node Controller (TNC) converts the
signals received from the ground station and forwards the data to the OBDH. Both the
satellite and ground station should follow a common protocol in order to establish a
proper communication channel between them. In this thesis, we are concerned with
modem design and development which is discussed in this chapter. The protocol used
is also later discussed in this chapter.
5
Figure 2.2 A general overview of the communication subsystem
2.1.3
Modem
Modem is a device which is combination of modulator and demodulator. At the
transmitting end, Modem acts as a device that accepts binary data from a data source
which is modulated to create a signal which is suitable for transmission and at the
receiving end it acts as complementary for transmitting end [7]. Modems are classified
based on the amount of data transferring capability which measured in terms of bits
per second (bps) or baud rate. In order to encode and decode the data we need to have
a proper modulation and demodulation technique. The designing of modem which can
establish a good communication link between the onboard data handler (OBDH) and
ground station with the available transmitter power is constrained by these above
mentioned power and size factors [3]. In this thesis we would design a modem based
on FPGA which is the main module in the communication subsystem. This modem is
embedded on a nanoRTU which is discussed later in this chapter.
2.1.4
Modulation and Demodulation techniques
Modulation is process of imposing the properties of a message signal onto a high
frequency carrier signal. Modulation schemes are mainly classified into Analog
modulation methods and Digital modulation methods. Analog modulation techniques
are further classified based on amplitude, frequency and phase modulation techniques.
Digital modulation schemes are further classified based on type of keying. The most
fundamental digital modulation techniques are Phase shift keying (PSK), Frequency
shift keying (FSK), Amplitude shift keying (ASK) and Quadrature amplitude
modulation (QAM). Demodulation is process of retrieving back the original
information from the modulated signal. In this thesis we are going to use BPSK
6
(Binary Phase Shift Keying) modulation and AFSK (Audio Frequency Shift Keying)
demodulation.
Binary Phase Shift Keying (BPSK)
BPSK is one of three basic binary modulation techniques. BPSK modulation is a
technique where the phase of a carrier sinusoidal signal changes abruptly by 180° or
phase reversal occurs for every transition of modulating binary sequence (input bit)
[6]. The general form of the BPSK signal is based on the following equation [23]. The
binary data is represented by two signals with different phases in BPSK. (t) and (t)
are the two signals at point of time t.
Where:
- A is the amplitude
is the frequency of the carrier and
- T is the time.
The phase of the transmitted signal remains the same if a “1” was transmitted and
is shifted by 180° if a “0” is transmitted [8].
ADVANTAGES:
 BPSK has high spectrum efficiency, good spectral characteristics, strong antiinterference performance, and has faster transfer rates [12].
 Due to its robustness it is extensively used in satellite communication systems.
DISADVANTAGES:
 It is simple to implement, but it is inefficient in terms of using available bandwidth.
Audio Frequency Shift Keying (AFSK)
AFSK is a modulation scheme in which the data is represented by changes in the
frequency of an audio tone. The changes in the frequency are between mark and space
frequencies represented by binary zero and one respectively.
ADVANTAGES:
 AFSK encoded signals pass through AC-coupled links that are included in most
devices designed to carry music or speech.
 Implementing an audio modulation scheme allows us operate on many digital modes
that have been developed by amateurs.
DISADVANTAGES:
 AFSK is inefficient in terms of using available bandwidth and power.
7
2.1.5
Communication Protocol
The AX.25 protocol is a data link layer protocol. It is basically designed for
amateur radio communications and extensively used in amateur radio networks [5].
The aim of this protocol is to establish a reliable communication link between two
terminals. When a connection is established between the two stations the AX.25 data
frames are passed from one station to the other and are traversed back. [10]
Flag (1)
Address(14)
Control(1/2)
PID(1)
Info(<256)
FCS(2)
Flag(1)
Figure 2.3 AX.25 protocol stack
The Fig 2.3 shows the frame format of the AX25 protocol. The functionality of
each field of the frame is
1.
2.
3.
4.
5.
6.
2.1.6
FLAG: indicates the start and stop of the frame.
ADDRESS: identifies the sender and the receiver
PID (protocol identifier): identifies the type of top level protocol
INFO: contains from zero to 256 bytes of data
FCS (frame check sequence): contains a cyclic redundancy check (CRC).
CONTROL: identifies the type of frame. There are three types of control field
formats.
I.
Information frame (I frame): Contains information about the sender’s
send and receive sequence number.
II.
Supervisory frame (S frame): Provide supervisory link control such as
acknowledging or requesting retransmission of I frames and link layer
window control.
III.
Unnamed frame (U frame): U frames are responsible for establishing
and terminating link connections.
NanoRTU
A Remote Terminal Unit (RTU) is a miniaturized flexible and microprocessor
controlled device used to interface with an OBDH. In order to reduce development
time, the US Air Force Research Laboratory proposed a plug and play standard known
as Space PnP Avionics (SPA). This standard allows for self- discovery and selfconfiguration of both hardware components and software applications within a satellite
network [20]. One such PnP component is the nanoRTU. It is low cost, low power
consumption, high reliability module which is a small size module that can be easily
mounted on the satellite subsystem. These units are basically designed for Nano/micro
satellites. In this thesis, we used a nanoRTU developed by ÅAC microtec AB,
Uppsala. The specifications, easy integration with payloads and the applications of the
nanoRTU are described in the data sheet [9].
2.1.7
Radiation
Space-based radiation is an important aspect to consider in system design. When
electronic systems are exposed to space radiation like high energy ion radiation,
magnetic fields and plasma interactions, there are chances of memory corruption,
8
degradation or permanent damage of components/systems [4]. It is therefore desirable
to use radiation tolerant devices in order to achieve high processing capabilities to
minimize this damage and its consequences [9].
2.1.8
FPGA and VHDL
A field programmable gate array (FPGA) is a programmable digital logic chip.
FPGAs contain two dimensional arrays of logic cells and switches. A logic cell is
programmed to perform certain actions and a switch can be used to interconnect these
logic cells. Once the desired logic is designed and synthesized, the design can be
dumped into the FPGA using a simple adaptor. The logic cells are programmed either
by the customer or by the manufacturer using a hardware descriptive language (HDL)
[11]. HDL is used to program the FPGA indeed to create a perfect logic device.
Verilog and VHDL (Very high speed integrated circuit HDL) are the most commonly
used hardware description languages. For this thesis the authors used VHDL to design
and implement the AFSK and BPSK modulation schemes.
2.1.9
FPGA vs. DSP
The implementation of a modem is a digital signal processing issue. Two types of
programmable platform could be used to realize the modem, i.e., a Digital Signal
Processor (DSP) or an FPGA.
 DSP’s are one time programmable devices whereas FPGA’s can be programmed as
many times as required. The FPGA also has an advantage of scalability and
expandability.
 FPGA has an advantage of parallel (multi)processing when compared to a DSP
which is a dedicated processor. As a result of this multi-processing the performance
of an FPGA is more than a DSP.
 FPGA’s are enormously faster, flexible and less expensive when compared to
DSP’s.
 DSP’s have better power efficiency than FPGA’s.
2.1.10 Software Defined Radio (SDR)
The term “software defined radio” was coined by Joseph Mitola in 1991,
refereeing to a shift from traditional hardware radio systems to software, where the
major functionality is defined [24]. It is defined as a radio communication system
whose hardware components are replaced by software implemented on embedded
computing devices [25]; also, systems that depend on software to perform their base
band functionalities are also called software defined radio’s [28] .The same hardware
can be used for communication on different transmission and reception channels just
by changing the software. Hardware defined radios are expensive compared to
software defined radios. Re-Configurability is one of the main advantage that software
defined radio have. The important blocks of SDR are intelligent antenna,
programmable RF modules, high performance Digital-to-Analog (DAC) and Analogto-Digital (ADC) converters, DSP techniques and the interconnect technology [26].
9
2.1.11 Plug and Play
The plug and play architecture for the avionics subsystems of a satellite allows
distinct applications to make use of a common set of avionics modules in a common
enclosure. Multiple subsystems can operate simultaneously to perform redundant
operations or joint operations. To develop avionics subsystems using a plug and play
methodology, three elements should be considered. They are: the open bus
architecture, radiation tolerant FPGA implementation of bus interfaces and use of a
common enclosure design [20].
2.2
Survey of related work
In [14] the overall operation of the communication subsystems in satellites is
discussed. A successful design of low-complexity GMSK (Gaussian minimum shift
keying) modem software which acts as a backend process in eliminating the use of an
external modem was reported. A DSP (TMS320C54x) processor was chosen for the
Telemetry/Telecommand (TM/TC) circuit. The design of TM/TC circuit adopted in
[14] helped to cut down the Nano-satellite cost. The TM/TC circuit is small and can be
easily mounted on any small satellite. The authors described the hardware and
software solutions they achieved to design a good TM/TC system.
Along with the above implementation, an additional AFSK modem was designed
to store and forward automatic packet reporting system (APRS) payload data in [15].
A general purpose DSP microcontroller TMS320C5416 was chosen for designing the
modem. In both [14] and [15] the communication was based on the AX.25 protocol
which is data link layer protocol derived from the ITU-TX.25 protocol suite.
A low power frequency shift-keying (FSK) modem was designed using a DSP
processor for CubeSat satellites in [16] and the communication was achieved
successfully via a kiss protocol. The original intention was to implement a modem that
could switch between two different baud rates (1200 and 9600 baud), but due to time
constraints they eventually ended up only implementing a 1200 baud rate.
The authors of the [12] have designed and implemented a BPSK modem as part of
a Software defined radio (SDR) transceiver system. The paper clearly states the
algorithms used in designing the BPSK modulator and the demodulator. When
designing the demodulator the authors used a Costas loop to achieve carrier
synchronization. Both the modulation and demodulation algorithms are firstly
simulated in MATLAB. Verilog HDL was the language used to design the system and
then it was simulated again using Modelsim SE 6.5 in order to prove the feasibility and
superiority of the proposed solution. Then the design was tested in a Virtex5 series
FPGA (Xilinx). The modulation and demodulation techniques are the core content in
the SDR communication system.
The space systems research laboratory at Saint Louis University designed a
SLUCUBE as part of their space program [21]. The main aim of the program was to
cut down the cost and provide rapid access to space using commercial off the shelf
components. In this program, they developed a prototype software defined radio that
supports a wide range of communications and modes. A COTS Stensat UHF/VHF
transceiver that supports AX.25 packet standard was used as the primary device for
communication. The newly developed modem supports UHF (uplink) and VHF
(downlink) frequency bands using a narrow band Frequency modulation scheme. The
software defined radio modem was flight tested for the first time on the SLUCUBE.
10
In [22] the author proposes a solution to tackle the problems of traditional RF
hardware by designing a software defined radio modem that can operate in UHF and
VHF amateur radio bands. An ADSP-BF537 blackfin DSP is proposed and the
software for the DSP will be created with National instruments lab-VIEW using an
embedded module for ADI Blackfin Processors. Various tests including thermal,
vacuum and vibration testing will be performed on the newly developed modem by the
Saint Louis University.
An onboard communication subsystem was developed by the students of
Sathyabama University for their CubeSat mission. Various design aspects related to
both hardware and software were discussed in [27]. Microchip dsPIC32 DSC
controller was used for implementing AX.25 protocol. MHX425 Transceiver is used
for transmission and reception using a FSK modulation technique. The communication
subsystem source code is implemented using a C compiler and MPLAB IDE for
performing various functionalities including CW beacon, Transceiver interface and
software TNC.
11
3
RESEARCH METHODOLOGY AND
IMPLEMENTATION
This chapter is organized into two sections. The first section covers the research
methodology that should be followed in order to answer the research questions. The
second section describes the implementation of the research methodology.
3.1
Research Methodology
The type of methodology used to answer the research questions is constructive
research.
Constructive Research
The basic idea behind constructive research approach is to develop a construct for
a specific problem. A construct can either be a model, an algorithm, new software or a
framework which can be used a reference in future research works carried out in
similar fields. The problem which is identified during the process of constructive
approach is either derived from theory or from practice. Theoretical derivation of the
problem includes extensive literature review of similar works in the past and practical
derivation implies existing or previous experiments conducted.
Figure 3.1 Constructive research flow
12
Constructive approach
Fuzzy info
A state of the art literature review has been performed to gather all the information
pertaining to the design and development of communication subsystems in several
CubeSats which have been deployed into orbit and which are in development.
Theoretical body of knowledge
Fuzzy info provided us with a broad theoretical understanding of what has been
done in the fields of CubeSat communication design. This body of knowledge helped
us forming a base in framing the main problem for which this thesis is carried out. It
has also helped us in performing a thorough examination of a number of studies stating
the challenges that are encountered by CubeSat developers while developing a
communication subsystem.
Practical utility
A software defined radio modem based on nanoRTU has been designed and
implemented. This experiment has resulted in additional challenges faced during the
development.
Frame work
A list of all the challenges that evolved during practical implementation of a
modem design and from theoretical knowledge helped us designing a framework for
future reference. The framework designed in this thesis describes in detail all the
problems encountered during state of art and experiment and proposes suitable
solutions.
Practical relevance
In order to test the framework, the same experimental setup used to identify the
challenges faced during the design of a modem is used as a testing reference. All the
challenges encountered are tested on the implementation of software defined radio
modem based on nanoRTU.
3.2
Implementation
This section describes the implementation of the above described methodology.
An overview of the process is clearly explained in fig 3.2.
In order to answer the research questions, we needed to conduct a literature review
of the existing problems documented when implementing a CubeSat modem and
combine these with problems that are encountered during the practical process of
designing and implementing an FPGA based modem. Taking both the sets of problems
into consideration we create a set of criteria that are to be considered in designing a
modem for a CubeSat.
13
Figure 3.2 Research flow
3.2.1
Literature Review
The literature review is first important step in any research. It is a means of
identifying, evaluating and interpreting problems and existing solutions relevant to the
research based on study and analysis of publicly available scientific articles, books and
other resources relevant to particular research area [29]. There are different reasons for
undertaking literature, the most common reasons according to B. Kitchenham [29].
 Summarizing the existing evidence concerning the technology in a
comprehensive and fair manner.
 Identification of gaps in current research and to put forward some more
areas for further research.
 Providing a frame of reference for the future research investigation.
14
Qualitative approach was followed while conducting literature review. The
structure of the literature review conducted is described as a following process.
The main idea behind the literature review is to convey the reader that what others
have accomplished in your field and how different is your work from others and the
ability to summarize the work of others for the convenience of the reader [61]. The key
steps that must be followed in any literature review are 1) Define keywords. 2) Finding
the sources 3) Article selection criteria [30].
Defining Keywords
Keywords are very important in finding out the information. A set of keywords
like CubeSat, communication subsystem, modem challenges and criteria were defined
by taking the research questions and breaking it into words.
Finding the sources
In order to find out the sources related to the research question search strings
were formulated by combining the keywords with Boolean AND and OR’s. But there
were very few papers so the authors have limited their search by using keywords and
combination of keywords in well-known databases including IEEE Xplore, Science
direct, Scopus, ACM digital library and Google scholar.
Article selection criteria
The information regarding the communication subsystems of different CubeSats
were collected, the investigation of the challenges in designing the modem were done
critical and accurate manner by using the below stated inclusion and exclusion criteria.
Further papers are filtered by reading the abstract, introduction and conclusions.
Inclusion criteria
 The key words formed from the research questions were implemented on
the selected databases including IEEE Xplore, Science direct, Scopus,
ACM digital library and Google scholar.
 Research relevant to development of communication subsystem of CubeSat
was included.
 Studies covering different types of communication subsystems.
 Studies covering various challenges in development of CubeSat
communication subsystems.
 Studies covering the implementation of software defined radio in
communication subsystems.
 Studies of various CubeSat launches.
Exclusion criteria
 Papers which are not in English.
 Papers regarding development of traditional satellites and which are not
relevant to the topic are excluded.
 Studies those are irrelevant to the research questions.
15
3.2.2
Design and implementation of modem
After an extensive literature search regarding the challenges that are observed in
designing a modem for a good communication subsystem, we observed that there are
only a few instances that quoted problems related to hardware. We needed to develop a
modem in order to understand the software related challenges more deeply. Along with
the literature review here we have designed and implemented a modem which acts as
backend software for the nanoRTU. Some of the concepts that are related to the
development phase are stated above and the implementation process will be explained
and finally the challenges that are observed will be presented below.
Modem development
The development we aimed to design and develop a modem on the ÅAC nanoRTU
211 radiation tolerant hardware platform that is transparent to different RF-front ends,
for example for transmission on UHF (400-450 MHz) and reception on VHF (130-160
MHz).
UART
I2C
USB
SpW
Packet handling
ÅAC RTU family
Digital Modem
Backend
ÅAC nanoRTU
200
RF-front end (TX)
Commercial
RF-front end (RX)
Commercial
Figure 3.3 Illustration of the digital satellite communication modem
The fig 3.3 shows the modeling of the communication subsystem. The
communication subsystem should be capable of receiving and transmitting signals
from ground stations as well as providing full control of telecommand data decoding
and telemetry data encoding. These signals should be passed through a modem that
handles the modulation and demodulation schemes [54]. The received signal from the
ground station will be demodulated by the modem and given to the OBDH through the
UART. The data from the OBDH will be given to the modem through the UART for
modulation and the modulated data will be given to the transmitter for transmission to
the ground station.
Modem intercommunication
The modulation and demodulation schemes used in the modem are BPSK and
AFSK respectively. The signal received from the ground station to the onboard
receiver will be an AFSK modulated signal. The received AFSK modulated signal is
given to the analog to digital converter which converts the analog signal to digital bits.
The SPI ADC forms the interface between the ADC and AFSK demodulator. The
digitally converted AFSK modulated data is then given to the AFSK demodulator
through an SPI bus. The demodulated data will be sent to the UART transmitter. The
UART transmitter can be used to send the demodulated data to the OBDH from the
modem.
16
Figure 3.4 ÅAC nanoRTU 211
On the other side the data from the OBDH is received by the UART receiver
which sends it to the BPSK modulator for modulating the data. The modulated data is
then given to the SPI DAC which acts as the SPI interface between the BPSK
modulator and the Digital to Analog converter. The analog converted signal is
transmitted from the onboard transmitter to the ground station. The fig 3.5 shows how
the communication is done internally.
Figure 3.5 Modem intercommunication
17
Implementation
In order to implement this method first we have to figure out the required
technologies that are available to create a software modem. Designing a modem which
can transmit and receive data is a signal processing issue. The available chip
technologies to handle these signal processing technologies are FPGA and DSP. Here
we have chosen an ACTEL FPGA (ProAsic A3P600) on which the modem would be
designed and implemented. The reasons for choosing an FPGA are discussed in
chapter 2. The Actel Libero IDE (integrated design environment) software toolset was
used for coding, simulating, synthesizing and finally dumping the code into the FPGA.
VHDL is the hardware descriptive language that was chosen to program the FPGA
with the defined set of modulation and demodulation techniques. The process flow of
the implementation is shown in fig 3.6.
Figure 3.6 Process flow
One of the main steps for implementing a software defined modem is coding the
modulation and demodulation techniques in VHDL. The logic required in the design of
the modulation and demodulation techniques in VHDL is explained below.
AFSK Demodulation
The AFSK modulated data contains multiple tones. Each tone can be characterized
by counting the number of samples between two zero crossings for a given sampling
frequency. As we use 1200 Hz to be the mark frequency and 2200 Hz to be the space
18
frequency, the number of samples between two zero crossings of the mark frequency
will be more than the number of samples between two zero crossings of the space
frequency. For example, a 1200 Hz signal sampled at 264 KHz will have 110 samples
between two zero crossings and a 2200 Hz signal sampled at 264 KHz will have 60
samples. So the counter that resets for every zero crossing will have two discrete
values at the output. A threshold value between these two values 110 and 60 will be
taken as a limit. Based on the limit value the input modulated signal is demodulated
and output as a digital value. If the number of samples between two zero crossings is
greater than the limit value then it is taken as ‘1’ and if the number of samples between
two zero crossings is less than the limit value then it is taken as ‘0’. The AFSK
demodulation is performed for the 1200, 2400, 4800 and 9600 baud rates.
Figure 3.7 AFSK Demodulation algorithm [55]
The number of samples between two zero crossings are calculated by the formula:
N = fs/ (modulated signal frequency * 2).
BPSK Modulation
Figure 3.8 BPSK Modulation algorithm
19
The BPSK modulation is implemented by taking the basic idea of changing the
phase of the carrier wave by 180° whenever there is a change in the input bit. So if “1”
is given as the input, the modulated signal remains same as the carrier signal. And if
“0” is given as the input, then the modulated signal will be the carrier signal with 180°
phase shift [56]. For the Bpsk modulation, the carrier wave cosine wave is taken from
a look-up table instead of calculating the cosine value. Calculating the cosine value
would allow for a pure continuous phase modulated signal but constraints on
computing power and the excessive time it takes for calculating the cosine value for
each sample eliminates this option. BPSK modulation is done for 1200, 2400, 4800
and 9600 baud rates.
Once coding for the modulation and demodulation was completed, the test benches
were created for both the modulation and demodulation process. The code was then
tested using the test benches.
The system was then simulated using a modelsim simulator. The simulation results
for AFSK demodulation are shown in the fig 3.9.
Figure 3.9 AFSK demodulation simulation
From the simulation results of AFSK it can be seen that the given input AFSK
modulated signal was demodulated. We can see that the mark and the space
frequencies were demodulated to binary ‘1’ and binary ‘0’ respectively. The sampling
frequency used for the demodulation of the AFSK signal was chosen to be 264 KHz
and the clock rate chosen was 1 MHz. The process chosen for demodulation was
independent of the used baud rate. The simulations were carried for 1200, 2400, 4800
and 9600 baud rates.
The simulation results of the BPSK modulation are shown in the fig 3.10. From
the BPSK simulation results, we can see that the input digital data was modulated. It
can be seen that for a given binary ‘1’ input, the output was applied carrier cosine
wave and for a given binary ‘0’ input, the output was the applied carrier cosine wave
with 180° phase shift. The simulations were carried out for 1200, 2400, 4800 and 9600
baud rates.
After the simulation process the HDL code is synthesized by synplify synthesis
tool. The synthesis tool translated text based HDL to a circuit and then optimized the
circuit. Once the synthesis was performed, the code was then passed to the place-androute block of the Libero IDE as the next step where the inputs and the outputs were
assigned with proper pins assignments according to the FPGA requirements.
20
Figure 3.10 BPSK modulation simulation
After synthesize and place-and-route stage of the libero IDE we performed post
synthesis simulation and post layout simulation to verify the output before it was
dumped into the FPGA.
Reset
nanoRTU 211
JTAG connector
USB to TTL level
converter cable
nanoRTU cable
Figure 3.11 Programming FPGA
The last and the final step would be to generate a programming file that was ready
to be dumped into the FPGA. For dumping the generated files into the FPGA, we need
an ACTEL PROASIC cable.
Once the programming was successfully completed it needed to be tested to verify
that the set of modulation and demodulation schemes were implemented successfully.
21
4
RESULTS & ANALYSIS
4.1
State of Art
The State of Art review was done by the authors in order to understand the design
techniques employed by different CubeSat developers. The results of state of the art
describe in detail, the various successful CubeSat programs that have been created for
different educational purposes and applications. The answer to research question 1 is
divided into two parts to provide a clearer and more precise explanation of results.
4.1.1
Successful CubeSat launches
The first part describes in detail, the design of communication subsystems and
payloads used in those CubeSats which have been successfully launched during the
Vega booster launch on February 9th 2012, from CSG space centre in Kourou, French
Guiana.
[email protected]
1. Purpose of lunch
The [email protected] CubeSat is an educational program by Politecnico di Torino,
Italy. It was designed to demonstrate the feasibility of active control of a
CubeSats attitude by magnetic actuation [31]. The main aim of the
CubeSat was the demonstration of an active 3-axis attitude determination
and control system including an inertial measurement unit.
2. Major payload
The satellite carries an Active Attitude determination and control system
as the major payload.
3. Communication subsystem – Design and Specifications
The satellite's communication subsystem used a dipole antenna for
transmission and reception. The global weight of the communication
subsystem (including the antenna) was 60g [32]. The maximum power
consumption of the transmitter was 650 mW with BPSK modulation. The
system used a 1200 bit/s link for communications. It had a half-duplex
communication channel which was activated by the ground station on
demand. AX.25 was the protocol chosen for communication on downlink
to share the telemetry data with other CubeSat communities [32].
22
Goliat
1. Purpose of launch
This was the first CubeSat that developed by students of University of
Bucharest and the Romanian space agency. The primary goal of the
satellite was to make scientific measurements and earth observations.
2. Major payload
The Goliat CubeSat carries three primary payloads –
a. CICLOP - To take pictures of Romanian territory
b. SAMIS - A sensor to measure energy
c. DOSE-N - A detector to measure the total dose of radiation inside
the satellite
3. Communication subsystem – Design and Specifications
The communication subsystem of this satellite had two transceiver
architectures, one for a beacon and the other for data transmission [33]. An
amateur radio transceiver and a micro hand MHX-2400 transceiver
functioned independently with two processors commanding the operation.
The MHX 2400 transceiver was initially set to 9600 bps at 1W power
output and depending upon the link performance, the baud rate may
increase or decrease. It used GFSK modulation for data transfer and
followed a proprietary protocol. For the beacon, the modulation schemes
were AFSK/FM and the protocol used was AX.25 [33].
PW-SAT
1. Purpose of launch
PW-Sat was a CubeSat project designed by the Faculty of Electronics and
Information Technology and by the Faculty of Power and Aeronautical
Engineering at Warsaw University of Technology. It was the first Polish
CubeSat deployed into space. The main objectives of the CubeSat were
testing a deployable atmospheric drag de-orbiting device, using solar sail
material. This method may be used in future to remove payloads from
LEO orbit [34].
2. Major payload
The PW-SAT had two primary payloads –
a. Gadadget - To collect data from the satellite via distributed ground
stations
b. Loenidas - To study the effects of satellite de-arbitration
3. Communication subsystem - Design and Specifications
The PW-SAT communication subsystem used the 70cm amateur radio
band for communication between the ground stations and the satellite. The
radio could send 1200 baud AFSK with the AX.25 packet format. It could
communicate at two frequency ranges - 435.032 MHZ (uplink) and
23
145.902 MHZ (downlink).The antenna module had four tape antennas of
55 cm length. The antennas were folded during take-off and it took 30
seconds to unfold the antennas once the satellite was released into orbit
[35].
UNICubeSat-GG
1. Purpose of launch
UNICubeSat was the first CubeSat built by the students of Sapienza
University of Rome. This was designed and manufactured by the GAUSS
group which has vast experience in developing university satellites
(UNISAT) that weigh about 10kg. The on-board data handler had an
MSP430 microcontroller as the main processor that controlled all the onboard operations. The main aim of the CubeSat was to study the effects of
orbital eccentricity on attitude motion, enhanced by gravity gradient [36].
2. Major payload
The satellite carried a Broglio Drag Balance Instrument that aimed to
contribute to the development of accurate thermosphere models, achieving
in situ measurement of the atmosphere density and to accurately detect
weather forecasts [37].
3. Communication subsystem – Design and Specifications
The GAUSS group in collaboration with Morehead state university (USA)
has designed the communication subsystem. The communication
subsystem used a commercial-off-the-shelf UHF transceiver for ground
commands made by Astrodev LLC. The Frequency of the radio in UHF
band was 437.305 MHz [36] and the modulation was GMSK at 9600bps
baud rate and FSK for data transmission at 9600 bps [37].
Xatcobeo
1. Purpose of launch
Xatcobeo CubeSat was built by students from various departments from
the University of Vigo which was headed by Fernando Aguado. This was
the first Galician artificial satellite built in collaboration with the National
Institute of Aerospace Technology, Spain [38]. The main aim was to
provide students with an opportunity to work using space standards.
2. Major payload
The CubeSat was developed to carry three payloads.
a. Software radio for communication – Software defined
reconfigurable radio which uses FPGA
b. Radiation Displacement Damage Sensor- A system for measuring
the amount of ionizing radiation
c. Panel Deployment Mechanism- To test and validate a new
mechanism that will provide the CubeSat with additional electrical
power, extend lifetime and improve the performance of the
spacecraft [39]
24
The software radio was developed using the C language using an
embedded development Kit on a Xilinx Spartan-3E FPGA [40].
3. Communication subsystem – Design and specifications
The communication subsystem used two modes, one to transmit a beacon
in the 145.94 MHZ VHF band and the other to transmit digital data in the
437.36 MHZ UHF band with an FFSK modulation scheme and AX.25 as
the packet protocol [41].
Masat-1
1. Purpose of launch
Masat-1 was the first Hungarian CubeSat to be deployed into the LEO.
This satellite was designed and developed by Budapest University of
Technology and Economics. The main objectives of Masat-1 were,
a. To design and implement the basic subsystems of the satellite
using custom built modules.
b. To gain experience in various departments of designing a satellite
in order to develop more complex space projects in future [31].
All the subsystems onboard were considered as experimental by the
Masat team since the CubeSat was custom built.
2. Communication subsystem – Design and specifications
The communication subsystems, control functions were performed on gate
level. The RF transmit power of the subsystem had two operating modes a 100mW low power mode and a 400mW high power mode. Every 4th
telemetry transmission was set default to be in high power mode [42]. The
operating downlink frequency was 435 MHZ and for Uplink it was 145
MHZ. It used two modulation schemes On-Off Keying (OOK) and FSK
[43].
OUFTI-1
1. Purpose of launch
OUFTI-1 is a technology demonstration mission by university of Liege,
Belgium. The objective of the project was to demonstrate the feasibility of
using amateur radio D-STAR communication protocol to communicate
through a CubeSat [44]. D-STAR stands for Digital Smart Technologies
for Amateur Radio.
2. Major payloads
The OUFTI-1 has two major payloads –
a. D-STAR- to test the amateur radio protocol which has new builtin features like digital communication.
b. It carries an innovative electrical power system developed in
collaboration with Thales Alenia Space ETCA.
25
3. Communication subsystem – Design and specifications
The satellite communication subsystem uses four quarter-wave deployable
antennas, two about 17 cm long for downlink and two about 50 cm long
for uplink [46]. The frequencies used for communication are 145 MHz for
the uplink and 435 MHz for the downlink. D-STAR can use an Ethernet
connection at 128 kbps and digital data and digital voice at 4.8 kbps in
GMSK transmission [48].
ROBUSTA
1. Purpose of launch
Robusta is the CubeSat which was developed by the RADIAC group of
University Montpellier 2 (UM2). RADIAC group is the radiation effects
group of the university which has 30 years of experience, and one of the
world’s leading groups in its field. The main purpose of the mission was to
measure the effects of ionizing radiation on the on-board bipolar
electronics components. The degradation data of the key parameters were
sent back by the payload. Each parameter was measured for every 12
hours, while the radiation dose was measured every 90 minutes and the
temperature data every 6 minutes. The data received from the payload was
analyzed and compared with the results which were observed during the
ground tests. This information from the payload sensor provides
information on earths radiation belts [31].
2. Communication subsystem – Design and implementation
Communication subsystem of ROBUSTA used a custom built transceiver.
A PIC performs the data packetization using AX.25 protocol. The radio
sends 1200 baud data using AFSK modulation [47].
4.1.2
CubeSats in development
The second part of the answer explains the design of communication subsystems
of all the CubeSats which are presently in development stages. An elaborate review of
all these CubeSats has been performed and the extensive survey has resulted in
miniscule but very specific and important information.
JAXA Launch
The Japan Aerospace Exploration Agency is planning to launch three CubeSat
from the “kibo” module of the International Space Station (ISS) to test the capability
of the “kibo” module and also to provide more launch options for future CubeSats.
FITSAT-1, WE WISH and RAIKO are three CubeSats selected to deploy from the
module in September 2012 [49].
FITSAT-1
This CubeSat is developed by the Fukuoka Institute of Technology. This CubeSat
is a technology demonstration mission. The main objectives of the satellite are to
demonstrate the high speed transmission module for small satellites and also to test the
26
visible light communication with high power LEDS. Fitsat-1 will use a neodymium
magnet for attitude control [49].
An AX.25 transceiver will be used for telemetry and telecommand purposes in
437.445 MHZ VHF band and a CW beacon will also be provided on 437.250 MHZ
band. This would be the first satellite to transmit data at a rate of 115.2 Kbps on 5.8
GHZ band. It can send a JPEG picture within 6 seconds [50].
RAIKO
RAIKO satellite is being developed by the students at the University of
Wakayama. RAIKO is developed in collaboration with Science University of Tokyo
and Tohoku University. It is being developed as a part of “research and development
of ultra-small satellite network UNIFORM japan-led” [51]. The mission has several
objectives but the main purpose of the mission is to picture the earth by a fish eye
camera.
WE WISH
WE WISH is a non-educational satellite developed by the Meisei Electric co. ltd, a
Japan based company. This satellite is selected for the first experiment of the “kibo”
module to be allowed into the 350 km LEO orbit along with two other university
satellites. The primary mission of the CubeSat is to monitor and investigate the global
environment with a satellite [52].
This CubeSat will send IR pictures of the earth’s surface with a resolution of
320x256 pixels that will be downloaded in 110 seconds using SSTV. The downlink
will be SSTV, telemetry and Beacon in 437.505 Mhz UHF band whereas the uplink
will be in VHF band [53].
All in the information gathered from the state of art of a wide range of CubeSats
and their respective communication subsystems have been summarized briefly in the
table 4.1.
27
Cubsat
Size
Frequency
Power
Protocol
Baud
Rate/Modulation
[email protected]
1U
437.445 MHz
650mW
AX.25
1200 BPSK
Goliat
1U
437.485 MHz
1W
Propritery,
9600 GMSK
AX.25
1200 AFSK
AX.25
1200 AFSK
AX.25
9600 GMSK,
PW-SAT
1U
145.902 MHZ
2W
435.032 MHZ
UNICUBESAT
1U
437.305 MHZ
96OO FSK
XATCOBEO
1U
145.94 MHZ
AX.25
1200 FFSK
AX.25
OOK,
437.36 MHZ
MASAT-1
1U
145 MHZ
400mW
435 MHZ
OUFTI-1
1U
FSK
145 MHZ
AX.25
GMSK
AX.25
1200 AFSK
AX.25
FSK
PI
PI
PI
PI
435 MHZ
ROBUSTA
1U
437.325 MHz
FITSAT-1
1U
437.25 MHz
RAIKO
2U
PI
WE WISH
1U
437.505 MHz
800mW
PI
Table 4.1 CubeSat communication subsystems from 2011 to till date
The third part of the result presents a summary of all the CubeSats which have
been launched from 2003 until 2011 along with the design specifications of their
communication subsystems. This part also analyses all the communication subsystems
and infers the analytical results from the authors’ perspectives. The analysis of all the
designs would serve as important references for future developers of communication
subsystems.
28
Satellite
Size
Frequency
Power
Protocol
Baud rate/Modulation
AAU1 CubeSat
1U
437.475 MHz
500 mW
AX.25,
Mobitex
9600 baud GMSK
DTU sat-1
1U
437.475 MHz
400 mW
AX.25
2400 baud FSK
CanX-1
1U
437.880 MHz
500 mW
Custom
1200 baud MSK
Cute-1 (CO-55)
1U
437.470 MHz
350 mW
AX.25
1200 baud AFSK
QuakeSat-1
3U
436.675 MHz
2W
AX.25
9600 baud FSK
XI-IV (CO -57)
1U
437.490 MHz
1W
AX.25
1200 baud AFSK
XI-V (CO-58)
1U
437.345 MHz
1W
AX.25
1200 baud AFSK
NCube-2
1U
437.505 MHz
1W
AX.25
1200 baud AFSK
UWE-1
1U
437.505 MHz
1W
AX.25
1200/9600 baud AFSK
Cute-1.7 APD
2U
435.505 MHz
300 mW
AX.25
1200
GMSK
Ion
2U
435.505 MHz
2W
AX.25
1200 baud AFSK
Sacred
1U
467.870 MHz
400 mW
AX.25
1200 baud AFSK
Kutesat Pathfinder
1U
437.385 MHz
500 mW
AX.25
1200 baud AFSK
Ice Cube-1
1U
437.305 MHz
600 mW
AX.25
9600 baud FSK
Ice Cube-2
1U
437.425 MHz
600 mW
AX.25
9600 baud FSK
Rincon1
1U
437.870 MHz
400 mW
AX.25
1200 baud AFSK
SEEDS
1U
437.485 MHz
450 mW
AX.25
1200 baud AFSK
HauSat1
1U
437.465 MHz
500 mW
AX.25
1200 baud AFSK
Ncube1
1U
437.305 MHz
1W
AX.25
9600 baud GMSK
Merope
1U
145.980 MHz
500 mW
AX.25
1200 baud AFSK
AeroCube-1
1U
902-928 MHz
2W
AX.25
9600 baud GFSK
CP1
1U
436.845 MHz
500 mW
AX.25
15 baud DTMF
CP2
1U
437.425 MHz
1W
AX.25
1200 baud AFSK
Mea Huaka (Voyager)
1U
437.405 MHz
500 mW
AX.25
1200 AFSK
GeneSat-1
3U+
2.4 GHz
1W
Proprietary
15 Kbps
AFSK/
9600
29
CSTB1
1U
400.0375 MHz
1W
Proprietary
1200 baud AFSK
AeroCube-2
1U
902-920 MHz
2W
Proprietary
38.4 kbaud
CP4
1U
437.325 MHz
1W
AX.25
1200 baud FSK
Libertab-1
1U
437.405 MHz
400 mW
AX.25
1200 baud AFSK
CAPE1
1U
436.245 MHz
1W
AX.25
9600 baud FSK
CP3
1U
437.845 MHz
1W
AX.25
1200 baud FSK
MAST
3U
2.4 GHz
1W
Proprietary
15 kbps
Delfi-C3 (CO-64)
3U
435.55 MHz
200 mW
Linear
40 KHz wideband
Seeds-2 (CO-66)
1U
437.485 MHz
450 mW
AX.25
1200 baud AFSK
CanX-2
3U
2.2 GHz
500 mW
NSP
16-256 kbps BPSK
AAUSAT-II
1U
437.425 MHz
300 mW
AX.25 SRLL
1200 AFSK/9600 GMSK
Cute 1.7 APD-II
3U+
437.475 MHz
2W
AX.25
w/Pacsat
9600 baud FSK
Compass-1
1U
437.405 MHz
300 mW
AX.25
1200 baud AFSK/MSK
PREsat
3U
437.845 MHz
1W
AX.25
1200 baud FSK
NanoSail-D
3U
2.4 GHz
1W
Proprietary
15 kbps
PharmaSat
3U
2.4 GHz
1W
Proprietary
15kbps
CP6
1U
437 MHz
1W
CC1000
AX.25
1200 baud AFSK
HawkSat-I
3U
425 MHz
1W
MHX-425
NSP
1200 baud AFSK
AeroCube-3
1U
900 MHz
2W
Freewave
FHSS
Proprietary
Aggiesat-2
1U
436.25 MHz
1W
AX.25
1200 baud AFSK
ITUpSat1
1U
437.325 MHz
1W
Custom
GFSK 19.2 kbps
UWE-2
1U
437.385 MHz
500 mW
AX.25
FSK 9600 BPS
BeeSat
1U
436 MHz
500 mW
AX.25
4800 and 9600 GMSK
Hayato (K-Sat)
1U
13.275 GHz
Custom
10 kbps/1 Mbps
Waseda-Sat1
1U
437.485 MHz
1W
AX.25
9600 baud FSK
Negai
1U
427.305 MHz
1W
AX.25
1200 FSK
30
TiSat-1
1U
437.305 MHz
400 mW
Custom CW
110 WPM
StudSat
1U
437.505 MHz
450 mW
AX.25
9600 baud FSK
O/OREOS
3U
437.305 MHz
1W
AX.25
1200 FSK
RAX1
3U
437.505 MHz
2W
AX.25
9600 baud FSK
NanoSail-D2
3U
437.275 MHz
1W
AX.25
1200 baud FSK
Perseus (4)
1.5U
PI
PI
PI
PI
QbX (2)
3U
PI
PI
PI
PI
SMDC-ONE
3U
UHF
PI
PI
PI
Mayflower
3U
437.600 MHz
900 mW
AX.25
1200 AFSK
E1P
1U
437.505 MHz
1W
KISS/custom
1200 FSK
Hermes
1U
2.4 GHz
1W
MHX-2420
56.2 kbps
KySat
3U
436.790 MHz
1W
AX.25
1200 FSK
JUGNU
3U
437.275 MHz
500 mW
CW
20 WPM
DICE-1/2
1.5U
460/465 MHz
2W
PI
1.5 Mbps
M-Cubed
1U
437.485 MHz
1W
AX.25
9600 GMSK
RAX-2
3U
437.345 MHz
2W
AX.25
9600 FSK
E1P-2
1U
437.505 MHZ
850 mW
AX.25
1200 FSK
AubieSat-1
1U
437.475 MHz
708 mW
CW
20 WPM
Table 4.2 CubeSat communication subsystems from 2003-2011 [60]
Analysis of the data in tables 4.1 and 4.2 has resulted in the following points.
Frequency
Most of the CubeSats used UHF and VHF (amateur radio frequencies). The
following reasons explain why these frequencies were used –
1) Easy to get license.
2) Transceivers can be built easily with low cost.
3) Tracking is easy. Anyone with amateur radio equipment can track the satellite.
An important drawback of using amateur radio frequencies is that, the data must
be unencrypted and published. Hence, anyone can access the detailed information
31
about the satellite and its payload. Lower frequency allocations do not support high
speed.
Data rates
All the data which is transmitted use a packet protocol. Data rates for the CubeSats
studied as part of state of the art, varied between 1200 and 9600 baud rate but most of
the CubeSats used 1200 baud rate. 9600 baud rate is the maximum possible data
transmission rate available. ITUPSAT 1 is the only CubeSat to use 19200 baud rate.
For such baud rates, ground stations would receive data only from CW beacon.
Modulation
AFSK is the most common modulation scheme used because AFSK allows
operation on several digital modes which have been developed by amateurs.
4.2
Criteria for modem implementation
4.2.1
Challenges faced by CubeSat developers – Results after literature
review
From the analysis, the authors have identified eight potential challenges that are to
be considered by future CubeSat developers. The solutions to these challenges serve as
evaluation criteria for modem design and development in future.
1. Power consumption [57][17]
Challenge
Power is one of the most important and major requirements of the CubeSat.
Considering the amount of power that a CubeSat can generate, power
efficiency becomes an even more important factor. The amount of power
consumed by the communication subsystem should be as low as possible so
that the max power can used for payloads.
Solution
Using COTS components for the communication subsystems can be a better
choice since such devices consume low power thus reducing the power
consumptions of the overall subsystem.
2. Radiation [58][4]
Challenge
Space-based radiation is an important aspect to consider after a CubeSat is
launched as it has direct effects on subsystems. When electronic systems are
exposed to space based radiation like high energy ion radiation, magnetic
fields and plasma interactions, there are chances of memory corruption,
degradation or permanent damage of components/systems.
32
Solution
It would be efficient to use Radiation tolerant devices in order to achieve high
processing capabilities to minimize the damage and its consequences [9].
3. Thermal dissipation [58]
Challenge
After the launch of the CubeSat, as there is no air in the outer space for
convective cooling it can cause damage in the components of the
communication subsystem.
Solution
Replacing the electrolytic capacitors in the radio with tantalum along with
additional conductive foam around the power amplifier will help decrease in
thermal dissipation problem.
4. Fault tolerance [59]
Challenge
If there is a problem in the communication subsystem due to some unknown
reasons then the satellite may lose contact with the ground station. There
should be method that enables an alternative to compensate the loss.
Solution
Some CubeSats use two transceivers which operate in two different bands. If
there is a failure in one of the transceivers then there would be a backup
option.
5. Long beacon [60]
Challenge
By not including a long beacon, developers face severe tracking problems.
Solution
It would be easy to track a satellite by using a long beacon. Include as much
spacecraft telemetry data as possible on this beacon provides diagnostic
information about the CubeSat even if the uplink appears not to be working.
6. Flexibility [57][58][22]
Challenge
Some protocols and modulation schemes are proprietary and device specific,
requiring an identical radio at the command ground station. Moreover, most of
the satellites operate in different frequency ranges and modes.
33
Solution
This problem can be mostly solved by developing a prototype that uses
software defined radio technology.
7. Reset [60]
Challenge
In case the satellite becomes non responsive, the processor can be reset and
the space craft would be back in action.
Solution
Include a simple reset algorithm. A DTMF decoder chip which is easily
available must be attached to the radio to achieve the satellite hard reset.
8. Common modulation [60]
Challenge
Satellites tend to have different modulation schemes for beacon and telemetry.
Because of these differences, the telemetry data cannot be tracked by other
ground stations.
Solution
A common modulation scheme should be agreed upon by the CubeSat and
amateur radio communities so that all the universities and amateurs can track
any space craft easily and send the data.
4.2.2
Challenges encountered during experimentation
1. Hardware consumption
Challenge
During the experimentation phase, we developed VHDL source code for
AFSK demodulation. From the synthesis report, we could see the amount of
FPGA logic cells used by the source code. Unfortunately it used more than
available number of cells of the FPGA (figure 1 in the Appendix A shows the
FPGA usage for method 1). We used another algorithm for AFSK
demodulation which uses less logic cells (figure 2 in the Appendix A shows
the FPGA usage for method 2). Developing complex software for a CubeSat
modem results in usage of more hardware which in deed results in
performance degradation.
Solution
Software optimization should be done in order to achieve better performance
with minimum power or hardware usage.
34
2. Software integration
The developed modulation/demodulation software should be integrated with
the ADC and DAC’s in order to have a proper communication. Software needs
to integrate with other software’s on board so that it can produce reliable
communication.
4.3
Advantages and Disadvantages of nanoRTU
Evaluation of the nanoRTU requires considering the above issues and assessing
the nanoRTU in relation to them.
NanoRTU
Problems that are identified
in Literature review
Yes
Power consumption
very low , 250 mW
Radiation
The flight module is a radiation
tolerant board
No
Thermal dissipation
Fault tolerant
Error Detection, Analysis and
Correction algorithms were
implemented that makes the
device fault tolerant.
No
Long beacon
Flexibility
Because of the software
defined radio technology, it is
easy to support a wide range of
frequencies and modes
Reset
Yes
Common modulation scheme
When the common modulation
scheme is announced it can
configured with that particular
technique
Hardware consumption
Software optimization has been
done to use less data.
Software integration
No
At the moment, the digital
signal processing modules
code is not integrated with the
other modules, but it can be
achieved soon.
Table 4.3 Verification of nanoRTU with the stated problems in LR
35
5
CONCLUSION & FUTURE WORK
5.1
Conclusion
The results obtained from the literature review have been very informative and
have broadened the area of CubeSat research and the future of CubeSat development
which is evolving at a rapid pace with time and technology as the results have
provided with descriptive details of various parameters needed to design a
communication subsystems, the challenges faced and the criteria required for using
these parameters in future. The following conclusions were drawn from the entire
work carried out –
RQ1: What is the state-of-art in the current research and development of CubeSat
communication subsystems?
The results of the state of art survey proved very useful in understanding the
design specifications and development processes of almost all the CubeSats which
have been launched into orbit or are still in development. Most of the communication
subsystems used COTS components. The tables 4.1 and 4.2 provide the frequency
ranges, baud rates, modulation techniques and power used by various CubeSats.
RQ2: What are the criteria for evaluating alternative modem implementations
(software/hardware) for CubeSat spacecraft systems?
Results to research question 1 along with a thorough study of the state of the art of
current and future CubeSat communication subsystems have led to the conclusion that
there are 8 potential challenges which must be overcome by developers in future for
which appropriate solutions were suggested that could act as references for future
designs. In addition two more challenges were mentioned which evolved during the
experimentation. Suggestions were made by the authors to minimize the problems
which are discussed along with the problems in the results section there by establishing
a set of criteria for evaluating a modem for CubeSat.
RQ3: What are the respective advantages and disadvantages of the nanoRTU, in
relation to other systems and (how) can this be quantified?
In order to know the advantages and disadvantages of nanoRTU with respect to the
criteria answered in research question 2, an experiment was conducted using
nanoRTU. The table shows the effects of using nanoRTU in a communication
subsystem taking into consideration the challenges faced and criteria evaluated for
modem development.
36
5.2
Future work
The work in this thesis has established a set of challenges and their corresponding
solutions which serve as guidelines for future CubeSat developers. These challenges
have been tested by considering nanoRTU. From the literature review, it has been
clearly recognized that AFSK and BPSK are the predominant modulation schemes
used for developing a communication subsystem. AFSK and BPSK were also used
during the implementation of nanoRTU as part of the experimental setup. Future work
in this area is vast. The existing SPIADC, SPIDAC and UART entities can be
integrated with AFSK and BPSK schemes to develop future communication subsystem
modules. These modules can be evaluated and their uplink and downlink performances
can be tested. This can prove as a major research work in future considering the rapid
increase in the use of software defined radios for designing modems. Current
developers use AX.25 as the standard protocol for data transmissions. There are other
protocols like FX.25 which are available. The use of such protocols and their effects
on communication subsystems along with their ability to withstand the challenges
mentioned when implemented in a modem can form a huge ground in future CubeSat
research.
37
6
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7
APPENDIX A
Figure 7.1 Hardware consumption for the AFSK demodulator algorithm (code 1)
43
Figure 7.2 Hardware consumption for the AFSK demodulator algorithm (code 2)
44
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