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Communication System for the AAUSAT-II. Kresten K. Sørensen [email protected] Department of Control Engineering Aalborg University. AAUSAT-II Mission Objectives. Education Communication (One-way and two-way) Attitude Determination & Control System (Detumbling and pointing) - PowerPoint PPT Presentation
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Aalborg UniversityCommunication system for the AAUSAT-II
Communication Systemfor the AAUSAT-II
Kresten K. Sø[email protected]
Department of Control Engineering Aalborg University
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Aalborg UniversityCommunication system for the AAUSAT-II
AAUSAT-II Mission Objectives•Education
•Communication(One-way and two-way)
•Attitude Determination & Control System(Detumbling and pointing)
•Payload (Gamma Ray Detector)•Deploy extra solar panels•Upload new software for radio amateur use
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Aalborg UniversityCommunication system for the AAUSAT-II
MECH - Mechanical structureOBC – Onboard ComputerCDH – Command Data HandlerCOM – CommunicationEPS – Electrical Power SupplyP/L – PayloadADCS – Attitude Determination and Control SystemGND – Ground stationMCC – Mission Control Center
AAUSAT-II Overview
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Aalborg UniversityCommunication system for the AAUSAT-II
COM SystemPurpose
• To create a communication path between the satellite and Earth.
Demands• Simple, robust design• Power saving• Small • Interface to: CAN, RS232, and Radio
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Aalborg UniversityCommunication system for the AAUSAT-II
Block Diagram
• Components – MCU– Radio– CAN – UART– Modem
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Aalborg UniversityCommunication system for the AAUSAT-II
Specifications for COM 1/2Radio Frequency: 437.425 MHz (Half
duplex)6 kHz
bandwidth
Data rates: 1200/2400/4800 bit/sec
FFSK/MSK-modulation
Transmitter Power: 0.5 W
Link budget: (Worst case)Downlink
margin6,2 dBUplink
margin 20,5 dB CAN bus: 125 kbps
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Aalborg UniversityCommunication system for the AAUSAT-II
Specifications for COM 2/2Mass incl. Antenna: 110gSupply voltage: 3.3 V
Power Consumption Listening: 184.8 mW Receiving:
283.8 mW Transmitting:
2085.6 mW
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Aalborg UniversityCommunication system for the AAUSAT-II
Modulation options • FSK -
Frequency Shift Keying
• MSK - Minimum Shift Keying
• GMSK - Gaussian Minimum Shift Keying
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Aalborg UniversityCommunication system for the AAUSAT-II
MSK Modulation• Advantages of MSK modem– Good noise performance compared to FSK– Easy to use
• Disadvantages for MSK modem– Synchronous interface
• Synchronization– Data must be synchronized to clock– The modem synchronize automatically on bit level – Synchronization on byte level must be done in
software on the MCU
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Aalborg UniversityCommunication system for the AAUSAT-II
Clock Divider• Adds the possibility of bit rates down to 150
bps by dividing the data clock with 1, 2, 4 or 8• Gives more than one symbol per bit and makes
the signal easier to demodulate.
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Aalborg UniversityCommunication system for the AAUSAT-II
Basic BeaconBasic Beacon format: AAUSATII <data>
• AAUSATII sent as Morse code on FM• An 800 Hz Morse tone is sent to the radio• Makes it easier to find the satellite • Morse tone created with the MCU PWM
generator and a low-pass filter
• Data is modulated by the modem• 6 bit battery voltage + 2 bit counter
After low-pass filter
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Aalborg UniversityCommunication system for the AAUSAT-II
ProtocolConnectionless AX.25 using UI frames
• Flags used for synchronization • Header for identification of packets• Checksum for verification of packet
contents• Connection orientated AX.25 is to difficult
to implement considering our demands• The connection orientation is placed on a
higher level on the OBC and GND server.
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Aalborg UniversityCommunication system for the AAUSAT-II
SoftwareSoftware tasks
• Relay data between GND and CDH• Send basic beacon • Collect housekeeping data
Interfaces• CAN bus• USART for modem and radio• UART for GND segment and debugging
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Aalborg UniversityCommunication system for the AAUSAT-II
Software StructureInterrupt module
• External communication functions
• Time critical functions
Main module• Internal
functions• Non time
critical functions
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Aalborg UniversityCommunication system for the AAUSAT-II
SynchronizationSynchronous bit stream without start and stop bits
• Hardware synchronization difficult• The AX.25 flag is used for
synchronization• Synchronization is done by the ISR
101111001001111110011111100111111001ISR sync.
CD
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Aalborg UniversityCommunication system for the AAUSAT-II
Pictures – Radio prototype
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Aalborg UniversityCommunication system for the AAUSAT-II
Pictures – Modem prototype
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Aalborg UniversityCommunication system for the AAUSAT-II
Pictures – Modem prototype