Projects:2014S1-56 Inter-Satellite Links for CubeSats
QB50 is a project consisting of 50 international double and tripe CubeSats. The CubeSats will have various scientific instruments on board to carry out atmospheric research within the lower thermosphere, between 200 - 380km altitude, which is the least explored layer of the atmosphere. There will be 3 sets of scientific equipment onboard that will be used for the research. Another purpose of the project is to study the atmospheric re-entry process.
Our project aim is to design and implement intersatellite linking between satellites. In other terms this is passing information from one satellite to another until it hits the destination satellite which is in contact with the targeted ground station.
Since a CubeSat is only 30x10x10 cm satellite there are constraints that need to be considered such as:
- Wireless space links having high bit error rates
- Low power budget
- String of pearls constellation
The project will be using the MSP430 microcontroller range from Texas Instruments due to there low power outputs that will allow the low power budget to be achievable. The radio frequency chip that will be used is also made by Texas Instruments called the CC1120. This was chosen since it was easy to write software for the microcontroller and RF chip to communicate with each other. Also the RF chip has packet handling capabilities to combat the effect of high bit error rates.
The approach that was taken throughout the project was looking at a current complete internet model, the TCP/IP model, and applying this to a satellite network. The model splits the transmission into 4 different layers the transport layer, internet layer, data link layer and the physical (wireless) layer.
This layer is responsible for determining if the data has minimal errors, duplicate data is discarded and lost packets are resent.
This layer is responsible for determining the final destination satellite and how the information will be routed to this destination.
This layer is responsible for applying the final encapsulation of the packet from the higher level layers and putting them into frames that will be sent over the network. This layer also completes any error detection/correction.
This layer is what medium the information will be transmitted over. In the case of our project the physical layer is wireless radio communications.
The packet structure that is transmitted is split into 2 separate sections, the header field and the data field. The header field will contain several signalling bytes that will be used to determine where and how the packet will need to go. The data field will will contain the data that is being transmitted.
The header field be 6 bytes and will contain:
- Source: The source of the packet
- Destination: The final destination of the packet
- Next Hop: The next hop in the route of the packet
- Packet ID: To see whether the packet is a duplicate
- Type: To distinguish between routing, data and acknowledgement packets
- Time to live: A counter on the amount of hops the packet has taken ensures the packet doesn't infinitely route through the network
The work has been broken down so that both are able to work on the project concurrently whilst in the end being able to integrate there software.
- Design and Implementation of transport layer
- Random multiple access
- Packet handling
- Design and Implementation of internet layer
- Routing table population
- Choose shortest jump path
- Design and Implementation of wireless link
- Create packet
- Transmission of packets
- Receiving of packets
Currently the system is able to communicate on a two terminal link. The features that are applied:
- Duplicate packets are handled
- Timed out packets are handled
- Reliable packet transmission (data with acknowledgements)
The internet layer has been designed for but still needs to be implemented to ensure that the linking can take place
As noted above in the conclusion the project wasn’t completed and therefore there is plenty of room for future work that will be able to be completed. These include:
- The power consumption constraint that was put upon us at the beginning of the project wasn’t applied at any stage of the project. The hardware that was chosen is capable of low power states so this is a definite outcome can could be achieved.
- Completing the internet layer is essential future work on the project. At the moment the project only allows for two terminal communications and it needs to exhibit routing behavior. This routing behavior has been designed for it will just need to be implemented.
- A more efficient method of automatic repeat requesting could be implemented. Other automatic repeat requests that could be implemented are go-back-n or selective repeat.
- A more efficient method of multiple access such as time division multiple access could be implemented. This will increase the efficiency of the data that is moved through the network since there will be set times that the CubeSats will know when to transmit and receive.
- Mr Robert Bulfon
- Mr Troy Maylin
- Associate Professor Michael Liebelt
- Associate Professor William Cowley
- Texas Instruments MSP430F2410
- Texas Instruments MSP430G2553
- Texas Instruments CC1120 V-Chip
- Code Composer Studio
- UART to USB serial interface
- DC power supply