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ISTNanosat-1 - Digital Communications and Processing Subsystem (Heart)

Ferreira, J.A. ; Rocha, R.

ISTNanosat-1 - Digital Communications and Processing Subsystem (Heart), Proc AMSAT-UK International Space Colloquium, Guildford, London, United Kingdom, Vol. --, pp. -- - --, September, 2012.

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Abstract
The ISTNanosat-11 - Heart subsystem presents a solution to manage the remote interaction with this cubesat through the space-link. This subsystem allows a ground operator to invoke remote commands into a specific subsystem or in the satellite as general. Also, this subsys- tem provides the satellite general health status information (telemetry) periodically in the downlink.
The telemetry information is encoded directly using one subset of the AX.25 link layer protocol - Unnumbered Information (UI) frames. Since the AX.25 protocol is commonly available in the Amateur Radio community, the number of operators that can easily decode and/or forward the telemetry is certainly high. The UI frames allow stateless and connectionless communication, which is useful in long-haul delayed links.
To allow a command-response mechanism to a particular subsystem or to the cubesat in gen- eral, the Cubesat Space Protocol (CSP)2 is used. As this protocol uses variable length packets it is also encapsulated inside AX.25 - UI frames. Here, the UI frames give limited size pay- loads, synchronism preambles and FCS. These characteristics are considered an advantage when facing very disruptive links, such as space-links. To allow the CSP to be carried inside AX.25-UI, one CSP/AX.25 adaptation layer was developed. This layer handles all the details associated with the interaction between AX.25 and CSP. One of the available remote com- mands is the download of gathered images from a tiny camera aboard. This feature requires fragmentation, which is not available in the transport implementations available from the CSP protocol. To address this issue, one light transport layer (tolerant CSP - T-CSP) was developed, which allows fragment assembling and retransmission in a SNACK basis.
This entire solution was developed and is being tested on a AT91RM9200 board with ARM9 processor, 8Mb of flash memory and 32Mb of DRAM. The operating system used is a parametrized Linux kernel 2.6.38 with AT91 patch and the userland is a buildroot3 image. In order to use the AX.25 lib and AX.25 tools inside the buildroot infrastructure the meta-packages were also developed and are widely available.
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1 http://istnanosat.ist.utl.pt/
2 https://github.com/GomSpace/libcsp
3 http://buildroot.uclibc.org/