47 lines
2.8 KiB
Markdown
47 lines
2.8 KiB
Markdown
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# Communication with sat-rs based software
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Communication is a huge topic for satellites. These systems are usually not (directly) connected
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to the internet and only have 1-2 communication links during nominal operation. However, most
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satellites have internet access during development cycle. There are various standards provided by
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CCSDS and ECSS which can be useful to determine how to communicate with the satellite and the
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primary On-Board Software.
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# Application layer
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Current communication with satellite systems is usually packet based. For example, the CCSDS space
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packet standard only specifies a 6 byte header with at least 1 byte payload. The PUS packet
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standard is a subset of the space packet standard which adds some fields and a 16 bit CRC, but
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it is still centered around small packets. `sat-rs` provides support for these ECSS and CCSDS
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standards to also attempts to fill the gap to the internet protocol by providing the following
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components.
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1. UDP TMTC Server. UDP is already packet based which makes it an excellent fit for exchanging
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space packets.
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2. TCP TMTC Server. This is a stream based protocol, so the server uses the COBS framing protocol
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to always deliver complete packets.
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# Working with telemetry and telecommands (TMTC)
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The commands sent to a space system are commonly called telecommands (TC) while the data received
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from it are called telemetry (TM). Keeping in mind the previous section, the concept of a TC source
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and a TM sink can be applied to most satellites. The TM sink is the one entity where all generated
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telemetry arrives in real-time. The most important task of the TM sink usually is to send all
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arriving telemetry to the ground segment of a satellite mission immediately. Another important
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task might be to store all arriving telemetry persistently. This is especially important for
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space systems which do not have permanent contact like low-earth-orbit (LEO) satellites.
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The most important task of a TC source is to deliver the telecommands to the correct recipients.
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For modern component oriented software using message passing, this usually includes staged
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demultiplexing components to determine where a command needs to be sent.
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# Low-level protocols and the bridge to the communcation subsystem
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Many satellite systems usually use the lower levels of the OSI layer in addition to the application
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layer covered by the PUS standard or the CCSDS space packets standard. This oftentimes requires
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special hardware like dedicated FPGAs to handle forward error correction fast enough. `sat-rs`
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might provide components to handle standard like the Unified Space Data Link Standard (USLP) in
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software but most of the time the handling of communication is performed through custom
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software and hardware. Still, connecting this custom software and hardware can mostly be done
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by using the concept of TC sources and TM sinks mentioned previously.
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