POCKET Packet Compression
Introducing innovative technologies
in support of mission operations
The Case Study – Selecting ...
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POCKET Compression: How to get ten times more information in your real-time data stream with minimal changes

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A problem with most compression techniques is that they require a certain amount of data to be stored before the block can be compressed efficiently. This introduces latency onto the link and makes them unsuitable for real-time control where typically small packets are generated and send to the user as fast as possible. POCKET is a unique method that can compress individual packets so fast that this latency is not noticeable. In fact it some cases it even decreases the link latency as the time saved in transmitting a smaller packet more than compensates for the time taken to compress it!
POCKET has been tested using real spacecraft housekeeping data and was found to compress significantly better than ZIP and significantly faster. Average compression ratios of 3 to 20 have been achieved depending the mission.
The result is a simple method that simply compresses packets as they are generated and outputs smaller packets. These can then be decompressed on the other end before being injected into a legacy control system. Hence the system impact is minimal.
POCKET has been prototyped and validated on on-board hardware (LEON2 processor) as part of the PROBA-3 mission. An end to end system is concurrently being bread boarded as part of an ARTES 5.1 study for use on telecommunications satellites. A TRP study will start soon on using it for remote control of robots. The ESA Patent Group has decided to protect POCKET by filing a patent application in the European Patent Office.

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POCKET Compression: How to get ten times more information in your real-time data stream with minimal changes

  1. 1. POCKET Packet Compression Introducing innovative technologies in support of mission operations The Case Study – Selecting which on- board measurements should be transmitted to the ground and at which sampling rate is a difficult and time consuming engineering job. This is because the overall bandwidth is limited and one never knows what data will be needed in the future. Sample too frequently and you run out of bandwidth, sample too little and you might miss something important. The Solution – POCKET compression allows high sampling on-board for many parameters by compressing standard housekeeping packets. Unlike must standard compression algorithms (e.g. ZIP) this can work on real-time data and stored data. This means that single packets are compressed into smaller single packets as they are generated. As it works on single packets it can be applied only when needed introducing much more flexibility into this chain and so reducing preparation and operations costs. Current Status – The POCKET compression has been implemented according to ECSS on-board software standards, prototyped and demonstrated in a end to end system, with real flight hardware and a real mission control system. Project Team – ESOC Advanced Mission Operations Concept Office. ESA/ESOC, Darmstadt, Germany Advanced Mission Operations Concept Office Contact: David Evans E-mail: David.Evans@esa.int Tel: +49 6151 90 2720 One way of estimating the information content of a message is to compress it. The information content of housekeeping telemetry packets is quite low as when they are stored in files they compress extremely well. Compressing packets and sending more of them in the same bandwidth would improve the information content of the data stream. This would bring benefits in terms of better reaction times and better spacecraft observability as well as reducing the upfront engineering effort and operations costs. Most methods proposed to compress housekeeping data only work on data that has been stored on-board, usually in files. This requires major changes in on-board architecture. It also means that engineering work when designing packets/downlink schemes for packets that have to be sent in real-time still has to be done. This reduces the potential impact. POCKET is a method for compressing packets in real-time i.e. each individual packet is compressed into an equivalent smaller packet as soon as it is generated. The smaller packet can then be sent in real- time or stored for later transmission. This opens up the possibility of increasing the information content of both the playback and real-time telemetry streams with a single, unique process at generation time. Note that no major changes in on-board architecture are required to implement it. POCKET is a software plug-in that is added just after each packet is generated. The input and output are packets and so all normal operation concepts can be applied. Another software plug-in is added just before the mission control system which intercepts the compressed packets and expands them before injection. The disturbance is minimal. In 2014 an ARTES 5.1 study was completed by Spacebel SA who designed and implemented the algorithm and the associated PUS library to control it. This was tested in an end to end system with a real SCOS based control system. The results showed that telecom satellite data was compressing by a factor of 8 while the CPU usage remained under 0.15% even under adverse conditions. The on-board software was then slightly modified to exploit this extra bandwidth to increase memory dumping speed by a factor of 9, procedure execution rate by a factor of 20 or decrease the probability of frame loss from 60% to close to zero when the satellite was in safe mode. Compression performance is not the only important factor in such a system. It must be incredibly fast because it must not introduce any notable latency on the real-time link. The tests demonstrated that only a few microseconds are needed to compress a typical packet. These tests have also shown that the compression and speed performance are very stable over time and in different scenarios. POCKET has been selected to fly on the ESA missions PROBA-3 and OPS-SAT. It is also being considered for the TIA mission ELECTRA. Finally it has been spun out for use in an UAV control system.

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