RESEARCH :: A study on Deep Space Networks (DSN)
Written by: Kiran Kankipati
When you are dealing Deep Space Networks (DSN) one among the most challenging parts is the Interplanetary distances and communicating data across such vast distances. This is where we are not dealing with common Internet type traffic such as HTTP/FTP/VoIP/etc but it is completely different when it comes to DSN so far. So optimizing data in DSN becomes mandatory. For example if you think one of the Mars Rovers, they have used LZO lossless compression. Although they do to an extent lossy compression on images shot by these space-probes at times they we may also need high-resolution detailed high-quality images. And sometimes it is not just photos sent back to the earth, at times the space probes may also report their health status, keep alive messages as well transmit the scientific research data such as data recorded in various sensors situated on-board.
Although we got space probes across the space and ISS (International Space Station) orbiting over Earth, we do not have a scenario yet something like human colonies/bases on Moon or Mars and other planets. Eventually when such things happen in around 2020-2030 or so as the way NASA and scientists predict, DSN is going to be a case where more private companies may offer their solutions. But before that we need to still solve some of the fundamental data communication challenges involved in DSN. This is on of the fields which I am actively involved since a decade.
Unlike here on Earth upgrading a piece of hardware or communication technology is just impossible to do on a space probe which may exist millions
of miles away from Earth. This also makes this technology evolve quite slowly unlike Earth bound communication technologies such as Mobile
communications, Satellite networks and so on. For further complete coverage of this topic kindly refer my below detailed video titled
Deep Space Communication - Episode1.
Understanding Communication Speeds: Most DSN networks are radio-wave signal based and not light (photonic) based communication. Radio waves do not travel at the speed of light. It is also one of the reason for the slow-down of the DSN unlike ground or earth bound fibre optic links since in this case data travels almost (since the medium is not vacuum and speed of light depends on the medium) at the speed of light. Before we imagine network speeds in DSN, let us understand an ideal situation of speed of light between two points in space:
NOTE: Since we compute network speeds often in bits/sec (and latency in nano-seconds and milli-seconds), in the above chart I am converting everything in seconds to understand the scale.
So based on the above chart now we can understand the scale of complexity in DSN. This underscores a fundamental limitation of physics !
Communication Protocols for DSN: For DSN a complete new set of protocols are defined which is SCP (stands for Space Communications Protocol). There are various RFCs which are defined which is called as SCPS (where S stands for Specifications). There are various variants under SCPS are defined such as SCPS-FP, SCPS-TP, SCPS-SP and SCPS-NP. The biggest difference you may find in DSN is that the delay involved due to inter-planetary distances. So based on the distance you may experience communication delays, loss of packets, etc. Say for example if you think a successful connection is established (for example a TCP session/connection), you may have to-and-fro keep alive acknowledgement packets exchanged every few milliseconds. But whereas in a case of DSN you may experience this happening every few minutes or every few hours. So that is how bizarre it is. Although there is no packet exchanges happening in few minutes or hours you should understand this is due to vast distances involved.
These SCPS specifications are defined by a committee called as
(stands for Consultative Committee for Space Data Systems). This is a body which is formed as per collaborative effort of various space agencies
across the world. An Internet spanning across multiple planets is termed as
(stands for Interplanetary Network or in short InterPlanet). For further complete coverage of this topic kindly refer my below detailed video
titled Deep Space Communication - Episode2.
Lossless Compression Algorithms for DSN: A specific set of tailor made algorithms are required for space communications unlike the ones which are used in communications here on Earth. They have to be light-weight and at the same time super-efficient and should have least processing latencies. The communication data could be just anything such as scientific research data collected via space probe sensors or it could be hi-resolution photos sent back to earth or it could be commands sent to these probes via ground control crew. I have done extensive research on this for almost more than a decade on various lossless compression algorithms. This is a case where we are dealing optimizing real-time data. This is not a passive file compression something like creating a tar-ball or some zipfile. This is a case you are sending and receiving packets continuously and you are processing them in real-time.
NASA have their own lossless compression variants and often they are customized. One of the well known algorithms which NASA uses is the LOCO-I (stands for Low Complexity Lossless Compression) which is mainly meant for compressing images. LOCO-I is a kind of lossless compression variant of JPEG. Which is why it is also can be sometimes called as JPEG-LS (stands for JPEG-Lossless). Based on LOCO-I NASA did hardware based solution which is FPGA-LOCO. Since it is hardware based, it is good in performance, reliability and extremely energy efficient.
Apart from this CCSDS have their own variant of RICE lossless compression algorithm. For further complete coverage of this topic kindly refer
my below detailed video titled Space Lossless Compression.
Thank you, Kiran