Sending/receiving data on Earth from any spacecraft is a difficult task, mainly because of the large distances involved. Delay and data loss are common when communicating across thousands and millions of miles. To make these delays and losses as minimum as possible, NASA is working on a reliable solar system internet connection, called Delay/Disruption Tolerant Networking (DTN).
NASA uses 3 communication networks for transmitting and receiving signals across large distances that contain space relay satellites and distributed ground stations to support space missions.
- Near Earth Network (NEN)
- Space Network (SN)
- Deep Space Network (DSN)
What Exactly is DTN?
NASA’s previous mission used single relay or point-to-point links (like the phone system) to communicate with low-Earth orbit as well ass deep space spacecraft. However, the future exploration concepts include much more complex system having multiple nodes (instead of just two). It will operate like Internet networks on Earth, which include several hops via satellite and other intermediate nodes, building the foundation for the SSI (short for Solar System Internet).
Like Earth-internet, the SSI will provide a standard platform upon which a wide range of applications will work on end-to-end network services. The SSI will use DTN protocol suite in almost all cases, including those with frequent link or longer light times disruptions, where traditional IPs (Internet Protocols) don’t work.
DTN is the specific set of rules for transmitting data, often called a protocol suite, that extends the capabilities of the conventional internet to work in space environment across extreme distances. These environments are usually subject to frequent disruption, high error rates, long delays and single direction links.
Solar System Internet using DTN Protocol: Image Source – NASA
How DTN Works?
The DTN protocols can operate with the Earth-internet IPs or work independently. It uses automatic store-and-forward technique, thus guarantees the deliver of data. If packet forwarding is not currently possible, the system stores it for future transmission. Therefore, only the next hop requires to be available when using Disruption Tolerant Networking.
Like conventional internet suite, DTN involves network management, routing, security and quality-of-service features. Although it’s designed for space applications, it could also be beneficial for terrestrial apps where high error rates and frequent disruptions are common.
The image above shows the data being transmitted from space to Earth via Space habitat and 2 communication relays, using DTN. The communication links between assets are not always available.
Currently, the DTN protocols are being developed by the NASA AES (Advanced Exploration Systems), and they are supporting the DTN standardization by the IETF (Internet Engineering Task Force) and CCSDS (Committee for Space Data Systems). All these DTN protocols will be open international standards. A few DTN implementations already exist and are publicly available, such as ION (Interplanetary Overlay Network) implementation.
Enhanced Operations and Situational Awareness – The DTN offers more insight into events when communication outages occur due to ground station handover, poor atmospheric conditions or as result of relay. DTN significantly decrease the need of scheduling ground station to receive or send information, which typically takes up to 5 days of planning.
Space Link Efficiency and Robustness – DTN provides efficient and more reliable data transmission, enabling more usable bandwidth. The link reliability is improved by multiple network paths and assets for transmission hops.
Security – DTN protocols allow for authentication, integrity checks and encryption on all links.
Interoperability and Reuse – DTN protocols enable interoperability of spacecraft (operated by any government or private space agency) and ground stations. Moreover, it allows NASA to use the same protocols for upcoming missions, whether it’s a low-Earth orbit or deep space mission.
Quality of Service – DTN protocol enable several priority levels to set for different data types, in order to make sure that the important packet is sent ahead of less important one.
The first DTN experiment was conducted on 10 July, 2009, which involved downloading a particular set of images over a planned TDRSS (Tracking & Data Relay Satellite System) handover. During this experiment, ground-to-space and space-to-ground links were interrupted for a few minutes. This DTN-on-ISS network demonstration was successful.
In the next test, they used DTN for unattended operations. The test was conducted for 3 days, and during this period 14 files were generated every hour. The conventional transmission resulted in 3,504 redundant receptions per file (on average), while DTN performed astonishingly better and resulted in only 0.06 redundant receptions per file.
The ISS implemented an institutional DTN service in May 2016, which improved the reliability of payload science data transmission and reduced operational overhead and planning.
On 20 November 2017, a selfie taken at the National Science Foundation’s McMurdo Station, Antarctica was sent to ISS using DTN protocol suite. As you can see in the selfie, NASA engineers Mark Sinkiat, Peter Fetterer and Salem El nimri held a photo of Vint Cerf, who helped develop the technology.
The DTN software on a smartphone sent the selfie on its journey to the ISS. The packets travel from the McMurdo ground station to White Sands Complex, NASA via TDRS (Tracking & Data Relay Satellite). Then, a set of DTN nodes forwarded the packets to Marshall Space Flight Center in Alabama, which is the access point of DTN network. The packets were forwarded to the space station via another TDRS link where they routed to the TRek (Telescience Resource Kit) demonstration payload. The last DTN node extracted the image data from packets, and payload reassembled the original photo and displayed it onboard the ISS.
These are some recent experiments on DTN. For now the AES DTN research team is working with SCaN (Space Communications and Navigation) and IPNSIG (InterPlanetary Networking Special Interest Group) to help make the SSI a reality.