Unmanned Systems Technology 015 | Martin UAV V-Bat | William Sachiti | Sonar Systems | USVs | Desert Aircraft DA150 EFI | SeaCat AUV/ROV | Gimbals

39 Sonar systems | Focus format. It allows AUVs to announce the presence of a device to reduce conflicts and enable a group of underwater devices – AUVs, submarines, divers and so on – to organise themselves into a network. The protocol itself is open source, and there are many different implementations. It has been evaluated by collaborating partners at centre frequencies from 900 Hz to 60 kHz, and over distances up to 28 km. The physical layer coding scheme is known as frequency-hopped binary frequency shift keying, which was selected for its robustness in the harsh underwater acoustic environment. A baseline Janus packet consists of 64 bits of information, which includes a 34-bit application data block that is defined according to one of 64 possible schemes. There are 256 user classes, allocated to countries, specific organisations and special purposes. The order in which the 13 pairs of signals, or tones, are used to encode the binary data is chosen to provide the most robust link, avoiding problems with multi-path signal reflections or collision with Janus packets from other users. This sequence is fixed and therefore known to all potential receivers. The data block has a range of defined elements that are used to identify the country of origin and even how the craft is powered, whether by wave, wind or solar power. Much of the Janus development work is carried out on the Littoral Ocean Observatory Network in the harbour of La Spezia in Italy. One of its advantages is that it can make military and civilian, NATO and non-NATO underwater devices interoperable, providing them all with a common language with which to communicate and arrange to cooperate. This is especially important for AUVs from multiple navies that may all be hunting mines with a range of AUV or USV craft. Many systems are looking at combining USVs with towfish sonar to identify mines and then signal to a fleet of AUVs to approach and identify the mines with multi-beam sonar and magnetic sensors, and disable them. Janus has been tested extensively at sea in exercises involving universities, industrial partners and research institutes. For example, the Portuguese Navy has been working on new ways to exchange data with AUVs so that they don’t have to surface to provide a traditional radio link. The NATO standard specifies the initial Janus band at 11.520 kHz. That should provide good range performance up to 6 km, say developers, but with a low data rate of 80 bit/s using acoustic signals. However, acoustic comms systems with frequencies of less than 20 kHz or more than 60 kHz are generally subject to export licence restrictions. Given that many offshore applications now take place in water depths below 3000 m, a centre frequency slightly above 20 kHz would give the optimum data rate performance at an acceptable maximum range, while remaining licence-free. A packet would take about 700 ms to be transmitted, and a typical 64-bit data cargo packet could therefore be transmitted over a 110 bit/s link. There have been other standards for underwater comms. The Subsea Wireless Group (SWiG) developed a draft standard called SWIGradio that was based on wirelessHART, a wireless protocol used in factory automation. The draft has been submitted to the American Petroleum Institute Standards Subcommittee 17 on Subsea Production Equipment. Unmanned Systems Technology | August/September 2017 The advantage of using a synthetic aperture sonar system over a single-beam system, showing the wider swathe of measurement for a seabed survey (Courtesy of Kraken Sonar) Janus is the first international digital underwater comms protocol, and it can be used to contact underwater devices using a common format