Johns Hopkins University says an engineering team from its Applied Physics Laboratory (APL) has proven that a free-space optical (FSO) communications system can operate in maritime applications. The APL team demonstrated a compact system that enabled communications between two moving ships at the annual 2017 Trident Warrior Exercise, an event where sailors trial naval warfare innovations and provide feedback to commanders and developers.
Using wireless optical transmission to transmit signals at high bit rates, FSO communication systems are seen as an adjunct communications capability to conventional radio frequency (RF) and microwave communications.
"We demonstrated bandwidths that were several orders of magnitude higher than all current radio frequency [RF] communications capability on Navy vessels, and at longer ranges than previously demonstrated FSO technology for maritime applications," said Juan Juarez, technical lead for the developing technology team, via John Hopkins' writeup of the event. "This is the equivalent to having up to 2,000 users simultaneously watching high-definition video streams across the optical link."
APL is the first organization to successfully operate an optical communications capability of up to 10 Gbps between two moving ships, and in near-shore environments, according to Juarez. He stated that existing commercial FSO systems are not sufficient for meeting defense needs, as FSO demonstration systems built for terrestrial defense applications have lacked mobility, data rates, or ranges, or have not been small enough for naval platforms.
Based on the first week of testing, the APL FSO system demonstrated the capability to resolve several of these issues, APL says. The testing was ship-to-shore, from the motor vessel (M/V) Merlin off the coast of Naval Base Point Loma, San Diego, to the 3rd Fleet headquarters parking lot. The team achieved more than 14 hours of link-up time during 4- to 6-foot high seas, including:
- 1–2 gigabits of error-free data transport at ranges greater than 25 km.
- Voice communications at greater than 35 km
- Chat messaging out to 45 km, the maximum available line of sight.
- Repeatable, semiautomatic reacquisitions over the entire line-of-sight range.
The second set of hardware was installed during the second week of testing onboard the Sea Hunter, an autonomous continuous trail unmanned vessel (ACTUV) developed by the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research. A six-person Space and Naval Warfare Command crew and the APL test team were on board for this demonstration.
With ACTUV Sea Hunter going 24 knots and M/V Merlin going 12 knots in a "V" formation to enable the two ships to separate from one another, while maintaining the links at changing speeds and motions, multiple links were made in 3- to 5-foot swells, over 10 km in range between the ships.
"Weather conditions during the two weeks of testing were typical of San Diego's ‘June Gloom' and gave the APL team plenty of opportunities to show that our FSO technology can operate even through some levels of fog and haze," Juarez said. "While the fog layer was present, links of over 10 kilometers were achieved, even though visibility at times was reduced to 2–3 kilometers."
APL's compact, mobile FSO demonstration system is the result of over a decade of APL testing in the field of free-space optics.
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