Improving the stealth of undersea robots and submarines

The Global Positioning System (GPS) is the predominant means of obtaining positioning, navigation, and timing (PNT) information for both military and civilian systems and applications. However, the radio frequency basis for GPS also means that its signals cannot penetrate seawater, and thus undersea GPS is effectively denied. Undersea vehicles use inertial measurement units (IMU) and other dead reckoning sensors to navigate while submerged. Performance of inertial sensors for dead reckoning continues to improve and come in smaller packages. However, while dead reckoning sensors may provide sole-source navigation for short duration missions, accumulation of inertial error eventually requires external measurements to maintain or restore accurate performance. As a result, undersea vehicles must regularly surface to receive GPS signals and fix their position, and this presents a risk of detection. In addition, the cost and power draw of state-of-the art undersea navigation sensors, such as a navigation grade IMU and Doppler velocity log (DVL), are significant.

To combat these vulnerabilities and limitations, DARPA is soliciting proposals for the Positioning System for Deep Ocean Navigation (POSYDON) program to develop an undersea system that provides omnipresent, robust positioning. DARPA envisions that the POSYDON program will distribute a small number of acoustic sources, analogous to GPS satellites, around an ocean basin. By measuring the absolute range to multiple source signals, an undersea platform can obtain continuous, accurate positioning without surfacing for a GPS fix. POSYDON seeks to eliminate the need for expensive IMU and velocity sensors. Host platforms of interest include autonomous underwater vehicles (AUVs) of all types, although DARPA also encourages solutions that can be leveraged by other undersea platforms and systems. POSYDON will not require the platform to surface to obtain and maintain its PNT information, and the platform will not need to make any transmissions, thus maintaining minimum platform detectability

The POSYDON program seeks to develop an undersea, ocean basin-scale, PNT system that may be used in lieu of surfacing to obtain a GPS fix. Solutions should be relevant for the deep ocean and continental shelves, and describe both the solution’s degree of applicability to littoral regions and a technical pathway for use in the littorals. As stated previously, platforms of interest include AUVs of all types, although DARPA also encourages solutions that can be leveraged by other undersea platforms and systems.

DARPA is funding the development of a small number of acoustic transmitters that can be anchored to fixed locations around ocean basins to serve as an undersea navigation constellation. By measuring its range to multiple signals emanating from known coordinates, an undersea vehicle can operate continuously with accurate navigation information without needing to surface for GPS fixes or to use high cost inertial systems that are typical of current UUVs. DARPA awarded a contract on March 15, 2016 to a Draper-led team to begin development of a solution for the Positioning System for Deep Ocean Navigation (POSYDON).

By measuring its position relative to multiple signals emanating from known coordinates, a UUV can operate continuously with accurate navigation information without needing to surface for GPS positioning or use inertial systems.

“Stealth is a critical capability for counter-anti-access/area denial (C-A2AD) missions,” said Joel Parry, Draper’s maritime warfare and intelligence, surveillance and reconnaissance (ISR) lead. “Giving UUVs GPS-like accuracy anywhere in the ocean, without the need for surfacing or active signal emissions, will significantly boost the chances of success of the types of missions that UUVs take on today, and will enable them to conduct missions that would be considered too risky today because of the chance of detection.”

Draper’s work on POSYDON begins by developing high-fidelity models of how the positioning signals will travel through the ocean as well as developing the signal waveforms. DARPA’s current plans for the program call for at-sea demonstration of the system in 2018.

Draper’s work on this program leverages its proven technology used by U.S. government customers to enable systems such as guided munitions and highly maneuverable aircraft to maintain their lock on satellite signals, as well as Draper solutions that enable users to acquire satellite signals indoors and in other areas where a conventional receiver would struggle.

The engineering solution used on POSYDON could also be used for civilian applications, such as underwater surveys, helping avert delays while the vehicle surfaces and errors that can occur between GPS fixes, Parry said.

The Draper-led team includes the Massachusetts Institute of Technology (MIT); the Scripps Institute of Oceanography at the University of California, San Diego; Intific Inc.; Riptide Autonomous Solutions; and Hydroacoustics Inc.