UDT Europe 2002, La Spezia, Italy, 18-20 June 2002
AUV Based Mine Hunting Demonstrated from MCMV Per Espen Hagen and Nils Størkersen Norwegian Defence Research Establishment (FFI) P O Box 25, NO-2027 Kjeller, Norway
[email protected] –
[email protected] Karstein Vestgård Kongsberg Simrad AS P O Box 111, NO-3191 Horten, Norway
[email protected] Per Kartvedt Royal Norwegian Navy Mine Warfare Flotilla P O Box 24 Haakonsvern, NO-5886 Bergen
[email protected] Geir Sten Norwegian Defence Logistics Organisation/Sea P O Box 3 Haakonsvern, NO-5886 Bergen
[email protected]
Abstract The Norwegian long-term program for military use of autonomous underwater vehicles (AUVs) reached an important milestone in December 2001, when the concept of AUV based mine hunting and rapid environmental assessment (REA) was demonstrated from the Oksøy class mine hunting vessel KNM Karmøy. Within one day, the complete concept of AUV based mine hunting was demonstrated: The HUGIN AUV was programmed and launched from the MCMV, performed an acoustically controlled survey, proceeded to carry out an autonomous mine reconnaissance survey in a forward area with mine-like bottom objects, and returned to the MCMV for recovery. Through automatic mine detection and classification algorithms, a list of mine-like contacts was established, and data from these contacts was evaluated manually. The end result was that four out of four mine dummies were detected, classified, and accurately positioned on each of the demonstrations. Four months after the successful demonstrations, the Royal Norwegian Navy approved a project to develop the HUGIN Mine Reconnaissance System (HMRS), the first HUGIN system developed specifically for military applications. Current plans are to introduce the first HMRS vehicle to the Norwegian MCM forces in 2004/05.
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Introduction
The HUGIN AUV system developed by FFI and Kongsberg Simrad has enjoyed considerable success in the civilian survey market. The first generation HUGIN vehicles have been used for commercial surveys in the North Sea since 1997 by NUI AS [11]. The first HUGIN 3000 vehicle, delivered to C&C Technologies in 2000, has already logged well over 10,000 line km of operation in the Gulf of Mexico [1]. This summer, another HUGIN 3000 vehicle is being delivered to the survey company Geoconsult AS.
Fig. 1. The HUGIN 3000 vehicle and an example of data recorded by HUGIN 3000 for BP in deepwater Gulf of Mexico
From the outset, the HUGIN technology has been part of a “dual use” philosophy, developed with both commercial and military uses in mind [3]. The last year has represented a breakthrough for military applications, and full-capability military AUVs may be introduced to service within 2-3 years. 1.1 List of abbreviations and acronyms AUV COTS DVL GPS HiPAP HMRS HPR IFSAS IMU MBE MCMV MILO REA RNoN SAS SSS USBL
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Autonomous underwater vehicle Commercial off-the-shelf Doppler velocity log Global Positioning System High-Precision Acoustic Positioning HUGIN Mine Reconnaissance System Hydro-acoustic Position Reference Interferometric SAS Inertial measurement unit Multi-beam echo sounder Mine countermeasures vessel Mine-like object Rapid environmental assessment Royal Norwegian Navy Synthetic aperture sonar Side scan sonar Ultra-short baseline
Mine hunting concept demonstrations
2.1 Background The military HUGIN demonstrations concluded a three-year research and development project headed by FFI. The technology used is firmly rooted in the commercially available HUGIN AUV system, co-developed by FFI and Kongsberg Simrad over the past decade (see e.g. [12]). However, a number of enhancements were made to meet the challenges of military AUV operations. These include extensions for high-quality autonomous operation, as well as software for processing and visualisation of AUV data. 2.2 Objectives The goal of the demonstrations was to show the feasibility and technological maturity of AUVs for use in several military applications, including • Rapid environmental assessment • Military-grade seabed mapping • Deep water mine hunting • Covert mine reconnaissance Technological aspects to be demonstrated included • AUV/UUV operations with complete real-time control • Fully autonomous AUV operation in a remote area • Multi-purpose missions and in-mission AUV reconfiguration • Precise autonomous and non-autonomous navigation • Multiple payload systems • A network-centric concept of operation • Automatic detection of mine-like objects (MILOs) • Modern software tools for computer-assisted classification of MILOs 2.3 Equipment KNM Karmøy is one of four Norwegian mine hunting vessels in the Oksøy class, built by Umoe Mandal, Norway during the 1990s. The dual-hull surface effect vessel is 55 metres long, 14 metres wide, with a displacement of approximately 380 tons fully loaded.
The installation of AUV related equipment onboard KNM Karmøy was provisional and sub-optimal. The entire installation took only a few days. This included minor modifications to the vessel, the installation of a launch and recovery ramp (constructed for a completely different type of ship) on the aft deck, as well as an arm with the various acoustic positioning and communication systems. Several hundred metres of cables were drawn across the ship, for a makeshift operations room, bridge displays, and a large screen display of navigation, control and payload data in the officers’ mess. Less than 12 hours of sea time was available for system testing before the actual demonstrations took place.
Fig. 2. HUGIN I on the launch and recovery system onboard KNM Karmøy The AUV used was HUGIN I, FFI and Kongsberg Simrad’s research, development, test and demonstration vehicle. Built in 1995-96, the vehicle is upgraded on an almost continuous basis. HUGIN I is 4.8 metres long, and weighs approximately 700 kg in air. The payload sensors for the demonstrations were a Kongsberg Simrad EM3000 multi-beam echo sounder (MBE) and a dual-frequency (100/350 kHz) side scan sonar (SSS) built by Sonar Equipment Services. For accurate navigation, the AUV was equipped with a Honeywell HG9848A 1 nmi/h inertial measurement unit (IMU) aided by a 300 kHz RDI Doppler velocity log (DVL), a Leica magnetic compass, and a Digiquartz pressure sensor. Kongsberg Simrad HPR ultra-short baseline (USBL) acoustic positioning was used during the acoustically controlled operations, and combined GPS/HPR position fixes were fed to the HUGIN navigation processor every 15 seconds [9]. A battery of Lithium Ion cells powered the vehicle. A “minefield” consisting of two Manta shaped objects and two MP80 shaped objects had been established at 200 m water depth prior to the demonstrations. While this minefield is not entirely realistic – Manta mines are normally used in shallow water – it is certainly a difficult one for conventional mine hunting systems. Two of the mine dummies were equipped with acoustic beacons, and were thus accurately positioned for later reference. 2.4 Demonstrations The demonstrations were held on December 5 and 6, 2001. The program for each day was roughly as follows: 10:00 Departure from Karljohansvern naval base in Horten, transit to launch point, AUV mission set-up 11:00 Launch, supervised operation 12:00 Autonomous operation (lunch and presentations aboard the MCMV) 14:00 Rendezvous with AUV, ascent, recovery, data download and processing 15:00 Presentation of recorded data and automatically detected objects 16:00 Arrival at Karljohansvern
Fig. 3. Left: The mission plan. Right: HUGIN I being launched from KNM Karmøy The first part of the mission, performed with full operational control due to the “virtual tether” of the acoustic communication links, involved mapping of an area in a lawnmower pattern, and acoustic transmission of a subset of the data to the MCMV. To achieve stable communications, KNM Karmøy followed the trajectory of the AUV within a few hundred metres. This was facilitated by slave displays on the bridge, showing the AUV’s exact position, heading, speed and mission plan. In real time, the visitors were able to see detailed bathymetry data, side scan imagery of the seafloor, as well as important environmental parameters such as water temperature, sound velocity, and water currents.
Fig. 4. Georeferenced bathymetry data as received on the acoustic data link during the mission. Note the near-perfect match of the 1 m depth contours between survey lines For the second part of the mission, the surface vessel stopped while the AUV continued its pre-programmed mission by transiting approximately 1.5 nmi to the “minefield”, and carrying out a systematic mine search of an area of approximately 0.3 km2 (1000×300 m). Because the payload sensor suite was marginal for mine hunting purposes, the speed and altitude of the vehicle were reduced (to 3 knots and 15 m), to allow maximum detail level from the sensors at the expense of a reduced coverage rate. The rendezvous was accomplished by positioning the MCMV close to the planned return route of the AUV. Acoustic contact with the AUV was re-established before the vehicle initiated its pre-programmed ascent procedure. Upon surfacing, HUGIN activated its GPS receiver and RF link, and reported its position to the MCMV every second. After recovery, the recorded data was downloaded from hard disks on the vehicle, and processing began immediately. Within two hours, the dummy mines were detected and manually classified. The total number of “false alarms” (non-mine objects flagged as probable mines) in the list of automatic detections was below 10 both days, and the detections corresponding to the
mine objects were consistently ranked among the most likely mine candidates. Details about the automatic mine detection algorithms can be found in [6] and [10]. 2.5 Evaluation The exact same mission was performed on both days of the demonstrations. This allowed us to also demonstrate the repeatability of the mission and the collected sensor data. Fig. 5 illustrates the strong correlation of the data sets from the two missions.
Fig. 5. Comparison of sensor data between missions day 1 (left) and day 2 (right). Top: MBE response from Manta mine dummy. Middle: MBE response from non-mine bottom object. Bottom: Side scan seafloor data including reflection from a Manta mine dummy. (The trajectory shift between the two missions is due to random errors in the autonomous navigation.)
As expected, the demonstrations showed that the payload sensor suite was marginal for mine hunting. Careful programming of vehicle altitude and speed helped compensate for this, but even today’s state of the art COTS sensors would allow 2-5 times better resolution or coverage rate. The two mines for which an acoustic position reference was established, were positioned within 20 m of the reference position both days using the real-time navigation solution. The mission was later repeated from the research vessel M/S Simrad Echo with a survey quality topside navigation and positioning system (differential GPS, Kongsberg Simrad HiPAP acoustic positioning). The result was a real-time position error of approximately 1 m in non-autonomous mode, and 4-6 m in autonomous mode [6][8]. The larger errors during the demonstrations were mainly due to the lower quality of the topside navigation systems (GPS vs. differential GPS, HPR vs. HiPAP) onboard the MCMV, which meant that the AUV’s position estimate was already off by perhaps 8-10 metres when going autonomous. Even so, the accuracy obtained is adequate for re-localisation during mine destruction.
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The HUGIN Mine Reconnaissance System
Following the successful concept demonstrations, the Royal Norwegian Navy approved a three-year project to develop the HUGIN Mine Reconnaissance System (HMRS) in April 2002. The project is a collaboration between FFI, Kongsberg Simrad AS, and the RNoN. The defence funding of the project is approximately €4M. 3.1 Timeline The first phase of the HMRS project, to be completed in early 2003, will define the system requirements and the resultant vehicle and system design. The design work will be based extensively on the massive research and development effort that is embodied in the existing HUGIN AUV technology base, but demand for new research and new capabilities is foreseen in several areas. The second phase will finalise the development of the HMRS system, and is expected to be accompanied by a procurement of one or more prototype vehicles on commercial terms from Kongsberg Simrad AS. The first prototype HMRS vehicle may be delivered in late 2004 or early 2005, for operational use in Norway, in international operations as a part of MCMFORNORTH, and possibly also in other roles. In parallel with the HMRS development, the RNoN is currently in the process of permanently modifying one of the Oksøy class mine hunters for full-scale AUV operations. This includes installation of a launch and recovery system, Kongsberg Simrad HiPAP acoustic positioning system, acoustic and RF communication links, and better integration with the MICOS tactical mine warfare system. The modifications will allow rapid mobilisation of the R&D vehicle HUGIN I onboard the MCMV for e.g. REA type operations. As a result, the RNoN will be able to gain experience with AUV operations even before the delivery of the first HMRS vehicle, and thus be in a better position to give advice to the HMRS development project. 3.2 System features As the requirements and design of the HMRS components are not yet finalised, many important decisions are not yet made. However, the following list is assumed to be close to the final design: • Vehicle dimensions: 4 m long, diameter 53-65 cm • Cruising speed 4 knots, operational speed 2-5 knots • Power source: Lithium Ion batteries • Turn-around time: Less than 5 hours (limited by battery recharge) • Endurance: 12-24 hours, depending on speed and sensor use • Navigation system: Aided inertial navigation system (AINS) using a 1 nmi/h class IMU, aided by - a Doppler velocity log, - synthetic aperture sonar (SAS) micronavigation (when available), - differential GPS (at surface), - HiPAP USBL acoustic positioning (when followed by surface craft), - acoustic positioning using one or more subsea beacons with known or unknown position (when available), - a range of bathymetric and feature based navigation techniques. Many of these features are already present in the current HUGIN vehicles. The payload sensors are the limiting factor for the system delivery. Depending on the desired delivery time, the payload sensor suite may be delivered in two steps. The initial sensor suite may consist of a COTS multi-beam side scan sonar and a COTS multi-beam echo sounder, while the full capability sensor suite will comprise an interferometric synthetic aperture sonar (IFSAS) and a volume search sonar (VSS). The SAS system is currently being developed in a parallel project between FFI and Kongsberg Simrad [5]. Target capabilities are 5×5 cm resolution in both imagery and bathymetry, with a 500 m swath width. A prototype system is scheduled for sea
trials around the end of year 2002. Together with a VSS, this will allow classification quality coverage of an area of approximately 1000 m2/s, for proud, short-tethered, and long-tethered mines. The initial system will not be efficient in countering buried mines, but the plug-and-play capability of the HUGIN payload system means that other payloads may easily be fitted (limited by space, buoyancy and power) [7]. 3.3 The future After the delivery of the prototype vehicle(s), the RNoN is expected to gain operational experience over a period of 1-3 years. This will allow the Navy to make a procurement decision based on own experiences. A possible result would be a decision to equip each of the four Oksøy class mine hunters with HMRS vehicles. The delivery of these systems could then start around 2007-08.
4
Conclusions
Despite the compromises made during the provisional installation of the HUGIN system onboard the MCMV, the installation proved sufficient for reliable, accurate and comfortable operation. This demonstrates two important points: 1. The HUGIN subsystems are robust, and work well even under highly sub-optimal conditions. 2. Modifying a traditional MCMV for AUV operation does not have to be a major undertaking. Even with the performance degradation caused by a sub-optimal installation, the HUGIN system was able to achieve the goals of realistic mission scenarios. This can be taken as further proof that AUV technology is now fast approaching the technological maturity required for routine military use in a number of roles. With the development of the HUGIN Mine Reconnaissance System, the Royal Norwegian Navy will be at the very forefront in operational use of AUVs in the years to come. This shows that even a small navy can attain a leading position in such an advanced, multi-disciplinary field, provided a long-term determination and commitment is maintained.
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References
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