Design and Deployment of a 3D Autonomous Subterranean Submarine Exploration Vehicle

DESIGN AND DEPLOYMENT OF A 3D AUTONOMOUS SUBTERRANEAN SUBMARINE EXPLORATION VEHICLE William C. Stone, Stone Aerospace / PSC, Inc. 3511 Caldwell Lane, Del Valle, TX 78617, Ph: (512) 247-6385, [email protected] ABSTRACT likely include the following components: The NASA Deep Phreatic Thermal Explorer • the parent spacecraft, which will remain in orbit (DEPTHX) project is developing a fully autonomous either about Jupiter or about Europa and which will underwater vehicle intended as a prototype of primarily serve as a data relay back to Earth from the the Europa lander third stage that will search for Lander. microbial life beneath the ice cap of that Jovian • the Lander, which will be a 3-stage device: moon. DEPTHX has two principal objectives: First, to develop and test in an appropriate environment Stage 1: the physical landing system that will contain the ability for an un-tethered robot to explore into propulsion systems, power, and data relay systems to unknown 3D territory, to make a map of what it sees, communicate with the orbiter, and which will control and to use that map to return home; and second, to and carry out the descent and automated landing on demonstrate that science autonomy behaviors can the moon. identify likely zones for the existence of microbial life, to command an autonomous maneuvering Stage 2: the “cryobot” second stage, which will melt platform to move to those locations, conduct localized a hole through up to ten kilometers of ice cap before searches, and to autonomously collect microbial reaching the sub-surface liquid ocean. Although the life in an aqueous environment. The concept and design is far from completely defi ned, the concept will prototypes are being tested in an unusual terrestrial most likely involve a nuclear powered thermal melter analog that presents many of the likely morphologic that will be attached to the bottom of the third stage. It regimes where life may exist on Europa: the 300- is highly likely that the second stage will also deploy meter-deep (or more) hydrothermal cenote of Zacatón, an armored fi ber optic bundle behind it as it melts Mexico, which contains diverse microbial mats, but deeper. The fi ber bundle will be frozen into the ice remains uncharted, both spatially and biologically. after the melter passes through with the third stage and In this presentation we summarize the fi nal vehicle will provide the communications link to the surface architecture and control systems approach to lander; autonomous exploration in fully 3D environments Stage 3: by far the most sophisticated element of in which apriori knowledge of the environment is the entire mission will be the lander third stage. In non-extant and for which there exists no external fact, the lander third stage will likely be a multi- navigation system. The latest fi eld work at Cenote stage device itself, roughly along the following lines: La Pilita will be presented describing the February 5, the melter tip from the second stage -- really just 2007 mission during which DEPTHX became the fi rst a shield and a sophisticated heat exchanger -- will fully autonomous cave exploring robot. likely be jettisoned upon reaching the subsurface ocean. The remaining vehicle will detach from the 1. Overview fi ber optic system and will leave behind a powerful communications and navigation beacon as well as a DEPTHX is a prototype AUV (autonomous rechargeable power pack. At that point what remains underwater vehicle) for developing and testing two is the lander third stage proper: a novel, fast-moving of the most critical capabilities that will be needed cylindrical-shaped AUV that will be propelled by by the Europa lander third stage. These two advances the nuclear power plant . The “fast mover” will carry are essential to the success of the Europa mission. To a collection of redundant, electrically powered self- review quickly, the Europa mission architecture will mobile smart sub-payloads that are sensor-rich devices capable of not only mapping into unknown territory 2. DEPTHX Vehicle Design and bringing the fast mover back to the navigation beacon at the bottom of the melt hole, but also able The vehicle design morphed dramatically during the to “sniff” for indicators of environmental variables course of the project and it will be worthwhile here that might indicate the presence of microbial life and to summarize how we arrived at the fi nal design. steer the combined 3rd stage vehicle towards those Initially, the thinking had been to canabalize a com- sites. At that point one of the smart sub-payloads mercial ROV chassis and add science and navigation will be deployed and will “fl y” off in pursuit of instrumentation to that platform, mainly to cut costs microorganisms. To fi nd them it will have to employ and speed schedule. This initial design [Stone et al, a multi-stage (hierarchical) detection system that 2005 UUST05] involved the use of clustered banks of improves the odds of fi nding something at each stage. sonar transducers with three clusters of 8 serving as Ultimately, using these tools, and if successful, it will forward, port, and starboard obstacle avoidance arrays detect and characterize microorganisms (most likely and a single lateral-looking barrel sensor that was to Bacteria or Archaea) in the Europan ocean and return be used for map building. A commercial ROV is not to the fast mover, which will then drive it back to the a very hydrodynamic device and an exterior “hydro- navigation beacon whereupon it will upload what it shell” was envisioned to cover the active components fi nds to the surface lander, which will then relay the to both reduce drag and to minimize the likelihood of info back to the orbiter, which will then relay it back a snagged vehicle during actual exploration in a lay- to Earth. bryinthine overhead environment of the type that was anticipated at the Cenote Zacaton test site as well as in DEPTHX, then, is a prototype for the smart sub- the ice cracks in the surface crust of Europa. Figure payload of the 3rd stage fast mover. 1 shows the “potato”, a second-generation concept The purpose of DEPTHX was to test the following in which the sonar clusters are un-grouped and the two hypotheses: discrete transducers are mounted on the surface of the hydroshell, with shielded co-ax cables carrying 1) can we design a fully autonomous underwater the signal to a common digital signal processor array. vehicle that can explore into completely unknown This design began to approach the idea of a fl exibly- territory, create maps of that unknown territory in deployed low profi le sensor system in which the bot three dimensions, and use those just-created maps could approach near 4π steradian viewing. The op- to return itself “home” without any other navigation aids? 2) can we design a fully autonomous science system that will seek out places where there is a high likelihood of microbial life and make a decision to collect samples that will demonstrate, unambiguously, that science autonomy is feasible on Europa. Specifi cally, can we demonstrate that by tracking environmental sub-aqueous variable gradients (temperature, chemical concentrations etc), along with spectral (color) differentiation, we can improve the likelihood of machine detection of microbial life. These two capabilities form the fundamental underpinning of the success of the Europa mission, and thus became the focus of the DEPTHX fi eld missions, the results of which will be described below. Figure 1: The “potato” hydroshell. Gray disks represent discrete sonar elements. erative word is “near”. In the potato design there still was actually going to work on an exploration mission. had to be a propulsion system and this occupied the The Drop Sonde developed for the May 2005 expedi- stern of the craft, as in traditional ROV design, since tion [Stone et al, 2005 UUST05] utilized the original in fact we were still thinking of an ROV chassis on the barrel scanner concept for map building. The data inside of the potato shell. coming in from Zacaton, however, began to evidence weaknesses in the line of sight limits for a purely pla- This remained the design through the spring of 2005 nar array in attempting to provide real-time maps in as hardware and code preparations for the May 2005 3D of the vehicle environment. A number of potential fi eld campaign to Zacaton dominated work on the array concepts - spring boarding off the breakthrough project. The express purpose of that expedition was potato skin idea - were sketched out at Zacaton. All to obtain early real (noisey) data from Zacaton in a of the sketches had the common concept of providing format that would allow testing of the crucial SLAM true 4π steradian viewing (for the moment we left out algorithm in the context of a vehicle-created 3D point the issue of propulsion). Later simulation in the con- cloud from the actual test site. While at Zacaton there text of the SLAM algorithm [see Fairfi eld et al 2005, were a number of discussions about how the system 2006, 2007] in representative 3D labryinthine environments favored one design that came to be called the “three Great Circle” concept. On paper there were three orthogonal, intersecting planes and one could envision the intersection of those three planes lying at the center of a sphere. The lines on the sphere penetrated by the planes produced the “Great Circles” and it was on those lines that the discrete sonar elements would be placed. In this design the concept of “mapping” and “obstacle avoidance” sonar became blurred and all sensors could be piped to both code segments through a publish-subscribe data system.

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