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Fat Man underwater propulsion vehicle ENDURANCE lands at Lake Bonney, Antarctica
Variable depth underwater habitat DEPTHX testing at the Applied Research Lab at UT
Bill Stone reads a paperback while spending 24 hours underwater using the MK1 rebreather Map of Wakulla Springs created using the Digital Wall Mapper MK1 rebreather
News: Zacaton 1 Field Campaign

Simulations can only go so far. The DEPTHX SLAM development team requested some initial geometry data from the Cenote Zacaton (see aerial image below, courtesy Bev Shade) during the December 2004 design team workshop in order to verify the 3D mapping and localization algorithm against real-world sensor data. This turned into a crash program to rapidly assemble key elements of the DEPTHX vehicle core in the form of a drop sonde that could be lowered into the cenote via cable (using a winch mounted on a floating platform). Stone Aerospace produced the drop sonde in 5 months. The spectacular results are shown below. A total of seven missions were run to a maximum depth of 200 m, limited only because of the need to use a legacy 200 m-rated sonar housing due to time limitations. However, by using a sling cradle and rotating the vehicle sideways it was possible to image to a depth of -290 m (see maps below). The result is striking, and represents the first human knowledge of the geometry of the world’s deepest cenote. “Fly-Through” movies created from the 3D map are available here.

Zacaton I was not without it’s difficulties. The Drop Sonde assembly was completed on site but problems arose with onboard power supplies in the 105F sun. During disassembly to determine a fix to the power supply problem a thunderstorm lightening strike fried two of the onboard computer cards by high voltage hopping through the auxiliary power supply being used to test the sonar. It took a day to diagnose the bad boards and another day for Team DEPTHX personnel to fly in from the States with the replacement boards, leaving just 36 hours to conduct the mapping runs.

We used all 36 hours, with teams working around the clock to collect as much data as possible before departure. The drop point was varied and each mission was geo-registered.

The data from all seven sonde drop missions were merged along with four previously acquired surface LADAR scans of Zacaton. SLAM tests over the summer, using “blind” data from one of the 7 runs as the “real-time” input to the SLAM algorithm, and the remaining merged data as the “stored map” we were able to converge location based purely on SLAM to 0.5 m resolution.


The Zacaton I investigation contributed significantly to the radical design changes in the vehicle that took place over the fall of 2005.


Clockwise from top left: debugging the sonar stack following the lightening strike; locking a drop point to the surface GPS benchmark grid; graveyard shift begins drop #6 at 4am; the Drop Sonde enroute to the floating platform. A laptop computer interface allowed for offboard control and programming as well as data download post mission. Once the sonde was activated the laptop was unplugged and the connector port sealed. At that point the system began automatically collecting geometry data.

At Right: Composite, registered 3D map of Cenote Zacaton. Plan View top and North-South Profile on the bottom. Dark green points at top represent surface-based laser radar data acquired prior to the drop sonde missions by Stone Aerospace and the University of Texas at Austin; drop sonde sonar data begins with the blue color and changes to red as depth increases.

The maximum depth imaged was 290 m which, together with the LADAR data, puts the depth of the Zacaton entry shaft at 307 m. The bright yellow line at 200 m depth represents the limit of cable travel; points in orange and red resulted from turning the drop sonde sideways and allowing it to spin on the cable to paint in the next 90 m to where the sonde lost sonar returns - evidencing the existence of a large, deep, continuing unexplored void to the northwest. More than 4.5 million data points were collected during the seven drop sonde missions to build the map seen here.

During autonomous exploration the bot will store a high resolution map of what it has previously explored (post-processed in between missions and constantly updated). This figure provides a visual interpretation of what an exploration mission will appear like to DEPTHX. The high resolution map ends at 200m. Beyond that the data become more sparse since only data collected during the new exploration run are present. Location certainty will decrease as the bot proceeds downward and will improve dramatically when it returns to “known” space.