Bottom Lander for Long Term Underwater Sensing (LoTUS)
This project is aimed at developing methods and technology for cost-effective long-term autonomous systems for measurements of conditions at the seabed in, primarely in polar regions, typically covered by seasonal ice. The probes are called bottom-landers. The basic requirements are that the system needs to be low-cost, endure ≈5 years at ≤ 1000 m depth after deployment, measuring and logging e.g. temperature and being able of transmitting data home from very remote areas under severe conditions. Since data transmission from submerges conditions to land stations are practically impossible the submerged part of the system needs to surface to establish satellite communication. The trade of between time-submerged and need for data transmitted to shore will be especially addressed by aiming at system capability of several consecutive dive cycles without need of manned-manipulation. The basic concept is a system that stays submerged for periods of months/years before surfacing for satellite communication with land based stations. Measured data is uploaded and new “mission control values” are downloaded. Possibly, another dive-cycle is then initiated with duration of months/years. This reconfiguration in combination with low system cost will enable measurements including spatial distributed grid of measurement in contrast to single point-measurements as the alternative with more expensive systems.
The LoTUS system is a small, lightweight, expendable, bottom landing, moored, multiyear (0-10 years) sensor platform tailored for collection of time series of CTD-data in polar regions. The LoTUS buoys are stand-alone, pop-up data collection and communication platforms. The physical components consist of the positive buoyant buoy, a mooring line and a dead weight anchor. The LoTUS is lightweight, (≈3 kg) and small (≈diameter 250mm) to enable deployment from any boat, helicopter or even aircraft.
The moored buoy deep-hibernates between real-time clock scheduled data collection when sensors are read and data is stored internally on EEPROM. At pre-set calendar time intervals the buoy activates the anchor-release consisting of electrolytically accelerated corrosion of a stainless steel coil. When surfaced, the Short Burst Data (SBD) Iridium protocol is used to transfer data to the land based host computer system. Adaptive routines for power saving re-hibernation is used for handling e.g. the situation of surfacing buoy being entrapped under ice, for hours, weeks or months. Once data is securely transmitted the LoTUS buoy reconfigures to serve as a drifter transmitting its GPS-location at pre-set intervals, for as long as the batteries allows. All of the parametrized pre-deployment programming and calibration of the buoy is done via the SBD. An SBD operated pre-operation calibration mode enables the exact adjustment of each individual buoy prior to the deployment.
The buoy hull structure consists of an all polystyrene spherical shell designed for depths of down to 2000 m. The electronics comply industrial standards and is designed for harsh environment with full functionality in the range from -40 degrees Celsius up to 85 degrees Celcius. The expected life length is well beyond 20 years (limited by power storage) with expected MTBF over 10^6 hours. Components have been selected with durability as a major factor, e.g. without electrolytic capacitors, with exception using an ultra-capacitor instead of a battery for the real time clock. Data storage is done on an EEPROM with a data retention of over 100 years.