Triaxus towing body on deck of RV Polarstern. (Photo: Jan Gerken, Alfred-Wegener-Institut)
The topAWI Triaxus (towed ocean profiler of the AWI) is a towed system manufactured by MacArtney A/S that serves as a device carrier for various sensors measuring parameters relevant to physical, chemical, and biological oceanography. It performs a saw-tooth flight pattern between a few meters below the surface and up to 350m depth at a vertical speed of 1m/s and it can be towed between 2 and 8 knots (1-4m/s) speed through water.
Depending on the exact parameters chosen, this results in a horizontal resolution of 3km or much better, suitable for the synoptic study of mesoscale and submesocale features. Alternatively, it can stay at a constant water depth or a constant distance from the surface (e.g. underside of sea ice) or bottom. AWI has developed a “depressor” that allows operation in medium-severity sea ice conditions. It can also be deflected horizontally to fly outside of the ship's wake. The Triaxus acts as a pass-through for the sensors such that each sensor is controlled by a separate laptop on the ship.
The following list shows the sensors currently integrated in the topAWI, but the integration of additional sensors is possible/ongoing:
Seabird "SBE9Plus" CTD with dual temperature, conductivity, and oxygen sensors as well as chlorophyll fluorescence, transmissivity, PAR, and pH
Seabird "Deep SUNA" nitrate sensor
2 Teledyne 1200kHz "Workhorse II" ADCPs (upward- and downward-looking)
WETLabs "AC-S" Spectrophotometer
TriOS "RAMSES SAM" Hyperspectrometer
Simrad "EK80" scientific echo sounder
Particle camera
At the beginning of 2020, AWI has developed a so-called "depressor," enabling operations in regions with light to moderate sea ice cover. It was used for the first time during the Polarstern PS131 expedition and allowed the collection of valuable data in the marginal ice zone north of Svalbard.
Further below, you can find additional information about the system. The intranet page (accessible only to AWI employees) can be found here.
The topAWI (towed ocean profiler of the AWI) steerable towing system, is a cable-towed system that serves as a device carrier for various physical, biological and chemical-oceanographic measuring instruments. The System is able to record oceanographic profile data while the ship is in motion between 2 and 8 knots (STW), up to maximum diving depth of 350 meter and a sidewise steering up to 100 meter port and starboard of the ship. In the area of these parameters, the towed system can be controlled by software in three different modes:
Permanent distance of the towed system to a reference source in vertical and horizontal direction
Fixed depth and/or lateral shear of the towed system
Undulation in the vertical and/or horizontal direction
Due to the system parameters and the possibility of control, the system enables sensors to be used on a new scale, for example:
Outside the influence of the ship
At a fixed distance under the ice
In a "profiling" mode with a distance between consecutive profiles of a few kilometers.
The following set of sensor is connected to the interfaces of the towed sensor carrier:
Seabird "Deep SUNA" nitrate sensor
Teledyne "Workhorse II" ADCP (upward and downward)
The topAWI system consists of the steerable towed system "Triaxus" from the company MacArtney in the extended Version "Modell E" and an exchangeable set of sensors and sensor equipment. The Triaxus system itself has no scientific sensors and only provides various interfaces for sensors. The sensors can be viewed completely separately from the Triaxus system and therefore allow universal replacement. The only limitation is the provided interfaces and system properties of the Triaxus system, like seen under "Sensors". The system and sensor properties together result in the possible uses of the system, as described under "Deployment options".
Components
The different components of the Triaxus towed system (Photo: Alfred-Wegener-Institut)
The "Triaxus" system consists of five components, as also seen in the diagram above:
"Control PC" and Software "ROTV" for computer based control of the system
"Deck Unit" consisting of "Topside Unit" and "Control console" to communicate, power and control the system
"Winch" to spool in and out the cable and to tow the system
"Fibre optic towing cable" to connect the system with the deck unit and pull the towed system
Steerable towing system "Triaxus" for carrying the sensors.
Next to the parts of the "Triaxus" system there are also various devices for evaluating the sensor data, called "Sensor 1" to "Sensor 12" in the diagram. In addition to that and not in this diagram, various communication and power connections from the ship and a 2 ton crane at the aft of the ship are required to use the system.
The system is able to work between 2 to 8 knots (STW) with a diving depth up to 350 m at a vertical speed up to 1 m/s and a simultaneous sidewise steering up to 100 m port and starboard of the ship. Depeding on the diving depth of the system and the speed of the ship, the winch can pay out up to 2500 m of cable with 60 m/min spooling speed. The winch is of the type "Mermac S20" and can be controlled directly, via remote controller or by PC in manual mode or fully automatic mode. For communication and power supply a 10 mm towing cable, with two conductors and two redundant glass fibers, is used. Due to the cable connection, the operating time is not technically limited and only a question of available measurement time and personnel. Basic equipment of the system is an Attitude sensor, a Depth sensor and an up- and downward looking Altimeter for seabed and under ice operation. As an emergency backup, the sensor carrier "Triaxus" without a connected "Fibre optic towing cable" has always a slight positive buoyancy. Therefore, the cable connection on the Triaxus is also equipped with a predetermined breaking point. As a security, if the underwater sensor carrier comes up to the surface without any connection, a "Xeos" "XMB-11K" radio beacon is installed for easier location.
The entire system is connected to power and data connections as shown in the overview above. Next to system's internal connections, the following connections from the ship are required to use the system: 400 VAC / 32A power supply for the winch, 230 VAC alternating current for the top side units and data acquisition computers, and data about the ship's speed and position provided through the NMEA protocol. An optional external input is an "Ultra Short Baseline" (USBL) system for determining the position of the Triaxus system and at the same time as a horizontal reference for controlling the system. As standard, a GAPS transponder ("MT912S - HD") with an external trigger input (see diagramm: "NEXUS MK IV", "Data Channel" number "5") is mounted on the Triaxus. Other USBL systems are not included in the system, but are possible.
The "Triaxus" system provides nine standard serial communication interfaces as well as three ethernet interfaces at 12V, 24V and 48 V power supply. This allows the use of standard sensors and is only limited by the system properties of the "Triaxus" sensor carrier itself. The sensors can be mounted in free areas inside of the sensor carrier, as well as onto the extra built-in sensor holders. A position at other points on the sensor carrier is still possible, but is associated with more planning.
CTD Sensors: starboard and port side of the "Triaxus" sensor carrier (Photo: Alfred-Wegener-Institut)
The specialty of the CTD measurement with the Triaxus system is that both the upcast and the downcast can be used for data acquisition. Often only the upcast or downcast can be used in a "normal" CTD-rosette measurement. This is because, due to the structure of the "normal" CTD-rosette, the sensors in the upcast or downcast are in the swirl caused by the measuring unit and the sensor data are therefore compromised. In the sensor carrier "Triaxus" the CTD Sensors are mounted in front area of the system, as seen in the pictures below, and therefore exposed to a permanent water flow without any swirling of system parts.
The "Triaxus" software "ROTV" allows three different types of vertical and horizontal control of the underwater sensor carrier:
Permanent distance of the towed system to a reference source
Fixed depth and/or lateral shear of the towed system
Undulation
These control options in connection with the system properties, of the "Triaxus" system, permit the following deployment options:
Profile measuring in vertical "Undulation" mode
Undulation mode (Photo: Alfred-Wegener-Institut)
Triaxus data example based on PS113 024_02 "equator crossing" (Photo: Alfred-Wegener-Institut)
With the setting "Undulation" in the ROTV software, successive vertical profiles up to a water depth of 300 meters can be measured, see picture "Triaxus Undulation using the example of PS113...". Both the upcast and the downcast can be used to determine the data, described in chapter "Specilty of CTD data" for the CTD as an example. The Triaxus profiles are not purely vertical profiles, rather the falling or rising flank of the wave-shaped movement of the system, see picture "Enlargement of triaxus undulation over period" below. The vertical profiles can be measured with a vertical speed up to 1 m / s, at a ship speed between 2 - 8 knots (STW). The distance between two of this profiles varies between 0.3 - 12.4 km, depending on the measuring depth, vertical speed and ship speed, as seen below under "Profile measurement distance". This type of profile measurement allows a high horizontal profile resolution with low ship time requirements, see picture. After the towed body has been deployed, the ship can continue travel with a speed up to 8 kn, depending on the horizontal resolution, and only has to reduce the speed again to recover the system.
Profile measurement distance
Theoretical horizontal profile resolution at a depth of 300 meters (Photo: Alfred-Wegener-Institut)
The profile measurement distance or horizontal resolution of the topAWI system mainly depends on the measurement speed of the sensors and the shape of the undulation profile. Since the measuring speed of the sensors is often a fixed variable and depends heavily on the respective scientific questions, the distance between to measurement profiles can only be changed by the shape of the undulation profile. The undulation profile is set in the control software of the topAWI system by the measuring depth, vertical speed and the ship speed. At a maximum measuring depth of 300 meters, the vertical speed can be between 0.1 - 1 m / s at 2 - 8 knots ship speed (SOG). This results in distance between two measurement profiles of about 0.3 - 12.4 km.
