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:

1. Permanent distance of the towed system to a reference source in vertical and horizontal direction
2. Fixed depth and/or lateral shear of the towed system
3. 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)
• WETLabs "AC-S" Spectrophotometer
• TriOS "RAMSES SAM" Hyperspectrometer
• Seabird "SBE9Plus" CTD
• Simrad "EK80" scintific echo sounder
• Particle camera

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. 

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:

1. Permanent distance of the towed system to a reference source
2. Fixed depth and/or lateral shear of the towed system
3. Undulation 

These control options in connection with the system properties, of the "Triaxus" system, permit the following deployment options:

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.

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.