The design of the system involves interfacing smart controllers, two way communication systems and a range of sensors including those used in traditional deep water oceanography. The main component is the data logger that interfaces to the differing sensor systems, the communication systems as well as controlling the power systems. The project used the Campbell Scientific CR1000 data loggers with in-house code.
The loggers have a library of XML sensor descriptors (using simplified sensorML stubs) that they use to find and describe the sensors that are attached to the logger, this information is pushed back to a data centre so that any change in the sensors attached to the logger results in entries in the master deployment database. In this way the system is somewhat self-describing so that changes in instruments (such as during servicing) are reflected when the logger re-starts back in the data centre.
A range of sensors have been used with a focus on using standard oceanographic instrumentation so that the data collected has the same utility as that collected by other parts of IMOS, in most cases the instruments are the same and are calibrated and set up the same. The main difference is that, via the loggers, we have two way full control of the instruments either via the logger directly or via being able to communicate directly with the instrument via the logger. This allows for adaptive and smart sampling where the sampling rate and behaviour of the sensor can be controlled manually or automatically from the remote data centre. It was hoped to be able to do more of this at the buoy level but computing power is still lacking in low power remote equipment.
At a number of sites underwater loggers have been installed complete with empty ports that can be used by new sensors or sensors deployed by other projects. The support for standardised RS-232, analogue, SDI-12 or Ethernet type interfaces means that the systems are ‘plug-and-play’ for a large range of sensors. Support for Ethernet also means easy support for more complex instruments and devices such as cameras, in this mode the Ethernet connectivity is handled as a transparent link across the network making a remote device work the same as if it was directly attached to the scientist’s computer. The use of transparent Ethernet and serial links also means that proprietary software can interact with the remote sensors giving the scientist complete control over the remote system.
The typical deployment of a Wireless Sensor Network at a reef consists of a Base Station that transmits the data back to the mainland and which interfaces the back-haul network to the on-reef network. For most sites the back-haul link uses existing 3G data networks but for remote stations satellite communications are also used and for the Davies Reef link a novel over the horizon surface ducted microwave link is used.
A series of network relay poles are then deployed around the reef to create the on-reef wireless network using 850MHz Spread-Spectrum radio and 802.11 2.4 and 5.0 GHz Wi-Fi technologies. The poles form an ad-hoc network and are set up to transmit data from any device that is present in the range of the pole. Typically poles are located at two kilometre intervals in shallow sandy locations. Poles are able to support a range of sensors and typically are used to mount sensors that need to be fixed (such as light metres and weather stations) of where more power and communication capacity is required (such as cameras), for list of sensors deployed see the Heron, One Tree, Davies, Myrmidon, Rib, Orpheus and Lizard station pages.
Two designs of buoys have been deployed into the wireless network, normally these are moored but as they have a GPS unit it is possible for them to be configured as drifters. The smaller buoy is 1350mm in diameter and is called a Sensor-Float, it is suitable for shallow protected deployments. Sites that use these extensively include Heron Island, Davies Reef, Orpheus Island and Lizard Island. A larger 1750mm Multi-Float is used where the depth is greater than 12m or where waves and currents are stronger. The larger buoy is better able to remain steady in areas of stronger waves and currents and has been used at both the Rib Reef and Myrmidon Reef deployments (both now de-commissioned) where the buoy is effectively in the open ocean and not in a protected lagoon.
The base station provides the interface between the on-reef wireless network and the back-haul data link. The system uses a Campbell Scientific CR1000 logger with an Ethernet module and an appropriate modem to provide the back-haul IP network link and a Campbell Scientific RF411 spread-spectrum radio or Ubiquity 2.4 or 5.0 GHz 802.11 Wi-Fi modem to provide the on-reef wireless network. The back-haul modem can be a 3G phone system modem or it can hook into other communications links such as a microwave or satellite modem.
The base station is powered off main supply where possible running through a 120 Amp-Hour sealed battery system to give backup power if the main supply fails. The system can also run off solar power with the addition of a solar regulator and a solar panel. The CR1000 draws only 30 mA, the RF411 radio draws 75 mA when transmitting and a typical 3G modem draws around 800 mA when transmitting giving an overall power budget of less and one Amp at 12 volts or 12 watts and so a 20watt solar panel can power the system although normally 2 x 20 watt panels are used. The Wi-Fi equipment draws more power and so stations using this have larger solar panels - typically a 60 watt panel. In new installations both the Campbell radio and Wi-Fi networks are installed, with the Campbell network providing a slower back up network as well as a control network for the Wi-Fi equipment.
The aerials used depend on the deployment but the typical installation uses the RF411 radio for the on-reef network and a 3G CyberTec modem for the back-haul link using the Telstra NextG network. Both systems operate around the same frequency, 850 MHz for the NextG network and 900 MHz for the spread-spectrum radios.
Relay and Senor poles
The primary function of the network relay poles are to provide the on-reef wireless network using the 900 MHz spread-spectrum radio and 2.4/5.0 GHz Wi-Fi. The poles are 6m tall and made from galvanised steel, they are self-supporting and normally sit on a large sandy area within the reef lagoon although they have also been deployed on reef flats.
The poles provide a stable platform for the network and so are suitable platforms for attaching sensors and other equipment that needs a stable base. They are used, for example, to mount the Vaisala weather stations and above water cameras. Due to their stability high-speed (300 MB/Sec) network links can be installed between poles allowing high speed transmission of data (such as high definition video).
The top of the pole contains the instrumentation and power systems. Power is provide by solar panels (initially by 4 x 5 watt panels but now typically 1 x 20 watt panel or larger) that feed into a power regulator and then into a small sealed battery that is housed in a small Pelican case. In a similar Pelican case the CR1000 logger and radio and Wi-Fi equipment are mounted along with an optional inductive modem for connection to an inductive coupler. The instrument box has connections for an SDI-12 sensor string, RS-232 based instruments as well as a direct analogue connection to the logger to allow the system to be programmed without opening the Pelican case. A small solar powered navigation light is mounted at the top of the pole along with the spread-spectrum radio aerial, in this case an RFI CD1795 aerial and Wi-Fi aerials (often both 2.4 and 5 GHz).
Small round buoys are used to mount the main sensors being deployed. Two main types are being deployed, a larger 1.8m buoy used for deeper deployments and a smaller 1.35m buoy for shallower areas.
The buoys are moored with a dual point mooring to keep them aligned so as to maximise the solar panel performance. The moorings consist of a two railway wheels (each 250Kgs) attached to galvanise chain and then to a stainless mooring wire and then to the buoy. A bungy is put in between the bottom part of the mooring and the buoy to keep the mooring tight.
The buoys themselves are made up of two parts, the lower float and an upper cover that creates an area for the housing of the instruments. The buoy is mounted on a frame or set of ‘legs’ that ensures that in periods of low tide the buoy will sit on the bottom and then re-float as the tide rises. This is to ensure they don’t fall over or become lodged in the event of an extreme low tide.