In recent years, the number and variety of autonomous instruments studying the world’s oceans has increased dramatically.
Virtually all of these systems are able to deliver increased functionality and scientific benefit if they are able to communicate with scientists and engineers who are on shore.
Often the communication is in one direction only, from the instrument back to land, but increasingly two-way communication is used allowing an operator to send commands or new programs to the remote instrument.
Satellites are the key technology that allows instruments to communicate from anywhere on the ocean surface. In particular the Iridium and ARGOS satellite systems require only small, low-power transmitters and wide-angle antennas that make them suited to battery-powered instruments bobbing around on the sea surface.
Satellite communication systems
The current telemetry technologies used cover a wide range of telecommunications platforms from the standard dial-up modem connection through to high-speed broadband satellite communication systems. In our ever-changing world, the need for up-to-date, real-time data has never been more important. For this reason, satellite communication has become the preferred technology of choice, providing data from remote regions of the world with improved bandwidth and speed.
The following information provides further details on the current satellite communications platforms currently in use.
Is based on a message-type system (email) and communicates through a constellation of low Earth orbit (LEO) Microstar satellites. The user generates messages using a unit called a Subscriber Communicator (SC), which communicates directly with the satellite network to ensure successful delivery of the message. The SC formats the data in to packages that are added to the body of the email message or are sent as additional attachments. These messages are delivered as per a normal email, into the user’s inbox using standard Internet protocols. The advantages of this system include low power, cost effective, near real time if Ground Station is in view.
Data Collection Platform (DCP)
Is used primarily for Metrological and Environmental data. The user is allocated a timeslot within a specific frequency channel in which to transmit their data. Global Positioning System (GPS) timing is used to ensure that the user transmits during their allocated timeslot only, as there is no interaction between the DCP transmitter and the orbiting satellite. The transmitted data are then made available to the end user to download via the Global Telecommunication System (GTS).
Can be used to make standard voice calls as well as the sending of data. There are a number of methods for transmitting data via Iridium, these include standard dial up, Point to Point Protocol (PPP), Short Burst Data (SBD) and SMS. Each of the data methods have their advantages and disadvantages ranging from cost, throughput and power.
Broadband Global Area Network (BGAN)
BGAN is a type of broadband technology using the latest satellite communications. This system has the advantage of providing an ‘always on’ Internet connection with bandwidth speeds of up to 0.5Mbits from terminal no bigger than a small laptop. This technology is continuing to evolve with new services being added for applications that require less power, lower data rates, smaller data payloads.
Some of the scientific instruments used to collect data, such as ocean bottom seismometers, are permanently underwater. Radio transmissions only travel a very short distance through salt water and so a different technology is required. There are some underwater observatories that provide direct cable connections for underwater instruments but these systems are hugely expensive and usually close to shore.
The most widely used alternative is to use underwater loudspeakers and microphones to communicate by sending sound signals, in the same way that whales, dolphins and other marine creatures communicate underwater. Sound travels at 1,500ms-1 through seawater and under the right conditions can be heard tens of kilometres away.
Instruments located on land or research ships can sometimes be connected directly to the Internet, either by cable or through a satellite system such as Inmarsat BGAN. This is particularly useful for instruments such as cameras that produce large amounts of data.
Communication, intelligent sensor data management and Web-enabling remote observation in a constrained environment have long been a challenge due to unreliable connection and limited communication bandwidth. Often, tasks are separated between constrained on-site nodes and more resource-abundant laboratory environments on dry land.
Traditionally, observations are sampled and brought back regularly by telecommunication though satellite and on-site visits to collect the physical memory storage and carry out sensor replacement and calibrations. These are inevitably needed as the measurements drift due to bio-fouling and other reasons.
Calibration history is recorded to allow easy management for future re-use. Datasets of the observations are then archived by data managers with various metadata and descriptions attached so that domain scientists can investigate a particular environmental phenomenon, such as a plankton bloom and its relationship with temperature and salinity.
When external communication is not available, intelligent on-site nodes operate autonomously for data acquisition using latest command set. Synchronised Web communication enables observational data and command synchronisation when communication becomes available. Preparing of commands, post processing of observations and sensor data management happen on dry land where resources are more abundant. Serial communication and Extensible Markup Language(XML) are used for data acquisition and transformation. Database management is used to allow scalable and reliable observational data storage with post-processing capability.