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This photo shows an Argo float being deployed into the ocean, although they are not normally thrown off the side of a ship as shown here. The usual method is to lower them gently into the ocean in a cardboard box to protect them hitting the side of the ship. The box is in a sling with a quick release on the bottom. When the box hits the water, a starch tablet in the bottom dissolves and the biodegradable box floats away, releasing the float. Argo: drones of the deep oceans By Dr DAVID MADDISON Right now, thousands of drones are floating deep in the oceans of the world, monitoring temperatures and other data. They are fully autonomous and they can change their buoyancy to sink deeper or rise to the surface to send data to satellites. M OST PEOPLE know about drone aircraft and their many types and capabilities but did you know that there are thousands of drones in the deep seas? Over 3600 such drones are quietly floating at around 1000 metres deep in the oceans of the world, monitoring temperatures, salinity and other parameters. Not only that, they also regularly descend to 2000 metres, then slowly float up to the surface, taking measurements as they go and then they beam their collected data to satellites. After transmitting their data they submerge again, endlessly repeating the cycle, unseen and autonomous for 14  Silicon Chip many years, until they reach the end of their lives due to misadventure or battery failure. This is Argo, an international project involving 30 countries including Australia. It consists of thousands of freeranging ocean floats that monitor the temperature, salinity, currents and other parameters of the ocean. Data from the floats is used in the study of oceanography and climatology. Important data The data obtained from Argo is important because it is acquired rapidly in near real-time and can assist in short and long-term weather forecasting, monitoring of long-term trends in the ocean, monitoring of ocean currents and for other weather, climate and oceanographic research. Until recent times, ocean temperature and other measurements have been made by research ships or commercial or military ships participating in the Voluntary Observing Ship scheme. But such measurements are limited in scope and follow the main shipping routes. In addition, because of the greater volume of shipping in the Northern Hemisphere, there was far more data from there than from the siliconchip.com.au Southern Hemisphere, where there also happens to be a greater volume of ocean. How does it work? So how do the Argo floats sink to 2000 metres deep or rise to the surface? They do it by controlling their buoyancy. Fig.1 shows a cross-section of a typical Argo float; they are essentially a cylinder which is more than 1.1 metres long and they float vertically. At depth, the buoyancy is controlled by an external hydraulic bladder at the bottom. To make it rise, a geared motor drives a rod which pushes down a piston in a cylinder filled with hydraulic fluid (oil). The hydraulic fluid inflates the bladder and the float then displaces more water, increasing its buoyancy and up it goes. To reverse the process, the motor retracts the piston and the fluid from the bladder is forced back into the siliconchip.com.au cylinder, reducing the buoyancy and accordingly, the Argo float sinks. The process is quite precise as the pressure is monitored by a sensor adjacent to the bladder. As we shall see, the water depth is directly proportional to the pressure, and vice versa. Extra buoyancy is required when the float reaches the surface to ensure that the antenna is clear of the water. This is provided by a pneumatic bladder which can be inflated by another pump. A typical float weighs 20-30kg. Sensors at the top of the float monitor temperature, salinity and other par­ameters, depending on the particular model of Argo float. An antenna at the top of the unit sends the data to a satellite. That broadly describes an Argo float but there are many variations, as described later in this article. Argo was conceived in 1999 when international organisations met to discuss creating a more coordinated approach to the gathering and distribution of oceanographic data. Following this meeting, a group of scientists developed a plan to have a 3000-float array in place by 2007 and this objective was achieved, the first floats having been deployed in late 1999. The figure of 3000 floats was arrived at by a requirement for each float to sample a roughly 3° x 3° latitude by longitude area between 60° north and 60° south. Higher latitudes were initially excluded because of the problem of the floats becoming entangled with sea ice and polar ice-sheets. There is now a program to deploy polar floats which will be discussed later. In 2009, suggestions for further improvements to the array were made such as providing extra coverage in certain areas and adding chemical and biological sensors to the floats. By November 2012, the one millionth “profile” (data set) of temperature and salinity had been gathered which represented twice as much data as had been collected by research vessels over the entire 20th century. At the time of this one millionth profile, 120,000 profiles were being collected every year or about one every four minutes, each profile consisting of up to 1000 temperature and salinity measurements. The information that can be gained from the study of Argo data includes: •  Measurement of ocean temperature over a range of depths. Fig.1: cross-section diagram of typical Argo float. Note the pneumatic bladder in this model. This is inflated near the surface to ensure the float rides high enough so that the satellite antenna is clear of the water. •  Measurement of ocean salinity over a range of depths that can reveal where the ocean has become less salty due to rainfall or river outflows and more salty due to evaporation or by the flow of ocean currents with various levels of salinity. This leads to insights into the hydrological cycle. •  Measurement of ocean circulation and temperature characteristics which lead to phenomena such as El Niño (an abnormal band of warm water of greater than 0.5°C above average that periodically develops off the coast of South America causing adverse weather events in Australia and many other countries); the Pacific Decadal Oscillation – sea surface temperature anomalies which affect climate in western North America, Siberia, India and Australia; and other similar phenomena. •  Accurate mapping of ocean circulation. •  Seasonal variations in the ocean and long-term variations. July 2014  15 Figs.2&3: these two plots show data from a float deployed off Western Australia, in the Leeuwin Current that runs south along the WA coastline. Fig.2 at left shows salinity versus depth, while Fig.3 at right shows temperature versus depth down to 500 metres. The x-axis of each is time in years while the y-axis is depth, in metres. The legend at bottom shows the correspondence between colour and either salinity (in parts per thousand) or temperature (in °C). Of special interest to some researchers is the heat content of the oceans. A 3-metre column of ocean water contains as much thermal energy as the entire height of the atmosphere of the same column diameter. Knowing the temperature and other parameters of the ocean and how heat is exchanged between the ocean and the atmosphere is important for understanding the climate system. A typical Argo mission A typical Argo float mission is 10 days. It involves sinking from the surface to a depth of about 1000 metres and parking at that location for around nine and a half days while it takes temperature, salinity, pressure (equivalent to depth) and other measurements the float is equipped to take; see Fig.8. A depth of around 1000 metres is typically chosen as it is usually a region with minimal current and the float will not drift away too far from its desired location. Following the parking period, the float drops to a depth of about 2000 metres and then proceeds to rise to the surface over a period of eight hours during which it takes further temperature, salinity, pressure and possibly other measurements along the approximately 2000-metre water column, depending on which sensors the float is equipped with. Pressure equals depth Note that in oceanography, water pressure, measured in decibars (dbar or db), is used as a measure of depth (in metres). One bar roughly equals one atmosphere and a decibar is roughly Fig.4: this is the path of an Argo float revealing the Antarctic Circumpolar Current. At the time of this image, the float had been deployed for six years, reporting a 2000 metre profile every 10 days while drifting at a depth of 1000 metres between reports. 16  Silicon Chip 0.1 atmospheres. The pressure in deci­ bars is for most practical purposes the same as the depth in metres, so that an increase in depth of one metre equates to increase in pressure of one decibar; 100 decibars is 100 metres. While pressure in the ocean would comprise the depth of water plus the atmosphere, the relatively small contribution of the atmosphere is ignored so at the surface, the pressure is considered to be 0 decibars. The precise conversion formula between decibars and metres of depth in the ocean can be found in a panel later in this article. Argo floats can phone home Argo floats communicate by one of two methods. Older floats typically communicate to the Argos satellite which is a general-purpose environmental data receiving satellite, not specifically associated with the Argo program despite the similar name. Newer floats use the Iridium satellite phone network. Essentially, they make a phone call to the relevant Argo data centre. Older floats which communicate with the Argos satellite have to sit on the surface for 12-26 hours in order to transmit their 78 data points to the satellite. They can only store one profile at a time. These long surface times mean that wind and surface currents can move the floats away from their intended location and they can even wash up on shore. Another risk of long surface times is that they will be spotted by fishermen and picked up when they should be left alone. This is a major reason for Argo floats, particularly in the tropics, siliconchip.com.au Fig.5: this is a general model of oceanic circulation, also known as thermohaline circulation or the “Global Conveyor Belt”. It’s driven by differences in water temperature and salinity which affect the density of seawater. In general, warm shallow water cools and sinks in the North Atlantic and deep cold water returns to the surface in the Indian and Pacific Oceans where it again warms. Argo can help monitor these currents, measuring temperature and salinity, and determine if any changes take place. sometimes ending up in remote fishing villages in the middle of nowhere! Since the older floats don’t have GPS, their location is determined by calculations involving Doppler shift of the radio signal. Newer floats which communicate via the Iridium network only require a surface time of around 15 minutes and can store up to 1000 data points per profile and 60 profiles. Their location is determined by GPS. One might wonder if the floats constitute a shipping hazard but there have been no incidents. Their time at the surface is relatively short and since they are generally far away from shore they are not likely to be hit by small speedboats. In any case, there is vastly more natural and man-made debris floating in the ocean, much of it larger than the floats. Australia is a big player The USA has the largest number of Argo floats while Australia has the second largest, representing about 11 percent of the total number (see Fig.6). Argo in Australia is operated by CSIRO Marine and Atmospheric Research in Hobart, with support from the Bureau of Meteorology, IMOS (Australia’s Integrated Marine Observing System), the Antarctic Climate and Ecosystem siliconchip.com.au   Jason & The Argonauts The name Argo derives from Greek mythology and is the name of the vessel in which Jason and the Argonauts went looking for the Golden Fleece. Argo also works in a complementary manner with the NASA Jason satellites to measure sea levels. Jason provides extremely accurate measurements of the sea level (to a few centimetres with complimentary gravity data from the NASA GRACE mission), while Argo provides measurement of salinity and temperature. This gives the contribution of water density (derived from temperature and salinity) to sea level which helps both validate Jason satellite data and also helps determine the contribution of sea level due to changes in the density of the water as opposed to extra water mass being added to (or removed from) the oceans such as that due to melting (or formation) of land-based ice. Cooperative Research Centre, the Royal Australian Navy and the Department of Climate Change and Energy Efficiency. Worldwide, the Argo program is sponsored by the World Climate Research Programme’s Climate Variability and Predictability project (CLIVAR) and by the Global Ocean Data Assimilation Experiment (GODAE). It is a pilot project of the Global Ocean Observing System (GOOS). There are about six major manufacturers of floats plus some minor ones. The Argo program does not specify the exact design of each float but does specify required performance data such as accuracy, type of sensors and float and battery life. Since the exact specifications are not defined it allows manufacturers to come up with better, more efficient and more capable designs and also allows float costs to be reduced. A typical float costs around $21,400 although the total deployed cost including the cost of the float, a ship to deploy the float and staff is around $35,000. Argo floats used in Australia are disassembled and undergo a thorough check before deployment and older models had their alkaline battery packs replaced with lithium ones. As a result of these pre-deployment checks, July 2014  17 Fig.6: this diagram shows the global distribution of floats and country of origin. The US has the highest number of floats (2000) while Australia has the second highest with 386. France has the third highest number of floats, with 256. Note that these are representative locations for a certain point in time only as the floats do drift around. Argo floats in the Australian fleet have very good longevity. The lifetime of older floats was manufacturer-rated at 3.5-4.5 years Fig.7: Argo data in its most basic form, showing a plot of temperature and salinity versus pressure (depth in metres) for a given position in the ocean. 18  Silicon Chip but due to the battery upgrade they have lasted up to 10 years. Currently deployed floats have a typical lifetime of 7-8 years because of a more complex mission profile and more measurements being taken, resulting in a reduced battery life. This lifetime refers to time out in the ocean before the batteries go flat as the floats are not usually retrieved. When the battery fails, the float is usually unable to rise from its approximate 2000 metre depth as there is insufficient battery capacity to reinflate the buoyancy bladder. There it will remain indefinitely, never to be retrieved. Note that while failed Argo floats are not usually retrieved due to the difficulty and expense of doing so (which would exceed the value of the float), if it comes to the attention of the Argo organisation that one has come ashore in an inhabited area or has actually fallen into someone’s possession, it is important that it is recovered. This is because the large lithium-ion battery pack could be a safety hazard in the wrong hands, especially if treated inappropriately. In addition, if traces of water have  Making An Argo Globe You can make your own world globe in the form of an icosahedron showing the location of Argo drifters for a given day. The image can be found at: http:// www-sci.pac.dfo-mpo.gc.ca/data/ projects/argo/images/icosa.tif and assembly instructions can be found at: http://www.pac.dfo-mpo.gc.ca/ science/oceans/Argo/documents/ Argo_icos.pdf entered the float, there could be an explosive and toxic mix of hydrogen, oxygen and chlorine gas, due to electrolysis of seawater by the battery. Sensor accuracy Since large amounts of scientific data are derived from the floats and that data is further incorporated into climate, oceanographic and other models, it is extremely important that the float sensor data be as accurate as possible. Temperature accuracy is ±0.002°C, salinity is within 0.02 parts per thousand and pressure is within 2.4 decibars. This is a very high level of accuracy siliconchip.com.au so scientists can have great confidence in the results. In regards to the missing Malaysian Airways flight MH370, the floats obviously have no capability to directly locate the wreckage. However, data from the floats feeds into and is the major contributor to the ocean current models that were used to track and predict the possible location of the crash debris. The most energy consuming process in the floats is changing the buoyancy to make the float rise or fall. Forcing hydraulic fluid into an external bladder at a depth equivalent to 2000 decibars, or around 1975 metres (ie, where Argo descends), requires a significant amount of energy. At that depth the pressure is around 200 atmospheres (20 megapascals or 2900 psi). Note that the exact depth where 2000 decibars occurs varies slightly according to the latitude. It equates to 1971.7 metres at ±60° degrees latitude, 1976.1 metres at ±35° degrees latitude and 1979.55 metres at the equator. Accessing the data Anyone, including SILICON CHIP readers, can access the Argo data for free and make their own discoveries. A website at http://wo.jcommops.org/ cgi-bin/WebObjects/Argo has gateways to the two global data centres and also other information. The US Global Data Center (the other is French) can be accessed at http://www.usgodae.org/ argo/argo.html Data for the Australian Argo array can also be seen at: www.cmar.csiro. au/argo/tech/index.html and www. marine.csiro.au/~gronell/ArgoRT/ index.html   Fig.8: a typical Argo float mission. The float descends first to 1000m and then to 2000m, switches on its sensors and then floats to the surface so that the collected data can be transmitted to a satellite. The satellite data is then downloaded to a ground station. Interestingly, in March 2013 the data centres were hit with a huge number of downloads involving computers from all over the world and hundreds of gigabytes of data. The reason was a mystery until it was discovered that it corresponded to the film “Argo” being given three Academy Awards and people were looking for free downloads of the movie. Naturally, they would only have downloaded raw Argo data, not video. Recent developments The Argo platform is very flexible and as noted above, is not strictly defined in terms of shape etc. This allows floats to be developed with a What Happens When They Float Ashore? Occasionally, Argo floats wash up on beaches or are otherwise found and it can involve some real detective work to track them down. The main reason for the need to recover such floats is that the large lithium battery inside them can be a safety hazard in the wrong hands. In one case, a float was deployed by the San Diego-based Scripps Institution of Oceanography near New Caledonia. It failed to surface after about 18 months and its last known location was off the coast of Mooloolaba in Queensland. It was then actually trawled by a fisherman who thought it would be a good idea siliconchip.com.au to turn it into a letterbox but it was spared that fate due to the intervention of CSIRO scientist Dr Ann Thresher who is in charge of Argo Operations. Once the float was brought to the surface by the fisherman, it started broadcasting its location again. The precise location could be determined to only about a block and Dr Thresher travelled from Hobart to find it. She initially failed to do so and decided to return home but then changed her mind, more determined than ever to recover it. She went to the yacht club and then the fishing boats and after showing a picture of the device eventually found This photo shows the sensor head of the float recovered off Queensland. After spending 18 months on the sea floor, it was fouled with barnacles. the fishing boat crew that had retrieved it. The device was then collected and returned to the CSIRO for examination. July 2014  19 Fig.9: how the Argo floats cope with surface ice. The float will only rise to the surface to transmit data if the surface is ice-free, otherwise the data is stored until a break in the ice is detected. Contrary to what is shown in this diagram, in the current operational scheme, if there is overhead ice detected the float descends again to about 1000 metres and continues its 10-day mission cycle. As the floats used in such areas can store a large number of profiles, they can make many attempts to surface (at intervals of 10 days) until success is achieved. wide variety of sensors to suit different applications. Newer floats may contain oxygen sensors, transmissometers to measure water turbidity (a measure of the biological productivity of water), an FLBB device (fluorometer/ backscatter combination sensor) for chlorophyll measurement and measurement of nitrates, and a variety of other sensors. In Australia, these new “Bio Argo” floats will be deployed this year in places such as the Bay of Bengal, as part of an Australia-India collaboration and off the north-west coast of Western Australia. These floats will mainly work at a depth of 300 metres. Incidentally, some of the more restrictive countries of the world will not allow Argo floats that collect biological data into their oceanic territories, presumably since it has implications for fishing policies etc.   A particularly interesting sensor has been developed that measures the electric field produced when (conducting) seawater currents move through the Earth’s magnetic field. This is usually called motional induction. It allows the direction and speed of ocean currents to be determined. The specific type of Argo float that carries this sensor package is called the EM-APEX or ElectroMagnetic Autonomous Profiling Explorer. The float contains a compass, accelerometers, magnetometers and a processing system to convert voltage differences measured by sensor electrodes to velocity components of the ocean current. This float also measures salinity, temperature and pressure, as do the other floats. Coping with ice Looking into the future, a number of Converting Pressure To Depth Based on UNESCO Technical Papers in Marine Science No. 44, gravity at a specific latitude and pressure is given by the following empirical, computationally-friendly equations: g (m/sec2) = 9.780318 * [ 1.0 + ( 5.2788 * 10-3 + 2.36 * 10-5 * x ) * x ] + 1.092 * 10-6 * p where x = [sin (latitude ÷ 57.29578) ]2 and p = pressure (decibars) Depth is calculated from pressure as follows: depth (metres) = [(((-1.82 * 10-15 * p + 2.279 * 10-10) 9.72659) * p] ÷ g where p = pressure (decibars) and g = gravity (m/sec2) -5 * p - 2.2512 * 10 ) * p + These formulae assume a certain water temperature and salinity. In reality, the difference between depth in metres and decibars is so small as to be of little practical significance. 20  Silicon Chip new varieties of Argo are envisaged. Bio Argo has already been mentioned. As stated, the initial deployment model for Argo excluded the high-latitude regions because of the possibility of entanglement and destruction in the sea ice. These issues have now been resolved with new ice-hardened floats with features such as antennas that are resistant to ice and also methods of detecting overhead ice. Overhead ice can be avoided by the float sensing a temperature close to the surface consistent with sea ice and then descending again if ice is expected. The float can stay submerged for a long time if necessary as numerous data sets can be stored and then transmitted to the Iridium satellites. Australia has deployed 29 floats in the seasonal ice region of Australia’s section of the Southern Ocean. Other future planned developments include a total fleet of 4500 floats and deep-profiling floats that go to 4500 or 6000 metres. Argo is providing unprecedented amounts of information about the ocean environment. It is a major part of the world’s ocean observing system. Among many other things, it should increase the power of predictive models of short-term and long-term climate forecasting, patterns of ocean currents and present and future trends in the global climate, as well as provide information on the interaction of both the shallow and deeper ocean with the atmosphere. New developments also allow monitoring of the biological productivity siliconchip.com.au   Doppler effect The Argos System Satellites received frequency received frequency > transmitted frequency time received frequency < transmitted frequency Doppler curve O LDER AUSTRALIAN Argo floats transmit their data via the Argos System satellites. While the names are similar, there is no direct relationship between the two programs, apart from the fact that Argo uses the general purpose Argos satellite system. These satellites are designed to receive and disseminate data of a primarily environmental nature from both fixed and mobile platforms around the world. Applications include but are not limited to: •  Tracking land and marine wildlife such as sea turtles, fish, birds and land animals fitted with miniature transmitters; •  Receiving environmental data from fixed and floating marine platforms (manned and unmanned); •  Monitoring of disease outbreaks, food shortages, therapeutic drug availability and human­itarian aid resource utilisation in Third World countries (via aid-worker mobile data terminals). This data is relevant to public health and aid authorities and the system can even monitor school attendance rates; •  Monitoring the climate via Argo and many other floats and buoys; •  Monitoring of global water resources such as river levels, snow fall, dams and the status of water distribution infrastructure; •  Monitoring fishing vessels via transmitters installed on them to ensure compliance with national and international fishing agreements; •  Tracking of adventurers in extreme environments and international yacht races; •  Improving maritime security by allowing shipping operators to keep constant track of their fleets, with all ships of over 500 tonnes gross being required by the of the ocean which might lead to new sources of sustainable fishing and other marine food sources (and may also indicate where these resources siliconchip.com.au Satellite Satellite orbit going away er g clos gettin Argos transmitter Fig.10: diagram showing the direction of Doppler shift as an Argos satellite approaches and then retreats from a transmitter. International Maritime Organisation (IMO) to have a Ship Security Alert System (SSAS) installed. Argos satellites are able to receive location data from GPS equipped transmitters but can also compute position data from platforms not equipped with GPS by utilising the Doppler shift of several received signals over a period of time. This is the same technique by which the rough location of the missing Malaysian Airlines Flight MH370 was determined. In practice, locations can be determined with an accuracy of 150 metres using Doppler shift as opposed to a few metres with GPS. In Doppler location, the Argos satellite records the precise frequency of the received signal for every message received. Several messages need to be received in order to obtain a positional fix in order to generate a Doppler shift ‘profile’ of how the frequency changes as the satellite first approaches and then recedes from the transmitter. are being depleted, to give fisheries a rest). Other benefits of Argo are that it fosters international collaboration and helps in the development of global At some point in the frequency versus time profile there is an inflection point representing the true transmitter frequency. The orbit of the satellite is known, as is the altitude of the transmitter, leaving the latitude, longitude and the true transmission frequency of the signal unknown for each transmission. These unknowns can be determined with two or three messages but a fourth message is required to completely solve the equations and determine the positional accuracy. The solution to the equations provides two possible locations and then plausibility tests are used to determine the actual location as one solution will most likely represent an unrealistic position of the platform. The latest Argos-3 satellites rep­resent a significant improvement over previous versions and have 2-way communication, better transmission management (eg, acknowledgement that data was correctly received) and the possibility of platform remote control and programming. environmental information databases. It is widely supported internationally, Australia is a major player and the SC future looks very bright. July 2014  21