Getting deep data from diving robots

Imagine, a year from now, you’re offshore, motorsailing in light air through a nearly flat sea. A thousand miles from land, there isn’t another vessel to relieve the glassy vista.

Suddenly, a slim, yellow cylinder pops to the surface 100 yards ahead.

What is it? There aren’t any lobster pots in water this deep. Sporting a shiny black antenna and some official-looking stickers, the object looks like a radio-equipped scuba tank. As you pass slowly to leeward of the mysterious object, you notice a change; it appears to slide lower in the water. Soon just the antenna points above the surface. Then even that is gone, disappearing into the sea like a diving submarine.

While it may seem like you’ve just had a visitation from an alien culture of the deep, what you’ve just witnessed in this mid-ocean encounter was the data transfer from an autonomous research robot to an orbiting satellite, one element in an ambitious data-gathering project called Argo. (For dramatic effect, I’ve compressed the time the robot is on the surface; in reality the floats spend about an hour bobbing in the sunlight before slipping away again.)

This deployment of free-floating robots, which began in 2000 and is projected to include about 3,000 units worldwide by 2004, is designed to gather data on ocean temperature and salinity. These 5-foot-long cylinders will record vertical profiles of the ocean waters by diving to a depth of 2 kilometers and rising to the surface. Once on the surface, they broadcast their data by satellite and then sink again, repeating this cycle every 10 days during their projected five-year life spans.

Data from the Argo project will allow researchers to better forecast the cycles of ocean and atmosphere, the most famous of which is El Niño/Southern Oscillation (ENSO). “We know the ocean modulates weather patterns,” said Stan Wilson, director of international ocean programs at NOAA. “The biggest potential contribution [of Argo] is laying the basis for improved seasonal forecasts.”

The conditions spawned by ENSO events can result in major storms, flooding in some areas, droughts in others, coastal erosion and more. And ENSO isn’t the only of these oceanic/atmospheric oscillations. The North Atlantic Oscillation (NAO), for example, affects wind strength and direction over the North Atlantic, influencing both the severity of storms and their direction of movement. With a better understanding of the forces that drive these cyclic oscillations, we can expect improvements in both long-term and short-term forecasts. For ocean voyagers, the benefit of more accurate forecasts is obvious. Thus, offshore sailors as a group should be heartened that the Argo program will help to fill in the gaps of our knowledge of the earth’s complex dance.

Unlike a submarine, which uses an onboard supply of compressed air to change its density and thus rise or sink in the water column, the Argo floats will change their density using hydraulic fluid and an expandable bladder at the bottom of the unit. When the unit is ready to descend, an onboard hydraulic pump draws fluid out of the bladder, the density of the unit increases, and it sinks. For ascent, the pump fills the bladder, which expands, decreasing the overall density of the robot, and it rises toward the surface. Since this method doesn’t change the density of the unit by much, the rise from 2 kilometers down up to the surface requires six hours. When it is rising toward the surface, the unit takes roughly 50 temperature and salinity readings and stores this data in its onboard memory. Each temperature and salinity reading is indexed to a specific depth, which is determined using a pressure sensor.

Once the unit breaks the surface, it transmits its accumulated data to an Argos Data Collection and Relay System package aboard a NOAA polar-orbiting weather satellite. These Argos (a data-collection system that long predates the Argo floats) packages determine the position of the float to within roughly a mile by measuring the Doppler shift of the float’s signals as the satellite passes over (this is the same technique that was used by the now-defunct Transit satnav system, the father of GPS). Some of the newer floats are using Iridium and Orbcomm communications satellites for relaying data. These spacecraft allow for two-way communications and also have a much higher data bandwidth. Since Iridium and Orbcomm spacecraft aren’t set up to measure Doppler shift, these floats have onboard GPS receivers for positioning. After the data has been sent and received, the float sinks again to repeat the process.

While viewed on a small-scale map of the oceans, it seems the seas will be peppered with these robot floats, but the separation between them will be substantial, roughly 300 kilometers between units. This buffer zone will change, however, as ocean currents carry the units hither and yon. Even then, these units will be sufficiently rare, and they will spend so much of their operational lives underwater that most voyagers will probably never catch sight of one, let alone hit one. Of course, these units are small enough that they wouldn’t significantly damage a well-found oceangoing vessel.

While Argo represents the first use of autonomous buoys that can measure below the surface, other types of ocean buoys are used to record data for studying the world’s oceans. These include surface drift buoys, expendable bathythermographs (called XBTs, these are sensors that are released by participating commercial ships and measure temperature down to 5,000 feet), and an array of moored deep-ocean ATLAS buoys arranged along the equator in the Tropical Atmosphere Ocean (TAO) array, part of a program called Tropical Ocean Global Atmosphere (TOGA).

While accidentally hitting an Argo float wouldn’t hurt your boat, the same can’t be said for an ATLAS buoy in the TOA array. These buoys weigh roughly 1,450 lbs, are 16 feet tall and certainly aren’t something you’d want to hit with your boat. Luckily, they are well-equipped with radar reflectors and can be picked up on radar at 4 to 8 miles away, depending on sea conditions. Also, they are so widely dispersed. You would probably have to hunt around to find one, so there’s little danger of an impact.

Like the weather balloons that are released several times daily on land, these robot floats will provide scientists with a wealth of data on the state of the oceans. And this information will almost certainly lead to better weather forecasts for offshore sailors.

By Ocean Navigator