FAQ

Below we answer some frequently asked questions about drifters. If you don't see your question addressed here, please contact us and we'll get back to you promptly.

A drifter is an oceanographic instrument that floats on the sea surface and is tracked to produce a time series of positions that are analyzed to yield a time series of velocities along that track. Oceanographers usually deploy drifters to infer surface current velocities from drifter velocity.

There are two important questions to ask in evaluating how well drifter motion follows surface currents: 1: what currents are the drifters measuring? and 2) how accurately does the drifter follow those currents?

Since a drifter must have physical size (where else to place the transmitter and batteries), its area and mass are acted upon by all of the forces at the ocean surface. Besides the surface current is is supposed to measure, parts above the surface are affected by winds and the parts from the surface to below are affected by waves. So a first principal of drifter design is to minimize the parts exposed at the surface.

Waves can create velocity contamination called rectification. This is a physical process in which waves periodically lift the drifter as their crest pass by the drifter and the drifter responds by bobbing up and down. If the drifter has the wrong shape, like a cylinder weighted at its bottom, it will bob vigorously, and that vertical oscillation will be transformed into horizontal velocity. For example, spar buoys have been observed to "walk" themselves into the approaching wave field.

And lastly, if ocean currents were uniform from top to bottom, it would be much easier to design the parts of a drifter that lie below the surface: just make it large enough and of a drag-neutral shape to overcome the effects of winds blowing on the parts of the drifter that stick out of the water. Unfortunately, that is not the case. Vertical velocity is not uniform. The ocean surface is physically active: surface heating and cooling, and precipitation, remove and add heat and water to the surface, causing density stratification based on vertical variations of temperature and salt content. The stronger these stratifications, the less good the physical coupling between layers; because of stratification one layer's momentum may not be effectively transferred to an adjacent layer. At the surface where winds may blow strongly, the vertical velocity structure can be quite complex. So, the correct drifter design is the one that will best measure the currents of interest.

The answer to this question is another question: what surface current are you trying to measure?

The key to designing a drifter to measure a surface current is to know the vertical current structure between the surface and the depth at which the current of interest resides. The guiding principle behind drifter design is make the forces exerted on the drifters by that current as large as possible compared to those exerted by other forces which would contaminate that velocity by pushing the drifter in any direction.

Clearwater makes two different types of drifters that measure currents at two different levels: very close to the surface and below the surface. Our ClearSat-1 measures currents between the surface and 1 meter depth. The ClearSat-1 is used usually in shallow waters near the shore: for example, in studies of river discharge, where the river's fresh water may lead to a thin fresh less salty layer of seawater at the surface. The ClearSat-1 is modeled after the CODE (California Ocean Dynamics Experiment) drifter, which primarily addresses minimizing the contaminating forces of winds and waves. The ClearSat-1 accomplishes this by suspending the vertical cylindrical hull of the drifter, which is negatively buoyant, below the surface, supported by floats at the ends of dihedral plane sails extending from the hull. Those features keep the drifter close to the surface and minimize the effects of waves.

The ClearSat-15 III is a drifter that is usually deployed in the open ocean. It was originally designed for the World Ocean Circulation Experiment (WOCE) and often is referred to as a WOCE SVP drifter. Initially, the intended region of deployment for the WOCE SVP drifter was the tropics where the challenge was to measure the currents in the well-mixed layer (constant temperature and salinity resulting in constant density) which often is found to extend from the surface to 20 m. Besides the waves which are to be found everywhere in the ocean and can contaminate drifter surface velocity measurements, it had been noted that these areas often experience afternoon winds acting on a warmed thin, layer, causing a jet, or high velocity transient. This diurnal velocity jet was seen as noise on the steady velocity of the well-mixed layer.

To counter the effects of wind, waves and diurnal thin jets, the components of the drifter at the heart of the well-mixed layer were made large in cross-section compared to the components from that level through the exposed parts of the surface float. Basically, the idea is to make the forces on the drifter caused by the currents in the well-mixed layer much larger than the forces of winds, waves and currents int the upper parts of the drifter. This concept is expressed as the drag-area ratio: the ratio of the cross-section of the drogue times the drag coefficient for the drogue shape divided by the cross-section of the stress reliefs, tether and surface float each multiplied by their respective drag coefficients. Based on models of vertical surface current structure in the tropics, this drag area ratio should be larger than 40:1. Thus the ClearSat-15 has a small surface float, thin tether and a long cylindrical drogue and a drag area ratio of greater than 40:1. In effect the drogue acts as a sea anchor locked into the heart of the well-mixed layer which drags along the tether and surface float containing the controller, transmitter and batteries needed to relay information and position back to the user.

Although, the WOCE SVP drifter specification was initially designed for use in the tropics, it has become the standard surface velocity measurement tool for oceanographer in all oceans.

The Argos system for data collection and location has been around for about 20 years. It requires a drifter to have nothing more than an Argos transmitter. Locations and their times are calculated by CLS and wrapped up with the data returned to the user. Data are time-stamped as it is received by the satellite. Argos locations are timed to the second and locations are reported to a precision of 0.001 degree. GPS has been around about half as long and requires a drifter to have a GPS engine, antenna and firmware to format a time and location message from the GPS engine output; that location data become part of the drifter’s data message returned by satellite. Clearwater reports GPS locations to 1 second and 0.00001 degree.

Differences between the determination of a location between Argos and GPS lead to different properties of the two measurements. Argos locations are determined by the reception of transmissions from an Argos PMT by the Argos satellite system which consists of several low earth orbiting satellites that periodically pass within sight 20 to 30 times a day. Every point on the earth is visible to Argos satellites. If a satellite receives several good receptions from the PTT, a location usually can be calculated. The time associated with the location is also part of the calculation. The location is derived from the Doppler shift measured by the satellite as it passes over the PMT. If there is no Argos satellite present, no position can be calculated. By contrast, the much higher flying GPS satellites are visible at every point on the globe all of the time. For those of us in medium latitudes there may be as many as 10 satellites visible at one time. GPS locations are determined by differences in arrival times for signals sent by several satellites. The locations are available as a 1 Hz time stamped data stream.

Argos positions are accurate to approximately 500 m and are available only when an Argos satellite passes overhead. GPS accuracy is usually about 10 m; relative positions can be accurate to inches. GPS locations are “on demand”; you can have a GPS location when you want it.

How do you determine which location technique to use? Really, we are talking about time and location; these are the parameters which fix other measurements in a temporal-global context. The first question to ask is: what are the time and distance scales of the processes to be observed and described? The WOCE Surface Velocity Program set about to characterize the surface current of the world’s ocean at the largest dimensions over the expanses of the Pacific, Atlantic, Indian and Antarctic Oceans. There the major features are tens of kilometers in extent varying slowly from one day to the next. The accuracy of Argos in time and position was more than enough to measure the surface currents without serious aliasing (the contamination of lower frequency information with that from higher frequencies that have been under sampled). The cutoff frequency for Argos-derived locations is probably between 1/3 and 1/6 hours; it should be higher at higher latitudes, because a PMT there sees more Argos satellite passes as the orbits converge near the poles. In contrast, GPS can be configured to sample at any frequency.

Argos has no limitations concerning how much Argos-generated location data the user receives. She gets a number of locations determined by the number of satellite passes. Because the overhead of Argos locations is contained entirely within the processing preformed at the downlink centers, the user is not concerned with a cost associated with battery power or data bandwidth. If you are using GPS for location, sampling rate is a concern for the effect it has on battery power and data bandwidth. With the advent of evermore efficient GPS engines the power budget needed for GPS location has dropped considerably so the limitations related to battery power and bandwidth are really the same thing. Most of the power of a drifter is consumed by transmitting data; the more data your drifter collects the more power it will take to send that data back to you. The associated part of the equation for the user is the cost of sending that data through the satellite link. This can be a taxing problem to untangle as systems charge in different ways: blanket daily charges, per byte charges, and combinations. See your data provider.

Although the system is evolving, Argos data has been limited to about 1 KB per day (the introduction of Argos 3 includes intelligent relay between PMTs and satellites and even higher data speed transmissions; all of these enhancements will boost daily to 3 KB per day and more). Iridium can carry considerably more data. We think the limit for locations and times for Argos is around 30 minute updates with the use of formatting to compress data. That does not leave much room for other data. Iridium can go considerably higher, but one needs to budget for the cost.