POGO was a modern variant of the dropsonde technique Richardson and Schmitz used to measure currents and transport in the Florida St. At various sites across the current they would release a free-falling instrument that would sink either to the bottom or to a predetermined depth, drop ballast and return to the surface for recovery. Its horizontal displacement between release and resurface is an accurate measure of transport at that site and time. They used an extremely accurate local navigation system called Decca Hi-Fix to determine its displacement. This is in the 1960s when the art and skill of measuring ocean currents was still in its infancy (and in some ways still is).
POGO was our version of their dropsonde. Between 1980 and 1983 we ran a program called Pegasus to map out the velocity structure of the Gulf Stream on a bi-monthly schedule. Sometimes we needed to extend our coverage farther north or south beyond the range of the transponder sites we had set up for Pegasus (see the Instrument section). We pick a site to release POGO, take a CTD or an XBT, and then maneuver to be close to where we expect POGO to surface. We knew how far away it was acoustically so when it surfaces and we hear the radio beacon, we record the ship’s LORAN-C position, which together with range and bearing to POGO gives us its position. Distance traveled between release and re-surface, divided by time gives us its average velocity. The release of ballast at turn-around depth was prescribed by a brass screw whose neck has been narrowed so that it will fail when hydrostatic pressure acting on the piston to which it is attached exceeds its breaking strength. At which point ballast falls off and POGO returns to the surface. A very simple yet reliable approach.
Now here’s a suggestion: fabricate expendable POGOs! No bigger than an XBT so it can fit in AXIS, the automated XBT deployment system, XPOGOs are released sequentially on a regular and repeat schedule. Each probe sinks to a predetermined depth or to the bottom, releases ballast and returns to the surface, whereupon it immediately acquires its GPS position, and broadcasts the finding to Service Argos or similar. Yes, it will need a GPS receiver and a transmitter but nowadays these are fabricated on chips at very low cost. Even tiny birds can be outfitted with Service Argos transmitters! Sure, POGO could profile temperature as well, but the key information will be the displacement vector between release and resurface.
Think about it, we can explore galaxies billions of lightyears away, but we oceanographers are still severely handicapped regarding what the ocean is doing at depth.