Lagrangian techniques have proved effective at quantifying initial rates of relative dispersion using accurately tracked clusters of floats, both at the surface and at depth. At larger space and time scales chemical tracer techniques show where stuff spreads. The former provides insight into local eddy activity while the latter informs on how water masses eventually intermix. But what happens between the sub-mesoscale and the gyre scale? As particles disperse their relative motion soon becomes incoherent, but the subsequent trajectories may still be highly constrained. Obvious examples of this include shelf-breaks as barriers to lateral mixing, and topography that suppress interbasin exchange except at fracture zones. Here we propose a further development of the RAFOS technology to enable studies of processes spanning these intermediate space and time scales. The floats will use the newly developed fishtag technology that allows for very energy-efficient acoustic reception; these also measure pressure and temperature. They will be small, ~ 5 kg in weight, and self-ballasting. We envision the basic unit cost to be well under 2 k$ in large numbers, but we are open to including other sensors such as for O2 and pH. Standard RAFOS sound sources provide acoustic navigation, but to save cost these may drift with the floats as SOFAR floats. Acoustic receivers on surfacing Argo floats would track the SOFAR floats. For applications that focus on the transition to geochemical applications at gyre scales high-resolution tracking can be replaced with occasional acoustic fixes to constrain their overall movements. An extreme Lagrangian challenge might to use floats to quantify mid-ocean abyssal drift.