Northwest Dive Club

Wired Magazine: "Darpa, the Pentagon’s research arm, [is] asking for proposals from industry. [...] A far better technology for underwater communication [than radio-based GPS] is acoustic signals—sound waves—at very low frequencies that travel well over long distances. Instead of satellites transmitting radio frequencies, an underwater GPS system could use a constellation of acoustic transmitters—powered underwater beacons, tethered to the seafloor, sending pings that propagate through the water. [...A]ny proposal will be cognizant of wildlife concerns. [...] Darpa wants to see at-sea demonstrations by 2018. If it has its way, there soon won’t be anywhere on the planet where you can still get lost."
https://www.wired.com/2016/05/darpa-wan ... ng-robots/ (link fixed, thx 60south)

Bad link. Mo' bettah: https://www.wired.com/2016/05/darpa-wants-underwater-gps-system-seafaring-robots/

Using low frequency acoustics on a basin-wide scale, I wouldn't expect the accuracy of a football bat.

Yeah, low-frequency and low accuracy was my first thought too... Plus, I'm sure that any tethered buoy is going to have some motion of its own which would add to the variability in the system. Seems like the development of small, cheaper inertial guidance systems might be a better direction to point the research dollars.

Jim

At least one sidescan sonar has been able to get high precision and the longer range of low frequency by adjusting the pulse repetition rate. When I think of all of the difficulties that were overcome to create GPS, this seems easy in comparison. With fixed beacons, they don't have to account for the motion of satellites. The relative speed of sound being so much slower that time measurement can be much less precise. A bigger problem may be the lack of demand. GPS, while initially designed to target cruise missiles, has a huge effect on the global economy. How much demand is there for submarine navigation?
-Curt

True, there are bound to be techniques that will help, like GPS differential baselining. I guess my definition of "high-precision" (millimeters accuracy) might be a little high too... If a few meters accuracy is acceptable, then it would be a lot easier to achieve. Still harder to define the position of a buoy randomly bouncing about on a tether than one in a clearly defined orbit, so that would be one source of 'slop'. Varying speed of sound in water according to density, salinity, temp, etc is another.

I would think the biggest potential demand would end up for preprogrammed autonomous 'bots, whether floaters or crawlers, that would want to either follow a course, or follow something and record a waypoint track. Either would be useful for all sorts of marine research.

Jim

This is one of those problems that seems easy enough but is actually tricky.

Not only do you have to contend with the varying speed of sound due to water density and temperature, but also problems of reflections, multipath, and signal attenuation. Even if the basin is relatively small (say <500 km diameter) you'll still have to use low frequencies. Above 10 KHz the attenuation is too much to go any significant distance; my guess is they'll have to use frequencies around 100 Hz, give or take, and rely on the SOFAR layer. If you go too low, however, you run into problems of power consumption. And at high powers you could harm animals. There's lots of trade-offs.

GPS has the advantage of speed-of-light radio waves, line-of-sight reception, and you can "see" an entire hemisphere of sky with all the satellites. In contrast, sound waves are slow, meandering, and you may not hear them unless you're close. Methinks they're going to need a boatload of low-frequency, low-power transmitters, all with accurate clocks, pulsing regularly on different wavelengths; the receiver will also need an accurate clock to estimate range, and an internal model of the basin characteristics and transmitter locations to estimate sound velocities. Even so, I would be amazed if they averaged better accuracy than 10's of meters.

The navy already has LOTS of toys on the seafloor. This is just transmitting versions instead of receiving hydrophones. They did say a constellation of them in the RFP, that implies they are thinking lots of these things.