Question: If you were to make an underwater GPS work how would you do it?

Tom Rooney answered on 9 Jun 2017:

The technique is called Long Baseline Acoustic Positioning (LBL for short). We place acoustic (sonar) transponders on the seabed in a circle around where we want to navigate. Once their positions are fixed we use them as references for our underwater vehicles. Unlike GPS, which sends signals all the time, our acoustic transponders only send a signal when interrogated by another sonar transponder on the vehicle. It’s a bit like your sat nav asking the satellites for a fix. Using an accurate speed of sound (it varies underwater) and measuring the time taken to receive the responses from the fixed transponders the computer can determine how far the vehicle is from each reference, and simply triangulates the position (good old fashioned pythag!). For more info type “Long Baseline acoustic positioning” into google. Or look at these links:

https://www.hydro-international.com/content/article/lbl-underwater-positioning