Hi @robotmark,
groundspeed is the autopilot’s best estimate of what its current speed is relative to the ground, based on its sensor inputs. The internal sensors (e.g. accelerometers, gyroscopes, magnetometer) in the autopilot’s IMU (inertial measurement unit) can provide a rough estimate, but additional accuracy requires sensors that are better suited to that kind of measurement. As some examples,
- a DVL can measure speed relative to the ground directly
- USBLs and similar systems measure position directly, relative to an external reference
- scanning sonars (like our Ping360) and other imaging sonars can also help with local positioning
- echo-sounders (like our Ping Sonar) can be used to measure distance to an obstacle or wall in a particular direction
- computer vision can sometimes be used for positioning around detectable objects of known shape+size / spacing
- and so forth
Note that accurate positioning underwater is a challenging problem, and there’s no “one size fits all” solution that is affordable and suits every situation (e.g. see this recent discussion on USBL prices). If it’s important to your application then you’ll need to determine a suitable option given your particular constraints.