JRC Doppler Sonar, such technology
How do they do that? The new JRC JLN-550 Speed Log at right is displaying not just 18k of forward speed over the ground (SOG), but is also showing that the bow is going to starboard at 1.8k and the stern to port at the same speed. The trick starts with a 4 beam 240 khz sonar transducer in the bow. The doppler shifting of the sonar pings off the bottom is used to get two axis SOG. The third axis (the stern motion) is calculated by adding rate-of-turn input from a gyro or ROT capable electronic compass. When bottom depths exceed 250 meters, the unit can switch to speed through the water (STW) using ultrasonics (2 mhz) to measure passing particles. Now this is big ship gear for sure—it starts at about $26,000, and the 265 pound transducer is termed “compact”—but accurate STW underway and multi axis SOG around docks would be very useful on medium size boats too. I’m hopeful. Airmar already has an ultrasonic speed transducer scaled for yachts (pdf brochure here). Now we just need a small, reasonably priced 4 beam sonar transducer.
