USBL and SBL (Water Linked) systems use different principles for calculating the position. USBL systems measure phase shift between the transmitter and receiver and estimate an angle. When adding “time of flight” of the signal you get the position. This principle works very well in many applications. The main challenges of USBL based systems are:
- Poor/no performance in shallow water. USBL can have a hard time when it is less than 10 meters depth. This is not an absolute number, and there are differences between the systems. Our UGPS works as long as you get all the four receivers down in the water.
- Poor/no performance when operating close to large reflective areas (ship hulls, harbor facilities etc.)
- Poor/no performance when the signal is transmitted through obstacles (scholes of fish in fish farming etc.)
- Poor/no performance inside pools/tanks etc.
- Relatively large physical size
SBL systems like the Water Linked system find the position by performing triangulation of the signal received by multiple receivers. This is exactly the same principle as the GPS system uses. Historically the SBL systems have had poor price/performance, but Water Linked has changed all that. The main advantages of the Water Linked systems are:
- Excellent performance in shallow water
- Excellent performance when operating close to large reflective areas Excellent performance in fish cages
- Excellent performance in pools/tanks (except inside steel tanks)
- Extremely small physical size
- Low cost
The main downside of the Water Linked system is its limited range. With the boat (baseline) in the middle, you cover an area of 300m radius and 300m deep. Many USBL systems can go further than this. So if you definitely need very long range and are operating in open waters without acoustic reflections, then go for USBL. If not, the Water Linked system is the better choice.