FOSREM team replaced the previous FOS1 sensor with the new FOS5LL with a 15 km long fiber loop. The old one was working since July 2017 and give us a lot of data. The new FOS5LL (Long Loop) is ten times more accurate, has a long 15000 m fiber optic loop and 1.5 m diameter. It’s working in a closed-loop configuration with 1310nm light.