Playing with WoS mod - need help!

[IcyWaters]

I'm just picking up SH5 again but I never really played it thoroughly before. My last sub sim was SH3 so many, many years ago!

I've installed SH5 and added the Wolves of Steel mod. I'm playing the first mission, Baltic Operations. I've spotted a convoy of three ships while my sub was on the surface and I've spent most of the night just following it. Now the convoy has made a right turn and I've cut the corner to bring them closer. Rough estimate from the Nav map is that they're 10-15km away. I submerge and try to use the hydrophone to work out their speed and course. My operator can get the contacts and bearing, but cannot tell me their distance. Why is this? Is 10-15km too far away?

The sonar operator will just say "contact warship, moving slow, moving away, bearing 093, long range."