BEA says the detection range of the Underwater Locator Beacon (ULB) also called as Emergency Locator Pingers (ELP) is up to 2,000 m.
Based on what figure of merit, what receiver, and what water conditions?
Given that the pinger is apparently under 3900m of water, and it was detected by a submarine sonar (with post mission FOM enhanced) at more than 2000m slant range, their range estimates appear to be "rough" and not "fine" in resolution. Not sure what sort of range resolution the sub might have been able to discern, even in post mission analysis, with
1) a signal that weak, at a comparatively high frequency (for sound in water)
2) angular direction being mostly "down" rather than lateral
Tricky sonar tracking and locating problem to resolve, for sure. Even more curious, I wonder if it was detected via a convergence zone sound ray path.
( I am about to draw blood from my scalp via head scratching on that one ... I can still only grok Direct Path from the geometry ... )