The original question is not about what altitude the output is declared NCD, but how groundspeed may effect it.
The initial "common sense" responses hinted that groundspeed has nothing to do with it. I've tried to point out that it is not as simple as that. Particular when you bring pitch angle into it. As Mono pointed out, the radalt's primary output is altitude, and so a clear limit can be set at which you declare NCD. However, various other factors can cause the output to become NCD, but it is not as straight forward as saying that it's this particular vertical speed or that particular groundspeed. It is a set of conditions that cause a loss of "track", and the OP is unlikely to get a specific answer to that question without the tech spec.
There has been much recent internet discussion about whether a specific radalt would go NCD at a certain groundspeed, it made this thread seem very familiar to questions I have been asked by colleagues.