Seems like the spoofers could work around that and still do damage if they wanted to, by gradually shifting away from reality. The open-loop position estimate

Seems like the spoofers could work around that and still do damage if they wanted to, by gradually shifting away from reality. The open-loop position estimate will never be perfect, so some margin would have to be designed into such a check, and that could be exploited. It would still be better than nothing though.