- for ill-conditioned antenna-pattern matrix, the determinant 'D' can
  be set to inf, yielding 1/D=0, and allowing the F-stat calculation
  to continue past such singular cases (which should only happen for
  small number of sfts ~O(2-3). Instead of returning 2F=0, however,
  we return the 'noise-expectation' 2F=E[2F]=4, which should overall
  be less disruptive and closer to a 'no information' result.