Required effective area versus assumed source distance for different localization uncertainties. The curves show the equivalent area, in units of the current GBM effective area, required to retain a finite upper bound on the source distance for the GBM–KW baseline. We adopted σ_t,stat = 0.5 ms, consistent with short-GRB timing at a ∼0.1 ms time resolution. The horizontal dashed line marks the current GBM-equivalent area, and the vertical dotted lines mark the critical distances D_crit at which the constraint becomes localization-limited. The right axis gives the equivalent statistical delay uncertainty. This figure quantifies how improved timing sensitivity and, in particular, better localization help expand the distance range over which the method can directly measure (rather than merely constrain, the source distance).