  The requirement of electroweak naturalness in simple supersymmetric models
implies the existence of a cluster of four light higgsinos with mass $\sim
100-300$\,GeV, the lighter the better. While such light compressed spectra may
be challenging to observe at LHC, the International Linear $e^+e^-$ Collider
(ILC) with $\sqrt{s}>2m_{\rm higgsino}$ would serve as both a SUSY discovery
machine and a precision microscope. We study higgsino pair production
signatures at the ILC based on full, \texttt{Geant4-}based simulation of the
ILD detector concept. We examine several benchmark scenarios that may be
challenging for discovery at HL-LHC due to mass differences between the
higgsino states between $20$ and $4$\,GeV. Assuming $\sqrt{s}= 500$\,GeV and
1000\,fb$^{-1}$ of integrated luminosity, the individual higgsino masses can be
measured to $1-2\%$ precision in case of the larger mass differences, and at
the level of $5\%$ for the smallest mass difference case. The higgsino mass
splittings are sensitive to the electroweak gaugino masses and allow extraction
of gaugino masses to $\sim 3-20\%$ (depending on the model). Extrapolation of
gaugino masses via renormalization group running can test the hypothesis of
gaugino mass unification. We also examine a case with natural generalized
mirage mediation where the unification of gaugino masses at an intermediate
scale apparently gives rise to a natural SUSY spectrum somewhat beyond the
reach of HL-LHC.
