  Radiatively-driven natural supersymmetry, a theoretically and experimentally
well-motivated framework, centers around the predicted existence of four light,
nearly mass-degenerate Higgsinos with mass $\sim 100-200$ GeV (not too far
above $m_Z$). The small mass splittings amongst the higgsinos, typically 4-20
GeV, results in very little visible energy arising from decays of the heavier
higgsinos. Given that other SUSY particles are considerably heavy, this makes
detection challenging at hadron colliders. On the other hand, the clean
environment of an electron-positron collider with $\sqrt{s}>2m_{higgsino}$
would enable a decisive search of these required higgsinos, and thus either the
discovery or exclusion of natural SUSY. We present a detailed simulation study
of precision measurements of higgsino masses and production cross sections at
$\sqrt{s}$ = 500 GeV of the proposed International Linear Collider currently
under consideration for construction in Japan. The study is based on a Geant4
simulation of the International Large Detector concept. We examine several
benchmark points just beyond the HL-LHC reach, with four light higgsinos
directly accessible by the ILC, and the mass differences between the lightest
SUSY particle and the heavier states ranging from about 4 to 20 GeV. It can be
shown that their masses and production cross sections can be precisely measured
to approximately 1\% precision or better. These precise measurements allow for
extracting the underlying weak scale SUSY parameters, giving predictions for
the masses of heavier SUSY states. These provide motivation for future
high-energy colliders. Additionally, dark matter properties may be derived.
Evolution of the measured gaugino masses to high energies should allow testing
the hypothesis of gaugino mass unification.
