  The Polarized Atomic Hydrogen Gas Jet Target polarimeter (HJET) is operated
at the Relativistic Heavy Ion Collider (RHIC) since 2004 to measure the
absolute polarization of each colliding proton beam. Polarimeter detectors and
data acquisition were upgraded in 2015 to increase the solid angle, energy
range, and to improve the energy and time resolution. These upgrades along with
an improved beam intensity and polarization allowed us to greatly reduce the
statistical and systematic errors for the proton polarization measurements in
RHIC Runs 15 ($E_\text{beam}\!=\!100~\text{GeV}$) and 17 ($255~\text{GeV}$).
For a typical 8 hour RHIC store, the measured proton beam average polarization
was about $P_\text{beam}\!\sim\!55\pm2.0_\text{stat}\pm0.3_\text{syst}\%$. The
elastic $\mathit{pp}$ analyzing power, $A_\text{N}(t)\!\sim\!0.04$, was
determined with a precision of about $|\delta A_\text{N}(t)|\!\sim\!0.0002$ in
the momentum transfer squared range $0.001\!<\!-t\!<\!0.020\,\text{GeV}^2$. In
this paper we present a detailed systematic error analysis of the polarization
measurements at HJET. Perspectives of using the HJET based absolute
polarimeters in the future Electron Ion Collider (EIC) will be also discussed.
