AbstractAmbiguity resolution enabled precise point positioning (PPP-RTK) can provide fast, potentially even instantaneous, centimeter-level positioning results, given that the phase ambiguities are correctly resolved. Without external ionospheric corrections, a time-to-first-fix the ambiguities of around 30 min is often reported for GPS-only solutions. In this contribution we investigate the capabilities of almost-instantaneous PPP-RTK without any a-priori ionospheric information. The key aspects are the mean square error-optimal best integer-equivariant estimator, a multi-GNSS solution using GPS, Galileo, BDS, and QZSS, and a proper weighting of the satellite clock and bias corrections with their inverse covariance matrix in order to obtain realistic observation models. Real data experiments with dual-frequency observations show that centimeter-level horizontal positioning errors are reached within one and two epochs in 87.6% and 99.7% of the cases, thereby demonstrating that almost-instantaneous PPP-RTK without atmospheric corrections is indeed possible with the current constellations.