We have shown that Rydberg atoms can be used for high-sensitivity, absolute sensing of radio frequency (RF) electric fields. We achieved a sensitivity of 3 μVcm-1Hz-1/2 for two read-out strategies. Results using a Mach-Zehnder interferometer2 and frequency modulated spectroscopy both achieve similar photon shot noise limited sensitivity. Fundamental limits to the sensitivity of the Rydberg atom-based RF electric field sensing have been addressed. Depending on the spectral resolution of the read-out, either the RF induced transmission line frequency splitting, the Autler-Townes regime, or a change in the on-resonant absorption, the amplitude regime, can be used to determine the RF electric field. Here, we present theoretical results of a 3-photon read-out scheme which enables the Autler-Townes regime of Rydberg atom-based RF electrometry to be extended to lower RF electric field strengths. We show that the residual Doppler shifts can, in principle, be reduced to ~11.8 kHz, on the order of the Rydberg atom natural lindewidths, using the approach.