No, the on-board GPS does not support this.... but if the user has a GPS that does, this external GPS can input its 'RTK-groomed' measurements into the 3DM-GX3-45 and the 3DM-GX3-45 will incorporate those measurements into the Kalman filter. To be clear: the on-board GPS will be bypassed in favor of the external GPS.
Wikipedia's definition: http://en.wikipedia.org/wiki/Real_Time_Kinematic
Real Time Kinematic (RTK) satellite navigation is a technique used to enhance the precision of position data derived from satellite-based positioning systems, being usable in conjunction with GPS, GLONASS and/or Galileo. It uses measurements of the phase of the signal′s carrier wave, rather than the information content of the signal, and relies on a single reference station to provide real-time corrections, providing up to centimetre-level accuracy. With reference to GPS in particular, the system is commonly referred to as Carrier-Phase Enhancement, or CPGPS. It has application in land survey and in hydrographic survey.