RTK Base

The base station set up transmitting an RTK correction . The other bit of kit is a rover; identical hardware but mounted atop a pole. It reconciles its view of the sky (in terms of the orbiting space vehicles (SVs)) which are being simultaneously tracked by the base; hence the term differential GPS surveying.,
Strictly speaking GPS relates only to the American SVs and others are available. The other main group in common use being the Russian Glonass SVs. Their introduction, even when only 2 or 3 are in the sky makes an enormous difference to the quality of the solution.