With GPS you are able to achieve a theoretical accuracy of 3 meters. GPS-RTK-SMA has mastered dialing in the accuracy on their GPS modules to 1cm!
The SparkFun GPS-RTK-SMA Board is a powerful breakout board for the ZED-F9P module from u-blox. The ZED-F9P is a top-of-the-line module for high accuracy GNSS and GPS location solutions including RTK. RTK is short for Real Time Kinematics. A GPS receiver capable of RTK takes in the normal signals from the Global Navigation Satellite Systems along with an RTCM correction stream and then calculates your location with 1cm three-dimensional accuracy in real time.
The ZED-F9P is unique in that it is capable of both rover and base station operations. It features a survey-in mode allowing the module to become a base station and produce RTCM 3.x correction data.
The GPS-RTK-SMA even has five communications ports: USB-C (which enumerates as a COM port), 2 UART (with 3.3V TTL), I2C (via the two Qwiic connnectors or broken out pins), and SPI. Sparkfun has also written an Arduino library for u-blox modules to make reading and controlling the GPS-RTK-SMA easy.
The ZED-F9P requires a good quality GPS/GNSS antenna and a 10cm metal disc is required to be placed under the antenna as a ground plane.
More information is available on the website of Sparkfun.