The M9P device from U-blox (see tech specs) uses four GNSS constellations (BeiDou, Galileo, GLONASS, GPS / QZSS) and has the abililty to run in Real-Time-Kinematic (RTK) mode where the location of a moving object can be tracked in realtime. The module only uses 36 mA while in use, making it ideal for battery-operated systems like ours.
Sparkfun and Ardusimple make convenient low cost boards.
Ardusimple’s simpleRTK2B has a link to a high power radio module (868MHz) which allows rovers to be km away.
We used a pair of these to learn about the ublox settings; they are a perfect starting point.
We chose the Sparkfun board because we wanted to break the link between the GPS and radio (and allow our MCU to use the radio). We are still using the radio modules (Xbee Pro SX) which we will mount on our PCB.