The standard positional accuracy of VBOX’s is 3 m 95 % CEP for Longitude and Latitude.
The height accuracy is 6 m 95 % CEP.
CEP = circle error probable.
95 % CEP means 95 % of the time the position readings will be within a circle of diameter 3 m of the true position. This error is due to the changing state of the ionosphere, constantly changing the time taken for the satellite signals to reach the earth, see GPS Errors and Noise.
The accuracy of the positional and height data can be improved using different types of Differential GPS correction methods, see Differential GPS.