How many nanometers is the American gps chip?

Are you asking how many nanometers the chip is made of? That shouldn't be. At present, everyone's GPS chips basically use ublox, Taidou, Zhongke Wei and MTK, and basically do not use American ones. Recently Xiaomi 8 used Broadcom's dual-frequency GPS.

You should ask about GPS positioning accuracy, right?

Using single-point positioning method, GPS single frequency can now be 5 meters, and dual frequency is 3-5 meters (open area).

Using differential positioning (RTD or RTK, there must be a reference station to transmit differential data to the rover), RTD can be accurate to the meter level.

RTK can be accurate to centimeter level.