A gyro, or gyroscope, is a device that uses Earth's gravity to determine orientation; in the case of directional drilling, tool-face magnitude and orientation (TFMO). A gyro consists of a freely rotating disc mounted on a spinning axis in the center of a larger and more stable wheel.
As the spinning axis turns, the freely rotating disc remains stationary to indicate central gravitational pull. The gyro is among the three methods used for wireline steering in HDD.
The others are walkover and magnetic wireline steering.
Trenchlesspedia Explains Gyro
A gyro is an essential part of drilling as it gives the correct direction and inclination data where other measurement drilling (MWD) tools don't work that well. Advancements in gyro survey technology have seen little advancement and depend on mechanical gyroscopes that can impact the drilling operation negatively.
Ordinary shocks and vibrations from drilling activities also affect them. The gyro cannot be tracked and its position cannot be verified, but it can be used in conjunction with a DC tracking system and magnetic steering tool to get the desired accuracy. Only using a gyro for drilling is the same as drilling blind.