Goodrich Gyroscopic Modules, as recommended by the manufacturer should not go below a -60 Deg Dip. However we find that with due care being administered in the start-up plane and smooth operation of the winch controls we can generally confidently survey to -40 deg Dip. Although with the potential for more spin-outs, the servicing requirements increase due to the wearing of the gyro’s gimble housing. 

Gyro Repeatability is an important factor in determining the integrity of the borehole trajectory data being acquired. The manufacturers of the instrumentation generally quote the accuracy to be greater than 1% for each positional shot. The system takes a reference bearing with an inertial spin motor and transfers this information through a wireline system to determine its relative position at the ascribed depths by the operator. Therefore the surface orientation of the instrument is of paramount importance.

A basic view of the gyro mechanics.

A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. The device is a spinning wheel or disk whose axle is free to take any orientation. This orientation changes much less in response to a given external torque than it would without the large angular momentum associated with the gyroscope's high rate of spin. Since external torque is minimized by mounting the device in gimbals, its orientation remains nearly fixed, regardless of any motion of the platform on which it is mounted.

Accuracy
X-Y Inclinometers (XY) 0 to 90 degrees +/-.5 degrees
Gyro (Goodrich DG-69 or Welnav) 0 to 45 degrees from vertical +/-1 degrees