MWD. A wireless system for making downhole measurements of azimuth, inclination, and tool facing from sensors mounted inside of a nonmagnetic drill collar located near the drill bit. Formation properties can be measured, such as: natural radioactivity, resistivity, and temperature. These measurements are sent to the surface by means of pressure pulses induced in the drilling mud by a mud-pulse transmitter, or stored downhole and brought to the surface when the bit and MWD collar are brought out of the hole.