a modular system for packaging of sensors and related electronics for an mwd system . a drill collar housing is provided with one or more cavities for receiving sensor modules that are adapted to sense one or more wellbore conditions . the sensor modules are removable and replaceable so that a desired sensor package may be installed within the drill collar housing . the drill collar housing is installed within the drill string , and a desired sensor module or modules are secured within the cavity of the drill collar housing . replacement or repair of the sensor portions requires only that the module or modules be removed from the cavity . the drill collar housing need not be removed from the drill string . the replaceable sensor modules may be interchangeably used in drill collar housings of different sizes without resulting in a degradation of sensed information .