A magnetic screening system uses directional gradiometers with high resolution and accuracy to measure magnetic field signatures of target objects (e.g., gun, knife, cell phone, keys) in a volume of interest. The measured signatures can be compared to signatures of known objects stored in a local database. Various mathematical processes may be used to identify or classify target object signatures. The magnetic screening system can operate in multiple modes, such as a tracking mode, measurement mode, calibration mode, and self-test mode. Calibration may be performed in substantially continous manner to allow calibration parameters to be adjusted during system operation (e.g., tracking mode) to compensate for changes in conditions that may impact calibration parameters. Through use of unique processes and designs, the magnetic screening system can achieve a high rate of processing persons for target objects.