Manufacturing of magnetometer units (20′) employs a test socket (41) having a substantially rigid body (43) with a cavity (42) therein holding an untested unit (20) in a predetermined position (48) proximate electrical connection (50) thereto, wherein one or more magnetic field sources (281, 332, 333, 334, 335, 336) fixed in the body (43) provide known magnetic fields at the position (48) so that the response of each unit (20) is measured and compared to stored expected values. Based thereon, each unit (20) can be calibrated or trimmed by feeding corrective electrical signals back to the unit (20) through the test socket (41) until the actual and expected responses match or the unit (200) is discarded as uncorrectable. In a preferred embodiment, the magnetic field sources (281, 332, 333, 334, 335, 336) are substantially orthogonal coil pairs (332, 333, 334) arranged so that their centerlines (332-1, 333-1, 334-1) coincide at a common point (46) within the predetermined position (48). Because the test-socket (41) is especially rugged and compact, other functions (e.g., accelerometers) included in the unit (20) can also be easily tested and trimmed.