Abstract: A method for magnetic ranging includes switching an electromagnet deployed in a target wellbore between at least first and second states and acquiring a plurality of magnetic field measurements at a magnetic field sensor deployed on a drill string in a drilling wellbore while the electromagnet is switching. The magnetic field measurements may be sorted into at least first and second sets corresponding to the first and second states of the electromagnet. The first and second sets of magnetic field measurements are then processed to compute at least one of a distance and a direction from the drilling well to the target. The electromagnet may be automatically switched back and forth between the first and second states independently from the acquiring and sorting of the magnetic field measurements.