Active magnetic ranging while drilling

A system for drilling first and second boreholes in a formation, the system comprising:

• a first drilling assembly in the first borehole, the first drilling assembly including a first drill string, a first drill bit and a first bottomhole assembly (BHA) connected to the first drill bit, the first BHA including a first measurement-while-drilling (MWD) system, a first bi-directional MWD telemetry interface, a first steering component, and a first magnetic field source comprising at least one permanent magnet having a north-south axis perpendicular to the longitudinal axis of the first BHA, so as to create an elliptically polarized magnetic field during longitudinal rotation of the first BHA;
• and
• a second drilling assembly in the second borehole, the second drilling assembly including a second drill string, a second drill bit and a second BHA connected to the second drill bit, the second BHA including a second MWD system, a second bi-directional MWD telemetry interface, a second steering component, and at least one ranging magnetometer incorporated into the BHA, wherein the first ranging magnetometer is configured to collect ranging measurements of the elliptically polarized magnetic field generated in the first drilling assembly from a location behind the second drill bit;
• wherein the second BHA further includes a second permanent magnet having a north-south axis perpendicular to the longitudinal axis of the second BHA, and wherein the first BHA further includes a second ranging magnetometer incorporated into the first BHA, wherein the second ranging magnetometer is configured to collect ranging measurements of the elliptically polarized magnetic field generated in the second drilling assembly from behind the first drill bit.