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We are developing a Bell-Bloom magnetometer based on pulses of optical pump light approximately resonant with the D1 optical transition in 87Rb, capable of operating in Earth-field-scale magnetic fields in an unshielded environment and exhibiting noise levels consistent with biomagnetic measurement capabilities (eg magnetocardiography and magnetoencephalography). The optical pump pulse repetition rate is adjusted to be on resonance with the Larmor frequency of the precessing spins by continuously monitoring the phase response of the spins relative to the optical pump pulses. When tested in a magnetically shielded environment, the single-channel demonstrated noise floor is approximately 10 fT/rtHz, and a differential pair of sensitive elements separated by a distance of approximately 9 cm demonstrated differential noise of approximately 1 fT/cm/rtHz. With closed-loop feedback and oscillating …