A method for onboard determination of a roll angle of a projectile. The method including: transmitting a polarized RF signal from a reference source, with a predetermined polarization plane; receiving the signal at a pair of polarized RF sensor cavities positioned symmetrical on the projectile with respect to a reference roll position on the projectile; measuring a difference between an output of the pair of polarized RF sensor cavities resulting from the received signal to determine zero output roll positions of the projectile; and comparing an output of the pair of polarized RF sensor cavities at each of the zero output positions to determine when the projectile is parallel to the predetermined polarization plane. The method can also include analyzing an output of at least one third sensor positioned on the projectile to determine whether the roll angle position of the projectile is up as compared to the horizon.