A fiber-optic gyroscope (FOG) system comprising: an optical fiber coil comprising an optical fiber wound around a spool of a FOG in a first orientation and a second orientation opposite the first orientation, the optical fiber comprising a first input/output and a second input/output ; and an optical beam controller comprising an optical switch configured to provide a first optical beam (OPT 1 , OPT A ) to the first input/output and a second optical beam (OPT 2 , OPT B ) to the second input/output during a first switching state and to provide the first optical beam (OPT 1 , OPT A ) to the second input/output and the second optical beam (OPT 2 , OPT B ) to the first input/output during a second switching state; characterized in that it further comprises: a controller configured to mitigate bias error in a determination of rotation (ROT) of the FOG about a sensitive axis by: measuring the rotation (ROT) of the FOG about the sensitive axis in the first switching state with a first scale factor; measuring the rotation (ROT) of the FOG about the sensitive axis in the second switching state with a second scale factor, wherein the first and second scale factors are equal and opposite to each other; estimating the bias error based on the measured rotation (ROT) of the FOG in the first and second switching states; and subtracting the estimated bias error from the determined rotation (ROT) in a feedback manner. an optical fiber coil comprising an optical fiber wound around a spool of a FOG in a first orientation and a second orientation opposite the first orientation, the optical fiber comprising a first input/output and a second input/output ; and an optical beam controller comprising an optical switch configured to provide a first optical beam (OPT 1 , OPT A ) to the first input/output and a second optical beam (OPT 2 , OPT B ) to the second input/output during a first switching state and to provide the first optical beam (OPT 1 , OPT A ) to the second input/output and the second optical beam (OPT 2 , OPT B ) to the first input/output during a second switching state; characterized in that it further comprises: a controller configured to mitigate bias error in a determination of rotation (ROT) of the FOG about a sensitive axis by: measuring the rotation (ROT) of the FOG about the sensitive axis in the first switching state with a first scale factor; measuring the rotation (ROT) of the FOG about the sensitive axis in the second switching state with a second scale factor, wherein the first and second scale factors are equal and opposite to each other; estimating the bias error based on the measured rotation (ROT) of the FOG in the first and second switching states; and subtracting the estimated bias error from the determined rotation (ROT) in a feedback manner. measuring the rotation (ROT) of the FOG about the sensitive axis in the first switching state with a first scale factor; measuring the rotation (ROT) of the FOG about the sensitive axis in the second switching state with a second scale factor, wherein the first and second scale factors are equal and opposite to each other; estimating the bias error based on the measured rotation (ROT) of the FOG in the first and second switching states; and subtracting the estimated bias error from the determined rotation (ROT) in a feedback manner. <EOS>