A fiber-optic gyroscope system (10, 52) comprising: an optical fiber coil comprising an optical fiber wound around a spool of a FOG (18, 64) in a first orientation and a second orientation opposite the first orientation, the optical fiber comprising a first input/output (20, 66) and a second input/output (22, 68); and an optical beam controller (16, 58) comprising an optical switch (28, 62) configured to provide a first optical beam (OPT 1, OPT A) to the first input/output (20, 66) and a second optical beam (OPT 2, OPT B) to the second input/output (22, 68) during a first switching state and to provide the first optical beam (OPT 1, OPT A) to the second input/output (22, 68) and the second optical beam (OPT 2, OPT B) to the first input/output (20, 66) during a second switching state; characterized in that it further comprises: a controller (26, 72) configured to mitigate bias error in a determination of rotation of the FOG (18, 64) about a sensitive axis by: measuring the rotation of the FOG (18, 64) about the sensitive axis in the first switching state with a first scale factor; measuring the rotation of the FOG (18, 64) 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 of the FOG (18, 64) in the first and second switching states; and subtracting the estimated bias error from the determined rotation in a feedback manner. an optical fiber coil comprising an optical fiber wound around a spool of a FOG (18, 64) in a first orientation and a second orientation opposite the first orientation, the optical fiber comprising a first input/output (20, 66) and a second input/output (22, 68); and an optical beam controller (16, 58) comprising an optical switch (28, 62) configured to provide a first optical beam (OPT 1, OPT A) to the first input/output (20, 66) and a second optical beam (OPT 2, OPT B) to the second input/output (22, 68) during a first switching state and to provide the first optical beam (OPT 1, OPT A) to the second input/output (22, 68) and the second optical beam (OPT 2, OPT B) to the first input/output (20, 66) during a second switching state; characterized in that it further comprises: a controller (26, 72) configured to mitigate bias error in a determination of rotation of the FOG (18, 64) about a sensitive axis by: measuring the rotation of the FOG (18, 64) about the sensitive axis in the first switching state with a first scale factor; measuring the rotation of the FOG (18, 64) 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 of the FOG (18, 64) in the first and second switching states; and subtracting the estimated bias error from the determined rotation in a feedback manner. measuring the rotation of the FOG (18, 64) about the sensitive axis in the first switching state with a first scale factor; measuring the rotation of the FOG (18, 64) 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 of the FOG (18, 64) in the first and second switching states; and subtracting the estimated bias error from the determined rotation in a feedback manner.