An antenna system comprising: an antenna; a vertical support assembly, a cross-level frame assembly, and an elevation frame assembly configured to movably support the antenna; an azimuth driver for rotating the vertical support assembly about an azimuth axis; a cross-level driver for pivoting the cross-level frame assembly relative to the vertical support assembly about a cross-level axis; an elevation driver for pivoting the elevation frame assembly and the antenna relative to the cross-level frame assembly about an elevation axis; a motion platform assembly affixed to and movable with the elevation frame assembly, wherein the motion platform assembly includes: a first angular rate sensor configured to sense motion about a first axis, a second angular rate sensor configured to sense motion about a second axis that is orthogonal to the first axis, a third angular rate sensor (60") configured to sense motion about a third axis that is orthogonal to the first axis and the second axis, and one or more accelerometers (65, 65'), wherein the one or more accelerometers are configured to determine an earth gravity vector; and a control unit that is communicatively connected to the first angular rate sensor, the second angular rate sensor, the third angular rate sensor, and the one or more accelerometers, wherein the control unit is configured to determine a location relative to the earth gravity vector of at least one of the first axis, the second axis, and the third axis. an antenna; a vertical support assembly, a cross-level frame assembly, and an elevation frame assembly configured to movably support the antenna; an azimuth driver for rotating the vertical support assembly about an azimuth axis; a cross-level driver for pivoting the cross-level frame assembly relative to the vertical support assembly about a cross-level axis; an elevation driver for pivoting the elevation frame assembly and the antenna relative to the cross-level frame assembly about an elevation axis; a motion platform assembly affixed to and movable with the elevation frame assembly, wherein the motion platform assembly includes: a first angular rate sensor configured to sense motion about a first axis, a second angular rate sensor configured to sense motion about a second axis that is orthogonal to the first axis, a third angular rate sensor (60") configured to sense motion about a third axis that is orthogonal to the first axis and the second axis, and one or more accelerometers (65, 65'), wherein the one or more accelerometers are configured to determine an earth gravity vector; and a first angular rate sensor configured to sense motion about a first axis, a second angular rate sensor configured to sense motion about a second axis that is orthogonal to the first axis, a third angular rate sensor (60") configured to sense motion about a third axis that is orthogonal to the first axis and the second axis, and one or more accelerometers (65, 65'), wherein the one or more accelerometers are configured to determine an earth gravity vector; and a control unit that is communicatively connected to the first angular rate sensor, the second angular rate sensor, the third angular rate sensor, and the one or more accelerometers, wherein the control unit is configured to determine a location relative to the earth gravity vector of at least one of the first axis, the second axis, and the third axis.