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校正的前提是獲取相機姿態，所以慣性測量元件必不可少，但是單一的慣性測量元件無法給出準確的姿態角，需要對多個慣性測量元件的數據進行數據融合。使用比較普遍的慣性測量元件的組合是加速度計和陀螺儀。它們輸出的數據各有優缺點，可以使用Kalman濾波對其進行數據融合以得到準確的姿態角[4-5]。
現有的實現Kalman濾波的方式主要有兩種：PC機和DSP，兩者都存在著系統性能和成本不能兼顧的問題。而FPGA卻可以很好地解決這個問題。本文主要講述的便是用FPGA實現Kalman濾波來獲取準確的相機姿態。
1 坐標模型
在進行航拍作業時，相機搭載在云臺上，云臺和飛機是一體的，當相機拍攝地面照片時，相機拍攝方向應與地面垂直，如圖1所示。
為獲取相機姿態，首先以相機為原點，建立坐標系，如圖2所示。其中，XY平面與地面平行，Z軸垂直于地面，Y軸方向為飛機飛行方向。
將加速度計固定在相機上，當相機姿態發生改變時，加速度的3個軸便可以輸出在其相應方向產生的重力分量信號，如圖2中的Gx，Gy，Gz所示。陀螺儀是一種角速度傳感器，它可以輸出對應軸上的瞬時角速度，如圖2中的Wx，Wy，Wz所示。
根據以上坐標系，可以得到姿態角θx，θy，θz（分別代表著俯仰角、橫滾角、偏航角），如式1所示，其中ΔT為采樣時間，Δθ為采樣時間內的姿態角變化。
可見，由加速度計或者陀螺儀都可以初步得到姿態角，但是無論是加速度計還是陀螺儀，它們輸出的原始數據都存在著固有缺陷：當相機處于變速運動狀態時，加速度計的輸出并不是重力加速度，而是重力加速度和其自身加速度的矢量和。陀螺儀是通過對角速度積分得到姿態角，隨著時間的推移，姿態角的誤差會越來越大。所以單一的加速度計或者陀螺儀都無法獲得正確的姿態角，必須對兩者進行數據融合。
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