基于Powell法的GNSS卫星轨道拟合算法

针对GNSS导航信号模拟源中卫星轨道计算的高精度实时性要求,提出了一种基于Powell最优化理论的卫星轨道拟合算法。该算法将有限点卫星位置拟合问题转化为无约束极小值问题,使用最优化理论求解卫星轨道模型参数,从而可以方便计算任意时刻的卫星速度、加速度等高阶量。算例结果表明计算卫星星历时,位置误差小于1×10-4m,速度误差小于1×10-6m/s,计算量为广播星历直接计算的1/3;计算精密星历时,位置精度在2cm左右,拟合精度较拉格朗日插值算法提高了大约1倍。通过实际应用,充分验证了算法的有效性。     

Pragmatism over principle: US intervention and burden shifting in Somalia, 1992–1993

ABSTRACT

The conventional wisdom about the 1992 US intervention in Somalia is that it was a quintessentially humanitarian mission pushed by President George H. W. Bush. This article challenges that interpretation, drawing on newly declassified documents. The Somalia intervention, I argue, was largely a pragmatic response to concerns held by the US military. In late 1992, as the small UN mission in Somalia was collapsing, senior American generals worried about being drawn into the resulting vacuum. Hence they reluctantly recommended a robust US intervention, in the expectation that this would allow the UN to assemble a larger peacekeeping force that would take over within months. The intervention ultimately failed, but the military learned useful lessons from this experience on how to achieve smoother UN handoffs in the future and thus effectively shift longer-term stabilisation burdens to the international community.