人造地球卫星在地面的投影点（或卫星和地心连线与地面的交点）称星下点，用地理经、纬度表示。卫星运动和地球自转使星下点在地球表面移动，形成星下点轨迹。
轨道维持的任务是将卫星的星下点轨迹保持在设计的参考轨迹附近。
The ground tracks of satellites required for mission should be maintained close to those of reference orbits over the lifetime of satellites.
阐述了基本轨道应用分析理论，包括星下点轨迹计算、可见性分析、覆盖评估和轨道寿命估算。
Expatiated on the application theory of basic orbit, including subastral point calculation, visibility analysis, coverage evaluating and spacecraft life-span estimate.
以轨道半径和轨道倾角为未知量依据星下点轨迹要求条件构建了非线性方程组，但直接求解过于复杂，采取迭代的方法解决。
Non-linear equations are(constructed) with the restriction of the satellite ground trace to resolve the unknown semi-axis and inclination, but it's too complicated to be resolved directly.
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