{
  "id": "sgp4-propagation",
  "title": "SGP4 propagation",
  "summary": "From a parsed element set to a state vector: element preparation, branch selection, and the propagation itself.",
  "nodes": [
    {
      "id": "start",
      "label": "TwoLineElement and GravityModel",
      "kind": "data",
      "group": "input",
      "summary": "Mean elements as parsed from the element set, plus the gravity model they were fitted against.",
      "api": "TwoLineElement",
      "label_zh": "TwoLineElement 与 GravityModel",
      "summary_zh": "按元素集解析得到的平均元素，以及其所拟合的引力模型。"
    },
    {
      "id": "prepare",
      "label": "Prepare mean elements",
      "kind": "step",
      "group": "prepare",
      "summary": "Convert degrees to radians and revolutions per day to radians per minute, and recover the un-Kozai mean motion.",
      "api": "Satellite",
      "label_zh": "准备平均元素",
      "summary_zh": "度转弧度、圈每天转弧度每分钟，并恢复去 Kozai 平均运动。"
    },
    {
      "id": "valid",
      "label": "Elements propagatable?",
      "kind": "decision",
      "group": "prepare",
      "summary": "The eccentricity must be below one, the mean motion positive, and the recovered semi-major axis physical.",
      "api": "validate",
      "label_zh": "元素可传播？",
      "summary_zh": "偏心率须小于一，平均运动须为正，且恢复出的半长轴须物理合理。"
    },
    {
      "id": "invalid",
      "label": "InvalidElements",
      "kind": "terminal",
      "group": "error",
      "summary": "The elements do not describe a satellite the model can propagate.",
      "api": "PropagationErrorKind",
      "summary_zh": "元素不描述该模型可传播的卫星。"
    },
    {
      "id": "branch",
      "label": "Period at or above 225 minutes?",
      "kind": "decision",
      "group": "branch",
      "summary": "Selects the deep space branch. The period follows from the mean motion.",
      "api": "is_deep_space",
      "label_zh": "周期大于等于 225 分钟？",
      "summary_zh": "选择深空分支。周期由平均运动给出。"
    },
    {
      "id": "sgp4",
      "label": "SGP4 secular terms",
      "kind": "step",
      "group": "propagate",
      "summary": "Apply the secular rates from the oblateness, and the drag decay of the semi-major axis and the eccentricity.",
      "api": "Satellite",
      "label_zh": "SGP4 长期项",
      "summary_zh": "施加扁率带来的长期变化率，以及阻力导致的半长轴与偏心率衰减。"
    },
    {
      "id": "sdp4",
      "label": "SDP4 deep space terms",
      "kind": "step",
      "group": "propagate",
      "summary": "Add the resonance with the Earth's gravity field and the long period luni-solar perturbations.",
      "api": "Satellite",
      "label_zh": "SDP4 深空项",
      "summary_zh": "加入与地球引力场的共振以及日月长周期摄动。"
    },
    {
      "id": "anomaly",
      "label": "Solve Kepler's equation",
      "kind": "step",
      "group": "propagate",
      "summary": "Recover the eccentric anomaly from the mean anomaly by iteration, then the true anomaly and the radius.",
      "api": "eccentric_from_mean",
      "label_zh": "求解开普勒方程",
      "summary_zh": "由平近点角迭代求出偏近点角，再求真近点角与半径。"
    },
    {
      "id": "position",
      "label": "Perifocal position",
      "kind": "step",
      "group": "propagate",
      "summary": "Evaluate the position and velocity in the orbital plane.",
      "api": "Satellite",
      "label_zh": "近焦点系位置",
      "summary_zh": "在轨道面内求位置与速度。"
    },
    {
      "id": "rotate",
      "label": "Rotate to TEME",
      "kind": "step",
      "group": "propagate",
      "summary": "Apply the argument of perigee, the inclination, and the right ascension of the ascending node.",
      "api": "State",
      "label_zh": "旋转到 TEME",
      "summary_zh": "按近地点幅角、倾角与升交点赤经依次旋转。"
    },
    {
      "id": "decayed",
      "label": "Radius physical?",
      "kind": "decision",
      "group": "propagate",
      "summary": "A radius below the surface means the drag term has brought the satellite down.",
      "api": "PropagationErrorKind",
      "label_zh": "半径合理？",
      "summary_zh": "半径低于地表意味着阻力项已把卫星带入大气。"
    },
    {
      "id": "decayed_end",
      "label": "Decayed",
      "kind": "terminal",
      "group": "error",
      "summary": "The satellite has decayed.",
      "api": "PropagationErrorKind",
      "label_zh": "已再入",
      "summary_zh": "卫星已再入。"
    },
    {
      "id": "state",
      "label": "State",
      "kind": "terminal",
      "group": "output",
      "summary": "Position in kilometers and velocity in kilometers per second in the True Equator Mean Equinox frame.",
      "api": "State",
      "summary_zh": "真赤道平春分点坐标系中的位置（千米）与速度（千米每秒）。"
    }
  ],
  "edges": [
    {
      "source": "start",
      "target": "prepare",
      "kind": "flow",
      "label": null
    },
    {
      "source": "prepare",
      "target": "valid",
      "kind": "flow",
      "label": null
    },
    {
      "source": "valid",
      "target": "invalid",
      "kind": "branch",
      "label": "no"
    },
    {
      "source": "valid",
      "target": "branch",
      "kind": "branch",
      "label": "yes"
    },
    {
      "source": "branch",
      "target": "sgp4",
      "kind": "branch",
      "label": "no"
    },
    {
      "source": "branch",
      "target": "sdp4",
      "kind": "branch",
      "label": "yes"
    },
    {
      "source": "sgp4",
      "target": "anomaly",
      "kind": "flow",
      "label": null
    },
    {
      "source": "sdp4",
      "target": "anomaly",
      "kind": "flow",
      "label": null
    },
    {
      "source": "anomaly",
      "target": "position",
      "kind": "flow",
      "label": null
    },
    {
      "source": "position",
      "target": "rotate",
      "kind": "flow",
      "label": null
    },
    {
      "source": "rotate",
      "target": "decayed",
      "kind": "flow",
      "label": null
    },
    {
      "source": "decayed",
      "target": "decayed_end",
      "kind": "branch",
      "label": "below the surface"
    },
    {
      "source": "decayed",
      "target": "state",
      "kind": "branch",
      "label": "valid"
    }
  ],
  "title_zh": "SGP4 传播",
  "summary_zh": "从解析后的元素集到状态矢量：元素准备、分支选择与传播本身。"
}