一、双行根数TLE下载

网上有一些下载最新双行根数的网站，例如：

• http://celestrak.com/ 无需注册可直接下载
• https://space-track.org/ 需要注册才能下载

二、利用STK Components做预报

1. 新建控制台项目，并添加对AGI.Foundation.Core.dll和AGI.Foundation.Models.dll的引用
2. 完整代码如下

using System;
using AGI.Foundation;
using AGI.Foundation.Celestial;
using AGI.Foundation.Coordinates;
using AGI.Foundation.Geometry;
using AGI.Foundation.Propagators;
using AGI.Foundation.Time;

namespace Example005
{
    class Program
    {
        static void Main(string[] args)
        {
            string license = "....需要有效的lic....";
            Licensing.ActivateLicense(license);

            // 定义需预报的时间区间
            JulianDate start = new JulianDate(new DateTime(2020, 8, 13, 0, 0, 0));
            JulianDate stop = new JulianDate(new DateTime(2020, 8, 13, 0, 10, 0));

            // 定义1分钟间隔
            var oneMinute = new Duration(0, 60);

            // 给定卫星根数
            string line1 = "1 25544U 98067A   20226.06311231  .00000634  00000-0  19556-4 0  9992";
            string line2 = "2 25544  51.6462  66.9823 0001637  29.7739 108.2756 15.49160058240839";

            // 创建卫星的运动点
            var sgp4 = new Sgp4Propagator(new TwoLineElementSet(line1 + "\n" + line2));
            PropagatorPoint satPoint = sgp4.CreatePoint();

            // 直接预报TLE坐标系下的位置和速度
            // Evaluate的order参数，0阶只求位置，1阶求位置和速度，2阶求位置、速度和加速度
            DateMotionCollection<Cartesian> posList = satPoint.GetEvaluator().Evaluate(start, stop, oneMinute, 1);
            WritePosList("TLE坐标系", posList);

            // 获取地球
            EarthCentralBody earth = CentralBodiesFacet.GetFromContext().Earth;

            // 预报地固系下的位置和速度
            PointEvaluator satPointInFixed = GeometryTransformer.ObservePoint(satPoint, earth.FixedFrame);
            DateMotionCollection<Cartesian> posListInFixed = satPointInFixed.Evaluate(start, stop, oneMinute, 1);
            WritePosList("地固坐标系", posListInFixed);

            // 预报惯性坐标系下的位置和速度
            PointEvaluator satPointInIcrf = GeometryTransformer.ObservePoint(satPoint, earth.InternationalCelestialReferenceFrame);
            DateMotionCollection<Cartesian> posListInIcrf = satPointInIcrf.Evaluate(start, stop, oneMinute, 1);
            WritePosList("惯性坐标系", posListInIcrf);

            // 角度转弧度
            double d2r = Math.PI / 180;

            // 预报相对于测站的方位、仰角和距离
            // 先定义测站位置
            var stationPoint = new PointCartographic(earth, new Cartographic(120 * d2r, 30 * d2r, 100));
            // 定义测站坐标轴和坐标系，xyz三轴为北东下右手坐标系
            var stationAxes = new AxesNorthEastDown(earth, stationPoint);
            var stationReferenceFrame = new ReferenceFrame(stationPoint, stationAxes);
            // 在测站坐标系下观察卫星点
            PointEvaluator satPointInStation = GeometryTransformer.ObservePoint(satPoint, stationReferenceFrame);
            DateMotionCollection<Cartesian> posListInStation = satPointInStation.Evaluate(start, stop, oneMinute, 0);
            // 卫星在测站坐标系下的位置值转换为方位、仰角和斜距
            Console.WriteLine("测站坐标系");
            for (int i = 0; i < posListInStation.Count; i++)
            {
                JulianDate time = posListInStation.Dates[i];
                Cartesian pos = posListInStation.Values[i];
                var aer = new AzimuthElevationRange(pos);
                Console.WriteLine("{0:yyyy-MM-dd HH:mm:ss}  {1,9:0.0000} {2,9:0.0000} {3,14:0.000}",
                  time.ToDateTime(), aer.Azimuth / d2r, aer.Elevation / d2r, aer.Range / 1000);
            }

            Console.ReadKey();
        }

        /// <summary>
        /// 屏幕输出时间、位置和速度
        /// </summary>
        /// <param name="posList"></param>
        static void WritePosList(string header, DateMotionCollection<Cartesian> posList)
        {
            Console.WriteLine(header);
            for (int i = 0; i < posList.Count; i++)
            {
                JulianDate time = posList.Dates[i];
                Cartesian pos = posList.Values[i];            // Values中存储的是位置
                Cartesian vel = posList.FirstDerivatives[i];  // FirstDerivatives中存储的是速度
                Console.WriteLine("{0:yyyy-MM-dd HH:mm:ss}  {1,9:0.000}  {2,9:0.000}  {3,9:0.000}  {4,9:0.000}  {5,9:0.000}  {6,9:0.000}",
                    time.ToDateTime(), pos.X / 1000, pos.Y / 1000, pos.Z / 1000, vel.X / 1000, vel.Y / 1000, vel.Z / 1000);
            }

            Console.WriteLine();
        }
    }
}