CesiumJS tests - https://sandcastle.cesium.com/?gist=40de61993218206f78fa0737e5d843c9

Cesium-Sandcastle.js

const viewer = new Cesium.Viewer("cesiumContainer", { shouldAnimate: true });
viewer.scene.globe.enableLighting = true;

// Distance threshold in meters for label visibility
const LABEL_VISIBILITY_DISTANCE = 5000000;

const updateLabelVisibility = () => {
  const cameraPosition = viewer.camera.position;
  const entities = viewer.entities.values;
  
  for (let i = 0; i < entities.length; i++) {
    const entity = entities[i];
    if (entity.label) {
      const entityPosition = entity.position.getValue();
      const distance = Cesium.Cartesian3.distance(cameraPosition, entityPosition);
      entity.label.show = distance < LABEL_VISIBILITY_DISTANCE;
    }
  }
};

viewer.camera.moveEnd.addEventListener(updateLabelVisibility);

Sandcastle.addDefaultToolbarButton("Ham Radio Bounces", () => {
  viewer.entities.removeAll();

  const warsaw = { callsign: "SN5H", lon: 21.0122, lat: 52.2297 };

  const contacts = [
    { callsign: "K1ABC", lat: 40.7128, lon: -74.0060, frequency: 14.070 },
    { callsign: "G4XYZ", lat: 51.5074, lon: -0.1278, frequency: 7.070 },
    { callsign: "JA1DEF", lat: 35.6895, lon: 139.6917, frequency: 14.250 },
    { callsign: "VK2HIJ", lat: -33.8688, lon: 151.2093, frequency: 7.250 },
    { callsign: "DL1KLM", lat: 52.5200, lon: 13.4050, frequency: 14.060 },
    { callsign: "F6NOP", lat: 48.8566, lon: 2.3522, frequency: 7.040 },
    { callsign: "SM0PQR", lat: 47.4979, lon: 19.0402, frequency: 14.320 },
    { callsign: "ZS6RST", lat: 30.0333, lon: 31.2333, frequency: 7.090 },
    { callsign: "W1STU", lat: 29.7604, lon: -95.3698, frequency: 14.070 },
    { callsign: "OE1XYZ", lat: 48.2082, lon: 16.3738, frequency: 7.050 },
    { callsign: "DL7ABC", lat: 48.1351, lon: 11.5810, frequency: 14.080 },
    { callsign: "F1DEF", lat: 45.7640, lon: 4.8357, frequency: 7.030 },
    { callsign: "LU1GHI", lat: 50.1109, lon: 8.6821, frequency: 14.065 },
    { callsign: "OH2JKL", lat: 37.9838, lon: 23.7275, frequency: 7.085 },
    { callsign: "PY3MNO", lat: 55.9533, lon: -3.1883, frequency: 14.320 },
    { callsign: "VE3ABC", lat: 43.6532, lon: -79.3832, frequency: 3.750 },
    { callsign: "JH1DEF", lat: 35.6762, lon: 139.6503, frequency: 21.250 },
    { callsign: "EA4GHI", lat: 40.4168, lon: -3.7038, frequency: 28.450 },
    { callsign: "ZL2JKL", lat: -41.2865, lon: 174.7762, frequency: 10.120 },
    { callsign: "PA0MNO", lat: 52.3676, lon: 4.9041, frequency: 1.850 },
    { callsign: "HB9PQR", lat: 47.3769, lon: 8.5417, frequency: 5.350 },
    { callsign: "9A1STU", lat: 45.8150, lon: 15.9819, frequency: 21.300 },
    { callsign: "YB3VWX", lat: -6.2088, lon: 106.8456, frequency: 28.500 }
  ];

  const bands = [
    { label: "160m", altitude: 200000, color: Cesium.Color.PURPLE, freqRange: { min: 1.8, max: 2.0 } },
    { label: "80m",  altitude: 225000, color: Cesium.Color.BLUE, freqRange: { min: 3.5, max: 4.0 } },
    { label: "60m",  altitude: 250000, color: Cesium.Color.GREEN, freqRange: { min: 5.3, max: 5.4 } },
    { label: "40m",  altitude: 275000, color: Cesium.Color.YELLOW, freqRange: { min: 7.0, max: 7.3 } },
    { label: "30m",  altitude: 300000, color: Cesium.Color.ORANGE, freqRange: { min: 10.1, max: 10.15 } },
    { label: "20m",  altitude: 325000, color: Cesium.Color.RED, freqRange: { min: 14.0, max: 14.35 } },
    { label: "15m",  altitude: 350000, color: Cesium.Color.CYAN, freqRange: { min: 21.0, max: 21.45 } },
    { label: "10m",  altitude: 375000, color: Cesium.Color.MAGENTA, freqRange: { min: 28.0, max: 29.7 } },
  ];

  const earthRadius = Cesium.Ellipsoid.WGS84.maximumRadius;
  const addEntity = (entity) => { viewer.entities.add(entity); };

  addEntity({
    name: warsaw.callsign,
    position: Cesium.Cartesian3.fromDegrees(warsaw.lon, warsaw.lat),
    point: { pixelSize: 12, color: Cesium.Color.WHITE },
    label: {
      text: warsaw.callsign,
      font: "14pt sans-serif",
      fillColor: Cesium.Color.WHITE,
      verticalOrigin: Cesium.VerticalOrigin.BOTTOM,
      pixelOffset: new Cesium.Cartesian2(0, -14),
    },
  });

  for (const contact of contacts) {
    addEntity({
      name: `${contact.callsign} (${contact.frequency} MHz)`,
      position: Cesium.Cartesian3.fromDegrees(contact.lon, contact.lat),
      point: { pixelSize: 10, color: Cesium.Color.GRAY },
      label: {
        show: false,
        text: `${contact.callsign}`,
        font: "12pt sans-serif",
        fillColor: Cesium.Color.WHITE,
        verticalOrigin: Cesium.VerticalOrigin.BOTTOM,
        pixelOffset: new Cesium.Cartesian2(0, -12),
      },
    });
  }

  const groundPoint = (lon, lat, fraction, endLon, endLat) => {
    const startCart = Cesium.Cartographic.fromDegrees(lon, lat, 0);
    const endCart = Cesium.Cartographic.fromDegrees(endLon, endLat, 0);
    const geodesic = new Cesium.EllipsoidGeodesic(startCart, endCart);
    const pointCart = geodesic.interpolateUsingFraction(fraction);
    pointCart.height = 0;
    return Cesium.Cartesian3.fromRadians(pointCart.longitude, pointCart.latitude, pointCart.height);
  };

  for (const contact of contacts) {
    const band = bands.find(b => {
      const freq = contact.frequency;
      return freq >= b.freqRange.min && freq <= b.freqRange.max;
    });

    if (band) {
      const startCart = Cesium.Cartographic.fromDegrees(warsaw.lon, warsaw.lat, 0);
      const endCart = Cesium.Cartographic.fromDegrees(contact.lon, contact.lat, 0);
      const fullGeo = new Cesium.EllipsoidGeodesic(startCart, endCart);
      const totalDistance = fullGeo.surfaceDistance;
      const maxHopDistance = 2 * Math.sqrt(2 * earthRadius * band.altitude);
      const hops = Math.max(1, Math.ceil(totalDistance / maxHopDistance));

      const positions = [];
      for (let i = 0; i < hops; i++) {
        const p0 = groundPoint(warsaw.lon, warsaw.lat, i / hops, contact.lon, contact.lat);
        const p1 = groundPoint(warsaw.lon, warsaw.lat, (i + 1) / hops, contact.lon, contact.lat);
        if (i === 0) positions.push(p0);
        const hopStartCart = Cesium.Cartographic.fromDegrees(
          Cesium.Math.toDegrees(Cesium.Cartographic.fromCartesian(p0).longitude),
          Cesium.Math.toDegrees(Cesium.Cartographic.fromCartesian(p0).latitude),
          0
        );
        const hopEndCart = Cesium.Cartographic.fromDegrees(
          Cesium.Math.toDegrees(Cesium.Cartographic.fromCartesian(p1).longitude),
          Cesium.Math.toDegrees(Cesium.Cartographic.fromCartesian(p1).latitude),
          0
        );
        const hopGeo = new Cesium.EllipsoidGeodesic(hopStartCart, hopEndCart);
        const midCart = hopGeo.interpolateUsingFraction(0.5);
        midCart.height = band.altitude;
        const bounce = Cesium.Cartesian3.fromRadians(midCart.longitude, midCart.latitude, midCart.height);
        positions.push(bounce, p1);
      }

      addEntity({
        name: `${warsaw.callsign} → ${contact.callsign} (${band.label}, ${hops} bounce${hops > 1 ? "s" : ""})`,
        polyline: {
          arcType: Cesium.ArcType.NONE,
          positions,
          width: 2,
          material: band.color,
        },
      });
    }
  }

  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(warsaw.lon, warsaw.lat, 20000000),
    duration: 7,
  });
});

Sandcastle.reset = () => viewer.entities.removeAll();