rcRing = { lat: startLat, lng: startLng };
      setRingsData(curRingsData => [...curRingsData, srcRing]);
      setTimeout(() => setRingsData(curRingsData => curRingsData.filter(r => r !== srcRing)), FLIGHT_TIME * ARC_REL_LEN);

      // add and remove target rings
      setTimeout(() => {
        const targetRing = { lat: endLat, lng: endLng };
        setRingsData(curRingsData => [...curRingsData, targetRing]);
        setTimeout(() => setRingsData(curRingsData => curRingsData.filter(r => r !== targetRin