Medium-earth-orbit satellites

Some satellites revolve in the orbits higher than those normally considered low earth, but at altitudes lower than geostationary level of 22,300 miles. These intermediate birds are called as medium-earth-orbit (MEO) satellites. A MEO satellite takes several hours to complete each orbit. MEO satellites operate in fleets, similar to the way LEO satellites are deployed. As the average MEO altitude is higher than the average LEO altitude, each bird can cover a larger region on surface at any given time. A fleet of MEO satellites can be smaller than a comparable fleet of LEO satellites, and still provide continuous, worldwide communications.

The orbits of GEO satellites are essentially perfect circles, and most LEO satellites orbit in perfect circles. But MEO satellites often have elongated, or elliptical, orbits. The point of lowest altitude is called as perigee; the point of greatest altitude is called as apogee. The apogee can be, much greater than the perigee. Such a satellite orbits at a speed which depends on its altitude. The lower the altitude, the faster the satellite moves. A satellite  with the elliptical  orbit crosses sky rapidly when it is near perigee, and slowly when it is near the apogee; it is simple to use when its apogee is high above the horizon, as then it stays in visible sky for long time.
Every time a MEO satellite completes one orbit, earth rotates beneath it. The rotation of earth need not, and usually does not, correspond to orbital period of the satellite. Thus, successive apogees for a MEO satellite occur over different points on the earth’s surface. This makes the tracking of individual satellites a complicated business, needs computers programmed with accurate orbital data. For a MEO system to be effective in offering worldwide coverage without localized periodic blackouts, the orbits should be diverse, yet coordinated in a precise and predictable way. In addition, there should be enough satellites so that each point on earth is always on a line of sight with one or more satellites, and preferably, there should be at least one bird in sight near apogee all times.