What is a Highly Elliptically Orbit?

A Highly Elliptical Orbit (HEO) is a highly eccentrical orbit with a low perigee (the point of orbit closest to the Earth) altitude of under 1,000 km and a high apogee (the point farthest from the earth) altitude of over 35,756 km. The "Highly Elliptical" term refers to the shape of the ellipse and the eccentricity of the orbit. A Highly Eleptical Orbit follows the curve of an ellipse. Examples of inclined HEO orbits include Molniya orbits and Tundra orbits.

One of the key features of an elliptical orbit is that the satellite in this type of orbit moves much faster when it is close to the earth than when it is farther away. This is because when the satellite is in perigee (the point of orbit closest to the Earth), the gravitational pull from the earth is high as compared to when the satellite is in apogee (the point farthest from the earth). As a result of this, when in the apogee, satellites in an HEO are able to provide better coverage over any point on the earth, as it is visible for a longer period of time. However when it is near the perigee, the Satellite passes over any point of interest very fast so is not always useful.

To solve this problem, two satellites on similar orbits are timed in such a way that one of them is always over the desired coverage area at all times (as shown in image 2). Also, if several satellites are placed in the same orbit having equal space, permanent coverage can be achieved.