A highly elliptical orbit has an extreme difference between its low perigee and very high apogee — often 500 km versus 40,000 km. Satellites crawl through the high segment for hours, so a HEO parks long dwell time over a chosen region; Molniya and Tundra orbits are the classic examples.
Understanding HEO
The physics of dwell
At a Molniya apogee the spacecraft moves around 1.5 km/s — versus roughly 10 km/s at perigee — so the six hours bracketing apogee cover only a small arc of sky as seen from the ground. From the service region the satellite appears to rise, hang almost stationary at high elevation for hours, and set; receivers can follow it with modest steering. The perigee pass on the far side of the planet is a brief sprint the mission simply writes off.
Operating in the belt-crossing environment
Twice per orbit a HEO satellite punches through the Van Allen radiation belts, accumulating dose that GEO and LEO missions largely avoid — driving hardened electronics and thicker shielding. Thermal design must handle swings between deep-space cold at apogee and rapid passes through Earth's shadow, and conjunction screening must handle an object whose track crosses every altitude shell between 500 and 40,000 km.