Fundamentals of Satellite Navigation - Understanding GPS: Principles and Applications - page 44

• Circular orbits have zero (or nearly zero) eccentricity.
• Highly elliptical orbits (HEO) have large eccentricities (typically with e

> 0.6).

Another categorization of orbits is by altitude:

• Geosynchronous Earth orbit (GEO) is an orbit with a period equal to the dura-
tion of the sidereal day—substituting P

= 23 hours, 56 minutes, 4.1 seconds

42,164.17 km as the orbital semimajor axis for GEO, or
an altitude of 35,786 km;
• LEO is a class of orbits with altitude typically less than 1,500 km;
• MEO is a class of orbits with altitudes below GEO and above LEO, with most
practical examples being in the range of roughly 10,000-25,000 km altitude;
• Supersynchronous orbits are those with altitude greater than GEO (greater
than 35,786 km).

into (2.12) yields a =

Note that GEO defines an orbital altitude such that the period of the orbit
equals the period of rotation of the Earth in inertial space (the sidereal day). A geo-
stationary orbit is a GEO orbit with zero inclination and zero eccentricity. In this
special case, a satellite in geostationary orbit has no apparent motion to an observer
on Earth, because the relative position vector from the observer to the satellite (in
ECEF coordinates) remains fixed over time. In practice, due to orbital perturba-
tions, satellites never stay in exactly geostationary orbit; therefore, even so-called
geostationary satellites have some small residual motion relative to users on the
Earth.
Another categorization of orbits is by inclination:

• Equatorial orbits have zero inclination; hence, a satellite in equatorial orbit
travels in the Earth's equatorial plane.
• Polar orbits have 90° inclination; hence, a satellite in polar orbit passes
through the Earth's axis of rotation.
• Prograde orbits have nonzero inclination with a value less than 90° (and hence
have ground tracks that go in general from west to east).
• Retrograde orbits have nonzero inclination with a value greater than 90° and
less than 180° (and hence have ground tracks that go in general from east to
west).
• Collectively, prograde and retrograde orbits are known as inclined.

Theoretical studies of satellite constellations typically focus on some particular
subset of orbital categories. For example, Walker extensively studied inclined circu-
lar orbits [16], Rider further studied inclined circular orbits to include both global
and zonal coverage [17], and Adams and Rider studied circular polar orbits [18].
These studies all focus on determining the set of orbits in their categories that require
the fewest satellites to provide a particular level of coverage (i.e., the number of sat-
ellites in view from some region on Earth). The studies determine optimal orbital
inclinations for their category of orbits and then determine the configuration of sat-