Fundamentals of Satellite Navigation - Understanding GPS: Principles and Applications - page 49

1. Coverage needs to be global.
2. At least six satellites need to be in view of any user position at all times.
3. To provide the best navigation accuracy, the constellation needs to have
good geometric properties, which entails a dispersion of satellites in both
azimuth and elevation angle from a user (a discussion of the effects of
geometric properties on navigation accuracy is provided in Sections 7.1 and
7.3).
4. The constellation needs to be robust against single satellite failures.
5. The constellation must be maintainable given the increased frequency of
satellite failures with a large constellation. That is, it must be relatively
inexpensive to reposition satellites within the constellation.
6. Stationkeeping requirements need to be manageable. In other words, it is
preferable to minimize the frequency and magnitude of maneuvers required
to maintain the satellites within the required range of their orbital

parameters.
7. There are tradeoffs between the distance of the satellite from the Earth's
surface versus payload weight, determined, in part, by the transmitter power
required to send a signal to Earth with minimum received power.

2.3.2.5 Selection of the GPS Constellation
The need for global coverage (1) and the need for good geometric diversity world-
wide (3) eliminate the use of geostationary satellites for navigation, though a con-
stellation of geosynchronous satellites with enough inclination could theoretically
be used to provide global coverage including the poles. Considerations weighing
against the use of an inclined GEO constellation to provide global coverage for nav-
igation include constraint (7) and the increased satellite power (and thus payload
weight) required from GEO to provide the necessary power flux density at the sur-
face of the Earth relative to satellites at lower altitudes and the regulatory coordina-
tion issues associated with GEO orbits. Thus, the constraint of global coverage (1)
plus practical considerations drive the satellite navigation constellation to inclined
LEO or MEO orbits.
Constraint (2) for minimum sixfold coverage, plus the need to minimize the size
of the constellation for cost reasons, drives the desired constellation to higher alti-
tude for satellite navigation. With satellites costing in the $20 million-$80 million
range, even for relatively small satellites such as GPS, the differences in constellation
size drive the desired altitude as high as possible. To first approximation, an order
of magnitude more satellites would be required to provide the necessary sixfold cov-
erage from LEO versus that with MEO. When launch costs are factored in, the over-
all cost differential between LEO and MEO is billions of dollars. Moreover,
constellations of LEO satellites tend to have worse geometric properties from a dilu-
tion of precision perspective than MEOs—consideration (3). With LEO and GEO
altitudes shown to be undesirable, MEO altitudes were determined to be preferable
for GPS.
Ultimately, inclined 12-hour orbits were selected for GPS as the best compro-
mise between coverage, dilution of precision characteristics, and cost. Some desir-
able characteristics of this orbital altitude include repeating ground tracks, a