If GPS had been built for civilian instead of military use, it might have evolved as a regional U.S. system served by geosynchronous satellites, which would provide local coverage at a lower cost but would not support the nation's global defense capabilities. In actuality, the 24 primary and spare satellites orbit Earth at an altitude of 20,000 kilometers, circling the planet twice a day with precisely repeating ground tracks.
The altitude of the GPS constellation was influenced by two primary criteria: the need to support a system for detecting nuclear detonations, and the need to permit early testing of a small constellation with minimal risk. These criteria favored a semisynchronous orbit—but these criteria are no longer the primary drivers of constellation management.
Aerospace has been conducting studies to determine whether a new altitude would be preferable, but these studies consistently show a loss of efficiency in transitioning to a geosynchronous (or higher) orbit; from a cost-benefit perspective, the current altitude is still the most efficient.
One altitude change that may be beneficial, however, is a small boost that will allow the orbits to migrate from their repeating ground tracks. Currently, the ground track of each satellite crosses the equator at precisely the same longitude. As a result, each satellite experiences the same gravitational effects day after day. Thanks to the longitudinal variations in Earth's gravitational field, these gravitational effects eventually cause some satellites to speed up or slow down relative to each other. To preserve the beneficial satellite-to-user geometries, this acceleration or deceleration must be corrected every year or so with station-keeping maneuvers—thrusts that counteract the gravity-induced motion. This thrusting action expends fuel and induces error into the satellite position that must be corrected over time by reestimating trajectories through postmaneuver monitoring and site observations. The satellite is marked unhealthy during this period and does not contribute to a user's navigation calculations. A small altitude change would move the satellites off their repeating ground tracks so that they would all experience the same cumulative gravitational field effects over time, eliminating the need for station-keeping maneuvers.