Invisible Stars - GPS Declassified: From Smart Bombs to Smartphones - page 75

ground technicians to maneuver new ones into designated “slots” along an
orbital path, keep them aligned during use, and boost “decommissioned” sat-
ellites into higher orbits—the space-age equivalent of putting them out to pas-
ture. Some of these “residuals” remain available for reactivation, if needed. 15
Technicians also keep each satellite's eight spiky antennas, called a helix array,
properly aimed toward the earth to transmit signals.
As the number of satellites has increased and each generation's capabilities
have improved, the system's ground control segment also has expanded and
evolved. Since 1986, the Air Force's 2nd Space Operations Squadron (2sops)
has managed the constellation from the master control station at Schriever
(formerly Falcon) Air Force Base in Colorado. 16 An alternate master control
station at Vandenberg Air Force Base in California is “functionally identical”
and capable of assuming indefinite control of the constellation during any
downtime at the master station. 17 A half-dozen original monitoring sites scat-
tered around the world have grown to sixteen, and there are a dozen command
and control antennas. 18 Integrating ten National Geospatial-Intelligence Agency
ground stations into the control segment (a project begun in 2005 and com-
pleted in 2008) increased accuracy as much as 15 percent by placing each sat-
ellite under continuous active monitoring by three ground stations, whereas
the satellites previously were unmonitored for portions of their orbits. 19 Con-
tinuous monitoring also enhanced security, offering faster detection of hostile
attempts to disrupt the constellation. 20 Through this network of receivers and
antennas, 2sops technicians monitor each satellite's health, compile and uplink
almanac data containing details about each satellite's orbit (they wobble and
vary slightly), and keep all operational clocks synchronized. Crews at the mas-
ter control station generate an alert whenever any planned or unplanned out-
age or operational issue might affect signals. The Coast Guard Navigation
Center website posts each alert as a Notice Advisory to Navstar Users (nanu),
and users can sign up for automatic alerts. In 2007 the control segment's main-
frame computer system, based on 1970s technology, was replaced with a mod-
ern information technology architecture. Upgrades to the operational control
system continue. 21
The Invisible (and Inaudible) Part
Unlike the Internet, with which users must connect to a server to download or
upload information, gps is a “passive” system in which users do not interact
with the satellites or the ground stations. That means an unlimited number of