Global Positioning System Reference
In-Depth Information
available. Since the satellites always appear to be slowly moving across the sky, a
location may work at one time but not at another. The signals are quite weak
(when compared with other domestic radio systems), so good quality antennae
management is needed for reliable use.
People take the availability of the service for granted but it has a number of
potential problems. First, it was built and is paid for by the U.S. Department of
Defense and is under the political management of the U.S. government. The initial
decision to open GPS in the early 1980s for nonmilitary use was taken after an
international Cold War incident when an airliner in the wrong place was shot
down. It was felt that it was wrong to deprive basic GPS when perhaps people's
lives could be saved by better civilian navigation systems. After the end of the
Cold War in 2000, the system was improved for public use by the removal of an
artificial degradation (called selective availability, or SA), which improved
performance by an order of magnitude. At any time, however, the system could
theoretically be either degraded or denied, especially in times of crisis or in
regions with conflicts.
Second, aircraft may not rely on a single GNSS because of the inherent
unreliability of a single complex navigational system (three would be needed so
that if one failed the other two could check each other's outputs). Such a situation
could occur if there was a catastrophic fault, for example, with a number of
satellites due to natural cosmic disruptions, or perhaps acts of space warfare, or
with the ground station's failure (without which the satellite's accuracy would
soon drift).
Finally, GNSS are radio systems are prone to all the problems inherent with
all radio systems. Perhaps the biggest threat is that of service denial by
interference that may be deliberate. GPS and similar systems use wideband
spread-spectrum broadcasts and are more difficult to jam than conventional
narrowband domestic radio systems, but radio jamming may become a problem
similar to computer hacking, especially as jamming equipment becomes more
sophisticated. To sum up, by all means use and benefit from GPS but keep
mindful of what is behind the scenes, politically, economically, and
technologically, because each area has many potential problems that could disrupt
GPS in the future. Assuming, therefore, that a user has access to four viable
satellites, how does the position get fixed?
