Data Hacking

304 Appendix A — GPS Primer
Several incarnations of GPS satellites have been put into orbit. The first set, called Block I,
were launched between 1978 and 1985, none of which are now operational. Replacements for
these were called the Block II and Block IIA. Additional replacements are called Block IIR,
and the latest satellites are called IIF.
The 27 satellites currently in use are a combination of Block II, Block IIA, Block IIR, and
Block IIF satellites.
The satellites were built by a variety of U.S. defense contractors:
Block II/IIA: Rockwell International (Boeing North American)
Block IIR: Lockheed Martin
Block IIF: Boeing North American
The orbital period (the time it takes for a satellite to orbit around the Earth) is twelve hours.
This means that at any given location, each satellite appears in the sky four minutes earlier each
day. The apparent groundtrack of the satellites (the path that their orbits would draw on the
surface of the Earth) is not the same each day because it is shifted westward slightly with each
orbit (a drift of 0.03 degrees each day).
The orbits of the satellites form a birdcage around the Earth such that there should always be
four or more satellites above the horizon at any one time. Two places on the globe, however, do
not fully benefit from the way in which the GPS satellite orbits are orientated: the north and
south poles. The orbital coverage here is not as good (for example, satellites are never overhead
at the poles), but this was considered a good compromise given the limited use that GPS would
see at these locations.
Why 30 satellites? This is the number considered sufficient to ensure that at least four (and a maximum
of twelve) satellites are always visible, at all sites on the Earth, at all times.
The GPS space segment was supposed to be activated in the late 1980s, but several incidents
(one of which, sadly, was the Challenger Space Shuttle disaster on January 28, 1986) caused
significant delays, and the full system of 24 SVs wasn’t deployed until 1994.
Some of the SVs that you will be using are now well over a decade old. This exceeds their initial
design life span of around 8 years!
The job of the satellites is multifold:
To provide extremely accurate, three-dimensional location information (latitude, longitude,
and altitude), velocity, and a precise time signal
To provide a worldwide common grid reference system that is easily converted to any
local grid in use