Data Hacking

GPS Primer
This appendix is a quick GPS primer for anyone who wants to know
a little more about the GPS network and how it works; and how the
little plastic box they have in their hand is capable of giving them such
a tremendously accurate positional fix anywhere on the globe.
The GPS Network
The GPS network consists of three distinct levels, or segments:
Space segment
Control segment
User segment
The Space Segment
The space segment consists of around 30 NAVSTAR satellites (also known
as Space Vehicles, or SVs for short). The exact number varies, but is normally
between 27 and 30. These satellites are the property of the U.S. Department
of Defense and are operated and controlled by the 50th Space Wing, located
at Schriever Air Force Base, Colorado.
NAVSTAR is an acronym for NAVigation Satellite Timing And
Ranging.
Of these 30 SVs, about 24 are active, and three are kept as spares in case of
problems with any of the others.
The spare satellites are positioned so that they can be quickly moved to the
appropriate orbit in the event of a failure of one of the operational satellites.
Satellites that are not working properly are considered sick, and you may
occasionally notice such a satellite oddly labeled on your GPS screen (its
icon might appear gray or the lock-on bar may show a good signal but no
lock). This is likely to be during testing when the Department of Defense
deliberately marks a “healthy” satellite as “sick” to see how the system copes.
The 24 operational satellites are arranged in six orbital planes around the
Earth, with four satellites in each plane. The satellites have a circular orbit
of 20,200 km (10,900 nm), and these orbits are arranged at an inclination
angle of 55 degrees to each other.
appendix
in this appendix
˛ The GPS Network
˛ How GPS Works