The Global Positioning System
The global positioning system (GPS) is a satellite-based navigation
system, consisting of a network of 24 orbiting satellites travelling
on six different orbital paths. These satellites, referred to as
NAVSTAR satellites, are constantly moving, making two complete orbits
around earth per day.
The first GPS satellite was launched in February, 1978. Satellites now
weight approximately 1 tonne, and are about 5 metres across with
extended solar panels.
GPS needs at least 24 satellites to provide full coverage of every
point on the earth, all the time. To calculate one position on the
earth, at least 3 satellites are needed. Currently, there are 28
working GPS satellites, out of roughly 750 currently in miliary,
civilian and commercial use.
GPS satellites, quite simply, broadcast data - each satellite knows
two things: its exact location in obit, and exactly what time it is.
It knows its position within a few feet, whilst moving a t 17 000
miles per hour, and knows the time within approximately 100
picoseconds (1 trillion picoseconds to a second). The satellite signal
also contains a 'psuedo-random code' - its identification, ephemeris
and almanac data. Ephemeris data contains the status of the satellite
(able to function or no), the information mentioned earlier, that is
the current date and time. The almanac data tells the GPS receiver
where the GPS satellite should be at any time throughout the day. Each
satellite transmits data not only on its own location, but the
location of every other satellite in the system.
To condense this, each satellite transmits a message which informs of
its identity, current position, and the time at which the message was
To now determine your position, the GPS receiver compares the time a
signal was transmitted by a satellite, with the time that it was
received. The time difference then determines how far away the
particular satellite is. Through trigonometry, with a minimum of three
satellites, the GPS receiver is then able to determine a
latitude/longitude position, and with a further satellite, also the
altitude. Through constant updates of your position, a receiver can
also provide accurate measures of speed and direction of travel.
The accuracy of the GPS system used to be hindered principally by a
source of position error called Selection Availability (SA),
intentionally imposing inaccuracies of up to 100m on civilian GPS by
the US Defense, the idea being that SA would prevent hostile terrorist
organisations holding the maximum power of GPS.
Some other factors affect accuracy - the relative positions of each
satellite in the sky, multi-pathing (where the satellite reception is
blocked/reflected by buildings, obstructions of the like) and
propagation delay due to atmospheric...