System segmentation
The current GPS consists of three major segments. These are the space segment
(SS), a control segment (CS), and a user segment (US).
Space segment
The space segment (SS) is composed of the orbiting GPS satellites, or Space
Vehicles (SV) in GPS parlance. The GPS design calls for 24 SVs to be distributed
equally among six circular orbital planes. The orbital planes are centered on
the Earth, not rotating with respect to the distant stars. The six planes have
approximately 55° inclination (tilt relative to Earth's equator) and are
separated by 60° right ascension of the ascending node (angle along the equator
from a reference point to the orbit's intersection).
Orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or
10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)),
each SV makes two complete orbits each sidereal day, so it passes over the same
location on Earth once each day. The orbits are arranged so that at least six
satellites are always within line of sight from almost anywhere on Earth.
As of April 2007, there are 30 actively broadcasting satellites in the GPS
constellation. The additional satellites improve the precision of GPS receiver
calculations by providing redundant measurements. With the increased number of
satellites, the constellation was changed to a nonuniform arrangement. Such an
arrangement was shown to improve reliability and availability of the system,
relative to a uniform system, when multiple satellites fail.
Control segment
The flight paths of the satellites are tracked by US Air Force monitoring
stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado
Springs, Colorado, along with monitor stations operated by the National
Geospatial-Intelligence Agency (NGA). The tracking information is sent to the
Air Force Space Command's master control station at Schriever Air Force Base,
Colorado Springs, Colorado, which is operated by the 2d Space Operations
Squadron (2 SOPS) of the United States Air Force (USAF). 2 SOPS contacts each
GPS satellite regularly with a navigational update (using the ground antennas at
Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates
synchronize the atomic clocks on board the satellites to within one microsecond
and adjust the ephemeris of each satellite's internal orbital model. The updates
are created by a Kalman Filter which uses inputs from the ground monitoring
stations, space weather information, and other various inputs.
User segment
The user's GPS receiver is the user segment (US) of the GPS system. In general,
GPS receivers are composed of an antenna, tuned to the frequencies transmitted
by the satellites, receiver-processors, and a highly-stable clock (often a
crystal oscillator). They may also include a display for providing location and
speed information to the user. A receiver is often described by its number of
channels: this signifies how many satellites it can monitor simultaneously.
Originally limited to four or five, this has progressively increased over the
years so that, as of 2006, receivers typically have between twelve and twenty
channels.
GPS receivers may include an input for differential corrections, using the RTCM
SC-104 format. This is typically in the form of a RS-232 port at 4,800 bps
speed. Data is actually sent at a much lower rate, which limits the accuracy of
the signal sent using RTCM. Receivers with internal DGPS receivers can
outperform those using external RTCM data. As of 2006, even low-cost units
commonly include WAAS receivers.
Many GPS receivers can relay position data to a PC or other device using the
NMEA 0183 protocol. NMEA 2000[10] is a newer and less widely adopted protocol.
Both are proprietary and controlled by the US-based National Marine Electronics
Association. References to the NMEA protocols have been compiled from public
records, allowing open source tools like gpsd to read the protocol without
violating intellectual property laws. Other proprietary protocols exist as well,
such as the SiRF protocol. Receivers can interface with other devices using
methods including a serial connection, USB or Bluetooth.