GPS Overview Part 2 - GPS System Segments
The original theory behind Location-Based Services -
or LBS - is to help you find out where you are or where something
else is.
One part of LBS is the GPS satellite
constellation. The following overview describes the history and workings
of GPS, as well as its uses and the future for it.
The Global Positioning System is comprised of three
segments: satellite constellation, ground control/monitoring
network and user receiving equipment. Formal GPS Joint Program
Office (JPO) programmatic terms for these components are space,
operational control and user equipment segments, respectively.
- The satellite constellation contains the satellites in orbit that
provide the ranging signals and data messages to the user equipment.
- The operational control segment (OCS) tracks and maintains the
satellites in space. The OCS monitors satellite health and signal
integrity and maintains the orbital configuration of the satellites.
Furthermore, the OCS updates the satellite clock corrections and
ephemerides as well as numerous other parameters essential to
determining user position, velocity, and time (PVT).
- Lastly, the user receiver equipment performs the navigation, timing
or other related functions (e.g. surveying).
The satellite constellation consists of the nominal
24-satellite constellation (the first was launched in 1978 and the 24th
in 1994). They transmit signals (at 1575.42 MHz) that can be detected by
receivers on the ground. The satellites are positioned in six
Earth-centred orbital planes with four satellites in each plane. This
means that signals from six of them can be received 100 percent of the
time at any point on earth. The nominal orbital period of a GPS satellite
is one half of a sidereal day or 11 hr 58 min. The orbits are nearly
circular and equally spaced about the equator at a 60° degree separation
with an inclination relative to the equator of nominally 55° degrees. The
orbital radius is approximately 26,600 km (i.e., distance from satellite
to centre of mass of the earth).
GPS satellites transmit two low power radio signals,
designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in
the UHF band.A GPS signal contains three different bits of information —
a pseudo-random code, ephemeris data and almanac data. The pseudo-random
code is simply an I.D. code that identifies which satellite is
transmitting information.
Several different notations are used to refer to the
satellites in their orbits. One particular notation assigns a letter to
each orbital plane (i.e., A, B, C, D, E, and F) with each satellite within
a plane assigned a number from 1 to 4. Thus, a satellite referenced as B3
refers to satellite number 3 in orbital plane B. A second notation used is
a NAVSTAR satellite number assigned by the U.S. Air Force. This notation
is in the form of a space vehicle number (SVN) 11 to refer to NAVSTAR
satellite 11.
The OCS has responsibility for maintaining the
satellites and their proper functioning. This includes maintaining the
satellites in their proper orbital positions (called station keeping) and
monitoring satellite subsystem health and status. The OCS also monitors
the satellite solar arrays, battery power levels, and propellant levels
used for manoeuvres and activates spare satellites. The overall structure
of the operational ground/control segment is as follows: Remote monitor
stations constantly track and gather C/A and P(Y) code from the satellites
and transmit this data to the Master Control Station, which is located at
Falcon Air Force Base, Colorado Springs. There is also the ground uplink
antenna facility, which provides the means of commanding and controlling
the satellites and uploading the navigation messages and other data. The
unmanned ground monitor stations are located in Hawaii, Kwajalein in the
Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the
Atlantic and Colorado Springs, Continental United States. Ground antennas
are located in these areas also. These locations have been selected to
maximise satellite coverage.
Location of GPS ground stations
The user receiving equipment, typically referred to
as a GPS receiver, processes
the L-band signals transmitted from the satellites to determine PVT. There
has been a significant evolution in the technology of GPS receiving sets
since they were initially manufactured in the mid-70’s. Initially, they
were large, bulky and heavy analog devices primarily used for military
purposes. With today’s technology, a GPS receiver of comparable or more
capability typically weighs a few pounds or ounces, and occupies a small
volume. The smallest of today’s are those of a wrist watch size, while
the largest is a naval shipboard unit (weighing about 32 kgs). The basic
structure of a receiver is the antenna, the receiver and processor, the
display and a regulated dc-power supply. These receivers can be mounted in
ships, planes and cars, and provide exact position information, regardless
of weather conditions.
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