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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