DEFINITION OF GLOBAL POSITIONING SYSTEM

Global positioning systems,

commonly referred to by their acronym GPS, were developed

by the U.S. Department of Defense to provide the U.S.

military with a superior tool for navigation, viable at any

arbitrary point around the world. As of 2002, the U.S.

Department of Defense has paid more than $12 billion for

the development and maintenance of the GPS program,

which has matured a great deal since its conception in the

1960s.

The current configuration of the global positioning system

includes three main components: the GPS satellites, the

control segment, and the GPS receivers. Working together,

these components provide users of GPS devices with their

precise location on the Earth’s surface, along with other basic

information of substantial use such as time, altitude, and

direction. Key in understanding how GPS functions is the

understanding of these components, and how they interrelate

with one another.

GPS satellites, dubbed the Navstar satellites, form the core

functionality of the global positioning system. Navstar satel

lites are equipped with an atomic clock and radio equipment

to broadcast a unique signal, called a “pseudo-random code,”

as well as ephemeris data. This signal serves to identify one

satellite from the other and provide GPS receivers accurate

information regarding the exact location of the satellite. These

satellites follow a particular orbit around the Earth, and the

sum of these orbits is called a constellation. The GPS Navstar

satellite constellation is configured in such a way that at any

point on the Earth’s surface, the user of a GPS receiver should

be able to detect signals from at least six Navstar satellites. It is

very important to the proper functioning of GPS that the precise

configuration of the constellation be maintained.

Geosynchronous satellite orbit maintenance is performed

by the control segment (or satellite control centers), with stations

located in Hawaii, Ascension Island, Diego Garcia,

Kwajalein, and Colorado Springs. Should any satellite fall

slightly in altitude or deviate from its correct path, the control

segment will take corrective actions to restore precise

constellation integrity.

Lastly, the component most visible to all who use GPS

devices is the GPS receiver. It is important to understand that

GPS receivers are single-direction asynchronous communication

devices, meaning that the GPS receiver does not broadcast

any information to the Navstar satellites, only receives

signals from them. Recent years have seen the miniaturization

and mass proliferation of GPS receivers. GPS devices are now

so small that they can be found in many other hybrid devices

such as cellular phones, some radios, and personal desk accessories.

They are also standard on many vehicles for land, sea,

and air travel. Accuracy of consumer GPS receivers is typically

no better than nine feet, but advanced GPS receivers can measure

location to less than a centimeter.

GPS resolves a location on the surface of the Earth

through a process called trilaterating, which involves determining

the distances to the Navstar satellites and the GPS

receiver. In order to do this, two things must be true. First,

the locations of the Navstar satellites must be known, and

second, there must be a mechanism for precision time measurements.

For very precise measurements, there must be a

system to reconcile error caused by various phenomena.

GPS receivers are programmed to calculate the location

of all the Navstar satellites at any given time. A combination

of the GPS receiver’s internal clock and trilateration signal

reconciliation, performed by the GPS receiver, allow a very

precise timing mechanism to be established.

When a GPS receiver attempts to locate itself on the surface

of the planet, it receives signals from the Navstar satellites.

As mentioned previously, these signals are intricate and

unique. By measuring the time offset between the GPS receiver’s

internal pseudo-random code generator, and the pseudorandom

code signal received from the Navstar satellites, the

GPS receiver can use the simple distance equation to calculate

the distance to the Navstar satellite.

To accurately locate a point on the Earth’s surface, at least

three distances need to be measured. One measurement is only

enough to place the GPS receiver within a three-dimensional

arc. Two measurements can place the GPS receiver within a

circle. Three measurements place the GPS receiver on one of

two points. One possible point location will usually be floating

in space or traveling at some absurd velocity, and so the GPS

receiver eliminates this point as a possibility, thus resolving the

GPS receiver’s location on the surface of the planet. A fourth

measurement will also allow the correct point to be located, as

well as provide necessary geometry data to synchronize the

GPS receiver’s internal clock to the Navstar satellite’s clock.

Depending on the quality of the GPS device, error correction

may also be performed when calculating location. Errors

arise from many sources. Atmospheric conditions in the ionosphere

and troposphere cause impurities in the simple distance

equation by altering the speed of light. Weather modeling can

help calculate the difference between the ideal speed of light

and the likely speed of light as it travels through the atmosphere.

Calculations based on the corrected speed of light then

yield more accurate results.

As weather conditions rarely fit models, however, other

techniques such as dual frequency measurements, where two

different signals are compared to calculate actual speed of the

pseudo-code signal, will be used to reduce atmospheric error.

Ground interference, such as multipath error, which arises

from signals bouncing off objects on the Earth’s surface,

can be detected and rejected in favor of direct signals via very

complicated signal selection algorithms.

Still another source of error can be generated by the

Navstar satellites being slightly out of position. Even a few

meters from the calculated position can throw off a high-precision

measurement.

Geometric error can be reduced by using satellites that are

far apart, rather than close together, easing certain geometric

constraints.

Sometimes precision down to the centimeter is needed.

Only advanced GPS receivers can produce precise and accurate

measurements at this level. Advanced GPS receivers utilize

one of several techniques to more precisely pinpoint

locations on the Earth’s surface, mostly by reducing error or

using comparative signal techniques.

One such technique is called differential GPS, which

involves two GPS receivers. One receiver monitors variations

in satellite signals and relates this information to the second

receiver. With this information, the second receiver is then

able to more accurately determine its location through better

error correction.

Another method involves using the signal carrier-phase

as a timing mechanism for the GPS receiver. As the signal carrier

is a higher frequency than the pseudo-random code it

carries, carrier signals can be used to more accurately synchronize

timers.

Lastly, a geostationary satellite can be used as a relay

station for transmission of differential corrections and GPS

satellite data. This is called augmented GPS and is the basic

idea behind the new WAAS (Wide Area Augmentation System)

system of additional satellites installed in North America.

The system encompasses 25 ground-monitoring stations

and two geostationary WAAS satellites that allow for better

error correction. This sort of GPS is necessary for aviation,

particularly in landing sequences.

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