What is GPS, how does it work, advantages/disadvantages

GPS Global Positioning SystemThe GPS is a Global Navigation Satellite System (GNSS) developed by the United States

Department of Defence. It is the only fully functional GNSS in the world. It uses a

constellation of between 24 and 32 earth orbit satellites that transmit precise radio signals,

which allow GPS receivers to determine their current location, the time, and their velocity.

These satellites are high orbit, circulating at 14,000km/hr and 20,000km above the earth's

surface. The signal being sent to the earth at the speed of light is what is picked up by any

GPS receiver that are now commonplace worldwide.

The first satellite navigation system, used by the United States Navy, was first successfully

tested in 1960. Using a constellation of five satellites.

A GPS receiver calculates its position by precisely timing the signals sent by the GPS

satellites high above the Earth. Each satellite continually transmits messages containing

the time the message was sent, precise orbital information (the ephemeris – orbit path and

speed of each satellite), and the general system health, current date and time of all GPS

satellites (the almanac). The receiver measures the transit time of each message and

computes the distance to each satellite. A form of triangulation is used to combine these

distances with the location of the satellites to determine the receiver's location. The

position is displayed, perhaps with a moving map display or latitude and longitude;

elevation information may be included. Many GPS units also show information such as

direction and speed, calculated from position changes.

Basic Concept of GPSIt might seem three satellites are enough to solve for position using triangulation maths,

however a very small timing error multiplied by the very large speed of light (the speed at

which satellite signals travel) —results in a large positional error. The receiver uses a

fourth satellite to solve for x, y, z, and t which is used to correct the receiver's timer.

Although four satellites are required for normal operation, fewer apply in special cases. If

one variable is already known (for example, a ship or plane may have known elevation), a

receiver can determine its position using only three satellites. Some GPS receivers may

use additional clues or assumptions (such as reusing the last known altitude or including

information from a vehicle computer) to give a degraded position when fewer than four

satellites are visible.

Absolute v's Relative PositioningA key question asked about GPS is “How accurate is it”? This is based on the generally

publicised information that GPS has a positional error of between 5m & 10m globally.

Considering the satellite signal is travelling 20,000km to its destination, having a 10m error

works out to be a % error of only 0.0000005%!!

Being the fussy species that Humans are, this error margin is still seen to be too

inaccurate for the purposes of positioning of a human being on, for example; a pavement (I

guess 10m could see this person walking in the middle of the road if they were following

the positioning information carefully!) or on a sporting field.

With further developments in GPS technology itself we will see great improvements in

absolute positioning accuracy over the next 10 years (see my next article – GPS System

Accuracy).

From a sporting (physical performance) perspective, it is the relative positioning accuracy

that is of real interest to the sporting coach and player. By relative positioning I mean how

far the person has travelled, their speed and time over a set course. Whilst the real start

position might vary due to the 10m absolute error, the actual distance travelled measured

by sports specific GPS devices have this error down to <1% (so in a 100m race, a GPS

will accurately measure 99.0m+). Due to the huge benefit of now being able to measure

these variables real time in an actual game or training environment, this application has

been widely accepted in elite sport programs worldwide.

GPS Strengths and Weaknesses?GPS has several strengths but just as many weaknesses. Understanding this ensures that

the most is gained from the technology without expecting more than is possible from this

current system.

STRENGTHS

• The system is self calibrating – Just turn on and use.

• Can be used in the field – doesn't require a laboratory or artificial environment.

• The technology is relatively small (typical GPS system is now no larger than a small

mobile phone).

• Supplies the user with Location Based information that can be used for mapping

(cars), location (geocaching), performance analysis (sport), GIS (Geographic

Information Services – Google Earth as an example – pick a street and the

technology can link to a database showing what retail outlets are in that vicinity).

• Works anywhere on earth

• Can give bearings, directions.

• There is currently no charge to use the signal (US Department of Defence bears the

cost of system maintenance and upgrade).

• Several new GPS systems are being installed globally over the next 5 years giving

greater accuracy and usability.

WEAKNESSES

• The technology is very power hungry, most systems will only last 8-12 hours before

needing a battery replacement or recharge.

• The GPS signal is unable to pass through solid structures so is unable to work

indoors, underground, under the water, or under a dense canopy of trees.

• Can be affected by large buildings and is typically unreliable in CBD areas.

• GPS accuracy is related to the quality of signal reception, the larger the antenna the

better the signal – so absolute miniaturisation is not possible whilst maintaining

good positioning accuracy.

What causes the 5m - 10m error in GPS?Atmospheric

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they

pass through the Earth's Atmosphere, especially the ionosphere (the uppermost part of the

atmosphere) These effects are smallest when the satellite is directly overhead and

become greater for satellites nearer the horizon since the path through the atmosphere is

longer.

Multipath

GPS signals can also be affected by multipath issues, where the radio signals reflect off

surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can

cause inaccuracy

Ephemeris and clock errors

While the ephemeris data is transmitted every 30 seconds, the information itself may be up

to two hours old. Data up to four hours old is considered valid for calculating positions, but

may not indicate the satellite's actual position.

Selective Availability

GPS includes a (currently disabled) feature called Selective Availability (SA) that can

introduce intentional, slowly changing random errors of up to a hundred meters into the

publicly available navigation signals to confound, for example, the guidance of long range

missiles to precise targets. When enabled, the accuracy is still available in the signal, but

in an encrypted form that is only available to the United States military & its allies.

Artificial sources

Man-made electromagnetic interference can also disrupt, or jam, GPS signals. In one well

documented case, the entire harbour of Moss Landing, California was unable to receive

GPS signals due to unintentional jamming caused by malfunctioning TV antenna pre-

amplifiers. Intentional jamming is also possible. Generally, stronger signals can interfere

with GPS receivers when they are within radio range, or line of sight.

In my next article I will look at what other forms of GPS technology are available, how to

make the current system cm level accurate as well as what's in store for users of GPS

enabled mobile phones.

Adrian Faccioni

GPSports Systems

ReferencesTrimble Website: http://www.trimble.com/gps/index.shtml

Wikipedia: http://en.wikipedia.org/wiki/GPS