GPS

Intro to GISSpring 2012

GPS

GPS = Global Positioning System GNSS = Global Navigation Satellite

Systems Satellite based technologies that

give location on Earth’s surface

Why create GPS?

Navigation: knowing where you are and where to go

Defense: precise locations for targets Now: many uses – commercial, field

mapping and surveying, automobile travel, recreation

Some history

Navigating the oceans Compass: points to magnetic north so

know direction traveling Sextant: instrument that can determine

angles of stars, moon and sun over horizon. Indicates your latitude

Chronometer: shipboard timepiece that indicates your longitude

More history

Early 20th century: radio-based navigation systems used during WWII

Limitations of ground radio systems Very accurate but doesn’t cover wide

area Or, one that covers wide area but is not

very accurate

GNSS

NAVSTAR: operated by the US Department of Defense, 1st satellite launched in 1978, last satellite launched in 1994

GLONASS: Russian, little used internationally

Galileo: being developed by a consortium of European governments and industries

Chinese Compass Satellite Navigation System: in development

GPS Components

Satellites Receivers(users) Control stations

http://www.aero.org/

Satellite Component

24 satellites orbiting the Earth

About 20,000 km above surface

Complete orbit in 12 hours

http://www.aero.org/

Satellites

Each satellite is carefully placed and monitored in orbit

Each contains a very accurate clock – to 3 billionths of a second, or 0.000000003

http://www.aero.org/

Satellite signals

Each broadcasts a signal that includes: Pseudorandom code: unique to identify which

satellite Ephemeris data: identifies satellite position in

space at any given moment Almanac data: exact time signal was sent

Electromagnetic radiation Low power radio waves that pass through

clouds, glass and plastic, but not Earth or buildings

Speed of light

Distance between satellite and receiver

Distance = velocity x time Know velocity: signals traveling at

speed of light 3 x 108 m/s Time: determine time between when

signal was sent by satellite and when received by the receiver (GPS unit)

Time

Need precise clocks Each satellite emits a

pseudorandom code Signal so complicated

that it looks random Receiver compares the

signal it receives with the signal should be exactly when it is received

•Satellite directly overhead: takes about 0.06 second to reach receiver

Source Bolstad, 2008. p. 180

Receiver component

Detect, decode and process signals from satellites within range

Contain accurate clock, although not as accurate as the ones on satellites

Measure the distance between the time the signal was sent and the time it was picked up by the receiver. Used to determine the distance to satellite

Need signals from at least 4 satellites to determine location and elevation More is better

Triangulation

Know location of satellites (at least 4)

Know distance of each satellite from receiver

Source Bolstad, 2008. p. 181

Control component

5 ground stations around the world Hawaii, Ascension Island (South Atlantic Ocean),

Diego Garcia (Indian Ocean), Kwajalein (Marshall Islands), and Colorado Springs

Master station in Colorado, USA Responsible for:

Tracking Communications Data gathering Integration Analysis

Factors and limitations that affect accuracy and use

Cannot pass through buildings, underground, sides of mountains, and dense foliage

Buildings and terrain can reduce visible sky and block signal reception

Signals can reflect off buildings and outcrops, thus increasing the length of time the unit receives the signal

Signal slows through the atmosphere. GPS uses a correctional factor

Locations of satellites should be at wide angles to each other

Differential GPS = DGPS

Use 2 receivers: 1 stationary and the other roving (e.g., hand-held unit)

Location of stationary known. Can be used to apply correction to signals due to atmospheric interference Works in reverse: known

location so will know the length of time a satellite signal should take to reach it

If the roving unit is within a few hundred kilometers, can use the error correction

Source Bolstad, 2008. p. 186

DGPS

Improves accuracy Some GPS

receivers can receive correction while collecting data

Others require post processing: corrections are applied later in a lab

Source Bolstad, 2008. p. 188