Global Positioning SystemGlobal Positioning System

Geographic Information Systems Dep’tDavao Light and Power Company

GPS History• US military began studies for improved

navigation system in mid-1960’s for: Cruise missiles Ballistic submarines Other strategic weapons purposes • Consolidated efforts were made in 1974 by

forming NAVSTAR GPS Joint Program OfficeFirst GPS satellite was launched in 1978Complete network of satellites in 1994

NAVSTAR GPS

NA NAVIGATIONVS SYSTEMT TIMINGA AndR RANGING

GPS Services

Standard Positioning Services (SPS)

- Unrestricted access provided to everyone

- Specified in Federal Radio Navigation Plan

- Accuracy – 100 meters 2D RMS. Further degradation in case of national emergency.

Precise Positioning Services (PPS)

- Restricted access, controlled by US Dep’t of Defense (DOD)

- Requires cryptographic equipment

- Accuracy – 16 meters 2D RMS

GPS APPLICATIONS

• Navigation• Surveying• Mapping• Research• Plate Tectonic

• GIS• Construction• Tracking System• Geodetic Control• & Etc.

GPS SEGMENTS

• SPACE SEGMENT

CONTROL SEGMENT

USER SEGMENT

SPACE SEGMENT

• 24 Satellites• 6 Orbital Planes• 55o - 60o Inclination• 20,200 km Above Earth• 4-12 Visible Satellites• 12 hrs – Orbit• Max.5hrs Above

Horizon

CONTROL SEGMENT

USER SEGMENT

How does GPS work?

GPS derivesposition relative tosatellite as“reference points,”

using trilateration.Not by triangulation.

The GPS unit on theground figures out itsdistance to each of severalsatellites

Determining Positions

• GPS uses the principle of ranging (Distance from the satellite to the user) to determine positions.

• You need to know the range to at least 3 satellite signals in order to locate your position (X,Y of the Lat-Long) coordinates

• The fourth satellite signal will give you the 3rd dimension (the height or elevation) and the time dissemination

Determining Positions

• To determine the distance to the satellite, you need to know how long a message takes you from the satellite.

• Distance = rate x time• The rate will be the speed of light (186,000 mi/sec

or 300,000 km/sec)• Time is the propagation time from the satellite to

the GPS receiver• Time is calculated using a clever technique of code

matching which compares signals from the satellite and GPS receiver

Determining PositionsThe Sphere Concept

ACCURACYThree Methods of Positioning Observation

Autonomous < 100 meters (SA on) < 12 meters (SA off)Differential 2 to 5 meters Submeter DecimeterPhase Differential Centimeter

GPS Accuracy Issues

GPS position accuracy depends upon a number of issues:

Available Satellite GeometryAtmospheric Errors

Receiver ErrorsMulti-path Errors

Selective Availability

Satellite GeometryPDOP (Positional Dilution of Precision)

The more separated thesatellites are – the

better the geometry, and the better the position solution

PDOP = Low

The closer the satellites are in the space, the worse the geometry, and the poorer the solution

PDOP = High

GPS Receiver Categories

• Navigational Units- Mostly handheld units- Non-downloadable and non-DGPS- 5 to 10 meter level accuracy

• Mapping Grade- Handheld and pole mounted units- Downloadable and DGPS capable- Submeter to Decimeter accuracy

• Survey Grade- Usually mounted in tripod or pole- Downloadable and DGPS / RTK capable- Centimeter accuracy

Navigational and Mapping Grade

Survey Grade A New Era of Surveying

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