Global Positioning System

Your location is:37o 23.323’ N

122o 02.162’ W

Control Segment (B)

GPS Infrastructure: Segments

• Control Segment

• Space Segment

• User Segment

Control Segment

•A set of ground stations that monitor the health of the satellites and transmit corrections to their orbit

•The exact position of each satellite is known at all times

Control Segment Locations

Control Segment: Ephemeris

• Exact location of satellite is also needed to establish GPS position

Your location is:37o 23.323’ N

122o 02.162’ W

Differential GPS

Space Segment

• The complete constellation of orbiting Navstar GPS satellites

• 21 operational, 3 backups

Space Segment: Orbits

User SegmentThe person roving with the GPS unit.

GPS by TrilaterationDistance D = Speed of Light x Time

D

Clock Offset

How Does GPS Work?

600 miles

How Does GPS Work?

600 miles 600 miles

How Does GPS Work?

600 miles 600 miles

600 miles

Location at yellow dot,where the three lines intersect

Why Four Satellites?

• To compensate for inaccuracy of quartz receiver clocks.

• A timing error of 1/1,000,000 of a second equals a positional error of 300 meters or more.

• Required for height positioning.

Autonomous Positioning

• Using one receiver

• No Differential Correction

• Not using carrier-phase positioning

DGPS: How does it work?

• Two GPS receivers are used for DGPS.• A high precision “Base” GPS receiver (Base

Receiver or Base Station) is placed at a known “controlled” point of reference such as a National Geodetic Survey marker. This receiver collects GPS signals and compares the results to the actual known coordinate of the Base.

• A “rover” receiver collects autonomous information in the field.

DGPS: How does it work?

DGPS: Important to know

• The base station and the rover must be collecting GPS data at the same time, and from the same satellites to be effective.

• The base station must be within 500 km. from the rover to insure that they are reading the same satellites. Also changes in mask angle increase with distance as do atmospheric error.

DGPS: Post-Processing

• There is no real-time connection between the base and the rover.

• Each receiver collects data independently of the other.

• Data collected from the rover is brought back to the office.

• Data from the base station is then obtained (available from a variety of sources).

• Base stations can be permanent or portable.

DGPS: Real-Time

•Link made to real-time correction system in the field

•WAAS (Wide Area Augmentation System), beacon

Sources of GPS Error

• Satellite/Receiver clock error• Satellite Ephemeris error• Atmospheric Refraction

– Ionospheric Refraction– Tropospheric Refraction– Satellite Mask Angle

• Receiver Noise• Multipath• Selective Availability

Atmospheric Delay

GPS signals are delayed as they pass through the atmosphere

Ionosphere

< 10 km > 10 km

Troposphere

Review Questions

Multipath

Atmospheric Delay (B)

Obstruction

Multipath

Selective Availability• Off-setting satellite clocks.

• Introduction of ephemeris error by the Space Command control center

• Only the military has the correction information.

S/A Status• In 1996 it was announced that S/A would

be phased out and turned off within 4 years.

• In 2000 S/A was turned off, but the military can still turn it on when necessary.

• The current plan is for S/A to be turned off permanently by the year 2006

Differential GPS: What errors can be corrected?

Satellite/Receiver clock errorSatellite Ephemeris errorAtmospheric Refraction

– Ionospheric Refraction– Tropospheric Refraction– Satellite Mask Angle

• Receiver Noise• MultipathSelective Availability

Poor Dilution of Precision

Good Dilution of Precision

Coordinate Systems

•A system for assigning location information to the Earth’s surface

•Measured in 3-Dimensions- X, Y, Z

•UTM Zone, Easting, Northing, HASL/HAE

•Latitude, Longitude, HASL/HAE

WGS 84

• The current best-fit ellipsoid. Adopted in 1984 as a global datum.

• Used as the official standard of the GPS

• Will eventually be replaced as Geodesists make improvements to their models

X

• Area Features

GPS Positions and Graphic Features

X XXX X

XXX X

X

X11:00.15

XX

XX11:00.05

11:00.10

11:00.20 11:00.25

11:00.30

X

XX

X11:02.50

11:02.45

11:02.4011:02.35

11:02.30

• Point Features

• Line Features

User Applications: Military

• Troop Deployment and Observation

• Overland Navigation

• Air Navigation

• Sea Navigation

• Target Tracking for Remote Sensing Applications

• Smart Weapons

User Applications: Civilian

• Many more than military

• Land, Sea, and Air Navigation

• Intelligent Vehicle Highway Systems (IVHS)

• Mapping/GIS

• Surveying

• Search and Rescue

• Recreation

• Tracking and Logistics

• More ?????

Any Questions?

Hmmm . . . Where are we?