Global Positioning Systems

Why GPS? Challenges of finding exact location by

traditional methods

Astronomicalobservation

Adjustmentsbased on gravity

variation

Development ofdatums

An exact location is found by measuring back to the nearest

control point

+ =

Challenges:• tedious• monuments move!• really not practical over large areas

A continent-sized series of control

points

A new approach … GPS

US Dept of Defense, cir. 1994

24 satellites ~11,000 miles above earth

Transmit precise location and time, 24/7

Three Segments of GPS:

Satellites

Receivers Ground stations

The Satellites

24 satellites: 4 “visible” at any one time, anywhere

Super-accurate atomic clocks

Constantly broadcast radio signals

The Receiver

Receives signals from satellites

Identifies transmitting satellites

Calculates distance from each satellite

Calculates present location

The Ground Stations

5 precisely-located stations that monitor and correct… the orbit of the satellites … and the accuracy of their clocks

These allow the satellites to broadcast their location correctly

GPS unit displays of satellites

Accuracy of the GPS: Limitations

Selective Availability Turned off on May 2, 2000

Created inaccuracy of up to 150 meters

Typical accuracy for today’s “consumer

GPS units”:

10 – 15 meters

Sources of error in GPS

Precision of receiver

Satellite clock errors

Atmospheric conditions

Lower atmosphere – weather – water vapor

Upper atmosphere (ionosphere) – ions – electrically charged particles

Slows downsatellite signals:

Result is like clock error

How can we compensate for all these errors?

Visibility of satellites

Other sources of error in GPS:

Positions of satellitesBetterBetter

WorseWorse

VISIBILITY ISSUES

How can we compensate for all these errors?

A solution: Differential GPS

Stationary GPS receiver, + radio transmitter,

at known location1. Receives signals from all visible satellites

2. Calculates timing delays in signal from each satellite

3. Radio transmits this info to nearby “roving” GPS receivers

4. Roving receiver can correct for same timing delays

Base station(e.g. WAAS)

Error with Differential GPS:

WAAS < 3 meters

Custom base station < 1 meter