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Global Positioning Systems (GPS). Jeroen Verplanke Department of Urban and Regional Planning and Geo-Information Management. What is GPS ?.  A world wide radio-navigation system  Uses satellites as reference points to calculate positions  Three components:  Space segment

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Global positioning systems gps

Global Positioning Systems (GPS)

Jeroen Verplanke

Department of Urban and Regional Planning and Geo-Information Management


What is gps
What is GPS ?

 A world wide radio-navigation system

 Uses satellites as reference points

to calculate positions

 Three components:

 Space segment

 Control segment

 User segment


Gps components
GPS components

Spacesegment


Gps components1
GPS components

Control segment

User segment


How gps works
How GPS works

1. The basis for GPS is resection from satellites (triangulation)

2. For resection a GPS receiver calculates

distance to satellites

using travel time of radio signals

3. To measure travel time, GPS needs exact timing

4. Along with distance you need to know the satellite position

5. For accurate positioning you must correct for errors


What gps accuracy do we need
What GPS accuracy do we need?

  • Soil map (1:250,000) :

  • Road map (1:100,000) :

  • Topographical map (1:50,000) :

  • Ecological map (1:25,000) :

  • Cadastral map (1:10,000) :

  • Architectural plan (1:1,000) :

  • Sketch map (1: ???) :


?

?

?

?

?

?

Cálculo de la posición usando la tecnología GPS

X

X

Superficie de la Tierra


1. Resection from satellites

Resection from satellites: determination of an unknown location using 1, 2 and 3 distances to known locations of satellite(s).

X

Da

A

Da

1 satellite

A

X


1. Resection from satellites

X

A

Da

Db

B

Da

Db

X

2 satellites

A + B


1. Resection from satellites

C

Dc

X

Da

Db

A

B

3 satellites

A + B + C


2 measuring distance to satellites
2. Measuring distance to satellites

Distance

= velocity * travel time

Distance is about 22,000 km

 We cannot see satellites

We cannot measure exact distance

Calculating distance to satellite

Using travel time of radio signal

Travel time = ?

approx. 0.07 sec !

Using radio signal to calculate distance

This signal travels with speed of light

Speed of light = 299,174 km/sec


3 exact timing
3. Exact timing

How to measure travel time

 satellite and receiver generate radio signal

at the same time

 travel time = phase difference between signals

1 msec

Satellite’s signal

GPS receiver’s signal


3 exact timing1
3. Exact timing

 Very precise clocks for exact timing

 satellites : highly accurate ‘atomic’ clocks

(about USD 100,000 each)

 receivers : moderately accurate quartz clocks

 Clock error due to difference in clock accuracy

 use a 4th satellite to correct for clock error


4 the satellite position in space
4. The satellite position in space

 Using satellites as reference points for positioning

also requires that you know the exact position in space

of each satellite, at any place and at any time.

 The GPS control segment monitors the satellite position

in space.

 All details of satellite orbits is available in an ‘almanac’

 This satellite status information can be downloaded

to the GPS receiver


5 correcting for errors
5. Correcting for errors

Main GPS error sources

 Clock errors

 Signal errors (noise)

 Interference in ionosphere and troposphere

 Multipath error

 Satellite position (“ephemeris”) error

 Geometrical error (Geometric Dilution of Precision - GDOP)

 Intentional errors (Selective Availability - SA)

 Human errors

 Receiver errors (hardware, software, antenna)



5 correcting for errors1

POOR GDOP (2-6)

GOOD GDOP ( 2)

5. Correcting for errors

Geometric Dilution Of Precision (GDOP)





5 correcting for errors2
5. Correcting for errors

Selective Availability (SA)

SA off (after May 2000)

SA on (before May 2000)


5 correcting for errors3
5. Correcting for errors

  • Some typical errors

  •  Satellite clock error 2 meter

  •  Receiver noise 0.5 meters

  •  Interference in

  • ionosphere and troposphere 5 meters

  •  Multipath error 1.4 meter

  • Satellite position (“ephemeris”) error 2 meters

  •  poor GDOPup to 200 meters

  •  Human errors up to hundreds of meters

  •  Receiver errors

  • (hardware, software, antenna) any size possible


5. GPS settings

  • units’ settings: position format: hddd.dddddo, map datum: WGS 84, and units: metric

  • ‘interface’ settings: i/o format: NMEA out, and baud rate: 4800.


About gps accuracy
About GPS accuracy

  • Accuracy can be from 100m to 1mm

  • Accuracy depends on purpose

    • Soil map (1:250,000) :

    • Road map (1:100,000) :

    • Topographical map (1:50,000) :

    • Ecological map (1:25,000) :

    • Cadastral map (1:10,000) :

    • Architectural plan (1:1,000) :

    • Sketch map (1: ???) :


Using a handheld gps receiver
Using a handheld GPS receiver

Typical accuracy: 10 m Horizontal

( civil use,good GDOP)

Results for a Month (Garmin 12XL)

Horizontal Accuracy (50%) 3.9 meters

Vertical Accuracy (50%) 9.6 meters

Horizontal Accuracy (95%) 9.3 meters

Vertical Accuracy (95%) 21.9 meters

Source: GPS ACCURACY MONITOR by Dennis Milbert (http://mywebpages.comcast.net/dmilbert/handacc/accur.htm)


In conclusion
In conclusion

3. Exact timing

1. Resection from satellites

2. Distance to satellites

5. errors

4. Position in space


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