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# GPS Basic Theory - PowerPoint PPT Presentation

GPS Basic Theory. Contents. GPS General Characteristics GPS System Components Outline Principle: Range Position Range Determination from: Code Observations Phase Observations. Error Sources Differential GPS Initial Phase Ambiguity Resolving the Ambiguity Dilution of Precision

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Presentation Transcript

Basic Theory

• GPS General Characteristics

• GPS System Components

• Outline Principle:

• Range

• Position

• Range Determination from:

• Code Observations

• Phase Observations

• Error Sources

• Differential GPS

• Initial Phase Ambiguity

• Resolving the Ambiguity

• Dilution of Precision

• Summary

• Developed by the US Department of Defense

• Provides

• Accurate Navigation 10 - 20 m

• Worldwide Coverage

• 24 hour access

• Common Coordinate System

• Designed to replace existing navigation systems

• Accessible by Civil and Military

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lX

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X

X

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Xl

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GPS Principle : Range

Space Segment

Satellite Time and Ranging

24 Satellites

20200 Km

Control Segment

1 Master Station

5 Monitoring Stations

User Segment

R2

• 3 Spheres intersect at a point

• 3 Ranges to resolve for Latitude, Longitude and Height

2 Spheres intersect as a circle

GPS Principle : Point Positioning

R1

We are somewhere on a sphere of radius, R1

• The satellites are like “Orbiting Control Stations”

• Ranges (distances) are measured to each satellites using time dependent codes

• Typically GPS receivers use inexpensive clocks. They are much less accurate than the clocks on board the satellites

• A radio wave travels at the speed of light

• (Distance = Velocity x Time)

• Consider an error in the receiver clock

• 1/10 second error = 30,000 Km error

• 1/1,000,000 second error = 300 m error

• 4 Ranges to resolve for Latitude, Longitude, Height & Time

It is similar in principle to a resection problem

Point Positioning with at least 4 GPS satellites and Good Geometry

• Like all other Surveying Equipment GPS works in the Real World

• That means it owns a set of unique errors

• Satellite Clock Model

• though they use atomic clocks, they are still subject to small inaccuracies in their time keeping

• These inaccuracies will translate into positional errors.

• Orbit Uncertainty

• The satellites position in space is also important as it’s the beginning for all calculations

• They drift slightly from their predicted orbit

GPS signals transmit their timing information via radio waves

It is assumed that a radio wave travels at the speed of light.

GPS signals must travel through a number of layers making up the atmosphere.

As they travel through these layers the signal gets delayed

This delay translates into an error in the calculation of the distance between the satellite and the receiver

19950 Km

Ionosphere

200 Km

Troposphere

50 Km

• Unfortunately not all the receivers are perfect. They can introduce errors of their own

• When the GPS signal arrives at earth it may reflect off various obstructions

• First the antenna receives the signal by the direct route and then the reflected signal arrives a little later

Accuracy 10 - 30 m

In theory a point position can be accurate to 10 - 30m based on the C/A Code

Improve my Accuracy ?

Use

Differential GPS

A

B

Differential GPS

• The position of Rover ‘B’ can be determine in relation to Reference ‘A’ provided

• Coordinates of ‘A’ is known

• Simultaneous GPS observations

• Differential Positioning

• Eliminates errors in the sat. and receiver clocks

• Minimizes atmospheric delays

• Accuracy 3mm - 5m

A

B

Differential Code / Phase

• If using Codeonly accuracy is in the range of 30 - 50 cm This is typically referred to as DGPS

• If using Phase or Code & Phase accuracy is in the order of 5 - 10 mm + 1ppm

Time (1)

Ambiguity

Ambiguity

Initial

Phase Measurement

at Time (0)

Measured

Phase Observable

at Time (1)

Initial Phase Ambiguity

Initial phase Ambiguity must be determined to use carrier phase data as distance measurements over time

Once the ambiguities are resolved, the accuracy of the measurement does not significantly improve with time

The effect of resolving the ambiguity is shown below:

Accuracy (m)

1.00

Ambiguities

Not resolved

0.10

Ambiguities

Resolved

0.01

Time (mins)

0 120

Rapid Static

Static

0 2 5

Rapid Static

Poor GDOP

Dilution of Precision (DOP)

• A description of purely geometrical contribution to the uncertainty in a position fix

• It is an indicator as to the geometrical strength of the satellites being tracked at the time of measurement

• GDOP (Geometrical), Includes Lat, Lon, Height & Time

• PDOP (Positional) Includes Lat, Lon & Height

• HDOP (Horizontal)Includes Lat & Lon

• VDOP (Vertical)Includes Height only

• Point Positioning :

• 10 - 30 m (1 epoch solution, depends on SA)

• 5 - 10 m (24 hours)

• Differential Code / Phase :

• 30 - 50 cm (P Code)

• 1 - 5 m (CA Code)

• Differential Phase :

• 5 mm + 1 ppm

### Many Thanks for Your Attention.Leica Geosystems Heerbrugg Switzerland

GPS Surveying

• What is Real Time ?

• What is Real Time GPS ?

• Point Positioning

• Real Time Differential Code

• Real Time Differential Phase

• Real Time Differential Requirements

• Advantages of Real Time GPS

• Limitations

• Real Time Industry Standards

• Real Time Modes Supported

• Applications

• Planning a Real Time Survey

• Important Considerations - On Site

In a scientific sense Real Time can be defined as any action undertaken that results in an instantaneous response. Look at your watch. The time displayed is happening in Real Time.

• 3 Distinct Categories:

• Point Positioning ( Navigated Position )

• Real Time Differential Code

• RTIME Code

• RTCM All Version

• Real Time Differential Phase

• RT-SKI

• RTCM All Version

• 3 Distinct Operation Methods:

• Accuracy

• Limitation

• Complexity

Accuracy 10 to 20m in each component

Dependent on DoD Selective Availability

Not suited for Surveying or Precise Navigation

• At Reference Station

• Reference Station on a Known Point

• Tracks all Satellites in View

• Computes corrections for each satellite

• Transmits corrections via a communication link in either propriety format or in the RTCM format

• At the Rover Station

• Rover position corrected by applying the received corrections

• ACCURACY 0.3m - 0.5m

• At Reference Station

• Reference Station on a Known Point

• Tracks all Satellites in View

• Transmits via a communication link GPS

• Measurements along with the Reference Station Coordinates

• At the Rover Station

• Rover receives the GPS Measurements and Reference Station Coordinates via the communication link

• Rover undertakes computations to resolve Ambiguities

• ACCURACY 1 – 2cm + 2ppm

• Initial Coordinates (WGS84)

• Known Coordinates

• Single Point Positioning

• Range to be covered.

• Inter-visibility

• Weight and Power requirements

• Operational Costs

• Getting into Local Coordinate Systems

• Local Ground

• State Plane

• GPS Hardware

• Dual Frequency

• Single Frequency

• Good Accuracy

• No post processing

• Immediate Results

• One man operation

• One Base multiple rovers increases production

• Collect raw data

• Increased confidence

• Ease of operation

• The two largest limitations effecting Real Time GPS Surveying

• Obstructions

• Multipath

• Loss of lock

• Range

• Location of Transmitter

• Power Consumption

• Real Time GPS has become an acceptable tool within the Survey Industry. It is not always the correct tool for the task.

• Radio Technical Commission for Maritime

• RTCM message typically consists of

• Reference station parameters

• Pseudorange Corrections

• Range Rate Corrections

• Corrections are based on the L1 Pseudorange observation

• UHF radios up to 40 Km

• VHF radios up to 100 Km

• Communication Satellites

• Every measurement is independent, no need for ambiguity resolution

• E.g: US Coast Guard Nav Beacons:

• Service is free

• Accuracy in the range of 1 - 5 m

• Ideal for GIS Surveys and hydographic work

• Boundaries

• Seismic Stakeout

• Profiles

• Establishing Portable Control Stations (sharing with Total Stations)

• Slope Staking

• Topo and Locations

• Mapping

• Monitoring

• Volumes

• Photo control

• Construction Control and Stakeout

Ground Surface

Design Surface

in DXF format

Applications (Real Time)

DTM Stakeout

• Horizontal

• Tangents, Spirals, Curves

• Profiles

• Parabolic Curves

• Cross Sections

• Accuracy Requirements

• Code = meter / sub-meter

• Phase = centimeter

• Availability of Control

• Horizontal

• Vertical

• Both

• Type of Transformation

• Local Grid

• WGS84

• Availability of satellites

• Installation of Reference Station

• Minimum obstructions

• Known Coordinates

• Check stations

• Check Hardware

• Check Battery and Memory capacity

• Check Stations

• Verifiy transformation

• Verifiy Base Station coordinates

• Verifiy Heights of Instruments, Ant. Offsets

• Quality Assurance

• Coordinate Quality Indicator

• Averaging Limit

• ### Many Thanks for Your Attention.Leica Geosystems Heerbrugg Switzerland

Operation Types

and

Applications

• CONTENTS

• Using GPS for Surveying

• Static

• Rapid Static

• Kinematics

• Real Time

• Accuracy and Observation Time

• Recommended Recording Intervals

• Using GPS for Surveying

• All GPS Surveying is carried out using differential techniques. That is to say a baseline is measured from a fixed point, (a reference station) to an unknown point (a rover station).

• This is undertaken using one of two methods :

Post ProcessingThe raw GPS data from the satellites is recorded and processed in the office using software LGO

Real TimeThe processing of the data is carried out as you work, giving an instantaneous and accurate position

• All GPS Surveying is carried out using differential techniques. That is to say a baseline is measured from a fixed point, (a reference station) to an unknown point (a rover station).

• This is undertaken using one of two methods :

Post ProcessingThe raw GPS data from the satellites is recorded and processed in the office using software used to create control points by putting one GPS unit on a known point and the second on the unknown point and collect a data. After that post processing must be done using a software to solve the unknown point

Real TimeThe processing of the data is carried out as you work, giving an instantaneous and accurate position

• Short observation time for baselinesup to 20 km.

• Accuracy is5-10 mm + 1 ppm

• Applications

Control Surveys, GIS city inventories, detail surveys. Replace traversing and local triangulation. Any job where many points have to be surveyed

Easy, quick, efficient

Ideal for short range survey

Rover

Rover

Rover

Rover

Rover

Rover

Rover

Rover

8

Rapid Static Survey (STS) - 2/2

• 1 Reference and 1 Rover

5

4

6

7

3

Ref 1

1

2

Training GPS System 1200 June 2007 DJE-3192

Rover

Rover

Rover

Rover

Rover

Rover

Rover

Rapid Static Survey (STS) - 2/2

• 2 Reference and 1 Rover

• 1 Reference and 1 Rover

5

4

6

7

3

Ref 1

Ref2

1

2

Training GPS System 1200 June 2007 DJE-3192

Reference

Reference

Reference

Reference

Reference

Rover

Rover

Rover

Rover

Rover

Rover

Rover

Rover

Rover

Reference

6

7

7

1

1

2

Rapid Static Survey (STS) - 2/2

• 2 Reference and 1 Rover

• 1 Reference and 1 Rover (leap frog)

• 1 Reference and 1 Rover

5

4

6

7

3

Ref 1

Ref2

1

2

Training GPS System 1200 June 2007 DJE-3192

23 : 10 : 20

23 : 10 :27

23 : 10 :18

23 : 10 :30

23 : 10 :26

23 : 10 : 20

23 : 10 :24

23 : 10 :18

23 : 10 :16

23 : 10 :22

23 : 10 :14

23 : 10 :12

True Kinematic (KIS)

• Stop Mode

• The rover must first initialize

• Accuracy : 10 - 20 mm + 1 ppm

• Moving Mode

• Once enough data is collected to resolve the ambiguities, user can now move the receiver

• Lock must be maintained on a minimum of 4 satellites at all time

• Rover records data at a specific time interval

• If lock is lost, the system must re-initialize

Training GPS System 1200 June 2007 DJE-3192

23 : 10 : 20

23 : 10 :27

23 : 10 :18

23 : 10 :30

23 : 10 :26

23 : 10 : 20

23 : 10 :24

23 : 10 :18

23 : 10 :16

23 : 10 :22

23 : 10 :14

23 : 10 :12

Kinematic on the Fly (KOF) - 1/2

• Accuracy : 10 - 20 mm + 1 ppm

• Moving Mode

• This technique does not require a static initialization

• While moving, once the rover is continuously tracking a minimum of 5 satellites on the L1 & L2 for a period of time, the ambiguities can be resolved

• Travelling under an obstruction will cause a loss of lock

Training GPS System 1200 June 2007 DJE-3192

23 : 10 :55

23 : 10 :54

23 : 11 :00

23 : 10 :52

23 : 10 :58

23 : 10 :53

23 : 11 :02

23 : 10 :57

23 : 10 : 51

23 : 11 : 01

23 : 10 : 56

23 : 10 : 28

23 : 10 : 20

23 : 10 :27

23 : 10 :18

23 : 10 :30

23 : 10 :26

23 : 10 : 20

23 : 10 :24

23 : 10 :18

23 : 10 :16

23 : 10 :22

23 : 10 :14

23 : 10 :12

Kinematic on the Fly (KOF) - 1/2

• Moving Mode

• Ambiguity resolution will re-establish once 5 satellites on L1 & L2 are acquired and tracking is consistent for a short period of time

• This technique allows positions to be determined up to the point that the minimum satellites were re-acquired

Training GPS System 1200 June 2007 DJE-3192

• Real Time Code, Real Time Phase

• No post processing required

• Results are instantly available

• Can operate in two modes

• RT-SKI

• RT-DGPS

B

A

Training GPS System 1200 June 2007 DJE-3192

Observation

Time

Baseline

Length

Number of

Satellites

Accuracy

GDOP

• Static :

• 6

• 6

• 6

• 4

• 4

• 4

2 - 3 hr

min. 3 hr

min. 4 hr

5 mm + 1 ppm

5 mm + 1 ppm

5 mm + 1 ppm

20 - 50 Km

50 - 100 Km

> 100 Km

Rapid Static :

Observation

Time

Baseline

Length

Number of

Satellites

Accuracy

GDOP

• 4

• 4

• 4

• 5

• 5

• 5

5 - 10 min

10 - 15 min

10 - 20 min

5 - 10 mm + 1 ppm

5 - 10 mm + 1 ppm

5 - 10 mm + 1 ppm

0 - 5 Km

5 - 10 Km

10 - 30 Km

Training GPS System 1200 June 2007 DJE-3192

Operation

Type

Recording

Interval

Static

RapidStatic

Kinematic

10 sec

5 - 10 sec

0.2 sec or more

Training GPS System 1200 June 2007 DJE-3192