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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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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

  • 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


Range = Time Taken x Speed of Light

Xll

Xll

l

l

lX

lX

ll

ll

X

X

lll

lll

Xl

Xl

Vlll

Vlll

lV

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GPS Principle : Range


GPS System Components

Space Segment

NAVSTAR : Navigation

Satellite Time and Ranging

24 Satellites

20200 Km

Control Segment

1 Master Station

5 Monitoring Stations

User Segment

Receive Satellite Signal


R3

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


Outline Principle : Position

  • 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


  • Point Positioning

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

    It is similar in principle to a resection problem


    Point Positioning

    Point Positioning with at least 4 GPS satellites and Good Geometry


    Error Sources

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

  • That means it owns a set of unique errors


  • Satellite 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


    Observation Errors

    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


    Receiver Error

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

      • Internal receiver noise

      • Receiver clock drift


    Multipath Error

    • 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


    Point Positioning Accuracy

    Accuracy 10 - 30 m

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


    How do I

    Improve my Accuracy ?

    Use

    Differential GPS


    Baseline Vector

    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


    Baseline Vector

    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 (0)

    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


    Resolving Ambiguities

    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


    Good GDOP

    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


    Summary of GPS Positioning

    • 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

    Many Thanks for Your Attention.Leica Geosystems Heerbrugg Switzerland


    Real Time

    GPS Surveying


    Contents

    • 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


    What is Real Time ?

    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.


    What is Real Time GPS ?

    • 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


    Point Positioning

    Accuracy 10 to 20m in each component

    Dependent on DoD Selective Availability

    Navigation Applications

    Not suited for Surveying or Precise Navigation


    Real Time Differential Code (RTIME Code)

    • 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 unit receives the corrections via the communication link

      • Rover position corrected by applying the received corrections

      • ACCURACY 0.3m - 0.5m


    Real Time Phase (RTSKI)

    • 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


    Real Time Differential Requirements

    • Initial Coordinates (WGS84)

      • Known Coordinates

      • Single Point Positioning

    • Communication Link

      • 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


    Advantages of Real Time GPS

    • Good Accuracy

    • No post processing

      • Immediate Results

  • One man operation

    • One Base multiple rovers increases production

  • Collect raw data

    • Increased confidence

  • Ease of operation


  • Limitation

    • The two largest limitations effecting Real Time GPS Surveying

      • Obstructions

        • Multipath

        • Loss of lock

      • Communication Link

        • 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.


    Real Time Industry Standard: RTCM

    • 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

  • Corrections are broadcast by:

    • UHF radios up to 40 Km

    • VHF radios up to 100 Km

    • Communication Satellites

  • Every measurement is independent, no need for ambiguity resolution


  • Real Time Industry Standard: RTCM

    • E.g: US Coast Guard Nav Beacons:

      • Broadcast RTCM

      • Service is free

      • Accuracy in the range of 1 - 5 m

      • Ideal for GIS Surveys and hydographic work


    Applications

    • 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


    Existing

    Ground Surface

    Design Surface

    in DXF format

    Applications (Real Time)

    DTM Stakeout


    Applications (Real Time)

    • Road Alignments

      • Horizontal

        • Tangents, Spirals, Curves

      • Profiles

        • Parabolic Curves

      • Cross Sections


    Planning a Real Time Project

    • Accuracy Requirements

      • Code = meter / sub-meter

      • Phase = centimeter

  • Availability of Control

    • Horizontal

    • Vertical

    • Both

  • Type of Transformation

    • Local Grid

    • WGS84


  • Planning a Real Time Project

    • Availability of satellites

    • Installation of Reference Station

      • Communication Link

      • Minimum obstructions

      • Known Coordinates

      • Check stations


    Important Considerations - On Site

    • 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 switzerland1

    Many Thanks for Your Attention.Leica Geosystems Heerbrugg Switzerland


    Different GPS

    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


    Static Survey (STS)

    • 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


    Rapid Static Survey (STS) - 1/2

    • 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

    • Advantages

      Easy, quick, efficient

      Ideal for short range survey


    Rapid static survey sts 2 2

    Reference

    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


    Rapid static survey sts 2 21

    Reference

    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


    Rapid static survey sts 2 22

    Reference

    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


    True kinematic kis

    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

    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


    Kinematic on the fly kof 1 2

    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

    • 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


    Kinematic on the fly kof 1 21

    23 : 10 :59

    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
    Real Time

    • 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


    Accuracy and observation times
    Accuracy and Observation Times

    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


    Recommended recording intervals
    Recommended Recording Intervals

    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


    Many thanks for your attention leica geosystems heerbrugg switzerland2

    Many Thanks for Your Attention.Leica Geosystems, Heerbrugg Switzerland


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