GPS Basic Theory

GPSBasic Theory

GPS General CharacteristicsGPS System ComponentsOutline Principle: Range Position

Range Determination from: Code Observations Phase Observations

Error SourcesDifferential GPSInitial Phase AmbiguityResolving the AmbiguityDilution of PrecisionSummary

Contents

Developed by the US Department of Defense

ProvidesAccurate Navigation 10 - 20 mWorldwide Coverage24 hour accessCommon Coordinate System

Designed to replace existing navigation systems

Accessible by Civil and Military

VllVlll

Range = Time Taken x Speed of Light

GPS Principle : Range

Control Segment1 Master Station5 Monitoring Stations

Space SegmentNAVSTAR : NavigationSatellite Time and Ranging24 Satellites20200 Km

User SegmentReceive Satellite Signal

GPS System Components

We are somewhere on a sphere of radius, R1

2 Spheres intersect as a circle

•3 Spheres intersect at a point•3 Ranges to resolve for Latitude, Longitude and Height

GPS Principle : Point Positioning

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 clock1/10 second error = 30,000 Km error1/1,000,000 second error = 300 m error

Outline Principle : Position

4 Ranges to resolve for Latitude, Longitude, Height & TimeIt is similar in principle to a resection problem

Point Positioning

Point Positioning with at least 4 GPS satellites and Good Geometry

Point Positioning

Like all other Surveying Equipment GPS works in the Real World

That means it owns a set of unique errors

Error Sources

Satellite Clock Modelthough they use atomic clocks, they are still subject to small inaccuracies in their time keepingThese inaccuracies will translate into positional errors.

Orbit Uncertainty

The satellites position in space is also important as it’s the beginning for all calculationsThey drift slightly from their predicted orbit

Satellite Errors

GPS signals transmit their timing information via radio wavesIt 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

50 KmTroposphere

Ionosphere200 Km

Observation Errors

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

Internal receiver noise

Receiver clock drift

F1 F2 F3 F4 F5 F6

ESC SFT CE

Receiver 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

Multipath Error

Accuracy 10 - 30 mAccuracy 10 - 30 m

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

Point Positioning Accuracy

How do I Improve my Accuracy ?

UseDifferential GPS

• The position of Rover ‘B’ can be determine in relation to Reference ‘A’ providedCoordinates of ‘A’ is knownSimultaneous GPS

observations

• Differential PositioningEliminates errors in the sat.

and receiver clocksMinimizes atmospheric

delaysAccuracy 3mm - 5m

Baseline VectorBaseline VectorBAA

Differential GPS

Baseline VectorBaseline VectorBAA

• If using Code only 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

Differential Code / Phase

Time (0)

Ambiguity

InitialPhase Measurement

at Time (0)

Ambiguity

Time (1)

MeasuredPhase Observable

at Time (1)

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

Initial Phase Ambiguity

Rapid StaticRapid Static

Accuracy (m)

Static 0 120

Rapid Static 0 2 5

Time (mins)

AmbiguitiesNot resolved

Ambiguities Resolved

Once the ambiguities are resolved, the accuracy of the measurement does not significantly improve with timeThe effect of resolving the ambiguity is shown below:

Resolving Ambiguities

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

Good GDOPGood GDOPPoor GDOPPoor GDOP

Dilution of Precision (DOP)

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

Summary of GPS Positioning

Many Thanks for Your Attention.

Leica Geosystems Heerbrugg Switzerland

Real TimeGPS Surveying

• Limitations• Real Time Industry

Standards• Real Time Modes Supported• Applications• Planning a Real Time Survey• Important Considerations -

On Site

• 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

Contents

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 ?

3 Distinct Categories:

• Point Positioning ( Navigated Position )

• Real Time Differential CodeRTIME Code

RTCM All Version• Real Time Differential Phase

RT-SKIRTCM All Version

3 Distinct Operation Methods:

• Accuracy• Limitation• Complexity

What is Real Time GPS ?

Accuracy 10 to 20m in each component

Dependent on DoD Selective Availability

Navigation Applications

Not suited for Surveying or Precise Navigation

Point Positioning

• At Reference StationReference Station on a Known PointTracks all Satellites in ViewComputes corrections for each satelliteTransmits corrections via a communication link in either

propriety format or in the RTCM format

• At the Rover StationRover unit receives the corrections via the communication linkRover position corrected by applying the received corrections

ACCURACY 0.3m - 0.5m

Real Time Differential Code (RTIME Code)

• At Reference StationReference Station on a Known PointTracks all Satellites in ViewTransmits via a communication link GPSMeasurements along with the Reference Station

Coordinates

• At the Rover StationRover receives the GPS Measurements and Reference

Station Coordinates via the communication linkRover undertakes computations to resolve Ambiguities

ACCURACY 1 – 2cm + 2ppm

Real Time Phase (RTSKI)

• Initial Coordinates (WGS84)

Known CoordinatesSingle Point Positioning

• Communication LinkRange to be covered.Inter-visibilityWeight and Power

requirementsOperational Costs

• Getting into Local Coordinate Systems• Local Ground• State Plane

• GPS Hardware• Dual Frequency• Single

Frequency

Real Time Differential Requirements

Good Accuracy• No post processing

Immediate Results• One man operation

One Base multiple rovers increases production

• Collect raw dataIncreased confidence

• Ease of operation

Advantages of Real Time GPS

• The two largest limitations effecting Real Time GPS SurveyingObstructions

MultipathLoss of lock

Communication LinkRangeLocation of TransmitterPower Consumption

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

Limitation

RRadio TTechnical CCommission for MMaritimeRTCM message typically consists of

Reference station parametersPseudorange CorrectionsRange Rate Corrections

Corrections are based on the L1 Pseudorange observation

Corrections are broadcast by:UHF radios up to 40 KmVHF radios up to 100 KmCommunication 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 freeAccuracy in the range of 1 - 5 mIdeal for GIS Surveys and

hydographic work

Real Time Industry Standard: RTCM

• Topo and Locations• Mapping• Monitoring• Volumes• Photo control• Construction

Control and Stakeout

• Boundaries• Seismic Stakeout• Profiles• Establishing

Portable Control Stations (sharing with Total Stations)

• Slope Staking

Applications

Existing Ground Surface

Design Surfacein DXF format

DTM Stakeout

Applications (Real Time)

Road AlignmentsHorizontal

Tangents, Spirals, CurvesProfiles

Parabolic CurvesCross Sections

Applications (Real Time)

• Accuracy RequirementsCode = meter / sub-meterPhase = centimeter

• Availability of ControlHorizontalVerticalBoth

• Type of TransformationLocal GridWGS84

Planning a Real Time Project

• Availability of satellites

• Installation of Reference Station

Communication LinkMinimum obstructionsKnown CoordinatesCheck stations

Planning a Real Time Project

• Check HardwareCheck Battery and Memory capacity

• Check StationsVerifiy transformationVerifiy Base Station coordinatesVerifiy Heights of Instruments, Ant.

Offsets

• Quality AssuranceCoordinate Quality IndicatorAveraging Limit

Important Considerations - On Site

Leica Geosystems Heerbrugg Switzerland

Different GPSOperation Types

andApplications

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

Static Survey (STS)

Short observation time for baselines up to 20 km.

Accuracy is 5-10 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, efficientIdeal for short range survey

Rapid Static Survey (STS) - 1/2

48 Training GPS System 1200 June 2007 DJE-3192

Ref 1Ref 1

1 Reference and 1 RoverRapid Static Survey (STS) - 2/2

RoverRover

RoverRoverRover

Reference

Ref 1Ref 1

2 Reference and 1 Rover

Ref2Ref2

Reference

Rover Rover

Ref 1Ref 1

Reference

RoverRover

Reference

Ref2Ref2

Reference

Reference Rover

Rover Rover

1 Reference and 1 Rover (leap frog)

2 Reference and 1 Rover

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Accuracy : 10 - 20 mm + 1 ppm Stop Mode The rover must first initialize

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

True Kinematic (KIS)

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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

Kinematic on the Fly (KOF) - 1/2Accuracy : 10 - 20 mm + 1 ppm

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

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Kinematic on the Fly (KOF) - 1/2

Real Time Code, Real Time Phase No post processing required Results are instantly available Can operate in two modes

RT-SKI RT-DGPS

Real Time

Baseline Length

Number ofSatellites GDOP Observation

Time Accuracy

20 - 50 Km50 - 100 Km> 100 Km

2 - 3 hrmin. 3 hrmin. 4 hr

5 mm + 1 ppm5 mm + 1 ppm 5 mm + 1 ppm

Static :

Rapid Static :Baseline Length

Number ofSatellites GDOP Observation

Time Accuracy

0 - 5 Km5 - 10 Km

10 - 30 Km

5 - 10 min10 - 15 min10 - 20 min

5 - 10 mm + 1 ppm5 - 10 mm + 1 ppm 5 - 10 mm + 1 ppm

Accuracy and Observation Times

OperationType

RecordingInterval

Static

Rapid Static

Kinematic

10 sec

5 - 10 sec

0.2 sec or more

Recommended Recording Intervals

Leica Geosystems, Heerbrugg

Switzerland

GPS Basic Theory

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