Leica - Introduction to GPS

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

Provides

Accurate Navigation 10 - 20 m

Worldwide Coverage

24 hour access

Common Coordinate SystemDesigned to replace existing

navigation systems

Accessible by Civil and Military

Xll

Vl

Xl

lll

lll

lVV

VllVlll

X

lX

Xll

Vl

Xl

lll

lll

lVV

VllVlll

X

lX

Range = Time Taken x Speed of Light

GPS Principle : Range

Control Segment1 Master Station5 Monitoring Stations

Control Segment1 Master Station5 Monitoring Stations

Space SegmentNAVSTAR : NavigationSatellite Time and Ranging24 Satellites20200 Km

Space SegmentNAVSTAR : NavigationSatellite Time and Ranging24 Satellites20200 Km

User SegmentReceive Satellite Signal

User SegmentReceive Satellite Signal

GPS System Components

We are somewhere on a sphere of radius, R1

R1

2 Spheres intersect as a circle

R2

•3 Spheres intersect at a point

•3 Ranges to resolve for Latitude, Longitude and Height

R3

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 clock

1/10 second error = 30,000 Km error

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

Outline Principle : Position

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

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 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

Satellite 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

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’ 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 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)

1.00

0.10

0.01

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 time

The 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 Code

RTIME Code

RTCM All Version• Real Time Differential Phase

RT-SKI

RTCM 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 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 Differential Code (RTIME Code)

• 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 Phase (RTSKI)

• 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

Real Time Differential Requirements

Good Accuracy• No post processing

Immediate Results• One man operation

One Base multiple rovers increases production

• Collect raw data

Increased confidence• Ease of operation

Advantages of Real Time GPS

• 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.

Limitation

RRadio TTechnical CCommission for MMaritimeRTCM message typically consists of

Reference station parameters

Pseudorange Corrections

Range Rate CorrectionsCorrections are based on the L1 Pseudorange

observationCorrections are broadcast by:

UHF radios up to 40 Km

VHF radios up to 100 Km

Communication SatellitesEvery 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

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 Surface

in DXF format

DTM Stakeout

Applications (Real Time)

Road Alignments

Horizontal

Tangents, Spirals, Curves

Profiles

Parabolic Curves

Cross Sections

Applications (Real Time)

• 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

Planning a Real Time Project

• 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

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, efficient

Ideal for short range survey

Rapid Static Survey (STS) - 1/2

48 Training GPS System 1200 June 2007 DJE-3192

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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) - 1/2

Accuracy : 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

BAA

Real Time


Baseline Length

Number ofSatellites

GDOPObservation

TimeAccuracy

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

GDOPObservation

TimeAccuracy

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

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5 5 5

4 4 4

6 6 6

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

Leica - Introduction to GPS

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