GPS Basic Theory
Mar 18, 2016
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
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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
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 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)
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 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
Many Thanks for Your Attention.
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
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1 Reference and 1 RoverRapid Static Survey (STS) - 2/2
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Rover
RoverRover
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RoverRoverRover
Rover
Reference
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1 Reference and 1 RoverRapid Static Survey (STS) - 2/2
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Rover
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Rover
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Rover
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Rover
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RoverRover
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Rover
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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
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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
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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
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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
4 4 4
5 5 5
4 4 4
6 6 6
Accuracy and Observation Times
56 Training GPS System 1200 June 2007 DJE-3192
OperationType
RecordingInterval
Static
Rapid Static
Kinematic
10 sec
5 - 10 sec
0.2 sec or more
Recommended Recording Intervals
Many Thanks for Your Attention.
Leica Geosystems, Heerbrugg
Switzerland