Download - GPS Basic Theory
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GPSBasic Theory
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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
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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
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Range = Time Taken x Speed of Light
GPS Principle : Range
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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
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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
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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
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4 Ranges to resolve for Latitude, Longitude, Height & Time
It is similar in principle to a resection problem
Point Positioning
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Point Positioning with at least 4 GPS satellites and Good Geometry
Point Positioning
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Like all other Surveying Equipment GPS works in the Real World
That means it owns a set of unique errors
Error Sources
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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
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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
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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
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• 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
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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
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How do I Improve my Accuracy ?
UseDifferential GPS
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• 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
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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
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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
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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
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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)
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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
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Many Thanks for Your Attention.
Leica Geosystems Heerbrugg Switzerland
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Real TimeGPS Surveying
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• 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
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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 ?
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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 ?
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Accuracy 10 to 20m in each component
Dependent on DoD Selective Availability
Navigation Applications
Not suited for Surveying or Precise Navigation
Point Positioning
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• 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)
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• 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)
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• 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
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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
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• 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
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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
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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
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• 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
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Existing
Ground Surface
Design Surface
in DXF format
DTM Stakeout
Applications (Real Time)
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Road Alignments
Horizontal
Tangents, Spirals, Curves
Profiles
Parabolic Curves
Cross Sections
Applications (Real Time)
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• 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
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• Availability of satellites
• Installation of Reference Station
Communication Link
Minimum obstructions
Known Coordinates
Check stations
Planning a Real Time Project
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• 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
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Many Thanks for Your Attention.
Leica Geosystems Heerbrugg Switzerland
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Different GPSOperation Types
andApplications
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CONTENTS
• Using GPS for Surveying
• Static
• Rapid Static
• Kinematics
• Real Time
• Accuracy and Observation Time
• Recommended Recording Intervals
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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
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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)
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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
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48 Training GPS System 1200 June 2007 DJE-3192
Ref 1Ref 1
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1 Reference and 1 Rover
Rapid Static Survey (STS) - 2/2
88
Rover
RoverRover
Rover
Rover
RoverRover
Rover
Reference
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49 Training GPS System 1200 June 2007 DJE-3192
Ref 1Ref 1
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Rapid Static Survey (STS) - 2/2
2 Reference and 1 Rover
Ref2Ref2
Reference
Rover
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50 Training GPS System 1200 June 2007 DJE-3192
Ref 1Ref 1
44 55
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1 Reference and 1 Rover
Rapid Static Survey (STS) - 2/2
Rover
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Rover
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Rover
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Reference Rover
RoverRover
1 Reference and 1 Rover (leap frog)
2 Reference and 1 Rover
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51 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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52 Training GPS System 1200 June 2007 DJE-3192
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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
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53 Training GPS System 1200 June 2007 DJE-3192
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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54 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
BAA
Real Time
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55 Training GPS System 1200 June 2007 DJE-3192
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
4 4 4
5 5 5
4 4 4
6 6 6
Accuracy and Observation Times
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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
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Many Thanks for Your Attention.
Leica Geosystems, Heerbrugg
Switzerland