1. GPS
Basic Theory

2. 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

3. Developed by the US Department of Defense
Provides
Accurate Navigation m
Worldwide Coverage
24 hour access
Common Coordinate System
Designed to replace existing navigation systems
Accessible by Civil and Military

4. Range = Time Taken x Speed of Light
GPS Principle : Range
Range = Time Taken x Speed of Light
Xll Vl Xl
lll l ll lV V
Vll
Vlll X lX
Xll Vl Xl
lll l ll lV V
Vll
Vlll X lX

5. GPS System Components Space Segment Control Segment User Segment
NAVSTAR : Navigation
Satellite Time and Ranging
24 Satellites
20200 Km
Control Segment
1 Master Station
5 Monitoring Stations
User Segment
Receive Satellite Signal

6. 2 Spheres intersect as a circle
GPS Principle : Point Positioning R1
3 Spheres intersect at a point
3 Ranges to resolve for Latitude, Longitude and Height R3
2 Spheres intersect as a circle R2
We are somewhere on a sphere of radius, R1

7. 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

8. Point Positioning
4 Ranges to resolve for Latitude, Longitude, Height & Time
It is similar in principle to a resection problem

9. Point Positioning
Point Positioning with at least 4 GPS satellites and Good Geometry

10. Error Sources
Like all other Surveying Equipment GPS works in the Real World
That means it owns a set of unique errors

11. 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

12. 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

13. Receiver Error
Unfortunately not all the receivers are perfect. They can introduce errors of their own
Internal receiver noise
Receiver clock drift

14. 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

15. Point Positioning Accuracy
Accuracy m
In theory a point position can be accurate to m based on the C/A Code

16. How do I
Improve my Accuracy ?
Use
Differential GPS

17. 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 B A

18. Differential Code / Phase
If using Code only accuracy is in the range of cm This is typically referred to as DGPS
If using Phase or Code & Phase accuracy is in the order of mm + 1ppm
Baseline Vector B A

19. Initial Phase Ambiguity
Initial phase Ambiguity must be determined to use carrier phase data as distance measurements over time
Time (0)
Ambiguity
Initial
Phase Measurement
at Time (0)
Ambiguity
Time (1)
Measured
Phase Observable
at Time (1)

20. 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)
Rapid Static
Static
Rapid Static

21. 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
Poor GDOP
Good GDOP

22. Summary of GPS Positioning
Point Positioning :
m (1 epoch solution, depends on SA)
m (24 hours)
Differential Code / Phase :
cm (P Code)
1 - 5 m (CA Code)
Differential Phase :
5 mm + 1 ppm

23. Many Thanks for Your Attention. Leica Geosystems Heerbrugg Switzerland

24. Real Time
GPS Surveying

25. Contents What is Real Time ? Limitations Real Time Industry Standards
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

26. 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.

27. 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
3 Distinct Operation Methods:
Accuracy
Limitation
Complexity

28. Point Positioning Accuracy 10 to 20m in each component
Dependent on DoD Selective Availability
Navigation Applications
Not suited for Surveying or Precise Navigation

29. 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

30. 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

31. 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

32. Good Accuracy Advantages of Real Time GPS No post processing
Immediate Results
One man operation
One Base multiple rovers increases production
Collect raw data
Increased confidence
Ease of operation

33. 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.

34. 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

35. Real Time Industry Standard: RTCM
E.g: US Coast Guard Nav Beacons:
Broadcast RTCM
Service is free
Accuracy in the range of m
Ideal for GIS Surveys and hydographic work

36. Applications Boundaries Topo and Locations Seismic Stakeout Mapping
Profiles
Establishing Portable Control Stations (sharing with Total Stations)
Slope Staking
Topo and Locations
Mapping
Monitoring
Volumes
Photo control
Construction Control and Stakeout

37. Applications (Real Time)
DTM Stakeout
Existing
Ground Surface
Design Surface
in DXF format

38. Applications (Real Time)
Road Alignments
Horizontal
Tangents, Spirals, Curves
Profiles
Parabolic Curves
Cross Sections

39. 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

40. Planning a Real Time Project
Availability of satellites
Installation of Reference Station
Communication Link
Minimum obstructions
Known Coordinates
Check stations

41. 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

42. Many Thanks for Your Attention. Leica Geosystems Heerbrugg Switzerland

43. Different GPS
Operation Types
and
Applications

44. CONTENTS Using GPS for Surveying Static Rapid Static Kinematics
Real Time
Accuracy and Observation Time
Recommended Recording Intervals

45. 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 Processing The raw GPS data from the satellites is recorded and processed in the office using software LGO
Real Time The processing of the data is carried out as you work, giving an instantaneous and accurate position

46. 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 Processing The 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 Time The processing of the data is carried out as you work, giving an instantaneous and accurate position

47. Rapid Static Survey (STS) - 1/2
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

48. Rapid Static Survey (STS) - 2/2
1 Reference and 1 Rover
Rover
Rover
Rover 5 4 6
Rover
Rover 7
Reference
Rover 3
Ref 1 8
Rover
Rover 1 2
Training GPS System June 2007 DJE-3192

49. Rapid Static Survey (STS) - 2/2
1 Reference and 1 Rover
2 Reference and 1 Rover
Rover
Rover
Rover 5 4 6
Rover
Rover 7
Reference 3
Ref 1
Ref2
Rover
Rover 1 2
Training GPS System June 2007 DJE-3192

50. Rapid Static Survey (STS) - 2/2
1 Reference and 1 Rover
2 Reference and 1 Rover
1 Reference and 1 Rover (leap frog)
Rover
Rover
Reference
Reference
Rover 6 7
Reference
Rover 5 4 6 7 1
Reference
Rover
Rover
Reference 7
Rover
Reference 3
Ref 1
Ref2 2 1
Reference
Rover 1 2
Training GPS System June 2007 DJE-3192

51. True Kinematic (KIS) Moving Mode Stop Mode
The rover must first initialize
Accuracy : 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
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23 : 10 : 20
Training GPS System June 2007 DJE-3192

52. Kinematic on the Fly (KOF) - 1/2
Moving Mode
This technique does not require a static initialization
Accuracy : mm + 1 ppm
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
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Training GPS System June 2007 DJE-3192

53. 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
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23 : 11 :00
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Training GPS System June 2007 DJE-3192

54. Real Time Real Time Code, Real Time Phase B A
No post processing required
Results are instantly available
Can operate in two modes
RT-SKI
RT-DGPS B A
Training GPS System June 2007 DJE-3192

55. Accuracy and Observation Times
Static :
Baseline
Length
Number of
Satellites
Observation
Time
GDOP
Accuracy Km Km
> 100 Km 4 6
2 - 3 hr
min. 3 hr
min. 4 hr
5 mm + 1 ppm
Rapid Static :
Baseline
Length
Number of
Satellites
Observation
Time
GDOP
Accuracy
0 - 5 Km Km Km 4 5
min
min
min
mm + 1 ppm
Training GPS System June 2007 DJE-3192

56. Recommended Recording Intervals
Operation
Type
Recording
Interval
Static
Rapid Static
Kinematic
10 sec
sec
0.2 sec or more
Training GPS System June 2007 DJE-3192

57. Many Thanks for Your Attention. Leica Geosystems, Heerbrugg Switzerland