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GPS Basic Theory. GPS General Characteristics GPS System Components Outline Principle: Range Position Range Determination from: Code Observations Phase.

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Presentation on theme: "GPS Basic Theory. GPS General Characteristics GPS System Components Outline Principle: Range Position Range Determination from: Code Observations Phase."— Presentation transcript:

1 GPS Basic Theory

2 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 Contents

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

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

6 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

7 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

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

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

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

11 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 its the beginning for all calculations They drift slightly from their predicted orbit Satellite Errors

12 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 Km 50 Km Troposphere Ionosphere 200 Km Observation Errors

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

14 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

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

16 How do I Improve my Accuracy ? Use Differential GPS

17 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 Differential GPS

18 Baseline Vector B A 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 Differential Code / Phase

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

20 Rapid Static Accuracy (m) Static Rapid Static Time (mins) Ambiguities Not 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

21 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 Good GDOP Poor GDOP Dilution of Precision (DOP)

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

23 Many Thanks for Your Attention. Leica Geosystems Heerbrugg Switzerland

24 Real Time GPS Surveying

25 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

26 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 ?

27 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 ?

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

29 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)

30 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)

31 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

32 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

33 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

34 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 Real Time Industry Standard: RTCM

35 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 Real Time Industry Standard: RTCM

36 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

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

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

39 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

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

41 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

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 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 Static Survey (STS)

47 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 48Training GPS System 1200 June 2007 DJE-3192 Ref Reference and 1 Rover Rapid Static Survey (STS) - 2/28 Rover Reference

49 49Training GPS System 1200 June 2007 DJE-3192 Ref Reference and 1 Rover Rapid Static Survey (STS) - 2/2 2 Reference and 1 Rover Ref2 Reference Rover

50 50Training GPS System 1200 June 2007 DJE-3192 Ref Reference and 1 Rover Rapid Static Survey (STS) - 2/2 Rover Reference Rover Reference Rover ReferenceRef2 6 7 Rover 7 1 Reference Rover 2 1 Reference Rover 1 Reference and 1 Rover (leap frog) 2 Reference and 1 Rover

51 51Training GPS System 1200 June 2007 DJE : 10 :22 23 : 10 :24 23 : 10 :26 23 : 10 :27 23 : 10 : : 10 :30 23 : 10 :12 23 : 10 :14 23 : 10 :16 23 : 10 :18 23 : 10 : : 10 :18 23 : 10 : 20 Accuracy : 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)

52 52Training GPS System 1200 June 2007 DJE : 10 :22 23 : 10 :24 23 : 10 :26 23 : 10 :27 23 : 10 : : 10 :30 23 : 10 :12 23 : 10 :14 23 : 10 :16 23 : 10 :18 23 : 10 : : 10 :18 23 : 10 : 20 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 : mm + 1 ppm

53 53Training 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 23 : 10 :5523 : 10 :54 23 : 10 :53 23 : 10 :52 23 : 10 : : 11 :00 23 : 10 :59 23 : 10 :58 23 : 10 :57 23 : 10 : : 11 :02 23 : 11 : : 10 :22 23 : 10 :24 23 : 10 :26 23 : 10 :27 23 : 10 : : 10 :30 23 : 10 :12 23 : 10 :14 23 : 10 :16 23 : 10 :18 23 : 10 : : 10 :18 23 : 10 : 20 Kinematic on the Fly (KOF) - 1/2

54 54Training 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 B A Real Time

55 55Training GPS System 1200 June 2007 DJE-3192 Baseline Length Number of Satellites GDOP Observation Time Accuracy Km Km > 100 Km hr min. 3 hr min. 4 hr 5 mm + 1 ppm Static : Rapid Static : Baseline Length Number of Satellites GDOP Observation Time Accuracy Km Km Km min min min mm + 1 ppm Accuracy and Observation Times

56 56Training GPS System 1200 June 2007 DJE-3192 Operation Type Recording Interval Static Rapid Static Kinematic 10 sec sec 0.2 sec or more Recommended Recording Intervals

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


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