GPS Basic Theory

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

Developed by the US Department of Defense Provides Accurate Navigation 10 - 20 m Worldwide Coverage 24 hour access Common Coordinate System Designed to replace existing navigation systems Accessible by Civil and Military

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

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

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

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

Point Positioning 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

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

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

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

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

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

Point Positioning Accuracy Accuracy 10 - 30 m In theory a point position can be accurate to 10 - 30m based on the C/A Code

How do I Improve my Accuracy ? Use Differential GPS

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

Differential Code / Phase 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 Baseline Vector B A

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)

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 0 120 Static Rapid Static 0 2 5

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

Summary of GPS Positioning 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

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Real Time GPS Surveying

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

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.

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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Different GPS Operation Types and Applications

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 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

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

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

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 1200 June 2007 DJE-3192

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 1200 June 2007 DJE-3192

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 1200 June 2007 DJE-3192

True Kinematic (KIS) Moving Mode Stop Mode The rover must first initialize Accuracy : 10 - 20 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 23 : 10 :22 23 : 10 :24 23 : 10 :26 23 : 10 :27 23 : 10 : 28 23 : 10 :30 23 : 10 :12 23 : 10 :14 23 : 10 :16 23 : 10 :18 23 : 10 : 20 Training GPS System 1200 June 2007 DJE-3192

Kinematic on the Fly (KOF) - 1/2 Moving Mode This technique does not require a static initialization Accuracy : 10 - 20 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 23 : 10 :22 23 : 10 :24 23 : 10 :26 23 : 10 :27 23 : 10 : 28 23 : 10 :30 23 : 10 :12 23 : 10 :14 23 : 10 :16 23 : 10 :18 23 : 10 : 20 Training GPS System 1200 June 2007 DJE-3192

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 23 : 10 :55 23 : 10 :54 23 : 10 :53 23 : 10 :52 23 : 10 : 51 23 : 11 :00 23 : 10 :59 23 : 10 :58 23 : 10 :57 23 : 10 : 56 23 : 11 :02 23 : 11 : 01 23 : 10 :22 23 : 10 :24 23 : 10 :26 23 : 10 :27 23 : 10 : 28 23 : 10 :30 23 : 10 :12 23 : 10 :14 23 : 10 :16 23 : 10 :18 23 : 10 : 20 Training GPS System 1200 June 2007 DJE-3192

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 1200 June 2007 DJE-3192

Accuracy and Observation Times Static : Baseline Length Number of Satellites Observation Time GDOP Accuracy 20 - 50 Km 50 - 100 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 5 - 10 Km 10 - 30 Km 4 5 5 - 10 min 10 - 15 min 10 - 20 min 5 - 10 mm + 1 ppm Training GPS System 1200 June 2007 DJE-3192

Recommended Recording Intervals Operation Type Recording Interval Static Rapid Static Kinematic 10 sec 5 - 10 sec 0.2 sec or more Training GPS System 1200 June 2007 DJE-3192

Many Thanks for Your Attention. Leica Geosystems, Heerbrugg Switzerland