GPS Theory and applications Atlas Gis

Atlas Gis Positioning Systems Positioning Systems Around The World Is Three Global Positioning System GPS ( USA ) GLONASS (RUSSIAN SYSTEM ) GALILEO (EUROPEAN SYSTEM ) Atlas Gis

Other Positioning Systems Russian SYSTEM Know as GLONASS Around 9 Operational Satellites Hasn’t Reached Full Operational Capability European System Known as GALILEO 2 Satellites launched yet in System Will Start To Operate 2013. All Systems Will Combine To Form GNSS, Global Navigation Satellite Systems Atlas Gis

Global Positioning System GPS Global Positioning System GPS Developed by the US Dep. of Defense in 1974( Civil Access In 1995 ) Navigational GPS Up to 30 - 50 m Accuracy Deferential GPS Up to 0.1 mm Accuracy Atlas Gis

General Characteristics Navigation System with Time and Ranging -Global Positioning System Designed to replace existing navigation systems Developed by the US Dep. of Defense Development started in 1974 According to military aspects Worldwide Coverage 24 hour access Accurate Navigation to 10 m Atlas Gis

General Characteristics Common Coordinate System WGS84 Weather and Sight independent One-way ranging system Cheap end-user receivers Military safe Accessible by Civil and Military Full Operational Capability since June 95 Atlas Gis

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

Atlas Gis GPS Constellation Nominal 24 Satellites Today we have 29Sats 6 orbital planes 55 ° Inclination 60 ° right ascension seperation Almost circular orbits Orbital radius 26.600 km Orbital period 12 h sideral time Repeated satellite constellation every day, 4 min earlier 4 min/day, 2h/month, 24 h/Year Atlas Gis

Fundamental Frequency 10.23 MHz GPS Signal Structure Each GPS satellite transmits a number of signals The signal comprises two carrier waves (L1 and L2) and two codes (C/A on L1 and P or Y on both L1 and L2) as well as a satellite orbit message Fundamental Frequency 10.23 MHz ÷ 10 L1 1575.42 MHz C/A Code 1.023 MHz P-Code 10.23 MHz x 154 L2 1227.60 MHz P-Code 10.23 MHz x 120 50 BPS Satellite Message Atlas Gis Notes :

Navigation Principle with GPS Simultaneous measurement of distances to at least 4 sats Satellites have known coordinates Receiver position: Intersection of 4 spheres around the sats Analytical view: Following unknowns have to be determined LATITUDE, LONGITUDE, HEIGHT RECEIVER CLOCK ERROR 4 observations <=> 4 unknowns Atlas Gis

Range = Runtime x Velocity of Light GPS Principle : Range Range = Runtime x Velocity 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 Atlas Gis

GPS Principle : Point Positioning 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 Atlas Gis

Outline Principle : Position The satellites are like “Orbiting Control Stations” Ranges (distances) are measured to each satellites using time dependent codes Runtime: Time from the transmission of the signal at the satellite till the reception at the receiver Multiply the runtime with the velocity of light Range = Runtime x velocity of light Notes :

Runtime Determination (Code) One-way ranging system: One clock inside the satellite and another one inside the receiver Receiver creates a duplicate of the code Cross correlation between received and generated code sequence Maximum correlation gives the runtime t Satellite Signal Receiver Signal Atlas Gis

Range Determination from Code Observation Pseudoranges (Code) Each satellite sends a unique signal which repeats itself approx. every 1 msec Receiver compares self generated signal with received signal From the time difference (DT) a range observation can be determined Receiver clock needs to be synchronized with the satellite clock Received Code from Satellite Generated Code from Receiver DT D = V (DT) Atlas Gis Notes :

Atlas Gis Pseudorange The Pseudorange Typically GPS receivers use inexpensive clocks. They are much less accurate than the clocks on board the satellites Runtime measurement is done in the receiver Receiver time scale has an offset Consider an error in the receiver clock 1/10 second error = 30,000 Km error 1/1,000,000 second error = 300 m error A radio wave travels at the speed of light actual signal velocity differs from theoretical value Instead of the true distance Satellite - Receiver we obtain the Pseudorange Atlas Gis

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

Range Determination from Phase Observation Phase Observations Wavelength of the signal is 19 cm on L1 and 24 cm on L2 Receiver compares self-generated phase with received phase Number of wavelengths is not known at the time the receiver is switched on (carrier phase ambiguity) As long as you track the satellite, the change in distance can be observed (the carrier phase ambiguity remains constant) Received Satellite Phase Generated Phase from Receiver DT D = c DT + lN Atlas Gis Notes :

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

Atlas Gis 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 Atlas Gis

Atlas Gis 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 Atlas Gis

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

Atlas Gis 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 Atlas Gis

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

Point Positioning Accuracy Accuracy 10 - 100 m Point Positioning under SA : +/- 100 m Horizontally +/- 160 m Vertically Atlas Gis

Point Positioning Accuracy As the owner of the GPS System the USA have decided to limit the access to the full system accuracy for civil users SPS Standard Positioning Service Based on the C/A-Code Worldwide available without limitation Navigation Accuracy von ±100 m in position and ±160 m in height (~95 % probability) PPS Precise Positioning Service Provides the full System accuracy C/A-Code gives ±10-30 m, P-Code ±20 m (~95 %) PPS is available for authorized users only Atlas Gis

How do I Improve my Accuracy ? Use Differential GPS Atlas Gis

Atlas Gis 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 Atlas Gis

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 3 mm + 0.5 Baseline Vector B A Atlas Gis

Initial Phase Ambiguity Initial phase Ambiguity must be determined to use carrier phase data as distance measurements over time Once the ambiguities are resolved, the accuracy of the measurement does not significantly improve with time Atlas Gis Notes :

Resolving Ambiguities The effect of resolving the ambiguity is shown below : Rapid Static Accuracy (m) 1.00 0.10 0.01 Static 0 120 0 2 5 Time (mins) Ambiguities Not resolved Resolved Atlas Gis Notes :

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 Atlas Gis

Summary of GPS Positioning Point Positioning : 10 - 100 m (1 epoch solution, depends on SA) 5 - 10 m (24 hours) Differential Code : 30 - 50 cm (P Code) 1 - 5 m (CA Code) Differential Phase : 3 mm + 0.5 1 cm Atlas Gis Notes :

Thank You For Your Attention Atlas Gis