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GPS ERRORS

GPS provides precise location information globally in all weather conditions using a constellation of satellites. However, GPS measurements can be affected by various errors. A regular consumer GPS receiver has an accuracy of around 10 meters, while higher accuracy of a few centimeters is possible using techniques like dual frequency monitoring, relative kinematic positioning, and augmentation systems that integrate external correction information. Sources of errors include multipath interference, atmospheric delays, satellite geometry, clock errors, and receiver quality.

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GPS ERRORS AND ACCURACY MEASUREMENT By: Maxim Rodrigues K
Content • Introduction • Conclusion • Reference
Introdution What is GPS ? • GPS is a space based satelite navigation system • A very precise positioning system • Provides location & time information in all weather • Maintained by United States Government & freely accessible by anyone with a GPS reciever • Developed and maintained by the US Department of Defense (DOD). • GPS is a constellation of 24 or more satellites flying 20,350 km above the surface of the earth. • Each one circles the planet twice a day in one of six orbits to provide continuous, worldwide coverage. • Its applications are too vast & new applications will come as technology is developing day by day. • It consisit of ground control stations, Satelite in orbit and reciver units
• GPS reciever come in all different shapes & sizes are widespread and are affordable • Today GPS recievers can be found in watches, phones, tablets, computers & wide variety of other devices.
• Radio-based navigation system funded and developed by DoD • First Satellite Launch in 1978 • Initial operation in 1993 • Fully operational in 1995 • System is called NAVSTAR • NAVigation with Satellite Timing And Ranging • Referred to as GPS • Limitation of GPS Must be able to see the satelites • Requires power • Multiple sources of error
Characteristics of GPS • Free • Precise • Reliable (trusted) • Anytime & anywhere • All weather • Unlimited user capacity
• NAVSTAR – USA • GLONASS – RUSSIA • IRNSS – INDIA
Accuracy of positional measurement by GPS reciever • Regular GPS reciver offers accuracy in possitional measurement of the order 10 meter • One should not infer that the system is not capable of delevering anything better • Accuracy of the GPS is more controlled by US-DoD rather than GPS itself
Technical facts to understand how one can control the accuracy of GPS measurement • Signal is transmitted to user from a GPS satelite with the help of GPS receiver which consist of two carrier frequencies called L1 & L2 frequency • L1 frequency is of 1575.42 MHz • L2 frequency = 1227.6 MHz • L1 frequency further modulated by a Precision Code (P-code) and a Coarse acquisition code (C/A-code) • Each GPS satelite is assigned with unique C/A-code along with its P- code
• P-code is made available only to authorised users such as high precisison military users • P-code is protected by encryption technique so that one other than the authorised ones can be able to use this code • On the other hand C/A-code is freely available for civilians & here the accuaracy is meticulously brought down by introducing arbitary errors in the measurement of posistion.
• Both codes are pseudorandom binary codes • L1 & L2 frequencies are also modulated with a 50-bit per second data transmission providing satelite orbit, system, time, sateliteb clock behaviour and the status information of all satelites to the ground control facilities. • Pseudorandom codes helps in the use of cheaper, low-power satelites • It also helps in using small antenna on GPS reciever • This code simplifies the system’s functioning by providing a way to have all satelites work on the same frequency without interfering with others. Here each satelite is alloted just a different code only. • Pseudorandom code gives the Department of Defence, USA (DoD-USA) a method of restricting the access of the user to the system • In the casev of DoD changes the code, then one can not use the GPS any more.
How accurate is GPS? Depends on some variables • Design of receiver • Relative positions of satellites, technically known as PDOP (Position dilution of precision) • Post processing • Time spent on measurement
Accuracy This calculated by a GPS reciever relies o 3 accurate measurement 1. Current time 2. Position of the satelite 3. Time delay for the signal GPS time is accurate to about 14 nanoseconds Higher accuracy is available today by using GPS in combination with augmentation systems. This enables real time positioning with a few centimeters
• Accuracy is how well the information or data matches the true values. • How close you are to the actual value • Precision refers to the level of measurement • How consistent a series of values are to each other
Determining Position • GPS reciever use a mathematical process called trilateration • GPS recievers use 3 dimensional trilateration to tell you • * where you are on the earth • *your current height
GPS POSITIONING • Static Positioning • A GPS reciever is resigned to be stationary while collecting GPS data • Kinematic Positioning • Stationary reciever called the base reciever is placed at a known point while a second reciever called rover will visit all known points • Kinematic GPS survey is suitable for an area where there are no large over hanging trees over passes or such structures in rovers route
Static v/s Kinematic positioning • In static positioning a GPS reciever is set to be stationary wheteas in Kinematic a reciever collects GPS data while moving • For kinematic positioning one reciever is left as stationary on a known point while a second reciever reffered to as rover, is moved over the path.
Triangulation • GPS reciever’s job is to locate 4 or > of satelites, figure out the distance to each and use this information to deduce its own location. • This operation is based on a simple mathematical principle called triangulation or trilateration. • Basic trilateration • Use measurement from 4+ satelites • Distance= travel time * speed of light
• Global Navigation Satelite System (GNNS) positioning is based on the pseudorange between satelite & recievers • The time of flight of radio signals from several satelites to a reciever is used to calculate pseudorange or pseudo-distance. • Even in smaller timing errors can present in large position errors • Long time error might imply a 3 m pseudorange error.
Errors in GPS • GPS measurement are both affected by several types of random errors & systematic errors which affects accuracy measurement
Sources of Error in GPS • Multipath • Atmospheric Delays • Dilution of Precision (DOP) • Clocks • Orbits ephemeris • Receiver electronics/ quality
Dilution of Precision (PDOP) • DOP is an indicator of three dimensional positioning accuracy as consequence of relative position of GPS satellites with respect to a GPS receiver.
• Position Dilution of Precision Good PDOP Poor PDOP
Multipath • When GPS signals arrive at the receiver having traveled different paths
Multipath error • Signal that bounces of a smooth object & hits the reciever antenna • Common in urban, forested areas • Reflection from the objects near the ground
• Multipath errors appear when a GNSS signal arrives at the receiver GNSS antenna after having been reflected from an object such as the surface of a building (see diagram below). The reflected signal clearly has to travel further to reach the antenna and so it arrives with a slight delay. This delay can cause positional error.
Ephemeris or Orbital Error • Satelite position are a funtion of time • Forces on GPS satelite are not perfect • Errors in estimated satelite position known as ephemeris errors
Satelite availability (SA) • Technique to deny accurate real time autonomous positioning to unauthorised users • SA turned on nominal horizontal, vertical errors could be upto 100 mtr & 156 mtr respectively
Satelite clock reciever error • Each GPS block II & block IIA satelite contain two cesium & two rubidium automic clocks • Satelite clock error is about 8.64 to 17.28 nanosecond/ day • Corresponding range error is about2.59 mtr to 5.18 mtr • GPS use inexpensive crystal rocks • Reciever clock error is larger than GPS satelite clock
Ionosphere • Upermost layer of earth’s atmosphere • UV & X-rays radiation from the sun interact with gas molecules and atoms here, which results in gas ionization. • Altitude and thickness of this layer vary with time, as a result of the changes in the sun’s radiation & earth’s magnetic field • Ionosphere is a dispersive medium, it bends the GPS radio signals & change its speed & it passes through various ionopsphere sub layers to reach GPS reciever.
Dispersive Medium • Dispersion is the phenomenon in which the phase velocity of a wave depends on its frequecy & such type of medium is called medium dispersive
Accuracy This calculated by a GPS reciever relies o 3 accurate measurement 1. Current time 2. Position of the satelite 3. Time delay for the signal GPS time is accurate to about 14 nanoseconds Higher accuracy is available today by using GPS in combination with augmentation systems. This enables real time positioning with a few centimeters
Methods of improving accuracy • # precision method • 1. dual frequency monitoring • Carrier phase enhancement (CPGPS) • Relative kinematic positioing (RKP) • # Augmentation
Dual frequency monitoring • Refers to system that can compare two or more signals • Two frquencies are affected in two different ways • After monitoring errors can be calculated
Relative kinematic positioning • Determination of a range signal can be reduced to an accuracy of less than 10 cm • Resolves in no of cycles in which signals is transmitted & received by the recievers
augmentation • Relies on external info bieng integrated into the calculation process • Some system transmit additional minfo about sources and errors • Some provide direct measurement of how much the signal was off in the past • Augmentation system • Nationwide diffrerential GPS system • Wide area augmentation system (WAAS) • Global differential GPS (GDGPS)
Real time correction • The base station calculates and broadcasts corrections for each satellite as it receives the data. • The correction is received by the roving receiver via a radio signal and applied to the position it is calculating. • As a result, the position displayed on the roving GPS receiver is a differentially corrected position
Post Processing Correction • Differentially correcting GPS data by post-processing uses a base GPS receiver that logs positions at a known location and a rover GPS receiver that collects positions in the field. • The files from the base and rover are transferred to the office processing software, which computes corrected positions for the rover's file. • This resulting corrected file can be viewed in or exported to a GIS.
Conclusion • GPS system is self caliberating and we just need to turn on it. • It can be used in the field and it is nut restricted to laboratory. • It works anywhere on earth. • It is widely used in various fields such as collection & mapping, navigation & recreation.
Meyburg et al., 2017
Schofield et al., 2007
References • 1. Ferguson, M.G. (2000). Global Positioning System (GPS) Error Source Prediction (Thesis). Department of the Air Force, AIR University. Wright-Patterson, Air Force Base, Ohio. • 2. Panda, B.C. (2005). Remote sensing principles and application. Viva book private limited, New Delhi. ISBN: 978-81-7649-630-8. • 3. Tsui, J.B. (2000). Fundamentals of Global Positioning System Recievers, A software approach. John Wiley & sons, Inc. ISBN 0-471- 38154-3.
• 4. https://www.slideshare.net/mobile/KutubuddinANSARI/03- gpserrors-59304987. Accessed on 14 August 2019. • 5. https://www.slideshare.net/mobile/maneeb/errors-and-biases-in- gps. Accessed on 15 August 2019. • 6. https://www.slideshare.net/mobile/Atiqa_khan/sources-of-gps- errors-in-a-glance-2016. Accessed on 15 August 2019. • 7. https://www.gps.gov/systems/augmentations/. Accessed on 18 August 2019.