Submitted by:
Kali Kedar Nath Behera
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GLOBAL POSITIONING SYSTEM
INTRODUCTION
GPS is a Global Navigation Satellite System(GNSS).
Uses a constellation of at least 24 satellites orbiting around the earth.
Transmits precise microwave signals, enabling GPS receivers to determine their location, speed, direction and time.
First developed by United States Department of Defense- official name is NAVSTAR-GPS.
GPS satellite navigation system includes
* Russian GLONASS
* European Galileo positioning system.
* Compass navigation system of China.
* IRNSS of India.
Fig: GPS Navigation
2. TECHNICAL DESCRIPTION
2.1 System Segmentation
GPS consists of three major segments:
Space Segment(SS)
Control Segment(CS)
User Segment(US)
Fig: Segments of Navigation
2.1.1 Space Segment:
• Comprises the orbiting GPS satellites or Space Vehicles(SV) in GPS parlance.
• GPS design originally called for 24 SVs, 8 in each three circular orbital planes. Now modified to 6 planes with 4 satellites each.
• Orbiting at an altitude of approx. 20,200 km, each SV makes two complete orbits.
2.1.2 Control Segment:
The flight paths of the satellites tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado.
The tracked information sent to the Air Force Space Commands master control station at Schriever Operation Squadron(2 SOPS) of USAF.
SOPS contacts each GPS satellites regularly with a navigational updates which are created by a Kalman filter using inputs from the ground monitoring stations, space weather information.
2.1.3 User Segment:
GPS receiver is the user segment of GPS.
GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock.
It include a display for providing location and speed information to the user.
A receiver is often described by its number of channels: signifies how many satellites it can monitor simultaneously.
2.2 Navigation Signals:
Each GPS satellites broadcasts a Navigation Message at 50 bits/sec giving the time-of-week, GPS week number and satellite
* health information (all transmitted in the first part of the message).
* ephemeris( transmitted in the second part of the message).
* almanac(later part of the message).
• The messages are sent in frames, each taking 30 sec to transmit 1500 bits. The first 6 sec of every frame contains data describing the satellite clock & its relationship to GPS time.
• The next 12 sec contain the ephemeris data, giving the satellite’s own precise orbit.
• The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model & information to relate GPS derived time to Coordinated Universal Time(UTC).
2.2 Navigation Signals
Each satellite transmits its navigation message with at least two distinct spread spectrums codes:
* Coarse/Acquisition (C/A) code.
* Precise(P) code.
(C/A) code:
It is freely available to the public.
C/A code is 1023 chip pseudo-random number(PRN) code at 1.023 million chips per second so that it repeats every millisecond.
Each satellite has its own C/A code so that it can be uniquely identified and received separately from other satellites transmitting on the same frequency.
2.2 Navigation Signals:
Precise(P) code:
It is encrypted and reserved for military applications.
It is a 1023 mega chip per second PRN code that repeats only every week.
P code is encrypted by the Y-code to produce the P(Y) code, which can only by decrypted by units with a valid decryption key.
Both C/A and P(Y) codes impart the precise time-of-day to the user.
2.3 GPS Frequencies:
2.4 Accuracy and error sources:
2.4.1 Atmospheric Effects:
* The travel time of GPS satellite signals can be altered by atmospheric effects; when a GPS signal passes through the ionosphere and troposphere it is refracted, causing the speed of the signal to be different from the speed of a GPS signal in space. Sunspot activity also causes interference with GPS signals.
2.4.2 Multipath Effects:
* It arises when signals transmitted from the satellites bounce
off a reflective surface before getting to the receiver antenna.
When this happens, the receiver gets the signal in straight
line path as well as delayed path (multiple paths). The effect
is similar to a ghost or double image on a TV set.
2.4.3 Ephemeris and Clock errors:
* Errors in the ephemeris data (the information about satellite orbits) will also cause errors in computed positions, because the satellites weren't really where the GPS receiver "thought" they were (based on the information it received) when it computed the positions.
* Small variations in the atomic clocks (clock drift) on board the satellites can translate to large position errors; a clock error of 1 nanosecond translates to 1 foot or 0.3 meters user error on the ground.
2.4.4 Selective Availability:
Selective Availability, or SA, occurred when the DoP(Dilution of Precision)
intentionally degraded the accuracy of GPS signals by introducing artificial clock and ephemeris errors.
When SA was implemented, it was the largest component of GPS error, causing error of up to 100 meters. SA is a component of the Standard Positioning Service (SPS),
which was formally implemented on March 25, 1990, and was intended to protect national defense. SA was turned off on May 1, 2000.
2.4 Accuracy & error sources:
2.4.5 Sagnac Distortion
GPS observation processing must also compensate for the Sagnac effect. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.
*Potential errors are one of several accuracy-degrading effects outlined in the table below:
2.5 GPS Interference & Jamming:
2.5.1 Natural sources:
Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.
Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun.
GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field.
2.5.2 Artificial sources:
In automotive GPS receivers, metallic features in windshields such as defrosters, or car window tinting films can act as a Faraday cage, degrading reception just inside the car.
Man-made EMI (electro-magnetic interference) can also disrupt, or jam, GPS signals.
Stronger signals can interfere with GPS receivers when they are within radio range, or line of sight.
3.Techniques to improve accuracy:
3.1 Augmentation:
This method of improving accuracy rely on external information being integrated into calculation process.
Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.
3.2 Precise Monitoring:
• This method uses two approaches:
Carrier-Phase Enhancement(CPGPS)
Relative Kinematic Positioning(RKP)
3.Techniques to improve accuracy:
CPGPS:
* This technique resolve the uncertainty that arises in GPS due to pulse transition of PRN is not instantaneous & thus correlation operation is imperfect.
* It utilizes the L1 carrier wave to resolve the uncertainty.
RKP:
*In this approach, determination of range signal can be resolved to a precision of less than 10cm. This is done by resolving the number of cycles in which the signal is transmitted and received the receiver.
3.Techniques to improve accuracy:
3.3 GPS time and date:
* While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC.
* To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal.
3.4 GPS modernization:
*This approach aims to improve the accuracy and availability for all users and involves new ground stations, new satellites & four additional navigation signals.
4.Applications:
GPS has significant applications for both military and civilian industry.
Military Applications:
Navigation
Target tracking
Missile and projectile guidance
Search and rescue
Map creation
4.Applications:
Civilian:
GPS receivers act as a surveying tool to determine the absolute location.
GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere & gravity field.
The capacity to determine relative movement enables a receiver to calculate local velocity & orientation, useful in vessels or observations of the Earth.
5.Conclusion
GPS continues to perform as the world’s premier space-based positioning, navigation and timing-service.
Endeavors such as mapping, aerial refueling, geodetic surveying, search & rescue operations have all benefitted greatly from GPS’s accuracy.
GPS receivers are incorporated into every type of system used by aircraft, ground vehicles and ships.