Global Positioning system

—  THE GPS SYSTEM HAS THREE SEGMENTS

     1.GROUND BASED SEGMENT

     2.SPACE SEGMENT

     3.RECEIVER SEGMENT

  

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Space Segment

—  GPS satellites fly in circular orbits at an altitude of 20,200 km and with a period of 12 hours.

—  Powered by solar cells, the satellites continuously orient themselves to point their solar panels toward the sun and their antenna toward the earth.

—  Orbital planes are centered on the Earth

—  Each planes has about 55° tilt relative to Earth's equator in order to cover the polar regions.

—  Space Segment (Continued)

—  Each satellite makes two complete orbits each sidereal day.

—  Sidereal - Time it takes for the Earth to turn 360 degrees in its rotation

—  It passes over the same location on Earth once each day.

—  Orbits are designed so that at the very least, six satellites are always within line of sight from any location on the planet.

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—  There are currently 30 actively broadcasting satellites in the GPS constellation.

—  Redundancy is used by the additional satellites to improve the precision of GPS receiver calculations.

—  A non-uniform arrangement improves the reliability and availability of the system over that of a uniform system, when multiple satellites fail

—  This is possible due to the number of satellites in the air today

—  Control Segment

—  The CS consists of 3 entities:

—  Master Control System

—  Monitor Stations

—  Ground Antennas

—  Master Control Station

—  The master control station, located at Falcon Air Force Base in Colorado Springs, Colorado, is responsible for overall management of the remote monitoring and transmission sites.

—  GPS ephemeris is the tabulation of computed positions, velocities and derived right ascension and declination of GPS satellites at specific times for eventual upload to GPS satellites.

—  Monitor Stations

—  Six monitor stations are located at Falcon Air Force Base in Colorado, Cape Canaveral, Florida, Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in the South Pacific Ocean.

—  Each of the monitor stations checks the exact altitude, position, speed, and overall health of the orbiting satellites.

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—  The control segment uses measurements collected by the monitor stations to predict the behavior of each satellite's orbit and clock.

—  The prediction data is up-linked, or transmitted, to the satellites for transmission back to the users.

—  The control segment also ensures that the GPS satellite orbits and clocks remain within acceptable limits. A station can track up to 11 satellites at a time.

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—  This "check-up" is performed twice a day, by each station, as the satellites complete their journeys around the earth.

—  Variations such as those caused by the gravity of the moon, sun and the pressure of solar radiation, are passed along to the master control station.

—  Ground Antennas

—  Ground antennas monitor and track the satellites from horizon to horizon.

—  They also transmit correction information to individual satellites.

—  User Segment

—  The  GPS receiver is the USER of the GPS system.

—  GPS receivers are generally composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock, commonly a crystal oscillator).

—  They can also include a display for showing location and speed information to the user.

—  A receiver is often described by its number of channels this signifies how many satellites it can monitor simultaneously. As of recent, receivers  usually have between twelve and twenty channels.

—  User Segment (continued)

—  Using the RTCM SC-104 format, GPS receivers may include an input for differential corrections.

—  This is typically in the form of a RS-232 port at 4,800 bps speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.

—  Receivers with internal DGPS receivers are able to outclass those using external RTCM data.

—  GPS SIGNAL

—  The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.

—  Three binary codes shift the L1 and/or L2 carrier phase.

—  The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS.

—  The P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P (Y)-Code is the basis for the PPS.

—  The Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters.

—  GPS SIGNAL

—  THE GPS SIGNAL CONSISTS OF

a)Pseudo random code:This code identifies the satellite because each satellite has got unique code

b)Ephemeris data:this provides the status of the satellite,current date and time

c)Almanac data:this provides the the status of all satellites  including theirs locations through out the day

—  P code and C/A code:

—  The C/A code is made up of a sequence of 0’s and 1’s called the chip,having a frequency of 1.023MBits/sec and in terms of distatnce ,it is 293m.the duration of each chip is 1µs and entire sequence is of one millisecond.the code sequence of 1023  such chips arranged in pseudo random order known to the receiver  and repeated every after one millisecond.

—  The C/A code can be decoded by the civilian GPS

—  P code and C/A code

—  In case of P code the chip frequency is 10.23MBits/sec,the duration of each chip is 0.1µs and in terms of distance 29.3m.the full code length is of 267 days and each satellite is allocated only 7days piece of code,during this interval there is no repetition of the code.Every 6 secs the satellite transmits the time that has lapsed since P code started.

—  Precise position system

—  Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use the PPS.

—  PPS Predictable Accuracy

—  22 meter Horizontal accuracy

—  27.7 meter vertical accuracy

—  200 nanosecond time (UTC) accuracy

—  The Precise Positioning Service (PPS) is a highly accurate military positioning, velocity and timing service which will be available on a continuous, worldwide basis to users authorized by the U.S. P(Y) codecapable military user equipment provides a predictable positioning accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically and time transfer accuracy to UTC within 200 nanoseconds (95 percent).

—  PPS will be the data transmitted on the GPS L1 and L2 frequencies. PPS was designed primarily for U.S. military use. It will be denied to unauthorized users by the use of cryptography. PPS will be made available to U.S. and military and U.S. Federal Government users. Limited, non-Federal Government, civil use of PPS, both domestic and foreign, will be considered upon request and authorized on a case-by-case basis, provided:

—  It is in the U.S. national interest to do so.

—  Specific GPS security requirements can be met by the applicant.

—  A reasonable alternative to the use of PPS is not available.

—  Standard positioning system

—  The Standard Positioning Service (SPS) is a positioning and timing service which will be available to all GPS users on a continuous, worldwide basis with no direct charge. SPS will be provided on the GPS L1 frequency which contains a coarse acquisition (C/A) code and a navigation data message. SPS provides a predictable positioning accuracy of 100 meters (95 percent) horizontally and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340 nanoseconds (95 percent).

—  These GPS accuracy figures are from the 1999 Federal Radionavigation Plan. The figures are 95% accuracies, and express the value of two standard deviations of radial error from the actual antenna position to an ensemble of position estimates made under specified satellite elevation angle (five degrees) and PDOP (less than six) conditions.

—  For horizontal accuracy figures 95% is the equivalent of 2drms (two-distance root-mean-squared), or twice the radial error standard deviation. For vertical and time errors 95% is the value of two-standard deviations of vertical error or time error.

—  Receiver manufacturers may use other accuracy measures. Root-mean-square (RMS) error is the value of one standard deviation (68%) of the error in one, two or three dimensions. Circular Error Probable (CEP) is the value of the radius of a circle, centered at the actual position that contains 50% of the position estimates. Spherical Error Probable (SEP) is the spherical equivalent of CEP, that is the radius of a sphere, centered at the actual position, that contains 50% of the three dimension position estimates. As opposed to 2drms, drms, or RMS figures, CEP and SEP are not affected by large blunder errors making them an overly optimistic accuracy measure

—  Some receiver specification sheets list horizontal accuracy in RMS or CEP and without Selective Availability, making those receivers appear more accurate than those specified by more responsible vendors using more conservative error measures

—  GPS DATA

—  The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.

—  Data frames (1500 bits) are sent every thirty seconds. Each frame consists of five subframes.

—  Data bit subframes (300 bits transmitted over six seconds) contain parity bits that allow for data checking and limited error correction.

—  Contents of Navigational Message

—  The navigation message is made up of three major components. The first part contains the GPS date and time, plus the satellite's status and an indication of its health. The second part contains orbital information called ephemeris data and allows the receiver to calculate the position of the satellite. The third part, called the almanac, contains information and status concerning all the satellites; their locations and PRN numbers.

—  Navigational message

—  GPS Frequencies

—  L1 (1575.42 MHz) - Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code.

—  L2 (1227.60 MHz) - P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.

—  L3 (1381.05 MHz) - Used by the Defense Support Program to signal detection of missile launches, nuclear detonations, and other applications.

—  GPS Proposed Frequencies

—  L4 (1379.913 MHz) - Being studied for additional correction to the part of the atmosphere that is ionized by solar radiation.

—  L5 (1176.45 MHz) – To be used as a civilian safety-of-life (SoL) signal.

—  Internationally protected range for aeronautical navigation.

—  The first satellite that using this signal to be launched in 2008.

—  Position Calculation

—  The coordinates are calculated according to the World Geodetic System WGS84 coordinate system.

—  The satellites are equipped with atomic clocks

—  Receiver uses an internal crystal oscillator-based clock that is continually updated using the signals from the satellites.

—  Receiver identifies each satellite's signal by its distinct C/A code pattern, then measures the time delay for each satellite.

—  Position Calculation (cont’d)

—  The receiver emits an identical C/A sequence using the same seed number the satellite used.

—  By aligning the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange.

—  Orbital position data from the Navigation Message is used to calculate the satellite's precise position. Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it.

—  Position Calculation (cont’d)

—  When four satellites are measured at the same time, the point where the four imaginary spheres meet is recorded as the location of the receiver.

—  Earth-based users can substitute the sphere of the planet for one satellite by using their altitude. Often, these spheres will overlap slightly instead of meeting at one point, so the receiver will yield a mathematically most-probable position.

—  ERRORS IN GPS

—  1.IONOSPHERIC AND TROPOSPHERIC DELAY

—   As a GPS signal passes through the charged particles of the ionosphere and then through the water vapour in the troposphere it gets slowed down a bit, and this creates the same kind of error as bad clocks.

—  There are a couple of ways to minimize this kind of error. For one thing we can predict what a typical delay might be on a typical day.

—  This is called modeling and it helps but, of course, atmospheric conditions are rarely exactly typical.

—  ERROR IN GPS

—  This “ dual frequency” measurement is very sophisticated and is only possible with advanced receivers.

—  2.USERS CLOCK ERROR

     If the user clock is not perfectly synchronised with satellite’s clock,the range measurement will be wrong

3.SATELLITE CLOCK ERROR

   This error is caused due to  the error in the satellite’s clock wrt the  GPS time.this is monitored by the ground based segments and any error in the satellite clock forms part of the navigational message

—  ERRORS IN GPS

  4.MULTIPLE PATH

—  The GPS signal may bounce off various local obstructions before it gets to the receiver.

—  This is called multipath erro

5.DEVAITION OF SATELLITE FROM PREDICTED PATH:the satellite are monitored and their paths are predicted by the ground based stations.however between the two consecutive monitoring of the same satellites,their may be minor drifts from their predicted paths resulting in small position inaccuracy.

—  ERROR IN GPS

—  6.GEOMETRIC DILUTION OF PRECISION: This depends on the number and the geometry of the satellites used.

—  If four satellites are clustered near each other, then one meter of error in measuring distance may result in tens or hundreds of meters of error in position.But if many satellites are scattered around the sky, then the position error may be less than 1.5 meters for every meter of error in measuring distances.

—  The effect of the geometry of the satellites on the position error is called Geometric Dilution Of Precision (GDOP), which can roughly be interpreted as the ratio of the position error to the range error.

—  Imagine the tetrahedron that is formed by lines connecting the receiver to each satellite used.

—  The larger the volume of this tetrahedron, the smaller (better) the GDOP.

—  In most cases, the larger the number of satellites the smaller the GDOP.

—  DGPS

—  Working of DGPS
The reference station established on ground receives signals from the satellites, after which it calculates the difference in the positioning of its own location. The reference station provides the users with the necessary corrections in the distance measured by the GPS system. These corrections are transmitted by means of ultra high frequency waves (UHF). Only those users, within the range of 370 km of the reference stations can benefit from the service. However, even the DGPS can generate errors resulting from the distortions produced in the troposphere and ionosphere. The ephemeris errors may also lead to the users receiving incorrect information. Thus, the information provided by the DGPS loses accuracy as we move away from the reference station. The errors in the DGPS may range from 0.22 to 0.67 km per 100 km.

—  Limitations Of DGPS

—  The corrections which will be applied to the position is valid for particular position i.e. for the reference station and roving stations close to the reference station.As the distance from the reference station increases, the  error in the wave will be different .Ionospheric and tropospheric correction will vary.

—  There may be chance that ephemeris data received at the reference may be different and that will lead to the incorrect correction

—  The multipath error will remain uncorrected or correction based  on the this will lead to the error in position.

—  Some area which is far away from the transmitting  will not be to receive data from the Reference station.