GPS guide

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G.P.S.
(Global Positioning System)
Presented by DEBANJALI BANERJEE 2nd year, CSE
Department S.V.I.S.T.
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Basics of GPS

• GPS, that is, Global Positioning System is a
  satellite-based navigation system made up of a
  network of 24 satellites placed into orbit.
• GPS measures 3-D locations on Earth surface using
  satellites.
• GPS operates using radio signals sent from
  satellites orbiting the Earth.
• The orbital height is 20,200km approximately.
• The systems as a whole consists of three segments
  
• 1. Satellites (space segment)
• 2. Receivers (user segment)
• 3. Ground stations (control segment)

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History of GPS

• Global Positioning Systems (GPS) is a form of
  Global Navigation Satellite System (GNSS)
• Official name of GPS is NAVigational Satellite
  Timing And Ranging Global Positioning System
  (NAVSTAR -GPS)
• First developed by the United States Department
  of Defense for military purpose on February, 1973
  and became fully functional from 1995.
• Consists of two dozen GPS satellites in medium
  Earth orbit (The region of space between 2000km
  and 35,786 km)
• Till 31st of January 2016 the total number of
  launches are 65.

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GPS Architecture
As said earlier, GPS architecture consists of
three segments Space, User and Ground (control)
segment. The Space Segment transmits
radio-navigation signals, stores and retransmits
navigation signal sent by the Control Segment.
The Ground or Control Segment is mainly
responsible for the proper operation of the GPS
signals. User Segment receives the GPS signal and
solve navigation equations. These transmissions
are controlled by highly stable atomic clocks on
board the satellites. Space Segment is formed
by a satellite constellation, Control Segment is
composed by a network of Monitor Stations, a
Master Control Station and the Ground Antennas.
User Segment is composed by GPS Receivers.
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Space Segment

• Currently 26 Block II, 2 Block IIR, no Block I
  satellites are active.
• (Picture of a Block II Satellite as follows )

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• Control Segment has 5 major stations which
  measure the distances of the overhead satellites
  every 1.5 seconds and send the corrected data to
  Master control and these Master control and
  Monitor Station are

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User Segment

• There are two services SPS and PPS
• The Standard Positioning Service
• SPS- is position accuracy based on GPS
  measurements on single L1 frequency C/A code
• C/A ( coarse /acquisition or clear/access) GPs
  code sequence of 1023 pseudo random bi phase
  modulation on L1 freq

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• The other type of service is The Precise Position
  Service
• PPS is the highest level of dynamic positioning
  based on the dual freq P-code
• The P-code is a very long pseudo-random bi phase
  modulation on the GPS carrier which does not
  repeat for 267 days
• Only authorized users, this consists of SPS
  signal plus the P code on L1 and L2 and carrier
  phase measurement on L2

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Working

• GPS functioning have 5 basic steps
• Triangulation.
• Distance Measurement.
• Timing.
• Satellite Positioning.
• Correction of Errors.

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Triangulation

• Triangulation is a process by which the location
  of a radio transmitter can be determined by
  measuring either the radial distance, or the
  direction, of the received signal from two or
  three different points. Triangulation is
  sometimes used in cellular communications to
  pinpoint the geographic position of a user.
• The position is calculated from distance
  measurement. Mathematically we need four
  satellites but three are sufficient by rejecting
  the ridiculous answer.

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How Triangulation works..

• This picture and the chart exactly shows how
  triangulation works

Intersection Equivalency Result
Intersection of two spheres Circle Circle
Intersection of three spheres Circle Sphere Two points
Intersection of four spheres (Not in the picture) Two points Spheres One point
n
n
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Distance measurement

• Distance to a satellite is determined by
  measuring how long a radio signal takes to reach
  us from that satellite. 
• To make the measurement we assume that both the
  satellite and our receiver are generating the
  same pseudo-random codes at exactly the same
  time. 
• By comparing how late the satellite's
  pseudo-random code appears compared to our
  receiver's code, we determine how long it took to
  reach us. 
• Multiply that travel time by the speed of light
  and you've got distance.

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Perfect Timing

• If the clocks are perfect sync the satellite
  range will intersect at a single point.
• But if imperfect the four satellite will not
  intersect at the same point.
• The receiver looks for a common correction that
  will make all the satellite intersect at the same
  point

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Satellite Positioning

• The Air Force has put each GPS satellite into a
  very precise orbit, according to the GPS master
  plan.
• 11,000 mile altitude is important because
  something that high is well clear of the
  atmosphere and it will orbit according to very
  simple mathematics.
• On the ground all GPS receivers have an almanac
  programmed into their computers that tells them
  where in the sky each satellite is, moment by
  moment.
• The basic orbits are quite exact but just to make
  things perfect the GPS satellites are constantly
  monitored by the Department of Defense.
• They use very precise radar to check each
  satellite's exact altitude, position and speed.
• The errors they're checking for are called
  "ephemeris errors" because they affect the
  satellite's orbit or "ephemeris.
• These errors are caused by gravitational pulls
  from the moon and sun and by the pressure of
  solar radiation on the satellites.

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Error Sources

• Satellite orbits -
• 1. IGS Final or Rapid orbits (available within
  24h) virtually eliminate orbit errors (lt.1m) for
  post-processing at any epoch
• 2. In real-time the broadcast orbit errors
  (3m) can be reduced (lt 1m) by DGPS (Differential
  GPS) or by using IGS Predicted or the Ultra-Rapid
  orbits
• Satellite clocks -
• 1. For relative positioning double differencing
  nearly eliminates satellite clock errors
• 2. IGS Final or Rapid satellite clocks
  (available within 24h) virtually eliminate
  satellite clock errors (lt.1m) for post-processing
  (currently only at the 5 min epoch sampling)
• 3. In real time the broadcast clock errors (SA)
  reduced by DGPS
• Tropospheric refraction -
• 1. Virtually eliminated by estimating
  corrections to a model (also used in GPS
  meteorology) or at IGS stations, by using the
  IGS tropospheric delay products!
• 2. Nearly eliminated (lt .1m) by using a model
  with measured met data
• 3. Reduced by a nominal model and/or
  differential positioning (e.g. DGPS)

dtSatellite clock error(s), drelrelativity
correction(s) af22nd order coefficient
(s-1),af11st order coefficient
(unitless),af0Zero order coefficient (s) ttime
of measurement(s), toetime of ephemeris(s),
tgdgroup delay(s)
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Error Sources (Contd.)

• Ionospheric refraction -
• 1. For dual frequency receivers, the use of
  (P1, P2) or (L1, L2) virtually eliminates
  ionospheric refraction (also used for ionospheric
  delay determination/monitoring, e.g. by IGS, the
  IGS ionospheric delay products )
• 2. For single frequency receivers, the use of
  IGS ionospheric delay products (available within
  a few weeks) significantly reduces ionospheric
  refraction errors
• 3. For single frequency receivers, ionospheric
  refraction errors reduced in differential
  (relative) positioning (e.g. DGPS) for baselines
  up to 100 km
• Antenna phase center variations -
• 1. Virtually eliminated in relative positioning
  over moderate baseline lengths (lt500km) when
  using the same antenna types
• 2. Antenna phase center corrections (e.g. IGS
  antenna phase center tables) must be used for
  different antenna types and precise positioning
  (lt.1m)
• Multipath -
• 1. Difficult to mitigate, errors can reach a few
  cm for the phase and up to a meter or more for
  pseudorange positioning/navigation
• 2. Reduced by improved site selections and
  hardware (receiver/antenna) designs

ffrequency of L1 or L2 (Hz) ?wavelength of L1
or L2 (m) L1,L2Phase Ninteger ambiguity on L1
or L2 (cycles) ?carrier phase measurement on L1
or L2 (cycles)
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Error Sources
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• A summative picture of how GPS works

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Dilution of Precision (DOP)

• Dilution of precision is a term used in
  satellite navigation and geometric engineering to
  specify the additional multiplicative effect of
  navigation satellite geometry on positional
  measurement precision. Dilution of precision is
  may be of two types depending upon its values
  GDOP (Good DOP) when the DOP value is between 2-5
  and PDOP (Poor DOP) when the DOP value is greater
  than 20.

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GPS Signaling

• GPS receiver on getting a signal from satellite
  compares it with the known 37 codes to determine
  which satellite is sending the signal.
• The receiver then decodes the timing information,
  multiplies by the speed of light to find the
  radius of the sphere.
• After its completed for at least three(3)
  satellites the GPS can successfully determine the
  position.
• This information is sent to the user via binary
  satellite codes.
• All these signals are run by some carrier signals
  and some pseudo-random code.
• The signal travel time is given by, Signal travel
  time offset between the satellite signal and
  the receiver signal .
• The signal travel time is approximately 186
  miles/second.

Satellite Signal
Receiver Signal
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• L1, L2 are two microwave carrier signal that are
  transmitted from Satellite Vehicle. L1 carriers
  the navigation message of frequency 1575.42 MHz
  and L2 measures the ionospheric delay of
  frequency 1227.6MHz. These are derived from the
  system clock of 10.23 MHz (phase quadrature).
  There are three (3) types of pseudo random code
  because they repeat themselves every 1023 bits.
  These codes are i) C/A Code (Coarse Acquisition
  Code) that modulates L1 carrier phase with a
  frequency 1MHz, ii) NAV/System Code that
  modulates L1-C/A and describes the satellite
  orbit and clock corrections, iii) P-Code (Precise
  Code) AS (anti-spoofing) encrypts P-code into
  Y-code and the P-code modulates both L1 and L2.
  Modulation used is Direct Sequence Spread
  Spectrum.

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Navigation

• GPS satellites broadcast three different types of
  data in the primary navigation signal.
• Almanac sends time and status information about
  the satellites.
• Ephemeris has orbital information that allows
  the receiver to calculate the position of the
  satellite.
• This data is included in the 37,500 bit
  Navigation Message, which takes 12.5 minutes to
  send at 50 bps.

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Navigation (Contd.)

• Satellites broadcast two forms of clock
  information
• Coarse / Acquisition code (C/A) - freely
  available to the public. The C/A code is a 1,023
  bit long pseudo-random code broadcast at 1.023
  MHz, repeating every millisecond.
• Restricted Precise code (P-code) - reserved for
  military usage. The P-code is a similar code
  broadcast at 10.23 MHz, but it repeats only once
  a week. In normal operation, the anti-spoofing
  mode, the P code is first encrypted into the
  Y-code, or P(Y), which can only be decrypted by
  users a valid key.

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Wide Area Augmentation System

1. Wide Area Augmentation System (WAAS) is an
   experimental system designed to enhance and
   improve satellite navigation over the continental
   United States and portions of Mexico and Canada.
2. WAAS is a highly advanced real-time differential
   GPS that requires no additional equipment to
   work.
3. WAAS uses its own geostationary satellites in
   fixed orbit over North America. There are 25
   ground reference stations positioned across the
   U.S. that monitor GPS satellite signals.
4. These stations continuously receive and correct
   GPS satellite information against their own known
   precise positions.
5. Each WAAS ground station then sends its corrected
   GPS data to one of two master control stations
   located on U.S. These master control stations
   create a correction message that weeds out
   atmospheric distortion, GPS satellite orbit and
   clock errors and time errors which is then
   broadcast to 2 WAAS satellite.
6. These in turn rebroadcast the correction
   information using the basic GPS signal structure
   to any WAAS capable GPS receiver.

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Wide area augmentation system

• The application of WAAS are improvement to
  aviation operation, software development that
  uses GPS and Space Segment upgrades.
• The advantage of WAAS is it addresses all
  navigation problem and give highly accurate
  positioning.
• The disadvantage of WAAS is that it is subjected
  to space weather and space debris threats

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Advantages of GPS

• GPS is extremely easy to navigate to reach the
  desired navigation.
• GPS works in all weather.
• The GPS costs you very low in comparison other
  navigation systems, so it is very easy to
  integrate into other technologies like cell
  phone.
• The most attractive feature of this system is its
  100 coverage on the planet and gives full time
  access.
• The system is updated regularly by the US
  government and hence is very advance.
• This is the best navigating system in water also.

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Disadvantages of GPS

• Sometimes the GPS signals are not accurate due to
  some obstacles to the signals such as buildings,
  trees.
• If we are using the GPS on a battery operated
  system, then due to battery failure we may have a
  problem, so for that reason we must need to carry
  a external battery.
• Sometimes GPS may fail due to certain reasons so
  we need to carry backup map for directions.
• Launching satellites in orbit is too costly.
• Impossibility to repair and maintain.

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Applications

• GPS helps in road transport and heavy vehicle
  guidance.
• The system helps in commercial aviation when
  aircrafts use GPS for route navigation.
• GPS helps in environmental and atmospheric
  monitoring, animal behavior studies, botanical
  specimen location, meteorology and climate
  research.
• GPS helps in tracking of vehicles and cargoes.
• GPS helps in surveying, geophysics and mapping.
• GPS timing is important for telecommunications
  applications, particularly for mobile telephone
  networks.
• Global financial systems increasingly need
  precise timing systems to schedule and priorities
  local and international money transfers,
  settlements and trades and to provide an audit
  trail for financial transactions.
• Last but not the least, GPS has immense
  importance in social networking.

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Conclusion

• GPS is a space-based navigation system that
  transmit precise microwave signals, provides
  location and time information in all weather
  conditions where there is an unobstructed line of
  sight to four or more GPS satellites.
• The system provides critical capabilities to
  military, civil, and commercial users around the
  world. Even in social networking also GPS plays
  an integral part.

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References

• http//www.gps.gov/students/
• https//en.wikipedia.org/wiki/Global_Positioning_S
  ystem
• www.esf.edu/for/herrington/557/557pps/GPS.ppt
• www.nps.gov/gis/gps/gps4ics/2_prework/prework.ppt
• www.clarkson.edu/class/cs463/wireless.../GlobalPos
  itioningSystem.ppt
• www.casde.iitb.ac.in/IMSL/motion/gps/Seminar20on
  20GPS.ppt
• https//www.rgs.org/NR/rdonlyres/95D99DBD-CE9B-4B8
  9-81F3-22D12B3B976E/0/Chapter6TheGlobalPositioning
  SystemGPSPrinciplesandConcepts.pdf
• http//www.nhdfl.org/library/pdf/Forest20Protecti
  on/Introduction20to20Global20Positioning20Syst
  em.pdf
• http//www.nhdfl.org/library/pdf/Forest20Protecti
  on/Introduction20to20Global20Positioning20Syst
  em.pdf

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Acknowledgement

• I would like to thank all the people who helped
  me in completing this power point presentation
  starting from my Parents, my teachers, my guide,
  my friends and definitely a special thanks to
  those who created the challenge that initiated
  the urge in myself to complete this project which
  I finally did.
• Thank you everyone for all your support and
  cooperation throughout the project.