GPS -Global Positioning System

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					                                                                                                         GPS – Global Positioning System

                                                                                                                                        Illustration of a GPS Satellite in orbit.
      What Every CS Graduate Needs to Know About                    GPS                                                                                            Images from Wikimedia Commons

                                                        Ananth Vadlamani       GPS consists of a constellation of satellites around
                                               Garmin AT, Salem, Oregon        the Earth transmitting radio-frequency signals. Using
                       CS Capstone Presentation at Oregon State University     these signals, a receiver near the Earth’s surface can
                                                         January 28, 2010
                                                                               determine its Position, Velocity and Time (PVT).

                 GPS – Global Positioning System                                                                                                             GPS Signals
The system consists of three segments:                                         Signals that are broadcast on the L1 frequency of 1575.42 MHz
• Space Segment                                                                • Carrier – Sinusoid with a frequency of L1.
      The GPS satellites.                                                      • Ranging Code – A periodic, pseudo-random noise (PRN) sequence
                                                                                 of bits.
• Control Segment
      A network of control stations that perform maintenance on the                 Two signals are transmitted on L1:
                                                                                    • Coarse Acquisition (C/A) code – intended for civilian users
• User Segment                                                                        The C/A code sequence is 1023 chips long and repeats every 1
      GPS receivers, in general.                                                      millisecond, i.e., a chipping rate of 1.023 MHz.
      All users and businesses that benefit from PVT obtained through               • Precision (encrypted) [P(Y)] code – intended for military users.
      GPS: Transportation, Cell phones, Banking, Electricity, Internet, etc.        The PRN code sequences have some unique correlation properties that
                                                                                      allow multiple satellites to transmit on the same frequency.
                                                                               • Navigation Data – A 50 Hz stream of Satellite and System

                                                                               GPS Satellites also broadcast navigation signals on frequencies of L2, L5*.

             GPS Signals – C/A Code Correlation                                                  Some Basic Concepts in Positioning
                                                                                                         Transmitter 1


                                                                                                                               ?          Transmitter 2



                                                                                                                     Transmitter 3
                Think of it as tuning into a radio station
Using Spread Spectrum technology, GPS signals for all satellites can be
put on the same frequency.                                                     Estimation of user position based on range measurements is known as

                                          GPS Trilateration Concept                                                       Estimation of User Position
                Now Imagine What Happens in 3D…                                      So, (one of the) basic measurements a GPS receiver makes
                                                                                     is: RANGE to the satellite (SV ).
                                                                                     Range is measured by the time-of-arrival (TOA) of the SV

                                                                                      R = (Tr ,USER − Tt , SV )* SPEED_OF_LIGHT

                                                                                     Non-linear range equation:
                                    R1                                                 ( X SV − x )2 + (YSV − y )2 + (Z SV − z )2   =R
                                                                                     So by linearizing and solving 3
                                                                                     equations for 3 unknowns, we
                                                                                     should be able to solve for user                               R2
Intersection of:
                                                                                     position !!!
• 2 Spheres = Circle                                                                 Hmmm… What’s the catch?
• Circle & Sphere = 2 points

                                     Estimation of User Position                                                          Estimation of User Position
TOA Range Equation                                                                   Also, in our 2D example, we assumed the transmitters were

                R = (Tr ,USER − Tt ,SV )* SPEED_OF_LIGHT
                                                                                     stationary at a known location.
                                                                                     But the satellites are not stationary!!!
                                                                                     Turns out that it doesn't matter as long as we know precisely
      Receiver Clock                 Satellite Clock                                 where the satellites are.
There s
There’s a clock bias (CB) on each                                                               i      Kepler’s bit l       t   broadcast b
                                                                                     GPS receivers use K l ’ orbital parameters b d      t by
of the SV range measurements.                                                        the SVs (almanac and ephemeris) to determine precise SV
  ( X SV − x )2 + (YSV − y )2 + (Z SV − z )2 + CB = PR                               positions.

The ranges are now called                                                            Think of it as identifying a radio station
Pseudo-Ranges (PR).                                             R1                   after you’ve been listening to it !
We need a 4th Satellite to solve for
the clock bias.                                            R4         R3

                                     Estimation of User Velocity                                                                    Estimation of User Time
1. Velocity = time derivative of Position.                                           GPS maintains a system time known as GPS Time
                           d ( x)
                                                                                     that is defined in weeks.
                     v=                                                              • Week number.
                                                                                          (beginning , Sat. 5 Jan.1980 – Sun. 6 Jan. 1980, modulo 1024)
2. In GPS, use the carrier signal and take advantage                                 • # seconds into the week. (0 to 604,800)
   of the Doppler Effect.
                                                                                                              g (            ),
                                                                                     The week starts at midnight (GPST), at the transition
                             ⎛ v ⎞                                                   from Saturday to Sunday.
                    f R = fT ⎜1 − LOS ⎟
                             ⎝    c ⎠
                                                          PRN code
     Where,                                                                          GPS Time maps to UTC (coordinated universal time),
     fR = Received frequency.
                                                                                     after accounting for leap seconds.
     fT = Transmitted frequency.
                                                                                     Local times are derived from UTC.
     c = SPEED_OF_LIGHT.
     vLOS = Line-of-Sight velocity.

                                           Error Sources                                                 Differential GPS (DGPS)
GPS measurements have errors, primarily due to:                The largest         errors         are           spatially                    and             temporally
• Ionosphere, troposphere, multipath and receiver noise.       correlated.
• Satellite Orbit and clock errors.                            This means two nearby receivers will experience similar
These errors can be mitigated in a variety of ways…
                                                               If the position of one receiver is known, it can compute
• Differential GPS (DGPS)                                       h            d
                                                               the errors and transmit corrections to the other. Th ’
                                                                                      i        i       h     h    That’s
• Using error models to (approx.) compensate for the errors.   DGPS!

Now, what’s Differential GPS?
                                                                   Wide Area Augmentation
                                                                   System (WAAS) is a type
                                                                   of satellite-based DGPS

                                     GPS Applications                                                                         GPS Applications
• GPS was initially developed as a military system, and the    1. Aviation
  U.S. Air Force maintains the Space and Control Segments.     •     Aircraft Guidance, Navigation & Control (GNC)
• However, civilian users and applications far outnumber              • A GPS receiver is one of the primary sensors*.
  military applications.                                       •     Used in Military and Civilian aircraft, UAVs for providing
                                                                     position, velocity, time (PVT).
• GPS chips are now being used in most consumer
                                                               •     GPS, other sensors and modern display technologies have
   l t i       Smartphones, C
  electronics: S   t h               Laptops, etc.
                            Cameras, L t       t
                                                                     revolutionized the way a pilot interacts with cockpit
• New uses for GPS chips are envisioned continuously and             avionics.
  developed rapidly.                                           •     For example:

                                                                                      * GPS is generally considered supplemental means of navigation except for oceanic/remote operations.
                                                                                                         In the US, WAAS-enabled GPS receivers can be used as primary navigation means.

Traditional Avionics                                           The New General Aviation Flight Decks
                                                                                                                                          Cessna Citation Mustang

Aviation Portables                                                                                           GPS Applications
                                                                     2. Marine
                                                                     •   Provides guidance to ships entering harbors.
                                                                     •   Provides reliable, worldwide, all-weather navigation
                                                                         capability for watercraft.
                                                                     •   Commercial fishing, for finding and returning to good
                                                                         fishing sites.

                                                                     3. Automotive
                                                                     •   Variety of automobile navigation devices providing driving
                                                                         directions, points of interests (POI), weather and traffic

                                       GPS Applications                                                      GPS Applications
4. Agriculture                                                       6. Outdoor/Handheld GPS receivers
•   Precision farming, position information for the application of   •   Ranging from basic handhelds that provide position,
    fertilizers, pesticides.                                             bearing to advanced models that have integrated
•   Identification of insect or weed infestation.                        topographic maps.
•   Automatic plowing, harvesting.

5 E i        t
5. Environment
•   Surveying and mapping of disaster areas such as forest           7. Cell phones, Smart-phones
    fires, earthquakes, hurricanes, etc.                             •   Many of the latest cell phones, from all leading
                                                                         manufacturers are equipped with a GPS chipset.
                                                                     •   Provide positioning and emergency services, such as
                                                                     •   Examples: iPhone, Blackberry, nüvifone, etc.

                                       GPS Applications                                                      GPS Applications
8. Fitness products                                                  10. Law Enforcement/Search & Rescue Ops.
•   Variety of functions including distance traveled, speed,         •   Traffic monitoring, Fire-fighting, Disaster management.
    heart rate monitoring, etc.

9. Urban & Indoor Navigation
•   GPS is integrated with other sensors such as inertial
    measurement units (IMU) and/or cameras to provide
                                                                     11. Freight Tracking
    navigation in urban (city streets, between high-rise
    buildings) and indoor environments.                              •   To keep track of vehicles, fleet and inventory.

                                                                                       GPS Applications                                                       GPS Applications
12. Mapping, Surveying & Geodesy                                                                                       14.    Location Based Services (LBS)
•              High Precision GPS receivers used in Terrain mapping,                                                   •   A booming market for consumer electronics and services.
               City modeling and database generation.                                                                  •   Based on the idea of determining user position and
•              Surveying for Civil engineering and construction.                                                           interacting with a remote server hosting a variety of
•              Geodesy – measuring tectonic plate movements, Earth’s                                                       information and services.
               spin and rotation changes, etc.                                                                         •                                        g
                                                                                                                           Examples: 411 service, finding local businesses,
                                                                                                                           entertainment, travel/transportation, turn-by-turn directions,
                                                                                                                           localized advertizing, … in short, never feeling lost or
13. Timing                                                                                                                 lacking required information.
•              Precise global timing standard for Cell phone towers,
               Banking, Electric grids, Internet servers, etc.
                                                                                                                       • And much more …

                                      References and Further Reading
[1] Misra, P., and Enge, P., “Global Positioning System – Signals,
    Measurements, and Performance,” 2nd Ed., Ganga-Jamuna Press, ISBN: 0-
[2] Kaplan, E. D., and Hegarty, C. J., Eds., “Understanding GPS Principles and
    Applications,” 2nd Ed., Artech House, ISBN: 1-58053-894-0.
[3] US Coast Guard GPS information webpage.
[4] Garmin International website.

Garmin product images are copyrighted by Garmin or its subsidiaries.
Other images used in this presentation are from Microsoft Clipart and Wikimedia Commons.

               Garmin GPS Offerings in Multiple Markets
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                                                                                                                                                Ananth Vadlamani
                                                                                                                                                    Garmin AT
                                                                                                                                                2345 Turner Rd SE
                                                                                                                                                 Salem, OR 97302
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