GPS Global Positioning System Lecture 11

What is GPS?  The Global Positioning System.  A system designed to accurately determining positions on the earth  The Global Positioning System.  A system designed to accurately determining positions on the earth

What are the parts of GPS?  Space Segment  Control Segment  User Segment  Space Segment  Control Segment  User Segment

GPS Components

 Developed by the DoD  Department of Defense  Funded in 1976 by Congress  Became fully functional in the early 1980’s  Developed by the DoD  Department of Defense  Funded in 1976 by Congress  Became fully functional in the early 1980’s

Space Segment  Satellites  Atomic Clock  Solar Panels  Transmitter / Receiver (High frequency, low energy radio waves)  Antenna  There are 24 active satellites in orbit  3 extra satellites in orbit that are not in use  Satellites  Atomic Clock  Solar Panels  Transmitter / Receiver (High frequency, low energy radio waves)  Antenna  There are 24 active satellites in orbit  3 extra satellites in orbit that are not in use

Satellite  Atomic Clock  9.2 billion oscillations per second  Each satellite transmits on two L Band frequencies  Every satellite can transmit its own unique code  The satellites can be repositioned by the DoD  Atomic Clock  9.2 billion oscillations per second  Each satellite transmits on two L Band frequencies  Every satellite can transmit its own unique code  The satellites can be repositioned by the DoD

GPS Satellite

GPS Satellite Orbits  4 satellites in each of 6 orbits  Pass over a monitoring station once every 12 hours  12,600 mile high orbit  There are always more than 4 satellites visible anywhere on the planet  4 satellites in each of 6 orbits  Pass over a monitoring station once every 12 hours  12,600 mile high orbit  There are always more than 4 satellites visible anywhere on the planet

GPS Satellite Orbits

Control Segment  5 monitoring stations  Ground station at Schreiver AFB in Colorado  Coordinates time with the Space Segment  Master control station  Hawaii  Ascension Island  Diego Garcia  Kwajalein  Correction factors issued twice per day  Timing  Orbital adjustments  5 monitoring stations  Ground station at Schreiver AFB in Colorado  Coordinates time with the Space Segment  Master control station  Hawaii  Ascension Island  Diego Garcia  Kwajalein  Correction factors issued twice per day  Timing  Orbital adjustments

User Segment  1984 first commercial GPS receivers on the market  Receivers that interpret the signals received from the space segment and ground based transmitters.  Quartz clocks in the GPS receivers are corrected by the satellite signals  GPS units  Accuracy ~10m - sub centimeter  1984 first commercial GPS receivers on the market  Receivers that interpret the signals received from the space segment and ground based transmitters.  Quartz clocks in the GPS receivers are corrected by the satellite signals  GPS units  Accuracy ~10m - sub centimeter

GPS Units landsurvey.html Multimedia_presentations.htm

Result  Segments work together to establish a box where your location is

Trilateration

 Satellites send out a stream of coded radio signals that indicate their location in space and the exact time that the signal is being sent.  The time difference between the same part of the code indicates how far the satellite is from the receiver  Distance = time * velocity  Velocity = 186,000 miles/second  ~.06 seconds travel time  Satellites send out a stream of coded radio signals that indicate their location in space and the exact time that the signal is being sent.  The time difference between the same part of the code indicates how far the satellite is from the receiver  Distance = time * velocity  Velocity = 186,000 miles/second  ~.06 seconds travel time

Trilateration  Each satellite signal radiates outward in a sphere  2 satellite signals overlap to produce a circle of probable location  3 satellite signals overlap to produce 2 points of probable location  One can usually be discarded as not relevant  4 satellite signals overlap to produce 1 point of location with elevation  Each satellite signal radiates outward in a sphere  2 satellite signals overlap to produce a circle of probable location  3 satellite signals overlap to produce 2 points of probable location  One can usually be discarded as not relevant  4 satellite signals overlap to produce 1 point of location with elevation

Trilateration

Possible Sources of Error  Interference  Atmosphere, Solid Objects  Ephemeris  Disturbances of satellite orbit:  Sun and moon gravitation  Pressure of solar radiation  Clock Problem  Atomic clocks are nearly perfect so even very small errors can be significant  Interference  Atmosphere, Solid Objects  Ephemeris  Disturbances of satellite orbit:  Sun and moon gravitation  Pressure of solar radiation  Clock Problem  Atomic clocks are nearly perfect so even very small errors can be significant

Sources of Error  Receivers  What the receiver is designed for  Kind of GPS chip  Can the unit reject errors  Multi Path  Reflection of signals off of objects so that the same signal reaches the receiver with different offsets  1/100 second discrepancy  Misread of 1,860 miles  Receivers  What the receiver is designed for  Kind of GPS chip  Can the unit reject errors  Multi Path  Reflection of signals off of objects so that the same signal reaches the receiver with different offsets  1/100 second discrepancy  Misread of 1,860 miles

Multipath

Selective Availability (S/A)  A disruption to the signal to make commercial units less accurate  A random timing introduced into the satellite signal  Accuracy off by up to 100 m  Military units filter out S/A  S/A was shut off so that commercial units received an unaltered signal  m accuracy without correction  The government can turn on S/A or turn off the signal if needed.  A disruption to the signal to make commercial units less accurate  A random timing introduced into the satellite signal  Accuracy off by up to 100 m  Military units filter out S/A  S/A was shut off so that commercial units received an unaltered signal  m accuracy without correction  The government can turn on S/A or turn off the signal if needed.

Signal Correction  Differential GPS  Base stations are used to receive the same satellite signal (Base Station)  Signals to the base station and the mobile GPS receiver are compared to eliminate some sources of error  m accuracy  WAAS - Wide Area Augmentation System  Currently only in North America  Asia - MSAS, Europe - EGNOS  Issues local correction factors based on local conditions  Improves accuracy of GPS location  < 3 m accuracy (to subcentimeter)  Differential GPS  Base stations are used to receive the same satellite signal (Base Station)  Signals to the base station and the mobile GPS receiver are compared to eliminate some sources of error  m accuracy  WAAS - Wide Area Augmentation System  Currently only in North America  Asia - MSAS, Europe - EGNOS  Issues local correction factors based on local conditions  Improves accuracy of GPS location  < 3 m accuracy (to subcentimeter)

Signal Correction  Tricks that are not correcting errors:  Map-matching  Estimated location is snapped to a known path on the map  Interpolated position  GPS unit continues to track estimated location based on speed and direction of travel even when a signal is lost  Instant averaging  GPS unit collects more than one position at each waypoint and averages them  Tricks that are not correcting errors:  Map-matching  Estimated location is snapped to a known path on the map  Interpolated position  GPS unit continues to track estimated location based on speed and direction of travel even when a signal is lost  Instant averaging  GPS unit collects more than one position at each waypoint and averages them