GPS,%20Inertial%20Navigation%20and%20LIDAR%20Sensors - PowerPoint PPT Presentation

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GPS,%20Inertial%20Navigation%20and%20LIDAR%20Sensors

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The Global Positioning System ... Position is determined by the travel time of a signal from four or more ... Code used to determine user's gross position ... – PowerPoint PPT presentation

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Title: GPS,%20Inertial%20Navigation%20and%20LIDAR%20Sensors


1
GPS, Inertial Navigation and LIDAR Sensors
  • Brian Clipp
  • Urban 3D Modeling
  • 9/26/06

2
Introduction
  • GPS- The Global Positioning System
  • Inertial Navigation
  • Accelerometers
  • Gyroscopes
  • LIDAR- Laser Detection and Ranging
  • Example Systems

3
The Global Positioning System
  • Constellation of 24 satellites operated by the
    U.S. Department of Defense
  • Originally intended for military applications but
    extended to civilian use
  • Each satellites orbital period is 12 hours
  • 6 satellites visible in each hemisphere

4
GPS Operating Principles
  • Position is determined by the travel time of a
    signal from four or more satellites to the
    receiving antenna
  • Three satellites for X,Y,Z position, one
    satellite to cancel out clock biases in the
    receiver

Image Source NASA
5
Time of Signal Travel Determination
  • Code is a pseudorandom sequence
  • Use correlation with receivers code sequence at
    time shift dt to determine time of signal travel

6
GPS Signal Formulation
7
Signal Charcteristics
  • Code and Carrier Phase Processing
  • Code used to determine users gross position
  • Carrier phase difference can be used to gain more
    accurate position
  • Timing of signals must be known to within one
    carrier cycle

8
Triangulation Equations Without Error
9
Sources Of Error
  • Geometric Degree of Precision (GDOP)
  • Selective Availability
  • Discontinued in 5/1/2000
  • Atmospheric Effects
  • Ionospheric
  • Tropospheric
  • Multipath
  • Ephemeris Error
  • (satellite position data)
  • Satellite Clock Error
  • Receiver Clock Error

10
Geometric Degree of Precision (GDOP)
  • Relative geometry of satellite constellation to
    receiver
  • With four satellites best GDOP occurs when
  • Three satellites just above the horizon spaced
    evenly around the compass
  • One satellite directly overhead
  • Satellite selection minimizes GDOP error

11
Good Geometric Degree of Precision
12
Bad Geometric Degree of Precision
13
Pseudorange Measurement
  • Single satellite pseudorange measurement

14
Error Mitigation Techniques
  • Carriers at L1 and L2 frequencies
  • Ionospheric error is frequency dependent so using
    two frequencies helps to limit error
  • Differential GPS
  • Post-Process user measurements using measured
    error values
  • Space Based Augmentation Systems(SBAS)
  • Examples are U.S. Wide Area Augmentation System
    (WAAS), European Geostationary Navigational
    Overlay Service (EGNOS)
  • SBAS provides atmospheric, ephemeris and
    satellite clock error correction values in real
    time

15
Differential GPS
  • Uses a GPS receiver at a fixed, surveyed location
    to measure error in pseudorange signals from
    satellites
  • Pseudorange error for each satellite is
    subtracted from mobile receiver before
    calculating position (typically post processed)

16
Differential GPS
17
WAAS/EGNOS
  • Provide corrections based on user position
  • Assumes atmospheric error is locally correlated

18
Inertial Navigation
  • Accelerometers measure linear acceleration
  • Gyroscopes measure angular velocity

19
Accelerometer Principles of Operation
  • Newtons Second Law
  • F mA
  • Measure force on object of known mass (proof
    mass) to determine acceleration

20
Example Accelerometers
  • Force Feedback Pendulous Accelerometer

21
Example Accelerometers
  • Micro electromechanical device (MEMS) solid state
    silicon accelerometer

22
Accelerometer Error Sources
  • Fixed Bias
  • Non-zero acceleration measurement when zer0
    acceleration integrated
  • Scale Factor Errors
  • Deviation of actual output from mathematical
    model of output (typically non-linear output)
  • Cross-Coupling
  • Acceleration in direction orthogonal to sensor
    measurement direction passed into sensor
    measurement (manufacturing imperfections,
    non-orthogonal sensor axes)
  • Vibro-Pendulous Error
  • Vibration in phase with pendulum displacement
  • (Think of a child on a swing set)
  • Clock Error
  • Integration period incorrectly measured

23
Gyroscope Principles of Operation
  • Two primary types
  • Mechanical
  • Optical
  • Measure rotation w.r.t. an inertial frame which
    is fixed to the stars (not fixed w.r.t. the
    Earth).

24
Mechanical Gyroscopes
  • A rotating mass generates angular momentum which
    is resistive to change or has angular inertia.
  • Angular Inertia causes precession which is
    rotation of the gimbal in the inertial coordinate
    frame.

25
Equations of Precession
  • Angular Momentum vector H
  • Torque vector T
  • Torque is proportional to
  • Angular Rate omega cross H plus
  • A change in angular momentum

26
Problems with Mechanical Gyroscopes
  • Large spinning masses have long start up times
  • Output dependent on environmental conditions
    (acceleration, vibration, sock, temperature )
  • Mechanical wear degrades gyro performance
  • Gimbal Lock

27
Gimbal Lock
  • Occurs in two or more degree of freedom (DOF)
    gyros
  • Planes of two gimbals align and once in alignment
    will never come out of alignment until separated
    manually
  • Reduces DOF of gyroscope by one
  • Alleviated by putting mechanical limiters on
    travel of gimbals or using 1DOF gyroscopes in
    combination

28
Gimbal Lock
29
Optical Gyroscope
  • Measure difference in travel time of light
    traveling in opposite directions around a
    circular path

30
Types
  • Ring Laser Gyroscope
  • Fiber Optic

31
Ring Laser Gyro
  • Change in traveled distance results in different
    frequency in opposing beams
  • Red shift for longer path
  • Blue shift for shorter path
  • For laser operation peaks must reinforce each
    other leading to frequency change.

32
Lock In and Dithering
  • Lasers tend to resist having two different
    frequencies at low angular rates
  • Analogous to mutual oscillation in electronic
    oscillators
  • Dithering or adding some small random angular
    accelerations minimizes time gyro is in locked in
    state reducing error

33
Fiber Optic Gyroscope
  • Measure phase difference of light traveling
    through fiber optic path around axis of rotation

34
Example Complete GPS/INS System
  • Applanix POS LV-V4
  • Used in Urbanscape Project
  • Also includes wheel rate sensor

35
Pulse LIDAR
  • Measures time of flight of a light pulse from an
    emitter to an object and back to determine
    position.
  • Sensitive to atmospheric effects such as dust and
    aerosols

36
Conceptual Drawing
37
The Math
  • d Distance from emitter/receiver to target
  • C speed of light (299,792,458 m/s in a vacuum)
  • ?t time of flight

38
Determining Time of Flight
39
From Depth to 3D
  • Use angle of reflecting mirror to determine ray
    direction
  • Measurement is 3D relative to LIDAR sensor frame
    of reference
  • Transform into world frame using GPS/INS system
    or known fixed location

40
Error Sources
  • Aerosols and Dust
  • Scatter Laser reducing signal strength of Laser
    reaching target
  • Laser reflected to receiver off of dust
    introduces noise
  • Minimally sensitive to temperature variation
    (changes path length inside of receiver and clock
    oscillator rate)
  • Error in measurement of rotating mirror angle
  • Specular Surfaces
  • Clock Error

41
Example Pulse LIDAR Characteristics
  • Sample specification from SICK

42
Doppler LIDAR
  • Uses a continuous beam to measure speed
    differential of target and emitter/receiver
  • Measure frequency change of reflected light
  • Blue shift- target and LIDAR device moving closer
    together
  • Red shift- target and LIDAR device moving apart

43
Application of Doppler LIDAR
  • Speed Traps

44
Combined Sensor Systems
45
Questions?
46
References
  • Grewal, M. Weil, L, Andrews, P. Global
    Positioning Systems, Inertial Navigation and
    Integration, Wiley,New York, 2001.
  • Titterton, D.H. Weston, J.L. Strapdown Inertial
    Navigation Technology. Institution of Electrical
    Engineers, London 1997
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