CE 316 Geomatics - Chapter 9

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CE 316 Geomatics - Chapter 9
2012-04-09 18:39:42
Geomatics Satellite Positioning

Geomatics - Satellite Positioning
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  1. Define Ambiguity
    The unknown interger number of cycles of the reconstructed carrier phase between the satellites and the sensor at the beginning of the observation
  2. Anti-Spoofing (A.S.)
    When P-codes becomes encrypted and no longer availabe to the civilian users
  3. Baseline
    Consists of a pair of stations for which simultaneous GPS data has been collected
  4. C/A Code
    The coarse/acquisition GPS code- a pseudo random (PRN) code transmitted at 1.023 MHz that repeats every milli-second. Each satellite has its own C/A code so that it can be uniquely identified and received seperately from other satellites transmitting on the same frequency
  5. Ephemeris
    A list of positions or locations of a celestial object as a function of time
  6. Epoch
    A particular fixed instant of time used as a reference point on a time scale
  7. Alerts
    Indicates the positions of the satellites in view to an observer on the earths surface at a designated time and location
  8. Cut-off angle
    The minimum elevation angle used by the sensor to track GPS satellites
  9. Geomagnetic Activity
    Occurs when solar flares cause irregular ionization of the ionosphere which in turn causes irregular refraction delays of a radio wave passing through the non-homogeneous medium
  10. GDOP
    Geometric dilution of precision
  11. GPS Window
    The period during the 24-hour day when there are a sufficient number of satellites in view to meet the positioning requirement
  12. Mission
    The manner in which the GPS will be used in the field
  13. Multipath
    Refers to a reflected signal that combines with the true signal resulting in a weaker position
  14. PDOP-Position of Dilution of Precsion
    A numerical indicator of the geometric strength of the satellite constellation in relation to positioning at a certain location
  15. Point Positioning
    The independent reduction of observations made by a particular receiver using the pseudo-range with information broadcast from satellites
  16. Post Processing
    Computer processing of data after the observation session
  17. Project
    Description of a survey job. There is no limit to the number of stations
  18. P-Code
    Precise GPS code - usually encrypted and reserved for military applications
  19. What frequency does the P-code run at and what is its repeating interval?
    P-Code is a 10.23 MHz PRN code that repeats every week
  20. PRN (Pseudo-random Noise) Number
    A number assigned by the GPS system designers to a given set of pseudo-random codes. Typically a satellite will keep its PRN indefinitely. Used as a way of labeling satellites.
  21. Pseudorange Measurements
    Measurements made using one of the pseudo-range codes on the GPS signal. They provide an unambiguous measure of the range to the satellite including the effects of the satellite and user clock biases.
  22. Selective Availability (S.A.)
    Used to describe the situation when the operators of the GPS system degrade the broadcast GPS message by dithering the clock or orbital information.
  23. UTC Time
    Coordinated Universal Time is a high precision atomic clock standard, with uniform and "leap" seconds. It is aligned with GMT
  24. Three major componets of the GPS system are:
    • The satellite system
    • The control segment
    • The user
  25. The GPS receiver can compute the transmit time by knowing the _____ time a message was received.
    The GPS reciever can compute the transmit time by knowing the exact time a message was recieved.
  26. A GPS is message is assumed to travel at what speed?
    The speed of light
  27. Based on the GPS system for USA, answer the following questions:
    A) What is the minimum number of satellites?
    B) What is distance from the satellites to Earth?
    • A) 24
    • B) 20200 km
  28. Based on the GPS system for USA how many orbital planes are there, how many satellites follow each plane and at what degree are the satellites inclined with respect to the equator?
    6 orbital planes with 4 satellites each @ 55˚ inclincation to the equatorial plane
  29. What is the expected life of a satellite?
    7.5 years
  30. How many satellites are currently in orbit and healthy?
  31. How many microwave signals are typically emitted from a GPS satellite? What are the names?
    Two microwave signals, L1 and L2
  32. What are the five primary signals from which observations are and used by surveyors and engineers?
    • L1
    • L2
    • L1 C/A
    • L1 P
    • L2 P
  33. Precise Position Systems (PPS) use the ______ signal to measure the ______ delay.
    Precise Position Systems (PPS) use the L2 signal to measure the ionosphere delay.
  34. Which code is unique to each satellite?
    PRN Code
  35. Which code is the basis for civilian GPS?
    C/A Code
  36. Ground stations around the world are used to...
    • Monitor the health of satellites
    • Upload orbital parameters to the satellite
  37. Master control stations for the USA GPS are located where?
    • Schriever Air Force Base, Colorado Springs
    • Sunnyvale, California
  38. How does single point positioning work?
    • One receiver is used to determine the 3 dimensions position of a unknown point
    • The measurements of the PRN (C/A or P) code are taken
    • GPS receiver correlates the incoming PRN with a similar stable signal generated in the reciever
    • ∆T∆c = pseudo-range between transmitting satellite and receiver
    • Position is affeted by timing errors, dt
  39. What are the sources of error associated with Single Point Positioning?
    • Error in satellite position
    • Satellite timing
    • Receiver timing
    • Site dependant (height of instrument, satellite geometry)
    • Atmospheric conditions (ionosphere, troposphere)
  40. What is differential GPS?
    • GPS system that requires two or more receivers simultaneously receiving signals from the same satellites
    • It is a form of corrected point positioning
    • Uses short wavelength carrier phase measurements
  41. Benefits of using differential measurement are:
    • Elimination of errors in the satellite and receiver clock
    • Elimination of erros in satellite ephemerides
    • Minimization of atmospheric effects
  42. Sources of error associated with differential GPS are:
    • Multipath
    • Receiver Noise
    • Site dependent factors
    • Not reading same satellites
    • Distance between stations is too great
    • Ambiguity
  43. Precision in Differential GPS increases when?
    Precision increases with a decrease in wavelength
  44. The wavelengths of L1 and L2 are:
    0.19m and 0.24m
  45. In Differential GPS, L1 and L2 can provide how much precision?
    Can provide cm to mm of precision
  46. In Differential GPS, C/A and P code can provide how much precision?
    Can provide decimeters to meters of precesion.
  47. What must be considred when preparing for a GPS survey?
    • Satellite availability
    • Satellite azimuth
    • Mask angle
  48. What are the 5 types of GPS surveying methods?
    • Static surveying
    • Rapid Static surveying
    • Reoccupation surveying
    • Stop and go surveying
    • Kinematic surveying
  49. What is Static GPS Surveying?
    • Most reliable surveying method
    • Involves to multiple GPS stations
    • Minimum 4 satellites for readings
    • Classical method for long lines and highest accuracy
  50. What is Rapid Static Surveying?
    • One or more roving receivers occupying points for 1 - 20 minutes
    • Reduced occupation time
    • One or more fixed reference recievers
    • Short observation time for baselines up to 15 kms
  51. What is Relative Kinematic Positioning (RKP)?
    • Dynamic system
    • Used to provide relative survey control
    • One or more receivers occupy points for severeal minutes
    • Receiver must track a minimum of 4 identical satellites
  52. Precise GPS RTK Positioning
    Combination of differential GPS correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistcal tests- possibly with processing in real-time positioning (RTK)
    Russian based satellite system
  54. How many satellites does GLONASS have in its system?
    At what altitude do they orbit?
    • 24 satellites
    • 19000km
  55. Based on the GLONASS system for Russia how many orbital planes are there, how many satellites follow each plane and at what degree are the satellites inclined with respect to the equator?
    Three orbital plane 120˚ apart, 8 satellites equally spaced 45˚ apart. Inclined 64.8˚ from the equatorial plane.
  56. GLONASS references what geocentric coordinate system?
  57. How is GLONASS controlled?
    • Controlled by a system of laser and radar tracking stations
    • System is based entirely on UTC and a hydrogen master clock.
  58. What is the GALILEO?
    • A GPS satellite with 26 SV's at an altitude of 23222 km
    • 3 orbital planes at 56˚ inclination
    • Low accuracy system open to all users
    • High accuracy system for military use and paying customers
    • Cost 3.4 Billion Euros
    • China's GPS satellite launched in April 2007
    • 35 SV's
    • 5 SV's in geogyncronus orbit directly above equator