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Class 20 – More GPS, GLONASS and OPUS

Class 20 – More GPS, GLONASS and OPUS. 2 April 2008. Topics. Homework review/answers Recap of GPS range solution GPS phase measurements GLONASS GLONASS/GPS interoperability. Homework. How accurate does a clock need to be to achieve measurement precision at the millimeter level (< 1 cm)?

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Class 20 – More GPS, GLONASS and OPUS

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  1. Class 20 – More GPS, GLONASS and OPUS 2 April 2008

  2. Topics • Homework review/answers • Recap of GPS range solution • GPS phase measurements • GLONASS • GLONASS/GPS interoperability

  3. Homework • How accurate does a clock need to be to achieve measurement precision at the millimeter level (< 1 cm)? • c = 299,792,458 meters per second • c *(10e-12 seconds) = 0.003m • What is the orbital period of SV1? • P2/a3=4π2/GMe • a = 26,559,122 m; Geometric gravitational constant = GMe = 3986005*108m3/s2 • period (seconds) = sqrt((a3*4π2)/GMe) • period (hour) = 11 hours 58 minutes

  4. Homework continued • How many seconds in a week? • (#days * #hours in day * #seconds in hour * #seconds in minute) = 7*24*60*60 = 604,800 • What is the current GPS week? • week of 30 March to 5 April 2008 is 1473

  5. Signal Processing on-board

  6. Frequency to Wavelength • We can track the phase of the signal and accumulate the number of wavelengths (and the fractional first phase) as a measurement. • λ = c / f ;wavelength = speed of light divided by frequency L1 = c/f1=19 cm L2 = c/f2 = 24.4 cm L5 = c/f5 = 25.5 cm c = 299792458m/s

  7. Frequency Combinations • Narrow-lane = f1 + f2 ≈ 11 cm • Wide-lane = f1 – f2 ≈ 86 cm • Iono-Free ≈ f1/(f1-f2) ≈ 5 cm • Why do this? • Iono-free effectively eliminates this effect • Other combinations assist integer fixing.

  8. Integer bias ambiguity

  9. GNSS • Global Navigation Satellite Systems • NAVSTAR GPS operational • GLONASS operational • Galileo (not yet) • COMPASS (from The Space Review) • “China’s existing Beidou navigation network is a clumsy system based on three satellites, (two operational and one reserve) in geosynchronous orbit, launched between 2000 and 2003.” 19 June 2006

  10. GLONASS

  11. GLONASS • Global'naya Navigatsionnaya Sputnikovaya Sistema • Intended 21 SV with 3 on-orbit spares • 3 orbital planes separated by 120 degrees • orbits inclined 65 degrees • orbit period 11h 15m • first launch 1982; most recent 25 Dec 2007 http://www.glonass-ianc.rsa.ru

  12. Orbital elements

  13. Current March 2K7 Status

  14. GLONASS Note multiple frequencies!

  15. GPS and GLONASS ultra-rapid orbit file

  16. Interoperability questions • GLONASS uses a different geocentric datum (PZ-90) • GLONASS time and GPS time are not the same. • Leap seconds are an issue • Hardware biases • Use of different frequencies means more difficulties when fixing integers. • Some broadcast negative frequencies!

  17. GPS only planning Nsats – Number of satellites PDOP – Position Dilution of Precision

  18. See G19 that sets at 23:30 and rises again at 05:30 (6 hr period)

  19. SKYPLOT

  20. Dilution of Precision • A planning measure measuring the effect of satellite geometry wrt the satellite constellation. Smaller values are better. • PDOP – Position (East, North and Up) • GDOP – Geometric (E,N,U and Time) • VDOP – Vertical (Up) • TDOP – Time (Time) • DOP combined with UERE to estimate positioning accuracy.

  21. Short Occupation Times

  22. VDOP

  23. Best VDOP

  24. GPS Baselines

  25. OPUS Solution Extended Output

  26. GPS Antenna Calibration Issues Obviously, GPS antennas have different physical dimensions. Less obviously, they track the satellites differently. Note the different values for the same elevation angle for these two antennas.

  27. Absolute Antenna Calibrations Required reading: [IGSMAIL-5189] Planned changes to IGS antenna calibrations http://igscb.jpl.nasa.gov/mail/2005/msg00111.html

  28. This is NOT JCAD 28. It is a tidal station near Rainbow Bridge. It is not yet in the NGS data base.

  29. These three G-file sections show (line “C”) the baseline components and their standard deviations for the three ties to CORS sites. The “D” line shows the correlations between baseline components (DX to DY, DX to DZ and DY to DZ).

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