Class 20

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Class 20 More GPS, GLONASS and OPUS

• 2 April 2008

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Topics

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

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Homework

• How accurate does a clock need to be to achieve
  measurement precision at the millimeter level (lt
  1 cm)?
• c 299,792,458 meters per second
• c (10e-12 seconds) 0.003m
• What is the orbital period of SV1?
• P2/a34p2/GMe
• a 26,559,122 m Geometric gravitational
  constant GMe 3986005108m3/s2
• period (seconds) sqrt((a34p2)/GMe)
• period (hour) 11 hours 58 minutes

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Homework continued

• How many seconds in a week?
• (days hours in day seconds in hour
  seconds in minute) 7246060 604,800
• What is the current GPS week?
• week of 30 March to 5 April 2008 is 1473

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Signal Processing on-board
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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.
• ? c / f wavelength speed of light divided
  by frequency

L1 c/f119 cm L2 c/f2 24.4 cm L5 c/f5
25.5 cm
c 299792458m/s
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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.

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Integer bias ambiguity
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GNSS

• Global Navigation Satellite Systems
• NAVSTAR GPS operational
• GLONASS operational
• Galileo (not yet)
• COMPASS (from The Space Review)
• Chinas 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

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GLONASS
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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

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Orbital elements
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Current March 2K7 Status
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GLONASS
Note multiple frequencies!
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GPS and GLONASS ultra-rapid orbit file
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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!

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GPS only planning
Nsats Number of satellites PDOP Position
Dilution of Precision
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See G19 that sets at 2330 and rises again at
0530 (6 hr period)
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SKYPLOT
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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.

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Short Occupation Times
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VDOP
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Best VDOP
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GPS Baselines
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OPUS Solution Extended Output
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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.
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Absolute Antenna Calibrations
Required reading IGSMAIL-5189 Planned changes
to IGS antenna calibrations http//igscb.jpl.nasa.
gov/mail/2005/msg00111.html
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This is NOT JCAD 28. It is a tidal station near
Rainbow Bridge. It is not yet in the NGS data
base.
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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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