1. Class 19 – NAVSTAR GPS, GLONASS and Galileo
31 March 2008

2. NAVSTAR GPS NAVigation by Satellite Timing And Ranging (NAVSTAR)
Shown (L to R): Block I, Block IIA and Block IIR space vehicles (SV)

4. NAVSTAR GPS Satellite Orbits

7. Satellite Characteristics
All data transmitted by the satellite based on a fundamental frequency generated by on-board atomic clocks.
(f0 = MHz)
L1 = 154 * f0 = MHz
L2 = 120 * f0 = MHz
C/A = 0.1 * f0 = MHz
P(Y) = f0
L5 = 115 * f0 = MHz (NEW civilian frequency NOT yet implemented)

8. How accurate a clock do we need?
Electromagnetic waves travel at the speed of light (c). In a vacuum is 299,792,458 m/second.
A pseudorange is c * Δt.
A clock accurate at 10-4 yields an error of 299,792 meter error.
A clock accurate at 10-9 yields an error of 3 meters.
To obtain millimeter level precision we a clock accurate to what level?

9. How long does the signal take to get to a ground-based receiver?
Nominal distance from geocenter to satellite is 26,560,000 m. From surface of earth (26,560,000 – 6,378,137 ≈ 20,182,000 m). Speed of light is 299,792,458 m/s. Therefore a signal reaches the earth in seconds.
For SV 1, (circled in red) we compute the orbital radius from the square root of the satellite orbital radius ( m). Orbit radius is: 26,559,122 m

10. Calculate orbital period
P2/a3=4π2/μ
where P is period
a is orbital radius
μ is geocentric gravitational constant (GM) = *1014m3/s2

11. GPS Time Started 0000 UTC 6 January 1980
No provision for leap seconds (continuous)
Time represented by GPS Week and Seconds of week.
How many seconds are in a week?
What is the current GPS week?
GPS software often uses the Modified Julian Date as a way to keep track of data.
JD count is from 0 at 12 (noon) 1 JAN (4713 BCE)
MJD = JD

12. Time/Date Conventions in the GPS World
Most GPS data available for use in post processing is organized by Year and Day-of-Year.
Today, 31 March 2008, is day 091
Precise orbit files (*.SP3) are organized by GPS Week and Day of Week
In this system, Sunday is Day 0
Today is day 1, GPS Week 1473

14. GPS Calendar Sources Canadian Geodetic Survey Division
US National Geodetic Survey (under Instructions option on CORS page)
NGS site above (under Utilities/Software) also has links to two DOS programs: gpscal.exe and gpswk.exe

15. RINEX Navigation Message
Clock bias, drift and drift rate
GPS Week
SV accuracy
Health

16. RINEX Observation File

17. How pseudoranges are measured

18. Pseudo-Range Measurement

19. Error Sources

20. Errors Illustrated
(Baseline error / baseline length) is proportional to (orbit error / dist to SV)

22. Precise Ephemeris (GPS)
++ = SV accuracy
c – time-related information
f – information for time/velocity calculation
i – currently unused
N.B. these values are the result of an international effort and reflect a weighted mean.

23. Precise Ephemeris (GPS)
Column values
SV Number X (km) Y (km) Z (km) clock (microseconds) X,Y,Z,C stdev

24. Orbit Sources International GNSS Service US National Geodetic Survey
US National Geodetic Survey
National Geospatial Intelligence Agency
Note that these orbits are in SP3 enhanced format

25. Signal Processing on-board

26. 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 = m/s

27. 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.

28. Integer bias ambiguity

29. 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

30. GLONASS

31. 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

32. 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!

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

37. 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.

38. SKYPLOT