Transcript Class 18 – NAVSTAR GPS, GLONASS and Galileo

Class 19 – NAVSTAR GPS,
GLONASS and Galileo
31 March 2008
NAVSTAR GPS
• NAVigation by Satellite Timing And
Ranging (NAVSTAR)
Shown (L to R): Block I, Block IIA and
Block IIR space vehicles (SV)
NAVSTAR GPS Satellite Orbits
http://gge.unb.ca/Resources/GPSConstellationStatus.txt
Satellite Characteristics
• All data transmitted by the satellite based
on a fundamental frequency generated by
on-board atomic clocks.
– (f0 = 10.23 MHz)
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L1 = 154 * f0 = 1575.42 MHz
L2 = 120 * f0 = 1227.6 MHz
C/A = 0.1 * f0 = 1.023 MHz
P(Y) = f0
L5 = 115 * f0 = 1176.45 MHz (NEW civilian
frequency NOT yet implemented)
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?
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 0.067 seconds.
For SV 1, (circled in red) we compute the orbital radius from the square root of the satellite
orbital radius (5153.55429268 m). Orbit radius is: 26,559,122 m
Calculate orbital period
• P2/a3=4π2/μ
– where P is period
– a is orbital radius
– μ is geocentric gravitational constant
(GM) = 3.986005*1014m3/s2
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 -4712
(4713 BCE)
– MJD = JD - 2400000.5
Time/Date Conventions
in the GPS World
• Most GPS data available for use in post
processing is organized by Year and Dayof-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
http://www.geod.nrcan.gc.ca/tools-outils/pdf/gps_calendar_2008.pdf
GPS Calendar Sources
• Canadian Geodetic Survey Division
• US National Geodetic Survey (under
Instructions option on CORS page)
– http://www.ngs.noaa.gov/CORS/Instructions3/
• NGS site above (under Utilities/Software)
also has links to two DOS programs:
gpscal.exe and gpswk.exe
RINEX Navigation Message
Clock bias, drift and drift rate
GPS Week
SV accuracy
Health
RINEX Observation File
How pseudoranges are measured
Pseudo-Range Measurement
Error Sources
Errors Illustrated
(Baseline error / baseline length) is
proportional to (orbit error / dist to SV)
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.
Precise Ephemeris (GPS)
Column values
SV Number X (km) Y (km) Z (km) clock (microseconds) X,Y,Z,C stdev
Orbit Sources
• International GNSS Service
– http://igscb.jpl.nasa.gov/components/prods_c
b.html
• US National Geodetic Survey
– http://www.ngs.noaa.gov/CORS/download2/
• National Geospatial Intelligence Agency
– http://earthinfo.nga.mil/GandG/sathtml/PEexe.html
• Note that these orbits are in SP3 enhanced format
Signal Processing on-board
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
Frequency Combinations
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•
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•
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.
Integer bias ambiguity
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
GLONASS
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
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!
GPS only planning
Nsats – Number of satellites
PDOP – Position Dilution of Precision
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.
SKYPLOT