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)

• 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 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

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

GPS Week

SV accuracyHealth

Clock bias, drift and drift rate

RINEX Navigation Message

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://earth-info.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

• 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.

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