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)?

• 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

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

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

Orbital elements

Current March 2K7 Status

GLONASS

Note multiple frequencies!

GPS and GLONASS ultra-rapid orbit file

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

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

SKYPLOT

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.

Short Occupation Times

VDOP

Best VDOP

GPS Baselines

OPUS Solution Extended Output

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.

Absolute Antenna Calibrations

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

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

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).