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04/22/02 EGS G6 2002 1
STABILITY OF GLOBAL GEODETIC RESULTS
Prof. Thomas Herring
Room 54-611; 253-5941
http://bowie.mit.edu/~tah
04/22/02 EGS G6 2002 2
Overview
• Motivation for talk: Anomalies in apparent positions of phase centers of GPS satellites, and variations in global scale
• Analysis of GPS results with satellite phase center position estimated and ground antenna phase center models
• Comparison of scale variations with VLBI results
04/22/02 EGS G6 2002 3
Motivation
• Launch of Block IIR satellite in July 1997 showed that the apparent phase center of satellite could over 1 m from the manufactures specification. Not until Oct 1999 when second Block IIR launched that this could be confirmed.
• So many years, use of anechoic chamber measurements for ground antennas causes large scale change (14 ppb)
• Scale rate estimates showed values that could be artifact or due to global deformation
04/22/02 EGS G6 2002 4
-4
-3
-2
-1
0
1
2
3
4
1994.0 1995.0 1996.0 1997.0 1998.0 1999.0 2000.0 2001.0 2002.0
GAMIT Δ ( )scale ppb JPL Δ ( )scale ppb
Δ
( )scale ppb
Year
2000 51- (0.9 / , 0.14 / )ITRF site average mm yr ppb yr
-0.24 /GAMIT Rate ppb yr -0.10 /JPL Rate ppb yr
Scale evolution from daily scale estimates
04/22/02 EGS G6 2002 5
Analysis GPS phase centers
• Two locations need to be considered:– Ground antenna phase center pattern– Satellite antenna phase center– In both cases: Pattern must be referred to a
physical location. Antenna Reference Point for ground antennas; position relative to center of mass for satellite.
• Recent insitu absolute phase center calibrations match most of the previous anechoic chamber results
04/22/02 EGS G6 2002 6
Absolute calibration
• Hannover System:
•
• http://www.ife.uni-hannover.de/~web/AOA_DM_T/
04/22/02 EGS G6 2002 7
Absolute phase center corrections for choke ring antennas
-15
-10
-5
0
5
10
0.0 20.0 40.0 60.0 80.0
L1 (mm)L2 (mm)LC Ionospheric free (mm)
Elevation angle (deg)
04/22/02 EGS G6 2002 8
Effects of satellite phase center offset (radial)
= dh cos ; sin = (R/a) sin ; (R/a) = 0.24
Zenith Distance
γ
dhδργ
R
a
EarthSatelliteDetail at Satellitedh=Radial phase centerchange
GroundReciever
04/22/02 EGS G6 2002 9
Similarity of satellite phase center and receiver height
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.0 20.0 40.0 60.0 80.0
Δ Heght30 - times satellite DPC constant
( )Elevation angle deg
04/22/02 EGS G6 2002 10
Relationship between DM phase center and effects of station height and satellite PC
-15.0
-10.0
-5.0
0.0
5.0
10.0
0.0 20.0 40.0 60.0 80.0
-75*(Δ -30* ) + Height satellite constant ( )LC Ion free DM Phase center mm
( )Elevation angle deg
04/22/02 EGS G6 2002 11
Summary of expected phase changes
• The absolute phase center corrections for choke ring antenna are ~20 mm
• Satellite radial phase center position changes map to 1/30 of station height changes when clock offset included
• Difference between satellite phase center change ands station height times 75 is comparable to choke ring phase center
• Expectation: meter level changes in satellite phase center position when choke ring phase center used.
04/22/02 EGS G6 2002 12
GPS and VLBI analyses
• To understand these effects, we have performed the following analyses:– GPS scale variations using standard GPS analyses
(zero phase center correction for choke ring; fixed satellite phase center). SOPAC h-files from sopac.ucsd.edu+JPL results)
– Satellite phase center offsets; zero choke ring– Satellite phase center offsets with Hannover Choke
ring model– VLBI SINEX file analysis for scale (now available at
http://www-gpsg.mit.edu/~tah/VLBI_SINEX).
04/22/02 EGS G6 2002 13
Networks used: Black 45 GPS sites; Red VLBI
VLBI:
1885 sessions1990-2000
GPS:
Monthly 1999-2001
04/22/02 EGS G6 2002 14
Estimated Satellite Z-offsets
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5 10 15 20 25 30
Z-offset No Choke ring phase centerZ offset Hannover Choke ring model
PRN
Block IIR
Block IIR
Apriori Block II/IIA offset
04/22/02 EGS G6 2002 15
Time series estimates
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
1999 2000 2001 2001 2002 2002 2003
PRN 11 Abs 1.44±0.03 mPRN 14 Abs 1.71±0.06 mPRN 28 Abs 1.45±0.07 mPRN 11 Rel -2.14±0.04 mPRN 14 Rel -1.87±0.06 mPRN 28 Rel -2.18±0.05 m
Year
Block IIR satellites
04/22/02 EGS G6 2002 16
Zoom of Absolute series only
0.5
1.0
1.5
2.0
2.5
3.0
2000.8 2001.0 2001.2 2001.4 2001.6 2001.8 2002.0 2002.2 2002.4
PRN 11 Abs 1.44±0.03 mPRN 14 Abs 1.71±0.06 mPRN 28 Abs 1.45±0.07 m
Year
Block IIR satellites. Absolute PC model only
04/22/02 EGS G6 2002 17
Effects on radial orbit position of satellite
-0.4
-0.2
0.0
0.2
0.4
0.6
2000.0 2000.4 2000.8 2001.2 2001.6 2002.0
Abs Mean -0.01±0.03 RMS 0.15 m σ 0.20 m -0.01±0.03 0.15 Rel Mean RMS mσ 0.21 m No Satellite PC 0.12±0.02 0.13 Rel Mean RMS mσ 0.18 m
Year
Orbit Adjustment relative to constrained model
Apriori orbit: No satellite or choke ring PC, sites constrained
04/22/02 EGS G6 2002 18
Summary of phase center
• The effects of ground antenna phase center model only satellite phase center estimates are large (~3.6 meters)
• Block II/IIA definitely different from Block IIR and some indication of differences between satellites within the same type (differences are a few centimeters)
• Radial orbit changes are small (<1 cm on average). Interestingly better agreement of loose solution with constrained when satellite PC estimated (10 cm differences),
04/22/02 EGS G6 2002 19
Scale effects
• From the different analyses and VLBI analysis we can estimate scale and its rate of change:
• Scale in ppb and scale rate ppb/yr (1ppb=6mm)
Soln Scale +- Srate +-
Abs -6.04 0.25 -0.24 0.06
Rel 11.99 0.25 -0.22 0.06
VLBI -0.21 0.04 -0.02 0.01
04/22/02 EGS G6 2002 20
Temporal Variations in Scale
-10.0
-5.0
0.0
5.0
10.0
1990 1992 1994 1996 1998 2000 2002
GPS Scale (ppb)VLBI Scale (ppb)
Year
04/22/02 EGS G6 2002 21
VLBI + GPS Detrended with Annual
-10.0
-5.0
0.0
5.0
10.0
1990 1992 1994 1996 1998 2000 2002
GPS Detrended with AnnualVLBI with annual
Year
04/22/02 EGS G6 2002 22
Zoom of Previous Figure
-2.0
-1.0
0.0
1.0
2.0
1995 1996 1997 1998 1999 2000 2001 2002
GPS Detrended with AnnualVLBI with annual
Year
VLBI Cos -0.31 Sin -0.24 ppb AnnualGPS Cos -0.24 Sin -0.29 ppb
04/22/02 EGS G6 2002 23
Conclusions
• GPS has difficulty in separating ground antenna and satellite phase center effects and positions. Limits the accuracy of global geodetic results
• Precision of GPS much better • Secular scale rates of loose GPS solutions
-0.1 to -0.2 ppb/yr (0.6-1.2 mm/yr heights) probably artifact: not seen in VLBI
• Annual scale is probably real: Amplitude 0.4 ppb (2.5 mm): Water and atmospheric pressure loading are likely origin