04/22/02 EGS G6 2002 1

STABILITY OF GLOBAL GEODETIC RESULTS

Prof. Thomas Herring

Room 54-611; 253-5941

tah@mit.edu

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