STABILITY OF GLOBAL GEODETIC RESULTS

Transcription

1 STABILITY OF GLOBAL GEODETIC RESULTS Prof. Thomas Herring Room ; /22/02 EGS G 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 G

2 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 G Scale evolution from daily scale estimates GAMIT Dscale (ppb) JPL Dscale (ppb) GAMIT Rate ppb/yr JPL Rate ppb/yr ITRF site average (0.9 mm/yr, 0.14 ppb/yr) Dscale (ppb) /22/02 EGS G

3 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 G Absolute calibration Hannover System: 04/22/02 EGS G

4 Absolute phase center corrections for choke ring antennas 10 5 L1 (mm) L2 (mm) LC Ionospheric free (mm) Phase Correction (mm) Elevation angle (deg) 04/22/02 EGS G Effects of satellite phase center offset (radial) Ground Reciever R y Zenith Distance a g Satellite Earth g dr dh Detail at Satellite dh=radial phase center change dr = dh cos g ; sin g = (R/a) sin y ; (R/a) = /22/02 EGS G

5 Similarity of satellite phase center and receiver height 0.2 Phase correction per unit change DHeght 30 times satellite DPC - constant Elevation angle (deg) 04/22/02 EGS G Relationship between DM phase center and effects of station height and satellite PC *(DHeight-30*satellite) + constant LC Ion free DM Phase center (mm) Phase change (mm) Elevation angle (deg) 04/22/02 EGS G

6 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 G 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 04/22/02 EGS G

7 Networks used: Black 45 GPS sites; Red VLBI VLBI: 1885 sessions GPS: Monthly /22/02 EGS G Estimated Satellite Z-offsets 4.0 Z-offset No Choke ring phase center Z offset Hannover Choke ring model Satellite Z-phase center position (m) Block IIR Apriori Block II/IIA offset Block IIR PRN 04/22/02 EGS G

8 Time series estimates 3.0 Block IIR satellites 2.0 Z Phase center offset (m) PRN 11 Abs 1.44±3 m PRN 14 Abs 1.71±6 m PRN 28 Abs 1.45±7 m PRN 11 Rel -2.14±4 m PRN 14 Rel -1.87±6 m PRN 28 Rel -2.18±5 m /22/02 EGS G Zoom of Absolute series only Block IIR satellites. Absolute PC model only PRN 11 Abs 1.44±3 m PRN 14 Abs 1.71±6 m PRN 28 Abs 1.45±7 m Z Phase center offset (m) /22/02 EGS G

9 Effects on radial orbit position of satellite Apriori orbit: No satellite or choke ring PC, sites constrained DSemimajor Axis PRN 11 (m) Orbit Adjustment relative to constrained model Abs Mean -1±3 RMS 0.15 m s 0.20 m Rel Mean -1±3 RMS 0.15 m s 0.21 m No Satellite PC Rel Mean 0.12±2 RMS 0.13 m s 0.18 m /22/02 EGS G 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 G

10 Scale effects From the different analyses and VLBI analysis we can estimate scale and its rate of change: Soln Abs Scale Srate Rel VLBI Scale in ppb and scale rate ppb/yr (1ppb=6mm) 04/22/02 EGS G Temporal Variations in Scale 1 GPS Scale (ppb) VLBI Scale (ppb) 5.0 Dscale (ppb) /22/02 EGS G

11 VLBI + GPS Detrended with Annual 1 GPS Detrended with Annual VLBI with annual 5.0 Scale (ppb) /22/02 EGS G Zoom of Previous Figure 2.0 GPS Detrended with Annual VLBI with annual 1.0 Scale (ppb) VLBI Cos Sin ppb Annual GPS Cos Sin ppb /22/02 EGS G

12 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 ( 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 04/22/02 EGS G