1 / 35

OVERVIEW OF IGS PRODUCTS & ANALYSIS CENTER MODELING

OVERVIEW OF IGS PRODUCTS & ANALYSIS CENTER MODELING. Status of core products focus on Ultra-rapid predicted orbits issues with current products Comparisons of AC analysis strategies evidence for systematic errors, esp. fortnightly harmonics Recommendations. Jim Ray, NOAA/NGS

mcleodl
Download Presentation

OVERVIEW OF IGS PRODUCTS & ANALYSIS CENTER MODELING

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. OVERVIEW OF IGS PRODUCTS & ANALYSIS CENTER MODELING • Status of core products • focus on Ultra-rapid predicted orbits • issues with current products • Comparisons of AC analysis strategies • evidence for systematic errors, esp. fortnightly harmonics • Recommendations Jim Ray, NOAA/NGS Jake Griffiths, NOAA/NGS IGS 2008 Workshop, Miami Beach, 2 June 2008

  2. * indicates AC contributions that are weaker than others

  3. Predicted IGU Orbit WRMS IGU Orbits (1st 6 hr of predictions) wrt IGR Orbits • WRMS of IGU orbit predictions have improved to <5 cm RMS • PRN29 (IIA) decommissioned • GOP solutions improved

  4. Scale & Rotations of Predicted IGU Orbits IGU Orbits (24 h of predictions) wrt IGR Orbits (shifted) • Z rotations (UT1 prediction error) reach 1 mas level • equivalent to equatorial shift of 12.9 cm at GPS altitude (shifted)

  5. Issues with Current Products • IGU orbit combination only marginally robust • sometimes AC predictions are better than combo Ultra-Rapid IGS Orbit Comparison for 1478_6_06 (10 May 2008 06h) CENT STA| DX DY DZ RX RY RZ SCL RMS WRMS MEDI | [mm] [mm] [mm] [uas] [uas] [uas] [ppb] [mm] [mm] [mm] --------+-------------------------------------------------------- cou 73| 11 -1 -4 536 -389 254 -.29 64 34 33 emu 49| 7 0 0 486 38 -60 .03 84 44 21 esu 95| 4 5 -2 -396 687 -72 .13 77 77 29 gfu 65| 1 -2 -2 302 -21 127 -.34 77 78 29 gou 82| -5 -4 -1 260 334 48 -.35 89 78 31 siu 62| 0 17 -33 -221 1068 730 .02 130 131 71 usu 33| 19 9 0 297 -394 -20 .14 123 111 56 igu n/a| 5 0 -5 283 103 45 -.08 74 79 18 • would benefit from more high-quality ACs • accuracy now limited by ERP predictions, mostly • may also apply to IGR orbits (but less severe) • IGU combined clocks are very poor • clock predictions no better than BRDC • not enough clock ACs • even IGR clocks sometimes weak when some ACs miss

  6. effect of 15-deg cutoff

  7. Aliased Tidal Peaks in PM Discontinuities • Spectra of polar motion day-boundary discontinuities show signatures of aliased O1, Q1, & N2 tides + unknown 7.2 d line

  8. VLBI (1-hr) UT1 residuals white over full frequency range Kalman Filter of VLBI UT1 + GPS LOD(Senior, Kouba, Ray – EGU 2008) • GPS LOD residuals • approximately white • with small peak at 13.7 d • possible difference in a priori tidal models wrt VLBI EMR analysis upgrade • Gauss-Markov values for GPS LOD biases • peak-to-peak range = ± 40 μs • RMS = 9 μs • 14.19-d periodic • treated as GPS artifact • amplitude varies between 5 & 11 μs • phase varies linearly w/ time due to changing period

  9. Fortnightly Band – Spurious IGS LOD(Senior, Kouba, Ray – EGU 2008) LODS – (AAM+OAM) spectra 14.12 d signal in IGS & C04 probably due to mix of GPS errors

  10. Day-boundary Orbit Discontinuities • Orbit discontinuities between days show temporally correlated errors & broad fortnightly spectral peak • From Griffiths & Ray (AGU 2007)

  11. Conclusions • Despite huge progress by IGS since 1994, numerous small systematic errors remain in products • see EGU 2008 presentation by J. Ray http://www.ngs.noaa.gov/IGSWorkshop2008/docs/igs-errs_egu08.pdf • Applications to cutting-edge science are currently limited • need to focus on identifying, understanding, & mitigating errors • should avoid rush to premature science conclusions • must renew basic GNSS research efforts, not just in geophysical applications • requires accurate knowledge of AC processing strategies • Improvements will probably require better station installations (to reduce near-field multipath biases) & analysis upgrades • more research into field configuration effects badly needed • need better leadership to popularize lessons learned • need better cooperation & coordination between analysts & network

  12. Recommendations • For more robust products: • recruit new or improved IGU ACs & more IGR clock ACs • investigate improved near-RT & predicted ERPs • should IGS start (UT1 + LOD) service ? (à la Senior et al., EGU08) • Reject GGOS UAW actions for: • SINEX parameter & naming extensions • piecewise, continuous segment parameterization as SINEX standard • Reject rigidly standardized AC procedures & parameterizations • would lead to stagnation & end of progress • would eliminate basis for multi-solution product combinations • but ACs must agree on conventional choices & use of modern models • Instead, set up inter-service SINEX & combinations WG • investigate technique-specific systematic errors • maintain SINEX format

  13. Recommendations (cont’d) • Updated AC summaries are required: • EMR 23 Jan 2002 • GFZ 27 Feb 2003 • JPL 13 Apr 2004 • SIO 31 Oct 2005 • (USNO 12 Sep 2006) • Suggest suspending ACs with no updates by 30 Sep 2008 • if processing summary is older than 2 years • submissions would be rejected from IGS products after Sep 2008 • Rescind AC status if no updates by 31 Dec 2008 • would need to formally rejoin IGS ACs after Dec 2008 • Or ask above ACs for effective alternative proposal

  14. Backup Slides 1

  15. A SURVEY OF SOME SYSTEMATIC ERRORS IN IGS PRODUCTS • Clock jumps at day boundaries & near-field multipath • Position time series show N * 1.04 cpy harmonics • N/S distortions in IGS frames • Earth rotation parameters smoothed & filtered • Spurious tidal lines in EOPs • Orbit discontinuities have fortnightly variations Jim Ray, NOAA/National Geodetic Survey EGU 2008 General Assembly, Paper G4 #A-01694, Vienna, 15 April 2008

  16. Context • GPS errors are propagated formally but true data noise is unknown • highly site-dependent & not white noise • e.g., variances of AC frame solutions differ by > x 100 • dealt with by empirical rescaling of covariance matrix • Evidence for systematic effects in IGS product covariances is well known • e.g., user velocity errors are routinely inflated to account for temporal correlation of position errors • but methods are purely empirical • Objective: Survey systematic errors in some IGS product values • underlying causes mostly unknown or not confirmed

  17. 1) Day-boundary Clock Jumps • clock bias accuracy is based on mean of code data per arc • for 24-hr arc with code σ = 1 m, clock accuracy should be ~120 ps • can study local code biases via clock jumps at day boundaries (H-maser stations only) • observed clock jumps vary hugely among stations: 110 ps to >1500 ps • presumably caused mainly by local code multipath conditions, esp. in near-field of antenna

  18. expect largest & longest-period MP errors when height H of antenna is small [Elósegui et al., 1995] • may have special problems when H is near multiples of λ/4 • reflected RCP GPS signals enter from behind as LCP • choke-ring design esp sensitive to L2 reflections from below [Byun et al. 2002] • most IGS RF antennas mounted over flat surfaces! Near-field Multipath Mechanism

  19. Correlated Clock & Position Effects: ALGO • ALGO day-boundary clock jumps increase in winters • every winter ALGO also has large position anomalies • IGS deletes outliers >5 σ • implies common near-field multipath effect is likely (phase & code)

  20. Probably better to mount antennas away from closereflecting surfaces!worse better

  21. Other Hardware Choices Also Important PIE1 AOA D/M_T antenna ASH701945E_M antenna + new cables • receiver health, firmware, antenna model, & cables also affect day-boundary clock jumps AOA firmware 3.2.32.8 3.2.32.11 Ashtech UZ-12 receiver Rogue SNR-8000 receiver Ashtech UZ-12 receiver

  22. 2) Stacked/Smoothed Spectra of Site Residuals (shifted) • for 167 IGS sites with >200 weekly points in 1996.0 – 2006.0 • large annual + semi-annual variations • plus harmonics in all components at N * (1.040 ± 0.008) cpy • flicker noise spectra down to periods of ~few months (shifted)

  23. Position Harmonics Linked to GPS Year • 1.040 ± 0.008 cpy fundamental does not match any expected alias or geophysical frequency • also not seen in VLBI, SLR, or fluid load spectra • Closely matches GPS “draconitic” year • rotation period of Sun w.r.t. GPS nodes (viewed from Earth) • GPS nodal drift is -14.16° per year (due to Earth’s oblateness) • period = 351.4 day or frequency = 1.039 cpy • Two possible coupling mechanisms suggested: 1) direct orbit modeling errors (e.g., related to eclipse periods & planes) 2) alias of site position biases (e.g., near-field phase multipath) due to beating of 24-hr processing arc against 23.93-hr GPS repeat period • useful distinguishing tests not yet made

  24. 3) N/S Distortions of IGS Frames N E U • Weekly mean biases of IGS frames compared to long-term frame

  25. IGS Frame Distortions • N/S mean component of IGS weekly frames shows largest annual variation • after weekly 7-parameter Helmert alignment • also largest dispersion among ACs in N/S direction • Not likely to be caused by annual inter-hemisphere fluid load cycle • load signal should be largest in heights, not N/S • Could possibly be related to along-track GPS orbit errors • but no mechanism identified • Likelier explanation: possible neglected 2nd order ionospheric effect

  26. 4) High-frequency Smoothing of EOPs • Day-boundary continuity constraint by some ACs smoothes & filters EOP estimates near Nyquist limit

  27. Filter/Smoother by Continuity Constraints • Some ACs estimate EOPs (& others) by continuous linear segments • attenuates power by factor 4 at Nyquist limit • smoothes estimates • filters certain phase components • To avoid contaminating IGS combination, such EOP solutions rejected since January 2008 (wk 1460) • but effects on other parameters probably still present • Past high-frequency studies should be reconsidered • Can use GFZ polar motion to estimate background, non-tidal, sub-daily variance: 13.6 to 20.7 μas2

  28. 5) Aliased Tidal Peaks in EOP Discontinuities • Spectra of polar motion day-boundary discontinuities show signatures of aliased O1, Q1, & N2 tides + unknown 7.2 d line

  29. 6) Day-boundary Orbit Discontinuities • Orbit discontinuities between days show temporally correlated errors & broad fortnightly spectral peak

  30. Conclusions • Despite huge progress by IGS since 1994, numerous small systematic errors remain in products • Applications to cutting-edge science must recognize limitations • need to focus on identifying, understanding, & mitigating errors • must renew basic GNSS research efforts, not just in geophysical applications • should avoid rush to premature science conclusions • Improvements will probably require better station installations (to reduce near-field multipath) & analysis upgrades • more research into field configuration effects badly needed • need better leadership to popularize lessons learned • need better cooperation & coordination between analysts & network

More Related