1 / 38

GNSS Observations of Earth Orientation

1. Polar motion observability using GNSS concepts, complications, & error sources subdaily considerations 2. Performance of IGS polar motion series compare Ultra-rapid, Rapid, & Final products assess random & systematic errors 3. Utility of IGS length -of- day (LOD)

niran
Download Presentation

GNSS Observations of Earth Orientation

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. 1. Polar motion observability using GNSS • concepts, complications, & error sources • subdaily considerations • 2. Performance of IGS polar motion series • compare Ultra-rapid, Rapid, & Final products • assess random & systematic errors • 3. Utility of IGS length-of-day (LOD) • assess value for combinations with VLBI UT1 • 4. Impact of errors in subdaily EOP tide model • effects on orbits, EOPs, & other IGS products GNSS Observations of Earth Orientation Jim Ray, NOAA/NGS Wuhan University, May 2013

  2. 02

  3. IGS aims for ~1 cm orbit & ~1 mm terrestrial accuracies • to satisfy most demanding mm-level user application requirements 03

  4. Ultra-Rapid Products 04

  5. 05

  6. Ultra-Rapid AC Orbit Comparisons (over 48 hr) • Performance among ACs is bimodal & widely dispersed • SIO & USN have been rejected for ~5 years; NGS & WHU added recently • AC quality is more uniform over first 6 hr of predictions • biggest differences come from 6 – 24 hr predictions 06

  7. Some IGU AC Orbits Have Large Rotations • SIO, USN, & NGS have had large Z rotations • NGS recently improved • CODE sometimes has moderately large Z rotations 0.5 mas = 64 mm error @ GPS hgt 07

  8. Orbit errors double when prediction interval increases by x4 • IGA total err only ~40% worse than IGRs (but 175% worse for RZ) 08

  9. Z rotation errors are largest RT error – • from UT1 prediction errors • Largest RT orbit prediction error comes from UT1 predictions • IGA accuracy also limited by RZ rotations 09

  10. due to modelling of orbit dynamics • large X, Y rotation errors – from PM prediction errors • Next largest RT limits from orbit modelling & PM prediction errors 10

  11. Ultra-rapid Observed Polar Motion Accuracy (Ultra Observed – Final) PM Differences • IGU accuracy improved • greatly after ~2003 • IGU dPM RMS errors <40 µas in recent years • errors in IGS Final PM reference no longer negligible • IGS PM errors • have low-frequency systematic components dPM-x dPM-y Polar Motion Differences (µas) improved . . .  even more . . .  (25 Mar 2000 – 31 Dec 2011: daily noon epochs only) Annual mean & std dev of (IGU-IGS) dPM dPM-x dPM-y Mean & Std Dev (µas) Final PM-x sigma Final PM-y -sigma 11

  12. Spectra of (Ultra Observed-Final) PM Differences dPM-x dPM-y • Subdaily tide model alias errors seen at 7.0 & 14.19 d periods • 7th GPS draconitic peak also strong in polar motion rates • 3rd GPS draconitic peak also prominent in IGS orbit discontinuities • note that differencing should remove common-mode errors! inter-annual power is largest (1461 d from 1 Jan 2008 – 31 Dec 2011) 12

  13. Ultra-rapid Predicted Polar Motion Accuracy (Ultra Predicted – Final) PM Differences dPM-x dPM-y • IGU PM prediction accuracy unchanged since ~2006 • recent IGU • 1-d RMS prediction • errors: • ~270 µas for PM-x • ~210 µas for PM-y • PM prediction errors appear more random than • systematic Polar Motion Differences (µas) (8 Nov 2006 – 31 Dec 2011: daily noon epochs only) Annual mean & std dev of (IGU-IGS) dPM dPM-x dPM-y Mean & Std Dev (µas) Final PM-x sigma Final PM-y -sigma 13

  14. Spectra of (Ultra Predicted-Final) PM Differences (1880 d from 8 Nov 2006 – 31 Dec 2011) dPM-x dPM-y • Subdaily tide model alias errors probably dominate sub-monthly band & perhaps annual, but no distinct lines • draconitic errors probably important in between 14

  15. Ultra-rapid Observed dLOD Accuracy (Ultra Observed – Final) dLOD Differences • IGU accuracy improved • steadily after ~2002 • IGU dLOD RMS errors <12 µs in most recent years • errors in IGS Final dLOD reference not negligible • IGS dLOD errors • have high-frequency systematic components Length-of-Day Differences (µs) improved . . .  (25 Mar 2000 – 31 Dec 2011: daily noon epochs only) Annual mean & std dev of (IGU-IGS) dLOD Mean & Std Dev (µs) Final dLOD sigma 15

  16. Spectra of (Ultra Observed-Final) dLOD Differences (1461 d from 1 Jan 2008 – 31 Dec 2011) • Subdaily tide model alias errors seen at ~7, ~9, & ~14 d bands • long-period errors muted by “calibration” of AC LOD biases via comparison with IERS Bulletin A over sliding window of recent past results 16

  17. Ultra-rapid Predicted dLOD Accuracy (Ultra Predicted – Final) dLOD Differences • IGU dLOD prediction accuracy slightly improved • since ~2006 • recent IGU • 1-d dLOD prediction • error ~50 µs • dLOD prediction errors have evident • systematic signatures Length-of-Day Differences (µs) (8 Nov 2006 – 31 Dec 2011: daily noon epochs only) Annual mean & std dev of (IGU-IGS) dPM Mean & Std Dev (µas) Final dLODsigma 17

  18. Spectra of (Ultra Predicted-Final) dLOD Differences (1880 d from 8 Nov 2006 – 31 Dec 2011) • Subdaily tide model alias errors seen at ~9 & ~14 d bands • plus strong 8th GPS draconitic & semi-annual peaks 18

  19. IGS ERP Predictions Compared to Other Services • IGS Ultra ERP predictions for 1 d after last observations compared to operational EOP services • IGU ERPs issued 9 hr before prediction epoch • predictions benefit from access to latest high-accuracy observations • Compare to IERS (USNO & Paris Obs) & JPL EOP services • results from IERS EOP Combination of Predictions Pilot Project (18.03.2012) • IGU PM predictions better than any others • IGU comparable to UT1/dLOD services due to their use of AAM predictions • EOP prediction services should consider assimilating IGU predictions, as well as most recent IGU observations 19

  20. Recent Ultra-Rapid ERP Accuracy • IGA observed EOPs updated every 6 hr • latency is 15 hr for each update • each EOP value is integrated over 24 hr • recent polar motion accuracy: <50 µas (1.5 mm) • recent dLOD accuracy: <12 µs (5.6 mm/day) • reported formal errors are generally reliable • IGU predicted EOPs updated every 6 hr • for real-time applications • issued 9 hr before EOP epoch • recent polar motion prediction accuracy: ~250 µas (7.7 mm) • recent dLOD prediction accuracy: ~50 µs (23 mm/day) • reported formal errors are too optimistic by a factor of 3 to 4 • IGU PM predictions better than use IERS service • IGU dLOD predictions similar to operational services • IGU ERP observations & predictions should be assimilated by operational EOP prediction services 20

  21. Rapid Products 21

  22. Rapid AC Orbit Comparisons • Orbit performance dispersion among ACs is reasonable • but ESA clearly dominates combination 22

  23. IGS (Rapid – Final) Polar Motion Differences  <55 RF sites >98 RF sites  • Clear improvement in PM accuracy when IGb00 reference frame adopted in 2004 • but systematic differences remain & dominate • probably mostly due to Analysis Center rotational deficiencies 23

  24. Spectra of (Rapid-Final) PM Differences • High-frequency noise consistent with ~30 µas accuracy recently • but longer period errors are most significant • fortnightly feature near 14.2 d signifies subdaily tide model errors PM-x PM-y (1024 d from Sep. 2006 – Jul. 2009) 24

  25. Rapid AC LOD Comparisons with Final LOD • EMR, JPL, & SIO show strong annual LOD variations • most other ACs show long-period variations • similar features in Final LODs 25

  26. Final Products 26

  27. Final AC Orbit Comparisons • Final AC orbit performance similar to Rapids • Rapid orbits are consistently & significantly better than any single AC Final 27

  28. A,C,R Spectra of IGS Orbit Day-Jumps • Jumps computed from Berne-model fit to adjacent orbit days • stacked over all SVs & lightly smoothed • “calibrated” for errors due to (fit + extrapolation) method • Background errors follow ~flicker noise on seasonal time scales • transition to whiter noise for <14 d odd GPS draconitic harmonics likely fortnightly signals (1024 d from Mar. 2005 – Dec. 2007) 28

  29. Along-track Spectra of AC Orbit Day-Jumps • AC along-track spectra show mostly flicker + white noise • Some AC peaks but good agreement only for fortnightly smoothing effect of CODE 3-d arcs (1024 d from Mar. 2005 – Dec. 2007) 29

  30. Cross-track Spectra of AC Orbit Day-Jumps fortnightly band • AC cross-track spectra show 3rd draconitic & fortnightly bands • Some spurious AC peaks & lower white noise floor 3rddraconitic harmonic for most ACs (1024 d from Mar. 2005 – Dec. 2007) 30

  31. Orbit/PM Rotational Inconsistencies • AC orbit & PM rotational offsets should be self-consistent • but orbit rotations show larger dispersion for all ACs • most ACs show internal rotational inconsistencies • part of problem was caused by IGS combination bug (fixed wk 1702) • IGS orbit accuracy probably limited by such rotational effects 31

  32. Compute Polar Motion Discontinuities midnight PM discontinuities daily noon PM offset & rate estimates • Examine PM day-boundary discontinuities for IGS time series • should be non-zero due to PM excitation & measurement errors 32

  33. Power Spectra of IGS PM Discontinuities • Common peaks seen in most AC spectra are: • annual + 5th & 7th harmonics of GPS year (351 d or 1.040 cpy) • probably aliased errors of subdaily EOP tide model (IERS2003) PM-x PM-y IGS Repro1 Combination (10 Mar 2005 – 29 Dec 2007) 33

  34. Spectra of Subdaily EOP Tide Model Differences • Compare TPXO7.1 & IERS2003 (used by IGS) EOP models • TPXO7.1 & GOT4.7 test models kindly provided by Richard Ray • assume subdaily EOP model differences expressed fully in IGS PM results PM-x PM-y 34

  35. Spectra of PM Discontinuities & Subdaily EOPs • Aliasing of subdaily EOP tide model errors probably explains: • annual (K1, P1, T2), 14.2 d (O1), 9.4 d (Q1, N2), & 7.2 d (σ1, 2Q1, 2N2, µ2) • Orbit errors presumably responsible for odd 1.04 cpy harmonics effects of orbit model coupling HFEOP (J. Gipson) - IERS2003 35

  36. 3 Cornered Hat Decomposition of ERP Errors • 3 cornered hat method is sensitive to uncorrelated, random errors • for time series {i, j, k} form time series of differences (i-j), (j-k), (i-k) • then Var(i-j) = Var (i) + Var(j) (assuming Rij = 0 for i ≠ j) • and Var(i) = [Var(i-j) + Var(i-k) – Var(j-k)] / 2 • but true errors also include common-mode effects removed in differencing • Apply to IGS Ultra (observed), Rapid, & Final PM & dLOD • consider recent 1461 d from 1 Jan 2008 to 31 Dec 2011 • Surprising results: • apparently, Rapids give best polar motion & Ultras give best dLOD • Ultras give similar quality polar motion as Finals • perhaps Finals affected by simultaneously solving for weekly TRFs 36

  37. 3 Cornered Hat PM Results with High-Pass Filtering • Apply Vondrak high-pass filter before 3 cornered hat for PM • test 4 cutoff frequencies: pass all, >0.5 cpy, >1 cpy, >2 cpy • results below from Paul Rebischung (IGN) • IGU & IGR PM errors nearly insensitive to frequency filtering • IGS Final PM appears to improve when high-pass filtered • implies low-frequency errors are in IGS Finals or common to IGU & IGR • source of low-frequency error (orbits?, frame?) not yet identified • but internal Analysis Center inconsistencies strongly suspected (filtered results from Paul Rebischung, IGN) 37

  38. Conclusions • Since 2004.0 IGS Final polar motion accuracy <~30 µas • robust global network is prime factor • Rapid PM is only slightly poorer, <~40 µas • GPS PM nearing asymptotic limit for random errors (~20 µas) • smaller systematic errors possible with new GNSSs, better orbit modeling, & better handling of solution constraints • new subdaily EOP tide model required – prospects currently unclear • IGS Ultra-rapid observed PM accuracy currently <50 µas • updated 4 times daily with 15 hr latency • could be used to provide some subdaily EOP resolution • should be more heavily used by EOP prediction services ! • Leading error sources are systematic • internal rotational inconsistencies by Analysis Centers • errors in IERS subdaily EOP tide model alias into all IGS products 38

More Related