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High Precision Applications of Global Navigation Satellite Systems

High Precision Applications of Global Navigation Satellite Systems. Brief introduction to GNSS About the International GNSS Service (IGS) IGS core products what, when and how? current quality state and limiting errors Plans for 2 nd reprocessing and next reference frame Ongoing challenges.

eagan-thomas
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High Precision Applications of Global Navigation Satellite Systems

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  1. High Precision Applications of Global Navigation Satellite Systems • Brief introduction to GNSS • About the International GNSS Service (IGS) • IGS core products • what, when and how? • current quality state and limiting errors • Plans for 2nd reprocessing and next reference frame • Ongoing challenges Jake Griffiths IGS Analysis Coordinator NOAA/NGS 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  2. Main Global Navigation Satellite Systems • U.S. – Global Positioning System (GPS) • currently 32 active satellite vehicles (30 healthy) in orbit • latest launch (GPS IIF) successful, under on-orbit testing • Russia – Globalnaya Navigatsionnaya Sputnikovaya Sistem (GLONASS) • currently 29 active vehicles (24 healthy) in orbit • 4 spares • 1 in test mode • Europe – Galileo • to be inter-operable with GPS and GLONASS • currently 4 active vehicles in orbit • initial operating capability (IOC; 18 satellites) expected by ~2015 • final operating capability (FOC; 30 satellites) expected by ~2020 • China – Beidou • currently 15 active vehicles in orbit • regional satellite system—5 geost. Earth orbit (GEO), 5 incl. geosync. orbit (IGSO) • plus global satellite system—30 medium Earth orbit (MEO) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  3. How a GNSS Works Source: boeing.comGPS IIF • Satellites in MEO • vehicle altitudes ~20,000 km • Transmit L-band radio signals (e.g., L1,L2,L5) • GPS: carrier waves modulated by C/A and P codes; other GNSS are similar • Ground antenna+receiver pairs track transmit signals • geodetic grade equip collects raw observations for precise positioning, navigation and timing applications • Service supporting high- precision GNSS apps? • International GNSS Service (IGS) GODE animation source: wikipedia.org Source: unavco.org Source: unavco.org 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  4. What is the IGS? • An International Association of Geodesy (IAG) Technique Service • Voluntary federation of >200 worldwide agencies aimed at providing the highest quality GNSS data and products in support of: • Earth science research and education • other high-precision applications • Organization: • Governing Board (Chair, U. Hugentobler) • Central Bureau (sponsored by NASA, managed by JPL) • Tracking Network (Coordinator, R. Khachikyan) • Data Centers (Chair, C. Noll) • Infrastructure Committee (Chair, I. Romero) • Analysis Centers (ACs) & Analysis Center Coordinator (ACC) • Working Groups, Pilot Projects, Product Coordinators • Associate Members & representatives from other IAG Services • Other IAG Technique Services? • ILRS (SLR), IVS (VLBI) and IDS (DORIS) (more details at igs.org) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  5. IGS GNSS Tracking Network 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  6. IGS Core Product Series • orbits, clocks, polar motion & LOD (ERPs), and station positions (Finals only) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  7. Outline for How IGS Core Products are Derived a priori datum (IGS08/IGb08) VLBI International Terrestrial Reference Frame (ITRF) IGS RF WG Chair (IGN) B. Garayt, A. Duret and P. Rebischung • Analysis Center (AC) Products • Latest IERS and IGS conventions generally adopted • Adjust all obs model parameters • Ultra-rapid and Rapid tightly constrained to a priori datum • Finals uses no-net-rotation (NNR) constraint over a priori coordinates of core set of RF stations • Finals realizes AC daily quasi-instantaneous “fiducial-free” frame w.r.t. a priori datum SLR AC SNX files(Finals only) Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS). IGS TRFprods Combined daily station positions and ERPs, stacked for long-term estimates and RF maintenance satellite orbits & clocks (SP3), receiver clocks (CLK), tropo delays (TRO), and polar motion & LOD (ERP) + daily station positions (SNX) DORIS AC SINEXrotations IGS AC Coordinator (NOAA/NGS) J. Griffiths and K. Choi - weighted average of AC products - Rapid and Final clocks are aligned to IGS timescale AC SP3, CLK & ERP files Main analysis difference between IGU/IGR & IGS is constraints on a priori RF station positions at AC level Combined Orbits, Clocks, and ERPs (Rapid & Ultra-rapid only) Ultra-rapid (IGU) orbits (GPS, GLO), clocks (SV), ERPs Rapid (IGR) orbits (GPS), clocks (SV, Rx), ERPs Final (IGS) orbits (GPS, GLO), clocks (SV, Rx), ERPs, and TRF prods IGS Core Products 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  8. Current Analysis Centers 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  9. Popularity of Core Products- download statistics @ NASA/CDDIS (06/2010 thru 06/2012) - • >3.6 million file downloads per month • 5 biggest users of CDDIS/IGS files: • U.S. 64.3%, Indonesia 19.3%, Canada 1.64%, Sweden 1.57%, Belgium 1.16% • Details 1/2012 thru 6/2012 … Courtesy: C. Noll (NASA/CDDIS) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  10. Core Product Accuracies • 5 cm (1D) orbit error = ~0.4 cm (3D) position error over 1000 km baseline (Beser & Parkinson, 1982) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  11. Limiting Errors in IGS Products • Harmonic errors • Griffiths and Ray (2012, GPS Solut.) showed that defects in IERS sub-daily EOP tidal model are major error source • probably main source of pervasive harmonic signals in all products • In addition, at 2012 IGS Workshop J. Ray et al. showed that: • systematic rotations are another leading error • they effect all core products (maybe clocks too??) • over ~annual scales, Final products appear rotationally less stable than Rapids • appears to affect IGS polar motion • also seems to affect X- & Y- rotational stability of IGS orbit and PPP results • and suggested: • may be due to inadequate intra-AC self-consistency in Finals • situation could improve (inadvertently) in switch to daily SINEX integrations • but quasi-rigorous combination method should be re-examined • because further study of long-term dynamical stability of IGS products would be limited till these issues are resolved More at acc.igs.org/orbits/igs12-rot-errs.pdf 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  12. Limiting Errors in IGS Products • Harmonic errors • Griffiths and Ray (2012, GPS Solut.) showed that defects in IERS sub-daily EOP tidal model are major error source • probably main source of pervasive harmonic signals in all products • In addition, at 2012 IGS Workshop J. Ray et al. showed that: • systematic rotations are another leading error • they effect all core products (maybe clocks too??) • over ~annual scales, Final products appear rotationally less stable than Rapids • appears to affect IGS polar motion • also seems to affect X- & Y- rotational stability of IGS orbit and PPP results • and suggested: • may be due to inadequate intra-AC self-consistency in Finals • situation could improve (inadvertently) in switch to daily SINEX integrations • but quasi-rigorous combination method should be re-examined • because further study of long-term dynamical stability of IGS products would be limited till these issues are resolved More at acc.igs.org/orbits/igs12-rot-errs.pdf 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  13. Harmonic Errors: Background (1/2) dE • GPS-sun geometry repeat period • “draconitic” year = 351.2 d • 1st & 2nd harmonics overlayseasonal signals • IGS station coordinates (2006, 2008) • in all dNEU components • up to at least 6th harmonic • later found in all parameters: • “geocenter” variations • polar motion rates (esp 5th & 7th) • LOD (esp 6th) • orbit discontinuities (esp 3rd) • strong fortnightly signals alsocommon dN % of GPS Stations dU Frequency (cycles per year) (figure from X. Collilieux et al., 2011) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  14. Harmonic Errors: Background (2/2) • 1) local multipath effect at stations • station-satellite geometry repeats every sidereal day, approximately • 2 GPS orbital periods during 1 Earth inertial revolution • actual GPS repeat period = (1 solar day - ~245 s) • sidereal period (K1) = (1 solar day - 235.9 s) • for 24-hr sampling (e.g., data analysis), alias period → GPS draconitic year • 2) mismodeling effect in satellite orbits • empirical solar radiation parameters intrinsically linked to orbital period • but no precise mechanism proposed yet • subsequent slides examine the impact of errors in a priori IERS model for sub-daily tidal EOP variations on GPS orbits • EOP tide errors at ~12 hr couple directly into GPS orbit parameters • EOP tide errors at ~24 hr may couple into other estimates • sub-daily EOP total magnitudes are ~1 mas = 13 cm shift @ GPS altitude • IERS model is known to have visible errors, which could reach the 10 to 20% level 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  15. Harmonic Errors: Sub-daily Alias and Draconitic (1/3) • Simulated impact ofsub-daily EOP tidalerrors on IGS orbits • generated “fake”model by changingadmittances by up to20%—assumed errorsderived fromcomparing IERS modelto test model from R.Ray (NASA/GSFC) • process ~3 years ofGPS orbits with IERS& “fake” models • difference conventional & EOP-test orbits @ 15 min intervals • compute spectra of differences for each SV, stack & smooth • compare spectral differences: input model errors vs. orbital response short- period errors go into orbits long-period errors absorbedmostly by ERPs, not orbits Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  16. Harmonic Errors: Sub-daily Alias and Draconitic (2/3) • Compare simulated EOP signatures with IGS Orbits • basic problem is a limited independent “truth” (via SLR) for IGS orbits • but can compute discontinuities between daily orbit sets • doing so aliases sub-daily differences into longer-period signals • to compare, also compute EOP-induced orbit differences once daily • IGS ORBIT JUMPS • fit orbits for each day withBERNE (6+9) SRP orbit model • parameterize fit as plus 3 SRPs per SV component • fit 96 SP3 orbit positions for each SV as pseudo-observations for Day A • propagate fit forward to 23:52:30 for Day A • repeat for Day B & propagate backwards to 23:52:30 of day before • compute IGS orbit jumps at 23:52:30 • SIMULATED EOP SIGNATURES • difference conventional & EOP-test orbits at 23:45:00 only • Compute IGS orbit jumps over ~5.6 yr, test orbits over ~2.8 yr 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  17. Harmonic Errors: Sub-daily Alias and Draconitic (3/3) • Offset peaks in ~14, ~9 and ~7 d bands due to simple daily sampling of input errors 10/√3 cm = ~5.8 cm (1D) annual errors Power Density (mm2 / cpd) ~1.0 cm white noise floor Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  18. Harmonic Errors: Summary • Harmonics of 351 d pervasive in all IGS products • Simulated orbital response to IERS sub-daily EOP tide model errors • compared conventional orbits to EOP-test orbits at 15 min intervals • Beating of sub-daily EOP tides causes spectral differences at other periods • long-period errors go into PM & LOD • short-period errors go mostly into orbits • bump in background noise at 2 cpd -> resonance with GPS orbital period • Compared IGS orbit discontinuities to EOP-test orbit differences at 23:45:00 • 24 h sampling causes sub-daily EOP tide errors to alias at ~14, ~9 and ~7 d bands -> peaks offset from expected periods • peaks at several (mostly odd) harmonics of 351 d • IERS diurnal & semi-diurnal tide model errors are probably main source for pervasive sub-daily alias and several draconitic errors in IGS orbits 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  19. Further Elaboration on Limiting Errors • Harmonic errors • Griffiths and Ray (2012, GPS Solut.) showed that defects in IERS sub-daily EOP tidal model are major error source • probably main source of pervasive harmonic signals in all products • In addition, at 2012 IGS Workshop J. Ray et al. showed that: • systematic rotations are another leading error • they effect all core products (maybe clocks too??) • over ~annual scales, Final products appear rotationally less stable than Rapids • appears to affect IGS polar motion • also seems to affect X- & Y- rotational stability of IGS orbit and PPP results • and suggested: • may be due to inadequate intra-AC self-consistency in Finals • situation could improve (inadvertently) in switch to daily SINEX integrations • but quasi-rigorous combination method should be re-examined • because further study of long-term dynamical stability of IGS products would be limited till these issues are resolved More at acc.igs.org/orbits/igs12-rot-errs.pdf 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  20. Switch to Daily TRFs in Finals • Finals now based on daily SINEX (terrestrial frame) integrations • prior to GPS Wk 1702 (19 Aug 2012) • products based on weekly SINEX—AC orbits pre-aligned using weekly-averaged AC SINEX rotations and daily AC PM-x and PM-y deviations from combined ERPs • daily AC SINEX rotations now used to pre-align AC orbits—ERPs rots. no longer used • higher scatter in combined orbits, ERPs and station positions • but less than sqrt(7) expected for random error • and smaller than other existing systematic errors • did not resolve rotational instability of Finals • mitigates impacts of unmodeled non-tidal atmospheric loading effects on IGS products • increased temporal resolution in station position time series • needed for continued study of non-tidal crustal loading models and impacts to IGS products • since exposed previously unknown sensitivity of GPS-derived ERP estimates to GLONASS orbit mismodeling • sensitivity is time-correlated with GLONASS eclipse seasons • CODE/ESA currently studying this effect 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  21. …and Correcting a Coding Error in Combo Software • Long-standing (since 2000) error in using AC SINEX rotations for AC Final orbit pre-alignment • prior to GPS Wk 1702 (19 Aug 2012), AC X- and Y- SINEX rotations were applied with incorrect sign convention • improved RX & RY in PPP using IGS by up to ~0.035 mas (~4.4 mm @ equator) in RMS • but systematic errors remain in RZ—clear ~60d signal (harmonic errors in AC clocks?) • Note: since Wk 1650, Final PPP using IGR (acc.igs.org/index_igsacc_ppp.html) gives: RX=-0.016 (RMS=0.041)RY=0.015 (RMS=0.039)RZ=-0.004 (RMS=0.022) • IGS RX & RY better than IGR for now • IGS RZ now biased w.r.t. IGR, and has higher scatter 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  22. Rotations of Current Final orbits (AC minus IGS) - weekly means - • Scatter of all AC rotations decreased markedly starting at Wk 1702 • no impact in switch to dailySNX • primarily from fixing combo software • Since revealed ESA self-consistency issues • poorly aligned to IGS frame • residual distortion between TRF and their orbits—see RX & RY • corrected on Wk 1732 • Now RY of IGR (violet) is biased • ESA consistency issues in IGR IGS05 IGx08 fixed AC orbit pre-alignment ESA fixed TRF issue • 1 mas = ~ 13 cm @ GPS altitude 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  23. WRMS of AC Orbit Residuals Since IG1- AC solutions minus IGS Final , after pre-alignment - • Inter-AC agreement approaches ~1 cm • switch to daily TRFs seems to have improved AC agreement for now • ESA dominates; EMR and JPL improved slightly to ~18 mm WRMS since IGx08 • IGS Final has ~4 mm WRMS difference with IGR—which prods are more precise? IGS05 IGx08 fixed AC orbit pre-alignment ESA fixed TRF issue 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  24. IGS vs IGR – More From PPP using Final Products- Mean station RMS after Helmert transformation to IGS frame - • w.r.t. IGS frame, IGR consistently more precise in all 3 components… • probably due to combination of errors in AC Final clocks • but could be from difference between IGR and IGS analysis approach Time [GPS Wk; April 22, 2012 thru May 12, 2013] 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  25. Other Known Systematic Errors • Ongoing efforts to address: • limitations of empirical solar radiation pressure (SRP) models • toward physical-based models (IGS Orbit Dynamics WG) • Rodriguez-Solano et al. (2009, 2011, 2012) SRP model w/ handling of eclipses (2013) • quality of non-tidal loading models and effects on IGS products • IERS Study (http://geophy.uni.lu/ggfc-nonoperational/uwa-call-data.html) • effects are negligible on secular frame • loading can be modeled at stacking level with equivalent results • time variations of low-degree terms in geopotential field • impacts on orbits: ~7 mm RMS (Melachroinos et al., AGU 2012) • effect on ~annual signal in IGS station position time series? • conventional model under development • tidal displacements at stations • ocean pole tide (JPL and EMR) & S1-S2 tidal atm loading model (pending update) • improved satellite attitude modeling (mostly benefits satellite clocks) • modeling higher-order ionosphere effects • most ACs working to implement 2nd-order correction • Unclear which of these developments will be ready for IG2 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  26. IGS 2ndReprocessing and ITRF2013 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  27. How will IG2 Differ from IG1 & Current Operations?- more details at http://acc.igs.org/reprocess2.html - • Longer data span (~1994 thru mid-2013) • IG2 + operational prods thru 2013 -> IGS contribution to ITRF2013 • Updated models, frames & methodologies • IERS 2010 Conventions generally adopted • NGA stations data w/ new antenna calibrations (for improved ITRF <-> WGS 84 tie)? • IGb08.SNX/igs08.atx framework (improved a priori datum) • combined products based on AC 1d TRF integrations • with corrected approach for applying AC SINEX rotations to AC orbits • no non-tidal atmospheric loading at obs level • 2nd-order iono corrections & S1-S2 atm. loading displacements @ stations • Earth-reflected radiation pressure (albedo) modeling (most ACs still to adopt) • reduce ~2.5 cm radial bias w.r.t. SLR [e.g. Urschl et al., 2007; Zeibart et al., 2007] • plus antenna thrusting [e.g., Rodriguez-Solano et al., 2009, 2011, 2012] • satellite attitude modeling by all clock ACs • Sub-daily alias and draconitic errors will remain • Final preps and initial processing by late June? Finalize in November? • Expect to deliver SINEX files for ITRF2013 by early 2014 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  28. Expected AC and IG2 Products- more details at http://acc.igs.org/reprocess2.html - • Daily GPS orbits & satellite clocks (in IGST?) • 15-minute intervals (SP3c format) • Daily satellite & tracking station clocks (in IGST?) • 5-minute intervals (clock RINEX format) • Daily Earth rotation parameters (ERPs) • from SINEX & classic orbit combinations (IGS erp format) • x & y coordinates of pole • rate-of-change of x & y pole coordinates (should not be used due to sensitivity to sub-daily tidal errors) • excess length-of-day (LOD) • Weekly (IG2 only) & daily terrestrial coordinate frames with ERPs • with full variance-covariance matrix (SINEX format) • May also provide (TBD) • daily GLONASS orbits & satellite clocks • 30-second GPS clocks (in IGST?) • ionosphere maps, tropospheric zenith delay estimates • new bias products 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  29. Who will Contribute to IG2?- more details at http://acc.igs.org/reprocess2.html- • All IGS Final‐product Analysis Centers: • CODE/AIUB – Switzerland – JPL – USA • EMR/NRCan – Canada – MIT – USA • ESA/ESOC – Germany – NGS/NOAA – USA • CNES/GRGS – Toulouse, France – SIO – USA • GFZ – Potsdam, Germany • Plus 1 reprocessing Center • ULR – University of La Rochelle TIGA (tide gauges), France • PDR – Potsdam-Dresden Reprocessing group (in IG1, but will not be in IG2) • Plus 1 Center contributing to TRF only: • GFZ TIGA – Potsdam, Germany 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  30. Expected Performance of IG2?- WRMS of AC repro1 orbits wrt IG1 - IGS05 Large scatter for some ACs in early IG1—expected to be improved in IG2 contributions By late 2007, inter-AC agreement bi-modal, approaching ~1.5 cm Time [GPS Wk; Dec. 26, 1993 thru Nov. 11, 2011] Courtesy of G. Gendt (GFZ Potsdam) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  31. WRMS of AC Orbit Residuals Since IG1- AC solutions minus IGS Final , after pre-alignment - • If current performance is any indication • could approach 1 cm inter-AC agreement for much of IG2 IGS05 IGx08 fixed AC orbit pre-alignment ESA fixed TRF issue inter-AC agreement reaches ~1.0 cm 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  32. Expected Performance of IG2 TRFs?- RMS of Recent AC TRFs wrt IGS - • Improvement in precision expected from: • horizontal tropo gradients estimated by all ACs • 2nd order iono corrections • Earth-reflected radiation pressure (albedo) modeling • Improvement in accuracy expected from: • igs08.atx (depends on antenna type) • Switch to daily AC TRFs: • should not impact quality of weekly combined TRFs (input to ITRF2013) • but will provide increased resolution of non-tidal displacements WRMS w.r.t. combination Courtesy: P. Rebischung (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  33. IG2 contribution to ITRF2013 • Contribution to the ITRF2013 scale rate? • satellite PCOs will be included in combination & stacking of IG2 TRFs. • assumption that PCOs are constant → “intrinsic GNSS scale rate” • No contribution to the ITRF origin yet • remaining unmodeled orbital forces • origins of IG2 TRFs likely not reliable enough • Some systematic errors still a challenge! • main source: antenna calibrations • > 1 cm errors revealed at stationswith uncalibrated radomes • few mm errors likely at stationswith “converted” antenna calibrations • will cause trouble in use of local tiesfor ITRF2013 colocation sites • consider to exclude in next ITRF Courtesy: P. Rebischung (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  34. Other Challenges: Mostly Network Issues(not addressed by IG2) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  35. Uncalibrated Radomes • 28/92 ( 30%) multi-technique sites have an uncalibrated radome • nearly half (13/28) operated by JPL 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  36. Uncalibrated Radomes: Impact on ITRF (1/2) • including all co-location sites • systematic VLBI <-> SLR scale discrepancy Courtesy: Z. Altamimi (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  37. Uncalibrated Radomes: Impact on ITRF (1/2) • when GNSS co-located sites with uncalibrated radomes are excluded • VLBI <-> SLR scale difference amplified by 0.2 ppb (network effect + calibration errors) Courtesy: Z. Altamimi (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  38. Loss of Core RF Stations (1/2) • core RF network • optimal spatial distribution • mitigate network effects in IGS SINEX combination (from X. Collilieux Ph.D. work) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  39. Loss of Core RF Stations (2/2) • Decrease in number of core RF stations • mostly due to anthropogenic impacts (antenna changes, etc.) • some displaced by earthquakes • IGS08 -> IGb08 update on 7 Oct 2012 • recovered sites with linear velocities before/after positional discontinuity • Overall (linear) rate of loss = ~0.13 sta/wk since end date of ITRF2008 • <IGb08: rate = ~0.16 sta/wk • >IGb08: rate = ~0.22 sta/wk • Today • best case: 71 core stations • actual: ~54 • Need for thorough studyof impacts on stability ofIGS reference frame • Station operators should limit disruptions, esp. at co-location sites 100% dataavailability actual dataavailability Courtesy: K. Choi (NOAA/NGS) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  40. Summary 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  41. Conclusions: IGS Errors • Current IGS products are of high accuracy and precision • GPS orbits • overall <2.5 cm (1D) • errors now dominated by Z- frame rotation scatter and possibly AC clock errors • X- & Y- frame rotations of Final orbits improved by ~0.035 mas (~4.4 mm @ GPS) • RMS scatter of AC orbits up to 1.6 cm • sub-daily alias and draconitic errors from IERS diurnal/semi-diurnal tides • ERPs • PM-x & PM-y: <30 mas • dLOD: ~10 ms • terrestrial frames • ~2 mm N&E • ~5 mm U • But Rapid products still slightly more precise than Finals • discrepancies have been reduced, but needs to be further study • may be due to combination of errors in AC Final clocks? • Because IGS products are of high quality, can measure subtle signals 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  42. Conclusions: Repro2 • Latest models, frames & methods to have largest impact since IG1 • IERS 2010 Conventions • IGb08/igs08.atx framework • Earth-reflected radiation pressure (albedo) modeling • sub-daily alias & draconitic errors will remain • To result in full history of IG2 products (1994 to mid-2013) • daily products: • GPS orbits & SV clocks (SP3c) @ 15 min intervals • GPS SV and station clocks (clock RINEX) @ 5 min intervals • Earth Rotation Parameters (IGS ERP) • terrestrial coordinate frames (IERS SINEX) • expected delivery for ITRF2013 -> early 2014 • And possibly some ancillary products • GLONASS orbits & clocks • 30-second SV & station clocks • bias products 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  43. Conclusions: More Repro2 and Other Challenges • IG2 quality should approach current IGS prods • quality for later (~2000 -> present) IG2 products will be best • early IG2 probably better than IG1 equivalents, but not as good as later IG2 • Ongoing Challenges • uncalibrated radomes at co-location sites • one recently available at SMST!! (co-located w/ SLR; unavail. for ITRF2008) • positional discontinuities at RF stations • 50% of IGS stations have discontinuities: harmful in co-location sites • GNSS/IGS is the link between the 3 other techniques in ITRF • loss of core RF stations • anthropogenic site disturbances (incl. many equip. changes) • data loss, and earthquakes & other physical processes • known biases and other systematic errors • harmonic and sub-daily alias errors in all IGS products • site-specific errors [e.g., Wetzell observations by Steigenberger et al., REFAG2010] 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  44. Questions? 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  45. Extra Slides 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  46. Spectrum of Daily ERP Differences due to sub-daily EOP Tidal Model “Errors” • M2 aliases into PM-x and PM-y; O1 aliases into LOD • 1st draconitic harmonic enters PM-x & LOD Power Density (mas2 or s2/ cpd) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 Frequency (cycles per day)

  47. Harmonic Errors: Sub-daily Alias and Draconitic • Simulated impact ofsub-daily EOP tidalerrors on IGS orbits • generated “fake”model by changingadmittances by up to20%—assumed errorsderived fromcomparing IERS modelto test model from R.Ray (NASA/GSFC) • process ~3 years ofGPS orbits with IERS& “fake” models • difference conventional & EOP-test orbits @ 15 min intervals • compute spectra of differences for each SV, stack & smooth • compare spectral differences: input model errors vs. orbital response Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  48. Harmonic Errors: Sub-daily Alias and Draconitic • Simulated impact ofsub-daily EOP tidalerrors on IGS orbits • generated “fake”model by changingadmittances by up to20%—assumed errorsderived fromcomparing IERS modelto test model from R.Ray (NASA/GSFC) • process ~3 years ofGPS orbits with IERS& “fake” models • difference conventional & EOP-test orbits @ 15 min intervals • compute spectra of differences for each SV, stack & smooth • compare spectral differences: input model errors vs. orbital response bump in background power – resonance of ~2 cpd sub-daily tide errors and GPS orbital period? Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  49. Harmonic Errors: Sub-daily Alias and Draconitic (3/3) • Aliasing of sub-daily errors responsible for some harmonics of 351 d • peaks at other harmonics likely caused by other errors other harmonics -- aliasing of other errors 10/√3 cm = ~5.8 cm (1D) annual errors 1st, 3rd, 4th, & 10th harmonics also caused by sub-daily EOP errors Power Density (mm2 / cpd) ~1.0 cm white noise floor Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

  50. Spectra of Orbital Responses tosub-daily EOP Errors – Near 1 cpd • at diurnal period, EOP model errors absorbed into orbits, esp cross- & along-track only 2 sub-daily tidal lines excited above background orbit noise unexpected peak in cross-track – probably a beat effect Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 05

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