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 Jake Griffiths IGS Analysis Coordinator NOAA/NGS 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 1
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 2
• Satellites in MEO How a GNSS Works Source: boeing. com GPS IIF – vehicle altitudes ~20, 000 km • Transmit L‐band radio signals (e. g. , L 1, L 2, L 5) – GPS: carrier waves modulated by C/A and P codes; other GNSS are similar • Ground antenna+receiver pairs track transmit signals GODE – geodetic grade equip collects raw observations for precise positioning, navigation and timing applications animation source: wikipedia. org Source: unavco. org • Service supporting high‐ precision GNSS apps? – International GNSS Service (IGS) Source: unavco. org 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 3
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: – – – – (more details at igs. org) 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) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 4
IGS GNSS Tracking Network 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 5
IGS Core Product Series Ultra‐Rapid (predicted half) Ultra‐Rapid (observed half) Rapid Final ID IGU IGA IGR IGS Latency Issue times (UTC) Data spans (UTC) real-time @ 03: 00, 09: 00, 15: 00, 21: 00 +24 hr @ 00: 00, 06: 00, 12: 00, 18: 00 -24 hr @ 00: 00, 06: 00, 12: 00, 18: 00 ● 3 - 9 hr @ 03: 00, 09: 00, 15: 00, 21: 00 ± 12 hr @ 12: 00 ● 17 - 41 hr @ 17: 00 daily 12 - 19 d weekly each Thursday ± 12 hr @ 12: 00 for 7 d Remarks for real-time apps ● GPS & GLONASS ● issued with prior IGA ● for near real-time apps ● GPS & GLONASS ● issued with following IGU for near-definitive, rapid apps ● GPS only for definitive apps ● GPS & GLONASS ● orbits, clocks, polar motion & LOD (ERPs), and station positions (Finals only) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 6
Outline for How IGS Core Products are Derived a priori datum (IGS 08/IGb 08) Analysis Center (AC) Products AC SNX files (Finals only) satellite orbits & clocks (SP 3), receiver clocks (CLK), tropo delays (TRO), and polar motion & LOD (ERP) + daily station positions (SNX) -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 quasiinstantaneous “fiducial-free” frame w. r. t. a priori datum International Terrestrial Reference Frame (ITRF) IGS RF WG Chair (IGN) B. Garayt, A. Duret and P. Rebischung Combined daily station positions and ERPs, stacked for long-term estimates and RF maintenance IGS TRF prods VLBI Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS). SLR DORIS AC SINEX rotations IGS AC Coordinator (NOAA/NGS) J. Griffiths and K. Choi AC SP 3, CLK & ERP files Combined Orbits, Clocks, and ERPs (Rapid & Ultra-rapid only) - weighted average of AC products - Rapid and Final clocks are aligned to IGS timescale Ultra‐rapid (IGU) orbits (GPS, GLO), clocks (SV), ERPs Rapid (IGR) orbits (GPS), clocks (SV, Rx), ERPs Main analysis difference between IGU/IGR & IGS is constraints on a priori RF station positions at AC level Final (IGS) orbits (GPS, GLO), clocks (SV, Rx), ERPs, and TRF prods IGS Core Products 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 7
Current Analysis Centers Final (IGS) Center Name Rapid (IGR) Ultra (IGU) cod Centre for Orbit Determination in Europe, Bern, Switzerland emr Natural Resources Canada (NRCan), Ottawa, Canada esa European Space Agency, European Space Operations Center (ESOC), Darmstadt, Germany gfz Geo. Forschungs. Zentrum, Potsdam, Germany gop Geodetic Observatory Pecny, Czech Republic grg CNES Groupe de Recherche de Geodesie Spatiale (GRGS), Toulouse, France jpl Jet Propulsion Laboratory, Pasadena, USA ngs National Oceanic and Atmospheric Administration (NOAA), Silver Spring, USA sio Scripps Institution of Oceanography, La Jolla, USA mit Massachusetts Institute of Technology, Boston, USA usn U. S. Naval Observatory, Washington, D. C. , USA whu Wuhan University, Wuhan, China 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 8
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 … SP 3 ERP 11, 711, 506 93. 7 3. 1 GPS 1, 359, 656 60. 7 6. 8 24. 8 Rapid GPS 887, 986 65. 6 8. 7 16. 9 6. 4 Final (IGL) GLO 225, 515 99. 1 0. 3 0. 6 Ultra‐rapid (IGV) GPS & GLO 223, 562 95. 0 Product GNSS Total Hits Ultra‐rapid GPS Final (IGS) ( 4 * 2, 927, 877 daily) (%) CLK (%) SNX (%) SUM (%) 3. 2 5. 8 2. 0 5. 0 Courtesy: C. Noll (NASA/CDDIS) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 9
Core Product Accuracies Series Product Types Accuracies GPS orbits ● GLONASS orbits ● GPS SV clocks ● ERPs: PM + d. LOD ~ 5 cm (1 D) ~10 cm (1 D) ~3 ns RMS / ~1. 5 ns Sdev ~250 µas / ~50 µs 15 min 6 hr GPS orbits ● GLONASS orbits ● GPS SV clocks ● ERPs: PM + d. LOD ~ 3 cm (1 D) ~5 cm (1 D) ~150 ps RMS / ~50 ps Sdev <50 µas / ~10 µs 15 min 6 hr GPS orbits ● GPS SV & station clocks ● ERPs: PM + d. LOD ~2. 5 cm (1 D) ~75 ps RMS / ~25 ps Sdev <40 µas / ~10 µs 15 min daily GPS orbits ● GLONASS orbits ● GPS SV & station clocks ● ERPs: PM + d. LOD ● Terrestrial frames <2. 5 cm (1 D) <5 cm (1 D) ~75 ps RMS / ~20 ps SDev <30 µas / ~10 µs ~2. 5 mm N&E / ~6 mm U 15 min 30 s (SVs) + 5 min daily ● Ultra‐Rapid (predicted half) ● Ultra‐Rapid (observed half) Rapid ● ● Final Output Intervals 5 cm (1 D) orbit error = ~0. 4 cm (3 D) position error over 1000 km baseline (Beser & Parkinson, 1982) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 10
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/igs 12 -rot-errs. pdf 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/igs 12 -rot-errs. pdf 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 12
Harmonic Errors: Background (1/2) • GPS‐sun geometry repeat period d. E • IGS station coordinates (2006, 2008) – in all d. NEU components – up to at least 6 th harmonic • later found in all parameters: – – “geocenter” variations polar motion rates (esp 5 th & 7 th) LOD (esp 6 th) orbit discontinuities (esp 3 rd) • strong fortnightly signals also common % of GPS Stations – “draconitic” year = 351. 2 d – 1 st & 2 nd harmonics overlay seasonal signals d. N d. U Frequency (cycles per year) (figure from X. Collilieux et al. , 2011) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 13
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 (K 1) = (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 14
Harmonic Errors: Sub‐daily Alias and Draconitic (1/3) • Simulated impact of sub‐daily EOP tidal errors on IGS orbits – generated “fake” model by changing admittances by up to 20%—assumed errors derived from comparing IERS model to test model from R. Ray (NASA/GSFC) – process ~3 years of GPS orbits with IERS & “fake” models Power Density (mm 2 / cpd) long-period errors absorbed mostly by ERPs, not orbits shortperiod errors go into orbits Frequency (cycles per day) • 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 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 15
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 with BERNE (6+9) SRP orbit model – parameterize fit as plus 3 SRPs per SV component – fit 96 SP 3 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 16
Harmonic Errors: Sub‐daily Alias and Draconitic (3/3) Power Density (mm 2 / cpd) • Offset peaks in ~14, ~9 and ~7 d bands due to simple daily sampling of input errors 10/√ 3 cm = ~5. 8 cm (1 D) annual errors ~1. 0 cm white noise floor Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 17
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 18
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/igs 12 -rot-errs. pdf 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 19
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 20
…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 ~60 d 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 21
Rotations of Current Final orbits (AC minus IGS) - weekly means - • Scatter of all AC rotations decreased markedly starting at Wk 1702 IGS 05 IGx 08 – no impact in switch to daily SNX – 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 fixed AC orbit pre‐alignment ESA fixed TRF issue • Now RY of IGR (violet) is biased – ESA consistency issues in IGR 1 mas = ~ 13 cm @ GPS altitude 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 22
WRMS of AC Orbit Residuals Since IG 1 - AC solutions minus IGS Final , after pre-alignment - IGS 05 IGx 08 fixed AC orbit pre‐ alignment ESA fixed TRF issue • 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 IGx 08 – IGS Final has ~4 mm WRMS difference with IGR—which prods are more precise? 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 23
IGS vs IGR – More From PPP using Final Products - Mean station RMS after Helmert transformation to IGS frame - Time [GPS Wk; April 22, 2012 thru May 12, 2013] • 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 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 24
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) & S 1‐S 2 tidal atm loading model (pending update) – improved satellite attitude modeling (mostly benefits satellite clocks) – modeling higher‐order ionosphere effects • most ACs working to implement 2 nd‐order correction • Unclear which of these developments will be ready for IG 2 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 25
IGS 2 nd Reprocessing and ITRF 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 26
How will IG 2 Differ from IG 1 & Current Operations? - more details at http: //acc. igs. org/reprocess 2. html - • Longer data span (~1994 thru mid‐ 2013) – IG 2 + operational prods thru 2013 ‐> IGS contribution to ITRF 2013 • Updated models, frames & methodologies – IERS 2010 Conventions generally adopted – NGA stations data w/ new antenna calibrations (for improved ITRF <‐> WGS 84 tie)? – IGb 08. SNX/igs 08. atx framework (improved a priori datum) – combined products based on AC 1 d TRF integrations • with corrected approach for applying AC SINEX rotations to AC orbits • no non‐tidal atmospheric loading at obs level – 2 nd‐order iono corrections & S 1‐S 2 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 ITRF 2013 by early 2014 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 27
Expected AC and IG 2 Products - more details at http: //acc. igs. org/reprocess 2. html - • Daily GPS orbits & satellite clocks (in IGST? ) – 15‐minute intervals (SP 3 c 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 (IG 2 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 28
Who will Contribute to IG 2? - more details at http: //acc. igs. org/reprocess 2. html - • All IGS Final‐product Analysis Centers: – – – CODE/AIUB – Switzerland EMR/NRCan – Canada ESA/ESOC – Germany CNES/GRGS – Toulouse, France GFZ – Potsdam, Germany – JPL – USA – MIT – USA – NGS/NOAA – USA – SIO – USA • Plus 1 reprocessing Center – ULR – University of La Rochelle TIGA (tide gauges), France – PDR – Potsdam‐Dresden Reprocessing group (in IG 1, but will not be in IG 2) • Plus 1 Center contributing to TRF only: – GFZ TIGA – Potsdam, Germany 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 29
Expected Performance of IG 2? - WRMS of AC repro 1 orbits wrt IG 1 - IGS 05 Large scatter for some ACs in early IG 1—expected to be improved in IG 2 contributions By late 2007, inter‐AC agreement bi‐modal, approaching ~1. 5 cm Time [GPS Wk; Dec. 26, 1993 thru Nov. 11, 2011] 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 Courtesy of G. Gendt (GFZ Potsdam) 30
WRMS of AC Orbit Residuals Since IG 1 - AC solutions minus IGS Final , after pre-alignment - IGS 05 IGx 08 fixed AC orbit pre‐ alignment ESA fixed TRF issue inter‐AC agreement reaches ~1. 0 cm • If current performance is any indication – could approach 1 cm inter‐AC agreement for much of IG 2 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 31
Expected Performance of IG 2 TRFs? - RMS of Recent AC TRFs wrt IGS - WRMS w. r. t. combination • Improvement in precision expected from: – horizontal tropo gradients estimated by all ACs – 2 nd order iono corrections – Earth‐reflected radiation pressure (albedo) modeling • Improvement in accuracy expected from: – igs 08. atx (depends on antenna type) • Switch to daily AC TRFs: – should not impact quality of weekly combined TRFs (input to ITRF 2013) – but will provide increased resolution of non‐tidal displacements Courtesy: P. Rebischung (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 32
IG 2 contribution to ITRF 2013 • Contribution to the ITRF 2013 scale rate? – satellite PCOs will be included in combination & stacking of IG 2 TRFs. – assumption that PCOs are constant → “intrinsic GNSS scale rate” • No contribution to the ITRF origin yet – remaining unmodeled orbital forces – origins of IG 2 TRFs likely not reliable enough • Some systematic errors still a challenge! – main source: antenna calibrations • > 1 cm errors revealed at stations with uncalibrated radomes • few mm errors likely at stations with “converted” antenna calibrations – will cause trouble in use of local ties for ITRF 2013 colocation sites • consider to exclude in next ITRF Courtesy: P. Rebischung (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 33
Other Challenges: Mostly Network Issues (not addressed by IG 2) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 34
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 35
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 36
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 37
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 38
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 • IGS 08 ‐> IGb 08 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 ITRF 2008 – <IGb 08: rate = ~0. 16 sta/wk – >IGb 08: rate = ~0. 22 sta/wk • Today – best case: 71 core stations – actual: ~54 100% data availability actual data availability • Need for thorough study Courtesy: K. Choi (NOAA/NGS) of impacts on stability of IGS reference frame • Station operators should limit disruptions, esp. at co‐location sites 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 39
Summary 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 40
Conclusions: IGS Errors • Current IGS products are of high accuracy and precision – GPS orbits • overall <2. 5 cm (1 D) • 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 • d. LOD: ~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 41
Conclusions: Repro 2 • Latest models, frames & methods to have largest impact since IG 1 – – IERS 2010 Conventions IGb 08/igs 08. atx framework Earth‐reflected radiation pressure (albedo) modeling sub‐daily alias & draconitic errors will remain • To result in full history of IG 2 products (1994 to mid‐ 2013) – daily products: • • GPS orbits & SV clocks (SP 3 c) @ 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 ITRF 2013 ‐> 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 42
Conclusions: More Repro 2 and Other Challenges • IG 2 quality should approach current IGS prods – quality for later (~2000 ‐> present) IG 2 products will be best – early IG 2 probably better than IG 1 equivalents, but not as good as later IG 2 • Ongoing Challenges – uncalibrated radomes at co‐location sites • one recently available at SMST!! (co‐located w/ SLR; unavail. for ITRF 2008) – 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. , REFAG 2010] 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 43
Questions? 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 44
Extra Slides 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 45
Spectrum of Daily ERP Differences due to sub‐daily EOP Tidal Model “Errors” Power Density (mas 2 or s 2/ cpd) • M 2 aliases into PM‐x and PM‐y; O 1 aliases into LOD • 1 st draconitic harmonic enters PM‐x & LOD Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 46
• Simulated impact of sub‐daily EOP tidal errors on IGS orbits – generated “fake” model by changing admittances by up to 20%—assumed errors derived from comparing IERS model to test model from R. Ray (NASA/GSFC) – process ~3 years of GPS orbits with IERS & “fake” models Power Density (mm 2 / cpd) Harmonic Errors: Sub‐daily Alias and Draconitic Frequency (cycles per day) • 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 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 47
• Simulated impact of sub‐daily EOP tidal errors on IGS orbits – generated “fake” model by changing admittances by up to 20%—assumed errors derived from comparing IERS model to test model from R. Ray (NASA/GSFC) – process ~3 years of GPS orbits with IERS & “fake” models Power Density (mm 2 / cpd) Harmonic Errors: Sub‐daily Alias and Draconitic bump in background power – resonance of ~2 cpd sub-daily tide errors and GPS orbital period? Frequency (cycles per day) • 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 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 48
Harmonic Errors: Sub‐daily Alias and Draconitic (3/3) • Aliasing of sub‐daily errors responsible for some harmonics of 351 d Power Density (mm 2 / cpd) – peaks at other harmonics likely caused by other errors other harmonics -aliasing of other errors 1 st, 3 rd, 4 th, & 10 th harmonics also caused by sub-daily EOP errors 10/√ 3 cm = ~5. 8 cm (1 D) annual errors ~1. 0 cm white noise floor Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 49
Spectra of Orbital Responses to sub‐daily EOP Errors – Near 1 cpd Power Density (mm 2 / 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 Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 50 05
Spectra of Orbital Responses to sub‐daily EOP Errors – Near 2 cpd Power Density (mm 2 / cpd) • at semi‐diurnal period, EOP model errors absorbed mostly into orbit radial (via Kepler’s 3 rd law) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 51 06
Spectra of Orbital Responses to sub‐daily EOP Errors – Near 3 cpd Power Density (mm 2 / cpd) • background power is lower • errors absorbed in all three components Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 52
Spectra of Orbital Responses to sub‐daily EOP Errors – Near 4 cpd Power Density (mm 2 / cpd) • same near 4 cpd Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 53
COMPARISON OF EXPECTED AC DATA USAGE ANALYSIS CENTER SYSTEM OBS TYPE ORBIT DATA ARC LENGTH DATA RATE ELEVATION CUTOFF ELEVATION INVERSE WGTS CODE GPS + GLO Db. Diff (weak redundant) 24 h 3 min 3 deg 1/cos 2(z) EMR GPS + GLO Un. Diff 24 h 5 min 10 deg none ESA GPS + GLO Un. Diff 24 h 5 min 10 deg 1/sin 2 (e) GPS + ? GLO? Un. Diff ? ? 24 h ? ? 5 min 7 deg 1/2 sin(e) for e < 30 deg GPS + GLO Un. Diff 3 + 24 + 3 h 15 min 10 deg none JPL GPS Un. Diff 3 + 24 + 3 h 5 min 7 deg none MIT GPS Db. Diff (weak redundant) 24 h (SRPs constr. — 9 d noise model) 2 min 10 deg a 2 + (b 2/sin 2(e)) a, b from site residuals NGS GPS Db. Diff (redundant) 24 h 30 s 10 deg [5 + (2/sin(e)) cm]2 SIO GPS Db. Diff (weak redundant) 24 h 2 min 10 deg a 2 + (b 2/sin 2(e)) a, b from site residuals ULR GPS Db. Diff (weak NASA/GSFC Summer Seminar 3 min ‐‐ 12 June 2013 24 h 10 deg 2013 Series redundant) a 2 + (b 2/sin 2(e)) 54 a, b from site residuals GFZ (& GTZ) GRG
COMPARISON OF EXPECTED AC SATELLITE DYNAMICS ANALYSIS CENTER NUTATION & EOPs SRP PARAMS VELOCITY BRKs ATTITUDE SHADOW ZONES EARTH ALBEDO CODE IAU 2000 AR 06; Bu. A ERPs D, Y, B scales; B 1/rev every 12 hr + constraints nominal yaw rates used E+M: umbra & penumbra impld. — turned off EMR IAU 2000 AR 06; Bu. A ERPs X, Y, Z scales stochastic none yaw rates estimated E: umbra & penumbra applied ESA IAU 2000; Bu. A ERPs D, Y, B scales; B 1/rev none; Along, Along 1/rev accelerations nominal yaw rates used E+M: umbra & penumbra applied + IR GFZ (& GTZ) IAU 2000; GFZ ERPs D, Y scales @ 12: 00 + constraints yaw rates estimated E+M: umbra & penumbra applied + AT IAU 2000; IERS C 04 & Bu. A ERPs D, Y scales; X & D 1/rev stoch. impulse during ecl. yaw rates estimated E+M: umbra & penumbra applied + IR JPL IAU 2000 AR 06; IERS C 04 X, Y, Z scales stochastic none yaw rates estimated E+M: umbra & penumbra applied MIT IAU 2000; Bu. A ERPs D, Y, B scales; B(D, Y) 1/rev none; 1/rev constraints nominal yaw rates used E+M: umbra & penumbra applied NGS IAU 2000; Bu. A ERPs D, Y, B scales; B 1/rev @ 12: 00 + constraints none; del eclipse data E+M: umbra & penumbra applied + AT SIO IAU 2000; Bu. A ERPs D, Y, B scales; D, Y, B 1/rev none; 1/rev constraints nominal yaw rates used E+M: umbra & penumbra applied ULR IAU 2000; Bu. A ERPs D, Y, B scales; none nominal yaw 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 D, Y, B 1/rev rates used E+M: umbra & penumbra applied 55 GRG
COMPARISON OF EXPECTED AC TIDAL MODELS ANALYSIS CENTER SOLID EARTH POLE OCEAN LOAD OCEAN POLE OCEAN CMC sub‐daily EOPs CODE IERS 2010; dehanttideinel. f eqn 23 a/b mean pole FES 2004; hardisp. f none sites & SP 3 IERS 2010; subd nutation EMR IERS 2010 eqn 23 a/b mean pole FES 2004; hardisp. f IERS 2010 sites & SP 3 IERS 2010 ESA IERS 2010; dehanttideinel. f eqn 23 a/b mean pole FES 2004; hardisp. f none sites & SP 3 IERS 2010 & PMsdnut. for GFZ (& GTZ) IERS 2010 eqn 23 a/b mean pole FES 2004 none sites & SP 3 IERS 2010; PMsdnut. for GRG IERS 2010 eqn 23 a/b mean pole FES 2004 none sites & SP 3 IERS 2010 JPL IERS 2010 eqn 23 a/b mean pole FES 2004; hardisp. f IERS 2010 sites & SP 3 IERS 2010 MIT IERS 2010 eqn 23 a/b mean pole FES 2004 none sites & SP 3 IERS 2010 NGS IERS 2010; dehanttideinel. f eqn 23 a/b mean pole FES 2004; hardisp. f none sites & SP 3 IERS 2010 & PMsdnut. for SIO IERS 2010 eqn 23 a/b mean pole FES 2004 none sites & SP 3 IERS 2010 ULR IERS 2010 eqn 23 a/b mean pole FES 2004 none sites & SP 3 IERS 2010 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 56
COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS ANALYSIS CENTER GRAVITY FIELD EARTH TIDES EARTH POLE OCEAN TIDES OCEAN POLE RELATIVITY EFFECTS CODE EGM 2008; C 21/S 21 due to PM IERS 2010 – FES 2004 none dynamic corr & bending applied EMR EGM 2008 IERS 2010 – FES 2004 none no dynamic corr; bending applied ESA EIGEN‐GL 05 C IERS 2010 – FES 2004 none dynamic corr & bending applied GFZ (& GTZ) JGM 3; C 21/S 21 due to PM IERS 2010 – FES 2004 none no dynamic corr & bending applied GRG EIGEN GL 04 S; C 21/S 21 due to PM IERS 2010 – FES 2004 none dynamic corr; bending applied JPL EGM 2008; C 21/S 21 due to PM; C 20, C 30, C 40 IERS 2010 – FES 2004 Desai & Yuan IERS 2010; eqn 6. 23 a dynamic corr & bending applied MIT EGM 2008; C 21/S 21 due to PM IERS 1992; Eanes Love # none no dynamic corr; bending applied NGS EGM 2008 IERS 2010 – FES 2004 none dynamic corr & bending applied SIO EGM 2008; C 21/S 21 due to PM IERS 1992; Eanes Love # none no dynamic corr; bending applied ULR EGM 2008; C 21/S 21 due IERS 1992; none 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 to PM Eanes Love # none no dynamic corr; 57 bending applied
CORS Branch Task Flow Map NSRS Realization (next page) IGS & ITRF NGS Goal 1: Support the Users of the National Spatial Reference System CORS Network/Data Support NGS Goal 2: Modernize and Improve the National Spatial Reference System GPS Metadata Maintenance GPS Ultra‐ Rapid products OPUS‐S OPUS‐RS OPUS‐NET (NGS internal) GPS Rapid products GPS Final products S h IG oug s Thr uct Prod Orbit Models CORS Data Analysis OPUS‐DB NGSIDB IGS Analysis Center Coordination (ACC) CORS Solution Experimental 2013 NASA/GSFC Summer Seminar Series ‐‐ 12 June 2013 NGS 2 nd Reprocessing GPS orbits, Earth Orientation Parameters CORS coordinates 58
NGS Strategic Goals Goal 1: Support the Users of the National Spatial Reference System Goal 2: Modernize and Improve the National Spatial Reference System Flowchart for NSRS Realization a priori datum (IGS 08) NGS 2 nd Reprocessing GPS orbits, Earth Orientation Parameters, IGS Station Positions - Adjust all obs model parameters in a minimally constrained (no-net rotation; NNR) solution - Realizes an NGS global frame w. r. t. a priori datum (IGS 08) using latest IERS and IGS conventions NGS 2 nd Reprocessing CORS coordinates - Tie CORS to global network and NGS Repro 2 orbits and ERPs at normal equation level using NNR Load NAD 83 (2013) coords into NGSIDB Final products from other IGS Analysis Centers International Collaboration IGS AC and RF Coordinators Finals Orbit, Clock, ERP and SINEX Combinations Daily IGS SINEX files to ITRF International Terrestrial Reference Frame (ITRF) Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS). ITRF 2013 VLBI SLR DORIS IGS Realization of ITRF 2013 IGS 2013 (station coordinates, satellite antenna calibrations) NGS CORS+global SINEX Stack SINEX files Realizes NGS‐derived using secular frame CATREF GS e. N S t ibu IG ntr ls to Co ina F CORS in NGS-derived global frame Align NGS‐derived frame to IGS 2013 Obtain NAD 83 coords via successive Adjust passive NAD 83 (2013) 14‐parameter network to NAD Coordinates NASA/GSFC Summer Seminar Series ‐‐ 12 Junetransformations 2013 83 (2013) NAD 83 (2013) 59
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