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

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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 How a GNSS Works Source: boeing.com GPS 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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

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

IGS Core Product Series ID Latency Issue times (UTC) Data spans Remarks Ultra-Rapid (predicted half) IGU real-time @ 03:00, 09:00, 15:00, 21:00 +24 hr @ 00:00, 06:00, 12:00, 18:00 ● for real-time apps ● GPS & GLONASS ● issued with prior IGA (observed half) IGA 3 - 9 hr 03:00, 09:00, 15:00, 21:00 -24 hr @ ● for near real-time apps ● issued with following Rapid IGR 17 - 41 hr 17:00 daily ±12 hr @ 12:00 ● for near-definitive, rapid apps ● GPS only Final IGS 12 - 19 d weekly each Thursday 12:00 for 7 d ● for definitive apps orbits, clocks, polar motion & LOD (ERPs), and station positions (Finals only) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Outline for How IGS Core Products are Derived a priori datum (IGS08/IGb08) VLBI IGS RF WG Chair (IGN) B. Garayt, A. Duret and P. Rebischung International Terrestrial Reference Frame (ITRF) 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 AC SNX files (Finals only) SLR IGS TRF prods Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS). 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 SINEX rotations 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 Combined Orbits, Clocks, and ERPs (Rapid & Ultra-rapid only) 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 Rapid (IGR) orbits (GPS), clocks (SV, Rx), ERPs Ultra-rapid (IGU) orbits (GPS, GLO), clocks (SV), ERPs IGS Core Products 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Current Analysis Centers Name Final (IGS) 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 GeoForschungsZentrum, 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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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 … Product GNSS Total Hits SP3 (%) ERP (%) CLK (%) SNX (%) SUM (%) Ultra-rapid GPS 11,711,506 ( 4 * 2,927,877 daily) 93.7 3.1 3.2 Final (IGS) 1,359,656 60.7 6.8 24.8 5.8 2.0 Rapid 887,986 65.6 8.7 16.9 6.4 Final (IGL) GLO 225,515 99.1 0.3 0.6 (IGV) GPS & GLO 223,562 95.0 5.0 Courtesy: C. Noll (NASA/CDDIS) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Core Product Accuracies Series Product Types Accuracies Output Intervals Ultra-Rapid (predicted half) ● GPS orbits ~ 5 cm (1D) 15 min ● GLONASS orbits ~10 cm (1D) ● GPS SV clocks ~3 ns RMS / ~1.5 ns Sdev ● ERPs: PM + dLOD ~250 µas / ~50 µs 6 hr (observed half) ~ 3 cm (1D) ~5 cm (1D) ~150 ps RMS / ~50 ps Sdev <50 µas / ~10 µs Rapid ~2.5 cm (1D) ● GPS SV & station clocks ~75 ps RMS / ~25 ps Sdev 5 min <40 µas / ~10 µs daily Final <2.5 cm (1D) <5 cm (1D) ~75 ps RMS / ~20 ps SDev 30 s (SVs) + 5 min <30 µas / ~10 µs ● Terrestrial frames ~2.5 mm N&E / ~6 mm U 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

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

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

Harmonic Errors: Background (1/2) GPS-sun geometry repeat period “draconitic” year = 351.2 d 1st & 2nd harmonics overlay seasonal 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 also common dE 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

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

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 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 Frequency (cycles per day) Power Density (mm2 / cpd) long-period errors absorbed mostly by ERPs, not orbits short- period errors go into orbits 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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 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

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

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

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

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

…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

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 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 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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 WRMS of AC Orbit Residuals Since IG1 - AC solutions minus IGS Final , after pre-alignment - IGx08 IGS05 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 IGx08 IGS Final has ~4 mm WRMS difference with IGR—which prods are more precise? 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

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

IGS 2nd Reprocessing and ITRF2013 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

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

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

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

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 stations with uncalibrated radomes few mm errors likely at stations with “converted” antenna calibrations will cause trouble in use of local ties for ITRF2013 colocation sites consider to exclude in next ITRF Courtesy: P. Rebischung (IGN/LAREG) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

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

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

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

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 study of impacts on stability of IGS reference frame Station operators should limit disruptions, esp. at co-location sites 100% data availability actual data availability Courtesy: K. Choi (NOAA/NGS) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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

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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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

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

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

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

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 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 Frequency (cycles per day) Power Density (mas2 or s2/ cpd) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Harmonic Errors: Sub-daily Alias and Draconitic 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 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 Frequency (cycles per day) Power Density (mm2 / cpd) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Harmonic Errors: Sub-daily Alias and Draconitic 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 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 Frequency (cycles per day) Power Density (mm2 / cpd) bump in background power – resonance of ~2 cpd sub-daily tide errors and GPS orbital period? 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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

Spectra of Orbital Responses to sub-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

Spectra of Orbital Responses to sub-daily EOP Errors – Near 2 cpd at semi-diurnal period, EOP model errors absorbed mostly into orbit radial (via Kepler’s 3rd law) Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 06

Spectra of Orbital Responses to sub-daily EOP Errors – Near 3 cpd background power is lower errors absorbed in all three components Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Spectra of Orbital Responses to sub-daily EOP Errors – Near 4 cpd same near 4 cpd Power Density (mm2 / cpd) Frequency (cycles per day) 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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 DbDiff (weak redundant) 24 h 3 min 3 deg 1/cos2(z) EMR UnDiff 5 min 10 deg none ESA 1/sin2 (e) GFZ (& GTZ) GPS + ?GLO? ?? 24 h ?? 7 deg 1/2sin(e) for e < 30 deg GRG 3 + 24 + 3 h 15 min JPL GPS MIT (SRPs constr.— 9d noise model) 2 min a2 + (b2/sin2(e)) a,b from site residuals NGS DbDiff (redundant) 30 s [5 + (2/sin(e)) cm]2 SIO ULR 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

COMPARISON OF EXPECTED AC SATELLITE DYNAMICS ANALYSIS CENTER NUTATION & EOPs SRP PARAMS VELOCITY BRKs ATTITUDE SHADOW ZONES EARTH ALBEDO CODE IAU 2000AR06; BuA ERPs D,Y,B scales; B 1/rev every 12 hr + constraints nominal yaw rates used E+M: umbra & penumbra impld.—turned off EMR X,Y,Z scales stochastic none yaw rates estimated E: umbra & penumbra applied ESA IAU 2000; BuA ERPs none; Along, Along 1/rev accelerations applied + IR GFZ (& GTZ) IAU 2000; GFZ ERPs D,Y scales @ 12:00 + constraints applied + AT GRG IAU 2000; IERS C04 & BuA ERPs D,Y scales; X & D 1/rev stoch. impulse during ecl. JPL IAU 2000AR06; IERS C04 MIT D,Y,B scales; B(D,Y) 1/rev none; 1/rev constraints NGS none; del eclipse data SIO D,Y,B scales; D,Y,B 1/rev ULR 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

COMPARISON OF EXPECTED AC TIDAL MODELS ANALYSIS CENTER SOLID EARTH EARTH POLE OCEAN LOAD OCEAN POLE OCEAN CMC sub-daily EOPs CODE IERS 2010; dehanttideinel.f eqn 23a/b mean pole FES2004; hardisp.f none sites & SP3 subd nutation EMR IERS 2010 ESA IERS 2010 & PMsdnut.for GFZ (& GTZ) FES2004 PMsdnut.for GRG JPL MIT NGS SIO ULR 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS ANALYSIS CENTER GRAVITY FIELD EARTH TIDES EARTH POLE OCEAN TIDES OCEAN POLE RELATIVITY EFFECTS CODE EGM2008; C21/S21 due to PM IERS 2010 IERS 2010 – FES2004 none dynamic corr & bending applied EMR EGM2008 no dynamic corr; ESA EIGEN-GL05C GFZ (& GTZ) JGM3; C21/S21 due to PM no dynamic corr & GRG EIGEN GL04S; C21/S21 due to PM IERS2010 dynamic corr; bending applied JPL EGM2008; C21/S21 due to PM; C20, C30, C40 IERS 2010 – FES2004 Desai & Yuan IERS 2010; eqn 6.23a MIT EGM2008; C21/S21 due to PM IERS 1992; Eanes Love # NGS SIO ULR 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

CORS Branch Task Flow Map NGS 2nd Reprocessing GPS Final products GPS Rapid products GPS Ultra-Rapid products CORS Data Analysis CORS Network/Data Support GPS orbits, Earth Orientation Parameters GPS Metadata Maintenance OPUS-S OPUS-RS OPUS-NET (NGS internal) IGS Analysis Center Coordination (ACC) NGS Goal 1: Support the Users of the National Spatial Reference System NGS Goal 2: Modernize and Improve the National Spatial Reference System CORS coordinates Through IGS Products CORS Branch Task Flow Map Orbit Models CORS Solution OPUS-DB NGSIDB Experimental NSRS Realization (next page) IGS & ITRF 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

Flowchart for NSRS Realization NGS Strategic Goals a priori datum (IGS08) Final products from other IGS Analysis Centers Contribute NGS Finals to IGS NGS 2nd Reprocessing - 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 (IGS08) using latest IERS and IGS conventions Goal 1: Support the Users of the National Spatial Reference System GPS orbits, Earth Orientation Parameters, IGS Station Positions International Collaboration IGS AC and RF Coordinators Goal 2: Modernize and Improve the National Spatial Reference System Finals Orbit, Clock, ERP and SINEX Combinations Daily IGS SINEX files to ITRF VLBI International Terrestrial Reference Frame (ITRF) NGS 2nd Reprocessing - Tie CORS to global network and NGS Repro2 orbits and ERPs at normal equation level using NNR SLR Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS). CORS coordinates DORIS ITRF2013 IGS Realization of ITRF2013 IGS2013 (station coordinates, satellite antenna calibrations) Stack SINEX files using CATREF NGS CORS+global SINEX Realizes NGS-derived secular frame CORS in NGS-derived global frame Align NGS-derived frame to IGS2013 Obtain NAD 83 coords via successive 14-parameter transformations Load NAD 83 (2013) coords into NGSIDB Adjust passive network to NAD 83 (2013) NAD 83 (2013) Coordinates 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013 NAD 83 (2013)