High Precision Applications of Global Navigation Satellite Systems

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

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 system5 geost. Earth orbit
  (GEO), 5 incl. geosync. orbit (IGSO)
• plus global satellite system30 medium Earth
  orbit (MEO)

3
How a GNSS Works

• 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 antennareceiver 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
Source unavco.org
4
What is the IGS?

• An International Association of Geodesy (IAG)
  Technique Service
• Voluntary federation of gt200 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)
5
IGS GNSS Tracking Network
6
IGS Core Product Series
Series Series Series ID ID Latency Latency Issue times (UTC) Issue times (UTC) Issue times (UTC) Data spans (UTC) Data spans (UTC) Data spans (UTC) Remarks Remarks Remarks Remarks
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) IGU IGU real-time real-time _at_ 0300, 0900, 1500, 2100 _at_ 0300, 0900, 1500, 2100 _at_ 0300, 0900, 1500, 2100 24 hr _at_ 0000, 0600, 1200, 1800 24 hr _at_ 0000, 0600, 1200, 1800 24 hr _at_ 0000, 0600, 1200, 1800 ? for real-time apps ? GPS GLONASS ? issued with prior IGA ? for real-time apps ? GPS GLONASS ? issued with prior IGA ? for real-time apps ? GPS GLONASS ? issued with prior IGA ? for real-time apps ? GPS GLONASS ? issued with prior IGA

Ultra-Rapid (observed half) Ultra-Rapid (observed half) Ultra-Rapid (observed half) IGA IGA 3 - 9 hr 3 - 9 hr _at_ 0300, 0900, 1500, 2100 _at_ 0300, 0900, 1500, 2100 _at_ 0300, 0900, 1500, 2100 -24 hr _at_ 0000, 0600, 1200, 1800 -24 hr _at_ 0000, 0600, 1200, 1800 -24 hr _at_ 0000, 0600, 1200, 1800 ? for near real-time apps ? GPS GLONASS ? issued with following IGU ? for near real-time apps ? GPS GLONASS ? issued with following IGU ? for near real-time apps ? GPS GLONASS ? issued with following IGU ? for near real-time apps ? GPS GLONASS ? issued with following IGU

Rapid Rapid Rapid IGR IGR 17 - 41 hr 17 - 41 hr _at_ 1700 daily _at_ 1700 daily _at_ 1700 daily 12 hr _at_ 1200 12 hr _at_ 1200 12 hr _at_ 1200 ? for near-definitive, rapid apps ? GPS only ? for near-definitive, rapid apps ? GPS only ? for near-definitive, rapid apps ? GPS only ? for near-definitive, rapid apps ? GPS only

Final Final Final IGS IGS 12 - 19 d 12 - 19 d weekly each Thursday weekly each Thursday weekly each Thursday 12 hr _at_ 1200 for 7 d 12 hr _at_ 1200 for 7 d 12 hr _at_ 1200 for 7 d ? for definitive apps ? GPS GLONASS ? for definitive apps ? GPS GLONASS ? for definitive apps ? GPS GLONASS ? for definitive apps ? GPS GLONASS

• orbits, clocks, polar motion LOD (ERPs), and
  station positions (Finals only)

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)
IGS Core Products
8
Current Analysis Centers
Center 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 ? ?
9
Popularity of Core Products- download statistics
_at_ NASA/CDDIS (06/2010 thru 06/2012) -

• gt3.6 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 GNSS Total Hits SP3() SP3() ERP() CLK() SNX() SUM()
Ultra-rapid GPS GPS 11,711,506 (? 4 2,927,877 daily) 93.7 93.7 3.1 3.2

Final (IGS) GPS GPS 1,359,656 60.7 60.7 6.8 24.8 5.8 2.0

Rapid GPS GPS 887,986 65.6 65.6 8.7 16.9 6.4

Final (IGL) GLO 225,515 225,515 99.1 0.3 0.6

Ultra-rapid (IGV) GPS GLO 223,562 223,562 95.0 5.0

Courtesy C. Noll (NASA/CDDIS)
10
Core Product Accuracies
Series Series Product Types Product Types Product Types Product Types Product Types Accuracies Accuracies Accuracies Output Intervals Output Intervals Output Intervals
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits 5 cm (1D) 5 cm (1D) 5 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits 10 cm (1D) 10 cm (1D) 10 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks 3 ns RMS / 1.5 ns Sdev 3 ns RMS / 1.5 ns Sdev 3 ns RMS / 1.5 ns Sdev 15 min 15 min 15 min
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD 250 µas / 50 µs 250 µas / 50 µs 250 µas / 50 µs 6 hr 6 hr 6 hr

Ultra-Rapid (observed half) Ultra-Rapid (observed half) ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits 3 cm (1D) 3 cm (1D) 3 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (observed half) Ultra-Rapid (observed half) ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits 5 cm (1D) 5 cm (1D) 5 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (observed half) Ultra-Rapid (observed half) ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks 150 ps RMS / 50 ps Sdev 150 ps RMS / 50 ps Sdev 150 ps RMS / 50 ps Sdev 15 min 15 min 15 min
Ultra-Rapid (observed half) Ultra-Rapid (observed half) ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD lt50 µas / 10 µs lt50 µas / 10 µs lt50 µas / 10 µs 6 hr 6 hr 6 hr

Rapid Rapid ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits 2.5 cm (1D) 2.5 cm (1D) 2.5 cm (1D) 15 min 15 min 15 min
Rapid Rapid ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks 75 ps RMS / 25 ps Sdev 75 ps RMS / 25 ps Sdev 75 ps RMS / 25 ps Sdev 5 min 5 min 5 min
Rapid Rapid ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD lt40 µas / 10 µs lt40 µas / 10 µs lt40 µas / 10 µs daily daily daily

Final Final ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits lt2.5 cm (1D) lt2.5 cm (1D) lt2.5 cm (1D) 15 min 15 min 15 min
Final Final ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits lt5 cm (1D) lt5 cm (1D) lt5 cm (1D) 15 min 15 min 15 min
Final Final ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks 75 ps RMS / 20 ps SDev 75 ps RMS / 20 ps SDev 75 ps RMS / 20 ps SDev 30 s (SVs) 5 min 30 s (SVs) 5 min 30 s (SVs) 5 min
Final Final ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD ? ERPs PM dLOD lt30 µas / 10 µs lt30 µas / 10 µs lt30 µas / 10 µs daily daily daily
Final Final ? Terrestrial frames ? Terrestrial frames ? Terrestrial frames ? Terrestrial frames ? Terrestrial frames 2.5 mm NE / 6 mm U 2.5 mm NE / 6 mm U 2.5 mm NE / 6 mm U daily daily daily

• 5 cm (1D) orbit error 0.4 cm (3D) position
  error over 1000 km baseline (Beser Parkinson,
  1982)

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
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
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)
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 _at_ GPS altitude
• IERS model is known to have visible errors, which
  could reach the 10 to 20 level

15
Harmonic Errors Sub-daily Alias and Draconitic
(1/3)

• Simulated impact ofsub-daily EOP tidalerrors on
  IGS orbits
• generated fakemodel by changingadmittances by
  up to20assumed 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 _at_ 15
  min intervals
• compute spectra of differences for each SV, stack
  smooth
• compare spectral differences input model errors
  vs. orbital response

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 (69) 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 235230 for Day A
• repeat for Day B propagate backwards to
  235230 of day before
• compute IGS orbit jumps at 235230
• SIMULATED EOP SIGNATURES
• difference conventional EOP-test orbits at
  234500 only
• Compute IGS orbit jumps over 5.6 yr, test orbits
  over 2.8 yr

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/v3 cm 5.8 cm (1D) annual errors
Power Density (mm2 / cpd)
1.0 cm white noise floor
Frequency (cycles per day)
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 -gt resonance
  with GPS orbital period
• Compared IGS orbit discontinuities to EOP-test
  orbit differences at 234500
• 24 h sampling causes sub-daily EOP tide errors to
  alias at 14, 9 and 7 d bands -gt 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

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
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 SINEXAC 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
  orbitsERPs 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

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 _at_ equator) in RMS
• but systematic errors remain in RZclear 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 (RMS0.041) RY0.015
  (RMS0.039) RZ-0.004 (RMS0.022)
• IGS RX RY better than IGR for now
• IGS RZ now biased w.r.t. IGR, and has higher
  scatter

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
  orbitssee RX RY
• corrected on Wk 1732
• Now RY of IGR (violet) is biased
• ESA consistency issues in IGR

IGx08
IGS05
fixed AC orbit pre-alignment
ESA fixed TRF issue

• 1 mas 13 cm _at_ GPS altitude

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
  IGRwhich prods are more precise?

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

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-nonoperation
  al/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

26
IGS 2nd Reprocessing and ITRF2013
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 -gt IGS
  contribution to ITRF2013
• Updated models, frames methodologies
• IERS 2010 Conventions generally adopted
• NGA stations data w/ new antenna calibrations
  (for improved ITRF lt-gt 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 _at_ 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

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

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

30
Expected Performance of IG2?- WRMS of AC repro1
orbits wrt IG1 -
Large scatter for some ACs in early IG1expected
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)
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

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)
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
• gt 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)
34
Other Challenges Mostly Network Issues(not
addressed by IG2)
35
Uncalibrated Radomes

• 28/92 (? 30) multi-technique sites have an
  uncalibrated radome
• nearly half (13/28) operated by JPL

36
Uncalibrated Radomes Impact on ITRF (1/2)

• including all co-location sites
• systematic VLBI lt-gt SLR scale discrepancy

Courtesy Z. Altamimi (IGN/LAREG)
37
Uncalibrated Radomes Impact on ITRF (1/2)

• when GNSS co-located sites with uncalibrated
  radomes are excluded
• VLBI lt-gt SLR scale difference amplified by 0.2
  ppb (network effect calibration errors)

Courtesy Z. Altamimi (IGN/LAREG)
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)

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 -gt 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
• ltIGb08 rate 0.16 sta/wk
• gtIGb08 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)
40
Summary
41
Conclusions IGS Errors

• Current IGS products are of high accuracy and
  precision
• GPS orbits
• overall lt2.5 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 _at_ 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 lt30 mas
• dLOD 10 ms
• terrestrial frames
• 2 mm NE
• 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

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) _at_ 15 min intervals
• GPS SV and station clocks (clock RINEX) _at_ 5 min
  intervals
• Earth Rotation Parameters (IGS ERP)
• terrestrial coordinate frames (IERS SINEX)
• expected delivery for ITRF2013 -gt early 2014
• And possibly some ancillary products
• GLONASS orbits clocks
• 30-second SV station clocks
• bias products

43
Conclusions More Repro2 and Other Challenges

• IG2 quality should approach current IGS prods
• quality for later (2000 -gt 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

44
Questions?
45
Extra Slides
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

47
Harmonic Errors Sub-daily Alias and Draconitic

• Simulated impact ofsub-daily EOP tidalerrors on
  IGS orbits
• generated fakemodel by changingadmittances by
  up to20assumed 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 _at_ 15
  min intervals
• compute spectra of differences for each SV, stack
  smooth
• compare spectral differences input model errors
  vs. orbital response

48
Harmonic Errors Sub-daily Alias and Draconitic

• Simulated impact ofsub-daily EOP tidalerrors on
  IGS orbits
• generated fakemodel by changingadmittances by
  up to20assumed 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 _at_ 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?
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/v3 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)
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)
05
51
Spectra of Orbital Responses tosub-daily EOP
Errors Near 2 cpd

• at semi-diurnal period, EOP model errors absorbed
  mostly into orbit radial (via Keplers 3rd law)

Power Density (mm2 / cpd)
Frequency (cycles per day)
06
52
Spectra of Orbital Responses tosub-daily EOP
Errors Near 3 cpd

• background power is lower
• errors absorbed in all three components

Power Density (mm2 / cpd)
Frequency (cycles per day)
53
Spectra of Orbital Responses tosub-daily EOP
Errors Near 4 cpd

• same near 4 cpd

Power Density (mm2 / cpd)
Frequency (cycles per day)
54
COMPARISON OF EXPECTED AC DATA USAGE COMPARISON OF EXPECTED AC DATA USAGE COMPARISON OF EXPECTED AC DATA USAGE COMPARISON OF EXPECTED AC DATA USAGE COMPARISON OF EXPECTED AC DATA USAGE COMPARISON OF EXPECTED AC DATA USAGE 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 GPS GLO UnDiff 24 h 5 min 10 deg none
ESA GPS GLO UnDiff 24 h 5 min 10 deg 1/sin2 (e)
GFZ ( GTZ) GPS ?GLO? UnDiff ?? 24 h ?? 5 min 7 deg 1/2sin(e) for e lt 30 deg
GRG GPS GLO UnDiff 3 24 3 h 15 min 10 deg none
JPL GPS UnDiff 3 24 3 h 5 min 7 deg none
MIT GPS DbDiff (weak redundant) 24 h (SRPs constr. 9d noise model) 2 min 10 deg a2 (b2/sin2(e)) a,b from site residuals
NGS GPS DbDiff (redundant) 24 h 30 s 10 deg 5 (2/sin(e)) cm2
SIO GPS DbDiff (weak redundant) 24 h 2 min 10 deg a2 (b2/sin2(e)) a,b from site residuals
ULR GPS DbDiff (weak redundant) 24 h 3 min 10 deg a2 (b2/sin2(e)) a,b from site residuals
55
COMPARISON OF EXPECTED AC SATELLITE DYNAMICS COMPARISON OF EXPECTED AC SATELLITE DYNAMICS COMPARISON OF EXPECTED AC SATELLITE DYNAMICS COMPARISON OF EXPECTED AC SATELLITE DYNAMICS COMPARISON OF EXPECTED AC SATELLITE DYNAMICS COMPARISON OF EXPECTED AC SATELLITE DYNAMICS 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 EM umbra penumbra impld.turned off
EMR IAU 2000AR06 BuA ERPs X,Y,Z scales stochastic none yaw rates estimated E umbra penumbra applied
ESA IAU 2000 BuA ERPs D,Y,B scales B 1/rev none Along, Along 1/rev accelerations nominal yaw rates used EM umbra penumbra applied IR
GFZ ( GTZ) IAU 2000 GFZ ERPs D,Y scales _at_ 1200 constraints yaw rates estimated EM umbra penumbra applied AT
GRG IAU 2000 IERS C04 BuA ERPs D,Y scales X D 1/rev stoch. impulse during ecl. yaw rates estimated EM umbra penumbra applied IR
JPL IAU 2000AR06 IERS C04 X,Y,Z scales stochastic none yaw rates estimated EM umbra penumbra applied
MIT IAU 2000 BuA ERPs D,Y,B scales B(D,Y) 1/rev none 1/rev constraints nominal yaw rates used EM umbra penumbra applied
NGS IAU 2000 BuA ERPs D,Y,B scales B 1/rev _at_ 1200 constraints none del eclipse data EM umbra penumbra applied AT
SIO IAU 2000 BuA ERPs D,Y,B scales D,Y,B 1/rev none 1/rev constraints nominal yaw rates used EM umbra penumbra applied
ULR IAU 2000 BuA ERPs D,Y,B scales D,Y,B 1/rev none nominal yaw rates used EM umbra penumbra applied
56
COMPARISON OF EXPECTED AC TIDAL MODELS COMPARISON OF EXPECTED AC TIDAL MODELS COMPARISON OF EXPECTED AC TIDAL MODELS COMPARISON OF EXPECTED AC TIDAL MODELS COMPARISON OF EXPECTED AC TIDAL MODELS COMPARISON OF EXPECTED AC TIDAL MODELS 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 IERS 2010 subd nutation
EMR IERS 2010 eqn 23a/b mean pole FES2004 hardisp.f IERS 2010 sites SP3 IERS 2010
ESA IERS 2010 dehanttideinel.f eqn 23a/b mean pole FES2004 hardisp.f none sites SP3 IERS 2010 PMsdnut.for
GFZ ( GTZ) IERS 2010 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2010 PMsdnut.for
GRG IERS 2010 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2010
JPL IERS 2010 eqn 23a/b mean pole FES2004 hardisp.f IERS 2010 sites SP3 IERS 2010
MIT IERS 2010 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2010
NGS IERS 2010 dehanttideinel.f eqn 23a/b mean pole FES2004 hardisp.f none sites SP3 IERS 2010 PMsdnut.for
SIO IERS 2010 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2010
ULR IERS 2010 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2010
57
COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS COMPARISON OF EXPECTED AC GRAVITY FORCE MODELS 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 IERS 2010 FES2004 none dynamic corr bending applied
EMR EGM2008 IERS 2010 IERS 2010 IERS 2010 FES2004 none no dynamic corr bending applied
ESA EIGEN-GL05C IERS 2010 IERS 2010 IERS 2010 FES2004 none dynamic corr bending applied
GFZ ( GTZ) JGM3 C21/S21 due to PM IERS 2010 IERS 2010 IERS 2010 FES2004 none no dynamic corr bending applied
GRG EIGEN GL04S C21/S21 due to PM IERS2010 IERS 2010 IERS 2010 FES2004 none dynamic corr bending applied
JPL EGM2008 C21/S21 due to PM C20, C30, C40 IERS 2010 IERS 2010 IERS 2010 FES2004 Desai Yuan IERS 2010 eqn 6.23a dynamic corr bending applied
MIT EGM2008 C21/S21 due to PM IERS 1992 Eanes Love none none none no dynamic corr bending applied
NGS EGM2008 IERS 2010 IERS 2010 IERS 2010 FES2004 none dynamic corr bending applied
SIO EGM2008 C21/S21 due to PM IERS 1992 Eanes Love none none none no dynamic corr bending applied
ULR EGM2008 C21/S21 due to PM IERS 1992 Eanes Love none none none no dynamic corr bending applied
58
(No Transcript)
59
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 SINEXfilesusingCATREF
NGS CORSglobal SINEX
Realizes NGS-derived secular frame
Align NGS-derived frame to IGS2013
CORS in NGS-derived global frame
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
NAD 83 (2013)