OVERVIEW OF IGS PRODUCTS

1
OVERVIEW OF IGS PRODUCTS ANALYSIS CENTER
MODELING

• Status of core products
• focus on Ultra-rapid predicted orbits
• issues with current products
• Comparisons of AC analysis strategies
• evidence for systematic errors, esp. fortnightly
  harmonics
• Recommendations

Jim Ray, NOAA/NGS Jake Griffiths, NOAA/NGS
IGS 2008 Workshop, Miami Beach, 2 June 2008
2
SUMMARY OF IGS CORE PRODUCTS SUMMARY OF IGS CORE PRODUCTS SUMMARY OF IGS CORE PRODUCTS SUMMARY OF IGS CORE PRODUCTS SUMMARY OF IGS CORE PRODUCTS SUMMARY OF IGS CORE PRODUCTS SUMMARY OF IGS CORE PRODUCTS
PRODUCT SUITES ACs CURRENT PRECISION LATENCY UPDATES SAMPLE INTERVAL QUALITY ASSESSMENT
Ultra-Rapid (predicted) orbits SV clocks ERPs 7 (2) 4 7 (2) lt 5 cm 5 ns lt 1 mas real time 03, 09, 15, 21 UTC 15 min 15 min 6 hr marginally robust extremely poor very weak
Ultra-Rapid (observed) orbits SV clocks ERPs 7 (2) 4 7 (2) 3 cm 0.2 ns 0.1 mas 3 - 9 hr 03, 09, 15, 21 UTC 15 min 15 min 6 hr fairly robust weak fairly robust
Rapid orbits SV, stn clocks ERPs 8 5 8 2.5 cm 0.1 ns 0.06 mas 17 - 41 hr daily 15 min 5 min daily robust marginally robust robust
Final orbits GLO orbits SV, stn clocks ERPs terr frame 8 4 6 8 8 2.5 cm lt 15 cm ? 0.1 ns 0.03 mas 3 (h), 6 (v) mm 13 - 20 d weekly 15 min 15 min 5 min, 30 s daily weekly robust not robust robust for 5 min robust robust
indicates AC contributions that are weaker than
others
3
Predicted IGU Orbit WRMS
IGU Orbits (1st 6 hr of predictions) wrt IGR
Orbits
PRN29 (IIA) decommissioned GOP solutions
improved

• WRMS of IGU orbit predictions have improved to lt5
  cm RMS

4
Scale Rotations of Predicted IGU Orbits
IGU Orbits (24 h of predictions) wrt IGR Orbits
(shifted)
(shifted)

• Z rotations (UT1 prediction error) reach 1 mas
  level
• equivalent to equatorial shift of 12.9 cm at GPS
  altitude

5
Issues with Current Products

• IGU orbit combination only marginally robust
• sometimes AC predictions are better than combo
• Ultra-Rapid IGS Orbit Comparison for 1478_6_06
  (10 May 2008 06h)
• CENT STA DX DY DZ RX RY RZ
  SCL RMS WRMS MEDI
• mm mm mm uas uas uas
  ppb mm mm mm
• -----------------------------------------------
  -----------------
• cou 73 11 -1 -4 536 -389 254
  -.29 64 34 33
• emu 49 7 0 0 486 38 -60
  .03 84 44 21
• esu 95 4 5 -2 -396 687 -72
  .13 77 77 29
• gfu 65 1 -2 -2 302 -21 127
  -.34 77 78 29
• gou 82 -5 -4 -1 260 334 48
  -.35 89 78 31
• siu 62 0 17 -33 -221 1068 730
  .02 130 131 71
• usu 33 19 9 0 297 -394 -20
  .14 123 111 56
• igu n/a 5 0 -5 283 103 45
  -.08 74 79 18
• would benefit from more high-quality ACs

6
COMPARISON OF AC DATA USAGE COMPARISON OF AC DATA USAGE COMPARISON OF AC DATA USAGE COMPARISON OF AC DATA USAGE COMPARISON OF AC DATA USAGE COMPARISON OF AC DATA USAGE
ANALYSIS CENTER OBS TYPE ORBIT DATA ARC LENGTH DATA RATE ELEVATION CUTOFF ELEVATION INVERSE WGTS
CODE DbDiff ( weak redundant) 24 24 24 h 3 min 3 deg 1/cos2(z)
EMR UnDiff 24 h 5 min 15 deg none
ESA UnDiff 24 h 5 min 10 deg 1/sin2(e)
GFZ UnDiff n x 24 h n 3 (Rapid 2) 5 min 7 deg 1/2sin(e) for e lt 30 deg
GRGS (new) UnDiff 48 h 15 min 10 deg 1/cos2(z)
JPL UnDiff 3 24 3 h 5 min 15 deg ? 7 deg none
MIT DbDiff (weak redundant) 24 h (SRPs over 9d) 2 min 10 deg a2 (b2/sin2(e)) a,b from site residuals
NGS DbDiff (redundant) 24 h 30 s 10 deg 5 (2/sin(e)) cm2
PDR (Repro) DbDiff (weak redundant) 24 24 24 h 3 min 3 deg 1/cos2(z)
SIO DbDiff (weak redundant) 24 h 2 min 10 deg a2 (b2/sin2(e)) a,b from site residuals
7
effect of 15-deg cutoff
8
COMPARISON OF AC TIDAL MODELS COMPARISON OF AC TIDAL MODELS COMPARISON OF AC TIDAL MODELS COMPARISON OF AC TIDAL MODELS COMPARISON OF AC TIDAL MODELS COMPARISON OF AC TIDAL MODELS COMPARISON OF AC TIDAL MODELS
ANALYSIS CENTER SOLID EARTH EARTH POLE OCEAN LOAD OCEAN POLE OCEAN CMC SUBDAILY EOPs
CODE IERS 2003 dehanttideinel.f eqn 23a/b mean pole FES2004 hardisp.f none sites SP3 IERS 2003 subd nutation
EMR IERS 2003 eqn 23a/b mean pole FES2004 gipsy none sites SP3 IERS 1996 no subd nutation
ESA IERS 2003 dehanttideinel.f eqn 23a/b mean pole FES2004 hardisp.f none sites SP3 IERS 2003 PMsdnut.for
GFZ IERS 1992 eqn 23a/b mean pole FES2004 hardisp.f none sites SP3 IERS 2003 subd nutation
GRGS (new) IERS 2003 nominal mean PM FES2002 none none IERS 2003 PMsdnut.for
JPL IERS 2003 eqn 23a/b mean pole FES2002 gipsy none none ? sites SP3 IERS 1996 ? IERS 2003
MIT IERS 2003 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2003 PMsdnut.for
NGS IERS 2003 dehanttideinel.f eqn 23a/b mean pole FES2004 hardisp.f none sites SP3 IERS 2003 PMsdnut.for
PDR (Repro) IERS 2003 dehanttideinel.f fixed mean pole GOT00.2 w/ 11 terms none none IERS 2003 no subd nutation
SIO IERS 2003 eqn 23a/b mean pole FES2004 none sites SP3 IERS 2003 PMsdnut.for
9
Aliased Tidal Peaks in PM Discontinuities
Peaks in PM Differences Peaks in PM Differences Peaks in PM Differences Peaks in PM Differences
AC 14 d 9 d 7 d
EMR PM-x 14.20.2 9.350.09 7.180.05
EMR PM-y 14.10.2 9.6 9.00.1 7.160.05
GFZ PM-x 14.20.2 9.40.1 7.210.05
GFZ PM-y 14.20.2 9.6 8.90.1 7.140.05
JPL PM-x 14.20.2 9.40.1 7.230.05
JPL PM-y 14.20.2 9.20.1 7.260.05

• Spectra of polar motion day-boundary
  discontinuities show signatures of aliased O1,
  Q1, N2 tides unknown 7.2 d line

10
Kalman Filter of VLBI UT1 GPS LOD(Senior,
Kouba, Ray EGU 2008)

• VLBI (1-hr) UT1 residuals
• white over full frequency range

• GPS LOD residuals
• approximately white
• with small peak at 13.7 d
• possible difference in a priori tidal models
  wrt VLBI

EMR analysis upgrade

• Gauss-Markov values for GPS LOD biases
• peak-to-peak range 40 µs
• RMS 9 µs

• 14.19-d periodic
• treated as GPS artifact
• amplitude varies between 5 11 µs
• phase varies linearly w/ time due to changing
  period

11
Fortnightly Band Spurious IGS LOD(Senior,
Kouba, Ray EGU 2008)
LODS (AAMOAM) spectra
14.12 d signal in IGS C04 probably due to mix
of GPS errors
12
Day-boundary Orbit Discontinuities

• Orbit discontinuities between days show
  temporally correlated errors broad fortnightly
  spectral peak
• From Griffiths Ray (AGU 2007)

13
COMPARISON OF AC GRAVITY FORCE MODELS COMPARISON OF AC GRAVITY FORCE MODELS COMPARISON OF AC GRAVITY FORCE MODELS COMPARISON OF AC GRAVITY FORCE MODELS COMPARISON OF AC GRAVITY FORCE MODELS COMPARISON OF AC GRAVITY FORCE MODELS COMPARISON OF AC GRAVITY FORCE MODELS
ANALYSIS CENTER GRAVITY FIELD EARTH TIDES EARTH POLE OCEAN TIDES OCEAN POLE RELATIVITY EFFECTS
CODE JGM3 C21/S21 due to PM IERS 2003 IERS 2003 CSR 3.0 none dynamic corr bending applied
EMR JGM3 C21/S21 due to PM freq-depend. Love IERS 2003 CSR none no dynamic corr bending applied
ESA EIGEN C21/S21 due to PM IERS 2003 IERS 2003 IERS 2003 none dynamic corr bending applied
GFZ JGM2 C21/S21 due to PM Wahr Love GFZ model GEM-T1 none dynamic corr bending applied
GRGS (new) EIGEN C21/S21 due to PM IERS 2003 IERS 2003 FES 2004 none dynamic corr applied no bending
JPL JGM3 C21/S21 due to PM IERS 2003 IERS 2003 CSR ? FES2004 none dynamic corr bending applied
MIT EGM96 C21/S21 due to PM IERS 1992 Eanes Love none none none no dynamic corr bending applied
NGS GEM-T3 C21/S21 due to PM IERS 1992 Eanes Love none none none no dynamic corr bending applied
PDR (Repro) JGM3 constant C21/S21 IERS 2003 except step 2 IERS96 fixed pole CSR 3.0 none dynamic corr bending applied
SIO EGM96 C21/S21 due to PM IERS 1992 Eanes Love none none none no dynamic corr bending applied
14
COMPARISON OF AC SATELLITE DYNAMICS COMPARISON OF AC SATELLITE DYNAMICS COMPARISON OF AC SATELLITE DYNAMICS COMPARISON OF AC SATELLITE DYNAMICS COMPARISON OF AC SATELLITE DYNAMICS COMPARISON OF AC SATELLITE DYNAMICS COMPARISON OF AC SATELLITE DYNAMICS
ANALYSIS CENTER NUTATION EOPs SRP PARAMS VELOCITY BRKs ATTITUDE SHADOW ZONES EARTH ALBEDO
CODE IAU 2000 BuA ERPs D,Y,B scales B 1/rev every 12 hr constraints none EM umbra penumbra none
EMR IAU 1980 extrap. past 3d X,Y,Z scales stochastic none yaw rates estimated E umbra penumbra none
ESA IAU 2000 BuA ERPs D,Y,B scales B 1/rev none Along, Along 1/rev accelerations none EM umbra penumbra applied IR
GFZ IAU 2000 GFZ ERPs D,Y scales _at_ 1200 constraints yaw rates estimated E umbra penumbra none
GRGS (new) IAU 2000 C04 BuA ERPs D,Y,B scales D,B 1/rev none none EM umbra penumbra applied IR
JPL IAU 1980 BuB ERPs ? BuA X,Y,Z scales stochastic none nominal yaw rates used 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 none
NGS IAU 2000 IGS PM BuA UT1 D,Y,B scales B 1/rev _at_ 1200 constraints none delete eclipse data EM umbra penumbra none
PDR (Repro) IAU 2000 BuA ERPs D,Y,B scales B 1/rev every 12 hr constraints none EM umbra penumbra none
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 none
15
COMPARISON OF AC TROPOSPHERE MODELS COMPARISON OF AC TROPOSPHERE MODELS COMPARISON OF AC TROPOSPHERE MODELS COMPARISON OF AC TROPOSPHERE MODELS COMPARISON OF AC TROPOSPHERE MODELS COMPARISON OF AC TROPOSPHERE MODELS COMPARISON OF AC TROPOSPHERE MODELS
ANALYSIS CENTER METEO DATA ZENITH DELAY MAPPING FNCT GRAD MODEL ZENITH PARAMS GRAD PARAMS
CODE GPT Saastamoinen dry GMF dry none 2-hr contin. w/ GMF wet 24-hr NS EW continuous
EMR ECMWF 6-hr grids ECMWF dry wet NMF dry wet none 5-min stochastic ZTD 5-min stochastic
ESA GPT Saastamoinen dry GMF dry none 2-hr contin. w/ GMF wet none
GFZ GPT Saastamoinendry wet? GMF dry wet ? none 1-hr constants w/ GMF ? 24-hr NS EW constants
GRGS (new) ECMWF 6-hr grids ECMWF dry wet Guo dry wet none 2.4-hr contin. w/ Guo dry none
JPL none ? GPT dryhgt scale wet0.1 m NMF ? GMF none 5-min stochastic ZTD 5-min stochastic
MIT GPT Saastamoinen dry wet GMF dry wet none 2-hr contin. w/ GMF wet NS EW vary linearly
NGS GPT Saastamoinen dry wet GMF dry wet none 1-hr constants w/ GMF wet NS EW vary linearly
PDR (Repro) Berg (1948) Saastamoinen dry IMF dry w/ ECMWF z200 none 2-hr contin. w/ NMF wet 24-hr NS EW continuous
SIO GPT Saastamoinen dry wet GMF dry wet none 2-hr contin. w/ GMF wet NS EW vary linearly
16
Conclusions

• Despite huge progress by IGS since 1994, numerous
  small systematic errors remain in products
• see EGU 2008 presentation by J. Ray
• http//www.ngs.noaa.gov/IGSWorkshop2008/docs/igs-
  errs_egu08.pdf
• Applications to cutting-edge science are
  currently limited
• need to focus on identifying, understanding,
  mitigating errors
• should avoid rush to premature science
  conclusions
• must renew basic GNSS research efforts, not just
  in geophysical applications
• requires accurate knowledge of AC processing
  strategies
• Improvements will probably require better station
  installations (to reduce near-field multipath
  biases) analysis upgrades
• more research into field configuration effects
  badly needed
• need better leadership to popularize lessons
  learned
• need better cooperation coordination between
  analysts network

17
Recommendations

• For more robust products
• recruit new or improved IGU ACs more IGR clock
  ACs
• investigate improved near-RT predicted ERPs
• should IGS start (UT1 LOD) service ? (à la
  Senior et al., EGU08)
• Reject GGOS UAW actions for
• SINEX parameter naming extensions
• piecewise, continuous segment parameterization as
  SINEX standard
• Reject rigidly standardized AC procedures
  parameterizations
• would lead to stagnation end of progress
• would eliminate basis for multi-solution product
  combinations
• but ACs must agree on conventional choices use
  of modern models
• Instead, set up inter-service SINEX
  combinations WG
• investigate technique-specific systematic errors
• maintain SINEX format

18
Recommendations (contd)

• Updated AC summaries are required
• EMR 23 Jan 2002
• GFZ 27 Feb 2003
• JPL 13 Apr 2004
• SIO 31 Oct 2005
• (USNO 12 Sep 2006)
• Suggest suspending ACs with no updates by 30 Sep
  2008
• if processing summary is older than 2 years
• submissions would be rejected from IGS products
  after Sep 2008
• Rescind AC status if no updates by 31 Dec 2008
• would need to formally rejoin IGS ACs after Dec
  2008
• Or ask above ACs for effective alternative
  proposal

19
Backup Slides 1
20
A SURVEY OF SOME SYSTEMATIC ERRORS IN IGS PRODUCTS

• Clock jumps at day boundaries near-field
  multipath
• Position time series show N 1.04 cpy harmonics
• N/S distortions in IGS frames
• Earth rotation parameters smoothed filtered
• Spurious tidal lines in EOPs
• Orbit discontinuities have fortnightly
  variations

Jim Ray, NOAA/National Geodetic Survey
EGU 2008 General Assembly, Paper G4 A-01694,
Vienna, 15 April 2008
21
Context

• GPS errors are propagated formally but true data
  noise is unknown
• highly site-dependent not white noise
• e.g., variances of AC frame solutions differ by gt
  x 100
• dealt with by empirical rescaling of covariance
  matrix
• Evidence for systematic effects in IGS product
  covariances is well known
• e.g., user velocity errors are routinely inflated
  to account for temporal correlation of position
  errors
• but methods are purely empirical
• Objective Survey systematic errors in some IGS
  product values
• underlying causes mostly unknown or not confirmed

22
1) Day-boundary Clock Jumps

• clock bias accuracy is based on mean of code data
  per arc
• for 24-hr arc with code s 1 m, clock accuracy
  should be 120 ps
• can study local code biases via clock jumps at
  day boundaries (H-maser stations
  only)
• observed clock jumps vary hugely among
  stations
• 110 ps to gt1500 ps

• presumably caused mainly by local code multipath
  conditions, esp. in near-field of antenna

23
Near-field Multipath Mechanism

• expect largest longest-period MP errors when
  height H of antenna is small Elósegui et al.,
  1995
• may have special problems when H is near
  multiples of ?/4
• reflected RCP GPS signals enter from
  behind as LCP
• choke-ring design esp
• sensitive to L2 reflections
• from below Byun et al.
• 2002
• most IGS RF antennas
• mounted over flat surfaces!

24
Correlated Clock Position Effects ALGO

• ALGO day-boundary clock jumps increase in winters
• every winter ALGO also has large position
  anomalies
• IGS deletes outliers gt5 s
• implies common near-field multipath effect is
  likely
• (phase code)

25
Probably better to mount antennas away from
closereflecting surfaces! worse better

26
Other Hardware Choices Also Important
PIE1
AOA D/M_T antenna
ASH701945E_M antenna new cables
AOA firmware 3.2.32.8
3.2.32.11
Ashtech UZ-12 receiver
Rogue SNR-8000 receiver
Ashtech UZ-12 receiver

• receiver health, firmware, antenna model,
  cables also affect day-boundary clock jumps

27
2) Stacked/Smoothed Spectra of Site Residuals
(shifted)
(shifted)

• for 167 IGS sites with gt200 weekly points in
  1996.0 2006.0
• large annual semi-annual variations
• plus harmonics in all components at N (1.040
  0.008) cpy
• flicker noise spectra down to periods of few
  months

28
Position Harmonics Linked to GPS Year

• 1.040 0.008 cpy fundamental does not match any
  expected alias or geophysical frequency
• also not seen in VLBI, SLR, or fluid load spectra
• Closely matches GPS draconitic year
• rotation period of Sun w.r.t. GPS nodes (viewed
  from Earth)
• GPS nodal drift is -14.16 per year (due to
  Earths oblateness)
• period 351.4 day or frequency 1.039 cpy
• Two possible coupling mechanisms suggested
• 1) direct orbit modeling errors (e.g., related to
  eclipse periods planes)
• 2) alias of site position biases (e.g.,
  near-field phase multipath) due to beating of
  24-hr processing arc against 23.93-hr GPS repeat
  period
• useful distinguishing tests not yet made

29
3) N/S Distortions of IGS Frames

• N
• E
• U
• Weekly mean biases of IGS frames compared to
  long-term frame

30
IGS Frame Distortions

• N/S mean component of IGS weekly frames shows
  largest annual variation
• after weekly 7-parameter Helmert alignment
• also largest dispersion among ACs in N/S
  direction
• Not likely to be caused by annual
  inter-hemisphere fluid load cycle
• load signal should be largest in heights, not N/S
• Could possibly be related to along-track GPS
  orbit errors
• but no mechanism identified
• Likelier explanation possible neglected 2nd
  order ionospheric effect

31
4) High-frequency Smoothing of EOPs

• Day-boundary continuity constraint by some ACs
  smoothes filters EOP estimates near Nyquist
  limit

32
Filter/Smoother by Continuity Constraints

• Some ACs estimate EOPs ( others) by continuous
  linear segments
• attenuates power by factor 4 at Nyquist limit
• smoothes estimates
• filters certain phase components
• To avoid contaminating IGS combination, such EOP
  solutions rejected since January 2008 (wk 1460)
• but effects on other parameters probably still
  present
• Past high-frequency studies should be
  reconsidered
• Can use GFZ polar motion to estimate background,
  non-tidal, sub-daily variance 13.6 to 20.7 µas2

33
5) Aliased Tidal Peaks in EOP Discontinuities
Peaks in PM Differences Peaks in PM Differences Peaks in PM Differences Peaks in PM Differences
AC 14 d 9 d 7 d
EMR PM-x 14.20.2 9.350.09 7.180.05
EMR PM-y 14.10.2 9.6 9.00.1 7.160.05
GFZ PM-x 14.20.2 9.40.1 7.210.05
GFZ PM-y 14.20.2 9.6 8.90.1 7.140.05
JPL PM-x 14.20.2 9.40.1 7.230.05
JPL PM-y 14.20.2 9.20.1 7.260.05

• Spectra of polar motion day-boundary
  discontinuities show signatures of aliased O1,
  Q1, N2 tides unknown 7.2 d line

34
6) Day-boundary Orbit Discontinuities

• Orbit discontinuities between days show
  temporally correlated errors broad fortnightly
  spectral peak

35
Conclusions

• Despite huge progress by IGS since 1994, numerous
  small systematic errors remain in products
• Applications to cutting-edge science must
  recognize limitations
• need to focus on identifying, understanding,
  mitigating errors
• must renew basic GNSS research efforts, not just
  in geophysical applications
• should avoid rush to premature science
  conclusions
• Improvements will probably require better station
  installations (to reduce near-field multipath)
  analysis upgrades
• more research into field configuration effects
  badly needed
• need better leadership to popularize lessons
  learned
• need better cooperation coordination between
  analysts network