overview of igs products & analysis center modeling jim ray, noaa/ngs jake griffiths, noaa/ngs...
TRANSCRIPT
OVERVIEW OF IGS PRODUCTS & ANALYSIS CENTER MODELING
Jim Ray, NOAA/NGS
Jake Griffiths, NOAA/NGS
• 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
IGS 2008 Workshop, Miami Beach, 2 June 2008
SUMMARY OF IGS CORE PRODUCTSSUMMARY OF IGS CORE PRODUCTSPRODUCT SUITES
# ACs
CURRENT
PRECISION
LATENCY UPDATES SAMPLE INTERVAL
QUALITY
ASSESSMENT
Ultra-Rapid (predicted)
• orbits
• SV clocks
• ERPs
7 (2)*
4
7 (2)*
< 5 cm
~5 ns
< ~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
< ~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
Predicted IGU Orbit WRMS
• WRMS of IGU orbit predictions have improved to <5 cm RMS
IGU Orbits (1st 6 hr of predictions) wrt IGR Orbits
• PRN29 (IIA) decommissioned• GOP solutions improved
Scale & Rotations of Predicted IGU Orbits
• Z rotations (UT1 prediction error) reach 1 mas level• equivalent to equatorial shift of 12.9 cm at GPS altitude
(shifted)
(shifted)
IGU Orbits (24 h of predictions) wrt IGR Orbits
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 33emu 49| 7 0 0 486 38 -60 .03 84 44 21esu 95| 4 5 -2 -396 687 -72 .13 77 77 29gfu 65| 1 -2 -2 302 -21 127 -.34 77 78 29gou 82| -5 -4 -1 260 334 48 -.35 89 78 31siu 62| 0 17 -33 -221 1068 730 .02 130 131 71usu 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– accuracy now limited by ERP predictions, mostly
• may also apply to IGR orbits (but less severe)
• IGU combined clocks are very poor– clock predictions no better than BRDC– not enough clock ACs
• even IGR clocks sometimes weak when some ACs miss
COMPARISON OF AC DATA USAGECOMPARISON OF AC DATA USAGEANALYSIS 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 < 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)) cm]2
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
effect of 15-deg cutoff
COMPARISON OF AC TIDAL MODELSCOMPARISON OF AC TIDAL MODELSANALYSIS 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
Aliased Tidal Peaks in PM Discontinuities
• Spectra of polar motion day-boundary discontinuities show signatures of aliased O1, Q1, & N2 tides + unknown 7.2 d line
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
• VLBI (1-hr) UT1 VLBI (1-hr) UT1 residuals residuals
– white over full frequency range
• 14.19-d periodic14.19-d periodic– treated as GPS artifact– amplitude varies between
5 & 11 μs – phase varies linearly w/ time
due to changing period
• GPS LOD residualsGPS LOD residuals– approximately white– with small peak at 13.7 d– possible difference in
a priori tidal models wrt VLBI
• Gauss-Markov values Gauss-Markov values for GPS LOD biasesfor GPS LOD biases
– peak-to-peak range = ± 40 μs
– RMS = 9 μs
EMR analysis upgrade
Kalman Filter of VLBI UT1 + GPS LOD(Senior, Kouba, Ray – EGU 2008)
14.12 d signal in IGS & C04
probably due to mix of
GPS errors
LODS – (AAM+OAM) spectra
Fortnightly Band – Spurious IGS LOD(Senior, Kouba, Ray – EGU 2008)
Day-boundary Orbit Discontinuities
• Orbit discontinuities between days show temporally correlated errors & broad fortnightly spectral peak
• From Griffiths & Ray (AGU 2007)
COMPARISON OF AC GRAVITY FORCE MODELSCOMPARISON OF AC GRAVITY FORCE MODELSANALYSIS 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
COMPARISON OF AC SATELLITE DYNAMICSCOMPARISON OF AC SATELLITE DYNAMICSANALYSIS 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 E+M: 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 E+M: umbra & penumbra
applied + IR
GFZ IAU 2000; GFZ ERPs
D,Y scales @ 12:00 + 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 E+M: umbra & penumbra
applied + IR
JPL IAU 1980; BuB ERPs → BuA
X,Y,Z scales stochastic
none nominal yaw rates used
E+M: 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
E+M: umbra & penumbra
none
NGS IAU 2000; IGS PM; BuA UT1
D,Y,B scales; B 1/rev
@ 12:00 + constraints
none; delete eclipse data
E+M: umbra & penumbra
none
PDR (Repro) IAU 2000; BuA ERPs
D,Y,B scales; B 1/rev
every 12 hr + constraints
none E+M: 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
E+M: umbra & penumbra
none
COMPARISON OF AC TROPOSPHERE MODELSCOMPARISON OF AC TROPOSPHERE MODELSANALYSIS 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
dry=hgt scale
wet=0.1 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
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
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
Recommendations (cont’d)
• 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
Backup Slides 1
A SURVEY OF SOME SYSTEMATIC ERRORS IN IGS PRODUCTS
Jim Ray, NOAA/National Geodetic Survey
• 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
EGU 2008 General Assembly, Paper G4 #A-01694, Vienna, 15 April 2008
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 > 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
1) Day-boundary Clock Jumps• clock bias accuracy is based on
mean of code data per arc
• for 24-hr arc with code σ = 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 >1500 ps
• presumably caused mainly by local code multipath conditions, esp. in near-field of antenna
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 λ/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!
Correlated Clock & Position Effects: ALGO
• ALGO day-boundary clock jumps increase in winters
• every winter ALGO also has large position anomalies
– IGS deletes outliers >5 σ
• implies common near-field multipath effect is likely(phase & code)
Probably better to mount antennas away from closereflecting surfaces!
worse better
Other Hardware Choices Also Important
• receiver health, firmware, antenna model, & cables also affect day-boundary clock jumps
Rogue SNR-8000 receiver Ashtech UZ-12 receiver
Ashtech UZ-12 receiver
AOA firmware 3.2.32.8 3.2.32.11
AOA D/M_Tantenna
ASH701945E_Mantenna+new cables
PIE1
2) Stacked/Smoothed Spectra of Site Residuals
• for 167 IGS sites with >200 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
(shifted)
(shifted)
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 Earth’s 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
3) N/S Distortions of IGS Frames
N
E
U
• Weekly mean biases of IGS frames compared to long-term frame
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
4) High-frequency Smoothing of EOPs
• Day-boundary continuity constraint by some ACs smoothes & filters EOP estimates near Nyquist limit
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
5) Aliased Tidal Peaks in EOP Discontinuities
• Spectra of polar motion day-boundary discontinuities show signatures of aliased O1, Q1, & N2 tides + unknown 7.2 d line
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
6) Day-boundary Orbit Discontinuities
• Orbit discontinuities between days show temporally correlated errors & broad fortnightly spectral peak
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