# Presentation: 750553

## 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**

## * indicates AC contributions that are weaker than others

| 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)* | 

< 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

**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****   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**
**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**

## Slide 6

| 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 < 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

**effect of 15-deg cutoff**

## Slide 8

| 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 |

## 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±  | 14.20.2  | 9.350.09  | 7.180.05  |
| EMR PM-y±  | 14.10.2  | 9.6 & 9.00.1  | 7.160.05  |
| GFZ PM-x±  | 14.20.2  | 9.40.1  | 7.210.05  |
| GFZ PM-y±  | 14.20.2  | 9.6 & 8.90.1  | 7.140.05  |
| JPL PM-x±  | 14.20.2  | 9.40.1  | 7.230.05  |
| JPL PM-y±  | 14.20.2  | 9.20.1  | 7.260.05 |

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

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

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

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

**Gauss-Markov values for 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)**

## Fortnightly Band – Spurious IGS 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)**

## Slide 13

| 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 |

## Slide 14

| 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 | 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 |

## Slide 15

| 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 | 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

**Near-field Multipath Mechanism**

**expect largest & longest-period MP errors when height H of antenna is small [El****ósegui 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 closereflecting 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_T
- antenna

- ASH701945E_M
- antenna
- +
- 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 2****nd**** 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 ****μ****as****2**

## 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±  | 14.20.2  | 9.350.09  | 7.180.05  |
| EMR PM-y±  | 14.10.2  | 9.6 & 9.00.1  | 7.160.05  |
| GFZ PM-x±  | 14.20.2  | 9.40.1  | 7.210.05  |
| GFZ PM-y±  | 14.20.2  | 9.6 & 8.90.1  | 7.140.05  |
| JPL PM-x±  | 14.20.2  | 9.40.1  | 7.230.05  |
| JPL PM-y±  | 14.20.2  | 9.20.1  | 7.260.05 |

## 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**