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
2013 NASA/GSFC Summer Seminar Series June 2013

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 system—5 geost. Earth orbit (GEO), 5 incl. geosync. orbit (IGSO)
plus global satellite system—30 medium Earth orbit (MEO)
2013 NASA/GSFC Summer Seminar Series June 2013

3. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
How a GNSS Works
Source: boeing.com GPS IIF
Satellites in MEO
vehicle altitudes ~20,000 km
Transmit L-band radio signals (e.g., L1,L2,L5)
GPS: carrier waves modulated by C/A and P codes; other GNSS are similar
Ground antenna+receiver pairs track transmit signals
geodetic grade equip collects raw observations for precise positioning, navigation and timing applications
Service supporting high- precision GNSS apps?
International GNSS Service (IGS)
GODE
animation source: wikipedia.org
Source: unavco.org
Source: unavco.org
2013 NASA/GSFC Summer Seminar Series June 2013

4. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
What is the IGS?
An International Association of Geodesy (IAG) Technique Service
Voluntary federation of >200 worldwide agencies aimed at providing the highest quality GNSS data and products in support of:
Earth science research and education
other high-precision applications
Organization:
Governing Board (Chair, U. Hugentobler)
Central Bureau (sponsored by NASA, managed by JPL)
Tracking Network (Coordinator, R. Khachikyan)
Data Centers (Chair, C. Noll)
Infrastructure Committee (Chair, I. Romero)
Analysis Centers (ACs) & Analysis Center Coordinator (ACC)
Working Groups, Pilot Projects, Product Coordinators
Associate Members & representatives from other IAG Services
Other IAG Technique Services?
ILRS (SLR), IVS (VLBI) and IDS (DORIS)
(more details at igs.org)
2013 NASA/GSFC Summer Seminar Series June 2013

5. IGS GNSS Tracking Network
2013 NASA/GSFC Summer Seminar Series June 2013

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

7. Outline for How IGS Core Products are Derived
a priori datum (IGS08/IGb08)
VLBI
IGS RF WG Chair (IGN)
B. Garayt, A. Duret and P. Rebischung
International Terrestrial Reference Frame (ITRF)
Analysis Center (AC) Products
Latest IERS and IGS conventions generally adopted
Adjust all obs model parameters
Ultra-rapid and Rapid tightly constrained to a priori datum
Finals uses no-net-rotation (NNR) constraint over a priori coordinates of core set of RF stations
Finals realizes AC daily quasi-instantaneous “fiducial-free” frame w.r.t. a priori datum
AC SNX files (Finals only)
SLR
IGS TRF prods
Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS).
Combined daily station positions and ERPs, stacked for long-term estimates and RF maintenance
satellite orbits & clocks (SP3), receiver clocks (CLK), tropo delays (TRO), and polar motion & LOD (ERP) + daily station positions (SNX)
DORIS
AC SINEX rotations
IGS AC Coordinator (NOAA/NGS)
J. Griffiths and K. Choi
- weighted average of AC products
- Rapid and Final clocks are aligned to IGS timescale
AC SP3, CLK & ERP files
Combined Orbits, Clocks, and ERPs (Rapid & Ultra-rapid only)
Main analysis difference between IGU/IGR & IGS is constraints on a priori RF station positions at AC level
Final (IGS) orbits (GPS, GLO), clocks (SV, Rx), ERPs, and TRF prods
Rapid (IGR) orbits (GPS), clocks (SV, Rx), ERPs
Ultra-rapid (IGU) orbits (GPS, GLO), clocks (SV), ERPs
IGS Core Products
2013 NASA/GSFC Summer Seminar Series June 2013

8. Current Analysis Centers
Name
Final (IGS)
Rapid (IGR)
Ultra (IGU)
cod
Centre for Orbit Determination in Europe, Bern, Switzerland 
emr
Natural Resources Canada (NRCan), Ottawa, Canada
esa
European Space Agency, European Space Operations Center (ESOC), Darmstadt, Germany
gfz
GeoForschungsZentrum, Potsdam, Germany
gop
Geodetic Observatory Pecny, Czech Republic
grg
CNES Groupe de Recherche de Geodesie Spatiale (GRGS), Toulouse, France
jpl
Jet Propulsion Laboratory, Pasadena, USA
ngs
National Oceanic and Atmospheric Administration (NOAA), Silver Spring, USA
sio
Scripps Institution of Oceanography, La Jolla, USA
mit
Massachusetts Institute of Technology, Boston, USA
usn
U.S. Naval Observatory, Washington, D.C., USA
whu
Wuhan University, Wuhan, China
2013 NASA/GSFC Summer Seminar Series June 2013

9. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Popularity of Core Products - download NASA/CDDIS (06/2010 thru 06/2012) -
>3.6 million file downloads per month
5 biggest users of CDDIS/IGS files:
U.S. 64.3%, Indonesia 19.3%, Canada 1.64%, Sweden 1.57%, Belgium 1.16%
Details 1/2012 thru 6/2012 …
Product
GNSS
Total Hits
SP3 (%)
ERP (%)
CLK (%)
SNX (%)
SUM (%)
Ultra-rapid
GPS
11,711,506
( 4 * 2,927,877 daily)
93.7
3.1
3.2
Final (IGS)
1,359,656
60.7
6.8
24.8
5.8
2.0
Rapid
887,986
65.6
8.7
16.9
6.4
Final (IGL)
GLO
225,515
99.1
0.3
0.6
(IGV)
GPS & GLO
223,562
95.0
5.0
Courtesy: C. Noll (NASA/CDDIS)
2013 NASA/GSFC Summer Seminar Series June 2013

10. Core Product Accuracies
Series
Product Types
Accuracies
Output Intervals
Ultra-Rapid
(predicted half)
● GPS orbits
~ 5 cm (1D)
15 min
● GLONASS orbits
~10 cm (1D)
● GPS SV clocks
~3 ns RMS / ~1.5 ns Sdev
● ERPs: PM + dLOD
~250 µas / ~50 µs
6 hr
(observed half)
~ 3 cm (1D)
~5 cm (1D)
~150 ps RMS / ~50 ps Sdev
<50 µas / ~10 µs
Rapid
~2.5 cm (1D)
● GPS SV & station clocks
~75 ps RMS / ~25 ps Sdev
5 min
<40 µas / ~10 µs
daily
Final
<2.5 cm (1D)
<5 cm (1D)
~75 ps RMS / ~20 ps SDev
30 s (SVs) + 5 min
<30 µas / ~10 µs
● Terrestrial frames
~2.5 mm N&E / ~6 mm U
5 cm (1D) orbit error = ~0.4 cm (3D) position error over 1000 km baseline (Beser & Parkinson, 1982)
2013 NASA/GSFC Summer Seminar Series June 2013

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
2013 NASA/GSFC Summer Seminar Series June 2013

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
2013 NASA/GSFC Summer Seminar Series June 2013

13. Harmonic Errors: Background (1/2)
GPS-sun geometry repeat period
“draconitic” year = d
1st & 2nd harmonics overlay seasonal signals
IGS station coordinates (2006, 2008)
in all dNEU components
up to at least 6th harmonic
later found in all parameters:
“geocenter” variations
polar motion rates (esp 5th & 7th)
LOD (esp 6th)
orbit discontinuities (esp 3rd)
strong fortnightly signals also common dE dN
% of GPS Stations dU
Frequency (cycles per year)
(figure from X. Collilieux et al., 2011)
2013 NASA/GSFC Summer Seminar Series June 2013

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 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 GPS altitude
IERS model is known to have visible errors, which could reach the 10 to 20% level
2013 NASA/GSFC Summer Seminar Series June 2013

15. Harmonic Errors: Sub-daily Alias and Draconitic (1/3)
Simulated impact of sub-daily EOP tidal errors on IGS orbits
generated “fake” model by changing admittances by up to 20%—assumed errors derived from comparing IERS model to test model from R. Ray (NASA/GSFC)
process ~3 years of GPS orbits with IERS & “fake” models
difference conventional & EOP-test 15 min intervals
compute spectra of differences for each SV, stack & smooth
compare spectral differences: input model errors vs. orbital response
Frequency (cycles per day)
Power Density (mm2 / cpd)
long-period errors absorbed mostly by ERPs, not orbits
short- period errors go into orbits
2013 NASA/GSFC Summer Seminar Series June 2013

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 with BERNE (6+9) SRP orbit model
parameterize fit as plus 3 SRPs per SV component
fit 96 SP3 orbit positions for each SV as pseudo-observations for Day A
propagate fit forward to 23:52:30 for Day A
repeat for Day B & propagate backwards to 23:52:30 of day before
compute IGS orbit jumps at 23:52:30
SIMULATED EOP SIGNATURES
difference conventional & EOP-test orbits at 23:45:00 only
Compute IGS orbit jumps over ~5.6 yr, test orbits over ~2.8 yr
2013 NASA/GSFC Summer Seminar Series June 2013

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/√3 cm = ~5.8 cm (1D) annual errors
Power Density (mm2 / cpd)
~1.0 cm white noise floor
Frequency (cycles per day)
2013 NASA/GSFC Summer Seminar Series June 2013

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 -> resonance with GPS orbital period
Compared IGS orbit discontinuities to EOP-test orbit differences at 23:45:00
24 h sampling causes sub-daily EOP tide errors to alias at ~14, ~9 and ~7 d bands -> peaks offset from expected periods
peaks at several (mostly odd) harmonics of 351 d
IERS diurnal & semi-diurnal tide model errors are probably main source for pervasive sub-daily alias and several draconitic errors in IGS orbits
2013 NASA/GSFC Summer Seminar Series June 2013

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
2013 NASA/GSFC Summer Seminar Series June 2013

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 SINEX—AC orbits pre-aligned using weekly-averaged AC SINEX rotations and daily AC PM-x and PM-y deviations from combined ERPs
daily AC SINEX rotations now used to pre-align AC orbits—ERPs rots. no longer used
higher scatter in combined orbits, ERPs and station positions
but less than sqrt(7) expected for random error
and smaller than other existing systematic errors
did not resolve rotational instability of Finals
mitigates impacts of unmodeled non-tidal atmospheric loading effects on IGS products
increased temporal resolution in station position time series
needed for continued study of non-tidal crustal loading models and impacts to IGS products
since exposed previously unknown sensitivity of GPS-derived ERP estimates to GLONASS orbit mismodeling
sensitivity is time-correlated with GLONASS eclipse seasons
CODE/ESA currently studying this effect
2013 NASA/GSFC Summer Seminar Series June 2013

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 equator) in RMS
but systematic errors remain in RZ—clear ~60d signal (harmonic errors in AC clocks?)
Note: since Wk 1650, Final PPP using IGR (acc.igs.org/index_igsacc_ppp.html) gives:
RX= (RMS=0.041) RY=0.015 (RMS=0.039) RZ= (RMS=0.022)
IGS RX & RY better than IGR for now
IGS RZ now biased w.r.t. IGR, and has higher scatter
2013 NASA/GSFC Summer Seminar Series June 2013

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 daily SNX
primarily from fixing combo software
Since revealed ESA self-consistency issues
poorly aligned to IGS frame
residual distortion between TRF and their orbits—see RX & RY
corrected on Wk 1732
Now RY of IGR (violet) is biased
ESA consistency issues in IGR
IGS05
IGx08
fixed AC orbit pre-alignment
ESA fixed TRF issue
1 mas = ~ 13 GPS altitude
2013 NASA/GSFC Summer Seminar Series June 2013

23. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
WRMS of AC Orbit Residuals Since IG1 - AC solutions minus IGS Final , after pre-alignment -
IGx08
IGS05
fixed AC orbit pre- alignment
ESA fixed TRF issue
Inter-AC agreement approaches ~1 cm
switch to daily TRFs seems to have improved AC agreement for now
ESA dominates; EMR and JPL improved slightly to ~18 mm WRMS since IGx08
IGS Final has ~4 mm WRMS difference with IGR—which prods are more precise?
2013 NASA/GSFC Summer Seminar Series June 2013

24. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
IGS vs IGR – More From PPP using Final Products - Mean station RMS after Helmert transformation to IGS frame -
Time [GPS Wk; April 22, 2012 thru May 12, 2013]
w.r.t. IGS frame, IGR consistently more precise in all 3 components…
probably due to combination of errors in AC Final clocks
but could be from difference between IGR and IGS analysis approach
2013 NASA/GSFC Summer Seminar Series June 2013

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 (
effects are negligible on secular frame
loading can be modeled at stacking level with equivalent results
time variations of low-degree terms in geopotential field
impacts on orbits: ~7 mm RMS (Melachroinos et al., AGU 2012)
effect on ~annual signal in IGS station position time series?
conventional model under development
tidal displacements at stations
ocean pole tide (JPL and EMR) & S1-S2 tidal atm loading model (pending update)
improved satellite attitude modeling (mostly benefits satellite clocks)
modeling higher-order ionosphere effects
most ACs working to implement 2nd-order correction
Unclear which of these developments will be ready for IG2
2013 NASA/GSFC Summer Seminar Series June 2013

26. IGS 2nd Reprocessing and ITRF2013
2013 NASA/GSFC Summer Seminar Series June 2013

27. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
How will IG2 Differ from IG1 & Current Operations? - more details at -
Longer data span (~1994 thru mid-2013)
IG2 + operational prods thru > IGS contribution to ITRF2013
Updated models, frames & methodologies
IERS 2010 Conventions generally adopted
NGA stations data w/ new antenna calibrations (for improved ITRF <-> WGS 84 tie)?
IGb08.SNX/igs08.atx framework (improved a priori datum)
combined products based on AC 1d TRF integrations
with corrected approach for applying AC SINEX rotations to AC orbits
no non-tidal atmospheric loading at obs level
2nd-order iono corrections & S1-S2 atm. loading stations
Earth-reflected radiation pressure (albedo) modeling (most ACs still to adopt)
reduce ~2.5 cm radial bias w.r.t. SLR [e.g. Urschl et al., 2007; Zeibart et al., 2007]
plus antenna thrusting [e.g., Rodriguez-Solano et al., 2009, 2011, 2012]
satellite attitude modeling by all clock ACs
Sub-daily alias and draconitic errors will remain
Final preps and initial processing by late June? Finalize in November?
Expect to deliver SINEX files for ITRF2013 by early 2014
2013 NASA/GSFC Summer Seminar Series June 2013

28. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Expected AC and IG2 Products - more details at -
Daily GPS orbits & satellite clocks (in IGST?)
15-minute intervals (SP3c format)
Daily satellite & tracking station clocks (in IGST?)
5-minute intervals (clock RINEX format)
Daily Earth rotation parameters (ERPs)
from SINEX & classic orbit combinations (IGS erp format)
x & y coordinates of pole
rate-of-change of x & y pole coordinates (should not be used due to sensitivity to sub-daily tidal errors)
excess length-of-day (LOD)
Weekly (IG2 only) & daily terrestrial coordinate frames with ERPs
with full variance-covariance matrix (SINEX format)
May also provide (TBD)
daily GLONASS orbits & satellite clocks
30-second GPS clocks (in IGST?)
ionosphere maps, tropospheric zenith delay estimates
new bias products
2013 NASA/GSFC Summer Seminar Series June 2013

29. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Who will Contribute to IG2? - more details at -
All IGS Final‐product Analysis Centers:
CODE/AIUB – Switzerland – JPL – USA
EMR/NRCan – Canada – MIT – USA
ESA/ESOC – Germany – NGS/NOAA – USA
CNES/GRGS – Toulouse, France – SIO – USA
GFZ – Potsdam, Germany
Plus 1 reprocessing Center
ULR – University of La Rochelle TIGA (tide gauges), France
PDR – Potsdam-Dresden Reprocessing group (in IG1, but will not be in IG2)
Plus 1 Center contributing to TRF only:
GFZ TIGA – Potsdam, Germany
2013 NASA/GSFC Summer Seminar Series June 2013

30. Expected Performance of IG2? - WRMS of AC repro1 orbits wrt IG1 -
IGS05
Large scatter for some ACs in early IG1—expected to be improved in IG2 contributions
By late 2007, inter-AC agreement bi-modal, approaching ~1.5 cm
Time [GPS Wk; Dec. 26, 1993 thru Nov. 11, 2011]
Courtesy of G. Gendt (GFZ Potsdam)
2013 NASA/GSFC Summer Seminar Series June 2013

31. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
WRMS of AC Orbit Residuals Since IG1 - AC solutions minus IGS Final , after pre-alignment -
inter-AC agreement reaches ~1.0 cm
IGx08
IGS05
fixed AC orbit pre- alignment
ESA fixed TRF issue
If current performance is any indication
could approach 1 cm inter-AC agreement for much of IG2
2013 NASA/GSFC Summer Seminar Series June 2013

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)
2013 NASA/GSFC Summer Seminar Series June 2013

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
> 1 cm errors revealed at stations with uncalibrated radomes
few mm errors likely at stations with “converted” antenna calibrations
will cause trouble in use of local ties for ITRF2013 colocation sites
consider to exclude in next ITRF
Courtesy: P. Rebischung (IGN/LAREG)
2013 NASA/GSFC Summer Seminar Series June 2013

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

35. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Uncalibrated Radomes
28/92 ( 30%) multi-technique sites have an uncalibrated radome
nearly half (13/28) operated by JPL
2013 NASA/GSFC Summer Seminar Series June 2013

36. Uncalibrated Radomes: Impact on ITRF (1/2)
including all co-location sites
systematic VLBI <-> SLR scale discrepancy
Courtesy: Z. Altamimi (IGN/LAREG)
2013 NASA/GSFC Summer Seminar Series June 2013

37. Uncalibrated Radomes: Impact on ITRF (1/2)
when GNSS co-located sites with uncalibrated radomes are excluded
VLBI <-> SLR scale difference amplified by 0.2 ppb (network effect + calibration errors)
Courtesy: Z. Altamimi (IGN/LAREG)
2013 NASA/GSFC Summer Seminar Series June 2013

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)
2013 NASA/GSFC Summer Seminar Series June 2013

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 -> IGb08 update on 7 Oct 2012
recovered sites with linear velocities before/after positional discontinuity
Overall (linear) rate of loss = ~0.13 sta/wk since end date of ITRF2008
<IGb08: rate = ~0.16 sta/wk
>IGb08: rate = ~0.22 sta/wk
Today
best case: 71 core stations
actual: ~54
Need for thorough study of impacts on stability of IGS reference frame
Station operators should limit disruptions, esp. at co-location sites
100% data availability
actual data availability
Courtesy: K. Choi (NOAA/NGS)
2013 NASA/GSFC Summer Seminar Series June 2013

40. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Summary
2013 NASA/GSFC Summer Seminar Series June 2013

41. Conclusions: IGS Errors
Current IGS products are of high accuracy and precision
GPS orbits
overall <2.5 cm (1D)
errors now dominated by Z- frame rotation scatter and possibly AC clock errors
X- & Y- frame rotations of Final orbits improved by ~0.035 mas (~4.4 GPS)
RMS scatter of AC orbits up to 1.6 cm
sub-daily alias and draconitic errors from IERS diurnal/semi-diurnal tides
ERPs
PM-x & PM-y: <30 mas
dLOD: ~10 ms
terrestrial frames
~2 mm N&E
~5 mm U
But Rapid products still slightly more precise than Finals
discrepancies have been reduced, but needs to be further study
may be due to combination of errors in AC Final clocks?
Because IGS products are of high quality, can measure subtle signals
2013 NASA/GSFC Summer Seminar Series June 2013

42. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Conclusions: Repro2
Latest models, frames & methods to have largest impact since IG1
IERS 2010 Conventions
IGb08/igs08.atx framework
Earth-reflected radiation pressure (albedo) modeling
sub-daily alias & draconitic errors will remain
To result in full history of IG2 products (1994 to mid-2013)
daily products:
GPS orbits & SV clocks 15 min intervals
GPS SV and station clocks (clock 5 min intervals
Earth Rotation Parameters (IGS ERP)
terrestrial coordinate frames (IERS SINEX)
expected delivery for ITRF2013 -> early 2014
And possibly some ancillary products
GLONASS orbits & clocks
30-second SV & station clocks
bias products
2013 NASA/GSFC Summer Seminar Series June 2013

43. Conclusions: More Repro2 and Other Challenges
IG2 quality should approach current IGS prods
quality for later (~2000 -> present) IG2 products will be best
early IG2 probably better than IG1 equivalents, but not as good as later IG2
Ongoing Challenges
uncalibrated radomes at co-location sites
one recently available at SMST!! (co-located w/ SLR; unavail. for ITRF2008)
positional discontinuities at RF stations
50% of IGS stations have discontinuities: harmful in co-location sites
GNSS/IGS is the link between the 3 other techniques in ITRF
loss of core RF stations
anthropogenic site disturbances (incl. many equip. changes)
data loss, and earthquakes & other physical processes
known biases and other systematic errors
harmonic and sub-daily alias errors in all IGS products
site-specific errors [e.g., Wetzell observations by Steigenberger et al., REFAG2010]
2013 NASA/GSFC Summer Seminar Series June 2013

44. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Questions?
2013 NASA/GSFC Summer Seminar Series June 2013

45. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Extra Slides
2013 NASA/GSFC Summer Seminar Series June 2013

46. 2013 NASA/GSFC Summer Seminar Series -- 12 June 2013
Spectrum of Daily ERP Differences due to sub-daily EOP Tidal Model “Errors”
M2 aliases into PM-x and PM-y; O1 aliases into LOD
1st draconitic harmonic enters PM-x & LOD
Frequency (cycles per day)
Power Density (mas2 or s2/ cpd)
2013 NASA/GSFC Summer Seminar Series June 2013

47. Harmonic Errors: Sub-daily Alias and Draconitic
Simulated impact of sub-daily EOP tidal errors on IGS orbits
generated “fake” model by changing admittances by up to 20%—assumed errors derived from comparing IERS model to test model from R. Ray (NASA/GSFC)
process ~3 years of GPS orbits with IERS & “fake” models
difference conventional & EOP-test 15 min intervals
compute spectra of differences for each SV, stack & smooth
compare spectral differences: input model errors vs. orbital response
Frequency (cycles per day)
Power Density (mm2 / cpd)
2013 NASA/GSFC Summer Seminar Series June 2013

48. Harmonic Errors: Sub-daily Alias and Draconitic
Simulated impact of sub-daily EOP tidal errors on IGS orbits
generated “fake” model by changing admittances by up to 20%—assumed errors derived from comparing IERS model to test model from R. Ray (NASA/GSFC)
process ~3 years of GPS orbits with IERS & “fake” models
difference conventional & EOP-test 15 min intervals
compute spectra of differences for each SV, stack & smooth
compare spectral differences: input model errors vs. orbital response
Frequency (cycles per day)
Power Density (mm2 / cpd)
bump in background power – resonance of ~2 cpd sub-daily tide errors and GPS orbital period?
2013 NASA/GSFC Summer Seminar Series June 2013

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/√3 cm = ~5.8 cm (1D) annual errors
1st, 3rd, 4th, & 10th harmonics also caused by sub-daily EOP errors
Power Density (mm2 / cpd)
~1.0 cm white noise floor
Frequency (cycles per day)
2013 NASA/GSFC Summer Seminar Series June 2013

50. Spectra of Orbital Responses to sub-daily EOP Errors – Near 1 cpd
at diurnal period, EOP model errors absorbed into orbits, esp cross- & along-track
only 2 sub-daily tidal lines excited above background orbit noise
unexpected peak in cross-track – probably a beat effect
Power Density (mm2 / cpd)
Frequency (cycles per day)
2013 NASA/GSFC Summer Seminar Series June 2013 05

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

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

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

54. COMPARISON OF EXPECTED AC DATA USAGE
ANALYSIS CENTER
SYSTEM
OBS TYPE
ORBIT DATA ARC LENGTH
DATA RATE
ELEVATION CUTOFF
ELEVATION INVERSE WGTS
CODE
GPS + GLO
DbDiff (weak redundant)
24 h
3 min
3 deg
1/cos2(z)
EMR
UnDiff
5 min
10 deg
none
ESA
1/sin2 (e)
GFZ (& GTZ)
GPS + ?GLO?
?? 24 h ??
7 deg
1/2sin(e)
for e < 30 deg
GRG h
15 min
JPL
GPS
MIT
(SRPs constr.— 9d noise model)
2 min
a2 + (b2/sin2(e))
a,b from site residuals
NGS
DbDiff (redundant)
30 s
[5 + (2/sin(e)) cm]2
SIO
ULR
2013 NASA/GSFC Summer Seminar Series June 2013

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

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

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

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

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 SINEX files using CATREF
NGS CORS+global SINEX
Realizes NGS-derived secular frame
CORS in NGS-derived global frame
Align NGS-derived frame to IGS2013
Obtain NAD 83 coords via successive
14-parameter transformations
Load NAD 83 (2013) coords into NGSIDB
Adjust passive network to NAD 83 (2013)
NAD 83 (2013) Coordinates
2013 NASA/GSFC Summer Seminar Series June 2013
NAD 83 (2013)