Brief introduction to gnss

Brief introduction to GNSS

• 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

U.S. – Global Positioning System (GPS)

• 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)

Satellites in MEO

• 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)

An International Association of Geodesy (IAG) Technique Service

• 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)

>3.6 million file downloads per month

• >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 …

Harmonic 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

Harmonic 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

GPS-sun geometry repeat period

• GPS-sun geometry repeat period

• “draconitic” year = 351.2 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

1) local multipath effect at stations

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

Simulated impact of sub-daily EOP tidal errors on IGS orbits

• 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 orbits @ 15 min intervals
• compute spectra of differences for each SV, stack & smooth
• compare spectral differences: input model errors vs. orbital response

Compare simulated EOP signatures with IGS Orbits

• 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

Offset peaks in ~14, ~9 and ~7 d bands due to simple daily sampling of input errors

• Offset peaks in ~14, ~9 and ~7 d bands due to simple daily sampling of input errors

Harmonics of 351 d pervasive in all IGS products

• 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

Harmonic 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

Finals now based on daily SINEX (terrestrial frame) integrations

• 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

Long-standing (since 2000) error in using AC SINEX rotations for AC Final orbit pre-alignment

• 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 @ 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=-0.016 (RMS=0.041) RY=0.015 (RMS=0.039) RZ=-0.004 (RMS=0.022)
• IGS RX & RY better than IGR for now
• IGS RZ now biased w.r.t. IGR, and has higher scatter

Scatter of all AC rotations decreased markedly starting at Wk 1702

• 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

Inter-AC agreement approaches ~1 cm

• 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?

w.r.t. IGS frame, IGR consistently more precise in all 3 components…

• 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

Ongoing efforts to address:

• 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-nonoperational/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

Longer data span (~1994 thru mid-2013)

• Longer data span (~1994 thru mid-2013)

• IG2 + operational prods thru 2013 -> 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 displacements @ 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

Daily GPS orbits & satellite clocks (in IGST?)

• 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

All IGS Final‐product Analysis Centers:

• 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

If current performance is any indication

• If current performance is any indication

• could approach 1 cm inter-AC agreement for much of IG2

Improvement in precision expected from:

• 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

Contribution to the ITRF2013 scale rate?

• 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

28/92 ( 30%) multi-technique sites have an uncalibrated radome

• 28/92 ( 30%) multi-technique sites have an uncalibrated radome

• nearly half (13/28) operated by JPL

including all co-location sites

• including all co-location sites
• systematic VLBI <-> SLR scale discrepancy

when GNSS co-located sites with uncalibrated radomes are excluded

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

core RF network

• core RF network
• optimal spatial distribution
• mitigate network effects in IGS SINEX combination (from X. Collilieux Ph.D. work)

Decrease in number of core RF stations

• 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 = ~0.22 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

Current IGS products are of high accuracy and precision

• 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 mm @ 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 as
• dLOD: ~10 s
• 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

Latest models, frames & methods to have largest impact since IG1

• 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) @ 15 min intervals
• GPS SV and station clocks (clock RINEX) @ 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

IG2 quality should approach current IGS prods

• 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]

M2 aliases into PM-x and PM-y; O1 aliases into LOD

• M2 aliases into PM-x and PM-y; O1 aliases into LOD

• 1st draconitic harmonic enters PM-x & LOD

Simulated impact of sub-daily EOP tidal errors on IGS orbits

• 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 orbits @ 15 min intervals
• compute spectra of differences for each SV, stack & smooth
• compare spectral differences: input model errors vs. orbital response

Simulated impact of sub-daily EOP tidal errors on IGS orbits

• 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 orbits @ 15 min intervals
• compute spectra of differences for each SV, stack & smooth
• compare spectral differences: input model errors vs. orbital response

Aliasing of sub-daily errors responsible for some harmonics of 351 d

• Aliasing of sub-daily errors responsible for some harmonics of 351 d

• peaks at other harmonics likely caused by other errors

at diurnal period, EOP model errors absorbed into orbits, esp cross- & along-track

• at diurnal period, EOP model errors absorbed into orbits, esp cross- & along-track

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

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

background power is lower

• background power is lower

• errors absorbed in all three components

same near 4 cpd

• same near 4 cpd