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Processing of groundbased GNSS Processing of groundbased GNSS
data to produce near realtime (NRT) tropospheric zenith data to produce near realtime (NRT) tropospheric zenith
path delays (ZTD)path delays (ZTD)
Jan Douša Jan Douša (jan.dousa([email protected]@pecny.cz))
Geodetic Observatory PecnGeodetic Observatory Pecný,ý,
Research Institute of Geodesy, Topography and Cartography,Research Institute of Geodesy, Topography and Cartography,
The The Czech RepublicCzech Republic
EGVAP Workshop EGVAP Workshop November 6, 2008November 6, 2008
EGVAP Workshop, November 6, 2008 2
OutlineOutline
• introduction to GNSSintroduction to GNSS
• the concept of GNSS contribution to meteorologythe concept of GNSS contribution to meteorology
• different GNSS processing approaches (PPP x Network)different GNSS processing approaches (PPP x Network)
• general aspects of the network processing (in brief)general aspects of the network processing (in brief)
• the requirements and features of near realtime (NRT) solutionthe requirements and features of near realtime (NRT) solution
• some some results and comparisonsresults and comparisons
• historhistorical viewical view
EGVAP Workshop, November 6, 2008 3
GNSS Global Navigation Satellite SystemsGNSS Global Navigation Satellite Systems
• GPS NAVSTARGPS NAVSTAR – – NNAVAVigation igation SSystem using ystem using TTiming iming AAnd nd RRanginganging
The United States’ military serviceThe United States’ military service
((1972, 1972, fully operationalfully operational since since 1994)1994)
• GLONASSGLONASS – – GLOGLObalnaja balnaja NANAvigacionnaja vigacionnaja SSputnikovaja putnikovaja SSistemaistema
Russian (the Soviet Union’) military serviceRussian (the Soviet Union’) military service
(1978, (1978, scheduled for restoration by 2010scheduled for restoration by 2010))
• GALILEOGALILEO European Space Agency (ESA) European Space Agency (ESA)
European commercial service European commercial service
(1999, (1999, scheduled to be fully operational by scheduled to be fully operational by 20201313))
DORIS (France), COMPASS or Beidou (China), QZSS (Japan), IRNSS (India) DORIS (France), COMPASS or Beidou (China), QZSS (Japan), IRNSS (India)
EGVAP Workshop, November 6, 2008 4
Satellite tracks projected onto the surfaceSatellite tracks projected onto the surface• GPS GPS – 31 (32) satellites / 6 orbital planes / 11h 58min– 31 (32) satellites / 6 orbital planes / 11h 58min• GLONASSGLONASS – 21 (24) satellites / 3 orbital planes / 11h 15min – 21 (24) satellites / 3 orbital planes / 11h 15min • GalileoGalileo – 27 (30) satellites / 3 planes – 27 (30) satellites / 3 planes
EGVAP Workshop, November 6, 2008 5
basic GNSSbasic GNSSobservablesobservables
GPS oscillator with fundamental GPS oscillator with fundamental frequency 10.23 MHz multiplied byfrequency 10.23 MHz multiplied by154x > 1575.42 MHz (L1)154x > 1575.42 MHz (L1)120x > 1227.60 MHz (L2)120x > 1227.60 MHz (L2)
code pseudorangecode pseudorangethe measure of the transit time from satellite to receiver using autocorrelation of received and replicated signal (the the measure of the transit time from satellite to receiver using autocorrelation of received and replicated signal (the time is coded in signal) time is coded in signal)
observablesobservables: : C1 = L1 C1 = L1 CC/A/A, , P1 = L1 P(Y) P1 = L1 P(Y), , P2P2 = L2 P(Y) and many others in future = L2 P(Y) and many others in future ≈≈ 1m absolute positioning for civil usage1m absolute positioning for civil usage
phase pseudorangephase pseudorangethe measure of the phase difference btw. received and replicated carrier frequencythe measure of the phase difference btw. received and replicated carrier frequency
observables: observables: L1, L2L1, L2 and others in future and others in futuresubcentimeterlevel relative positioningsubcentimeterlevel relative positioning
doppler datadoppler datathe measure of doppler shift due to a mutual motion of satellite and receiverthe measure of doppler shift due to a mutual motion of satellite and receiver
EGVAP Workshop, November 6, 2008 6
Error sources for GNSSError sources for GNSS
SatellitesSatellites:: e ephphemeriemeriss, , clocks, differencial code biasesclocks, differencial code biases(AS(ASantispoofingantispoofing, , S/S/AAselective availabilityselective availability before 2000) before 2000)
ReceiversReceivers:: clocksclocks, , phase center offsets and variations, differencial code phase center offsets and variations, differencial code biasesbiases
EnvironmentEnvironment:: troposphere, ionosphere, multipath, Earth’s kinematicstroposphere, ionosphere, multipath, Earth’s kinematicsProcessingProcessing:: cycleslips in phases, model errors cycleslips in phases, model errors
→→ EliminationEliminationby observable differencesby observable differencesby introducing precise models and productsby introducing precise models and products
EGVAP Workshop, November 6, 2008 7
Parameters in GNSS mathematical modelParameters in GNSS mathematical model
thus we have to handle somehow these parameters in GNSS processing:thus we have to handle somehow these parameters in GNSS processing:
• satellite and receiver positionsatellite and receiver position• satellite and receiver clock correctionssatellite and receiver clock corrections• Earth orientation parameters and geocenter coordinatesEarth orientation parameters and geocenter coordinates• satellite and receiver code differential biassatellite and receiver code differential bias• satellite and receiver phase center offsets and patternssatellite and receiver phase center offsets and patterns• troposphere effecttroposphere effect• ionosphere effectionosphere effect• ambiguitiesambiguities
EGVAP Workshop, November 6, 2008 8
Observable differencesObservable differences
to eliminate some of the errors in mathematical GPSto eliminate some of the errors in mathematical GPS model model, we often create, we often create and use differences from the and use differences from the original observablesoriginal observables::
singledifferencesingledifference (SD) (SD) – – difference between two stationsdifference between two stations ( (baseline generationbaseline generation)), which, which eliminates the eliminates the satellite clock errors observed at both stationssatellite clock errors observed at both stations
doubledifferencedoubledifference (DD) (DD) – – difference between two SDs difference between two SDs ( (measurement to two satellites from the single measurement to two satellites from the single baselinebaseline)), which eliminates, which eliminates reciever clock errorsreciever clock errors
trippledifferencetrippledifference (TD) (TD) – diferen – diferences between two ces between two DD DD in different epochs, which is useful to detect in different epochs, which is useful to detect the phase skips (e.g. when signal from satellite was discontinued)the phase skips (e.g. when signal from satellite was discontinued)
the original observables we often call asthe original observables we often call as zerodifferencezerodifference (ZD) (ZD)
EGVAP Workshop, November 6, 2008 9
GPSmeteorology conceptGPSmeteorology concept
GPS x NWP
we knowwe know precise receiver and orbit positions, precise receiver and orbit positions, we we eliminateeliminate i ionosonospherephere effect (receiver and satellite clock effect (receiver and satellite clock error), error), we introducewe introduce (PCVs, OCTIDE, ...) (PCVs, OCTIDE, ...)
we estimate: we estimate: zenith path tropospheric delay (receiver zenith path tropospheric delay (receiver and satellite clocks)and satellite clocks)
EGVAP Workshop, November 6, 2008 10
GPS observation equationGPS observation equationBasic GPS carrier phase observable (scale to distance):Basic GPS carrier phase observable (scale to distance):
LLrereccsatsat = = σσrecrec
satsat + c* + c*δδsatsat + c* + c*δδrecrec + + λλ*n*nrecrecsatsat + + ∆∆IONION + + ∆∆TRP TRP + + εεrerecc
satsat
σσrecrecsat sat .. .. receiversatellite distance in vacuum receiversatellite distance in vacuum
((receiverreceiver and and satellite coordinates satellite coordinates))c c .. speed of light.. speed of lightδδsatsat, , δδrec rec .. .. satellite and receiver clocksatellite and receiver clock errors errors
λλ .. wavelength of the carrier phase.. wavelength of the carrier phasennrecrec
sat sat .. unknown initial phase .. unknown initial phase ambiguitiesambiguities
∆∆IONION .. ionospheric (slant) delay.. ionospheric (slant) delay
∆∆TRPTRP .. .. tropospheric (slant) path delaytropospheric (slant) path delay
∆∆TRPTRP = = mmffhh(z) * ZHD + (z) * ZHD + mmffww(z) *(z) * ZWDZWD (z = zenith distance)(z = zenith distance)
ZTD ZTD = ZHD + ZWD= ZHD + ZWDZTD ZTD [m][m] Zenith Total DelayZenith Total Delay (usually site & timedependent parameters)(usually site & timedependent parameters)
mmffww // hh mapping function mapping function (wet / hydrostatic)(wet / hydrostatic)
LLklklijij = L = Lklkl
i i – L– Lklklj j = ( L= ( Lkk
i i – L– Llli i ) – ( L) – ( Lkk
j j – L – L llj j )) double differences in network sol. double differences in network sol.
← satellite and receiver position need satellite and receiver position need to be accurately knownto be accurately known
← eliminated using double-differences eliminated using double-differences (estimated in PPP !!!) (estimated in PPP !!!)
← need to be resolved integer or floatneed to be resolved integer or float← first-order eliminated infirst-order eliminated in
the ’ionospherthe ’ionospheree-free’ combination-free’ combination
EGVAP Workshop, November 6, 2008 11
Least Squares AdjustmentLeast Squares Adjustment
GPS „distance“ GPS „distance“ measurements measurements
(code and/or phase)(code and/or phase)
stochastic informationstochastic informationunknown unknown parametersparameters
• coordinatescoordinates
• ambiguitiesambiguities
• ztd‘sztd‘s
• after linearization
• user usually knows the models for the orbits, tides, etc.
Observations:Observations:
residualsresiduals
EGVAP Workshop, November 6, 2008 12
Normal Equations (NEQsNormal Equations (NEQs))
normalnormal equationequation
parameter estimationparameter estimation
minimizing the residuals: minimizing the residuals: e‘ P e e‘ P e min. min.
parameters of interest (coordinates, parameters of interest (coordinates, troposphere, ...) troposphere, ...)
parameters to be eliminated (ambiguities)parameters to be eliminated (ambiguities)
EGVAP Workshop, November 6, 2008 13
Sequential Adjustment: IdeaSequential Adjustment: Idea
often applied in two ways:often applied in two ways:
- time domain time domain
(sequential solutions)(sequential solutions)
- space domain space domain
(network clusters)(network clusters)
Processing of sequential Processing of sequential solutions :solutions :
identical identical with processing all with processing all observations in a common observations in a common adjustment, if there are no adjustment, if there are no correlationscorrelations of the original of the original observationsobservations
time
EGVAP Workshop, November 6, 2008 14
Processing strategies & softwareProcessing strategies & software
PPP approach:
Epos - Epos - GFZGFZ
Gipsy - Gipsy - NGAANGAA
Network approach:
Gipsy - Gipsy - ASI ASI
Bernese - Bernese - BKG, GOP, KNMI, LTP, ROB, METO, SGNBKG, GOP, KNMI, LTP, ROB, METO, SGN
• Large NEQ
• Increasing CPU with incr. number of sites/parameters
• Correlations btw stations are ignored
• Use of external products (orbits, clocks)
Disadvantages
• Correlations between parameters of all stations are taken into account
• Independence of external products (except for small networks)
• Small NEQ
• Keeping CPU with increasing number of sites / parameters (e.g. ZTD every 15 min, estimation of gradients)
• Investigations of site dependent effect
Advantages
Network using double differences Precise Point positioning (PPP)
EGVAP Workshop, November 6, 2008 15
PPP processing strategy PPP processing strategy (example GFZ)(example GFZ)
Part 1 - Network orbit improvement:
Adjustment of precise orbits & clocks Global network : ~20 IGS+German sites Input orbits: GFZ 3h Ultra-rapid (pred.)
CPU (Linux PC): ~6 to 8 minutes
Part 2 - PPP Analysis:
Estimation of trop. parameters Large set of parameters possible (high sampling rate, trop. gradients)
NEW: ‚slant delays‘ estimation CPU (Linux PC): <5 min for 220 sites
courtesy of Galina Dick (GFZ)courtesy of Galina Dick (GFZ)
GFZ EPOS SoftwareGFZ EPOS Software
EGVAP Workshop, November 6, 2008 16
General network processing stepsGeneral network processing steps
• creating data batches (xhourly or sliding window)creating data batches (xhourly or sliding window)• data quality checkdata quality check• single point positioning for rough receisingle point positioning for rough receiverver clock synchroniz clock synchronizationation• network design by double differencing (clusters possible)network design by double differencing (clusters possible)• data screening for phase cycleslips, ambiguities set updata screening for phase cycleslips, ambiguities set up• iterative site & satelliteiterative site & satellite quality check and outliers rejectionquality check and outliers rejection• ionosphere product & ambiguity resolutionionosphere product & ambiguity resolution• reference frame realireference frame realizzation & coordinate estimationation & coordinate estimation• ZTD product generationZTD product generation
EGVAP Workshop, November 6, 2008 17
Network processing strategy Network processing strategy (example GOP)(example GOP)
• preprocessingpreprocessing is based on twohours data batches is based on twohours data batches 1 hour redundancy with the previous run easier ambiguity resolution, 1 hour redundancy with the previous run easier ambiguity resolution, coordinates also for regularly ‘late’ RINEX ( > 30min ) coordinates also for regularly ‘late’ RINEX ( > 30min )
• normal equations (NEQ) normal equations (NEQ) – 1h for ZTD and 2h for coordinates– 1h for ZTD and 2h for coordinates• processingprocessing in clustersin clusters of the network of the network• coordinates coordinates are combined from last 28 days using 2hNEQs with ambiguity fixed, freeare combined from last 28 days using 2hNEQs with ambiguity fixed, free
network solution, IGS05 reference framenetwork solution, IGS05 reference frame• ZTD productZTD product based on last 12h stacking of 1hNEQs based on last 12h stacking of 1hNEQs• ionosphere product ionosphere product for ambiguity resolutionfor ambiguity resolution
EGVAP Workshop, November 6, 2008 18
GOP processing schemeGOP processing scheme
EGVAP Workshop, November 6, 2008 19
Ambiguity resolution in near realtimeAmbiguity resolution in near realtime• initial phase ambiguities represent a huge number ( > 90% !) of necessarilly estimated initial phase ambiguities represent a huge number ( > 90% !) of necessarilly estimated
parameters in mathematical GPS modelparameters in mathematical GPS model• in network solution, they can be resolved for integer numbers, which has strong impact in network solution, they can be resolved for integer numbers, which has strong impact
for the coordinate estimation in shorttime dataspan for the coordinate estimation in shorttime dataspan • ambituity resolution depends on timewindow and baseline lenghtambituity resolution depends on timewindow and baseline lenght• in GOP solution, for example, the ambiguities are resolved for in GOP solution, for example, the ambiguities are resolved for 70% in total70% in total within within
twohour data batch applying twostep approach (twohour data batch applying twostep approach (widelane ambiguitieswidelane ambiguities at at MelbourneWubbenna phase+code linear combination resolved in 8090% and MelbourneWubbenna phase+code linear combination resolved in 8090% and narrownarrowlane ambiguitieslane ambiguities at ionospherefree phase linear combination resolved with 70% at ionospherefree phase linear combination resolved with 70% success) success)
• resolved ambiguities are introduced ‘as known’ at least for the official coordinate resolved ambiguities are introduced ‘as known’ at least for the official coordinate estimation (estimation (North/East/UpNorth/East/Up coordinate repeatability improved from 10/10/25mm to coordinate repeatability improved from 10/10/25mm to 6/6/16mm6/6/16mm) )
• a positive bias of aprox. 1mm observed in ZTD solutions btw ambituity free and fix a positive bias of aprox. 1mm observed in ZTD solutions btw ambituity free and fix solution !solution !
EGVAP Workshop, November 6, 2008 20
NRT coordinate solutionsNRT coordinate solutionsThe coordinates, which are ‘fixed’ or ‘tightly constrained’ in NRT ZTD solution should be as The coordinates, which are ‘fixed’ or ‘tightly constrained’ in NRT ZTD solution should be as
good as possible ( good as possible ( ≈≈ 3:1 for CRD:ZTD) 3:1 for CRD:ZTD)example: GOP solution for the coordinatesexample: GOP solution for the coordinates• the coordinates are based on ambiguity fixed solution using last 28 days of twohourly the coordinates are based on ambiguity fixed solution using last 28 days of twohourly
NEQs, the solution is updated every hour.NEQs, the solution is updated every hour.• the coordinates are expressed in local datum close to the last ITRF realization by IGS the coordinates are expressed in local datum close to the last ITRF realization by IGS
(currently IGS05) by applying the Helmert transformation (fidutial stations are (currently IGS05) by applying the Helmert transformation (fidutial stations are iteratively checked)iteratively checked)
EGVAP Workshop, November 6, 2008 21
Troposphere model – Bernese GPS softwareTroposphere model – Bernese GPS software
Slant tropospheric path delays = wet + dry (hydrostatic) are mapped into zenith using a Slant tropospheric path delays = wet + dry (hydrostatic) are mapped into zenith using a mapping function (mf) mapping function (mf)
SSPPD = D = mmffHH(z)(z) ZHD + ZHD + mmffWW(z)(z) ZWD [z = zenith distance]ZWD [z = zenith distance]
where ZHD can be well a priori estimated if atmospheric pressure and station heiht+latitude are where ZHD can be well a priori estimated if atmospheric pressure and station heiht+latitude are
known (e.g. Saastamoinen, 1972)known (e.g. Saastamoinen, 1972)
Because its variability, ZWD should be estimated for baselines > 20kmBecause its variability, ZWD should be estimated for baselines > 20km
Extended model could apply additionally the azimuthal dependency expressed as horizontal Extended model could apply additionally the azimuthal dependency expressed as horizontal
tropospheric gradients (Gtropospheric gradients (GNN north, G north, GEE easth): easth):
SSPPD = D = mmffHH(z) ZHD + (z) ZHD + mmffWW(z)(z) ZWD +ZWD +
∂∂ mfmfWW//∂∂ z [ G z [ GNN cos(A) + Gcos(A) + GEE sin(A)] [A = azimuth]sin(A)] [A = azimuth]
Constant or Constant or piecewise linear functionpiecewise linear function is is applied for ZTDapplied for ZTD
Standard atmosphere (or insitu atm. pres. measurement) for a priori ZHDStandard atmosphere (or insitu atm. pres. measurement) for a priori ZHD
Dry and wet ‘Niell’ mf (‘Global’ or ‘Vienna’ mf in future) Dry and wet ‘Niell’ mf (‘Global’ or ‘Vienna’ mf in future)
EGVAP Workshop, November 6, 2008 22
Troposphere model – impact study exampleTroposphere model – impact study exampleSome impacts in past using older models:Some impacts in past using older models: 2.2. no a priori model (zero value) and dry Niell mapping function used for the total zenith delay estimated (used no a priori model (zero value) and dry Niell mapping function used for the total zenith delay estimated (used
until May 2005).until May 2005).3.3. a priori ZHD based on standard atmosphere and wetNiell mapping function estimating ZTD (hopefully a priori ZHD based on standard atmosphere and wetNiell mapping function estimating ZTD (hopefully
most of the ZWD).most of the ZWD).→ bias variablebias variable in time and space in time and space
Another sitedependentAnother sitedependentbias was introduced bias was introduced in 2006 due to changingin 2006 due to changingrelative relative →→ absolute absolutePhase Center VariationsPhase Center Variationsand Offsets model usedand Offsets model used(upto 5mm)(upto 5mm)
EGVAP Workshop, November 6, 2008 23
TroposphericTropospheric product product (GOP example) (GOP example)
• ZTDs for every hour (HH:00 + HH:59)ZTDs for every hour (HH:00 + HH:59)• a linear trend is considered between the valuesa linear trend is considered between the values• coordinates are heavily constrained to our estimated values realizing the IGb00 coordinates are heavily constrained to our estimated values realizing the IGb00
reference frame and written to the COST 716 format.reference frame and written to the COST 716 format.• ZTD product filtering:ZTD product filtering:
– Sites with less than 4 hours of data in ZTD solution are excluded from the productSites with less than 4 hours of data in ZTD solution are excluded from the product– Sites with less than 2 days of data in coordinate solution are excluded. Sites with less than 2 days of data in coordinate solution are excluded.
• ambiguityfree (AF) and ambiguityfixed (AX) ZTD solutions are provided ambiguityfree (AF) and ambiguityfixed (AX) ZTD solutions are provided (officially AF), both using the same a priori coordinates values (ambiguityfixed).(officially AF), both using the same a priori coordinates values (ambiguityfixed).
EGVAP Workshop, November 6, 2008 24
Requirements:Requirements:hourly GNSS data (IGS, EPN, national,...)
precise orbits (IGS ultrarapids, ...)precise orbits (IGS ultrarapids, ...)
for PPP: precise satellite clocks, DCB bias
Features:Features:processing started every hour
usually ZTD at the edge of the processing window
correlation with respect to previous estimates (physical, via processing, possible constraints – depends on timeresolution)
Other important models:Other important models:ocean and Earth tides (station coordinate, geocentr, satellite orbits)
receiver and satellite phase center offsets and variations
troposphere mapping function
2nd, 3rd order ionosphere
many others especially
in precise orbit determination
NNear realtime ear realtime aspects of aspects of ZTDZTD estimation estimation
last hour
EGVAP Workshop, November 6, 2008 25
GNSS hourly data availabilityGNSS hourly data availability
EGVAP Workshop, November 6, 2008 26
predominantly IGS ultrarapid orbits usedpredominantly IGS ultrarapid orbits used
Requirements on predicted orbits for ZTDRequirements on predicted orbits for ZTD
errors in ZTD
2001
2008
Synthetic error in orbit positionSynthetic error in orbit position 1m in alongtrack 1m in crosstrack 1m in radial (mostly eliminated in DD)
EGVAP Workshop, November 6, 2008 27
ZTD results PPP vs NetworkZTD results PPP vs Network
ZIMM and GOPE – one of the 12 ‘supersites’ZIMM and GOPE – one of the 12 ‘supersites’
EGVAP Workshop, November 6, 2008 28
Some ZTD/PWV comparison at GOPSome ZTD/PWV comparison at GOP20012003 comparison20012003 comparisonNRT x postprocessingNRT x postprocessing
StdDev : StdDev : 47mm 47mm
BBiasias : : 13mm13mm
EGVAP Workshop, November 6, 2008 29
weeklyweekly Sdev and Bias Sdev and Bias GPS ZTD from GOP near realtime GPS ZTD from GOP near realtime NWM Hirlam from DMI NWM Hirlam from DMI
StdDev: 816mm (28mm)Bias: upto 16mm (25mm) (strong seasonal variation)
ZTD comparisonZTD comparisonNRT GOP NRT GOP ×× HIRLAM (NWP) HIRLAM (NWP)
EGVAP Workshop, November 6, 2008 30
AC’s NRT AC’s NRT ZTD ZTD x postprocessing @ GOP x postprocessing @ GOPztd differences freqency & distribution functions (2004/2005)ztd differences freqency & distribution functions (2004/2005)
ACRI ASI BKG GFZ GOP IEEC LPT NKG NKGSBOR1
GOPE
HERS
POTS
WTZR
ONSA
MAR6
CAGL
MATE
EGVAP Workshop, November 6, 2008 31
Hour x day plots (ztd differences)Hour x day plots (ztd differences)NRT NRT xx postprocessingpostprocessing
EGVAP Workshop, November 6, 2008 32
Groundbased GPSmeteorologyGroundbased GPSmeteorology(Europe)(Europe)
COST-716 Action (1998-2003):
"Exploitation of Ground-Based GPS for Operational Numerical Weather Prediction and Climate Applications“ 15 Institutions 7 ACs> 200 GPS sites
TOUGH (2003-2006):
„Targeting Optimal Use of GPS Humidity Measurements in Meteorology“ 15 Institutions (Coordinator DMI) 12 ACs > 400 GPS sites
E-GVAP (2006 - 2009):„The EUMETNET GPS Water Vapor Programme“
13 Institutions 10 ACs > 800 GPS sites