GLOBK

1
GLOBK

• Thomas Herring
• Room 54-611 617-253-5941
• tah_at_mit.edu
• http//geoweb.mit.edu

2
Summary

• Applications of globk (why do we need it).
• Structure of how globk works and philosophy
• GLOBK Command file
• Use of process noise models
• Main commands that are encountered.
• GLORG command file
• Coordinate system definition module (or frame
  stabilization)
• Pos_org and rate_org commands

3
Function of GLOBK

• The number of parameters in GPS phase analysis
  are dominated by bias and atmospheric delay
  parameters. The bias parameters increase as the
  number of stations times the numbers of
  satellites. It is not practical to include large
  numbers of stations in an analysis without some
  method of eliminating these bias and atmospheric
  delay parameters Globk provides one way of
  eliminating these parameters.
• Phase analysis is CPU time intensive and another
  benefit of the Globk approach is that decisions
  on parameter constraints (such as site
  coordinates, Earth orientation parameters,
  orbits) can be done (quickly) at the Globk level.
• Principle is to carry forward solution and
  variance-covariance matrix for global
  parameters.

4
Introduction to Kalman filtering

• Estimation method in which parameters can be
  stochastic processes (i.e. parameters are not
  fixed, deterministic quantities but rather they
  vary with time according to statistical noise
  processes).
• Examples polar motion and UT1, radiation
  pressure effects on satellites, possible
  transient events, monument instability noise.
• Nearly all parameters in globk can be stochastic
  or deterministic
• Statistical character of parameters are set with
  commands of the form apr_XXX (specifies the
  apriori uncertainty of a parameter) and mar_XXX
  (specifies the stochastic nature of the
  parameter)
• The parameters that are estimated in Globk are
  based on those that are given apriori
  uncertainties (special method used to put zero
  apriori constraint on parameter, i.e., fix the
  parameter value).

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Stochastic processes

• Stochastic models in globk are random walks.
  Random walks are the sums of white noise they
  are a special case of a first order Gauss-Markov
  process with infinite correlation time.
• The variance of the change in a random walk over
  a time interval is proportional to the time
  interval. (Strictly, a random walk is not a
  stationary process).
• Globk uses random walks because process can be
  represented with a single parameter.
• If process noise is zero, then classed as
  deterministic

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Common applications of Globk

• Combine sessions to get average position over an
  observing campaign
• Allows connection between sites that were not
  simultaneously observed
• When large numbers of stations need to be
  processed, network can be broken into manageable
  sub-networks (e.g., 40-50 stations each) and the
  sub-networks combined with Globk. (Special case
  is combination of regional networks and global
  network for orbit determination).
• Combine averaged files together to estimate
  velocities
• Other parameters can also estimated such as
  earthquake offsets, possible scale variations for
  long duration combinations.
• Analysis of individual sessions and combined
  files for repeatability

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Things Globk can not do

• Non-linear effects in aproiri models can not be
  removed. In some campaign processing, this can
  be a problem. Rough rule of thumb 100-10001
  convergence rate (e.g., a 1 meter error in an
  apriori coordinate could result in 1-10 mm error
  in final position). Gamit detects and corrects
  (by default) this problem.
• Fix errors in original analysis such as cycle
  slips or antenna phase center models
• Sites can be deleted which often stops effect on
  other stations
• Ambiguities can not be resolved in Globk (would
  make files too large)
• (Antenna height errors can be corrected in Globk
  hfupd)

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GLOBK Command file

• Example given in gg/example/gsoln/globk_comb.cmd
• Basic features of command files (also common to
  other input files used by globk)
• Character in column 1 denotes comment
• only minimum redundant part of command needed
• In many cases, not all arguments need be given

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Preparing globk

• sh_glred automates standard runs with globk and
  can be used as a starting point.
• Assumes internet access but can be run without
• Globk needs
• Binary h-files (htoglb) These are the input
  files quasi-data files used by Globk. They
  contain parameter estimates, covariance matrices
  and ancillary information (such as receiver and
  antenna information).
• Command file to tell it what to do. File is read
  through a series of command lines. There are a
  few restrictions on order of commands. Each
  command acts on results from previous commands.
• Optional apriori information files such as site
  coordinates and velocities, polar motion/UT1
  information. (Much of this information can be
  carried from h-files themselves)

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Binary h-files

• There are number of methods and sources of binary
  hfiles
• From Gamit, they can be generated with htoglb
  from the hltexptgta.ltyygtltdoygt files in processing
  day directories
• Other major source Solution Independent Exchange
  Format files (SINEX). Available for GPS, VLBI,
  SLR and DORIS.
• Older formats from VLBI, SLR and GPS can be read
  but now replaced by SINEX. (JPL stacov files can
  be read but very poor meta data in these files).
• Program detects which type of file is given.
• De-constraining options are available for SINEX
  and should be used.
• Globk itself can write binary h-files
• Generally Globk assumes h-files are loosely
  constrained.

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HTOGLB

• Running htoglb
• Runstring
• htoglb ltdirgt ltephemeris filegt options ltlist of
  ascii filesgt
• dir is directory where binary files will be
  written
• ephemeris file is satellite orbit file (not used
  any more, make_svs). This file also contains the
  aproiri coordinates of the sites in the hfiles.
• options allow control of file names, and
  de-constraints
• -d options allows rotation and translation
  covariances to be added. Needed for IGS sinex
  and PBO frame SINEX files.
• -n allows name format to be changed.
• list of ascii files, usually with UNIX wild
  cards.
• If no arguments are given help is printed.

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Module sequence

• The sequence of modules are
• globk -- Main controlling program that calls
  other modules
• glinit -- Initialization program, reads the
  headers of all the binary h-files to determine
  their contents. (Allows wild cards in names etc.)
• glfor -- Forward kalman filter
• glbak -- Backards running filter and smoother.
  Only invoked when temporal variations in
  stochastic parameters is needed. For
  deterministic parameters such as positions and
  velocities, only forward solution is needed.
• glsave -- Saves combined binary h-files
• glout -- basic output program
• glorg -- origin definition program plus post
  analysis constraints (e.g., setting velocities of
  close sites to be equal)

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Stand-alone programs

• glsave and glorg are often run as separate
  programs
• Commands can be put in the globk command that
  will run these modules
• Sometimes these commands are not included in the
  globk command file and in most cases they can be
  run after the globk program completes running
  (explicit name must given for the com_file for
  post-globk runs.)
• Caution Internal files used by globk may be
  replaced with subsequent runs of globk so there
  needs to be some caution with multiple globk
  runs. Solution is to use wild-card names for
  com_file, srt_file and sol_file. (names are
  derived from gdl file name).

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Associated programs

• Other program associated with globk
• glist -- lists the contents of a series of
  h-files
• glbtosnx -- Generates SINEX files from binary
  h-files
• hfupd -- Updates binary h-files for changes in
  station.info or sinex header file (distributed by
  IGS)
• glred -- Convenient way of running globk
  multiple times. This program is commonly used for
  repeatability analyses which are in turn used to
  set the process noise statistics in globk.
• ensum, enfit -- time series analysis (batch)
• Matlab derived programs (interactive)
• velview -- displays and analyzes velocity fields
• tsview -- displays and analyzse time series.
• Both these programs are available as executables
  for Linux and Mac OSX.

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Running globk

• Typing globk will list the help Runstring
• globk ltcrtgt ltprint filegt ltlog filegt ltgdl filegt
  ltcommand filegt ltOPTgt
• where ltcrtgt 6 (screen output)
• ltprint filegt -- Print file name (can be 6 for
  screen)
• ltlog filegt -- log file name (can be 6)
• ltgdl filegt -- file containing list of hfiles to
  be processed. Options can be added at the ends
  of line to re-weight files
• ltglobk command filegt -- command file name
• ltOPTgt is a string (case sensitive) such that
  lines which start with this string will be
  interpreted as commands. This allows one command
  files to be for different purposes such as a
  velocity and repeatability runs.

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globk files

• The gdl file is generated normally with an 'ls'
  of the directory containing the binary hfiles
• command file is created with an editor (can be
  based on gg/example templates version of file)
• print and log files are written by globk. When
  glorg in invoked inside globk, the .prt extent
  from the print file name is replaced with .org
  and this file is written also. Depending on
  options, files may not be output at all or may be
  erased before output (default is to append to
  files).

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Main globk commands

• Commands commonly used
• Globk uses three "scratch" files which can be
  explicilty named
• com_file -- Common file contains information
  about run
• srt_file -- binary file with time-sorted list of
  hfiles
• sol_file -- binary file with solution and
  covariance matrix
• To use glorg stand-alone, com_file command must
  be used

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Estimation commands rules

• For a parameter to estimated in globk, apr_xxx
  command must be used where xxx is a parameter
  type (e.g., NEU, SVS, WOB, UT1, ATM)
• If zero given as aprori sigma, then parameter is
  not estimated (effectively left unconstrained)
• To force a parameter to apriori value, F is used
  as the apriori sigma
• eg, apr_neu SITE 10 10 10 F F Fwould force the
  velocity to apriori value
• When glorg is used to define reference frame,
  apr_neu, apr_wob and apr_ut1 should be kept
  loosely constained (equivalent of 1-10 meters)

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Estimation commands

• Site position estimation
• apr_neu ltsitegt ltsigNgt ltsigEgt ltsigUgt ltsigVNgt
  ltsigVEgt ltsigVUgt
• site is site name maybe ALL to apply to all
  stations. Wild card _at_ at end of line can be used
  to apply to multiple sites (e.g., AOA1__at_ would
  apply to AOA1_GNR and AOA1_GHT if these both were
  in solution).
• ltsigNgt ltsigEgt ltsigUgt aprior sigma on position (m)
• ltsigVNgt ltsigVEgt ltsigVUgt apriori sigma on
  velocities (m/yr)
• Stochastic noise process
• mar_neu ltsitegt ltRWNgt ltRWEgt ltRWUgt ltRWVNgt ltRWVEgt
  ltRWVUgt
• site name, may be all
• ltRWNgt ltRWEgt ltRWUgt Random walk positions (m2/yr)
• ltRWVNgt ltRWVEgt ltRWVUgt Random walk velocities
  ((m/yr)2/yr) Normally 0 0 0 used

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Estimation commands

• Satellites Orbit
• apr_svs ltPRNgt ltX Y Zgt ltVx Vy Vzgt ltRadiation
  parametersgt
• ltPRNgt is PRN_NN or all (normal)
• ltX Y Zgt Position sigma (3 values) (m)
• ltVx Vy Vzgt Velocity sigma (3 values) (mm/s)
• Radiation pameter sigmas (upto 11 values)
  (fraction 0-1)

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Satellite orbit parameters

• Normally apr_svs only used in to two forms
• When global GPS sites included so orbit can be
  estimated well
• apr_svs all 100 100 100 10 10 10 1R
• When only local data used and constrain to
  apriori orbit
• apr_svs all 0.1 0.1 0.1 0.01 0.01 0.01 0.01R
• R at end of line means remaining radiation
  parameters
• With modern data and IGS orbits, the orbits can
  be completely fixed i.e.apr_svs_all F F F F F F
  FR

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Earth Orientation Parameters (EOP)

• Apriori uncertainties
• apr_wob lt?Xgt lt?Ygt lt?Xdotgt lt?Ydotgt
• ?X, ?Y pole position apriori sigma (mas)
• ?Xdot ?Ydot apriori sigma for rate of change
  (mas/day)
• apr_ut1 ltsut1gt lt?lodgt
• lt sut1 gt in mas, lt ?lod gt in (mas/day)
• Process noise has same form
• mar_wob ltRWXgt ltRWYgt ltRWXdgt ltRWYdgt
• units are (mas2)/yr and (mas/day)2/yr
• mar_ut1 ltRW UT1gt ltRW lodgt
• Nominally EOP are well known today (0.1 mas
  typically), however in many analyses we like to
  keep these parameter loose to allow common-mode
  local rotation (for IGS runs still needed)

23
EOP statistics

• Normally used in two forms
• Global network of stations
• apr_wob 10 10 1 1
• apr_ut1 10 1
• Regional network (constrained). When constrained
  this way system is not free to rotate (see
  pos_org in glorg).
• apr_wob 0.2 0.2 0.02 0.02
• apr_ut1 0.2 0.02
• In many analyses, the global form is used even
  for local networks. (Care is needed if local
  network is not not surrounded by stations with
  well defined motions).

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UT1 issues

• Some care needs to be used with UT1 because it
  cannot be separated from the nodes of the
  satellites orbits
• UT1 tables and orbits must have used the same
  frame for constraints on UT1 to applied.
• If local processing, then should be OK
• If orbits were determined with different EOP
  tables, then UT1 should be loosened. (More of a
  problem with older analyses -- mid 1990s and
  earlier)

25
Frame estimation commands

• apr_tran ltX YZgt ltVx Vy Vzgt -- estimate
  translation of coordinate system
• apr_scale ltppbgt ltppb/yrgt -- estimate scale and
  scale rate
• mar_tran and mar_scale specify the process noise
• Normally only used in global networks but can be
  help full in regional networks to allow for
  common mode errors. (Generally, small networks
  can translate easily and so explicit translation
  not needed).
• apr_scale MUST be used if scale allowed in glorg
  frame definition.
• When combining data, pre- and post-absolute phase
  center models (Week 1400, Nov 2006), apr_scale
  should be used.

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File commands

• A number of commands are used control the files
  used by globk
• apr_file ltnamegt lets a new apriori coordinate
  file be used
• Format is
• Site_name X Y Z Vx Vy Vz epoch
• Site names are 8 characters, for GPS ABCD_GPS
• X Y Z are geocenter Cartesian (m)
• Velocities are m/yr
• Epoch is deciminal years

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Apriori position files

• apr_files commands can be issued multiple times
  with the latest values taken
• If not specified, GAMIT apriori's are used (no
  velocities)
• EXTENDED option allows more complex behaviors
  (see globk.hlp)
• Allows for jumps in positions and velocities
• Periodic signals
• Logarithmic and exponential decay after
  earthquakes.

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Orbit files

• When single days are processed, orbits from gamit
  will be used by default
• For multiple days, with globk, the make_svs
  command is needed to generate an orbit file.
• Form
• make_svs ltfile namegt
• ltfile namegt will be overwritten
• Command must be near top of command file.

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EOP files

• GAMIT EOP are used by default, but can be changed
  with in_pmu command
• Form
• in_pmu ltfile namegt
• Format of file is
• yy mm dd hr min Xpole - Ypole - UT1-AT -
• Must be uniformly spaced
• Pole in arc-seconds, UT1 in time-seconds

30
Earthquake files

• It is possible to automatically account for
  displacement caused by earthquakes with the
  earthquake file
• To main functions
• Earthquakes specified by 2-char code
• renames of sites either due to wrong names in
  gamit processing or problem with position change
  after equipment change.
• Rename command
• Rename ltoldgt ltnewgt HFile code Epoch ranges
  Position shift
• Changing positions with rename commands no longer
  recommended.

31
Earthquakes in eq_file

• For earthquake
• eq_def ltcodegt Lat Long Radius Depth Epoch
• eq_cosei ltcodegt ltStatic Variancegt ltdistance
  dependent variancesgt
• eq_post ltcodegt ltdurgt ltStatic RWgt ltdistance
  dependent RWgt
• eq_rename ltcodegt
• eq_log ltcodegt lttau (days)gt ltstatic NEU sigma
  (m)gt ltSpatial sigma NEUgt
• The rename option cause site to renamed from
  xxxx_GPS to xxxx_GCode

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GLORG

• Glorg allows coordinates systems to be defined by
  applying explicitly rotations and translations
  rather than tightly constraining some stations
• Allows post applications of constraints (e.g.
  equating velocities at near by sites)
• If loose constraints on site positions and
  velocities, apriori coordinates and velocities
  can be changed.
• It can be run as a separate program or during the
  globk run itself

33
Glorg commands imbedded in globk command file

• The globk command file imbedded glorg commands
  are
• org_cmd ltglorg command file namegt
• org_opt ltOptions for outputgt
• org_out ltoutput file namegt
• If org_out is not given then the extent on the
  print file name is replaced with .org

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Miscellaneous commands

• max_chi ltmax chi2 Incrementgt ltmax prefit
  differencegt ltmax rotationgt
• allows automatic deleting of bad h-files and bad
  coordinates
• app_ptid allows applying the pole tide correction
  if not included in gamit run.
• (Careful with SINEX files, since these don't
  specify if correction has been applied)

35
Miscellaneous commands

• crt_opt, prt_opt, org_opt specify output options
  for screen, print and org files
• glorg help gives all options, main ones are
• ERAS -- erase file before writing (normally files
  appended)
• NOPR -- Do not write output
• BLEN -- Baseline lengths
• BRAT -- baseline rates when velocities estimated
• RNRP -- generates reports on differences in
  parameter estimates after renames.
• FIXA -- makes apriori coordinates and velocities
  consistent when equates are used in glorg (can
  sometimes fail in complicated rename
  scenarios--best apriori files are consistent)
• VSUM -- Lat/long summary of velocity (needed to
  plot velocities)
• PSUM -- Lat/long position summary
• GDLF --Include list of hfiles and chi2
  increments from run
• CMDS -- Echos globk command file into output file
• MIDP -- Refer positions to mid-point of data span
• PBOP -- Output PBO formatted time series and
  velocities

36
Defining coordinate system in globk

• To define a coordinate system need origin,
  orientation and possibly scale
• Strictly for GPS scale should be defined but
  systematic errors in the position of the phase
  center of satellites cause a scale error and
  scale rate error (due to evolving GPS
  constellation Problem should be reduced with new
  absolute phase center model but all old data will
  need to be reprocessed once new orbits have been
  generated).
• Strictly, origin is also defined but mis-modeled
  perturbation on satellite orbits can cause shift
  in origin

37
glorg coordinate definition

• The glorg module in globk defines the coordinate
  system by applying rotation, translation, and
  scale to best align with coordinates of a
  selected set of stations
• Algorithm assumes these parameters are loosely
  constrained.
• Apriori coordinates and velocities define frame.
• User chooses which of the translation, rotation
  and scale are included
• If scale is used, it must be explicitly estimated
  in globk
• For velocity definition Same generalized system
  can be applied i.e. rates of translation,
  rotation, scale aligned with velocities of sites.
• In general No specific station coordinates are
  tightly constrained

38
GLORG pos_org and rate_org

• The two commands determine how coordinate system
  will be realized from loose globk analysis.
• Class of parameters
• Xtran ytran ztran allows translation
• Xrot yrot zrot allows rotation
• Scale allows rescaling of system
• Strictly when translation used the apr_tran and
  mar_tran commands should be used in globk
  analysis. (We are still not sure this is best
  thing to do)
• Absolutely if scale used, apr_scale and mar_scale
  commands should be used in globk

39
GLORG Rotation

• If rotation used in pos_org, EOPs must be loose
  in globk (apr_wob/mar_wob and apr_ut1/mar_ut1)
• If rotation not used then EOPs should be
  constrained (Often good on small networks) but
  position errors grow near edges of network
• There can be problems with zrot terms because of
  differences between the node position of the
  satellites and UT1 tables used in globk analysis.
  
• Rotation rate EOP tables are in
  Nuvel-not-net-rotation frame. If analysis done
  in another frame (e.g., North America fixed)
  rotation rates should be allowed or EOP file
  generated in corrected frame (no module yet to do
  this but will be done soon).

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USING PBO products

• PBO sinex files can be downloaded from Unavco and
  used in globk analyses.
• Two types of sinex files are made available
• Loose Loosely constrained files. These can be
  converted to binary hfiles with normal htoglb
  run.
• Frame Frame defined files. There are two
  choices here. A standard htoglb run will
  generate solution with coordinates well
  determined in the PBO reference frame. To use
  these binary hfiles to generate results in
  another reference frame, the apr_rot and apr_tran
  commands should be used to allow the solutions to
  rotate and translate. An alternative approach is
  to use the -dtr option in htoglb which add
  rotational and translational covariances to the
  binary hfiles.
• The orbits are fixed to IGS orbits in the PBO
  sinex files.

41
Summary

• Basics of Kalman filtering
• Inputs to globk
• Control of globk
• Glorg applications (discussed in more detail in
  reference frame section)
• Basic programs in the globk suite