User Controllable Options in GAMIT

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User Controllable Options in GAMIT

• Station/session information
• Models for orbits and station motion
• Models for signal propagation
• Controls for cycle-slip repair
• Sampling and estimation controls
• A priori values for coordinates and orbits

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Primary User files

• station.info
• receiver type, antenna type and height as f(t)
• guess.rcvant
• Converts RINEX header to GAMIT standard
• L-file
• Prior estimates of station coordinates
• sestbl.
• Controls modeling and estimation for session
• sittbl.
• Site specific controls

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station.info

• Fixed format file with station information
• TRCK SITE Station Name Ant Ht Ant N Ant E
  \
• 7ODM 7ODM Seven Oaks Dam 0.0083 0.0000
  0.0000 \
• Rcvr AntCod HtCod Vers \
• ASHZ12 ATDMRB DHPAB 9.10 \
• Year Doy SN Start Stop
• 2000 110 0 00 00 00 24 00 00
• (\ line break for this display)
• This file must be sorted by TRCK, then epoch

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TRCK SITE station_name

• TRCK and SITE
• 4-character code
• match the 4-char code in RINEX file name
• TRCK SITE for static analysis
• Change for kinematic analysis
• File must be sorted by this field, then epoch
• station_name
• is a long (12-char) description

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AntHt, N and E, HtCod

• AntHt
• Raw antenna height as measured
• Critical measurement!
• HtCod
• Defines geometry of AntHt measurement
• DHPAB is RINEX standard is vertical height to
  antenna reference point (ARP)
• N and E offsets
• e.g., uncalibrated tribrach
• rarely used
• occupied point w.r.t. benchmark

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RCVR Vers

• RCVR
• 6-character codes for the receiver type
• not critical except for some old receivers
• or "buggy" firmware versions
• Vers
• version number of firmware in the receiver
• Should be correct to handle SNR coding properly

7
AntCode

• AntCod and HtCod are critical
• Sets phase center model
• conversion of antenna height measurement
• SLBGP -- Slant bottom ground plane

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Epoch

• year and doy specify year and day of year when
  the values apply
• times set part of day
• (if it changes during UTC day)
• This representation will be augmented in future
  version of GAMIT

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gg/tables/rcvant.dat

• File contains all receivers and antennas known to
  GAMIT
• Translates user-specified 6-character code to IGS
  SINEX standard.
• The file in templates directory provides mapping
  from RINEX header information to 6-char codes.
• Example lines
• rcv ASHTECHZ-XII ASHZ12
• rcv Z-XII3 ASHZ12
• swv 5F 1.00

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gg/templates/guess_rcvant.dat

• Explanation given at top of file
• used a represent space
• Sometimes need to be careful that pattern
  correctly gets correct type. Should not be a
  problem for IGS rinex files
• default lines allowed, but be careful

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Updating station.info

• RINEX headers will be used to update station.info
  unless
• An entry already exists for the day being
  processed
• stinf_unique is set to -u in process.defaults and
  entry has not changed
• xstinfo set for a site in sites.defaults
• An error here can be fatal
• If systematic patterns appear in sky map residual
  plots, antenna type may be wrong

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Apriori coordinates

• L-file
• Geocentric latitude, longitude and radius
• (Spherical, not ellipsoidal)
• Coordinates (no velocities) for a single epoch
• Read by model and solve
• sh_gamit calls gapr_to_l
• Creates an L-file from (time-dependent)
  coordinates and velocities from globk apr file

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Apriori coordinates (2)

• sh_gamit merges entries from existing lfile for
  sites not in apriori file
• If script still cant find coordinates
• Uses pseudorange data in rinex file to generate
  point position or differential range position
  relative to site in sites.defaults
• Use Rinex header coordinates (can block this by
  setting use_rxc N in process.defaults)
• During the sh_gamit run, the coordinates are
  updated if they are in error by gt 30 cm

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sh_gamit runs autcln

• a priori coordinates need to be lt 10 m
• with default settings
• autcln can be "de-tuned" to allow worse
• Post-2000, No SA
• receiver-estimated coordinates fine
• Pre-2000 with SA
• differential range position needed
• Common problem with campaign data

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sestbl. controls

• file resides in tables directory
• important options
• parameters for running model
• estimation procedures for solve
• some site-specific info also in sittbl.
• if conflict, sittbl. takes precedence (automatic)

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Primary sestbl. entries

• Satellite constraints
• defaults for post-IGS (1992) regional data
• for broadcast ephemeris
• used, increase 6 Keplerian elements to 1 ppm
• increase radiation parameters to 100 ()
• Type of Analysis 0-ITER (keep)
• Assumes a-priori coordinates are good

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sestbl. observables

• LC_HELP
• uses pseudoranges to help fix ambiguities.
• works well for regional networks
• LC_only
• uses phase data only to fix ambiguities
• quicker, works well for global networks

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sestbl. observables (continued)

• L1, L1L2
• Single frequency phase combinations
• best for short baselines (e.g., ties)
• ionospheric effects are 1-10 ppm
• therefore 1-10 mm per km of site separation
• But use caution with mixed antenna types

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Decimation

• Speeds up processing
• factor 4 usual for 30-second data
• Our practice is to
• use short term position scatter with sigma at
  this decimation and data noise sigmas
• This has little effect on positions for sampling
  up to 5 minutes
• Uncertainty (sigma)
• goes as square root of number of data

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autcln control options

• Command file name autcln.cmd keep
• AUTCLN Postfit
• Y runs prefit run to get data noise model
• R iterates if pre-fit RMS is too high
• Use N-file
• Y autcln computes an elevation angle dependent
  noise estimates for each site, assuming
• data variance A2 (B/sin(elev))2

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autcln control options (continued)

• Delete AUTCLN input C-files
• Controls speed and size of run
• YES -- deletes all cfiles during run. Re-runs
  model between cleaning and estimation. Needed if
  bad apriori coordinates. Uses least disk space
• Intermediate -- Deletes intermediate C-file,
  similar to yes
• NO -- Keeps C-files and uses more disk space, but
  faster. Good for continuous array processing.

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Atmospheric delay estimation

• Zenith delays and gradients
• Number of points in piecewise continuous function
• Overall constraint
• First order Gauss Markov parameters (variance and
  correlation time)
• Number of gradient parameters
• overall constraint

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Atmospheric delays

• Defaults are good for most conditions
• If turbulent conditions, then constraints should
  be loosened (increased numerical values)
• Elevation cutoff angle (with N-file not so
  critical)
• Autcln should be consistent (cleaning minimum
  elevation lt estimation minimum elevation)

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model parameters

• Antenna Model
• ELEV is most common
• Important when mixing antenna types
• Tide Model (1 bit for each option)
• 1 -- Earth tide
• 2 -- Frequency dependent K1
• 4 -- Pole tide (IGS/ITRF standard)
• Can also be applied in GLOBK
• 8 -- Ocean loading (need ocean grid file)
• Binary sum defines the model, e.g., 156

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model options EOP, Yaw

• EOP model choices
• Diurnal and semidiurnal modes
• 1 -- Pole
• 2 -- UT1
• 4 -- Use Ray model (VLBI model if bit20)
• YAW model
• to handle satellite rotation
• default is Y
• some problems with old data during and after
  eclipses

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solve options

• Controls how solve performs the least squares
  estimation
• Estimate_EOP
• binary coded
• can be constrained later in globk run
• UT1 and wobble constraints.
• usually done later in GLOBK
• but can be tightened in solve for regional
  analysis

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Cleaning options

• Quick-pre
• sets estimation type for the prefit run
• can be made fast with decimation and observable
• Delete eclipse data
• POST -- 30 minutes after eclipse
• ALL -- POST but data in eclipse
• Mainly an issue for global data sets

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Ambiquity resolution

• Defaults optimized for regional networks (lt1000
  km). Default is 500.
• Wide lane (L1-L2) and narrow lane (L1)
• Ionospheric constraint plus range data
• default values work well (see Section 5.5)
• Constraints on good apriori coordinates help

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sittbl. site-dependent options

• Normally not changed
• Mainly constraints on apriori apriori station
  coordinates
• Distribution example has IGS sites plus others
• Takes precedence over sestbl.
• Not all columns are required

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Summary

• User-controllable files add flexibility
• Modifications seldom required
• Distribution example is for regional net
• Some changes appropriate for
• global data processing
• operation near real-time