3.3.3. sestbl.
The processing within GAMIT itself (arc, model, autcln, solve) is controlled by sestbl., sittbl., and autcln.cmd.
The sestbl. entries that you are likely to consider in setting up your processing are shown below.
The values are generally appropriate for regional or global processing for most data acquired after 1995.
Choice of Experiment = BASELINE ; BASELINE/RELAX./ORBIT
Satellite Constraint = Y ; Y/N (next two lines are read and free-format. only all needed on next line)
all a e i n w M D Y B 1UDC 1UDS 1UYC 1UYC 1UBC 1UBS 2UC 2US 4UC 4US SX SY SZ
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.001 0.001 0.001
By setting Choice of experiment = BASELINE, you are fixing the orbits and omitting the orbital parameters from your GAMIT processing and output h-files.
If you intend to combine your h-files with those from global processing at, e.g., MIT or SOPAC, or if you are processing a GNSS with poor orbits, then you should set Choice of experiment = RELAX and apply constraints to the orbital parameters.
The values given for orbital parameters (1 part in \(10^9\) or about 2 cm) are reasonable for current IGS orbits (see http://acc.igs.org/igsacc_final.html for orbital accuracy over time).
Choice of Observable = LC_AUTCLN ; L1_SINGLE/L1&L2/L1_ONLY/L2_ONLY/LC_ONLY/
; L1,L2_INDEPEND./LC_HELP/LC_AUTCLN
Ambiguity resolution WL = 0.15 0.15 1000. 99. 15000. ; used for LC_HELP only
Ambiguity resolution NL = 0.15 0.15 1000. 99. 15000. ; Allow long baselines with LC_AUTCLN
Ionospheric Constraints = 0.0 mm + 8.00 ppm
The next four entries control the observations used and ambiguity resolution.
LC_AUTCLN and LC_HELP are the usual choices with dual-frequency receivers and all but the shortest baselines.
With the LC_AUTCLN option, the widelane ambiguities are assigned and resolved in autcln using the pseudoranges.
With LC_HELP, the wide-lane ambiguities are resolved by applying an ionospheric constraint (see Dong et al. [1998]).
For data acquired with codeless receivers (before ~ 1994), LC_HELP should be used.
The sensitivity parameters for both wide-lane (with LC_HELP) and narrow-lane ambiguities are set conservatively but may be varied if you need to squeeze greater accuracy out of short observing sessions.
See Dong et al. [1998] and Section 3.5 of the GAMIT Reference Manual for a detailed discussion.
For baselines less than a few km, using L1 and L2 independently or only L1 may reduce noise compared to using the ionosphere-free combination.
To check for the size of ionospheric errors, it’s wise to try LC_HELP, L1_ONLY, L2_ONLY, and L1,L2_INDEP and compare the results.
If your data were acquired with a single-frequency receiver, you should set L1_RECEIVER in sestbl. and also add the L1only command to autcln.cmd since autcln by default will attempt to use both L1 and L2 in cleaning (even if L1_ONLY is requested for solve).
DMap = GMF ; GMF(default)/NMFH/VMF1
WMap = GMF ; GMF(default)/NMFW/VMF1
Use map.grid = Y ; VMF1 grid file with mapping functions and ZHD
Met obs source = UFL GPT 50 ; hierarchical list: RNX ufile GPT/STP [humid value]; default GTP 50
if [humid value] < 0, use RNX or UFL if available
Zenith Delay Estimation = Y ; Yes/No (default No)
Interval zen = 2 ; 2 hrs = 13 knots/day (default is 1 ZD per day)
Zenith Variation = 0.02 100. ; zenith-delay variation, tau in meters/sqrt(hr), hrs (default .02 100.)
Elevation Cutoff = 0 ; default 0 to use value in autcln.cmd
Atmospheric gradients = Y ; Yes/Np (default No)
Number gradients = 2 ; number of gradient parameters per day (NS or ES); default 1
Gradient Constraints = 0.03 ; gradient at 10 deg elevation in meters; default 0.03 m
Gradient Variation = .01 100 ; gradient variation
Output met = N ; write the a priori met values to a z-file (Y/N)
The next block of entries controls the modeling and estimation of the atmospheric delay.
The first command specifies the mapping function to be used to project the zenith delay to the elevation angle of the satellites.
Different functions must be used for the hydrostatic (“dry”) delay and the “wet” delay due to water vapor.
The default, sufficient for all but the most accurate studies of heights and for meteorological studies, is the “global mapping function” (GMF) developed by Boehm et al. [2006] from fitting numerical weather model (NWM) data over 20 years.
A more accurate reconstruction of the NWM data can be obtained by interpolating hydrostatic and wet mapping function coefficients as a function of time and location from the (large) global grid files compiled by the Vienna group [Boehm et al., 2006].
You have access to these values by downloading the VMF1 grid files for each year from ftp://everest.mit.edu/pub/GRIDS/, and setting Use map.grid = Y, DMap = VMF1 and WMap = VMF1.
The next command controls the source of pressure (most importantly, but also temperature) for the a priori zenith hydrostatic delay (ZHD).
The most accurate values, if they are available, are from local measurements of surface pressure, which can be written into RINEX met files and stored in a met/ directory for use by sh_gamit.
Setting the first option of Met obs source = RNX tells GAMIT to use these for any station for which they are available.
The next most accurate source would be the ZHD values from the VMF1 grid files.
Since GAMIT reads the grid file for all sites used for the day and writes the values into the U-file, you select this option by setting Met obs source = UFL and map.grid = Y.
(If you use a VMF1 station-list file, map.list, the second option, GPT or UFL can be used for stations missing from the list file.
Since the only source of NWM ZHD data currently tabulated for GAMIT is the VMF1 mapping-function grids, the met.grid and met.list files are not yet supported.)
Finally, as with the mapping functions, the Vienna group has constructed from the NWM data an analytical model, designated “global pressure and temperature” (GPT2), of Lagler et al. [2013].
It reads from table gpt.grid in ~/gg/tables/ values of zenith hydrotstatic delay (ZHD), temperature, lapse rate, and dry and wet mapping functions as a function of latitude, longitude, and day-of-year that are averages from a fit to 10-year monthly averages from a global numerical weather model.
The option STP implies standard constants (1013.25 hPa, 20°C), used prior to Release 10.3 but now effectively obsolete.
To keep errors in height estimates below 2 mm, you need an accuracy of about 10 hPa in a priori pressure [Tregoning and Herring, 2006].
The Interval zen command controls the number of zenith delay parameters estimated using the session.
For geodetic studies, estimating values at 2-hour intervals with the constraints given is more than adequate.
For meteorological studies, you may want to estimate more parameters and/or alter the constraints (see Chapter 7 of the GAMIT Reference Manual).
For atmospheric gradients, we allow a linear change during the session (two parameters each for N/S and E/W gradients).
Update T/L files = L_ONLY ; T_AND_L (default), T_ONLY, L_ONLY, NONE
The command to update the L-file assures that the second solve solution will have adjustments within a linear range and allows sh_gamit to apply updated coordinates to the processing of successive days.
The criterion for updating is set by default to be adjustments larger than 30 cm (though this can be changed with a sestbl. command, not shown).
If you know that all of your a priori coordinates are accurate to within a few cm and don’t want bad data to corrupt the coordinates in the L-file, you can set Update T/L files = NONE.
AUTCLN Command File = autcln.cmd ; Filename; default none (use default options)
Station Error = ELEVATION 10 5 ; 1-way L1, a**2 + (b**2)(L**2) in mm, ppm, default = 10. 0.
The next two commands control how the phase data are weighted in the preliminary and final solutions.
The Station Error entry shown tells solve to use elevation-dependent weighting for the phase date.
In the preliminary solution, the assigned error is 10 mm with negligible elevation dependence.
With AUTCLN postfit = Y (default with Type of analysis = 1-ITER) the second solve run will assign weights to the phase data based on the actual scatter computed by autcln in its “postfit” edit, and will contain both a constant and an elevation-dependent term, as recorded in the output print file autcln.post.sum.
Elevation-dependent weighting is recommended for almost all analyses;
an exception might be tests of models (e.g. mapping functions) sensitive to low-elevation observations.
Decimation Factor = 4 ; FOR SOLVE, default = 1
Quick-pre observable = LC_ONLY ; for 1st soln, default same as Choice of observable
Quick-pre decimation factor = 10 ; 1st iter or autcln pre, default same as Decimation Factor
Related to the error weighting is the sampling of data for the solution.
For cleaning purposes (model, autcln), the full sampling of the x-file (usually 30 s or 15 s) is used.
However, since the phase errors are correlated over many minutes, it is not necessary to use sampling this frequent in solve.
The default decimation factor (4, resulting in 2-minute sampling for the usual case) provides formal uncertainties that are usually reasonable within a factor of two while reducing time significantly.
(See the discussion of uncertainties in Section 2.3.)
For the preliminary solution used to get decimeter-level coordinates for editing and to avoid non-linearity in the final adjustments, 5-minute sampling is sufficient.
Antenna Model = AZEL ; NONE/ELEV/AZEL default = ELEV
Tides applied = 31 ; Binary coded: 1 earth 2 freq-dep 4 pole tide (zero mean pole)
; 8 ocean 16 pole tide (IERS2010 mean pole) 32 atmosphere S1/S2
; 64 pole tide (IERS20 secular pole) (31 default ITRF2014, 79 default ITRF2020).
Use otl.list = N
Use otl.grid = Y
There are over a dozen sestbl. entries controlling the models used in processing the data.
These are all given at the bottom of the template file in ~/gg/tables/ and discussed in Section 3.2 of the GAMIT Reference Manual.
The three shown here are those most likely to need editing for current applications.
Changing Antenna Model to NONE allows bypassing of the phase-center-variation (PCV) file antmod.dat, an option advisable only if you are using identical antennas within a small regional network.
The Tides applied option is binary coded, allowing any combination of the various tidal models to be applied in model.
For most applications you may keep the default, which is to apply all of the tidal effects currently available in GAMIT (though the 2-bit for K1 is not necessary with the current IERS 2003 model for the solid-body tides, and the pole tide can be applied later in GLOBK).
If your data are not strongly affected by ocean tidal loading (“OTL”) or you do not wish to copy over the large ocean loading grid files, you might omit this model, setting Tides applied = 23 instead of 31.
The OTL components are read from a station list file or a global grid, with station-list values taking precedent if Use otl.list = Y and the station is within 10 km of a station in the file.
If you have local stations close to an IGS station, it is advisable to use the grid exclusively so that you don’t introduce artifacts in the relative motions of two stations.