EAS-591T – Space Geodetic Measurements of Active Crustal
Motions
LAB 9
(see also http://www.seismo.berkeley.edu/~battag/GAMITwrkshp/wrkshp.html).
First, we estimate GPS station coordinates for each day of data in
loosely constrained solutions (h-files). That means that we tightly constraint
neither the coordinates of the tracking sites nor the GPS satellites orbits. In
our solutions, the orbits of GPS satellites and station coordinates are not in
a well-determined reference frame. Baseline lengths are determined very
precisely in the loosely constrained solutions and the entire GPS network and
GPS constellation can be rotated and translated as a rigid body.
To use the coordinates derived from these solutions, we need to transform
all the loosely constrained solution into a consistent reference frame so that
we can derive rates of deformation from the time series of the stations'
coordinates. The reference frame defines the origin, scale and orientation of
our geodetic coordinates. A reference frame is realized through the coordinate
and covariance of a number of reference stations. We include information about
the reference stations of the adopted geodetic reference frame (usually the
latest realization of the ITRF) by combining our loosely constrained solutions
with the IGS global h-files from SOPAC.
Finally, we apply the reference frame constraints and estimate the site
velocities (Fig. 2 below).
GAMIT is a comprehensive GPS analysis package developed at MIT and
Scripps for the estimation of three-dimensional relative positions of ground
stations and satellite orbits. The primary output of GAMIT is a loosely
constrained solution (h-) file of parameter estimates and covariances that can
be passed to GLOBK for combinations of data to estimate station positions,
velocities, orbital and Earth-rotation parameters.
The analysis software is composed of distinct modules, which:
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|
INPUT |
OUTPUT |
|
NGSTOT |
|
· T-file (earth-fixed tabular ephemeris) · G-file (orbital initial conditions) |
|
ARC |
|
· T-file (inertial tabular ephemeris) · G-file (interpolated from inertial T-file) |
|
ORBFIT |
|
· G-file |
|
|
INPUT |
OUTPUT |
|
MAKEXP |
|
· D-file (FIXDRV input file) · Session.info · Input batch file for MAKEX, MAKEJ |
|
MAKEJ |
|
· J-file (satellite clock file) |
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MAKEX |
|
· K-file (receiver clock) · X-file (input observations) |
|
FIXDRV |
|
· B-file (bexpy.bat ; primary batch file) · B-file (bexpy.nnn ; secondary batch file) · I-file (rcvr clock polynomials) |
|
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INPUT |
OUTPUT |
|
MODEL |
|
· C-file (observed-computed, partial derivatives)
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|
|
INPUT |
OUTPUT |
|
AUTOCLN |
|
· C-file (cleaned) |
|
|
INPUT |
OUTPUT |
|
SOLVE |
|
· Q-file (contain a record of the analysis) · H-file (loose constrained solution) · G, L (update) |
1. Create a working directory
mkdir ~/your_name_here/010
2. Go to your working directory
cd ~/your_name_here/010
3. Copy your data files to your working directory and convert to
rinex format
For instance, if your files are stored in /my_disk/data_directory/010 in
Hatanaka compressed format:
cp /my_disk/data_directory/010/????0100.03d.Z .
uncompress *Z
crx2rnx ????0100.03d
rm *.03d
4. Choose igs files for tie to itrf
Log on to http://igscb.jpl.nasa.gov/
Go to "tracking network" and choose 3 to 10 IGS sites well
defined in ITRF2000, that define a good network encompassing your local array.
5. Get igs data files
ftp lox.ucsd.edu [anonymous]
cd
/pub/rinex/2003/010
bin
get
nlib0100.03d.Z
get
usno0100.03d.Z
get
amc20100.03d.Z
get
auto0100.03n.Z
quit
6. Get cors data files
ftp ftp.ngs.noaa.gov [anonymous]
cd
/cors/rinex/2003/010/wlci
bin
get
wlci0100.03d.Z
quit
7. Uncompress and convert data files to rinex format:
uncompress *Z
crx2rnx nlib0100.03d
crx2rnx usno0100.03d
crx2rnx amc20100.03d
crx2rnx wlci0100.03d
rm *.03d
8. Get ephemerides file, then uncompress and change name:
ftp lox.ucsd.edu [anonymous]
cd
/pub/rinex/2003/010
bin
get
auto0100.03n.Z
quit
uncompress auto0100.03n.Z
mv auto0100.03n epgga3.010
9. Get precise igs orbit, uncompress:
gweek 010 2003 [answer = 1200 5]
ftp igscb.jpl.nasa.gov [anonymous]
cd
igscb/product
cd
1200
bin
get igs12005.sp3.Z
quit
uncompress igs12005.sp3.Z
10. Link tables:
link_tables 2003 /project/geodesy/mit_epicen/tables
11. Prepare l-file (a priori site positions):
Option 1:
gapr_to_l gt/itrf00.apr ltmp "" 2003 010
grep USNO itrf00.apr >! ltest3.010
grep WLCI itrf00.apr >> ltest3.010
grep AMC2 itrf00.apr >> ltest3.010
grep NLIB itrf00.apr >> ltest3.010
rm ltmp
Option 2:
grep USNO gt/litrf00.2003_010 >! ltest3.010
grep WLCI gt/litrf00.2003_010 >> ltest3.010
grep AMC2 gt/litrf00.2003_010 >> ltest3.010
grep NLIB gt/litrf00.2003_010 >> ltest3.010
If a station is not in litrf00.2003_010, then use its approximate
coordinates from the rinex file header and transform them to spherical with
'tform'.
Edit ltest3.010: add one line at the top of the file, for instance 'Epoch
2003.0247: From file itrf00.apr'
12. Prepare station.info (antenna heights, equipment type):
grep USNO gt/station.info >! station.info
grep WLCI gt/station.info >> station.info
grep AMC2 gt/station.info >> station.info
grep NLIB gt/station.info >> station.info
13. Edit station.info:
-
add info for
sites not present in station.info
-
add firt line:
test
pgga
-
add second
line, a format statement:
(A1,2(A4,1X),A16,F7.4,2(1X,F8.4),2(1X,A6),1x,a5,1X,F5.2,1X,I4,1X,I3,1x,I2,6(1X,I2))
14. Prepare sittbl. (site specific processing parameters)
grep NLIB gt/sittbl. > sittbl.
grep AMC2 gt/sittbl. >> sittbl.
grep WLCI gt/sittbl. >> sittbl.
grep USNO gt/sittbl. >> sittbl.
15. Edit sittbl.:
-
add info for
sites not present in sittbl.
-
add 2 comment
lines at the top:
line 1:
SITE
FIX WFILE --COORD.CONSTR.-- --EPOCH-- CUTOFF
APHS CLK KLOCK CLKFT
DZEN WZEN DMAP
WMAP ---MET. VALUE-
---
NZEN ZCNSTR ZENVAR ZENTAU
line 2:
<< MY SITES >>
-
IMPORTANT:
decide on a priori constraints for your sites. For instance, use 99.99/99.99/99.99 for your local sites (WLCI) and 0.005/0.005/0.050 for the IGS sites (NEU, meters).
-
Set the number
of zenith delays to be estimated to 7.
16. Prepare sestbl. (processing paremeters)
cp gt/sestbl. .
Edit sestbl. as necessary for your
experiment.
1. First, prepare session:
makexp test pgga epgga3.010 2003 010 3 ltest3.010 30
00 00 2880
2. Then, prepare orbit file with partial derivatives:
sh_sp3fit -f igs12005.sp3
sh_check_sess -sess 010 -type gfile -file gpgga3.010
3. Prepare satellite clock files:
makej epgga3.010 jpgga3.010
sh_check_sess -sess 010 -type jfile -file jpgga3.010
4. Convert rinex files to x-files:
makex test.makex.batch > /dev/null
5. Prepare gamit batch file:
fixdrv dtest3.010
In the foreground:
csh btest3.010
In the background:
csh btest3.010 > /dev/null &
(yes, that’s all…. Well, if all works fine!)
There are four criteria for determining if a solution is acceptable:
1.
The
uncertainties in the baseline components must have the same magnitude of the a
priori constraints applied to station coordinates and orbital parameters
2.
A good solution
produces a "normalized rms" (nrms) of about 0.25. Anything larger
then 0.5 indicates that there are problems (e.g., cycle slips that have not
been removed, bad coordinates of the fixed stations):
grep "Postfit nrms" qtesta.010
3.
The fractional
part of the solution (Fract) must be smaller than 10.
4.
The adjustments
on the site coordinates (only provided for the two constrained solutions) should
be small on the order of a few millimeters