Guide to using SATFIT31 to Calculate an Initial Orbit
-------------------------------
1. Create a directory on your hard drive - any suitable name will do
eg C:\SATFIT\
2. Unpack SATFIT31.ZIP in this directory. You should then get four files as
follows
Stations.in
Satfit.exe
Elfind.exe
Elcord.exe
Stations.in is a listing of observers, their station identification and their
geographical coordinates. This is necessary in order to identify the station
making the observations. If you do not yet have an identifying number, choose
for yourself a number NOT in this list, add it to this list and insert your
geographical coordinates and other details. Make sure you use this same number
when generating an observation report, in the so-called IOD format, for running
in this software. IOD format is explained here:
http://www.satobs.org/position/IODformat.html
3. In order for these programs to work you need to have a minimum of three
observations of the satellite, made over a short arc on the same pass. A
properly formatted observation is recorded as a single line in one of the four
IOD formats using right ascension and declination codes 1,2,3 or 7. A typical
file (called SAT.TXT in this example) can have any name but must have extension
.TXT, looks like the following:
90020 00 000A 0433 G 20070531185000000 56 15 0437346-644639 39 +080 05
90020 00 000A 0433 G 20070531185200000 56 15 0539091-730036 39 +070 05
90020 00 000A 0433 G 20070531185400000 56 15 0815553-785103 39 +070 05
90020 00 000A 0433 G 20070531190000000 56 15 1401224-563550 39 +070 05
90020 00 000A 0433 G 20070531191000000 56 15 1501390-182552 39 +070 05
90020 00 000A 0433 G 20070531192000000 56 15 1522555+005113 39 +070 05
90020 00 000A 0433 G 20070531193000000 56 15 1536559+120925 39 +060 05
Note that in this case there are more than three observations made on the same
pass.
4. Type SATFIT sat.txt
SATFIT will load and list the observations found in the file SAT.TXT with each
observation being given a line number. You may now either Q(uit) the program or
select which observations you want to process. Assume this is observations
#1, #5 and # 7 so type in
1 5 7
and press the ENTER key. Note that each observation is separated by a space. The
program allows one to enter as few as 2 observations but beware that the orbit
derived from this can be a long way out so where possible aim for at least three
observations on a pass.
Using these observations SATFIT will derive an orbit and display it in the
format of a standard TLE element set. If you are not happy with this
particular element set ( assuming you have more than three observations) you can
select a combination of any three observations and derive an orbit as long as
the observations are listed in chronological order. When satisfied with the
result press Q(uit) to exit the program.
NOTE: It is only necessary to type the first letter of each command, such as Q
etc. The meaning of the command will be shown as Q(uit) etc. The commands are
not case sensitive.
5. Upon quitting the program, a NOTEPAD window will automatically open where you
can delete TLE's you do not want to keep, change BStar values etc. You do not
really need to delete the unwanted TLE's as long as you make sure that the one
you want to use is the first TLE in the file.
When done with NOTEPAD save and close the NOTEPAD window.
6. The Sat.txt should now automatically open in SATFIT.
Type F(it) to check how well the observations match the element set. If
you have have more than three observations for the satellite pass you will see
all your observations made and how well the element set matches your
observations and show the "rms" overall error for the fit followed by the
element set used. The program now waits for your next instruction. If there is
any observation that stands out as being obviously wrong it can be removed with
the R(emove) command.
Now type S(tep). The resultant rms error will be shown and by simply pressing
the ENTER key after each value and you should see the rms values converge. Once
you are happy that no further improvement is possible quit the program by typing
Q(uit).
Your observations will again be listed along with the residuals for each
position, followed by the new rms value and the last element set derived from
your S(tepping) command. Again, if you do not like any observation it can be
R(emoved), then run F(it) and S(tep) again.
7. If you are happy with the final result then type W(rite), followed by U(pdate)
and then Q(uit). If you forget to do this you will get an error message in the
next step stating INCORRECT ELEMENT FORMAT. If this happens just type W,
followed by U and Q and it should solve this problem.
8. You will then be asked to enter a command. Note that the next two commands
are NOT listed in the command options shown.
Type C(el_C_ord).
The program will reply with how many observations found and list the element set
and wait for your next command.
9. Type I(nitial).
The computer screen will now display a lot of data in rapid succession as it
runs a batch run on improving the orbit to match the observations. At this stage
there is no input from the user.
10. Type Q(uit) to exit this part of the program (ELCORD) and type Q(uit) again
to exit SATFIT and that's it. The final orbit derived and the observations will
be found in SAT.TXT. Note that the final element set is at the top of the file
followed by the observations.
This orbit should be good enough to enable you to find the satellite again on
its next pass over your observation site. Multiple observations of a satellite
made over a span of several days will firmly establish the orbital elements.