ELFIND.5 approximates an unknown orbit from two or three observations. Even with only two observations, three parameters of the unknown orbit can be very closely determined : inclination, ascending node and longitude. ELFIND presents the computed orbit parameters as a TLE.

ELFIND finds and reads observations in the IOD format and computes a set of orbital elements matching the observations. The observations must be made on a short arc from a single pass.

The default file for ELFIND input is named "unid.txt". However, any file name may be used as a command line argument to ELFIND.


For example:
         elfind sat.txt

would search for observations in the file "sat.txt" and append the computed TLE at the end of the same file. If two command line arguments are found, ELFIND interprets the first as the input file and the second as the output file. If ELFIND is executed with no command argument i.e.


The file "unid.txt" is searched for properly formatted observations and the computed TLE is appended to the end of the file "unid.txt".

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. Two observations give the program enough information to find the orbital elements of the circular earth orbit through those observations. Three observations find all parameters but the atmospheric drag. A nonzero value is supplied for the drag element in both cases.

One important consideration while running ELFIND. Although the program will accept three observations as input, the orbit calculated EXACTLY fits the three observations. Unless the observations are very close to the exact orbit, wild values for eccentricity and mean motion will result and the program may fail altogether if the three observations describe an impossible orbit. For SATID a two observation fit to a circular orbit is usually the most appropriate and is sufficient to find the inclination, ascending node, and true longitude at epoch. SATID tries to identify the UNKNOWN by finding a match to these three parameters. ELFIND requires that the file "STATIONS.IN" be present in the same directory so it can read the observations.

When executed, ELFIND prompts the user to enter the row number of the observations to be used in calculating the preliminary orbit. Type two numbers separated by a space and press [ENTER]. No further user action is required. The preliminary orbit will be displayed in the DOS window and written to the output text file.

[elfind results]

ELFIND.4  adds the capability to make multiple calculations without exiting the program.  The program is now ended by entering a "Q" at the prompt.  All TLE's are appended to the input/output file.