ADVANCE uses a **12th order
Runge-Kutta-Nystrom integrator **that is current state of the art according
to Montenbruck and Gill. The algorithm and coefficients were found at http://www.josef-graef.de/baeume/rkn_i.html.

ADVANCE uses a **modified Harris-Priester density model in
the range from 100 km to 2000 km**. Model compiled with tables and
algorithms from Montenbruck and Gill and the Java Astrodynamics Toolkit (JAT).
Also calculates the atmospheric deviations due to the **solar diurnal bulge**.

Includes the **first 20x20 terms of the JGM3 model**. The
full 70x70 JGM3 model is also included in the zip file. This many terms
may not make much difference at our level of accuracy. I have
zipped a program, UNNORMALIZE.EXE, with source, which I wrote to convert the
JGM3 normalized coefficients to unnormalized coefficients and then print them
out according to the 3 column arrangement required by the program. UNNORMALIZE
can be easily recompiled to print out the unnormalized coefficients in
ascending order. The table of normalized gravity coefficients was obtained
from JAT. There are several other state of the art gravity and atmospheric
models available on the JAT site.

Uses the **RV2EL **algorithm to
convert back to TLE mean elements for output. An explanation of RV2EL can be
found here: RV2EL.txt.

The operation is always in batch mode, i.e., it will advance
every TLE in the default input file, advance.txt, to the first ascending node
after the YYYY MM DD on the top line. If you only want to advance one
satellite, you just put one satellite in the advance.txt file. Default output
is to advance.out and echoed to the screen. Satellites with TLE dates past the
advance date are skipped. Perhaps this program will prove useful to generate
search orbits for a list of lost satellites. The default choices for input and
output files are so I can just double click ADVANCE.EXE and have it run. You
can also run it from a command prompt with one argument (the input filename)
or two arguments (input and output filenames). Another way to begin operation
is to **drag and drop **an input file on ADVANCE.EXE.

As input, ADVANCE uses TLEs. TLE's are optimized for propagation by SGP4/SDP4. ADVANCE converts the elements internally from SXP4 mean elements to osculating elements using a differential correction fit to SXP4 generated pseudo obs. The TLE is then advanced to the target date using the numerical algorithm. Once advanced, the elements are converted back into SXP4 mean TLE elements.

Scott Campbell

campbel.7@hotmail.com