The package "org.orekit.propagation.analytical.tle" is a Java implementation
of the algorithms described in the paper "Revisiting Spacetrack Report #3
(AIAA 2006-6753)." I am using the Orekit implementation as a partial basis
for a re-coding of those algorithms in Swift.
That requires me to look closely at conversions involving language constructs
that are available in Java but not in Swift. One of these is the "for
( ; ; )" control statement, and I've tripped over a usage in Orekit that seems
odd .. it’s computationally correct, but unnecessarily complicated. In the
file DeepSDP4.java, a constant is defined:
private static final int SECULAR_INTEGRATION_ORDER = 2;
and later in that same file, two loops of the following form:
for (int j = 0; j < SECULAR_INTEGRATION_ORDER; j += 2) {
derivs[j] = term1a + term2a;
derivs[j + 1] = term1b + term2b;
if ((j + 2) < SECULAR_INTEGRATION_ORDER) {
term1a = -term1a;
term2a *= -4.0;
term1b = -term1b;
term2b *= -4.0;
}
}
as I read this, the "for" loop will only execute once (with j = 0), and the
contained "if" condition will never be true. That reduces the above code to:
derivs[0] = term1a + term2a;
derivs[1] = term1b + term2b;
to check this, I made this (and one other similar near-by) change and re-
ran the unit tests with no failure.
There is one more "for" loop that will execute only once (with j = 2):
for (int j = 2; j <= SECULAR_INTEGRATION_ORDER; ++j) {
xlpow *= xldot;
derivs[j - 1] *= xlpow;
delt_factor *= delt / (double) j;
xli += delt_factor * derivs[j - 2];
xni += delt_factor * derivs[j - 1];
}
Am I being incredibly stupid and missing something obvious?