I was trying to understand how SGP4 computes the propagated position I had a look on the original publication paper but i have not found a satisfactory answer. So I had a look to the Matlab implementation of the SGP4 library (and its usage example) on https://www.celestrak.com/publications/AIAA/2006-6753/. But i got really confused.
It seems that the original TLE is propagated linearly head until finish time without timesteps (function sgp4.m). Clearly this cannot be true, it must take into account a timestep placing sgp4 calls into a loop (as the example code usage shows) and intermediate results most probably kept in the "satrec" structure.
However the question remains and is the following:
What is the point of propagating for example, the rate of ascending node (nodedot) since the starting TLE, linearly and straight to a intermediate or final time (tsince)?
I mean, for example the "nodedot" is nonlinear and depends mostly on J2 perturbation (if i remember correctly) so "nodedot" would change between integration timesteps. It doesn't make any sense assuming the initial "nodedot" in all propagation steps!
I understand might be a very basic question, but what am I missing?