Perturbation theory
Cranford continued to work on the modelling, eventually leading Lane to publish "Spacetrack Report #2" detailing the Air Force General Perturbation theory, or AFGP4. The paper also described two simplified versions of the system, IGP4 which used a simplified drag model, and SGP4 (Simplified General Perturbations) which used IGP4's drag model along with a simplified gravity model. The differences between the three models were slight for most objects. One year later, "Spacetrack Report #3" was released, included full FORTRAN source code for the SGP4 model. This quickly became the "de facto" standard model, both in the industry as well as the astronomy field.