# Transformation between ICRF and Fixed in STK

I am building a satellite orbit propagator in MATLAB and I am using STK as a reference.

Comparing TwoBody propagator using RKN78 (MATLAB) with HPOP (degree 1, order 1), I am getting quite good results in the ICRF coordinate system, but when I transform the MATLAB results in the Inertial frame to the Earth fixed frame considering the precession, nutation and polar motion of Earth and compare the results with HPOP (degree 1, order 1) in the fixed frame, I get position error in few hundred meters for 1 orbit simulation period.

I am looking for more accurate result so is there a way to play with the ICRF-Fixed transformation in STK settings ?

AGI states the following in their online documentation for ICRF-Fixed transformation

"Earth realizes its Fixed frame from the transformation algorithm between it and the ICRF. The transformation includes precession, nutation, and rotation effects, as well as pole wander and frame corrections."

• I don't know how not to be blunt about this, so I'll just have to be blunt about this: Comparing one state (position+velocity) propagation technique against another is a fool's game unless one knows the true state. The group that developed the JSC Engineering Orbital Dynamics had a person dedicated to chasing down the true state of various vehicles for the purpose of comparisons. His task was not easy, nor was that of those who had to try to replicate those orbits using various propagation techniques. Aug 21 '18 at 5:15
• Neither his work nor that of the developers of those evaluations is answerable in the limited space of this website. Aug 21 '18 at 5:16
• @DavidHammen Technically, taking the question literally, "is there a way" only requires a mere "Yes" or "No" answer, so an answer saying something along these lines would be valid: "Yes, there is the way used by JSC, but it is a very involved approach for [reasons] so it cannot be fully specified here. Here are some resources you can use to find out more about this: [resources]". Aug 22 '18 at 14:00
• From your answers, I can say that you did not understand my question so let me clarify my point. I am asking if STK GUI has options which allow the user to control the way inertial-fixed frame transformation is performed. An example would be a checkbox to include or to exclude earth nutation and precession from the transformation algorithm. Aug 23 '18 at 13:03
• I see that after a while this post has comments but not an answer. Did you make any progress on this topic? Do you have your code shared on GitHub or similar? Jan 21 '19 at 14:50

STK provides various ways to report TrueOfDate and MeanOfDate which will help you compare intermediate steps in the ICRF to ECEF conversion chain. You also need to ensure that your pole wander/motion and UTC-UT1 constants match between STK and your Matlab code. Mismatch in polar motion will easily lead to a few hundred meter error.