Frame2 Tutorial - Purpose of Line 112

Orekit usage
1

I’m new to Orekit and Java.

I working to understand the lines 111 to 116 in Frame2, particularly lines 112 and 113 (114 and 115). It seems like line 116 could be written as:

final Transform gpsToItrf = new Transform(date, formerTransform.getRotation(), measuredTranslation);

eliminating the need for line 113. It is possible to incorporate Line 111 into Line 112 eliminating lines 113 to 116. Do lines have to do with updating the frame tree in the right order?

Thanks,
Scott

2

Hello @BigB welcome

Extracting only the rotation from the transform would lose the time derivatives (i.e. the rotation rate). We need these derivatives to transform velocities.

3

Thanks, @luc. So line 112 handles the “rotation” of the basis but not the translation. We exact the rotation line 113 and combine it with the translation in line 116 to get the final transform from gps to ITRF. Correct? And there is no way to incorporate the translation into line 112?

4

Your understanding is correct, we combine a rotation and a translation.

It is not possible to incorporate the translation in line 112 because this line computes the transform between two frames that are related at all to the measured position and velocity (set at lines 97 and 98 and merged into a Transform, really a translation, at line 111). So measuredTranslation (line 111) and formerTransform do not share any data; this is why we combine them later on.

5

Thank you,