I am working on an implementation of an SLR tool. At some points of the documentation of the CRD format the term telescope invariant point shows up. Apparently it's a point on an ILRS antenna system. From what I have got so far it is some (arbitrary?) point of the antenna system which is not known beforehand but has to be determined through observations.
The documentation states
Note that station transmit and receive times are nominally with respect to the system reference point (SRP) which will in many cases be the telescope invariant point. This requires a knowledge of both the transmit delay and receive delay, which is critical for transponder ranging. It is less critical for normal satellite (two-way) ranging since errors in distributing the system delay to these components will cancel.
If the telescope invariant point is coincident with the station reference point (SRP) I do not have to take care of any corrections at this point?
I also do not get the sentence "This requires a knowledge of both the transmit delay and receive delay". In how far could the transmit and receive delay of an antenna system influence a reference point?