In Ar tonelico II Sasha talks about launching a satellite. In Ar tonelico Qoga it's revealed that she has launched this satellite, and it's being used to extend Infel Phira's range far enough that Cocona has the ability to use song magic despite being far out of Sol Marta's range.
However, the only reason Cocona is at Sol Cluster in the first place is because Mir needs to deliver the Selfstabilizing Tesseractal Nuclear Loop / 4D Core Square Ring / Heart of Gaea (thank you, inconsistent translations). She can't go herself because Sol Cluster is outside of Sol Ciel's broadcast range and as a β-type reyvateil Mir will die if she leaves the range of her tower.
But Infel Phira's power output is tiny compared to Sol Marta (which is receiving its power from the even more powerful Sol Ciel in the first place). Why can't Sasha extend Sol Marta or Sol Ciel's output and just let Mir go herself?
After the games, Sasha develops a relay that lets Tyria visit Sol Ciel. Couldn't she just have done that, but the other way around?
Out-of-universe, this is obviously because they want to re-use Cocona as a character. But in-universe, it's totally out of character for Mir to trust anyone else if she could've done the task herself. As a reyvateil equal in power to an Origin, it would also likely have been trivial for her to deliver the Heart of Gaea. Is there any in-universe reason given for why Sasha couldn't rig up something to let Mir go herself, rather than spending years faffing around making v-boards?