Normally I don't do this because it borderlines tinted shipping... but sadly it's extremely clear that some people need to be taken by the hand and walked through these things.
Anyways, once and for all.. lets get started. I'm using thumbs because I don't want picture spam. Feel free to clicky and make things bigger though.
Here we can see Alto's purple trail flying up from behind Quarter. Hopefully you can distinguish Battle Frontier from Quarter.
And here we have the appearance of Sheryl on Ranka's screen. Take note Battle Frontier is on Ranka's right.
Now we have Ranka appearing on Sheryl's screen, to her left. And look! We also have Alto popping up in Gerwalk form in-between Frontier and Quarter!
And now we have Alto in Battleroid mode. Take special note of the positioning of the VF's legs and body facing towards Sheryl. Also take not of what Ranka is doing... oblivious? I think so. She's already looking straight forward before the salute even happened.
Note: Unless the animation team messed up the distance, depth perception in the scene... Given the size of the Battle Frontier, Quarter and Alto's VF... it'd impossible for Alto's VF to remain that visibly large to both crafts if he were actually floating in the between Frontier and Quarter.
Because of this... it is my belief that since Alto is CLEARLY seen on Sheryl's end that he had been floating near Frontier's bridge.
And the salute. Alto's VF head is turned to the left, which happens to be the side facing Sheryl. Also, take a look at the gif forgottendiary
so graciously provided us. Alto doesn't shift around extra to turn to Ranka.
Here we have the Sheryl and Ranka frame. Sheryl's background shows a partial space battle showing us that she's looking to her left, presumably at Alto due to the scene prior. Now Ranka, her background is the Quarter's bridge meaning she had already been looking directly ahead.
Honestly, how could Ranka have seen something to her far right if she'd been looking straight forward since Alto went Battleroid.
Now last, these two shots of him flying away help us decipher where he was actually facing and if he was closer to one or the other.
Take into consideration that the battle is straight in front of them.. if Alto were already floating in the middle to salute both, he'd just fly straight forward. Instead he flew away while drifting to his left... this hints to us that he had to turn a little to get back into the center of the fight... it also implies that he had been close to something on his left, our right.
Who just happened to be on the Alto's left when he was saluting... and then who was on the right when he was flying away? It was Battle Frontier and Sheryl.
Credits to Skull Fairy Crew.