The Aster missile computes the predicted intercept time according to the target and environmental data it receives during the launch sequence. From the missile flyout and up to the point at which the active RF seeker switches on, the missile is inertially guided, receiving periodic target position and environment updates from the Firing Control Unit via the up-link. …
Implying that Aster missiles have rather long minimum range of engagement, and that Aster ought to be supplemented with a closer-in air defense missile.
There is no reason why the guidance system couldn't have been designed to switch into RF seeker mode immediately after being launched.
Please explain the advantage of midcourse updates via data link over semiactive, provided that one’s system has multiple electronically steered antennae which can keep many targets “painted” simultaneously around 360 degrees az. and 180 degrees el. with mucho more kilowatts than transmitters in Aster missiles can emit. In addition, missiles used by brand AB can also get data link updates.
Taking a guess, a course update could be 'flashed' to the missile in a fraction of a second which would minimize the possibility of any system finding and blocking the ship->missile channel.
Alternatively it could be that the comms method used is similar to mobile phone technology that uses time domain multiplexing - each missile gets a time slot for communication and the command and control system iterates through each time slot updating one missile at a time.
There are many reasons why the system could have been designed as it is, but judging from what little is in the public domain it is quite an impressive piece of kit.
Also, I assume you meant 90-degrees elevation, not 180-degrees.