You've never heard of Radiant Silvergun?!? It was released for the Saturn (in Japan only, natch) in 1998. In fact, Ikaruga was originally going to be Radiant Silvergun 2. Have you noticed when you boot Ikaruga up, under the Treasure logo it says Project RS-2? There you go. If you're familiar with RS, you'll notice a lot of similarities in Ikaruga.
Here's a review of RS that I wrote for my OPCFG site, if you want to find out more about it:
http://www.geocities.com/opcfg/silvergunreview.html