How would you measure conversion for an iPhone App Download?

I want to test how users convert from my web page to downloading my iPhone app. Right now the best funnel I figured out was to go through a page that pings Google Analytics 开发者_JAVA百科and then redirects to an iTunes link. But this funnel only measures conversion for users who got redirected to iTunes and not if they actually downloaded the app once they saw it in iTunes.

Anyone knows of a way to measure conversion to the actual download?

(I know this is not a coding question but still a problem a app developers would encounter trying to market their app)