I've compared the Heat seeking features for all the halo games so far, and Halo Reach and Halo 4 both require to keep the crosshairs steady on the target which MUST be an airborne vehicle, but Halo 2, you just had to aim, hold the right trigger down which locks onto any vehicle (aerial or ground) and works great. Honestly, some things that were done in Halo 2 were just pure genius.