The AN/SPY-1 S-band volume search radar is the core of the AEGIS battle management system. It has had a number of upgrades; the AN/SPY-2 is an add-on component for ballistic missile defense, whose antenna mounts on top of that of the SPY-1. Other radars work with it, such as the AN/SPG-62 as the target illuminator on all AEGIS vesssels, and, only on the Ticonderoga-class cruisers, the AN/SPS-49 longer-range air search radar.
AN/SPY-1 is an S-band radar intended to track a large number of targests in a volume of airspace. AN/SPY-3 combines the S-band functions of the SPY-1 and the X-band features of the SPY-2, but is not physically compatible with the SPY-1 mounting and will go onto classes of ships other than the Ticonderoga, Burke and Kongo.
AN/SPY-1A was installed in the now-retired CG-47 Ticonderoga and other Ticonderogas until Philippine Sea (CG-58). The remaining Ticonderogas received the next major generation, AN/SPY-1B, starting with Princeton(CG-59) and the remaining AEGIS cruisers. The SPY-1B's were upgraded in place to SPY-1B(V).
Burke and Kongo destroyers received the AN/SPY-1D. The AN/SPY-1F was developed for the Spanish F-100 frigates, and is physically smaller and has lower power, but similar functionality to the SPY-1D.
The AN/SPY-1D(V), under development for installation in some Burke Flight IIA ships, is an improved system with improved clutter rejection (needed in the littoral), more moving target indicator (MTI) reatures, and better protection against electronic deception.