Standard Missile 2 Midcourse Guidance

The RIM-66 SM-2MR in its boost phase

One of the less appreciated aspects of naval warfare is how the same weapon can have rather different capabilities depending on the combat system controlling it. The neglect of this point is somewhat remarkable given that the importance of combat systems was made obvious well over a hundred years ago with the introduction of centralized fire control. But this post is about the RIM-66/RIM-67/RIM-156 SM-2 - the mainstay of Western naval air defense. This missile has been integrated with a variety of combat systems including Aegis, Terrier, and Tartar, as well as several foreign systems, but it actually functions in rather different ways depending on which system is controlling it.

As is widely known, SM-2 represented a massive advance over its predecessor (SM-1) because of its use of a programmable autopilot. This allowed SM-2 to fly a more efficient ballistic trajectory, almost doubling its range, as well as time share illuminators, allowing a ship to engage many more targets simultaneously. But while the abilities of SM-2 are well understood, the exact method by which it accomplishes these feats is not. However, these details are important because they help explain why Aegis was so revolutionary and why it remains the gold standard of naval air defense to this day.

When fired from an Aegis ship, SM-2 operates under a combination of radio command guidance and semi-active radar homing. After completing its unguided boost phase, the missile receives S-band uplinks from the AN/SPY-1 radar. These uplinks are used to directly steer the missile onto the target, with the ship’s computers calculating the optimal trajectory. Upon receiving each uplink, the missile transmits an S-band downlink (also through the AN/SPY-1) that confirms receipt of the uplink, communicates the missile’s position and status, as well as assists in kill assessment. This method of command guidance has several advantages. By using the ship’s computers to decide the trajectory, it only relies on the missile’s more limited processors during the terminal phase. Further, because it uses the downlink for positional information, the ship does not need to use its radars to track its outgoing missiles. Finally, it allows the missile to be placed very close to its target before hand off to the semi-active radar terminal guidance, which means the target only needs to be illuminated briefly and more missiles can be time shared per an illuminator.

But on the Terrier and Tartar ships, this method could not be employed for the simple reason that there was no AN/SPY-1 radar to send uplinks or receive downlinks. Instead, the AN/SPG-55 or AN/SPG-51 illuminators were modified to transmit an X-band subcarrier wave to serve as an uplink. However, this had three orders of magnitude less bandwidth than the Aegis S-band uplink and was insufficient for radio command guidance. Therefore, it was used to transmit an intercept point for the missile to fly to, with the missile itself calculating the trajectory. Further uplinks would only be sent if the target changed course and invalidated the original intercept point. Interestingly, in order to keep costs down, this different form of guidance was achieved by installing an emulator chip on the missile that fooled the guidance system into believing that it was receiving Aegis command guidance rather than calculations from its own autopilot.

The superstructure of a post-NTU Kidd-class destroyer showing the forward AN/SPG-51 illuminator (center) and AN/SYR-1 receivers (port and starboard just above the bridge windows).

Originally, Terrier and Tartar ships with SM-2 had no way to receive a downlink. This meant that they used their air search radars to track their missiles’ positions, which decreased precision and meant the hand off to semi-active homing had to be performed sooner and the target illuminated longer. Additionally, without information from the missile to aid kill assessment, there would likely have been a greater delay in follow up shots in the event the missile missed. These limitations were later mitigated when AN/SYR-1 receivers were installed on most Terrier and Tartar ships as part of the New Threat Upgrade. However, it is unclear if the NTU downlink had all of the functionality of the Aegis downlink.

Today, SM-2 is increasingly being used on foreign ships, the majority of which are the ten APAR frigates of Denmark, Germany, and the Netherlands. While APAR is a more advanced radar than AN/SPY-1, these ships actually control SM-2 using a derivative of the Terrier/Tartar method rather than than Aegis method. It is likely that this was done because APAR is an X-band radar and it was cheaper to adapt the existing X-band datalink. However, it is also possible that APAR (a smaller radar than AN/SPY-1) was incapable of command guidance - especially since it was already tasked with search, tracking, and illumination. But even though these ships went with the existing X-band system, SM-2 still required modifications to work with APAR’s interrupted continuous wave illumination, which theoretically allows a ship to illuminate sixteen targets simultaneously and should mitigate some of the limitations of the Terrier/Tartar method. Still, the APAR system apparently does not include a downlink receiver and is thus in some ways less capable than the NTU ships. In the future it will be interesting to see which method of controlling SM-2 was chosen for the Zumwalt-class, given the similarities between its AN/SPY-3 and APAR.