The U.S. Navy has apparently decided that to go with the +15 option (that is, a factor of 32 improvement in signal-to-noise) in the S-band part of the Air and Missile Defense Radar (AMDR) on its planned new Aegis destroyers, according to its DDG-51 Shipbuilding program manager, Navy Captain Mark Vandroff.   Speaking at a Surface Navy Association’s event on January 15, Vandroff stated that “What we’ve locked is the AMDR at SPY [radar] plus 15 decibels” for the new ships.[1]  Under current plans, the first of the new ships, designated DDG-51 Flight IIIs, would be procured in FY 2016 and would achieve an initial operational capability in 2023.

Figure source:  Capt. Vandroff’s Powerpoint slides, available at:  http://www.navsea.navy.mil/Media/SNA2013/DDG51%20UPDATE%20(CAPT%20Vandroff)%20-%20FINAL.pptx.

Background

The Aegis destroyers currently being procured and built are the DDG-51 Block IIA versions, equipped with the most recent and capable version of the Aegis radar, the SPY-1D(V).  See my blog post of August 3, 2012 for a description of the different Aegis radar variants.  All of these new ships will be delivered with ballistic missile defense capabilities built-in.

However, the current Aegis radar is significantly underpowered for many important missile defense applications.   For example, a 2011 Defense Science Board report stated that “The current Aegis shipboard radar is inadequate to support the objective needs of the EPAA mission” (p. 26) and “Radars of much more substantial operating range than the current radars on Aegis ships will be necessary for the full realization of a robust regional defense” (p. 8).

In particular, the ranges of the current S-band Aegis radars against missile targets are significantly less than those of the current TPY-2 ground-based X-band radars.  See the blog posts of September 21 and October 23  for estimates of the ranges of the current TPY-2 and Aegis radars against a missile target.  (S-band is between two and four GHz, and the current Aegis system operates between 3.1 and 3.5 GHz.  X-band is between 8 and 12 GHz, with the current TPY-2 likely operating between about 9 and 10 GHz.)

Starting in FY 2016 (with ship number 123) U.S. Navy destroyer production will switch over to the new DDG-51 Flight III ships, in which the current SPY-1 radar is replaced by the new Air and Missile Defense Radar (AMDR).  The AMDR replaces the current S-band Aegis SPY-1 radar, with two radars, the S-band AMDR-S (which is the radar Capt. Vandroff is referring to in the quote above) and the X-band AMDR-X, along with a common control system.  The AMDR-S will be a large four-faced S-band  phased-array (similar to the existing Aegis radar) and will be used for volume search and for air and missile defense.  The much smaller AMDR-X radar will used for roles such surface and horizon search.   Vandroff’s slides indicate that early purchases of the DDG-51 Flight III destroyers will use a single rotating SPQ-9b radar as the AMDR-X (in place of the current SPS-67 radar).  Later Flight III ships (possibly starting with FY 2024 purchases) would use a three-faced SPY-3 X-band phased-array radar as the AMDR-X.

A January 2012 report by the Government Accountability Office indicated that, at that time, two primary options in terms of antenna size were under consideration for the S-band radar of the AMDR, a 12-foot (roughly similar in diameter to the current antenna) SPY + 11 version and fourteen-foot SPY + 15 version, where the number following “SPY” gives the signal to noise advantage (in decibels) for the radar relative to the current Aegis SPY-1 radar.  Note that 11 dB = 12.6 and 15 dB = 31.6.  Thus the SPY+15 radar would have a signal-to-noise ratio against a given target about a factor of 32 greater than the current Aegis radar.

Discussion

Leaving aside factors outside the radar designer’s control (or that can be set equal), the S/N a radar can achieve against a given target will be proportional to its average power P_AV, its effective antenna area A_E and its antenna transmit gain G, and inversely proportional to its system temperature T_S and system losses L_S:

The current Aegis SPY-1D(V) and TPY-2 radars appear to have roughly equal average powers and antenna areas (the Aegis antenna is slightly physically larger).  However, the TPY-2 has a large advantage in gain G (nearly a factor of ten) because its wavelength is about a factor of three shorter than that of Aegis (and gain is inversely proportional to the square of the radar wavelength).  The TPY-2 radar, which uses solid-state transmit-receive modules on its antenna, also likely has significant advantages in both system temperature and system losses compared to the Aegis radar which is powered by centralized transmitters using vacuum tubes.    Taken together, then, the current TPY-2 might have an advantage of very roughly 20-30 times (or possibly even more) in S/N over the current Aegis radar.[2]  For long-range missile tracking or discrimination, a factor of 30 would correspond to roughly a factor (32)¼ = 2.3 in range.

If the above estimate of a factor of about 30 is correct, than the +15 dB ≈ 32 increase provided by the new S-Band AMDR radar would give a S/N capability about equal to that of the current TPY-2.  However, against warhead and missile targets the AMDR would likely see a somewhat larger (roughly  a factor of three) target radar cross-section due to its lower operating frequency) which could give it an advantage.

The figure below compares the current SPY-1 antenna to the planned S-band antenna of the AMDR based on slides from Capt. Vandroff’s presentation.

Figure 2: Comparison of sizes of current Aegis SPY-1 antenna and planned AMDR-S antenna.

This figure indicates (based on simply measuring the relative sizes) that the new AMDR-S antenna area will be about 75% greater than the area of the current SPY-1 antenna.  Assuming that the new radar operates at the same roughly 3.3 GHz frequency, than its gain will also increase by about 75%.  Thus the AMDR-S gains about a factor of three in S/N relative to the current Aegis radar simply due to its larger antenna.  The remaining factor of ten to get a total factor of about 15 dB = 32 will have to coming to from increasing the average power and decreasing the system temperature and system losses, increases which seem plausible.

Will the 14-foot, SPY +15 new radar be powerful enough?  According to the January 2012 GAO report, “Flight III with a 14-foot AMDR will not be powerful enough to meet the Navy’s objective, or desired IAMD capabilities.[3] [IAMD =Integrated Air and Missile Defense] .  The GAO cited two recent Navy studies.[4]  A 2009 “Radar/Hull Study red team” said that a SPY+15 capability would give a “marginally adequate” capability against the threats considered in that study.  An earlier Maritime Air and Missile Defense of Joint Forces (MAMDJF) study rejected the SPY+15 as inadequate and concluded that a radar close to a SPY+30  capability (30 dB = 1,000) would be needed for the most stressing threats.  This would require an array diameter greater than 20 feet.   However, according to the GAO, the Navy has concluded that 14-foot radar now planned is the largest that can be built into the existing DDG-51 hull.