AN/SPY-1 Radar

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Associated Systems:

The AN/SPY-1 is a multi-function, phased array, 3D air search radar that provides search, detection, tracking, and discrimination data of air and surface targets. It is the main fire control radar for the Aegis Combat System, which equips both Aegis ships and Aegis Ashore sites located in Poland and Romania.

First deployed in 1983, SPY-1 is an S-band, PESA radar capable of providing tracking and discrimination data for ballistic and cruise missiles, aircraft, and other air or space breathing threats. Each radar has four faces to provide a 360-degree azimuth field of view. These radars support not only the onboard Standard Missile interceptors that contribute to regional and fleet defense missions but also provide additional tracking and early-flight discrimination data for the U.S. homeland missile defense mission. Aegis BMD ships have contributed to every Ground-based Midcourse Defense system intercept test since 2002.1

SPY-1 Development


The Navy installed its first SPY-1 prototype aboard USS Norton Sound (AVM-1) in 1973. It was tested extensively over the following years, detecting and tracking commercial aircraft flying over the Pacific and providing guidance for live-fire intercept tests. The next iteration, SPY-1A, was the first operational version and equipped CG-47 to CG-58. It was first deployed operationally in November 1983 off the coast of Lebanon, with Aegis ships joining the Multinational Peacekeeping Force. Employing the advanced multifunction radar, USS Ticonderoga (CG-47) “became responsible for the automatic detection, tracking, and reporting of all surface and air traffic in the area […] She became an unofficial, de facto sea-based air traffic control center.”2 Given SPY-1A’s success, the Navy pushed forward on its development of several follow-on builds, as illustrated in the following table.

Following the 1991 Gulf War, U.S. officials found that SPY-1A track recordings from Aegis ships deployed in the Gulf showed Iraqi ballistic missiles in-flight. This was a significant finding, and it helped push Aegis into the ballistic missile defense mission. As one subsequent report explained, “Initial studies by APL verified the feasibility of modifying the Aegis Combat System, including the AN/SPY-1 radar and Standard Missile-2 Block IV, to add an Area BMD endo-atmospheric engagement capability to protect ports and forces ashore against ballistic missile threats such as the Scud variety seen in Desert Storm.”3 Since APL verification of this potential, the SPY-1 radar has undergone several developments to enhance its BMD capabilities.

The Aegis Combat System, however, was not originally designed for this encompassing mission. While policymakers have funded improvements to get ACS up to speed, the SPY-1 radar remains among the weaker links in the Aegis chain and a limiting factor in Standard Missile interceptions. SPY-1 provides interceptors with track quality data to a range of around 370 km.4 This satisfies the SM-2 requirement, which ranges 160 km, but is not enough for anti-ballistic SM-3 interceptors that can travel up to 700 km (Block IA/B) or 2,500 (Block IIA) km.5 It is also insufficient for the SM-6, which, because of its active seeker, has a potential range of 460 km. This is a primary conclusion noted in a Defense Science Board analysis, which asserts that the current SPY-1 radar is “inadequate” to fulfill ongoing mission requirements.6 Other reviews have reached similar conclusions.7

SPY-1 Upgrades

Until 2016, SPY-1 could not support simultaneous BMD and antiship cruise missile operations. The ships could either employ the radar in volume search mode for long-range, BMD capability, or anti-sea skimming mode to focus on short-range, horizon threats. This forced Aegis ships to travel and operate in pairs to cover both threat varieties.8

The Aegis Baseline 9 upgrade remedied this issue by adding the Multi-Mission Signal Processor to create an IAMD mode.9

The majority of the Navy’s contingent of Flight IIA Arleigh Burke-class Destroyers, which are not currently BMD capable, are also being upgraded to Baseline 9C, which would add their SPY-1 radars to the homeland defense network.10

SPY-1 has faced critical degradation and maintenance issues. This has been linked to the shift in SPY-1 maintenance from highly trained contractors over to Navy technicians, who had neither the time nor opportunity to develop needed skills. According to the infamous February 2010 Fleet Review Panel study, also known as the “Balisle report,” SPY-1 showed “declining materiel readiness,” and “SPY manpower, parts, training and performance are in decline.” Notably, the report also mentions that “BMD operational requirements drove up frequency of reporting in the recent year.” In 2011, Rear Adm. Rick Hunt and Rear Adm. Dave Thomas announced in a joint statement that “In the past 12 months […] cruisers submitted 107 casreps [casualty reports] and destroyers submitted 105 on equipment associated with the SPY radar transmitter.”11 SPY-1 maintenance issues have also been affirmed anecdotally in several publications.12

Service History

Aegis ships are equipped to perform a wide range of missions, which makes them a high-demand, low-density asset. As of June 2016, there are 33 Aegis BMD ships in the U.S. Navy, five Ticonderoga-class Cruisers and 28 Arleigh Burke-class Guided Missile Destroyers, equipped with SPY-1 radars.13


    1. Thomas Karako, Ian Williams, and Wes Rumbaugh, Missile Defense 2020: Next Steps for Defending the Homeland (Washington DC: CSIS, 2017), 73-75,
    2. McLane Jr. and McLane.
    3. Kenneth W. O’Haver et. al., “Radar Development for Air and Missile Defense,” Johns Hopkins APL Technical Digest, volume 34, no. 2 (2018): 8-9,
    4. George Lewis and Theodore Postol, “Ballistic Missile Defense: Estimating the Range of an Aegis Radar against a Missile Warhead Target,” mostlymissiledefense (blog), October 23, 2012,
    5. This is not to say that the Navy needs a ship-based radar with a target discriminating range of 2,500 km, but rather that ships should be prepared use a fair margin of their effectors’ capability without depending solely on offboard sensors.
    6. Defense Science Board, “Science and Technology Issues of Early Intercept Ballistic Missile Defense Feasibility,” September 2011, 8,
    7. The National Research Council writes that “The Achilles heel of the NTW [Navy Theater Wide] program is the Aegis SPY-1 radar, which is an excellent air defense radar but a marginal radar for the full range of NTW mission requirements.” See National Research Council et. al., Navy’s Needs in Space for Providing Future Capabilities, (Washington, DC: National Academies Press, 2005), 106,
    8. Walter L. Perry et. al., Measures of Effectiveness for the Information-Age Navy, (Santa Monica, CA: RAND, 2002): 21,
    9. “US Navy and MDA certify new Aegis Combat System Baseline 9.C1,” Naval Technology, January 13, 2016,
    10. Sam LaGrone, “Navy Again Reduces Scope of Destroyer Modernization, 5 Ships Won’t Receive Any Ballistic Missile Defense Upgrades,” USNI News, March 3, 2015,
    11. Aviation Week, “Come About: Aegis Maintenance Costs Could Threaten AMDR Development,” December 2011, 20,
    12. “…But the groom revealed that the ship’s inertial navigation system was not up to par. My jaw hit the deck when the technician told me that if tasked, I would not be able to complete the long-range surveillance-and-track mission.” See Jim Kilby, “Is Your SPY Radar Enhanced, Nominal, or Degraded?” Proceedings Magazine, January 2012,
    13. J. Michael Gilmore, “Sensors,” Director, Operational Test and Evaluation FY 2014 Annual Report, January 2015,