2009/04/01

SAMs with active radar homing

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The classic (albeit not exclusive) method of guiding surface-to-air missiles to their targets is the semi-active radar homing (SARH) technology.
A surface radar "illuminates" the aerial target and the missile seeker uses the reflected electromagnetic energy to find its target.
The technology has been refined to use this guidance only for the terminal approach, using an inertial navigation-based autopilot for most of the flight.


This technology has many disadvantages that have been accepted (far too long in my opinion):
- the need for huge & heavy illumination radars*
- the need for a line of sight between the illumination radar and the target
- the limitation that only one or few nearby targets can be illuminated by one such radar at once*
- the illumination radar gives away its position and thus the position of the ship or the battery

Good examples for late-generation missiles with SARH are early Patriot missiles (land-based area air defence) and the Standard SM-2 missile (naval area air defence missile).

A different guidance (ARH) was used for the 1970's AIM-54 Phoenix missile; it had its own active radar. It took till the early 1990's to field such missiles as standard medium range missiles for fighters (AMRAAM, Derby, R-77, MICA RF).

The same guidance technology (at times identical seekers!) was later introduced into surface-to-air missiles; the prime examples are Aster 15 and Aster 30 (relatives of MICA, photo), Patriot PAC-3 and SM-6 (SM-2 with AMRAAM seeker, yet to be introduced).

This happened with a very long delay. It took about ten to twenty years to adopt the air-to-air missile ARH seekers to surface-to-air missiles. The advantages are a bit different between SAM and AAM applications, but this delay seems to me to be a gross failure on part of the responsible authorities. A SM-6 (yet to be introduced) was already feasible in 1991.

ARH guidance for SAMs deserves much more attention than it really gets. The SARH missiles have proven themselves in peacetime range tests, but their deficiencies are gross.
Examples:

An ARH missile can be fired from a ship even without using the ship's radars. It needs merely a data link to a platform that has a line-of-sight to the target and provides the target information. An AEW (airborne early warning) aircraft, or another ship, for example.

An ARH missile can be fired from a ship at a target over land that's hidden behind hills, a city skyline or simply flying extremely low. The Royal Navy was unable to fire at the Argentinian jets that attacked it from land at San Carlos (Falklands War). An aerial radar + ARH SAM combination would have protected the fleet. A fleet that trusts in SM-2 wouldn't be able to use that missile in a similar scenario.

Anti-radar missiles (ARM) can suppress a ship's or battery's radar, thus turning SARH-guided SAMs temporarily useless. It requires a missile with autonomous homing (like ARH or passive infrared seeker) or a ECM-proof laser beam rider guidance (short-ranged) to keep defending without an active radar. There are also methods and tools to protect a radar against ARMs, of course.

Anti-ship missiles can use features of anti-radar missiles to home on their targets, using the defenders' illumination radar to detect the real target (and to ignore the decoys around it).

SARH-guided SAMs are also less trustworthy against saturation attacks. The dependency on illumination radars and the difficulty in timing its use on multiple targets may be solved in theory by math and computers. It helps that modern SARH missiles need the illumination only during the terminal phase. Nevertheless - a ship with 60 ARH SAMs can engage 30 targets at once with 2 SAMs each if its radar and fire control can handle that. It wouldn't be able to defend against 30 well-timed threats with SARH SAMs.


The newest SARH missile is ESSM (Evolved Sea Sparrow Missile).
It's widely accepted as new armament due to its strengths - although it still sticks to SARH instead of an ARH seeker like AMRAAM's.


ESSM is a very special case because of its unique strengths.
Four ESSMs fit into one standard U.S. vertical launch cell instead of one - the huge quantity of available missiles is a strength in its own right.
Even its predecessor has demonstrated an anti-surface capability in tests and an accident, even blowing a small boat apart.
It's also compatible to the older NSSM (NATO Sea Sparrow Missile), the most widespread Western ship-to-air missile.
The relatively short range (in comparison to SM-2) allows the use of electronically scanned search & track radars for illumination (similar to the use of SARH missiles on fighters in the 70's and 80's). It doesn't need dedicated illuminations radars any more.

I personally dislike ESSM's apparent total reliance on SARH terminal guidance. ESSM should in my opinion be available in ARH and IR seeker versions as well - well, maybe that will happen sometime. Again ten to 20 years later maybe?


Active radar homing air defence missiles are increasingly popular for their huge advantages and prove their worth and accuracy in tests. Such missiles deserve more attention than they seem to get, and the delay of their introduction is rather disappointing.


*: Unless a phased array antenna is used to illuminate, as for example a SPY-1 for an ESSM missile.

edit 2016-1: RIM-162 Evolved Sea Sparrow missile Block 2 is almost ready for production and features an active radar homing sensor mode, in addition to home-on-jam (passive radar homing) and semi-active homing modes. Its range is greatly increased over ESSM, but so will be its price (Block 1 was said to have cost up to 800,000 USD, Block 2 will probably b twice that or more).
My "ten to 20 years" quip  at the end was thus slightly over the top; it's likely going to be nine years counted from 2009 till operational usefulness of ESSM Block 2.

edit: A later SM-6 version is meant to become capable of hitting ships. The high speed reduces the targeting problem considerably compared to subsonic anti-ship missiles (ships move several miles while the missiles travel, which makes targeting specific ships difficult at long ranges). The modest effectiveness of the SM-6 warhead against ships will likely limit them to knocking out or suppressing ship radars or killing boats. 

edit 2016: news.usni.org/2016/03/07/navy-sinks-former-frigate-uss-reuben-james-in-test-of-new-supersonic-anti-surface-missile
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13 comments:

  1. Good post Sven, however ARH is not a silver bullet. There's only so much seeker that'll fit into the head of a missile. A ship's illuminator can be far more powerful.

    Ships using ARH missiles still have to provide rapid updates, flying the missile close enough to allow a lock by the narrow, short-ranged seeker cone.

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  2. I'd prefer a mix or SARH, ARH and IR as primary guidance techniques.
    I place a strong emphasis on the enemy's ability to counter systems - the reliance on a single guidance technology seems to be quite risky to me. The addition of ARH and IR seekers would diversify the abilities and make comprehensive countermeasures much more challenging.

    An illumination radar can create a stronger reflection than an active radar seeker, but that depends on many variables - including the respective distances to the target, respective output and emitter characteristics.

    So far both ARH and SARH missiles have demonstrated their capabilities in tests.

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  3. A mix of complementary guidance modes would be nice, but limited dollars often precludes this.

    The U.S. SM-2/3/6/ESSM suite is further along in this regard than the Euro Aster. SM-2 Block IVA has a dual-mode SARH/IR guidance. And, as you said in your post, SM-6 uses the ARH seeker from AMRAAM.

    Aster uses ARH only.

    Another thing to mention about SARH is that modern systems can fire more missiles against more targets than they have illuminators. They use datalinks to guide the missiles close to the target (aka Mid-Course Guidance mode), and only require illumination for the final intercept.

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  4. That's what I meant in the 2nd sentence.

    I suspect that we'll see an Aster version with MICA's or IRIS-T's IR seeker next decade.

    I also suspect that some missiles have a secondary guidance mode that's command control. That's easy to incorporate in missiles that have a one-way data link anyway.

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  5. IMHO, going the dual-mode guidance route like SM-2 Block IVA is preferable to just replacing the ARH head with an IR head. After all, there are only so many VLS tubes on a ship competing for a variety of weapon/seeker types.

    This is especially problematic for Aster/Slyver at the moment. Down the road they may get VL CAMM and VL Crotale multipacks, but those are point-defense missiles. Aster may just be too small for a dual-mode guidance.

    I wonder if they've considered building a quad-packed, boosted Meteor?

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  6. Crotale VL? No need for that.
    MICA VL is read in ARH and IR versions. It needs some navalization especially of sensor algorithms and fuze technology, though.
    MICA might even be a potential quadpack missile.
    MICA is planned as a later topic, albeit with a different background.

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  7. VL Crotale has already achieved test firings from Slyver A35 cells.

    http://www.defensenews.com/story.php?i=3790560

    It can be quad-packed and provides yet another means of guidance - CLOS.

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  8. "Because the fins are foldable, 16 Crotale missiles can be mounted in a single Sylver launcher.

    MBDA, meanwhile, has developed its agile integrated multispectral air defense system for warships, using vertical launch versions of the air-to-air Mica missile and the Aster weapon. A combination of Aster and Mica missiles using radio frequency and infrared technology seekers offers a stealthy passive capability to defeat countermeasures, an MBDA executive said."

    16 per cell is sick. Even a corvette could have more Crotales ready than a normal 76mm gun has ready shells.

    There's also something very interesting about CC missiles; some (like the Swedes for RBS-23) claim an ability to intercept supersonic ARMs with CC missiles.

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  9. Sven,

    I don't think they mean 16 Crotales per cell. It's 16 per 4-cell "launcher", or 4 per cell.

    http://www.dcnsgroup.com/files/pdf/Sylver.pdf

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  10. Yes ;-)

    Anyway, quad packs are in general a very interesting thing; the planners need to weigh four short-ranged missiles vs. one longer-ranged missile.

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  11. Yes, and there's a push to put even more munition types in VLS cells. I've seen mention of putting Nulka decoys and Netfires missiles in VLS cells to reduce ship RCS.

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  12. A disadvantage to active radar homing compared to SARH on SAMs that I've seen a USN Sea Sparrow tech talk about is that it makes them unusable for anti-ship work and that ARH can have severe issues with sea skimmers (apparently to the extent that early model AMRAAMs were completely useless against low altitude targets protected by ground clutter).

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  13. That depends on the specific design.

    Signal processing and fuze need to be navalized.

    A secondary anti-ship mode would still be possible using the datalink to the missile as command control link instead of the seeker.

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