Reuters: Navy destroyer targeted again by Yemen’s Houthis. Unlike the Sunday attack, electronic countermeasures alone were not sufficient to insure against impact, so two SM-2 missiles were launched, and one short-range Sea Sparrow. Popular Mechanics offers an everyman explanation of countermeasures. This is for the reader who would like just a little bit more, permitting a prediction of the actual culprit.

The simplest explanation of the most basic radar (radio-detection-and-ranging) is that some kind of radio signal is broadcast, or sent in the direction of a target, which echoes a very tiny amount of it back to the station. This description dates to before World War II. A system based solely upon it might have the ability to detect a bomber fleet several hundred miles away, but with barely enough precision for the RAF to send Spitfires to intercept. If one were to compare basic radar with the human eye, it would be a that of a person almost blind from cataracts. The bare radar image is not a point, but a cloud of uncertainty.

A radar system that can actually find a target requires the combination of:

• Frequencies.
• Pulses.
• Polarizations and modes.
• Scans, from one location, or integration (help) from multiple locations. Historically, the transmitter and the receiver have not always been in the same place.
• Detection in a cacophony of noise.
• Signal processing to remove all kinds of spurious indications, such as ground clutter.
• Estimation, linear stochastic  filtering, extracts the likely position of the target hiding within the echo cloud.

All of the above can be forged, faked by the target. This is ECM (electronic countermeasures.)

• Conversely, radar can potentially use the ECM signal of the target to gain information (locate) the target.
• Conversely again, two targets equipped with ECM can cooperate to make a phantom appear between them, guiding a missile to empty ocean.
• Conversely again, a radar station can change its parameters of operation in an unexpected way, defeating ECM unless it is agile.

The Chinese C-802 anti shipping missile is an old, widely exported weapon. As with most of these weapons, the actual, as opposed to advertised kill probability against a target with ECM is not publicly known. All antishipping missiles have two guidance systems:

• To get it on its way, the likely position of the ship can be programmed into the missile, which then knowingly adjusts for where it is and where it wants to be. The most simple type is inertial guidance. In more advanced systems, such as a hacked C-802, the missile can receive updated information in flight.
• As the missile approaches the ship, it switches to its own tiny radar set for terminal guidance. But it is hard for the missile to hear its radar echo, because the target is doing its best to yell louder — with false information.

The closer the missile gets to the ship before relying on its own radar, the greater the chance it will be able to hear its own, truthful echo. But it can have help. A large, stationary radar system can broadcast a signal which the missile’s radar receiver can listen to, in addition to its own weak transmitter. The C-802 missile has been widely hacked by small contractors — improved and customized in a multitude of ways.

In order to get around the ECM that frustrates shore based radar, the Houthis put some observers in tiny boats near the Mason. From the current open source reports,  these  possibilities cannot be distinguished:

• The boats were within optical range. Using something like a surveyor’s total station, the position of the Mason was obtained by optical triangulation.
• A radar station in Houthi territory, possibly for  civil aviation, was used to illuminate the Mason. The Houthi boats could have used directional antennas with widely available spectrum analyzers to perform a cruder-than-optical triangulation.

The attacks comprise an ingenious, if unsophisticated, attempt to leverage outmoded military technology against a sophisticated, high value target. The signature elements are:

• Small boats, potentially in large numbers.
• A low tech run around high-tech ECM.
• Possible innovative use of a technique derived from bistatic radar.
• The gadget on the boats, the product of a small industrial base, attempting an outside-the-box solution.
• A hacked Chinese missile.

The open source estimate is that this is an Iranian test of a “weapons system”, for which the Yemen conflict offers a scenario of perfect deniability.

In dollar cost of weapons expenditure, Iran won. Retaliation is likely in order to equalize expenditures.

Edit: As of 11:55 PM ET, Wed October 12, 2016, CNN reports “Three US strikes hit radar sites in Yemen, Pentagon says”.  The size and static nature of radar sites make easy targets. Some radars have limited mobility, but it’s a big trucking job. The strike is likely to be followed by a protracted effort to attrit the missile launchers, which are mobile.