January 05, 2018

Why the Carriers Are Not Doomed Part 1

Claims that US carriers are very vulnerable to missile attack, and will be sunk immediately in any upcoming war, are quite common. They’re also wrong. The carriers are surprisingly survivable, and the prowess of missiles is usually grossly exaggerated.

USS Nimitz, USS Ronald Reagan and USS Theodore Roosevelt operating together, November 2017

The first line of defense is the fact that the sea is very big[Citation Needed] and ships are small and mobile. Obviously, improved radar and modern navigational systems have made it easier to find and track ships at sea since WWII. Clouds and darkness are no longer serious obstacles, and it’s harder (though not impossible) to mess up your navigation. But there are still problems. A typical radar system will tell you that there is something there, but it won’t tell you what it is. You can make guesses based on where contacts are, but a smart opponent will do all he can to frustrate this. Carrier groups, for instance, never operate in the “bullseye” formation made famous in photos, and often do their best to look like merchant traffic.1

The George Washington Battle Group demonstrating the bullseye formation

The preferred way to solve the identification problem is electronic signal measures (ESM). At the most basic, if there is a radar that is only installed on carriers, and you detect it coming from a ship, then you know it’s a carrier. On a more sophisticated level, most military radars are essentially hand-built, and it is possible to take a ‘fingerprint’ and identify a specific radar remotely. The US, Russia, and China all have sea-surveillance ESM satellites. However, these can be defeated by emissions control, or EMCON.2 While this obviously limits a ship’s capabilities, satellite communications allow ships to avoid most of the disadvantages of EMCON. A satellite can transmit to the surface over a broad area, and a ship can send a tight beam back to the satellite, which is very difficult to intercept. The ship can rely on data from remote sensors (including ESM and IR satellites) to learn of an incoming attack, and only break EMCON after it thinks it has been located.

A long-duration exposure showing a pair of US Naval Ocean Surveillance Satellites

But what about other alternatives? Optical cameras don’t require cooperation from the target, but are stymied by clouds and darkness. IR cameras can't see through the atmosphere to sea level very well. Modern Synthetic Aperture Radar (SAR) platforms like TerraSAR-X/TanDEM-X are quite capable of identifying ships from space. However, they can only see a limited swath of the ocean, particularly in the higher-resolution modes that are required to get firm identification. The combined TerraSAR-X/TanDEM-X cost approximately $400 million. Jane’s says that it can image 90% of Earth’s surface within 2 days, although there are several different resolution modes and I don't know exactly how this was calculated.3 The X-band radar used is badly affected by rain,4 and the US used to hide ships from Soviet radar satellites in bad weather. I’m not sure how effective electronic warfare is against this kind of system, and sources differ on the matter.

TerraSAR-X and TanDEM-X in formation

But what about non-space-based options? Can’t we just send out planes or drones to look for the carriers?

The obvious problem we come to again is emissions. A radar can usually be detected at least twice as far away as it can detect anything. The obvious solution for the carrier is to use some form of low-power low probability of intercept (LPI) radio to vector fighters out to kill the snooper, or even simply brief them on deck and send them out under full EMCON. The E-2 Hawkeye has a very good ESM system, and with tankers and cleverness, could manage to at the very least significantly mitigate the ‘flaming datum’ problem. Optical surveillance, besides being flummoxed by clouds, has a very limited range compared to radar, and is best used for identifying targets found by radar. The obvious suggestion is drones, but drones have issues of their own. A search plane could be sent out under EMCON with orders to only report back if they find something. This isn’t really viable for drones, which can then be shot down on the basis of their transmissions.5 Also, drones of the size necessary for effective sea surveillance are large and relatively expensive.

An E-2D Hawkeye

But eventually, the carrier’s luck will run out, and the bad guys will locate them, and they’ll die, right?

Not so fast. A location, particularly one that is targeting-grade, is inherently perishable, and if your opponent knows you’ve got it, then it’s going to spoil even more quickly. Essentially, you have to get your weapons near the target before the area where the target can be gets bigger than the area the weapons can search, and then make sure that those weapons lock on to the right target. This is true regardless of the nature of the weapons, although different weapons have very different parameters for search and lock-on. One example would be an air strike. Spending a lot of time searching for the carrier is a good way to get killed by defending fighters, but the attacking aircraft can still compensate for some degree of imprecision in the fix, and make sure the target they're going after is what they want to hit instead of something else that happened to be nearby.

An F-14 intercepting a Tu-95 Bear

A missile is much less capable of such compensation, and this has been a long-term problem with anti-ship missiles. The US has stopped installing Harpoon missiles (range of ~70 nm) on destroyers, and removed the submarine-launched version from service, out of fear of hitting the wrong target. The original Tomahawk Anti-Ship Missile, which had an effective range of over 200 nm, was going to solve this problem by using ESM, looking for the radars of a hostile ship. However, this was problematic, and in exercises, the hit rate was around 25%. Some Soviet missiles had datalinks to allow updates after launch, but these required a platform within sensor range of the target.

Iowa launching a Harpoon

Ballistic missiles have an even bigger problem. Unlike a cruise missile, they cannot wait and search for a target. The first ASBM, the Soviet SS-NX-13, was designed to home in on radar emissions, but never entered service, as it would have displaced ballistic missiles on submarines, the numbers of which were limited by treaty. The guidance mechanism of the DF-21D is not known, but there are only a few options. Optical guidance is right out, as it doesn’t work at night or in bad weather. IR does work at night, but is still vulnerable to bad weather, a common problem at sea. Radar might work, but is vulnerable to countermeasures and there are limits on what can be done with a platform the size of a missile warhead, particularly in terms of non-cooperative identification. In plain English, the missile can’t tell the carrier from a destroyer with a blip enhancer or a merchantman. Home-on-radar is an option, but can be foiled by not using radar or using a different kind of radar than the missile is expecting.6

A DF-21D missile

All of this gives a very limited window for the attacker to take a targeting-grade sighting and get the data to the weapons. This is usually ignored in popular articles, but it’s very important. Every minute, a carrier at full speed gets another ~900 m away from the initial fix. The internet makes instant, reliable communication seem easy, but the list of outages and failures due to various bad actors should be a reminder that this kind of communication is difficult and vulnerable to disruption. Throw in an active and very hostile power, and the network is likely to be seriously disrupted at best, and brought down at worst. China might have made the investment in targeting platforms and communications to be able to effectively target something like the DF-21D, although we can’t be sure how well their efforts will work until it’s tried for real. Enemyistan almost certainly hasn’t. Early US attempts at this kind of targeting often saw data delayed by an hour or more, and they were not facing the full array of countermeasures that a major power can bring to bear. If the delay is too great, then there’s nothing in the missile’s field of view when it shows up, and it either plunges into the sea or takes out something it thinks is a carrier, but turns out to be a Danish container ship.

A Danish container ship

None of this should be taken to mean that the US carriers are completely safe from detection. There are ways to find ships at sea, but they are expensive and difficult, and their effectiveness can’t be absolutely certain. That said, these problems are an important part of the equation, and someone who ignores them when discussing the matter should not be taken seriously.

The difficulty of finding the carrier is only the first line of defense. Next time, I'll discuss the active defenses a carrier can mount to an attack.

1 See this article for a more detailed explanation of how this is done.

2 Basically, you just don't use the radio or radar. This is common practice in western navies.

3 The best data I have suggests that you can get data between 20 and 45 degrees off-nadir, which translates into the ability to look at areas between 170 and 460 km off the ground track, on one side. This is probably enough to fix and identify a carrier that has already been roughly located, although the finer modes have swath widths of only 10-30 km.

4 For some reason, Airbus fails to mention this in their marketing literature

5 Directional satcom works much better on ships than on airplanes.

6 The SPY-1 radar of Aegis has special modes to fool radar-homing missiles.

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