What sensors are needed to counter the hypersonic threat?

The old analogy of missile defense as “hitting a bullet with a bullet” no longer applies. The muzzle velocity of bullets is “only” 1,800 mph. A hypersonic missile travels at about 3,900 mph, twice as fast. A bullet can’t catch a hypersonic missile!

To implement a hypersonic missile defense, it takes a comprehensive suite of high-performance sensors in orbit, including optical sensors, radar, and infrared, supported by sensor fusion, ground tracking systems, high-speed signal processing, and more.

Detection, identification, and tracking

A space-based sensor suite not only detects the object but also provides the right data needed for classification (what type of missile or other object is it?) and identification (is it a friend, enemy, or neutral object?).

Visual and infrared optical sensors are combined to identify shapes and heat signatures, enabling classification. Identification friend or foe (IFF) transponders are used for hypersonic missile identification. Like all missiles, hypersonic missiles return a radio signal when interrogated, allowing friendly forces to recognize it as their own or identify it as an enemy missile.

Advanced hypersonic and ballistic tracking space sensor (HBTSS) satellite hypersonic tracking systems use multi-spectral imaging to improve performance in the most challenging conditions. High-resolution satellite-based radar can be especially important with limited ground-based radar coverage. The HBTSS provides continuous tracking even after the handoff to the missile intercept system (Figure 1).

Figure 1. The HBTSS satellite system uses sensor fusion to detect, identify, and track hypersonic missiles. (Image: Northrop Grumman)

HBTSS satellites can provide continuous tracking data to counter the hypersonic threat. A combination of space-based radar and infrared optical sensors can provide more accurate tracking data. Each type of sensor alone might have challenges acquiring the fast-maneuvering target, especially during the early phases.

The infrared sensor can be useful for target acquisition, and the space-based radar can track its trajectory. That requires high-speed processing to support continuous sensor fusion. If the radar loses its fix on the target, the infrared sensor can help with reacquisition.

Ground-based radar

Tracking the missile flight path also requires a layered sensor network. Ground radar is good for identifying high-flying aircraft and ballistic missiles but often can’t locate hypersonic missiles traveling and maneuvering at low altitudes due to line-of-sight limitations (Figure 2). Ground-based radar tracks and targets during the terminal phase of hypersonic missile attacks.

Two types of radar are used in a layered architecture in hypersonic missile defense systems. Over-the-horizon radar (OTH) can detect targets at long distances, beyond the radar horizon. OTH radar uses radio signals that bounce off the ionosphere to reach far distances and provide early warning, but are not useful for targeting.

360-degree ground-based radar

Active electronically scanned array (AESA) radar with high-speed scanning capabilities can track and target hypersonic and ballistic missiles and is being deployed in missile defense systems. The lower-tier air and missile defense sensor (LTAMDS) is the latest iteration of AESA technology.

LTAMDS has three antenna arrays, the primary in front and two secondaries in the back, that can simultaneously detect and track multiple threats from any direction (Figure 3). LTAMDS is also part of the Integrated Battle Command System (IBCS) that connects sensors and weapon systems to enhance situational awareness and decision-making in the battlespace command center.

Figure 3. The LTADMS has three AESA radars that can support 360-degree situational awareness and tracking of hypersonic missiles. (Image: U.S. Army)

The AESA in the LTAMDS uses gallium nitride (GaN) power devices in the transmit and receive modules to support longer range and higher resolution by strengthening the radar signal and enhancing system sensitivity. GaN also enables more compact radar systems. The LTAMDS is part of the Patriot Advanced Capability-3 Cost Reduction Initiative (PC3 CRI) interceptor system being deployed against the latest hypersonic threats.

Summary

Hypersonic missiles are fast and agile. That demands that the defensive systems be equally fast and agile. A combination of space- and ground-based sensor fusion using a variety of optical and infrared sensors plus multiple types of radar are required. Hypersonic missile defense systems are being continuously refined into existing systems like the PC3 CRI.