Sensors play a critical role in identifying, classifying and communicating ballistic missile threats for boost, ascent, midcourse and terminal intercept assets.

Infrared sensors on the ground, in aircraft, or on spacecraft can detect hot spots like motor-vehicle engines, jet engines, missile exhausts and even campfires – but must be 'looking' in the direction to gather the information. They have good location accuracy and high sensitivity to signals without registering false targets such as sun reflections.

Phased-array radars operate with antenna radiation patterns and are widely used in communications, defense, and space applications. Phased-array radars use a large number or individual elements 'arrayed' on a radar face; some systems use a single face, others use multiple faces. Like a mechanical antenna, each individual element can both transmit and receive. These functions are controlled electrically by timing or 'phasing' the signal to each element. While some phased array radars are fixed, systems like the Aegis system have a fixed array that is mounted on a ship and is a mobile radar.

No. The sensor layer for the BMDS will consist of several different types of sensors. These are; Overhead Persistent Infrared, UAV-Based Sensors, Space Tracking & Surveillance System, Sea-Based Radars, Upgraded Early Warning Radar, COBRA DANE radar, Midcourse X-Band Radar, AN/TPY-2 Radar and the SPY-1. For more information, please look at the Sensors page.

Some are currently active and some are still coming online in the near future.

Currently, the Overhead Persistent Infrared uses existing satellite technology to collect, process and communicate data for missile warning and defense.

Unmanned Aerial Vehicles-based sensors are in place and used to protect U.S. forces and allies in forward deployed areas against ballistic missile attack by identifying and communicating threats.

Sea-Based Radars consist of the operational Sea-Based X-band radar mounted on a mobile platform and Aegis ships. The SBX is able to detect, acquire, and track targets to provide data necessary to classify and engage. The SBX is a unique combination of advanced X-band radar with a mobile, ocean-going, semi-submersible platform that provides the BMDS a sensor capability that can be positioned to cover any part of the globe. Aegis destroyers detect and track ICBMs and report track data to the missile defense system. This capability shares tracking data to cue other missile defense sensors and provides fire control data to Ground Based Midcourse Defense (GMD).

Space Tracking & Surveillance System is planned to be a constellation of low-earth orbiting satellites that will provide a global capability to detect and provide critical tracking information about ballistic missiles. The system will provide end-to-end tracking, discrimination of warheads and decoys and transmission of data to other systems to cue radars and intercept assets. STSS will also be able to provide hit/kill assessments to process and improve BMDS intercepts.

There are three operational Upgraded Early Warning Radars (UEWR) using solid state, phased-array surveillance operating from Beale Air Force Base in California, Thule Air Base, Greenland, and RAF Fylingdales in the United Kingdom and an operational COBRA DANE Radar at Shemya, Alaska. The UEWR's provide integrated tactical warning and attack assessment as well as providing estimated launch and impact points. These radars operate in the Ultra High Frequency Band and can detect objects out to 3000 miles.