The AN/MPQ-64 "Sentinel" series radar serves as the core ground-based sensor of the U.S. Army's Forward Area Air Defense System. Since its fielding in 1995, it has been tasked with detecting and issuing early warnings against medium- and low-altitude threats. Nevertheless, as threats posed by low-altitude slow-speed targets such as unmanned aerial vehicles and cruise missiles grow increasingly complex, the original design of the Sentinel, developed at the end of the Cold War, can barely adapt to new battlefield environments. To address this, the U.S. Army launched a comprehensive upgrade program for the Sentinel radar, giving birth to the AN/MPQ-64A4 Sentinel A4. In 2019, Lockheed Martin outbid Raytheon to secure the development contract for the Sentinel A4, undertaking a complete redesign of the radar system. In July 2023, the program passed a milestone decision and officially entered the pre-production phase. As of 2026, the U.S. Army has awarded Lockheed Martin approximately $3 billion in contracts for the mass production and engineering services of the Sentinel A4, with plans to procure a minimum of 240 radar units.

 I. System Architecture and Basic Parameters
  The Sentinel A4 is a 3D X-band phased array radar system capable of supporting beyond-visual-range air defense engagements. The adoption of the X-band (8–12 GHz) delivers an optimal balance between resolution and resistance to weather interference, making it particularly suitable for detecting, tracking and classifying all types of aerial targets in tactical environments.

  In terms of system composition, the core radar assemblies of the Sentinel A4 are mounted on a modified M1095 Medium Tactical Vehicle Trailer, while the generator and communication equipment are integrated onto an M1083 Medium Tactical Vehicle. The entire system can be operated and maneuvered by a crew of only two personnel. Supported by the combination of the trailer and truck, the system forms a complete tactical workflow of "highway mobility – rapid deployment – instant network interconnection", with full setup achievable by a single vehicle in approximately 15 minutes. The radar delivers full 360-degree azimuth coverage and an elevation angle range from -10° to +55°.

 II. Core Technological Breakthroughs
  The most fundamental technological leap of the Sentry A4 lies in its upgrade from the Passive Electronically Scanned Array (PESA) used in the A3 and earlier models to an Active Electronically Scanned Array (AESA). This shift is not an incremental improvement, but a revolutionary restructuring of the radar system architecture.
  Each antenna element of an AESA radar is equipped with an independent transmit/receive module. It can electronically steer the beam to any direction within the coverage sector at microsecond-level speeds, without waiting for mechanical antennas to rotate into position.

The core advantages brought by this technological upgrade are as follows:
  First, the capability to track multiple targets simultaneously. AESA radars can search for and track numerous targets in different directions at the same time. By contrast, traditional mechanically scanned radars are limited by their physical rotation speed, making them ineffective against saturation attacks coming from multiple directions and in multiple waves.

  Second, drastically enhanced beam flexibility and scanning rate. The vertical scanning rate of the Sentry A4’s antenna reaches 90 degrees per second, far exceeding the A3 model’s 137 scans per minute. A faster scanning rate translates to a higher data refresh rate, which markedly improves continuous tracking performance against high-speed maneuvering targets such as cruise missiles, as well as low, slow, small targets including miniature unmanned aerial vehicles (UAVs).

  Third, fully digital AESA design. The Sentry A4 adopts a fully digital AESA architecture. Leveraging digital beamforming technology, it can generate multiple independent beams concurrently, enabling parallel processing of search and tracking missions.