Definition and Objective
The term "low probability of intercept" (LPI) does not have a single precise definition. By textbook definition, it generally refers to keeping a radar system's emitted signal below the detection threshold of an opponent's electronic intelligence receiver while still detecting targets at the required range.
The aim of an LPI radar is "I can see the adversary, but the adversary cannot see me." In other words, the radar can detect targets at ranges where an electronic warfare intercept receiver cannot detect the radar signal.
Intercept Conditions for EW Receivers
For an electronic warfare intercept receiver to intercept a radar signal, several conditions must be met: time alignment, spatial alignment, frequency alignment, polarization alignment, and sufficient gain and sensitivity.
Origins and Early Research
The origins of LPI radar trace to radio-frequency stealth techniques. As early as 1941, researchers in the UK proposed methods to reduce radar cross section (RCS). Theoretical exploration of LPI radar began in the 1970s. In 1979, Robert G. Siefker published "Interception of Stealthy Radars", which initiated research into low-probability-of-intercept radar.
Engineering research into LPI radar was driven by U.S. precision strike programs and developed in the context of radar stealth applications.
Sneaker, LPIR, and Pave Mover Programs
The first demonstration project on LPI radar technologies, the "Sneaker program", was conducted jointly by the U.S. Navy and Westinghouse, though reports are limited. The first systematic LPI project, LPIR, began in the mid-1970s and was initiated by DARPA, the U.S. Air Force, and the U.S. Navy, with Hughes Aircraft as the primary contractor.
LPIR provided a conclusive demonstration of LPI technology applied to airborne weapon systems. The first flight tests of an airborne LPIR system were completed in 1979 and 1980. The system conducted 57 flight-exchange tests against F-111 aircraft equipped with AN/ALR-62 radar warning receivers at McClellan Air Force Base. The ALR-62 provided 360-degree coverage over a wide frequency range and had a sensitivity of -65 dBm. The ALR-62 units used in these tests were capable of detecting and identifying LPIR waveforms. Notable tests included:
- January 1979: completion of ground-to-air tests;
- November 1979: completion of air-to-air tests, in which the LPI radar system could detect and track an F-111 at 40 km without being detected by the ALR-62;
- April to May 1980: completion of air-to-ground tests (moving ground-target detection and tracking) with a maximum detection range of 20 km.
After successful flight tests under the Sneaker program, DARPA and the U.S. Air Force immediately initiated a second LPI radar project, "Pave Mover". Contractors included Rog and Hughes Aircraft. That project continued research into LPI methods and characteristics, covering alternative LPI strategies, remote operation, intercepted data links, air-to-ground modes, adaptive ECM nulling, weapon delivery, and electronically scanned LPI techniques. These efforts contributed to the subsequent global development of LPI radars.
Approaches to Achieve Low Probability of Intercept
From the principles of radar transmitters and intercept receivers, achieving low probability of intercept in a radar system can be approached through:
- Maximizing emitted bandwidth;
- Stealthy antenna design with ultra-low sidelobes;
- Design of LPI waveforms, such as frequency modulation and phase modulation;
- Exploiting natural and electromagnetic environments;
- Power management;
- Control of transmission timing and frequency.
Therefore, is it appropriate to describe LPI radar as a "new-architecture" radar?
