Hey, There
These can actually be specialized radar variants, but I shared them under the general term ESM to avoid making the topic too long and complicated. ESM systems are extremely complex and many details about them are classified.
In general, they operate by detecting microwave-band electromagnetic emissions from other systems. In many modern fighter aircraft, ESM and AESA radars work together. A good example is the Sukhoi Su-57, which has additional antenna arrays on the sides of the fuselage and wings for passive detection.
For example, sensors operating in L-band can sometimes detect stealth aircraft at longer ranges because of the longer wavelength. This means a system might detect something like a Lockheed Martin F-35 Lightning II at around ~200 km. However, at that stage the system usually cannot identify exactly what the object is.
In simple terms, the warning to the pilot might be something like:
“Pilot, there is an unusual electromagnetic source here. I can detect it, but I cannot identify what it is yet.”
Once the target enters the low-RCS detection range of the main AESA radar, identification becomes much clearer.
In short, ESM is a broad and highly diverse military technology with many different subsystems used in modern electronic warfare.
ESM vs ELINT (Simple System Diagram)
ELECTROMAGNETIC SPECTRUM
│
▼
Enemy radar emissions
Communication signals
Missile seekers
│
▼
┌─────────────────┐
│ ESM SYSTEM │
│ (Electronic │
│ Support Measures)│
└─────────────────┘
│
┌─────────────┼─────────────┐
▼ ▼
RWR (Radar Warning) ELINT
Real-time pilot Intelligence analysis
situational awareness and signal study
"Radar detected" "What radar type?"
"Direction?" "Frequency?"
"Threat level?" "Country/system?"
│ │
▼ ▼
Immediate tactical use Strategic intelligence
(during combat) (databases & analysis)
Short Explanation
ESM (Electronic Support Measures)
ELINT (Electronic Intelligence)
This information is used to build radar databases so aircraft can later automatically recognize enemy systems.