Differences Between Active Radars and Passive Radars

The core difference between active radars and passive radars lies in whether they actively transmit detection signals. The two vary significantly in working principles, performance characteristics and applicable scenarios, with specific comparisons and analyses as follows. 

I. Core Working Principles

▪  Active radar operates in a "active transmission + receiving reflected signals" mode.

The radar itself emits electromagnetic waves (such as microwaves, infrared rays, etc.), which cover a preset detection range.

When a target object appears within the range, the electromagnetic waves are reflected back to the radar receiver.

By analyzing parameters like time difference, frequency offset and signal intensity of the reflected signals, the receiver determines the target’s position, distance and moving speed, and triggers corresponding actions (like automatic door opening).

In the automatic door field, active infrared sensing radars and 24GHz microwave radars fall into this category.

▪  Passive radar works in a "only receiving external signals radiated or reflected by the target" mode, without transmitting any detection waves itself .

It identifies targets by capturing signals emitted by the target itself (such as infrared radiation from the human body, electromagnetic signals from mobile phones) or environmental signals reflected by the target (such as sunlight, surrounding radio waves).

A typical example is the passive infrared sensor (PIR) used in automatic doors, which judges whether someone is approaching by detecting the temperature difference between the human body and the environment (i.e., far-infrared radiation emitted by the human body).

II. Comparison of Key Performance and Applicable Scenarios

▪  Signal Source. Active Radar, self-emitted, active detection. Passive Radar, Relies on inherent signals of targets/environment, passive reception.

▪  Detection Distance & Range. Active Radar,long distance, wide coverage,controllable by adjusting transmission power. Passive Radar, short distance, narrow coverage, limited by target signal intensity.

▪  Anti-interference Capability. Active Radar, less susceptible to external interference (like strong light, clutter waves), but easy to be identified. Passive Radar, vulnerable to environmental signal interference (like temperature fluctuations, strong light). 

▪  Stealth Performance. Active Radar, poor stealth due to signal transmission.  Passive Radar, extremely high stealth with no signal emission.

▪  Target Recognition Capability. Active Radar could detect both stationary and moving targets (for some models). Passive Radar,mainly detects targets with signal differences (like moving human bodies)

▪  Applications in Automatic Door Field . Active infrared radars, microwave radars (universal for indoor and outdoor use) . Passive infrared sensors (mostly used for indoor doors)

III. Specific Application Cases in the Automatic Door Field

▪  Application of Active Radar. The active microwave radar commonly used in automatic sliding doors and revolving doors is a typical active radar. It actively emits 24GHz microwaves. As long as there is a moving or stationary target—even a person wearing dark clothes or behind a transparent glass door—it can receive reflected signals to trigger door opening. It has strong resistance to strong light and dust, making it suitable for complex environments such as shopping malls and hotels.

▪  Application of Passive Radar. The passive infrared sensor used in office and residential automatic doors is a typical passive radar. It does not emit any signals, and only captures the far-infrared radiation from the human body. When someone approaches, the sensor detects the temperature change and triggers door opening. However, it tends to fail in high-temperature environments or when detecting stationary targets, and is also sensitive to direct strong light.

IV. Summary

▪  Active Radar. Features active detection, strong anti-interference performance and wide applicability. It is suitable for scenarios with high reliability requirements, but comes with a slightly higher cost .

▪  Passive Radar. Boasts excellent stealth performance, low power consumption and low cost. It is ideal for simple indoor scenarios, but is highly dependent on the environment .