Radar (RAdio Detecting And Ranging) systems, as their name implies, are systems used to detect objects and to evaluate the distance between them and a single antenna or a group of antennas. In this blog post, we’ll explore two radar mechanisms: active and passive.
Active radar is the type of radar most of us are familiar with. Its principle of operation is simple: a radio wave is emitted from an antenna and reflects off objects the wave encounters. The signal is reflected back to the emitter location, where a receiving antenna picks up the echoed signal. When the transmitter and the receiver of a radar system are collocated, the radar is said to be monostatic.
Once the echo is received, the distance between the radar system and the object can be determined with a simple time-of-flight calculation. Since the speed of an RF wave in the air is the speed of light (3×108 m/s), and since the time between the emission of the wave and its reception takes into account a round trip to the target and back, the distance to the object can be calculated by the simple formula D = t * c / 2, where:
- D is the distance in meter
- t is the time delay between the emission of the signal and its reception
- c is the speed of light (~3×108m/s)
The following figure explains the basic mechanism of an active radar system. In the figure, the variable t (time delay) equals the total time for the signal to be transmitted to the object and reflected back: ttransmitted + techo.
You can find good documents for radar beginners on the radartutorial.eu website.
Instead of using collocated transmission and reception antennas, a passive radar system relies on a signal transmitted from a different location. This type of radar system is called bistatic.
The ranging of this type of radar is done by calculating the delay between the signal received directly from the transmitter and the signal received after being reflected off a target.
Since only the time delay can be calculated from this technique with one transmitter and one receiver, the single conclusion that can be drawn is that the detected object is located somewhere on an ellipse whose foci are the transmitter and the receiver.
The figure below illustrates this concept.
In the figure, t1+t2 = t3+t4. This holds true for every object that is located on the ellipse of the figure. For both of the objects in the diagram, the time delay between the original signal and the reflected signal as seen by the receptor is exactly the same. Only by using multiple transmitters and receivers can this type of radar system precisely locate an object. The performance of the system is highly dependent on the number of transmitters and receivers and their geometry.
In this post, we’ve seen an overview of how active and passive radar work. These descriptions cover only the basic principles of range calculation for both types of radar. Radar systems can be based on either of these technologies, or even a combination of both (multistatic radar systems).