Systems & Technology Overview
The ABCs of Radar
The word radar is an acronym taken from RAdio Detection And Ranging. A radar is a device that transmits and receives electromagnetic energy as radio waves to detect and estimate distance to and/or velocity of an object. It generally includes a transmitter emitting radio waves toward an object and a receiver detecting the radio energy reflected by the object. Distance is estimated by measuring the propagation time to and from the object. Velocity is estimated by measuring the Doppler frequency shift. This so-called Doppler effect occurs when the frequency of a wave changes and results from the relative velocity between an object and the radar. The velocity can also be estimated by observing the rate at which the range of the target changes.
Transmitting and receiving radio waves requires the use of an antenna that acts as an interface between the electrical circuits and free space where the radio waves are radiated towards potential targets. Antenna characteristics are critical in shaping performance of a radar system, and high gain antennas yield more sensitive systems. The angular position of targets can be found with radar systems using directive antennas where the antenna scan angle is recorded for each target detected.
Radar receivers have to deal with very low level signals because of the power density decreasing with the distance traveled by the radio waves. When received, signals are amplified, conditioned, digitally sampled and fed to a radar processor that applies specific processing techniques to optimally extract targets.
To be detected by the radar, targets need to generate strong enough signals (by reflection of transmitted signal) to rise above the noise. Signal to Noise Ratio (SNR) is therefore an important measurement that determines whether targets can be seen at all by the radar. In general, SNRs in excess of 10 ~12 dB are required to assure reliable detection with an acceptable false alarm rate. False alarms are undesired detections that can occur when no target is present. The SNR for a received signal is given by the classic radar equation:
|Signal to Noise Ratio|
|Gain of the Transmitting Antenna|
|Gain of the Receiving Antenna|
|Wavelength of the Radio Wave|
|Radar Cross-section of the Object|
|Receiver Noise Equivalent Temperature|
|Processing Noise Bandwidth|
|Distance from the Transmitter to the Target|
As can be seen from the above equation, more transmitter power and more antenna gain directly improve signal to noise ratio and capability to detect targets. The detectability of an object by the radar also greatly depends on the distance between the object and the radar. The further away the object is, the smaller is the SNR and the more difficult it is for the radar to detect it. We can also see that the signal returning from a target varies as the 4th power of the range. For example, if a target moves from 1km to 2km from the radar, the strength of the target’s signal return is 16 times less.
Highly reflective targets will return more energy back to the radar and will be more easily detected. The reflectivity or scattering from an object depends on factors such as the material the object is made of, its dimension, its shape and the angle at which the signal hits the object. This is defined as the Radar Cross Section or RCS. The RCS is a measure of the target’s ability to reflect radar signals back in the direction of the radar receiver. In a surveillance context, a potential target or intruder could be a pedestrian. A pedestrian is often characterized in literature as having a RCS of 1.0 m2. This does not mean that the pedestrian is physically 1 m2 in cross section, it means that its apparent size as observed by the radar is equivalent to 1 m2. The table below provides examples of RCS for various targets that may be encountered in a security or surveillance environment.
|Crawler||0.03 ~ 0.1|
|Pedestrian||0.3 ~ 1.5|
|Light ground vehicle||5 ~ 50|
|Kayak||1 ~ 5|
|Small boat||5 ~ 100|
|Heavy ground vehicle||20 ~ 1000|
|Ship or vessel||50 ~ 10000|
|Small airplane||5 ~ 20|
|Helicopter||10 ~ 500|
In attempting to detect targets, radars are faced with reflections coming from undesired objects, landscape and precipitation from the environment. Those unwanted reflections are called clutter and compete with the targets of interest. A target can only be detected provided it rises above the system noise and clutter.