A signal transmitted and received by a radar is called RF signal. The available frequencies that can be used as RF signals are determined by the laws in each country (for example, 24GHz, 60GHz, 79GHz). On the other hand, as it is difficult to analyze the received wave of several tens of GHz, a mixer is used to convert it to a lower frequency. The signal after this conversion is called IF signal.
By analyzing the IF signal, it is possible to get information about "something " in front of the radar. The IF signal would be changed depending on the condition of "something" as well as the RF signal transmitted by itself. Here, we introduce three major measuring methods: Pulse method, Doppler method and FMCW method.
measuring method01
The pulse method is a method that measures the distance to "something" by the time it takes for the transmitted radio waves to be reflected by "something" and returned. After transmitting the radio wave, it stops and waits for the reflected wave to be received. This is similar to the echo. The echo uses sound/voice, but radar uses the radio wave.
measuring method02
The Doppler method is a method that can measure the speed of "something" which is moving. Also, it is called CW method. When the frequency of the transmitted radio wave is reflected by a moving "something", the frequency increases or decreases in proportion to the speed. This is similar to the doppler effect when the ambulance passes by. The figures below show the RF signal and IF signal with Doppler methods. The transmitted frequency is constant. By subtracting the frequency of the transmitted wave from the received wave that the frequency has changed due to reflection on the moving ", only the changed frequency appears in the IF signal, thus, the speed of "something" could be measured with this frequency. While the Doppler method can measure slight movements such as human breathing, it cannot measure the distance to "something".
measuring method03
The FMCW method is a method that measures the distance and speed of "something" from the frequency of the received wave that is reflected by "something" while changing the transmission frequency. As the transmission frequency is constantly changing, it is possible to measure the distance by knowing the frequency of the received wave (as it is possible to know how many seconds ago it was transmitted). If the frequency to be transmitted is increased by a certain value, the amount of change in the transmission frequency until it is reflected and returned can be known by subtracting the frequency of the transmitted wave. It is possible to measure the distance by this amount of the change. This amount of change is called the beat frequency.
However, as explained in the Doppler method, if "something" is moving, the received frequency changes depending on its speed, the actual beat frequency is influenced by the speed of "something" as well as the distance to. In order to solve this, by analyzing both the beat frequency measured while increasing the transmission frequency and the beat frequency measured while decreasing the transmission frequency, it is possible to measure the distance and speed. This is the same as having two equations to solve a quadratic equation.
While the effect of the speed of "something" on beat frequency is proportional to the speed, the effect of the distance on the beat frequency is increase rate in the transmission frequency. There is also a method reduces the effect of the speed of "something" on the beat frequency by increasing the increase rate of transmission frequency, which is called the first charp.
In order to know where "something" is, it is necessary to know information on distance and angle. Measuring the angle is called angle measurement.
The methods of angle measurement can be classified into such as a method which moves radar (antenna) physically, a method which uses phased array type antenna and MMO method which has multiple transmit antenna and multiple receive antenna.
Radar is not the only way to get information of "something". Especially for short-distance and medium-distance measurements,
there are other ways such as infrared sensor, ultrasonic sensor, camera and LIDAR.
The advantage of the radar over these sensors is that it gets less influence from the measurement environment.
It is not easily influenced, for example, the influence of the brightness or darkness, temperature, the background patterns and
climate influence (such as fog, rain and snow).
Also, as the gained information is not personal identifiable like camera, it can be used in private places such as toilets and bathrooms.
Besides, there is no need to install a window for the sensors. This gives more flexibility to the design of the products.
For a car, it is possible to hide the sensors behind the emblem, and for the toilets, it is not necessary to install a black window.
| Radar | Infrared | Ultrasound | Camera | LIDAR | |
|---|---|---|---|---|---|
| Short distance detection (2m or less) | ● | ● | ● | ● | ● |
| Medium distance detection (2-30m) | ● | ● | ● | ● | |
| Long distance detection (more than 30m) | ● | ● | |||
| Distance detection | ● | ● | ● | ● | |
| Speed detection | ● | ● | ● | ||
| Angle detection | ● | ● | ● | ● | |
| Environmental resistance | ● | ||||
| Freedom of product design | ● | ||||
| Size | ● | ● | ● | ● | |
| Price | ● | ● | ● | ● |
