Radar
- Farris Matar
Introduction
Radar systems are electronic systems that use radio waves to detect objects in the surrounding environment. The system transmits radio waves outward into the air, where they can bounce off of objects in the radar's transmission path. The waves that bounce and return to the radar, called pulses, are then received and analyzed by the radar system, which can then determine the shape, position, and velocity of the discovered objects relative to the radar.
How Radars Work
All radars work using the same basic components:
- Transmitter: Generates the desired RF wave at some power level. These waves can come directly from an oscillator or derived using an amplifier or a series of amplifiers.
- Antenna: Takes radiofrequency (RF) energy and transmits it from the transmission line into the environment. The antenna is also used to determine how sensitive the transmission/reception is and the direction of transmission/reception (i.e. unidirectional vs. omnidirectional).
- Receiver: Receives weak signals in the range of frequencies transmitted, amplifies them, then converts them from RF to the original data before it was transmitted (baseband).
- Duplexer: A switch that will either connect the transmitter or the receiver to the antenna. This allows one antenna to be used as both the transmitter and the receiver, may not be included in all radar systems.
- Synchronizer: Controls the timing for the entire radar system. Essentially it determines how often pulses are sent and resets the timing clock for each pulse.
- Indicator: Transforms the received information into an output that can be interpreted by the operator. This is often in the form of a display unit
Below is an example of a simple radar system:
The Doppler Effect
Most small-scale radar systems are considered Doppler radars. A Doppler radar uses the Doppler effect to calculate the velocity of moving objects relative to the radar. The Doppler effect describes how the observed frequency of a wave emitted from a moving object changes based on the relative velocity to the observer. The observed frequency will be higher if the object emitting the wave is moving towards the observer and lower if moving away from the observer. Using this principle, a Doppler radar can calculate the position and velocity of the moving object.
The specifications of Doppler radars vary by part, but in general they work as long-range, low-resolution sensors for measuring distance and velocity of obstacles.
Common Types of Radar Sensors
Doppler Radar
As explained previously, Doppler radars use the Doppler effect to calculate the velocity of moving objects. Most are also capable of determining the distance of these objects relative to the radar, simply by measuring the time taken for the transmitted wave to be received by the antenna.
Passive Radar
Passive radar systems do not include a transmitter, and as such they do not transmit their own waves to produce their readings. Instead, passive radars constantly receive waves transmitted by other transmitters in the environment, using the difference between waves received directly from the transmitter and waves reflected off objects to determine their location. This is known as bistatic range. Passive radars can also determine the speed and heading of objects by measuring the bistatic Doppler shift, which is essentially applying the Doppler effect but with a separated transmitter and receiver.
Bistatic Radar
Bistatic radar systems use both their own transmitter and receiver, but keep the two separate. Similar to passive radars, bistatic radars will use bistatic range and bistatic Doppler shift to measure the position and velocity of objects relative to the receiver. The difference is that the system uses one dedicated transmitter rather than any third-party transmitter in the environment. As such, they are more reliable and controllable than passive radars, but may suffer in terms of accuracy and difficulty in setting up the system.
Using Radar Sensors
Advantages
- Long range: Many radar sensors will have a range that easily surpasses 100m, with some capable of extending the range up to 1000m
- Reliable: Radar is not significantly affected by lighting, weather, or dust, and is suitable for nearly all outdoor conditions
- Relatively low power consumption
Disadvantages
Low field of view: FOV is especially low at long range, often falling below 20°. For a complete view of the surroundings, the sensor will need to be rotated in the direction that needs to be observed
- Low resolution & accuracy: Radar sensors are not good for getting precise readings of the position and velocity of obstacles. They are more so for getting a general sense of where obstacles are so they can be avoided
Sample Radar Sensors
The sample sensors below are equipped with a CAN bus interface.