With the growing excitement around autonomous driving, laser radar has become highly sought after due to its high precision, large data output, and immunity to visible light interference. However, it's worth noting that mainstream autonomous driving systems still rely on millimeter-wave radar. Why is that? Let's explore.
First, an introduction.
The radar we're discussing here is a type of electromagnetic wave-based radar, not a mechanical wave-based reversing radar. Those familiar with World War II history may know that radar technology originated from military applications. The first practical radar was used to detect enemy aircraft attempting to cross the English Channel at night, flying over the metal shells of the sky. Radar played a crucial role in battles like the Pacific night fights and even contributed to tragic events, such as Jewish anti-radiation missiles being destroyed in the Bekaa Valley.
After the war, radar transitioned into civilian use, becoming a key tool for drivers. Today, it's a vital sensor in adaptive cruise control, blind spot monitoring, and automatic emergency braking systems, helping ensure safer driving experiences.
Second, structure and principle.
Currently, on-board radar operates primarily in the 24 GHz and 77 GHz bands. While the 77 GHz band is considered the future trend, as it's specifically allocated by the International Telecommunication Union for vehicle radar, both 24 GHz and 77 GHz radars are often referred to as millimeter-wave radars.
In practice, many onboard radars use planar antenna arrays because they are compact and cost-effective compared to traditional rotating designs. Here’s a typical example of a flat panel antenna radar:
[Image: Comparison of millimeter wave radar and lidar]
Looking at the internal structure:
[Image: Comparison of millimeter wave radar and lidar]
One key component is the antenna array, which includes 10 transmit antennas (TX1), 2 transmit antennas (TX2), and 4 receive antennas (RX1–RX4). These antennas are responsible for detecting nearby and distant targets, with different field-of-view coverage.
[Image: Comparison of millimeter wave radar and lidar]
Nearby targets have a wider field of view (around 90 degrees), requiring more antennas, while distant targets have a narrower field of view (around 20 degrees), so fewer antennas are needed.
Here’s what the radar looks like on a car:
[Image: Comparison of millimeter wave radar and lidar]
The radar emits directional beams rather than uniform spherical waves, with varying intensity across directions:
[Image: Comparison of millimeter wave radar and lidar]
It measures three main parameters: position, velocity, and azimuth.
Position and speed are calculated based on the time it takes for the signal to return and the Doppler effect. Azimuth is determined by measuring the phase difference between signals received by multiple antennas.
Third, application examples.
Millimeter-wave radars are commonly used in ACC (Adaptive Cruise Control), BSD & LCA (Blind Spot Detection and Lane Change Assist), and AEB (Automatic Emergency Braking). While these functions are well-established, let's dive into something more interesting.
a) Radar data processing flow
Target recognition and tracking are central to ACC functionality. After receiving radar echoes, the system processes the signals using FFT to identify energy peaks. Multiple reflection points might belong to the same object, such as a truck generating 5-10 reflections. By matching clusters of reflection points and tracking them over time, the system can estimate object positions and movements.
b) Two small questions about radar
Can radar detect stationary objects?
Some early ACC systems didn’t respond to stationary objects like parked cars. This wasn’t due to radar limitations, but rather target classification issues. Early radars had low angular resolution, making it hard to distinguish between objects like manhole covers or street signs. To avoid false braking, systems were designed not to react to stationary objects unless they changed their motion state.
Compared to LiDAR?
While LiDAR offers higher precision and more data, millimeter-wave radar remains essential. It performs better in adverse weather conditions, such as fog, where LiDAR can fail. Additionally, millimeter-wave radar is much more affordable, with prices around $1,500 compared to tens of thousands for LiDAR. For simpler scenarios, engineers still prefer radar due to its cost-effectiveness and sufficient performance.
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