2020年11月1日星期日

Measurement methods of radar distance, target angle, target azimuth and target speed

Radar, derived from the abbreviation of radio detection and ranging, means "radio detection and ranging". Its basic function includes measuring the distance, angle, azimuth and speed information of the target. This article briefly summarizes its methods of measuring the above four information.


1. Distance measurement

Pulse method: the difference between arrival and return time;

Frequency modulation method;

Phase method: the difference between phases, 2Π corresponds to a wavelength;


2. Measurement of target angle

The effectiveness of the radar antenna beam is used for measurement.

Maximum beam method: Launch a beam, then the received signal is strongest when the center point is facing the target, and the angle of the center point is the target angle;

Dual beam method: Two beams are emitted. When the energy of the two received beams is equal, the angle at which the center is located is the target angle.

3. Measurement of azimuth 

Generally, a 360-degree scan is used to determine the target azimuth by using energy changes;

4. Target speed measurement

Utilizing the Doppler effect, the fixed point transmits a sine wave to the target, and the transmitting frequency and the receiving frequency are different. The speed v can be calculated by the frequency difference Δf

Attention mechanism of human eyes

The structure of human eyes

The front end of the human eye is the sclera (white of the eye), the pupil (the black circle in the middle), and the iris (the brown part between the pupil and the sclera)

The rear photosensitive part is divided into photosensitive cells and ganglion cells;

Photoreceptor cells are divided into rod-shaped cells (receiving the brightness of light) and cone-shaped cells (receiving color, divided into R, G, and B cone-shaped cells).

Ganglion cells are responsible for delivering stimuli to the optic nerve.


The reason for the attention mechanism

Source: Professor's PPT


As shown in the figure above, the distribution of cone-shaped cells on the retina with angle is not uniform. The yellow dot (that is, the 0 degree range) is the center of our field of vision, which is always moving. Therefore, the human eye has the strongest ability to discern color in the center of the field of vision, which is the attention mechanism. However, there is no visual cell in the blind spot, so there is no vision, but the compensation mechanism of the human eye will compensate it.

Eagle eyes have good vision because there are more cones. Because cone-shaped cells mainly perceive color, rod-shaped cells can only sense intensity, and more rod-shaped cells cannot identify objects such as snakes and voles. Therefore, the good vision of eagle eyes is mainly due to the large number of cone-shaped cells. In addition, the eagle eye has two centers, that is, two attentions.


Application of attention mechanism

Using eye movement experiments, you can see where the information is seen first, and the results are shown in the figure:
Source: Professor's PPT


Among them, the size of the circle represents the staying time, the color of the circle represents the time sequence of observation, the warm color represents first, and the cool color represents later.

The visual focus is on the center, label, and legend.

Therefore, when visualizing, we can mark titles and useful information where people are more likely to pay attention; legends and table examples should be as detailed as possible. Specific to the layout of the paper, you can try to put nice charts on it, especially the upper left.

In addition, the attention mechanism is also widely used in AI-related technologies such as CNN and GAN.

2020年3月13日星期五

Effect of operating frequency on system power, penetration ability, equipment volume, and system performance

Note:
This article considers empirical situations and may not apply to all situations.

1. Effect of frequency on power

Generally speaking, the higher the frequency, the smaller the power.
The power affects the working distance. Therefore, the system with a higher frequency often has a smaller working distance. The phenomenon that millimeter waves are commonly used in vehicles, 5G small cells, etc. can be explained with this.


2. Frequency vs. Penetration Capability

Generally, the lower the frequency, the stronger the penetration. (The penetrating ability here refers to the penetrating ability of the detection, that is, the diffractive ability. It does not refer to the energy penetration of gamma rays, lasers, etc.)

3. Relationship between frequency and device volume

Generally, when the other conditions are the same, the larger the operating frequency, the smaller the equipment size. This can be understood from the perspective of receiving and transmitting antennas. For example, millimeter waves can be chip-sized.

4. Relationship between frequency and system performance

When selecting a frequency, the cost is usually known, and the available frequency is determined based on the nature of the budget and frequency. For example, when the budget cost is low, a more mature frequency band should be selected.

Image aliasing (Moire pattern)

Image sampling and quantization are necessary steps in digital image acquisition and storage.

According to Nyquist's sampling law:

An analog signal can be perfectly reconstructed form its samples as long as the sampling frequency is at least twice the amount of the maximum frequency component present in the analog signal.

That is, the sampling frequency is at least twice the highest frequency in the analog signal. Otherwise, aliasing will occur.


Python regular expressions match specific input and output images

The saved results of an image generation experiment are the following pictures. Now I want to calculate the SSIM and PSNR between * _real_image * and * _synthesized * in the same iterations.


Calculate picture SSIM and PSNR using Python

This article mainly shows the calculation of two pictures psnr and ssim using Python.


Some reasons for error "Segmentation fault (core dumped)"

Sometimes we encounter "Segmentation fault (core dumped)", meaning that memory is used incorrectly, which may be caused by the following reasons.

1. Caused by breakpoints in the program

It may be caused by breakpoints in the program for debugging, such as pdb.set_trace (), etc.

In most cases, the pdb debugging tool that comes with Python will not cause problems, but individual cases do occur. The specific reason is not clear, but you can just remove pdb.set_trace () to solve it.

2. Mismatch between number of objects and number of categories

This is easy to appear in the debugging process of specific segmentation detection tasks such as yolo. You can change the number of filter files, or the number of categories to solve. The specific solution should be determined according to the specific situation.