问题 单项选择题 A型题

如果脉冲重复频率是10kHz,下面哪种多普勒频移可导致混叠()。

A.2kHz

B.3kHz

C.4kHz

D.5kHz

E.6kHz

答案

参考答案:E

解析:最大频移即最大速度受脉冲重复频率的限制(fd=PRF/2),当被检测目标的运动速度即频移超过PRF/2时,回声信号被截断为两部分,即发生频谱混叠或倒错。

单项选择题 A2型题
阅读理解

阅读理解

     Are you happy? Do you remember a time when you were happy? Are you seeking happiness today?

Many have sought a variety of sources for their feelings of happiness. Some have put their hearts and

efforts into their work. Too many have turned to drugs and alcohol. Meanwhile, untold numbers have

looked for it in the possession of expensive cars, exotic(异国的) vacation homes and other popular

"toys". Most of their efforts have a root in one common fact: people are looking for a lasting source of

happiness.

    Unfortunately, I believe that happiness escapes from many people because they misunderstand the

journey of finding it. I have heard many people say that, "I'll be happy when I get my new promotion."

Or "I'll be happy when I lose that extra 20 pounds." It is dangerous because it accepts that happiness is

a "response" to having, being or doing something.

     In life, we all experience stimulus(激励) and response. Today, some people think that an expensive

car is a stimulus, and happiness is a response; a great paying job is a stimulus, and happiness is a

response; a loving relationship is a stimulus, and happiness is a response. This belief leaves us the thinking

and feeling: I'll be happy when …

     It has been my finding that actually the opposite is true. I believe that happiness is a stimulus and a

response is what life brings to those who are truly happy. When we are happy, we tend to have more

success in our work. When we are happy, people want to be around us and enjoy loving relationships.

When we are happy, we more naturally take better care of our bodies and enjoy good health. Happiness

is not a response but a stimulus.

     Happiness is a conscious choice we make every day of our lives. For unknown reason to me, many

choose to be painful, unsuccessful and angry most of the time. Happiness is not something that happens

to us after we get something we want-we usually get things we want AFTER we choose to be happy.

1. From the second paragraph, we know too many people ______________.

  A. are not happy when they work hard

  B. are not happy when they drink or take drugs

  C. Are happy when they possess their own expensive cars

  D. all desire exotic vacation homes

2. Generally speaking, most people feel happy because ______________.

  A. they think happiness is rooted in their deep hearts

  B. they get what they want to have

  C. they get a great paying job

  D. they get an expensive car

3. Which of the following is right according to the author?

  A. If you want to get what you want, you first choose to be happy.

  B. We should try to get more and then we'll be happy.

  C. Most people today are happy.

  D. Work is a necessary part in our daily life

4. From the viewpoint of the author, happiness is _______________.

A. based on our needs          

B. unconditional

C. out of reach                

D. limited