The fifth article in the series of radar Fundamentals: the function of radar modulation style

One 、 PRF stagger

PRF stagger radar has a variety of pulse repetition periods （PRI）, Suppose the stagger number is m, Then the pulse repetition period has m A fixed value . Each pass m Pulse , The repetition period value of each pulse changes circularly once .
Purpose ： Solve distance blur .
problem ： For fixing PRI Radar of , When the target is far away, the echo does not fall on the current PRI in , There is distance blur .

Minimum detection range of radar （ Distance blind spot ）

For a single station pulse radar , Because it does not receive the target echo when transmitting the signal , So there is a blind area for distance measurement , That is, the radar has the minimum detection distance .
Distance blind zone and emission Pulse width relevant , For pulse width 1us Corresponding 150m The distance blind spot , For a signal with a larger pulse width, there will be a too large distance blind area , for example 100us The pulse width of 15km Distance blind spot .

Blind zone distance ：R=ct/2

c For the speed of light ;t Is the pulse width .

Radar range and speed blur

If the target's radar echo signal only comes back in the next or next few pulse echoes , Then there is distance ambiguity （ That is, when the target is far away, the echo does not fall on the current PRI in ）.

The formula of maximum unambiguous distance and maximum unambiguous speed is as follows ：

  When the target distance or speed exceed The maximum value , Pulse Doppler radar It will be impossible to directly judge the distance or speed of the target according to the detected target echo range gate or speed gate . Different PRF It will bring different degrees of distance / Speed blur .

The obvious , Low PRF Can increase Rmu And decrease Vmu, That is, it will reduce the distance ambiguity but increase the speed ambiguity ; high PRF Reduce Rmu And increased Vmu, That is, it increases the distance ambiguity but decreases the speed ambiguity ; therefore , If you want to avoid distance blur and speed blur at the same time, there will be contradictions .( resolvent ： PRF stagger , Solve ambiguity by algorithm )

Doppler shift

Blind speed

The reasons causing ： For moving target radar （MTI, Including pulse Doppler radar ）, When the Doppler frequency of the moving target is equal to the integral multiple of the pulse repetition frequency , These moving targets will be MTI Filter out .

Concept ： The radial velocity corresponding to Doppler frequency is called blind velocity .

resolvent ： use PRF stagger Methods , Increase the first blind speed .