Pulse repetition frequency

The pulse repetition frequency ( engl. pulse repetition frequency, PRF, literally " pulse repetition frequency " ) of a radar unit is the number of pulses transmitted based on the duration of the count. The radar transmits a short rf pulses of the transmitted pulse duration ( pulse width, Eng. Pulsewidth ) and waits between transmission pulses to the echo signals. The time from the start of a transmit pulse to the start of the next transmission pulse is called the pulse repetition period ( engl. pulse repetition time, PRT ) and is the reciprocal of the pulse repetition frequency:

The time between the transmit pulses, the reception time is generally. This is always smaller than the difference between the pulse repetition period and the transmission time, and in addition is sometimes limited by a so-called dead time. In the dead time before the next transmit pulse are performed in modern radar system test ( BITE Built In Test Equipment).

Ambiguous time measurements

The pulse repetition frequency of a radar device must ensure that the receiving time is long enough to receive echo signals from distant objects. If the pulse repetition frequency is too high, then you can echo signals that arrive too late to lead to measurement errors. There are now ambiguous results of the range calculation possible because two transmitted pulses are a possible cause for the echo signal in question. These echo signals are often called by the English term ambiguous returns, sometimes the German term removal convolution is also used.

The maximum distance for a clear measurement result (so-called " unambiguous return" ) can be calculated using the following formula:

With

The pulse repetition PRT must be sufficient for the round trip of the electromagnetic wave, so the range is only half of the distance traveled by the electromagnetic wave path. The pulse duration is therefore flows into this formula a, because the entire first transmission pulse must be received before a target mark for the visual display unit can be generated. (This restriction is replaced mainly with the radars weight, working with the pulse compression method and very long transmit pulses. )

Suppression of ambiguities

Despite an adequate reception time, it may happen that overreaches be achieved by anomalous propagation conditions of the electromagnetic waves. This overreach have a different meaning than the overreach in radio links of communication with radar equipment. In such overshooting can be produced at the time of the radar system is often not the reference. These echo signals are then displayed with an incorrect distance. To such ambiguities (English: ambiguous returns ) to prevent, there are several options:

  • Choosing a very low pulse repetition frequency;
  • Use of a Staggered PRT, ie a continuously changing duration of the reception time;
  • The monopulse method;
  • Certain intra Pulse modulation methods.

Size of the pulse repetition frequency

The pulse repetition frequency can not be arbitrarily reduced. This parameter of the radar device, the data renewal rate depends on the demands the highest possible pulse repetition frequency. The size of the pulse repetition frequency is therefore a compromise.

Staggered PRT

In earlier radars, a " staggered PRT " is a way to detect this incorrect measurements as such. Staggered is an English term and means as much as staggered or offset. As a protection against interference from overreaching and avoid blind speeds variable Infulsfolgeperioden (PRT ) can be used.

When the distance between the staggered PRT transmitting pulses is not constant. Echo signals from overreaching have no constant time reference so to the wrong PRT and are therefore recognizable as overreaching.

Modern radar sets with a digital radar signal processing can identify these gestaggerte answer and try to still represent the echo signals from overreaching at the right distance. This process is also called " distance unfolding".

Monopulse method

With radar sensors with monopulse process can not occur such overreach, since each transmit pulse is sent in a different direction and echo signals that arrive late can not be received. The main lobe of the antenna pattern shows with a new transmission pulse already in a different direction.

Intra- Pulse Modulation

In the intra pulse modulation method, an additional frequency is modulated within the transmit pulse. If this modulation is not constant, the radar signal processor can allocate a response to its transmission pulse, even if the echo comes from another pulse period. Non-constant modulation can be realized by coded pulse - phase modulation, and even by a pseudo-random pulse pattern, which is similar to white noise, as possible.

  • Radar
Built-in test equipment Monopulse radar Radiation pattern
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