As a coherent radar , a is the pulse radar system referred which with a stable phase reference works individual transmission pulses to a phase-stable reference oscillation. This stable phase reference is necessary in order to be able to distinguish interfering echoes from fixed targets such as elevations, vegetation and buildings from the echoes from moving targets (ships, vehicles, people or aircraft) with the help of a phase comparison between two or more pulse periods.
There are three different radar concepts:
- incoherent radar
- fully coherent radar
- pseudocoherent radar.
Self-oscillating transmitter oscillators have a random phase position from pulse to pulse and are therefore not coherent . Each transmission pulse from a non-coherent radar device begins with a different phase position. Non-coherent radars are still used today as ship radars. The moving water surface (sea clutter) makes it difficult to detect moving targets. In ship radar devices, moving targets are recognized in the target extractor by digital filters . For this purpose statistical methods and methods of non-linear dynamics are used.
For radar devices that are supposed to detect moving land vehicles or aircraft, the moving target is detected by comparing the phase position of at least two echo signals ( pulse-pair processing ). The echo of an immobile object always has the same phase relation to the current transmission pulse, since there is no change in distance and at the same time no Doppler shift . Moving targets, which move with a radial speed to the radar system, provide different phase positions for successive measurements. A non-coherent radar cannot meaningfully compare these phase positions with one another and thus cannot distinguish moving targets from fixed targets.
Fully coherent radar
Low power waveform generators with subsequent high power amplification produce a fixed phase relationship between the transmission pulses, i.e. coherence. The transmission frequency is generated from a stable and continuously oscillating mother generator. The modulation of the power output stage of the transmitter does not affect the phase position of the transmission pulse. If it is assumed that a constant pulse repetition frequency (PRF) is also derived from the frequency of the mother generator, then each transmission pulse really starts with the same phase position. Radar devices in which the phase position is so stable are called fully coherent . For some applications this rigid phase position is actually not necessary, the only condition is that a coherence oscillator supplies a phase-stable continuous oscillation and each transmission pulse has a phase position synchronous with the oscillation of the coherence oscillator.
A distinction between a fixed target and a moving target is often made in a fixed radar by measuring the phase difference between two successive echo signals. The distance can be viewed as a multiple of the wavelength. If the reflecting object moves even slightly, the phase position of the second echo signal changes compared to the previously received one. In order to avoid that the periodicity of the sinusoidal oscillation causes a phase difference of a multiple of 360 ° to simulate a stationary target (so-called blind speed ), different pulse train periods and often two different wavelengths are used in the transmission pulse ( frequency diversity radar ).
In the case of a side-looking airborne radar , the fixed targets of the observed landscape all have a movement relative to the radar. Full coherence is necessary here in order to achieve an improvement in the angular resolution from the change characteristics of the Doppler frequency that are individual for each point in the terrain. A detection of moving targets is not so easy here because they cause a measurement error in the angle determination (see: Synthetic Aperture Radar ).
Pseudo-coherent radars are sometimes also called "coherent-on-receive radar". With a pseudo-coherent radar, the transmitter oscillates like a non-coherent radar with a random phase position. It is therefore not coherent because two successive transmission pulses have a random phase difference and thus their echo signals can no longer be compared with one another.
With each transmission pulse, a free-running but very stable coherent oscillator is synchronized, which then continues to oscillate with the current phase position of the transmitter during the reception time until the next transmission pulse forces it to a new phase position. In this way, the phase position of the echo signal can at least be compared with that of the synchronizing transmission pulse. This determines a phase difference which can be compared with the phase differences at the same distance (range cell) from other pulse periods. The ratio of the amplitudes of moving targets to that of disruptive fixed targets can be improved in this way in the order of about +20 dB (factor 100).
Since the phase position is retained in modulation and demodulation, the coherent oscillator does not need to work on the transmission frequency, but can oscillate on a lower intermediate frequency, which is used in a heterodyne receiver anyway. This coherent oscillator is forcibly synchronized by the phase position of the transmitted signal and thus supplies the reference phase for the phase discriminator . Thus, the phase position of the last transmission pulse is retained for the duration of the reception time. The tuning of this coherent oscillator had to be done manually and often corrected several times during operation because of the temperature drift.
A second possibility of storing the phase position of the transmitter is to send the transmission pulse on a delay line (delay time is equal to the pulse duration of the transmission pulse) and thus generate a continuous oscillation at the transmission frequency through feedback. Because it is derived directly from the transmission pulse, this signal also has the same phase position as the transmission pulse. However, this procedure is no longer used today. Even cheap radars that still work with a self-oscillating high-performance oscillator ( magnetron ) use electronic circuit solutions for coherent oscillators today.
The synchronizer provides the time reference for the distance measurement. All time-critical components receive synchronization pulses. The modulator provides the high voltage for the transmission tube , usually a magnetron , for the transmission time . The duplexer switches the antenna to the transmitter for the transmission time and to the receiver during the reception time.
A small part of the transmission energy is coupled out between the transmitter and the duplexer in order to control the automatic frequency control ( AFC ) and to obtain the phase information for the coherent oscillator . As in the receiver, this HF component is mixed down to the intermediate frequency. The phase information is retained during mixing.
The coherent oscillator oscillates in a highly stable manner at the intermediate frequency, but is forcibly synchronized by this at the time of the transmission pulse. During the reception time, the coherent oscillator continues to oscillate with the phase position of the last transmission pulse and provides a reference frequency and the reference phase for the phase detector .
The disadvantages of the pseudo-coherent radar method can be summarized as follows:
- The synchronization process of the coherent oscillator cannot be as precise as with a fully coherent radar device. This reduces the visibility of slow-flying aircraft.
- With this technology, frequency changes can hardly be carried out. A frequency change with a magnetron requires mechanical changes to the resonators.
- This system is inflexible and can hardly make major changes to the PRF, the transmission pulse duration or other parameters. Such change options are reserved for the fully coherent radar device, which makes these changes even in assemblies with low power consumption. Frequency modulation of the transmission signal (as with the pulse compression method ) is also impossible.
- Overreaches of fixed targets still have the phase reference of the penultimate transmission signal. But since the coherent oscillator is already working with the next (random) phase position, they can no longer be recognized as fixed targets. With the pseudo-coherent radar method, they are therefore always visible as interference on the viewing device.
- A high PRF mode to improve the energy balance of the radar with shorter pulse periods than the required transit time is not possible with this method.
- Renato Croci, "Coherent-on-Receive Systems", a brief overview, December 28, 2003