COHO

COHO, short for Coherent Oscillator, is a technique used with radar systems based on the cavity magnetron to allow them to implement a moving target indicator display. Because the signals are only coherent when received, not transmitted, the concept is also sometimes known as coherent on receive. Due to the way the signal is processed, radars using this technique are known as pseudo-coherent radar.

Description

In contrast to other microwave devices, like the klystron, a magnetron produces a slightly different signal with every pulse, differing in both frequency and phase. This makes it very difficult to perform pulse-to-pulse comparisons for clutter removal and moving target detection.

COHO taps off a small amount of the signal from the magnetron using a directional coupler and produces a much smaller signal. This is passed into the stable oscillator (STALO) that is used to convert the high-frequency microwave signal to a lower intermediate frequency, typically around 50 MHz. Since the STALO is also used to down-convert the received signal, the tapped-off signal and received signal will be similar and can be directly compared.

The tapped-off signal now has to be stored for a period while the main signal travels out to the target and back. This can be accomplished with a phase locked loop or similar solution. This provides a stable continuous signal with the same phase and frequency as the original pulsed signal. It is this component that is properly the COHO, although the term is generally applied to any radar including one.

The continuous output of the COHO is then sent into one channel of a phase detector, with the received signal in the second. Objects in front of the radar will reflect the signal, and the phase of the signal will depend on its precise distance from the antenna. The output of the phase detector will thus be non-zero whenever it receives a target return. This signal is used as the video, and may be sent directly to a radar display.

This technique does not itself reveal moving targets, that still requires pulse-to-pulse comparison. This can be accomplished by splitting the output of the phase detector, and sending one half into a acoustic delay line with the same delay time as the pulse repetition frequency. This means the signal from the last pulse will exit the delay at the same time as the next pulse is being received. One of the signals is then inverted, typically from the delay line, and added to the new signal. This results in an output signal only where the two signals changed.

If the target is moving, and thus its exact distance is changing relative to the radar, the phase of the received signal will change from pulse to pulse and will produce a signal in the final video. Non-moving targets will have (roughly) the same phase pulse-to-pulse, and their signal will be eliminated. The velocity of the target along the line-of-sight determines the change in phase pulse-to-pulse, so over a series of pulses, the difference in phase will vary. The rate of variation in phase is the same as the Doppler frequency that would be seen in a fully coherent Doppler radar, and can be used to determine the radial speed of the target.

A significant disadvantage of the COHO design is that it cannot be used in frequency agile radars. Those change their frequency from pulse to pulse, so pulse-comparison techniques like COHO do not work. For these roles, a coherent signal source like a klystron is needed. It is also difficult to change the parameters of the radar like the pulse width or pulse repetition frequency, as these were typically implemented in mechanical devices that had to be changed, although a small selection of pre-set selections could be used.

COHO was widely used from the early 1950s into the 1960s, but the introduction of increasingly powerful klystrons and newer travelling wave tube systems displaced the magnetron from most military radars by the 1970s. More recent systems based on various solid-state elements are coherent as well. COHO is no longer used except in certain classes of low-cost radars.

References

"The COHO". Radar Tutorial.
"Pseudo-coherent Radar". Radar Tutorial.
Long, Maurice (2004). Airborne Early Radar Warning System Concepts. SciTech Publishing.

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