Thread by @heatloss1986

The early F-106's Airborne Moving Target Indication (AMTI) system was an unusual method for dealing with the threat of Soviet bombers flying at low altitudes to avoid detection by radar. Though no pictures or official explanations exist, I think I can describe this system.🧵

First, I must admit that AMTI is only a partially correct method of describing the "clutter" function on the F-106's MA-1.
While it provided MTI based on target Doppler shift, it did so with a "coherent on receive only" system rather than a fully clutter-referenced MTI system.

Radar coherency is critical to Doppler processing. A fully coherent radar has a single, frequency-stable, continuous oscillator generating the frequency to be used by the radar. This is then amplified to produce the output power by a Klystron or Traveling Wave Tube. (Image: TWT)

By comparing the shift in phase and frequency between the reference and the received pulse, a Doppler shift can be measured. This reference is on the top of this chart, and a coherent radar is shown below in green. A non-coherent radar, shown at the bottom, works differently.

A non-coherent radar uses an oscillator (normally a cavity magnetron, which does not require a separate amplifier) that is switched on and off, meaning it starts at a random phase. Therefore, without a way to compare phase shift, it cannot process target Doppler information.

When MA-1 entered service in 1957, nobody had yet figured out how to make a stable oscillator and amplifier for a coherent pulse-doppler radar. This would come about in around 1960 with Master-Oscillator Power Amplifier, a Hughes method of radar timing later applied to lasers.

So what is "coherency on receive"? This system saves the pulse information (starting phase and frequency) transmitted by the radar and compares it to the phase and frequency of radar returns.

On MA-1, this was probably done with a vacuum tube designed to store the pulse without degradation or changes in frequency. This was a tall order, shown by the failures during the development of the F-4E's CORDS system, which attempted the same system on a lower-weight radar.

When the radar mode selector was in Clutter mode and the range selector was at 4 or 16 miles, the MTI mode would be activated.
The system was unique in terms of its design for one major reason: it was optimized for "low overtake" targets rather than high closing rate targets.

Normally, high closing rates are easier for pulse Doppler radars to spot. In PD, the received pulse is processed to extract its component frequencies with an algorithm known as a Fourier transform. These are then compared with the expected frequency from the stable oscillator.

However, due to the rear-aspect limitations of the earliest AIM-4 Falcons, only rear-aspect attacks were considered relevant for this system. This heavily influenced the design of CORO.

Instead of using complex signal processing to spot targets, it likely applied a higher amplitude multiplier to the lower Doppler shift targets (probably inversely proportional to shift) while applying a lower multiplier to the higher closing rate targets--namely, the ground.

When this was projected on the radar screen, the low overtake targets would be brighter, and therefore easier to pick out of the clutter returns that would normally be brighter than it! (This image is not of the CORO system in action, but rather just a normal radar return)

Unfortunately, I have no hard sources for this as of right now, only some deductions based on the comments made by a few old maintainers and corroboration with some other mentions of the system in official documents.

If anyone out there has more concrete info or stories to tell, let me know! I'm totally alright about being wrong here, but as of right now, with the information I have access to, I feel confident that this is how it worked.