Range gate pull-off

Range gates and strobing

Even the earliest radar systems included a system to highlight a single selected target for further analysis. For instance, the Gun-Laying Mark I, the British Army's first operational radar, used an on-screen cursor known as the strobe to highlight a single target. This worked by filtering out, or gating, signals that were not within the strobe's short time period, typically a few microseconds, corresponding to a range of a few hundred meters. The signal within the strobe's window was then sent to secondary displays where two operators would determine the azimuth and elevation of that single target, by keeping its blip centered in their displays. Similar systems were used by many radars by the mid-war period.

By the end of the war, many experiments were being carried out on automatic target following, or radar lock-on. In these systems, the operator would select a target using the strobe, and then circuits in the radar would automatically track the target in azimuth and elevation. This eliminated the need for the additional operators. Since the target's range would continue to change as it moved, the circuitry also attempted to keep the strobe centered in range. Some systems automated even the strobing; the AI Mark V was designed for single-seat fighter aircraft where the pilot would be too busy to adjust the strobe, and instead had a second system to sweep the strobe through a wide range and then lock onto the first signal it saw.

In the post-war era the circuitry that produced the strobe and filtered out other returns became more widely known as a range gate.

Range Pull-off

While testing a late-war radar design, the AI Mk. IX, a serious problem with the auto-follow system was found.

While this system was being tested, Bomber Command was pressing the Air Ministry to use "window", better known today as chaff, as a radar countermeasure. Fighter Command pointed out that the Germans could easily copy the system and use it against England, potentially re-opening The Blitz.^{[lower-alpha 1]} It was suggested that the AI Mk. IX would ignore window because it deccelerated rapidly after it was dropped, and would thus quickly pass out of the range gates and not be tracked. But exactly the opposite occurred in testing; the radar unerringly locked onto the window and the target disappeared from the display.

Range gate pull-off is essentially an electronic version of window. Instead of producing the secondary return by dropping a packet of foil reflectors, the second return is created by a transponder in the target aircraft. The transponder initially responds as rapidly as possible to the radar's signal, producing a second blip that overlaps the original. Over a period of time it increasingly delays the return so that it falls "behind" the radar signal in time. The goal is to delay the signal so it counters the aircraft's motion, leaving a signal at what appears to be a (nearly) fixed location in space. If the radar was locked on to the aircraft, it may remain locked to this second pulse as the aircraft moves away from the original location. Eventually, the aircraft will fall outside the range gate and disappear, while the radar continues tracking the false signal. Thus, the false signal is said to pull the range gate off the target.

The system can be further improved through detailed control of the returned signal. One improvement is to have another signal sent out at "zero time", immediately on reception. This may require the transponder to listen to the radar's signal and determine its pulse repetition frequency so it can properly time the response to be exactly the same as the radar's reflection. This signal is then added to the one from the reflected radar signal, making the target signal stronger. The system then begins sending the second signal as well, at exactly the same strength as this doubled-up return, while at the same time reducing the false reflection as the second signal moves away in time. The result on the radar display is that the real signal fades back to its original smaller value, while the fake second signal remains a constant stronger strength. This makes the real return look like the fake.

One problem with a simple RGPO system is that the false signals will always appear further away from the radar, due to the slight delay in the transponder. On a plan-position indicator, the false signal will appear as a second dot at increasing distances from the first, which the operator can then manually strobe to regain lock. This can be offset by having the transponder store how rapidly the radar signals are being received, and then estimating the pulse repetition frequency so that it can predict future pulses. Then it can send its signal before the next pulse, allowing it to pull off in either direction and making manual re-lock more difficult.

Velocity pull-off

Doppler radars directly measure the target's velocity via the Doppler effect. In typical early implementations, the received signal was amplified and then sent into a bank of narrow-band filters, each one corresponding to a particular target velocity.

If an RGPO jammer responds to such a signal by sending out the same frequency it received, this additional signal will be sent into the same filter, adding to the original signal and making it stronger. It the transponder instead responds at a fixed frequency, it will fall into a different filter and can be easily distinguished. In either case, the original target return remains locked-on.

Modifying a transponder to deal with Doppler radars is easy, it simply requires it to be able to adjust its frequency. In this case, the system initially responds at the same frequency as the original signal, and then increasingly shifts the frequency over time in a manner similar to the RGPO case. This will cause a second signal to appear in adjacent filters, with no way to know which is the original. Since the frequency can be easily adjusted up or down, it does not have the added complication seen in RGPOs that want to pull-off in either direction.

Pulse-Doppler radars use both pulse timing and Doppler shifting to track targets, so by varying both the frequency and return timing, these can be pulled off as well. Such a transponder will continue to work against non-Doppler radars as well, as these generally have wide frequency response and continue to see the signal as long as its frequency shift does not become significant.

Countermeasures

The effectiveness of the pull-off can be reduced if the radar changes its pulse repetition frequency, thereby making it difficult for the transponder to continue smoothly delaying the fake signal. Frequency agility has the same effect, as the transponder cannot guess what frequency to send out the fake signals on until it hears the one from the radar.

Denying this capability means the signal from the transponder can only respond to signals after hearing them on its receiver. These signals will always represent returns from greater distances than the jammer aircraft. Pulse-to-pulse comparison techniques, like moving target indication, can be used to filter out these sorts of returns as they appear on the radar to be slower-moving targets.