What is the purpose of range gating in a tracking mode of a Radar?

Question

I am trying to understand what happened in the famous incident where the Patriot failed to intercept a Scud missle because of a software bug.

The reprot can be found here.

Why does the radar need to use range gates during the tracking mode (when it predicts where the target will be). Why can't the whole beam be processed? I doubt it is processing power limitation since during the search mode the whole beam is processed.

Answer 1

The term "range gate/gating" can mean different things depending on the context and the radar system being talked about.

In general, a "range gate" is a window in range observed for some purpose, mainly for detecting and tracking targets in a radar system.

In legacy radar systems, analog circuits were used to define a time window (which corresponds to range) where target energy was allowed to pass and fed to the detection and tracking circuits. Other energy existing outside of this window was not used. When it came to tracking, there are techniques where you can compare portions of the time window to detect if a target is in front or behind the center of the window. Usually an error signal is generated that would then drive some kind of servomotor which would move the window to center the target again. This action happened continuously when the radar had a good track. Many systems allowed the human operators to manually adjust this window as they saw fit, usually during the detection phase of an engagement.

With proper execution, jamming techniques like range-gate pull-off (RGPO) could be especially brutal.

In modern radar systems, tracking is accomplished digitally. Here, a "range gate" can refer to a few things including but not limited to:

1. One sample from the sampled data after pulse compression/DFT. This definition is one of the most common.
2. A collection of samples as defined in (1), who each are their own range gate, that defines a range span. This one can be confusing but with proper context it's usually not too bad.
3. A particular value that limits the search space for detection/tracking algorithms.

Your question is related to (3). First off, you can totally search the entire range span if you wanted to. As a matter of fact, this is done in systems that are responsible for detecting and acquiring a target. You can see that being done in the "search action" phase in Figure 3 of the document you linked.

This is avoided in tracking because in many situations you simply do not have enough time to search the entire range span. The tracking algorithms/filters need to be fed at some required measurement update rate in order to keep the track alive. These rates are especially fast in missile-on-missile engagements were closing velocities are easily in the multiple of Machs.

Answer 2

Range gate gives to the direction tracking process the timing "compare the monopulse beams now".

Servo tracking range gate can drift away if there's a strong artificial echo which is timed to give at first a plausible "it's on the peak" indication and slowly moved away.

Clever anti-jam systems and also an experienced radar operator can manually force the range gate follow the right target, no matter there's a strong fake echo.

Answer 3

I doubt it is processing power limitation since during the search mode the whole beam is processed.

Hm the figures suggests you have a scanning beam: the radar can't observe the whole sky at once. So, until anything happens, you scan (as in: focus the beam at a spot, wait for the farthest sensible reflect, then you look at the next spot) the part of the sky from which attacks might be coming.

Typically that part of the sky is "empty", there's not going to be civilian aircraft in there, for example

Of course, once you detect something, you want to make sure where exactly it's going, and that it's really what you think it is. Hence, you shift your beam to a predicted position of the target. But that might include other radar targets – steel structures, your own aircraft, civilian aircraft, your own artillery fire...

So, it doesn't suffice there's something in your beam, it needs to be in the right spot.

Now, if you can't choose where you're looking, your SNR suffers – as said, there's clutter, there might be your own reflectors etc. So, this operation basically requires you to increase your radar's sensitivity by gating out anything that's not what you're expecting.

Answer 4

In order to track an object in 3D space, azimuth, elevation and range must be translated from 3 antenna with time and mobile spacial fixed reference.

It is synchronous to rev/sec stored in integer relative time with 12bit counters and position rotated by the phase shift of the reflected pulse relative to each rotor angle to triangulate position and compute velocity and acceleration to next expected position. The clock was counting relative time with an error less than 1e-11 to track all event history and current events with an expected use of 2h rather than 100h used. As a consequence, the triangulation history of the 3 phase array had a cumulative error from OCXO drift. Resetting the system count to calibrate a reference location for absolute accuracy was being used. The fix may have been to provide a selective resync/rezero count to null the history and start a new accumulated real-time travel history in order to accurately predict the future location travelling faster than the speed of sound in short range.

This selective gating blocks fake delayed echoes.