RWR Launch Warning (SARH) - Page 4 - ED Forums
 


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Old 06-30-2020, 08:11 AM   #31
Fri13
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Quote:
Originally Posted by Cmptohocah View Post
If I understood correctly, this means that - theoretically - RWR should keep "buzzing" as long as there some sort of STT detected after the launch. Wether that be the same platform continuing the lock after the launch or a wingman - in a pair flight - having the target in STT even though the launching platform eventually lost the lock.
Am I understanding this right?
Forget for a moment a radar, missile and guidance and just concentrate to RWR side.

A RWR is typically a set of three or four antennas/sensors. All are linked to central computer.

Mathematically each sensor creates a point, thats position is known in space.
And it creates a known pattern of those 3 or 4 points.

Each sensor is designed to capture a specific frequency, it has sensitivity against a background noise and electronics noise in cables etc.
And each sensor has own blind spots, like a sensor in leading edge of wing can't see what is happening at the rear of the wing (let's keep it simple that the radio emission doesn't travel across the wing and it's shape).

The magic happens in the central computer, that is listening each sensor. When radio emission is transmitted, it will reach each sensor at different time because they are at different distance from the emission source.
Using this time difference, the central computer uses a mathematical calculation to triangulate the emission point from known 2-4 points.
Using just a 2 sensors the system can use time of receive to detect which side emission is coming from, as that side sensor that receives it first is the side of the source. But computer can not calculate anything else than a direction as it only knows "this received it first".

The challenge for RWR resolution is the accuracy of the computer clock. We are talking about pico seconds scale here.

The RWR computer clock is the key to calculate time between different sensors detection times, and so on direction. But it doesn't matter how far the emission really comes as RWR is only interested direction with clock.

The other challenge is the emission power. How much energy does it have. That can be used to estimate the emission distance if we know the emission power at source by some means. So RWR computer as well measures the emission power from each sensor and then calculates the distance to it. Simple inverse square law applies here.

That is limitation of the sensors, cables and computer own signal-noise-ratio. As sensor and all computers generate constantly some signal that is considered noise, you need to receive a signal that is higher in power than that to be detected. And this is where comes the "RWR has sensitivity 125% higher than radar max detection range." Meaning your RWR will detect emission power about twice longer range than radar can possibly have a change to detect anything at max range (maximal target size).
So when emission is transmitted, a RWR can detect it far sooner.

The emissions that RWR can receive are wide in frequency scale, and wide in power scale. And it can accurately calculate angles to source by timing on same plane, but weakly in parallel plane.

Now when basic radar is operational and emitting, it uses one frequency and transmits multiple pulses per second. By various means it would need to use one receiver/sensor to find a echo of emission and calculate direction, but range it finds easily. And radars typically rotates or sweeps the large space, so emission is directed to given position short period of time.

So how does a RWR detect an lock? By detecting time of exposure to emission. If a emission is received for a longer time than estimation of one sweep period is, then it is seen as lock as radar doesn't anymore scan the space but is focused to RWR position.

As each radar has own frequency and code it transmit, it can be detected is there one or more emission sources.

In a analogy form, if you play hide and seek and then seeker will start staring directly at your hiding location, you will know that seller has spotted you, concentrating to your location.

Now when a missile is launched, radar needs to start send a guidance information to missile in that same emission that is pointing to RWR.
And this change in the transmission is considered as a launch. As it is only estimation because there might not be real launch, just simulated. As radar can be set to start transmitting guidance commands even if missile has not launched.

As the launch process basically goes:

1) search a target.
2) spot a target.
3) calculate where target is located.
4) transmit target location to missile.
5) launch a missile.
6) start transmitting guidance commands relative to target location of specific periods.
(Possibly 7) transmit a missile destruction command.)

The missile is the other part of the combination.
It has only one job and that is, Fly as commanded and explode when commanded or when one of the fuze is triggered (proximity, impact, time).

The radar doesn't care is the missile launched or not. Why SEAD missions are critical that you will visually spot the launch so you can confirm that missile is coming. As launches can be simulated just to trigger launch warning in RWR. And so on unaware pilot starts to take preparations by maneuvering etc as his RWR is screaming a emission of lock and guidance.

Now, the RWR doesn't know is it really target or not. In close formation all RWR received warning if radar emission touches them long enough time.

The RWR doesn't either know is the radar locked to it or missile coming.
As many SAM systems are distributed to multiple phases and different directions.
Where example missile can be launched and guided by another radar than search radar. The RWR will only see a search radar, but not the guidance radar that is transmitting guidance commands with very narrow beam to missile, that is pointing totally another direction than RWR is.
Then only in the last seconds the tracking radar can be activated to lock on the target, guidance radar sends a command for missile to activate own seeker and lock on the tracking radar emission.

So a SAM launch from a 100 km distance in STT mode can be detected only visually or just last seconds when missile is already a kilometer or two from your position, while you are interpreter to RWR screaming at you.

And yet there might not be nothing. The track radar can be activated and just to fright you as RWR doesn't know is there a missile or not.
And you can't know by any means that is the missile coming or not and how much time you have to impact.

This same thing can be done in fighters that you start transmitting guidance as fake launch, and target RWR doesn't know is the missile coming or not.

The RWR systems as well have a memory. Based the radar type the RWR keeps warning until timer is up and no new emissions is detected in that time.
The RWR doesn't know is the missile destroyed, missed or coming (if there even is a missile). It is just reacting based emissions it can detect.

And as radars can maintain a lock on you, the missile is not a dumb one that will forget it's mission in millisecond when it doesn't receive guidance. But missile will keep listening that is there coming new commands.
Some missiles has a self-destruction feature with timer since last guidance commands if it can't detect a emission front of it.

Why you wouldn't want to have jammer On as missile can have a Home-On-Jammer feature. As typical jammer is one that starts jamming in same frequency as radar emission is, so missile can find it and use it for guidance.
And at given ranges their effective jamming is lost and it becomes a beacon to be targeted.

The jammer as well triggers the RWR. So if you go flying with jammer On, anyone with RWR will see you transmitting as you would have radar On.
And one with radar can go silent (no emission) but keep radar scanning and use your jammer as return, so they will find a vector to you, while not the speed or distance. But combine two radars, and now you have triangulated the target with jammer.
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