Would VHF radar+two stage IR SAM against stealth fighter?

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armedupdate

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Unread post05 Oct 2017, 02:52

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The problem with VHF radars is the relative inaccuracy. You will likely get kms off of distance. With radars seekers they would have trouble locking on unless it is guided very accuractly with datalinks. And with IR seekers they cannot measure range.
However, with modern two-stage missiles or dual pulse missiles that can burn at terminal phase, this may solve the problem. VHF radar could throw the missile close enough lets say by maybe 10 km of inaccuracy, however this may be enough, for an agile missile to use an IR seeker to find the target, and use the second burn to go straight at at target. No need to find range since with a rocket motor you could just move on bearing like a IR missiled fired at WVR.

Thoughts?
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wrightwing

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Unread post05 Oct 2017, 03:34

VHF isn't gonna get a missile, IR or otherwise, within detection/engagement range.
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laos

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Unread post09 Oct 2017, 12:39

I would be so sure if modern UHF AESA radar is employed. Monopulse tracking significantly improve precision.
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hornetfinn

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Unread post09 Oct 2017, 13:28

There are several problems with this:

1. Like already noted, VHF/UHF radar have pretty poor resolution. Another problem is that they have poor resolution in 3 dimensions, meaning also height resolution. So this means the missile search volume would be pretty large especially at longer ranges from radar.

2. VHF/UHF radar have quite long time between track updates as they use very large rotating antennas. This is usually in the region of 5 to 20 seconds which makes the real resolution even worse as any change in target speed or heading makes the volume a lot larger.

3. IR missiles have rather narrow Field of View and thus it takes a long time to search a large volume. Of course FoV could be wider, but then range and resolution suffer immediately. Missile going Mach 3+ would have very limited time to search the volume before it could no longer engage the target even if it detected it.

4. VHF/UHF radars are very large and cumbersome systems that are pretty easily detected and geolocated as they are powerful systems with narrow bandwidth (due to low frequency used). Stealth fighters like F-22 and F-35 would very likely know where they are and what they were doing. F-117 was brought down because it had no knowledge about the existence of enemy radars.

Basically it could be done and it could work in some cases, but it's likely not very effective overall especially against fighters with modern ESM/EW systems which could detect the radar. Even if radar could operate freely, the kill probability would be pretty low due to low radar resolution and slow volume search speed for missile seeker. There are a lot of reasons why VHF/UHF radars have not been used for direct target engagements except some anti cruise missile tests. VHF/UHF radars could be used to cue engagement radars though and using sensor fusion techniques could be more of a threat.
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armedupdate

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Unread post09 Oct 2017, 17:23

hornetfinn wrote:There are several problems with this:

1. Like already noted, VHF/UHF radar have pretty poor resolution. Another problem is that they have poor resolution in 3 dimensions, meaning also height resolution. So this means the missile search volume would be pretty large especially at longer ranges from radar.

2. VHF/UHF radar have quite long time between track updates as they use very large rotating antennas. This is usually in the region of 5 to 20 seconds which makes the real resolution even worse as any change in target speed or heading makes the volume a lot larger.

3. IR missiles have rather narrow Field of View and thus it takes a long time to search a large volume. Of course FoV could be wider, but then range and resolution suffer immediately. Missile going Mach 3+ would have very limited time to search the volume before it could no longer engage the target even if it detected it.

4. VHF/UHF radars are very large and cumbersome systems that are pretty easily detected and geolocated as they are powerful systems with narrow bandwidth (due to low frequency used). Stealth fighters like F-22 and F-35 would very likely know where they are and what they were doing. F-117 was brought down because it had no knowledge about the existence of enemy radars.

Basically it could be done and it could work in some cases, but it's likely not very effective overall especially against fighters with modern ESM/EW systems which could detect the radar. Even if radar could operate freely, the kill probability would be pretty low due to low radar resolution and slow volume search speed for missile seeker. There are a lot of reasons why VHF/UHF radars have not been used for direct target engagements except some anti cruise missile tests. VHF/UHF radars could be used to cue engagement radars though and using sensor fusion techniques could be more of a threat.

1. 2. But isn't resolution not incredibly bad? We aren't talking about 10 km off target? All we need is a missile to fly close enough to ignite the second stage.
3. From what I know IR missiles don't need huge FoVs if it's WVR.
4. True that.
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wrightwing

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Unread post09 Oct 2017, 18:11

VHF radar can't get a missile within 10km of a target, which is why nobody is designing air defense systems around VHF. VHF is sufficient to provide early warning, that something is out there, but a weapons grade track is impossible. As has been mentioned, it can't provide the 3D information necessary for a track, it has a slow refresh rate, and a resolution that's worse than within 10km. Aside from the radar limits, there are also missile limits. Think about how important heading, speed, altitude, etc.... are to missile range. Without high precision, the missile simply won't be within its WEZ much less NEZ.
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skyward

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Unread post09 Oct 2017, 19:32

3. From what I know IR missiles don't need huge FoVs if it's WVR.


That is because the IR missiles is being told where to look by the fighter before launch.
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hornetfinn

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Unread post10 Oct 2017, 11:41

armedupdate wrote:1. 2. But isn't resolution not incredibly bad? We aren't talking about 10 km off target? All we need is a missile to fly close enough to ignite the second stage.
3. From what I know IR missiles don't need huge FoVs if it's WVR.
4. True that.


It is pretty bad for missile guidance. Generally missiles need very good "weapons grade" target tracking for successful engagement like wrightwing said. Let's think about it for a moment:

VHF radar detects a target say 100 km away from the radar. Each detection has measurement accuracy of say 1000 m in azimuth, about 100 m in range and 3000 m in altitude and detection is updated every 6 seconds (pretty fast for VHF radar). These would be pretty good specs for VHF radar and range is just 100 km. What all this means is that target doing 250 m/s is going to move abut 1500 m between consecutive detections and there is going to be pretty large errors (within the measurement accuracy) in coordinates of each detection on average. If the target is going in straight line, it's fairly easy to predict where it will be next. However, when it's maneuvering even slightly, it's getting a whole lot more difficult with increasing errors in predicting where it will be. Remember that the radar must guide the missile to interception point which is always going to be in the future and missile is going to predicted point.Because of the aforementioned accuracy and slow update interval there is going to be huge uncertainty volume where the missile has to search for a target. For example just the altitude uncertainty is going to be several kilometers in best case.

Target speed calculation is also very tricky due to those errors and slow updates. Say there is 500 m error (half the measurement accuracy) in azimuth in two consecutive detections and target actually moves 1500 m between those two detections. Real speed is 250 m/s but radar could calculate the speed to be anything between 83 m/s to 416 m/s depending on error direction in two detections. Radar would naturally calculate the speed over say 10-20 detections, but then any acceleration or deceleration by the target would instantly create pretty large error until the tracking algorithm could follow. That is when radar does not miss any detections (not very uncommon especially in low frequency radars) and smoothly tracks the target. If it misses just one detection, the uncertainty volume instantly multiplies.

Basically low frequency radar is fine for targets flying straight and level at constant speed, but maneuvers and changes in speed are serious problem for them. That's why they are used for early warning and general surveillance and not for missile guidance.
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hornetfinn

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Unread post10 Oct 2017, 11:58

skyward wrote:
3. From what I know IR missiles don't need huge FoVs if it's WVR.


That is because the IR missiles is being told where to look by the fighter before launch.


Exactly. Missile seeker effective range and resolution are also inversely proportional to instantaneous FoV (IFoV). Missile seeker with 1 deg IFoV would pretty much have 90 times longer range than one with 90 deg IFoV. Usually IR missiles have seekers with IFoV in the region of 2-5 degrees for having enough range and resolution while seeing large enough area to search for target (with high accuracy cueing) and target tracking to work effectively.
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laos

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Unread post12 Oct 2017, 20:16

Hornetfinn  - I agree with your description of challenges faced by the users of VHF/UHF radars.
Despite this in recent years we can observe renaissance of UHF radars.
Just to name few : AN/APY-9, MEADS UHF Surveillance Radar, numerous Israeli, Polish, Ukrainian, Chinese and Russian surveillance radars.

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