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Wavelengths & radar contact orientation - 2/5/2021 9:46:26 AM   
pacman14k

 

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I'm watching P Gatcomb's YouTube video titled 'Command: Modern Operations Tutorial - Radar' (sorry, it won't let me post the actual link) to understand how radar works. From 6:10 he explains how a radar operating on the 'A band' has a long wavelength, then demonstrates this with a P-10 "Knife Rest B" search radar, the result being that the first targets it picks up are giant rectangles in "landscape" orientation relative to the emitting radar (as in landscape vs portrait, but I'm sure there's a proper way of describing it). But since we're talking a wave with a long wavelength, shouldn't the rectangle representation of the contact be in a giant portrait orientation relative to the radar? After all, it's waveLENGTH not waveWIDTH.

The other thing I don't understand is at around the 8:10 mark, after the radar has made the rounds a few times, the contact changes from that massive landscape orientated rectangle, to a smaller "portrait" orientated rectangle. Also, the other odd thing about this rectangle is that its length remains about the same size, ie, it's gone from incredibly wide to very narrow.

Can someone explain what is going on here, and how all this relates to the wavelength?

Thanks!
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RE: Wavelengths & radar contact orientation - 2/5/2021 11:12:03 AM   
Dragon029


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The length and width of a detection box are based on the angular and range resolution of a radar. Both of these are affected by the radar's wavelength, but there are other factors involved too.

For the length of a detection box (which shows the minimum and maximum range to the target) you're talking about the range resolution. This is how accurately a radar can measure the time that it's taken for a radar signal to reach the target and reflect back. To measure this time, a radar needs to detect key identifying features in return signals; usually it will measure things like the rising or falling edge of a pulse. If the radar gets a weak signal return and/or there's a lot of echos / multi-pathing then it'll have a number of potential distances to the target. A long radar wavelength means a low frequency as well, which typically results in pulses that have smoother rising and falling edges to their pulses, making the exact measurement of when a pulse arrives at the radar a bit more ambiguous. This value should improve over time as a target gets tracked / detected multiple times, but narrow down the target's range can take a while, especially when there are multiple potential targets being detected.

For the width of a detection box you're talking about angular resolution. This is mainly dictated by the width of a radar's beam, which in-turn is largely dictated by the types of antennas or dish used, and how the size of those antennas relates to the wavelength of the radar. For a phased array, the number of antennas is critical here. If you have an antenna array that's only 6 radars wide, you're going to have a much lower angular resolution than a radar that's 20 antennas wide (assuming too that the same type of antenna is being used in both arrays). For radars that don't use phased arrays, things like the gain of an antenna (which gets better as a parabolic dish gets larger) and the pulse repetition frequency (which is limited to an extent by radar wavelength / frequency) impact your ability to determine where a target is in relation to the direction that you're pointing / mechanically scanning the radar. This width of the detection box will get smaller more rapidly, because unless the target is manoeuvring very erratically and moving very quickly, the scanning of a radar will gradually narrow down the scope of where they are in azimuth (and elevation for a 3D radar). Keep in mind however that this narrowing won't continue forever; you'll reach a minimum angular resolution due to limits in hardware (being able to discern where a signal is strongest when noise is constantly varying the received signal strength) and due to how rapidly a radar can update its track.

(in reply to pacman14k)
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RE: Wavelengths & radar contact orientation - 2/5/2021 12:18:37 PM   
thewood1

 

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That's a great layman's explanation. Radar and detection explanations are usually to generic or so detailed I stop paying attention. That was perfect. It should be in the manual.

(in reply to Dragon029)
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RE: Wavelengths & radar contact orientation - 2/5/2021 1:06:38 PM   
SeaQueen


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Almost there. The ability of a radar to resolve range depends on the pulse width of the radar set. Nothing else. Beam width (which in CMO/CMANO is synonymous with antenna gain) what determines a radar's angular resolution.

Now... why does a track get better with time? Two things:

1) Kalman filtering (this is a mathematical technique involving matrix multiplications and Bayes theorem). It's way too complicated to explain here today, so... go read about it somewhere. There's got to be an engineering dork around here somewhere. Not sure if Command does this, but if it doesn't it'd probably make D's life easier.

2) Because angular resolution is an angle, 3 deg resolution is worse at 300 NM than it is at 2 NM, because that same angle subtends more or less space at different distances. That's just geometry.

Also Doppler radars might be able to discern range a little better using the Doppler shift and some signal processing, but this is the basics.

With that, the final dogfight scene in TOPGUN:

https://www.youtube.com/watch?v=jqfXXaOisKo

Is actually potentially reflective of a real phenomenology (awesome!). Aircraft sufficiently closely spaced MIGHT be able to fit within what's called the "resolution cell" of the radar (which the box of angular width equal to the beam width, and length equal to the pulse width times the speed of light). Since the aircraft are both in the same resolution cell, the radar thinks they're one thing!

This is important for sneaky MiG-28s looking to fire Exocet anti-ship missiles from 100 miles away for no obvious reason than the fact that they're MiGs, and they're black with a red star and pointy. You know how they are. Here's a question! Could a "MiG-28" (i.e. F-5) carry an Exocet anti ship missile? Maybe the centerline? What about ground clearance? I need to ask someone about that.

At any rate, here's an explanation of the "resolution box" which is slightly more nerdy.

https://www.radartutorial.eu/01.basics/The%20resolution%20cell.en.html

I'm not sure Command really captures that phenomena directly but there's database entries to get a similar(?) effect. Perhaps the developers can comment?


quote:

ORIGINAL: Dragon029
For the length of a detection box (which shows the minimum and maximum range to the target) you're talking about the range resolution. This is how accurately a radar can measure the time that it's taken for a radar signal to reach the target and reflect back. To measure this time, a radar needs to detect key identifying features in return signals; usually it will measure things like the rising or falling edge of a pulse. If the radar gets a weak signal return and/or there's a lot of echos / multi-pathing then it'll have a number of potential distances to the target. A long radar wavelength means a low frequency as well, which typically results in pulses that have smoother rising and falling edges to their pulses, making the exact measurement of when a pulse arrives at the radar a bit more ambiguous. This value should improve over time as a target gets tracked / detected multiple times, but narrow down the target's range can take a while, especially when there are multiple potential targets being detected.


< Message edited by SeaQueen -- 2/6/2021 12:38:36 PM >

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RE: Wavelengths & radar contact orientation - 2/5/2021 6:12:11 PM   
Schr75


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quote:

This is important for sneaky MiG-28s looking to fire Exocet anti-ship missiles from 100 miles away for no obvious reason the fact that they're MiGs, and they're black with a red star and pointy. You know how they are. Here's a question! Could a "MiG-28" (i.e. F-5) carry an Exocet anti ship missile? Maybe the centerline? What about ground clearance? I need to ask someone about that.


I don´t know about the avionics in the MiG-28, erhm, F-5, but weight and size wise, fitting an Exocet shouldn´t be a problem.

The missile weights about 780kg and the F-5 can carry up to 3200kg.
It has a wingspan of 1.35m, but with the wings angled at 45 degrees it only needs 95cm of ground clearance, and since it can carry a 275 gallon fuel tank on the center pylon I think the Exocet should be able to fit.

The attached picture of an Iranian MiG-28 with a drop tank on the centerline pylon shows just how big a thing it can carry there.




Attachment (1)

(in reply to SeaQueen)
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RE: Wavelengths & radar contact orientation - 2/6/2021 3:02:36 AM   
pacman14k

 

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Thanks for your responses.

So thanks to you guys I've done some more reading about beamwidth of the main lobe, if my understanding is correct it's the angle left and right of the central (most powerful) line along the main lobe where power has dropped by half. According to the Knife Rest B page on the Radar Tutorial website (again, which I can't link), the Knife Rest B has a beamwidth of 21 degrees. In other words, if you move 10.5 degrees clockwise or anti-clockwise from the centre of the main lobe while maintaining the same radius, the radar's power will have dropped by half. So I assume the first detection of a contact by a Knife Rest B will be ambiguous by 21 degrees in azimuth, then as the radar does more sweeps this azimuth angle gets smaller and smaller.

Also, I measured the angle of BOGEY #2 in P Gatcomb's video, and low and behold, exactly 21 degrees between the radar and the left and right edges of the box. The other contact, however, was inside a 37 degree box from the radar. Not quite sure what is going on there.

Thanks again, I've learnt a lot!

(in reply to Schr75)
Post #: 6
RE: Wavelengths & radar contact orientation - 2/6/2021 7:34:59 AM   
Dimitris

 

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quote:

ORIGINAL: SeaQueen
With that, the final dogfight scene in TOPGUN:

https://www.youtube.com/watch?v=jqfXXaOisKo

Is actually potentially reflective of a real phenomenology (awesome!). Aircraft sufficiently closely spaced MIGHT be able to fit within what's called the "resolution cell" of the radar (which the box of angular width equal to the beam width, and length equal to the pulse width times the speed of light). Since the aircraft are both in the same resolution cell, the radar thinks they're one thing!

This is important for sneaky MiG-28s looking to fire Exocet anti-ship missiles from 100 miles away for no obvious reason the fact that they're MiGs, and they're black with a red star and pointy. You know how they are. Here's a question! Could a "MiG-28" (i.e. F-5) carry an Exocet anti ship missile? Maybe the centerline? What about ground clearance? I need to ask someone about that.

At any rate, here's an explanation of the "resolution box" which is slightly more nerdy.

https://www.radartutorial.eu/01.basics/The%20resolution%20cell.en.html

I'm not sure Command really captures that phenomena directly but there's database entries to get a similar(?) effect. Perhaps the developers can comment?




It's on our near-term stack, along with a few other things. Until recently the biggest block was not the radar emission properties (I _think_ we have all those relevant for this effect) but how to handle the "contact management" logics. These were put in place when we added passive sonar masking (which is conceptually similar, though on different domain), so the actual implementation should be a bit easier now.

< Message edited by Dimitris -- 2/6/2021 7:37:20 AM >


_____________________________


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RE: Wavelengths & radar contact orientation - 2/6/2021 9:19:24 AM   
Parel803

 

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nice reading

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RE: Wavelengths & radar contact orientation - 2/6/2021 11:35:16 AM   
Joelsi


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If anyone is interested in radar stuff I can recommend George W. Stimson's Introduction to Airborne Radar

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RE: Wavelengths & radar contact orientation - 2/6/2021 4:22:20 PM   
avenger300

 

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This looks like a great place to ask a burning question that I have had.

Recently, I have gotten into some of the DB using SQLite in R. I've discovered the table "MiscSensorRangeCalculated" and it seems to have different sensor ranges for different radars/sensors vs. different kinds of targets.

I'm interested in radars that can sense periscopes. There seems to be two categories: RadarRangeSrchPeriscope and RadarRangeIllumPeriscope.

The values (in km?) in these two columns seem to be very different for each radar. For example, the AN/TPS-71 seems to have a "search range" of 127 for periscopes, but a illumination range of "0" (I'm surprised that an "A" band radar can even pick up periscopes at all!).

tl;dr: what is the difference between RadarRangeSrch and RadarRangeIllum?

Thank you!

(in reply to Dragon029)
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RE: Wavelengths & radar contact orientation - 2/7/2021 1:56:06 PM   
SeaQueen


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quote:

ORIGINAL: pacman14k

Thanks for your responses.

So thanks to you guys I've done some more reading about beamwidth of the main lobe, if my understanding is correct it's the angle left and right of the central (most powerful) line along the main lobe where power has dropped by half.


Not necessarily (the radar might be able to narrow it down more than that) but that's generally the first guess.


quote:


According to the Knife Rest B page on the Radar Tutorial website (again, which I can't link), the Knife Rest B has a beamwidth of 21 degrees. In other words, if you move 10.5 degrees clockwise or anti-clockwise from the centre of the main lobe while maintaining the same radius, the radar's power will have dropped by half. So I assume the first detection of a contact by a Knife Rest B will be ambiguous by 21 degrees in azimuth, then as the radar does more sweeps this azimuth angle gets smaller and smaller.


Only if there's some kind of Kalman filter in the processing to fuse the historical position of the target with the updated one.

quote:


Also, I measured the angle of BOGEY #2 in P Gatcomb's video, and low and behold, exactly 21 degrees between the radar and the left and right edges of the box. The other contact, however, was inside a 37 degree box from the radar. Not quite sure what is going on there.

Thanks again, I've learnt a lot!


Excellent.

(in reply to pacman14k)
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RE: Wavelengths & radar contact orientation - 2/8/2021 7:56:42 AM   
KnightHawk75

 

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@avenger300
quote:

tl;dr: what is the difference between RadarRangeSrch and RadarRangeIllum?

What DB are you referring too as I have no such table (MiscSensorRangeCalculated), nor columns RadarRangeSrchPeriscope, etc in my db3k databases, perhaps professional does?
But in general for cols that do exist in the actual sensors table in db3k that have search vs illum in the names the difference one applies to search features of the sensors, one applies to weapon illumination features of a sensor.



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RE: Wavelengths & radar contact orientation - 2/8/2021 4:52:07 PM   
avenger300

 

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quote:

ORIGINAL: KnightHawk75

quote:

tl;dr: what is the difference between RadarRangeSrch and RadarRangeIllum?

What DB are you referring too as I have no such table (MiscSensorRangeCalculated), nor columns RadarRangeSrchPeriscope, etc in my db3k databases, perhaps professional does?
But in general for cols that do exist in the actual sensors table in db3k that have search vs illum in the names the difference one applies to search features of the sensors, one applies to weapon illumination features of a sensor.





I opened the db3 game file using some R and SQL programming language. It's a large file that has technical info on most of the database entries, and numerous tables with various aspects of each entry.

Whats the difference between search and weapons illumination?

(in reply to KnightHawk75)
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RE: Wavelengths & radar contact orientation - 2/8/2021 6:20:15 PM   
Zanthra

 

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quote:

ORIGINAL: avenger300


quote:

ORIGINAL: KnightHawk75

quote:

tl;dr: what is the difference between RadarRangeSrch and RadarRangeIllum?

What DB are you referring too as I have no such table (MiscSensorRangeCalculated), nor columns RadarRangeSrchPeriscope, etc in my db3k databases, perhaps professional does?
But in general for cols that do exist in the actual sensors table in db3k that have search vs illum in the names the difference one applies to search features of the sensors, one applies to weapon illumination features of a sensor.





I opened the db3 game file using some R and SQL programming language. It's a large file that has technical info on most of the database entries, and numerous tables with various aspects of each entry.

Whats the difference between search and weapons illumination?



Generally speaking, semi active homing weapons require a target to be illuminated to be guided to the target, and for radar it usually tips the target off that they have been locked. I don’t know of any semi active radar guided weapons that would be used against a sub at periscope depth.

(in reply to avenger300)
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RE: Wavelengths & radar contact orientation - 2/9/2021 1:19:48 AM   
KnightHawk75

 

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quote:

ORIGINAL: avenger300


quote:

ORIGINAL: KnightHawk75

quote:

tl;dr: what is the difference between RadarRangeSrch and RadarRangeIllum?

What DB are you referring too as I have no such table (MiscSensorRangeCalculated), nor columns RadarRangeSrchPeriscope, etc in my db3k databases, perhaps professional does?
But in general for cols that do exist in the actual sensors table in db3k that have search vs illum in the names the difference one applies to search features of the sensors, one applies to weapon illumination features of a sensor.





I opened the db3 game file using some R and SQL programming language. It's a large file that has technical info on most of the database entries, and numerous tables with various aspects of each entry.

Whats the difference between search and weapons illumination?


Not sure what R is doing to it in terms of those names, but no such table exists nor do those cols in retail (I mean you can just simply open it in a sqlite db browser app like https://sqlitebrowser.org/ to see what I mean).

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RE: Wavelengths & radar contact orientation - 2/10/2021 11:21:29 AM   
c3k

 

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Beyond beam properties, azimuth accuracy depends on mechanical accuracy of a rotating radar. Gear lash, etc., play into that. Knowing where the radar antenna is pointing is not simple task when speaking of long range and trying to come up with targeting solutions.

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RE: Wavelengths & radar contact orientation - 2/10/2021 12:18:20 PM   
SeaQueen


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quote:

ORIGINAL: c3k

Beyond beam properties, azimuth accuracy depends on mechanical accuracy of a rotating radar. Gear lash, etc., play into that. Knowing where the radar antenna is pointing is not simple task when speaking of long range and trying to come up with targeting solutions.


Only with a mechanically scanned array. :-) AESA/PESA antennas don't have that problem, among many others...



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RE: Wavelengths & radar contact orientation - 2/10/2021 5:23:57 PM   
c3k

 

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quote:

ORIGINAL: SeaQueen


quote:

ORIGINAL: c3k

Beyond beam properties, azimuth accuracy depends on mechanical accuracy of a rotating radar. Gear lash, etc., play into that. Knowing where the radar antenna is pointing is not simple task when speaking of long range and trying to come up with targeting solutions.


Only with a mechanically scanned array. :-) AESA/PESA antennas don't have that problem, among many others...





Exactly!! AESA/PESA have many, many advantages. I was merely pointing out one more drawback/concern with radar azimuth accuracy for those radars that rotate. (A similar issue is extant with elevation.) Some AESA arrays (redundant, I know) do move, both horizontally and vertically.

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RE: Wavelengths & radar contact orientation - 2/14/2021 11:25:38 PM   
Zanthra

 

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Until fairly recently, an AESA radar with the same accuracy as a mechanical radar of similar size would be more expensive, due to the complex digital logic required, as such, they were only used where more accuracy was required than could be obtained from a mechanically scanned radar, despite the substantially higher expense.

Interestingly, telling where an object is is not as simple as knowing how the radar is positioned relative to the aircraft or ship. A ship rolling and tumbling on rough seas will have all the radar contacts whipping around it at crazy speeds if measured relative to the ship itself. Accurately knowing the position, as well as the yaw pitch and roll of the vehicle the radar is on is also important for integrating the radar contacts with those from other platforms, and that problem is not reduced with AESA radars.

< Message edited by Zanthra -- 2/14/2021 11:29:45 PM >

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