Question about Triangulation with infrared sensors

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gideonic

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Unread post28 Jun 2016, 14:47

hornetfinn wrote:
LWIR sensor can detect low temperature targets better than MWIR one, but in most cases high-performance MWIR sensor outperforms LWIR sensors. LWIR sensors are usually cheaper at same performance, but MWIR are able to reach higher performance. They have the advantage of higher resolution, higher sensitivity, higher temperature contrast and have wider temperature range. So MWIR systems create crisper and better images with less noise. LWIR sensor would not give much benefit for aircraft sensor. LWIR is used for example in anti-ballistic missile interceptors where background is very cool and homogenous (deep space) and targets can be very cool (no air friction to heat the ballistic missile warhead for example). At lower altitudes and against earth background they don't work as well as cooled MWIR sensors.

LWIR is easier and significantly cheaper to make (both detector and optics) and reasonable performance can be had without cooled sensor system. I would bet this is why AAS-42 and IRST21 are LWIR systems. LWIR is pretty good for searching targets, but it would not be as good for target recognition and identification due to reasons mentioned earlier.

And Block 4 Advanced EOTS will be SWIR anyway. Being even further away from LWIR.
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eloise

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Unread post28 Jun 2016, 17:13

hornetfinn wrote:
LWIR sensor can detect low temperature targets better than MWIR one, but in most cases high-performance MWIR sensor outperforms LWIR sensors. LWIR sensors are usually cheaper at same performance, but MWIR are able to reach higher performance. They have the advantage of higher resolution, higher sensitivity, higher temperature contrast and have wider temperature range. So MWIR systems create crisper and better images with less noise. LWIR sensor would not give much benefit for aircraft sensor. LWIR is used for example in anti-ballistic missile interceptors where background is very cool and homogenous (deep space) and targets can be very cool (no air friction to heat the ballistic missile warhead for example). At lower altitudes and against earth background they don't work as well as cooled MWIR sensors.

LWIR is easier and significantly cheaper to make (both detector and optics) and reasonable performance can be had without cooled sensor system. I would bet this is why AAS-42 and IRST21 are LWIR systems. LWIR is pretty good for searching targets, but it would not be as good for target recognition and identification due to reasons mentioned earlier.

Then what would be the point of giving LWIR system to aircraft that can already carry a high performance MWIR system ? ( F-15, F-16 both received poded version of AAS-42 even though they can both carry sniper-xr ATP )
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armedupdate

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Unread post28 Jun 2016, 21:59

Can IRSTs with triangulation measure range with obtuse angles? Or do they need parallaxes?
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hornetfinn

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Unread post29 Jun 2016, 08:47

gideonic wrote:And Block 4 Advanced EOTS will be SWIR anyway. Being even further away from LWIR.


To be accurate it adds SWIR detector to EOTS while retaining MWIR detector. SWIR system is useful because it sees reflected light, although invisible to human eye in that wavelength. It offers even higher resolution and contrast than MWIR systems, especially useful for detecting and identifying small ground targets like individual terrorists or their vehicles. Basically SWIR images look like monochrome visible light camera pictures, although they can also be artificially colored. SWIR sensors are also extremely small and fitting one inside EOTS is probably pretty easy. LWIR sensors are much larger and require more space. Another advantage of SWIR is that most tactical laser illuminators and target markers work in that part of spectrum and SWIR system can see those laser beams. They can also see through glass which is impossible for longer wavelength systems. SWIR also has the advantage of using just regular optics due to this fact, whereas MWIR and LWIR systems require special non-glass optics.

MWIR and LWIR are superior in many ways against "normal" military applications against military targets like aircraft, tanks, IFVs, APCs and fortified structures. They allow much longer range for detecting and identifying such targets as longer wavelengths attenuate slower in atmosphere than SWIR.
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hornetfinn

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Unread post29 Jun 2016, 09:35

eloise wrote:
hornetfinn wrote:
LWIR sensor can detect low temperature targets better than MWIR one, but in most cases high-performance MWIR sensor outperforms LWIR sensors. LWIR sensors are usually cheaper at same performance, but MWIR are able to reach higher performance. They have the advantage of higher resolution, higher sensitivity, higher temperature contrast and have wider temperature range. So MWIR systems create crisper and better images with less noise. LWIR sensor would not give much benefit for aircraft sensor. LWIR is used for example in anti-ballistic missile interceptors where background is very cool and homogenous (deep space) and targets can be very cool (no air friction to heat the ballistic missile warhead for example). At lower altitudes and against earth background they don't work as well as cooled MWIR sensors.

LWIR is easier and significantly cheaper to make (both detector and optics) and reasonable performance can be had without cooled sensor system. I would bet this is why AAS-42 and IRST21 are LWIR systems. LWIR is pretty good for searching targets, but it would not be as good for target recognition and identification due to reasons mentioned earlier.

Then what would be the point of giving LWIR system to aircraft that can already carry a high performance MWIR system ? ( F-15, F-16 both received poded version of AAS-42 even though they can both carry sniper-xr ATP )


I can see several reasons for this. For long range detection and tracking LWIR can be as good as MWIR or in some cases even better (cold targets against cold background), but identification performance is lower (lower resolution and contrast). LWIR systems with decent to good performance are significantly cheaper than similar MWIR systems and can be smaller due to not needing cooling. MWIR can outperform LWIR, but at higher cost. For comparison, Thales Catherine-XP thermal imager has tank recognition range of 5.5 km using MWIR and 4.1 km using LWIR detector with otherwise identical systems and optics. The difference doesn't seem huge, but is still about 1/3. Identification range difference is likely much larger (where MWIR resolution and contrast would be beneficial) and detection range difference smaller (LWIR is good for detecting sources of heat). LWIR systems generally cost about one fifth of equivalent MWIR systems. MWIR systems also need more maintenance than LWIR systems, mainly due to cooling system. MWIR would likely be preferred from performance standpoint, but other considerations probably dictated using LWIR. I think they went for separate IRST system instead of targeting pods as they can be made smaller and likely significantly cheaper. They would not be installed inside EFT otherwise. Targeting pods probably also lack some IRST functionality and adding those features would increase their cost even further.

I do think having whole thermal radiation (visible light, NIR, SWIR, MWIR, LWIR, VLWIR) covered would give advantages as each wavelength has their advantages, but given finite space, weight and money currently means that only one or two bands are covered by almost all systems. LWIR gives nice capabilities at low cost. MWIR gives great capabilities at high cost. SWIR gives some unique capabilities at low cost and small size. Same is true for visible light cameras.
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hornetfinn

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Unread post30 Jun 2016, 09:03

armedupdate wrote:Can IRSTs with triangulation measure range with obtuse angles? Or do they need parallaxes?


I'm not sure I understand the question, but generally triangulation will be most accurate the more known reference points (sensors in this case) there are and the wider they are separated from each other. Even two widely spaced sensors can result in good target position/range accuracy given that the sensors can see the target orthogonally or close to it. In air combat that's pretty unlikely to happen, but say 4 F-35s flying with tens of nautical miles separation will probably be able to calculate pretty accurate target position and range even at long ranges. If they get additional info for sensor fusion like ESM data, that info would further reduce uncertainty especially against radars with constant output power levels.
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