IR optics and weather

[Euri]

The colder the temperature the better the IR spotting?

[John Kettler]

Euri,

Are you talking a pure case of clear weather and really cold, or is it cold or really cold with, say, rain and fog thrown in? Also, are you looking for something running or just sitting there, such as an ATG,  or do you mean a tank fully warmed up? I can tell you from direct experience and subsequent debriefings that the M48A5 (bailed to Hughes Aircraft Company, Missile Systems Group for seeker tests)  I was temporary TC of flat out disappeared from view of a AN/TAS4 TOW Night Sight (8-12 micron band) , on a hill, at a range of ~1 km on a cold, rainy, at times foggy day. Temperature was in the 40s F. This was with the motor well fired up and the tank having moved hundreds of meters to get into position. Not only could I hear the consternation on the radio, but overhear things like "It disappeared! Where did it go?" In fact, I was even accused of having moved the vehicle, when I hadn't moved it at all following an earlier repositioning request! The combined effects of attenuation and the cool down (hugely reducing tank background temperature delta) of the vehicle caused by continuing cold rain wiped out the ability of the TOW Night Sight to detect enough thermal contrast and display the tank. Consequently, there is no black and white answer here. This is based not only on my tank outing, but having studied IIR guidance professionally and watched actual IR video from a number of systems. Hughes MSG built the IIR Maverick and the other versions as well.  Further, I would point out that there are times of day in certain environments in which there is zero thermal delta between the target's temperature and the background, making the target invisible to such systems. This is called crossover. The US had this happen with thermal equipped (Hughes gear again) A-6 Intruders attacking North Vietnam's vital Thanh Hóa (Dragon's Jaw) bridge during the Vietnam War. Like every other sensor out there, IIR systems have limitations. Today's equipment, though, has markedly higher detector sensitivity and way better signal processing than what I've described. Some of it, such as Raytheon's (was a Hughes product first )Thermal Weapon Sight also operates in an entirely different spectral region. While a hot target  against a cold background (with no WX, crossover or anything else interfering) is highly desirable, achieving that goal on the battlefield is another thing altogether.

Regards,

John Kettler

[panzersaurkrautwerfer]

The colder it is, the more clear "hot" targets are.  So on a cold clear day, the contrast between manmade object and the ground around it is pretty pronounced.

The more stuff there is in the air, the harder it is for the thermal to penetrate.  Which isn't to say it won't, but having operated a thermal optic in a monsoon, heavy snowfall, and lots of fog, there is some degradation.  

I think the best example is that gunnery in the deep winter of Korea on clear days was awesome, because the target was literally the only hot thing on the range, and spotting other AFVs was really quite clear.  On the heavy rain/foggy days, some stuff would be hard to acquire, like there's some calibration panels with a heat lamp bulb, those would take a little doing, and the targets are just plywood warmed on a headed concrete pad before being elevated, so if the pad was practically underwater, it'd be a lot less effective at heating and then somewhat screened by the rain, However generally AFVs and real humans remained pretty easy to spot out to the 1-2 KM range (which was about as long of a LOS as you get in Korea.

Hot days were always harder for me because it fills the optic full of "junk," rocks, scrap, pretty much anything that you don't want to touch on a hot day glows pretty well on a bright sunny day.  You have to slow down your "scan" (basically moving the optic back and forth to find targets) to better figure out what all the spots are, vs on a cold day, when ANY hot spot is something alive/with internal combustion going on.  

[Euri]

The trigger for the question was the spotting I witnessed in the recent video I uploaded