XM21 Remote Sensing Chemical Agent Detector

[mawyatt]

Per request and snipped from other related posts. These various posts discuss the battlefield M21 instrument development (XM21 was the experimental version) and some of the obstacles that were overcome.

In addition to the Theoretically Impossible task as stated by a Presidential Science Advisor, surviving a ridiculous Point Blank Thermo-Nuclear Detonation Thermal Blast Wave and Electro-Magnetic Pulse, and complete self calibration, the instrument had to have a UI that a child could operate and be simple to setup on a tripod. All of these challenges were met or exceeded in the development and the M21 went into full production and made the front page of Newsweek long ago.

The instrument utilized a SOTA 8-12um cooled detector, diamond turned optics, precision laser controlled tiny interferometer with flex pivot bearings, doped Germanium Window, internal thermally servoed Blackbody reference and many more unique and advanced features.

Remember flying back from Boston for a meeting with the prototype instrument, we had a special pass that allowed us to bypass airport security and were placed on the plane first. The instrument had a 1st Class seat, and I sat next to it. One of the flight attendants asked what it was (looked pretty ominous), and all the passengers took a long stare as they boarded. Later the pilot came back and asked about it and believe it or not he said "I'll fly to Cuba and we can sell this thing to Castro!! Joking of course but at a time when hijacking was common on airlines!!

Anyway, here's the various snips and hope you enjoy the discussions. If you have questions we'll try and answer if we can (memory isn't that good, and some "things" we still can't disclose)

Best,

https://www.semanticscholar.org/paper/CHEMICAL-DEFENSE-EQUIPMENT-O%27Hern-Dashiell/07889ce00d201692bc171aaee1c5914d9cbc145e/figure/22

"Back in 1980 we developed a battlefield instrument (XM21) capable of remotely and passively detecting chemical warfare agents at a distance of over 5 miles. The controversial technique was based upon passive spectral radiometry in the 8~12 micron region, and employed a tiny moving mirror interferometer, and ultra sensitive cooled (~70K) HgCdTe detector. The general idea was to use the finite but small temperature differences between air, clouds, terrain and such to help identify the potential chemical agents. So controversial was this passive technique that the program was halted because the presidental science advisor claimed the technique theoretically impossible!! A demonstration at the US Army Chemical Proving Grounds amongst congressional personnel and staffers proved the science advisor wrong!! The program continued and went into production protecting Allied Battlefield Soldiers and made the cover of News Week a few decades ago!

Fitting a complex multi-source & emissivity black body radiation (Stefan-Boltzmanns T^4) at 8~12 microns was not trivial, and required the XM21 to have a set of known BB emissivity calibration factors that included the instruments self-radiation effects. A technique was developed (later patented) where a known BB source (treated alumina substrate) was servoed to ambient temperature, then various delta temperatures above and below ambient "measured" for calibration. Also a known plastic with specific chemical "signatures" was also measured using the interferometer to create various calibration interferograms.

These calibration interferrorgrams allowed a complex curve fit to the input scene black body effects which was subtracted out leaving the net residue of the various emitters which included the net absorption and radiation of the scene. This residue also contained the various chemical "signatures" of the chemicals contained within the scene, including any potential agents which could be identified and alerts/alarms created.

Anyway, creating BB fit for a single emissive source is straightforward, however a multi-emissive source may prove more difficult, especially when one needs to separate the sources to help identify features."

"The X21 project was particularly interesting in that US President Regan's chief science advisor said it was impossible for this technique and instrument to work! We lobbied for a demonstration and proved the technique and instrument did indeed work as described, and the project went on to full production and deployment alerting solders in the battlefield of chemical attacks.

The miniature moving mirror interferometer was a masterpiece of engineering, utilizing flexure pivots for the moving mirror, voice coil motor, HeNe laser based closed loop servo control system,  diamond turned corner cube reflectors, small Split-Cycle Sterling Cryo cooler, an 18 bit ADC was phase-locked clocked with the interferometer position (derived from laser fring counts), thus producing coherent sampling interferograms that suppressed outside influences like vibration, temperature, and aging. All this and you could hold it in your hand"


"Fun story related to ZnSe and Ge optics. Back in ~1980 we were developing an important instrument to give early battlefield warning of a chemical attack and relied on passive spectral radiometry at a distance. This instrument was later called the XM21 and worked in the 8-12 micron region and employed all ZnSe optics as dictated by the optical physics purest. The instrument was to used by solders in the battlefield and required passing the classic military EM Compatibility tests in addition to the EM pulse from a point blank thermo-nuclear detonation (crazy but a customer requirement!).

So the EMC folks required the optical input lens to be covered with a dense wire mesh to pass these tests, basically causing an additional ~20% optical thru put loss. After an enormous amount of engineering had gone into getting the instrument sensitive enough to "see" the atmospheric details at up to 5 miles and doing so completely passively, this didn't seem like a good approach! We staring looking for an alternative and came up with Ge as a possibility, however the optical purest insisted on the ZnSe for all the optical components. After some serious discussion and challenges we got the the optical purest to evaluate a Ge lens for just the instrument front, and use ZnSe everywhere else. We devised a blind A/B lab test which showed both as having the same response with no real difference in performance. This was a rather large rectangular lens about 160mm by 80mm I recall and thick to pass the wavefront explosive force for the thermo-nuclear blast wave.

However we still had the EMC to pass, and these folks insisted on the wire mess approach which would have the same effect in reducing the sensitivity. The ace up our sleeve was doping the Ge lens to increase the conductivity to a level where the lens would act as a 3D transmission medium from free space to the front lens side and from the lens to free space on the interior side, so in effect just the opposite of an anti-reflective coating but a highly reflective on both sides at the frequencies of the EMC  test and EM pulse. Straight forward transmission line theory was applied to show the attenuation caused by the doped Ge lens was sufficient to pass all the tests. Needless to say the EMC and optical purest were highly skeptical and so another series of lab tests were established to confirm EMC compatibility and verify the 8-12 micron thru put wasn't significantly affected. This worked and the instrument passed all the EMC conducted and radiated tests, including the ridiculous EM pulse and blast wave tests. This was a fun project that went on to full production and help provide early warning for solders of impinging chemical attacks when deployed."

[RoGeorge]

Thank you for taking the time to answer my request. 

I'll have to read again and search about technical terms a little more before shooting questions.  For now I only understand the very generic goal of the device, and that it was a big engineering challenge to make it work.

The airplane paragraph was quite funny, tells a lot about how the world has changed since.

[mawyatt]

The device basically can be thought of as taking the input "scene" IR spectrum from 8-12um and mixing with a swept local oscillator (moving mirror interferometer) that creates a position sensitive optical intensity waveform. The optical output from the interferometer is sensed with a cooled HgCdTe detector and preamp which produces a voltage that resembles a Sin(X)/X function in the time domain and the frequency is related to the moving mirror interferometer. The interferometer is servoed to a HeNe Laser (lambda = 0.6328um) such that the moving mirror proceeds at a precise velocity & position linked to Quartz oscillator which is the reference. Basically the moving mirror is phase locked to the Quartz crystal by means of the HeNe laser fringe patterns (position feedback) which are used to create an interferometer coherent clock, where the moving mirror effectively becomes the VCO and the Laser the Descrete Phase Detector in a conventional PLL and the control voltage drives the moving mirror motor (Voice Coil Type).

A custom high resolution ADC is used to digitize the detector preamp output interferogram after some filtering and gain ranging. The ADC uses the coherent clock from the HeNe laser which provides a precise mirror position for each ADC sample and rejects out of band signals for external influences such as vibration, temperature, aging, pressure changes, humidity, power supply variations and so on. This essentially becomes a coherent synchronously sampled system and all the benefits from such.

The digitized interferorgram is then processed which includes zero filling and an FFT. At this point the result is a equivalent spectrum from 8 to 12um of the input scene, and various routines are performed to extract scene details, which includes, remove instrument artifacts, Black Body (SB T^4) fits, internal calibration and so on. The scene details are enhanced by subtracting a fitted BB curve(s) with internal calibration BB curves and the net absorption and re-radiation of all the various features in the scene are reveled. From here the chemical molecular signatures reveled are compared with chemical molecular ratios of known chemical agents and decision made regarding battlefield agents.

BTW the "scene" is the total integration of the field of view of the instrument as it scans across which includes the atmospheric aerosol out to ~ 5 miles. The sensitivity involves a concentration path length ratio, meaning a higher concentration at a longer distance has a similar "signature" as a lower concentration at a shorter distance.

This is obviously quite simplified but hopefully gives an idea of the measurement method and instrument details sufficient to appreciate what was achieved in spite of the Theroretically Impossible task as outlined by the Presidential Advisor long ago!!


Interesting side note, the HgCdTe detectors we utilized were more sensitive than the theory predicted at that time, everyone was using Honeywell's HgCdTe detectors and the facility was classified as a US National Asset back then (another interesting story).

Anyway, hopes this helps.

Best,

[coppercone2]

so this is essential like a fog detector? or does it pick up on gas flow like when you open a hot room window in the winter?

[mawyatt]

Not sure I understand?

The instrument essentially passively "looks thru" the atmosphere within the FOV out to ~5 miles as it scans across the horizon. Any chemical compounds and such in the FOV that have an absorption and/or re-radiation within the 8-12um band will be detected and the wavelength & amplitude characterized. Thus different chemical molecules will be detected and characterized by wavelength and intensity, then this data used to evaluate the potential for battlefield chemical agents.

Best,

[coppercone2]

Oh I thought it was visually seeing like turbulance because of the gas mixing. If you open a window near a radiator on a cold day you can see a haze (like desert haze), or asphalt on a hot day in some places

like a really sensitive version of that. I would imagine heavy gasses being 'funny' visually

[mawyatt]

Wish it was that easy!!

Basically you get the representation of the aerosol FOV from 8-12um out to ~5 miles, think of as a 3D chuck of air in that FOV space. Within that 3D air space you can ascertain the various molecular structures and determine the various chemicals and relative concentrations and this is done completely passively.

Best 

[coppercone2]

I wonder why you need chemical detector after a nuclear bomb?

[mawyatt]

That was a requirement imposed on the instrument by the customer. We argued that if the battlefield had just been nuked nobody cares about chemicals or left behind equipment, everyone's outta there for sure!! We didn't win the argument and this single ridiculous requirement almost quadrupled the instrument cost.

The blast wave travels near the surface speed of sound, very hot, and the EMP pulse is like getting hit by lightning!!! We survived and passed all the tests, however the mounting tripod did not (was developed by another company).

Best,

[5U4GB]

Coming in a bit late to the party, the FAS has an overview of the overall device.

[mawyatt]

Interesting article!!

They didn't mention the program was originally developed at Honeywell, starting in ~1981, and after the initial development the Honeywell division was sold to Brunswick. We worked on the program thru the development phase, then went on to other R&D projects. 

Also, they didn't mention the program was set to be terminated because the Chief Science advisor to US President Regan said it was theoretically impossible to remotely sense chemical agents passively in the battlefield. We lobbied for a demonstration, which was held at the US Army Proving Grounds, and attended by many research scientist, staffers, and other experts. After a highly successful demonstration the program continued.

The comment about the Winnebago RV is funny and true, this was the traveling lab that held a pair of Level 6 Minicomputers which did all the data collection, number crunching and control. This RV traveled to various testing sites around the US to collect data and help refine the detection and classification algorithms. After the algorithms were perfected, everything fit within a single Motorola 68000.

Best,

[coppercone2]

I figured out who made your design requirement, its the space marines.

just kidding , standard Chinese military strategy


And on a historical note, I thought back to what I know about the Korean war, and MacArthur's strategy, I bet you they figured just the radiation would not be enough (I assume they figured this out with a circle tool and a map), and they would need to gas and irradiate the border between China and NK, and since then the idea of chemicals and radiation became like cookies and cream for military planners.  Just shore it up with some gas rockets if there is a hole in the wall! And you know they had gas there already, because it was likely the ace in their sleeve for the thankfully averted Operation Downfall.... clearly a alot of it (i.e. red hat chemical cleanup team saga? that ended in the 70's, officially, so probobly most of it was gone by 1990)

The scary part IMO, is that a democracy is interested in knowing if the gas is there. Who on earth are they planning to send through that? One can hope it was just a way to verify the integrity of a supposed 'barrier' that could be created, and not meant for planning an attack! I hope its them wanting to be really sure that they don't get flanked, and not them hoping they can goad some 'allies' to go through that  ... But it seems to fit the other whacky strategies they had like nuclear land mines meant to be extra dirty (area denial craziness)

and suddenly the 'blasted lands' of judge dredd and such seem plausible. 

So it might be the chicken and the egg, some general was dying to know what the proper order of battle is to make a solid retaining wall. At first they were like "hmm... lets nuke it and then sprinkle some VX on it after it settles down" and then they started getting paranoid .... "hmmmm maybe there won't be time for that.... Oh I know... we should do a test to see how soon we can spray VX gas after a explosion, or maybe even spray gas BEFORE the explosion, it makes it easier for logistics.... how can we test this..." well I am kind of glad reagan's science advisor tried to put that idea down, its lunacy.


I know its just military science but IMO your project was seriously getting in some bad territory. Or maybe they just wanted to cover all the bases and I am being paranoid 

[mawyatt]

I figured out who made your design requirement, its the space marines.

Probably!! The tripod was from another company and during the blast wave test (explosive high air pressure wave) the tripod bent and the edge hit the release lever which sent the XM21 crashing into the thick steel side wall of the test chamber and shattered

I know its just military science but IMO your project was seriously getting in some bad territory. Or maybe they just wanted to cover all the bases and I am being paranoid 

We never tested with "live" agents, there was a harmless simulant that had similar molecular structures to agents in the 8-12 micron range that was used. The USSR actually used live agents during training, that eliminated those that didn't follow protocol, and they had massive stockpiles of chemical weapons back then.

We felt good about giving our solders an early warning of an impending chemical attack, but thank goodness that never happened. My British grandfather on my mothers side was attacked in WWI with Phosgene Gas, he was burned and forever lost his sense of smell, so helping our solders and allies has been a priority our entire career.

Best,

[coppercone2]

Oh I did not mean the testing was too dangerous, even with live gas we survived the 1960's, and we probobly eat more goats now then we gassed, its just another superfund site, not the end of the world, and I am glad we value the environment more then Russians but

What I meant that the nuclear proof part of the specification brings really nasty polluting strategies to mind, of making toxic barriers. If you have the tools someone will want to measure it.

I imagined this detector would lead to research like
1) nuclear bomb proof nerve gas
2) nerve gas that is even more persistent, toxic until the radiation goes away, your not supposed to go there anyway right!? that stuff wears off too quick!

And that this would result in judge dredd style blasted lands and walled off cities with 500 foot tall walls that prevent 'heavy agents' from billowing in too much.


Like at least there was enough sense to keep the bombs kind of clean. But there was defiantly a group of people that did not want this nice humanitarian specification.


I am just saying.. .you made that thing a bit too good! its scary

I guess that you can't bank on world leaders getting sad and calling it off after seeing a horizon of mushroom clouds

And BTW I think chemical weapons are very irresponsible and kind of cowardly. Generally I agree with the Geneva convention for avoiding inhumane weapons that maim so much, as broad as it is. But I am also saying I dont trust any military either. Not too much anyway. You really need to keep them in check because it goes crazy bananas

[coppercone2]

oh and as far as NBC goes, my absolute favortie, which I lost the video for, is one of those recon vehicles that is designed to drive around and sample things.

The best part about it was that on the back of the tank (iirc it looked like a stryker), they had a portal that opened up, and there was a glove box inside of the back of the tank, so one of the crew members would get on their stomach and stick a gloved hand out of the back of the tank so they could gather samples and put it into a carousel that would go into a sensor.

The concept is sound, but the implementation looked exactly like the rhino scene from Ace Ventura 2 (where it looks like a man is being born from a robo-rhino behind). The things man is willing to do!!!

[mawyatt]

There is still some places in France I believe where farmers get chemical burns from Phosgene gas from WWI that's sunk deep into the soil, nasty stuff that lasts forever!!

Regarding the clean nuclear bombs, yes those were developed and believe called Neutron Bombs. They had lower levels of blast wave and radiation, but very high levels of Neutron Flux which would kill all living things but leave the buildings standing.

It's shameful what man has done to develop weapons to kill others and so little respect one group of people has for another group, wether they are nations, religions, race, gender and so on.

Best,

[5U4GB]

There is still some places in France I believe where farmers get chemical burns from Phosgene gas from WWI that's sunk deep into the soil, nasty stuff that lasts forever!!

You can see that when driving through some rural areas, the result of the récolte de fer (harvest of iron) stacks of rusty shells piled up next to the road waiting to be picked up.  As you point out, the really scary ones aren't the generic rusty ones but those that slosh slightly when picked up.

[coppercone2]

hey did you ever see a clock made out of 1/4 rotation analog panel meters mawyatt?

Like 3 panel meters for each time unit

For some reason I just imagine you being around one of those some where within the MIC. I suspect they existed.
https://retrovoltage.com/2010/12/07/meter-clock-roundup/

maybe they added a 4th one too, for slow motion camera sync? that might be a impressive panel meter specification to survive counting to 1000ms over and over and over again, accurately to 0,5% (mirrored scale)

just to cheer it up a little bit. Or even a 1ms full scale clock, maybe developed on an alien world with evolved praying mantis like creatures that can process that kind of data (Yautja ?). Boring when you just let it rotate though.

[mawyatt]

Never seen one of those clocks, and how is this related to the discussions here??

Best,

[coppercone2]

imagined its in the bowels of a top secret gov lab

[coppercone2]

well, can you give us some interesting details about the EMC testing?

Z- pinch? explosive flux compression generator? Vircator? big thing like the "istra' soviet HV site? 
i.e.:
https://en.wikipedia.org/wiki/Istra_High_Voltage_Research_Center
https://en.wikipedia.org/wiki/ATLAS-I

Kind of sounds like putting it in front of a flux compression bomb would fulfill the mechanical and the electrical test requirements in 1 go.

[mawyatt]

The EMC was handled just like all military requirements, both conducted and radiated.  See Mil Std 461 for starters, probably lots of details online.

The EMP is a special case and handled with special equipment and facilities, which are not open for discussion. However it's like getting hit with a direct lightning strike with an extremely fast rate of rise voltage & current at very high levels. We passed by utilized Transorbs, fast clamp diodes, and gas discharge tubes working thru controlled dynamic impedances to limit the amount of energy impinging on the various circuits, the solid Aluminum case with proper bonding/sealing and the highly doped Germanium IR window provide the EM field shielding as in a Faraday Cage.

The blast wavefront was created by an ultra high pressure oxygen discharge which simulated the ~1,230km/h acoustic pressure wave (speed of sound) that had a large hooper above which dropped a load of ultrafine aluminum power into the wave front Oxygen stream. This powder was ignited with a flash ignitor (Aluminum dust is highly combustable, especially in a high Oxygen environment) and created the intense blast flame wavefront. Overall like a giant massive flashbulb from the old film camera days!!

Obviously all this required very complex and special facilities to handle these types of tests, not your usual UL certifications

Best,

[coppercone2]

lol when I was young I threw a handful of magnesium dust on a fire I can imagine

[mawyatt]

Aluminum dust was the "Fuel" in the Space Shuttle solid Fuel Boosters, Ammonium Perchlorate was the Oxidizer!!

Best,

[coppercone2]

Aluminum dust was the "Fuel" in the Space Shuttle solid Fuel Boosters, Ammonium Perchlorate was the Oxidizer!!

Best,

yeah it was like a space shuttle in my face, I thought it would sparkle (mass rate problem, I tired a pinch and it was amusing so I thought a handful would be colorful but the reaction rate I guess increased and god damn).. I think it was like 200 mesh

I also think I had my hand over the top of the fire and thought I could let it seep out from a closed fist (like a salt shaker) but as soon as I felt the heat from the first bit I just retracted and also opened my hand but all the magnesium got into the fire, I did not get burned (maybe singed facial hair but no painful burns) but it was billlllllowing smoke in that room and I had trouble seeing. Slower flash bang

their smart enough to use a hopper instead of a closed hand

might be fun at a beach bon fire or something to put a shot glass full of magnesium on the end of a long pole and dump it so a cloud drifts down into the flame

best part was to wonder if my parents were gonna walk in on me. the memory is as hazy as the house was