Fox Two — Infrared Missile Target Tracking

Analysis of IR tracking technology based on the AIM9 Sidewinder.

14 min readNov 9, 2017

1946. Germany has been defeated, and its military technology put under the microscope, the west began work on large-scale military projects based on findings from German rocket technology. Up until that point, these projects had been little more than research endeavors of the Third Reich scientists. At the Naval Ordinance Test Station (Present day NAWS China Lake), a Caltech physics pHd William McLean began working on an in house project that would dramatically change the way military aircraft would be built and operated. A precision guided, rocket powered, air-to-air high explosive armament; The Sidewinder. A name fitting not only because of its ability to snake through the skies like Crotalus cerastes, but because it was a cold-hearted heat-seeking killer. The purpose of this article is to explore the techniques the missile uses to track and attack targets through technical analysis.

Over the years, the design improved to meet modern performance standards and requirements such that new generations hardly resemble their predecessors in anything other than the ‘Sidewinder’ moniker. The latest operational evolution of the AIM-9, the AIM-9X Block II, was developed by Raytheon and is currently in service in the mix with some of the older variants. Do note however, that the AIM9X is a significant departure from the previous versions since substantial changes were made.

I’ve got tone!

The design of the missile consisted of multiple novel components but remained relatively simple for low cost:

The front of end of the missile was made out of a glass lens instead of a steel-shelled warhead. Inside the lens sat a gyroscopically spinning mirror in front off a lead sulfide IR photodetector. It operated as an eye that focused in on infrared contacts in the airspace while ignoring background infrared noise.

Step 1: Isolate Heat Signature

A key component of the seeker head is a dichroic filter, which is used to isolate and pass through the required infrared wavelengths. The principle behind a dichroic filter is thin-film interference; something commonly seen on wet road oil spills. Multiple layers of material with different refractive indexes reflect light that can combine constructively or destructively. Destructive interference eliminates the wavelength while constructive interference wavelengths pass through. Interference is determined by the optical path difference (OPD) between the rays, in the case below, between ray D and ray C.

Multiple layers of thin film coatings can be added with thicknesses down to micrometer/nanometer range using precise vacuum deposition. By precisely depositing thin film layers, the thicknesses of up to a hundred layers of film interact with the light depending on their various refractive indexes and generate a precise filter. We can use the anti-reflection coating equations for our infrared isolation lens:

Visible and ultraviolet light must be removed to prevent signal noise at the sensor corrupting the signal. However, this is an extremely wide band of wavelengths so they are more easily removed with an absorption filter (an alternative to dichroic). Selecting the ideal infrared frequency is performed using constructive interference by selecting a material of index n-coating, and applying an amount d, for some wanted 𝜆 using the first equation. Note that 𝞥2 also plays a role in the lens design. In general, dichroic filters are better than absorption filters since fine-crafting the passband is easier, the stopband light is not absorbed in the medium, and because microscopic film layers are less sensitive to stresses of temperature and bleach-out. The downside of such a filter is that the incident angle at which the filter is properly functional is much lower than an absorption filter and is more expensive to make. This property directly affects the ‘field of view’ of the missile. Modern state of the art filters are able to detect multiple IR bands.

Step 2: Estimate Position Magnitude

Beyond the lens, the IR signal that is filtered must enter a sensor and system for guidance. This part requires a clever bit of classical analog electro-mechanics because in the 1950’s, solid-state electronics were not well developed. The guidance system involves an interaction between the sensor and a position sensing system. In order to direct the missile to the target, its spatial position with respect to the current location of the missile must be used to electronically adjust the wings towards the target. The device that accomplishes the primary navigation is the optical modulator rotating reticle. In order to explain the functionality of this I will refer to patent US3690594 A ‘Method and apparatus for the determination of coordinates’ by Joseph F. Menke.

Imagine a disk with evenly spaced opaque and transparent portions being spun on the axis perpendicular to its face. This axis, the normal to the disk face, points towards the target and sits between the target and the sensor.

The sensor would see the infrared source only during the transparent portions of the reticle and so the output of the sensor will be a periodic pulse train of some frequency that is equal to the rate at which the reticle spins times how many transparent spaces it has. Keeping this setup in mind, let us consider a more professional diagram from an original patent by William B McLean himself US3216674 A ‘Proportional navigation system for a spinning body in free space.’

Assume you, the reader, are the IR sensor facing toward the enemy plane with your view obstructed by a spinning reticle. In this image, we see a slightly different reticle that is semi-opaque. The alternative transparent pie-slice regions reveal a dot, which is considered the IR source. Fig 13 shows square pulses are generated when the IR source is exposed and the magnitude of the pulses (an induced current in this case) is proportional to the position away from the center of the disk. Fig 15 shows that the radially further out the IR source is, the larger the magnitude. In fig 14, we see the signal generated by an IR spot that is near center for a complete revolution of the reticle. Note that half-opaque portion generates a 0 value signal. In order to understand why this method is used, change your co-ordinate system to polar coordinates as such:

Reticle polar coordinate (left) and general polar coordinate (right) systems

Assume vector 2 represents the axis that faces the region toward the enemy plane drawn in red in my earlier diagram. We replace x,y,z vectors with r,ϕ,ϴ. Using the magnitude of the pulsed signal, we have determined r (see fig 2 for reference). In simple terms, this is the magnitude of distance between the missile’s direction of travel (vector 2) and the enemy plane position.

Step 3: Estimate Angular Position

If we use a reference signal based on the spin of the reticle, we can time the sensed IR pulses against the reference signal. Suppose the IR signal was offset by some angle ϕ away from the 0 degree reference, then logically, the pulse train must be phase shifted from the reference by the same amount.

Note that from the above fig 5 and fig 6 we see points a, b and c produce not only different amplitudes (H1, H2, H3) due to the distance from center (14) but also a different frequency (phase ϕ1, ϕ2, ϕ3) based on the angular distance from the horizon 0 degree (15).

Using electronic circuitry to recover the phase difference, we have now effectively obtained ϕ. Determination of ϴ is not required. The information is sent to the control systems that navigate the wings of missile as such:

1) ϕ controls the direction in which the missile should go i.e. what orientation the wings should be

2) r controls the magnitude of the deviation of the wings

Intermission: Some Q&A

Naturally the some questions arise and for the most part I can only guess as to what the answer would be:

1) How does the sensor work?

A: The sensor/detector functionality is not really the focus of this study but I’ll try to shed some light onto this. While there are several compounds which are chemically sensitive to IR wavelengths that can be used for the sensor, one of the oldest is PbS (Lead II Sulfide) photoconductor. Specific wavelengths of photons in the IR band are able to excite the electrons on the surface from their valence bands into the conduction band inducing a current flow or reduction in resistance of the material. Both effects can be used for amplitude estimation. PbS specifically is sensitive to radiation between around 1um to 2.5um which are classified short wavelength infrared (SWIR).

I found an example of a modern PbS photoconductor from a website called thorlabs.com. This particular one has a sensing area that is 3mm x 3 mm and and costs a whopping $182.00 USD. This allows us to put into perspective how a 1960’s military grade PbS photoconductor built using a less refined process with a much larger sensing area would probably cost well into the 4-5 figure region. The PbS sensor is unfortunately strongly affected by its temperature: PbS has a negative temperature coefficient so cooling the sensor is a must for sensing a wider band of signals and getting a more sensitive response. The cooler the sensor, the more sensitive it is and so the missile will use something like a Peltier effect cooler (AIM-9E), nitrogen (AIM-9D/G/H). The first variants of the AIM-9B were actually uncooled and this was a major contributor to its relatively poor performance in hit rate (around 20% — Vietnam War). Furthermore, the PbS sensors coupled with AM modulation of the older designs were not sensitive enough for “all aspect” operation; the weapon could only track targets in tail chase engagements when aiming for the enemy’s hot exhaust gases. Future iterations of the missile, were much more sensitive, used a combination of better sensor material (InSb — Indium Antimonide), FM modulation, solid state electronics and Argon cooling (AIM-9L/M/P/R). These missiles were then classified as “all-aspect” as they could be shot from any orientation with respect to the enemy and hit rates improved dramatically (around 80% — Falklands War).

2) How many transparaent sections are required?

A: The number of sections and the spin rate effect the frequency of the generated signal. A large frequency means you are sampling the enemy position more frequently and getting a better result. Higher frequency also means lower flicker noise. Any constant background IR radiation sources such as sunlights reflecting from the clouds or terrain will appear uniformly over all the transparent sections while the enemy jet will only appear in one particular area. At the sensor, the uniform slow changing or unchaning signal will appear at the low frequencies. As in all electronics some noise will appear at higher frequencies such as thermal noise. Somewhere in between the extremes, we find our jet planes sampled frequency. This frequency is then bandpass filtered to get accurate readings.

3) Why even use a reticle?

A: The thorlabs website recommended using an optical chopper with their sensors to improve signal noise problems with continuous wave light.

You should be able to recognize this device from the earlier section:

Such devices are commonly used in the laboratory setting when experiments involve photonic devices. More fundamentally though, the chopper is also converting the DC continuous wave light signal into an AC signal which we can compare to the reference AC. The entire system is quite simply an AC signal phase checker — this is why we MUST have a rotating reticle.

The shape of the output AC signal is directly dependent on the reticle shape. The most fundamental base case for a reticle would be a 2 slice one:

A reticle that is half transparent and half opaque can still provide phase lead & lag information. We could in theory just spin it faster to compensate for not having as many slices but then we lose the IR noise rejection from the full coverage alternating slice reticle (rising sun reticle). Fig 13, 14 and 15 shows a reticle I like to call ‘opaque blocked semi rising run’ which combines the simple half opaque half transparent and rising sun for phase sensing and noise rejection. Note that this reticle is an AM modulation reticle and would produce this output:

Test And Evaluation Of the Tactical Missile — E.J. Eichblatt retrieved from [8]

The full rising sun reticle and other fancy reticles are considered FM reticles and we will get into this idea in the next section.

4) If the target is at the dead center of the disk what values of r and ϕ will it give the control system?

A: An excellent question ! The IR hot spot that the sensor sees is not a point but rather a circle with some width. As this circle comes closer to the center of the disk, the phase angle becomes harder to determine with respect to the reference. It would be like trying to use a compass while standing on the North Pole… every direction is south! In patent US3690594 A, I found an alternative that I believe would be able to tackle this problem and I’ll explain it in the next section.

Step 4: Zero In On The Target

Now that we have gotten the fundamentals of the reticle-sensor relationship down, we can look at some boundary conditions and how to potentially deal with those problems. The primary limitations are being able to determine angle ϕ when the IR spot approaches the disk center, or when r increases beyond the circumference of the disk. If the missile loses the target from the circumference of the disk, the signal filtering system will see no target in its range and background IR noise will make up the bulk of the signal information. I would assume the guidance system would erratically change directions if it does something at all. Beyond the field of view provided by the lens, the missile has a major limitation that it cannot course correct once it loses target to reacquire it into field of view. Perhaps by chance it could rotate itself back into the FOV or maybe the enemy fighter pulls back into the FOV itself. However, suppose the missile was not mid flight. In fact, if it hasn’t launched yet, the parent aircraft can feed the control system a set of signals to create an initial launch vector based on its on-board radar and IR tracker information.

With regards to the ϕ ambiguity, a possible solution is found in US 3690594 A wherein the complexity of the spinning reticle system increases further by adding a second spin. This time the disk center of the reticle itself moves in a ring thereby eliminating a fixed central location. This raises the complexity of the electronics because an IR spot would appear to move from the reticle axis due to reticle rotation PLUS by the movement of the plane itself. If the spot remains still however, the resulting trace would look like a non-concentric circle. These diagrams should clear up any confusion about that wordy description.

In this figure, a fixed light source (16) apparatus is used in place of the enemy plane IR signature. The light source travels through a set of lenses (17) into the reticle (18) which spins on its axis (27) and the axis itself rotates in circles. An IR point marked as an X (4) would trace out a path (6) with respect to the reticle axis generating a signal with varying Phase and Amplitude. The exposure pattern would result in something similar to this:

See if you can recognize the reticle in the head of this British made ‘Red Top’ missle.

https://www.youtube.com/watch?v=LR-yfiG14gQ

Surface Based Air Defense Systems Analysis — H.M. Macfadzeqn retrieved from [8]

Further away from the center of the reticle the path is exposed for longer and the pulse width increases while closer to the center the pulses will shrink. Disregard the amplitude for this example as we are only focusing on the phase calculation aspect and the magnitude is irrelevant. Keen readers with electronics knowledge will notice that this signal appears to be a frequency modulated (FM) signal. This is why this reticle is classified as an FM reticle. While monolithic IC PLL designs were just emerging and while most currently mainstream designs such as the gilbert cell had not been invented yet, demodulation would likely have taken passive or vacuum tube designs. Demodulation of this signal could be accomplished by means of a frequency discriminator and phase comparator. In the below figure, the control means (54) in a electronic unit responsible for taking signal level and phase information to guide the rocket wings.

A frequency discriminator is also known as a slope detector and is comprised of a differentiator and envelope detector. Recall that the derivative of a signal is the instantaneous slope of that signal as a function. Then you will recognize that the slope detector would determine the underlying baseband signal required. Slope detection consists of a circuit that is tuned to a frequency that has a slight offset from the carrier. The figure below shows a simple slope detector circuit that could be used for this task. Here, C1, T1 and C2 form a tuned transformer, which is tuned to the carrier frequency (based on reticle spin rate discussed earlier) plus some offset and serves to modulate the amplitude portion of the signal. Diode D1, capacitor C3 and resistor R1 forms the classic envelope detection circuit, which obtains the outlined, envelop of the input signal.

FM reticle systems are no doubt complex, but the processing is doable. If you aren’t satisfied with this yet have a look at modern ‘Rosette Scanning.’ Before the shot is fired, the pilot receives a set of auditory feedback signals indicating the tracking status of the missile. When the seeker head is searching a low growling noise/static is heard… as the seeker independently acquires its target, the pitch increases…with a strong lock, sharp loud squeals tells you that you’ve ‘got tone’ and you’re ready to fire.

Accurate sound tone recreation in Falcon BMS 4.32 Simulator

From here on out the signals control wing actuators and the solid fuel rocket motor pushes the missile toward the target till close enough to trigger a IR or RF activated proximity fuse and we finish with a bang.

There’s so much about this missile that I urge you to look into… the history and the rest of the technical additions to the missile such as: