January 1960 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
really on point regarding infrared guided missiles as reported in this 1960
issue of Radio−Electronics magazine, but the photo of a Sidewinder
missile on the wingtip of an F−104 Starfighter reminds me of back in the early
1980's when I visited the Smithsonian Air & Space Museum in Washington, D.C.,
and the wingtip of the F−104 hanging there was nearly close enough to touch. I
marveled over how incredibly thin it was for an airplane capable of flying at
Mach 2 (1,482 mph)*. Missiles or auxiliary fuel tanks (and sometimes
experimental instruments) could be attached to that diminutive, yet evidently
extremely strong wingtip.
AIM−9 Sidewinder missiles went into service
in 1956, so they were relatively new when this story was published. As many as
50 countries, including the U.S., still use them today. The current version is
AIM-9M(R). That's the same time (1955) that
Stratofortress bombers, also still in service, were deployed.
The Smithsonian website lists its top speed as 1,037 mph, while also claiming,
"...the first U.S. jet fighter in service to fly Mach 2, twice the speed of
Infrared Guides Missiles
The hot gasses of an enemy jet or rocket powered plane become the target for
infrared-guided missiles. They can even fly right zip into the plane's tailpipe
By James R. Spencer*
Recently, the US Air Force's F-104A Starfighter was armed with the Sidewinder,
an infrared guided missile, to form a deadly aircraft-firepower combination. The
155-pound Sidewinder, a 9-foot missile that has been in use for more than 2 years,
is guided to its target by infrared radiation emitted by the target. Thus the hot
exhaust gases of an enemy plane become the target for this weapon. A Sidewinder,
traveling at supersonic speeds, can actually zoom into the tailpipe of a jet aircraft
to destroy it.
Fig. 1 - Infrared radiation spectrum showing the three infrared regions.
Fig. 2 - Spectral response of a typical lead sulphide photocell.
Fig. 3 - Gold-doped, single-crystal germanium infrared detector with the miniature
cryostat that cools the detector to liquid nitrogen temperatures.
Fig. 5 - Spectral response of a gold-antimony doped germanium infrared detector.
Fig. 6 - The Sidewinder infrared-guided air-to-air missile. The detection equipment
is all in the nose, above the black band.
Although the actual guidance equipment aboard the Sidewinder is classified, many
general questions concerning infrared and the use of infrared in homing guidance
can he answered.
What is infrared?
Infrared is a form of electromagnetic radiation, similar to visible light and
radio waves. It is generated by thermal agitation and radiated by everything at
a temperature above absolute zero (-273°C). High temperatures create more thermal
agitation than low temperatures, so the infrared radiated by a hot object is more
intense than from a cool one.
The infrared portion of the electromagnetic spectrum ranges from 400,000,000
megacycles (the high limit where infrared borders on visible light) to approximately
1,000,000 mc (the low limit where infrared approaches the higher microwave frequencies).
It is easier to measure infrared in terms of wavelength than frequency, so a standard
unit of wavelength for infrared has been adopted. This unit is the micron - 1 micron
is 1/10,000 centimeter long. The infrared portion of the spectrum is divided into
three portions (see Fig. 1) - the near infrared from 0.75 to 1.5 microns; the middle
infrared from 1.5 to 10 microns, and the far infrared from 10 to 300 microns. The
horizontal scale in Fig. 1 is logarithmic.
Infrared radiation can travel astronomical distances. The warmth from sunlight
is infrared radiation that has traveled 93,000,000 miles. Only during the last few
miles, when the radiation penetrates the earth's atmosphere, does it suffer any
serious attenuation. Infrared is affected less by haze and light fog than visible
light, but heavy clouds or rain can have a significant adverse effect.
At high altitudes where air-to-air combat is likely, above 30,000 feet, infrared
radiation suffers little attenuation from the light, thin atmosphere.
Detecting infrared radiation
In the laboratory, thermal detectors can be used to pickup infrared radiation.
They sense the heating effect caused by absorbing the radiation. However, this method
can not be used for missile guidance.
Instead a photodetector forms the heart of the guidance system. In a photodetector,
the absorbed photon energy either creates a voltage (photovoltaic cell) or causes
a change in the conductivity of the detector (photoconductor). The photoconductive
process is usually used because of the detector's small size, ruggedness and excellent
infrared response characteristics. Lead sulphide photoconductors are one of the
most extensively used types.
Fig. 4 - Cross-section of the detector in Fig. 3.
In general, photodetectors have response characteristics that limit their usefulness
to the near infrared region and a small part of the middle infrared region. Fig. 2 shows the response curve of a typical lead sulphide cell as a function of wavelength.
Note that the response peaks at about 1 micron and begins to cut off quite sharply
at approximately 2.5 microns.
A recent development in photoconductors has extended their long-wave-length response.
The new photodetectors are the indium antimonide and n- and p-type gold-doped germanium
detectors. Fig. 3 shows one of them and the miniature cryostat (low-temperature
thermostat) used to hold it at the normal operating temperature. Fig. 4 is a cross-section
drawing of the detector's construction. The sensitive detecting elements are single
crystals of indium antimonide or gold-doped germanium. Fig. 5 is the spectral response
of an n-type gold-antimony-doped germanium detector. Compare this curve to the one
for the typical lead sulphide cell (Fig. 2).
An optical system in the nose of the missile gathers infrared radiation emitted
by the distant target and focuses this radiation upon the detector. Electrical signals
from the optical system are combined with the amplified detector signal to give
target direction information. These data are compared with the missile heading and
the result is an error signal which is fed to a servo control system. The end result
is that the heading of the missile is constantly corrected to keep it on a collision
course with the target. The equipment necessary for infrared guidance is relatively
simple and very compact. It is all contained within the missile - above the black
band in Fig. 6.
With most guidance systems, the weakness is that the missile gets less and less
accurate as it travels away from the guidance control source. But, if the source
of control information is moved closer to the target, accuracy improves. For the
ultimate in accuracy, the source of guidance control information should be at the
target - and that's where the guiding infrared radiation originates.
Whether an infrared-guided missile can be fooled or not is classified data. But
remember, both jet and rocket engines produce large quantities of heat. Removing
the source of this infrared radiation from either a jet or rocket powered target
also removes the target's source of propulsion.
* Technical Editor, Philco TechRep Division Bulletin.
Posted January 24, 2023