-Admin Alpha Mariner

An Alpha Defense Exclusive on FTIR

The Instruments Research and Development Establishment (IRDE) of DRDO is working on the development of Passive FTIR sensor. Alpha Defense has learned that two such sensors are to be made by the winning bid. They are to be developed, integrated, supplied, installed and commissioned within a year from the time of contract signing. Passive FTIR sensors are used for detection of chemical clouds from a stand off distance. This would mean such sensors can find its place onboard warships, drones, warplanes and unmanned recon vehicles. Typical stand off distances range from 300 to 400 metres. This is pretty much the news aspect but let us look into at what a passive FTIR sensor is.

What is Passive FTIR sensor?

Passive FTIR sensor i.e Passive Fourier Transform Infra Red sensor is a sensor that employs the very common Fourier transform technique developed by the great French mathematician Joseph Fourier. Fourier transform decomposes any function (be it time, space or any variable) into frequency domain components which are multiple sinusoidal waveforms. A very well illustrated example can be found here. This sensor employs this transform algorithm to obtain the wavelength i.e the color and hence the chemical composition of the cloud. Since this sensor indirectly measures these aspects by transforming the time domain radiations ( time intervals etc) data into frequency domain (radiations per second or Hertz etc) and eventually into wavelength, it is termed as a passive sensor. (If it directly measured this wavelength, it would have been called active.)

Working of a Passive FTIR sensor

The following schematic shows the various modules of this sensor and the various components in it.

Schematic of a passive FTIR sensor. Individual images belong to respective owners, but the schematic is an Alpha Defense creation.

Collimator Module and Calibration Module

The passive FTIR sensor employs FTIR spectroscopy, wherein the terrestrial black body radiations of ambient atmosphere and gas clouds in the long wave infrared wavelength are collected. These radiations are collected using a collimating receiver module which includes a Cassegrain Telescope and an off-axis parabolic mirror combination. There is a calibration module that consists of a Helium Neon Laser and a blackbody infrared source. The radiations from both these components of calibration module serve two purposes. One is to ensure that the system functionalities are working fine and another is to generate a reference point.

Michelson Interferometer

The Michelson Interferometer can be said to be the heart of the system. As shown in the above schematic, the infrared radiations from the outisde source (gas clouds etc) and the reference radiations from the calibration module are passed into the Michelson Interferometer. In Michelson Interferometer as shown below, a half silver mirror is used which allows some radiation to pass and some to reflect. This mirror is typically placed at 45 degrees to the line of incoming radiation source. The outside infrared radiations and calibration radiation interfere with each other forming interference patterns which are obtained onto a screen.

This mirror can be moved linearly using a voiceless coil actuator which develops interference patterns of different wavenumbers (wavenumber = 1 / wavelength). These linear motions are measured in terms of scan rates which typically is between 10 to 20 and the typical resolution achieved is 1 wavenumber per cm. The entire Interferometer is mounted on vibration isolators in order to damp the effect of any external vibrations.

Detector Module

The Interference fringes are detected using a detector module that consists of Stirling cooled MCT detector and a Silicon detector. The optical data is now converted into electrical data and sent to the FTIR Master Controller and Processing Unit.

Controller Module

The controller module controls the motion of voice coil actuator, the thermal and regular cameras, the calibration sources and the display unit. The Geographic Information System (GIS) module provides the coordinates and the measurement direction of the sensor. The data from the Silicon detector and MCT detector is captured using 24 bit ADCs (Analog to Digital Converters). Data from Silicon dectector is processed to generate the reference syncronization signal.

Schematic of the Controller and Processing Unit. Individual images belong to respective owners, but the schematic is an Alpha Defense creation.

The Fourier Transform algorithm is applied on the MCT detector data with respect to the reference signal using a Linux based real time operating system in the processing unit. This processed data is then compared with a predefined chemical library. This information so obtained is then fused with local GPS information and superimposed on the camera images captured. A GUI software is used that enables real time signal acquisition. An Internet of Things (IoT) module ensures the connectivity between various systems. The obtained information is then relayed to where needed through the control room.

Abilities of passive FTIR sensor

Such passive FTIR sensors can detect chemical clouds upto range of a few hundred metres and can even detect minutest concentration of identified chemicals. They are usually able to obtain high resolutions and can provide the location mapping using a GIS overlay on the geographical map and the images captured using the thermal and regular cameras. Such sensors are usually designed to work in normal conditions with some tolerance of a few degrees on either side. They compare the data obtained with the predefined library and can provide automatic warning along with the display of detected species.


With detection time of a few seconds, and thanks to IoT enabled network connectivity, these sensors provide almost realtime data which facilitate quick decision making without any ambiguity.

FTIR sensors will help the forces in taking the necessary precautions when it comes to chemical risks during an ongoing mission and help facilitate in taking the right call. During peace time, these sensors come very handy for situation risk assessment in the event of accidents.


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By Alpha Defense

Alpha Defense initially a solo venture but now a defense group by people from various demographics of India covering defense news and updates. We believe in unbiased analysis of every subject in hand. Our mission is to provide simplfiied defense information to the public.

One thought on “Passive FTIR sensor : New sharp eyes on the sky”
  1. 300-400 meter detection range is very less…unless it hv 100+ km detection range, it hv no practical use against stealth fighters.🙏

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