JED - December 2012 - (Page 48)

EW 101 Infrared Systems and Countermeasures – Part 1 IR Spectrum and Techniques By Dave Adamy he last time we talked about anything in the infrared (IR) spectrum was in mid-2001. A lot of water has gone under the bridge in the intervening years, and there have been significant developments – in weapons, sensors and countermeasures. In this series, we will talk about some principles and techniques and current developments. T THE ELECTROMAGNETIC SPECTRUM The purpose of Electronic Warfare is to deny an enemy the benefits of the use of the Electromagnetic Spectrum (EMS) while preserving those benefits for friendly forces. That means the whole EMS, from just above DC to just above daylight. That said, we have spent most of the last 20 years dealing with the radio frequency (RF) part of that spectrum in this column. We will remedy this shortfall during the next few months, and bring those long-ago infrared (IR) columns up to date. Figure 1 is a much expanded view of the EMS, with emphasis on the optical and IR range. Note that the horizontal scale is in both frequency and wavelength. The relationship between these two values is defined by the equation: λF = c where: λ = the wavelength in meters F = the frequency in Hz c = the speed of light (3 x 108 meters/second) In the RF portion of the spectrum, we normally use frequency for convenience; however the frequencies in the opti- cal and IR portion are inconveniently large, so we usually talk about these signals in terms of their wavelengths. There are three parts of the IR spectrum important to us in EW • Near IR (0.78 to 3 μmeter) • Mid IR (3 to 50 μmeter) • Far IR (50 to 1000 μmeter) There are other bands and other band-edge wavelengths defined in literature, but we will use these definitions in this series. Note that μmeters are also called microns. In general, the near IR signals are associated with high temperatures, Mid IR signals with lower temperatures and Far IR signals with much lower temperatures like those in which humans can survive. This will be explained and expanded later in this column during our black body theory discussion. IR PROPAGATION In the July 2007 “EW 101” column, we discussed “line of sight” attenuation for RF signals. In that discussion, it was stated that the formula comes from “optics.” Of course, we converted the units and the assumptions to make a convenient formula for RF applications. [Loss = 32 + 20log(F) + 20 log(d).] In the IR frequency range, we use the optics basics. Figure 2 shows the applicable geometry. The transmitter is located at the center of a unit sphere. The transmitting aperture is projected onto the surface of the sphere. The receiving aperture is projected back on the same unit sphere. The ratio of 48 The Journal of Electronic Defense | December 2012 Figure 1: The Electromagnetic Spectrum includes much more than the RF frequency range.

Table of Contents for the Digital Edition of JED - December 2012

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
2013 EW/SIGINT Resource Guide
Open Architectures for EW and SIGINT
EW 101
AOC News
Index of Advertisers
JED Quick Look

JED - December 2012