JED - December 2011 - (Page 64)

EW 101 Spectrum Warfare – Part 8 Digital Communication continued By Dave Adamy W 64 The Journal of Electronic Defense | December 2011 hen distributed (i.e., net-centric) military operations are implemented, the vulnerability of interconnecting links to jamming and the danger associated with hostile geolocation of transmitters are critical considerations. Both of these problems are reduced by implementing transmission security – which is different from message security. CLEAR TEXT DIGITAL MESSAGE ENCRYPTER PSEUDO-RANDOM BIT STREAM FROM MESSAGE BITS + PS BITS PSEUDO-RANDOM BIT STREAM AT MESSAGE SIGNAL RATE TRANSMISSION SECURITY VS. MESSAGE SECURITY Message security prevents an enemy from accessing the information carried in a signal by use of encryption. Highquality encryption requires that the signal be digital, and adds a pseudo-random bit stream to the signal bit stream as Figure 1: Message security is achieved by adding a pseudo-random bit stream to a digitized input message. shown in Figure 1. For clarity in this discussion, let’s call this the encrypting signal. The summed bit stream is itself pseudo-random and makes the message nonrecoverable. In commercial applications, it is often acceptable to use an encrypting Links with Maximum Transmission Security signal that repeats after as few as 64 to 256 bits. However, in secure military enControl Station Remote cryption, the encrypting signal may not Manned Platform Sensor or repeat for years. (The shorter the encryptManned Facility Jammer ing bit stream, the easier it is for an enemy Or Other High Value Asset to “crack the code.”) At the receiver, the Remote original encrypting bit stream is added to Sensor or Links with Less the received bit stream, which returns the Jammer Transmission signal to its original, non-encrypted form. Security On the other hand, transmission security involves spreading the spectrum of the transmitted signal in some pseudo-random Remote way that makes it very difficult for an enSensor or Jammer emy to detect the signal, jam the signal or locate the transmitter. The three ways to Figure 2: It is desirable to provide a higher level of transmission security on links from higher spread the signal are frequency hopping, value assets. chirp and direct-sequence spread spectrum. They are discussed (in the context of jamming) in the May to October 2009 “EW 101” columns. This month, we will HOP consider these techniques from a transmission security point PERIOD of view. Although there are other operational benefits, the principle benefit of transmission security is to prevent an enemy from locating the transmitter – and thus being able to fire on it or use a homing weapon against it. As shown in Figure 2, it is most important to provide transmission security for links from high-value assets to lower value assets. A frequency-hopped signal switches its full power to a difTIME ferent frequency every few milliseconds (for slow hoppers) or microseconds (for fast hoppers) as shown in Figure 3. This Figure 3: A frequency hopping signal moves its full transmit power to a makes it fairly easy to detect the presence of the signal, and new frequency many times during a message. HOPPING RANGE FREQUENCY

Table of Contents for the Digital Edition of JED - December 2011

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
2012 EW/SIGINT Resource Guide
EW Antenna Challenges
SEAD in Libya
EW 101
AOC News
AOC Industry and Institute/ University Members
Index of Advertisers
JED Quick Look

JED - December 2011