List of Abbreviations
Chapter 2. Literature Review
2.5 Electronic Warfare
2.5.4 Signal Detection and Interception
In order to exploit a communications signal, its presence first needs to be detected, then the emitter located through direction finding in order to optimise detection, and then it may be possible to demodulate the signal in an attempt to retrieve its contents; this process is shown in Figure 2.18.
The process of detecting the presence of signals to provide an image of the electromagnetic environment is SIGINT, and the interception of signals to access the contents is COMINT.
Figure 2.18: Process to Exploit Communications
The probability of intercept is defined as the electronic warfare system will detect the presence, and some parameters, of a particular signal from the time the signal first reaches the electronic warfare systems locations and before it is too late to complete the interception. Spread-spectrum communications are classified as LPI due to the fact that their characteristics make them very difficult to intercept (Adamy, 2009). Frequency hopping signals are only present at a specific frequency for a very short time period before they hop to another frequency; this makes interception
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difficult as the electronic warfare system will need to track the hops (Adamy, 2009; Poisel, 2004).
Direct-spread communications, such as those found in 3G mobile phone communications, often have the signal energy distributed over such a wide frequency bandwidth that the spread signal strength is below the background noise, making them difficult to detect and intercept (Adamy, 2009; Poisel, 2004). Adamy (2004) and Nicholson (1998) provide in depth discussion on the requirements and probability of detecting various signal types.
There are three basic search strategies when attempting to detect the presence of signals: a general search, where there is no prior knowledge of signal presence, and every possible frequency and direction needs to be considered without any priority. A directed search is possible when some characteristics are known, such as the frequency range, or possible direction from which the signal could be arriving; then any signals that fall within the search criteria are given priority over other signals. A sequentially qualified search measures specific parameters of possible signals in a sequential order; depending on each step the signal may be assigned a priority to determine the amount of effort that will be employed to further analyse the signal (Adamy, 2009).
The location of the emitters can then be located with some precision, depending on the technique used. Multiple direction finding stations could be used to triangulate the emitter location, more stations usually result in a greater degree of accuracy; the mobility of the direction finders and target also affects accuracy. Adamy (2009) provides more detail on location techniques and accuracy.
The interception of the signal attempts to demodulate the signal to access the data contained within;
however as the information is usually encrypted, the interception may not be practical unless the encryption can be broken while knowledge of the information carried by the signal is still of some worth to the interceptor (Adamy, 2009). As mentioned above, the characteristics of spread-spectrum communications hinder interception of the information; this depends on the length of the pseudo- random sequence used for the spreading, as longer codes are more secure (ibid.). Adamy (2009) discusses this in more detail.
2.5.4.1 Mathematical Calculations for Detection of Radio Communications
This section will describe how to calculate if a signal can be detected for a given sensitivity of an intercepting antenna, and the maximum range at which a transmitted signal can be detected. The equations presented are in a form taken from Adamy (2009); other sources such as Nicholson (1998) and Poisel (2004) provide more in-depth equations that will not be required for this
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dissertation. These equations are presented as they will be used in Chapter 7 to determine the maximum range that communications can be intercepted as to determine the effectiveness of an electronic warfare threat.
The first step is to calculate the signal strength at the receiving intercept antenna; the received signal power is related to the power that it was originally transmitted at, the gains of the transmitting and receiving antennas (in the direction of the intercept receiver), and the loss in power that the signal experiences during transmission. This can be calculated by:
2.4
Where PR is the received signal strength in dBm, PT is the transmitted power in dBm, GT is the gain of the transmitting antenna in dB, L is the propagation loss in dB, and GR is the gain of the receiving antenna in dB.
Two forms of propagation loss (L) will be considered, for line of sight and two-ray propagation.
Two-ray propagation occurs when the distance between the transmitting and receiving antennas is such that there are reflections from the ground in addition to the main signal. To determine which form of loss will occur, the Fresnel Zone needs to be calculated; if the distance between the antennas is greater than the Fresnel Zone, then two-ray propagation occurs. The Fresnel Zone is affected by antenna heights and the frequency of the signal. The formula for calculating the Fresnel Zone is:
2.5
Where FZ is the Fresnel Zone distance in kilometres, f is the transmitted frequency in MHz, hT is the height of the transmitting antenna in metres, and hR is the height of the receiving antenna in metres. The constant terms in Equations 2.5 to 2.7 are due to the fact that radio waves propagate spherically, In Equation 2.6 the speed of light (3 x 108 m/s) is also taken into account. The loss from line of sight propagation is a function of the distance between the two antennas and the signal frequency; this is calculated as:
2.6
where d is the distance between the two antennas in kilometres, and the other variables have been described before. The loss from two-ray propagation is a function of the distance, and the height of the antennas; this is calculated as:
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2.7
If the received power is greater than the intercepting antenna's sensitivity, S (in dBm), then the signal can be successfully intercepted. Therefore, to calculate the maximum range a signal can be intercepted for a given sensitivity, the received power, PR, should be replaced by the sensitivity, S, and then solve for the distance, d. The formula to calculate maximum distance at which the signal can be successfully intercepted for line of sight propagation is then:
2.8
The equivalent formula for two-ray propagation becomes:
2.9