The signal environment.
To repeat the often repeated generality, the signal environment is extremely dense and its density is increasing. Like most generalities, this one is usually true but does not tell the whole story. The signal environment in which an EW or reconnaissance system must do its job is a function of the location of the system, its altitude, its sensitivity and the specific frequency range it covers. Further, the impact of the environment is strongly affected by the nature of the signals the receiver must find and what information it must extract from those signals to identify the signals of interest.
The Signal Environment
The signal environment is defined as all of the signals that reach the antenna of a receiver within the frequency range covered by that receiver. The environment includes not only the threat signals which the receiver intends to receive, but also signals generated by friendly forces and those generated by neutral forces and non-combatants. There may be more friendly and neutral signals in the environment than threat signals, but the receiving system must deal with all of the signals reaching its antenna in order to eliminate the signals which are not of interest and identify threats.
Signals of Interest
The types of signals received by EW and reconnaisance systems are generally classified as pulsed or continuous wave (CW) signals. In this case, "CW signals" include all of the signals which have continuous waveforms (unmodulated RF carrier, Amplitude Modulation, Frequency Modulation, etc.). Pulse Doppler radar signals are pulsed but have such a high duty cycle that they must sometimes be handle.d like CW signals in the search process. In order to search for any of these signals, the receiver must, of course, have adequate bandwidth to receive enough of the signal to observe whatever parameters must be measured. For some kinds of signals, the bandwidth required to detect signal presence is significantly less than that required to recover signal modulation.
Altitude and Sensitivity
As shown in Figure 1, the number of signals that a receiver must consider increases directly with altitude and sensitivity.
For signals at VHF and higher frequencies, which can be considered to be restricted to line of sight transmission, only those signals above the radio horizon will be in the signal environment. The radio horizon is the Earth surface distance from the receiver to the most distant transmitter for which line of site radio propagation can occur. This is primarily a function of the curvature of the Earth, and is extended beyond the optical horizon (an average of about 15%) by atmospheric refraction. The usual way to determine radio horizon is to solve the triangles shown in Figure 2. The radius of the Earth in this diagram is 1.33 times the true Earth radius to account for the refraction factor (called the "4/3 Earth" factor). The line of sight distance between a transmitter and a receiver can be found from the formula:
D = 4.11 x [[-square root of [H.sub.T]] + [-square root of [H.sub.R]]]
D = the transmitter to receiver distance in kilometers
[H.sub.T] = the transmitter height in meters
[H.sub.R] = the receiver height in meters
Thus the radio horizon has a relative definition depending on the altitude of both the receiver and any transmitters present. All else being equal, you would expect the number of emitters seen by a receiver to be proportional to the Earth surface area which is within its radio horizon range - but of course the emitter density also depends on what is happening within that range.
For example, an antenna on the periscope of a submarine will receive only signals from the few transmitters expected to be located within a very few miles. While the submarine may see quite a few signals if it 'is operating close to a large surface task force or to a land area with much activity, the signal density will still be very low when compared to that seen by an aircraft flying at 50.000 feet. The high-flying aircraft can be expected to see hundreds of signals containing millions of pulses per second.
When the receiver is operating below 30 MHz, the signals have significant "beyond the horizon" propagation modes, so the signal density is not so directly a function of altitude. VHF and UHF signals can also be received beyond line of sight, but the received signal strength is a function of frequency and the Earth geometry over which they are transmitted. The higher the frequency and the greater the non-line-of-sight angle, the greater the attenuation. For all practical purposes, microwave signals can be considered to be limited to the radio horizon.
Another element determining signal density is the receiver sensitivity (plus any associated antenna gain). As discussed in detail in the July and September 1995 "EW 101" columns, received signal strength decreases in proportion to the square of the distance between the transmitter and the receiver. Receiver sensitivity is defined as the weakest signal from which a receiver can recover the required information and most EW receivers include some kind of thresholding mechanism so that signals below their sensitivity level need not be considered. Thus, receivers with low sensitivity and those using low gain antennas deal with far fewer signals than high-sensitivity receivers or those which benefit from high antenna gain. This simplifies the search problem by reducing the number of signals which must be considered by the system in identifying threat emitters.
Information Recovered from Signals
It is a reasonable generality that EW and reconnaissance receiver systems must recover all of the modulation parameters of received signals. For example, if the target signal is from a communications transmitter even though the system may not be designed to "listen to what the enemy is saying" it will still be necessary to determine the frequency, the exact type of modulation and some of the modulation characteristics to identify the type of transmitter (and thus the type of military asset with which it is associated). For radar signals, the receiver must normally recover the received signal's frequency, signal strength, pulse parameters and/or FM or digital modulation in order to identify the type of radar and its operating mode.
One significant difference between ESM and reconnaissance receiver systems is that the ESM system will usually recover only enough information about a received signal to allow it to be identified while the reconnaissance system will normally make a complete set of parametric measurements.
It should be noted that many ESM systems integrate emitter location with the search process, using a preliminary emitter location measurement as part of signal isolation and identification. The role of emitter location in threat identification will be discussed in a later column on processing. However, in the context of signal search it should be understood that a signal can be classified as friendly or neutral and thus removed from further search consideration based on the location of the emitter.
Next month we'll discuss search approaches which take advantage of wideband and direct frequency measurement receivers for various types of ESM and reconnaissance systems. For your comments and suggestions, Dave Adamy is at Internet:firstname.lastname@example.org.
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|Title Annotation:||electronic warfare|
|Publication:||Journal of Electronic Defense|
|Date:||Mar 1, 1998|
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