During this post will try to go through various communications jamming terminologies and concepts, emphasizing the concurrent significance of signal strength and spatial proximity between transmitting and receiving entities.
In this post, we will focus our attention on “parity distance,” a critical parameter that signifies the point at which a transmitter’s signal strength equals that of a victim receiver concerning a jamming signal. When a transmitter is positioned closer to the receiver than this parity distance, it dominates the communication, rendering the jamming effort ineffective.
Introduction
In the realm of electromagnetic warfare, achieving operational success in communications jamming necessitates a holistic understanding of the interplay between signal strength and spatial separation. Here, we are going to explain the notion of “parity distance” as a pivotal yardstick for evaluating the equilibrium between signal intensities of a transmitting entity and a victim receiver, with regard to a jamming signal. When the transmitter’s proximity to the receiver falls within the range defined by the parity distance, the communication remains undisturbed, while surpassing this threshold tips the balance in favor of the jamming signal.
Electromagnetic Warfare Axioms
Jamming effectiveness rests upon five core electromagnetic warfare axioms that collectively frame its efficacy.
These axioms encompass the role of:
- The antenna heights
- The receiver sensitivity for the ability of the receiver to decode weak signals
- The transmitter power for the more the power the better
- The antenna performance (ie. Antenna Gain, and Directivity)
- The spatial link distance between transceivers
Electromagnetic Spectrum Three-Body Problem
A complex component of the discussion involves the electromagnetic spectrum three-body problem. This scenario introduces a jammer (Jx) and two transceivers (TRx) with diverse geometric orientations. The diagram below shows the possible two linear formations and the three triangular formations, each with distinct implications for jamming and communication.
Receiver Sensitivity in Reactive Jamming Techniques
To comprehensively analyze the concept of receiver sensitivity, we need to highlight the importance of this attribute in reactive jamming techniques.
Such techniques rely on the jamming platform’s ability to intercept victim signals, analyze them, and subsequently emit a jamming signal. This approach involves machine learning, enhancing the reactive jamming solution capabilities.
There are three states of communication link jamming, mirroring the traditional communication models of simplex, duplex, and half-duplex.
- Neither transceiver can receive a signal from the other
- One transceiver can receive a signal from the other, but not vice versa and
- Both transceivers can receive a signal from the other
The ideal scenario is the one during which no information flows between the transceivers.
Jam to Signal Ratio
An integral aspect of assessing jamming disruption is the Jam to Signal Ratio (J/S). This parameter, typically expressed in dB notation, quantifies the power of a jamming signal relative to the desired signal at a specific point, such as the antenna terminals of a receiver. To find J/S ratio we just need to subtract the transmit signal strength from the signal strength.
Communications Link and Link Budget
Mathematical characterization of communication links between elements is crucial. In order to do so, we can use the Two-Ray Path Loss formula, by providing specific parameters such as frequency band, antenna type, noise jamming, and bandwidth matching.
L = 120 + 40log10d – 20log10ht – 20log10hr
Where:
L = path loss in decibels (dB)
120 = constant used for distance in kilometers and height in meters
d = distance in kilometers (km)
ht = transmitter antenna height in meters (m)
hr = receiver antenna height in meters (m)
Parity Distance Estimation
As mentioned previously, one of the most important criteria for effective jamming is the “parity distance.” This distance represents the maximum spatial separation between transceivers, allowing the jamming signal strength to match that of the victim receiver. The process to estimate the parity distance is through signal strength knowing the Effecting Radiated Power of the transmitter, the antenna heights, and other pertinent parameters. Using simple algebra we can solve the following equation in terms of distance.
S = Pt – (120 + 40log10d – 20log10ht – 20log10hr)
Where:
S = signal strength in dBm
Pt = ERP of the transmitter in dBm
120 = constant used for distance in kilometers and height in meters
d = distance in kilometers (km)
ht = transmitter antenna height in meters (m)
hr = receiver antenna height in meters (m)
Example
We need to first calculate the Jam to Signal Ratio, knowing the Jamming Signal Strength as well as the involved distances and other transmit/receive characteristics.
In the next picture, we can see an example of how Parity Distance can affect the jamming effectiveness.
In this particular case, the TRx-1 WILL BE JAMMED just because the Jamming parity Radius is less than the distance between TRx-1 and TRx-2, and TRx-2 is outside the Duplex Communication Area.
In other words, if TRx-2 wants to maintain communication with TRx-1 in the presence of a jammer, TRx-2 needs to be within the 2.511km parity distance and at the same time 4km away from the Jammer.
Conclusion
In conclusion, this work tries to stress the varied nature of evaluating jamming effectiveness, emphasizing the simultaneous consideration of signal strength and spatial proximity. We tried to present a simplified but systematic approach to estimate the parity distance between transceivers, thereby enabling strategic control over communication maintenance during jamming operations.
Ref:
[1] Adamy, David L. Tactical Battlefield Communications Electronic Warfare. 1st ed., Norwood, MA, Artech House, 2009, pp. 129–134.
[2] “Two-Ray Ground-Reflection Model.” Wikipedia, 23 Jan. 2021, en.wikipedia.org/wiki/Two-ray_ground-reflection_model.
[3] “Jamming Efficacy of Communications Links”, Journal of Electromagnetic Dominance
