As technology advances, adversaries have increasingly sophisticated capabilities to jam transmissions over geosynchronous satellites. There are multiple deployment scenarios to consider when it comes to implementing anti-jam technology. The question then becomes, what constitutes the best way to deploy anti-jam technology for SATCOM?
To overcome jamming challenges as well as cost constraints, governments are relying more and more on commercial off the shelf (COTS) equipment to leverage commercially available technologies and adapt these technologies to mitigate the very real threat of intentional signal jamming. Fortunately, the basic technologies used for jamming mitigation are already employed in many COTS modems. The COTS technologies currently used are designed to enable Communications on the Move (COTM) and provide better transponder resource allocations.
Anti-jam radios and satellite modems rely on spread spectrum technology to overcome a jamming signal. Spread spectrum comes in two broad groups: Frequency hop spread spectrum and direct sequence spread spectrum (DSSS).
In frequency hop spread spectrum, the transmission frequency rapidly hops from one frequency to another using a pseudorandom code known to both the transmitter and the receiver. The speed at which the carriers must hop in order to appropriately mitigate jamming interference is determined by two variablessophistication of the adversary and physical distance of the transmitter from the jamming device. Frequency hop spread spectrum works especially well as an anti-jam technique for free-to-air transmissions by moving the carrier so quickly that an adversary cannot match the frequency agility.
The challenge for COTS products is the modems frequency agility. Although most COTS time division multiple access (TDMA) systems leverage channel hopping to evenly distribute traffic loads across multiple in-bound channels, the frequency agility of the systems is not designed to meet anti-jamming needs. In addition, the channel hopping of a TDMA system is based on traffic loads and is not designed to work with a pseudorandom code.
With DSSS, a pseudorandom binary code is at a chosen chip rate and combined with a binary data stream through digital logic. The output is then modulated and transmitted. The chip rate of the pseudorandom code is the equivalent of the spread factor of the signal. DSSS works by lowering the signal-to-noise ratio needed to de-modulate a carrier to a point where an adversarys jamming signal can be overcome.
Many commercially available TDMA systems incorporate a type of channel hopping to evenly distribute network resource over a large number of terminals. Although this channel hoping is not orthogonal, nor is its frequency agile enough to serve an anti-jam function, the base capability could potentially be modified to be part of the total solution.
Fortunately, in many COTS systems, DSSS has already been implemented, at least for satellite modems with COTM capabilities. In the case of DSSS for COTM terminals, the lower ratio of Energy per bit (Eb) to the spectral noise density (N0) characteristics of the waveform are used to overcome adjacent satellite interference, which can be a problem in systems with ultra-small COTM antennas.
An innovative approach to introduce into the options for anti-jam technology is to deploy a hybrid solution. This approach may be the most successful implementation for a COTS product-based anti-jam solution. A hybrid solution could potentially accommodate COTS equipment in which the embedded electronics do not allow for the full frequency agility required by a pure frequency hop spread spectrum solution. Likewise, a hybrid solution could enable acceptably high data throughputs over a modem which is limited in transmission symbol rate and where the direct sequence spread spectrum chip rate required to address the threat would be prohibitive.
Regardless of the jamming mitigation strategy employed, systems operating under jammed conditions experience a greater number of errors than systems operating on clean channels. As such, the coding interleavers depth would have to be increased, which will negatively impact voice quality because deep interleavers must take in and process a much larger block, therefore increasing the latency of communications and affecting call quality.
Regardless of the spread spectrum technology used to mitigate jamming signals, anti-jam waveforms require an enormous amount of bandwidth. The exact spreading factor required will depend primarily on the capabilities of the adversary and the jamming strategies they employ. Spreading factors required are based on an end-users threat assessment. These classified threat assessments vary based on the adversary faced in various theaters. The classified nature of the threat poses another challenge to a COTS satellite transmission equipment manufacturer. How do you design to meet a threat which cannot be fully described to engineering?
A hybrid solution can help. The advantage of a hybrid solution is its ability to better address multiple types of jamming threats. In broad terms, an adversary may employ a frequency agile, narrowband, high-power spectral density waveform or a wideband, low-power spectral density carrier. An orthogonal frequency hop spread spectrum system is best suited for the former, while a DSSS system is best for the latter.
An ideal anti-jam capable system would be able to be dynamically tuned to optimally meet either threat. This capability is especially relevant when you consider different jamming threats may be encountered in different theaters of operations. Dynamic tuning may also be the solution to the classified nature of the threat as the manufacturer would no longer need to understand the full nature of the threat.
Regardless of the mitigation method implemented for anti-jam, there are a number of challenges faced by COTS manufactures of satellite modems. Purpose built anti-jam modems are designed not with mass production or reduced cost of goods sold in mind but rather with cutting-edge electronics which enable anti-jam technology. However, the basis for an anti-jam mitigation system is currently resident in many COTS satellite modem offerings.
About the author
Contributing editor Karl Fuchs is vice president of technology for iDirect Government Technologies (iGT). He joined iGT in 2004 as the director of sales engineering, just as the satellite-based IP communications company was expanding its very small aperture satellite (VSAT) market presence into the federal government and international Internet Protocol (IP) networking world. He now works as the vice president of technology. With more than 20 years of experience in technology and with the federal government, Fuchs leads iGTs team of federal systems engineers and serves as chief architect for new product integration. Prior to joining iGT, Fuchs was director of systems engineering at Nortel Networks, where he oversaw the Verizon account team of systems engineers, leading the design of IP, frame relay, asynchronous transfer mode (ATM) and dense wavelength division multiplexing (DWDM) networks. Before joining Nortel, he designed IP and ATM networks for Sprint and the federal government.
Active in the satellite industry for more than 10 years, Fuchs has contributed editorial to numerous publications including Federal Computer Week, Institute for Defense and Government Advancement, COTS Journal, Military Information Technology, Via Satellite, MILSATCOM and Satellite Evolution Global. In addition, he has been a featured speaker at leading industry events including the DoD SATCOM User Workshop, ISCe, IBC, Pacific Telecommunications Council and Emergency Management Talks.