Ideas on jamming digital communications

Analog

When we look at UCAV operations, we think of them mostly in isolation; that is the vehicles will be operating in some 'designed for' environment. That designed for environment may be pretty nasty: AAA, SAM's, manned aircraft adversaries, traditional jamming, complex tactical defense methods used by the opponent, etc. However, there is another growing issue, one that will have a profound effect on the communications demands placed on the operation of UCAV's and other similar types of quasi- autonomous air vehicles.

Traditionally, jamming has been the preferred way of preventing an adversary from using their radar or radio for either offensive or defensive purposes. In recent years such jamming has also been extended to services like GPS. In most of these activities, a jammer uses a high power transmitter that mimics the frequencies and modulation used by an opponent to confuse the opponents receivers into throwing away returns and signals due to their now corrupted nature. Additionally, jamming can be used to add spurious signals to a radar systems returns, making the receiving radar think there are more, or less, targets in an area. In some cases, particularly in depriving a user of radio comms, complete transmissions are recorded, altered and retransmitted, making the receiver unsure of the quality of their data.

I am going to label all these as Analog Jamming. Generalized, they all revolve around the manipulation of analog waveforms.

Digital

Digital radio communications have been with us for a decade or more, providing generally secure, high bandwidth transmission. They rely on the concept that the 1's and 0's of the transmissions are the only significant states a signal can take, and that by the use of error checking and correction algorithms, most errors that can occur are easily fixed. When an error cannot be fixed, probably due to the number of errors detected, either the data is tossed or it is requested again by the underlying protocols of the digital communications system. Encryption of the digital signals used by the military, and even some civilian systems, has strengthened the perceived security of these systems.

Jamming digital systems using traditional means is not easy. In fact it can be said to be extremely difficult. One subtlety of good jamming is not to be so obvious about the activity that the jammer openly discloses its position, as is the case with GPS jamming - retribution for such is a HARM missile, or a visit from ground forces dressed all in black.

Jamming a digital communications stream is going to need a slightly different approach, certainly in respect to traditional jamming approaches. One thing remains; jamming is an activity aimed at denying or restricting an opponents use of the high ground, in this case excellent communications.

Digital Comms

Digital signals and communications rely on the transmission of organized frames of data over fairly traditional radio waves. The actual transmission often takes the form of a signal piggy-backed to a conventional radio wave: frequency modulation. The signal is an analog representation of the digital signal. Some systems allow for techniques that were only common on advanced radar, namely frequency agility, a.k.a. frequency hopping. This is the means by which a signal is sent, not on one transmitted frequency, but in short bursts over a series of randomly selected frequencies. The frequency hopping method averts some forms of traditional jamming by making sure that some of a message gets through, the receiver knows they have some good data and can requery for damaged data.

A data frame consists of many parts, but generally there are sections that indicate the routing to be given to the message, information about the source of the message, information about the type of message, the message data itself, and data to help in the error checking and fixing (the checksums). Each part is critical to the overall message: a bad route and the receiver may not get anything, a bad source and the message can be thrown away, an invalid message type will not be processed, and if the data does not match its checksums, then it will be thrown away as damaged beyond repair. The data part of the message may be encrypted to reduce the ability of someone reading the message. However, encryption takes time, and heavy, or high levels of encryption take longer to do. If the message being transmitted is between two field commanders discussing a strategy or two bankers discussing a singificant funds transfer the encryption can be made high, so long as they don't mind the pauses, but if two computers are swapping targeting information or a stock trade, the encryption can be of a lower level due to the timeliness of the data being sent; data with a long useful life needs to be heavily encrypted, data with a short useful lifespan needs relatively low forms of encryption.

Jam It

By its nature digital communications offers many jam reducing features, but also exposes itself to two distinct forms of jamming: data bit corruption, and death-by-retry.

Data Bit Corruption

Because the data frames tend to be short, measured both in milliseconds and bit length, they are highly repetitive, though they don't contain the same data in each frame, many frames will be extremely similar. By setting up a low power jammer that mimics the frame length, frame content and checksum, a jammer could feed into the air a continuos series of messages that looked like an opponents transmissions, while each transmitted frame would contain only a mix of passable frame infrastructure data, random bits for data and a completely valid checksum. This data would be transmitted at variously, a slightly lower power and a marginally higher power for minimal bit lengths, than the opponents transmissions, but synchronously with the opponents transmissions. The effect should be to transplant bits from the junk frames to the real frames. By the nature of the only infrequently higher output, the jammer can be made to be unidirectional, but multi-frequency, hence reducing its necessary overall output. Such a jammer should be harder to detect, and nasty to work around, it also leads to the second effect, that is common to both analog and digital jamming, Death-by-retry.

Death-by-retry (DBR)

When systems cannot get a message straight, they request the thing again. In a perfect world this would be an instantaneous event with no significance, but in a world in which bandwidth implies restrictions on what can be received and transmitted, the system slows down a little. Remember that both the transmitter and receiver can end up requesting repeats of messages, providing a perfect recipe for madness. Most protocols limit the number of retries any individual frame may be re-requested, after that the entire message is often thrown away and either retransmitted or asked for again. This retransmission is noticed as the network slowing down. With enough trashed messages flying about, the network grinds to a halt. It happens with hardwired systems, it will happen with radio based systems.

In a battlefield environment with many protagonists, a digital jammer may inflict severe performance degradation on any communications systems. If a system is a communications hog, such as the proposed UCAV's are likely to be, the performance degradation will likely lead to mission terminations and the spectre of repeat visits to environments now warned of their value.

Operations

Large fleets of UCAVs' are going to be nearly impossible to manage in any circumstances, I suspect operations will be limited to small numbers of aircraft, but even these smaller numbers in the presence of heavy jamming, both conventional and more elaborate communications denial attacks, will make operations extremely suspect. Denied the data throughput needed to confirm targeting, or retargeting, UCAV's will become expensive toys.

Digital jamming will have a pernicious effect on all forms of electronic over-the-air communications on tomorrow's battlefields, because communications will become the single most important element in battle planning and execution. While much of what I have driveled on about here is ELINT, a great deal falls under the more general heading of EW (Electronic Warfare) - one of the blackest of military activities.

Sites with Jamming or general EW information