FM 24-18: Tactical Single-Channel Radio Communications Techniques
6-1. Importance to the Radio Operator
The radio operator has a vital position in every unit. Actions by the operator can either greatly aid or seriously hamper the accomplishment of the unit's missions. Any person, regardless of rank, who operates a radio must know what the consequences of his actions might be. If a radio is used properly, the unit will more easily accomplish its mission and its personnel will enjoy greater safety. If a radio is used improperly, the consequence may be the destruction of the unit.
Warfare today is not limited to just bombs and bullets. An important part of the commander's combat assets is the electronic warfare (EW) equipment that he can use to aid him and hinder the enemy.
6-2. Definition and Scope
Simply stated, EW is the military action involving the use of electromagnetic energy (radio frequency waves) to determine, exploit, reduce prevent hostile use of the electromagnetic spectrum (frequency bands) and actions which retain friendly use of the spectrum. EW includes three kinds of activities. The first, electronic warfare support measures (ESM), involves actions taken to search for, intercept, locate, record, and analyze radiated electromagnetic energy for the purpose of exploiting such radiation in support of military operations. Thus, ESM provides a source of information required to conduct the second EW action, electronic countermeasures (ECM). ECM involves actions taken to prevent or reduce effective use of the electromagnetic spectrum by the opposing force. The third part of EW, electronic counter-countermeasures (ECCM), involves actions taken to ensure effective use of the electromagnetic spectrum despite EW activity by the enemy. Figure 6-1 illustrates the relationships of ESM, ECM, and ECCM.
On implementation of Joint Interoperability of Tactical Command and Control System (JINTACCS), the JINTACCS format will be the standard operational report used to report MIJI incidents. If JINTACCS has not been implemented, or in cases where JINTACCS is not available, use the current format in this manual.
|NOTE: The JINTACCS format is in DA Pam 25-7, Annex 81 to chapter 3, pages 3-81-1 thru 3-81-13.
Figure 6-1. Electronic warfare functions.
6-3. Enemy EW Techniques Each radio operator must be aware of what the enemy will try to do. The enemy is well equipped to conduct EW and the different techniques he uses have specific purposes in his EW effort.
The first thing that the enemy must do is intercept our radio signal. All he needs to do this is a radio receiver that operates in the same mode and on the same frequency you are using to transmit. The mere fact that you are operating gives the enemy valuable information. It tells him that you are in the area. And, by the number of stations operating on the same frequency, he can estimate the size of the unit. If your net is operating in the clear, his language specialists can understand exactly what is said for even more information. When he analyzes the traffic pattern, he can figure out which station is the net control station (NCS) and identify the headquarters. Usually, in US forces, the NCS is the radio used by the operations officer/section of the highest headquarters operating in the net. By further traffic analysis, he can determine changes in the level of activity that could mean a movement or upcoming operation.
Radio Direction Finding.
Interception is only one of the many dangers that the radio operator will face. After the enemy knows that you are in the area, he will try to locate your position by using radio direction finding (RDF). A radio direction finder consists of a radio receiver, a directional antenna, and some other specialized equipment. With RDF equipment, the approximate azimuth (bearing) to a transmitting radio can be determined. One azimuth gives a general indication of direction. The intersection of two azimuths by different RDF stations is called a cut and gives a general indication of distance. The intersection of three or more bearings is called a fix and gives a general location. The ideal fix is the exact intersection of three or more bearings. However, exact intersection is seldom achieved. Terrain and weather conditions, together with variations in radio wave propagation characteristics, plus the inherent RDF equipment and operator inaccuracies, all tend to prevent an ideal fix. The fix that is obtained is called an actual fix. Although the actual fix may not be usable for immediate targeting purposes, it is more than enough for intelligence analysts to develop targeting data. Airborne direction finding is more accurate than ground-based direction finding but normally requires further analysis for targeting. RDF ability to intercept electronics equipment emissions and determine a bearing depends on the power output of the targeting transmitter and its antenna radiation patterns. Experience indicates RDF accuracy of 500-meter (547-yd) circular error probable (CEP) is considered a very good RDF fix. Normally, 50 percent of the CEPs are approximately 1,500 meters (1,640-yd) when the direction finder is located within 20 to 25 kilometers (12.4 to 15.5 mi) of the forward line of own troops (FLOT). Many Threat forces will fire on a 1,500-meter (1,640-yd) CEP if they have sufficient massed artillery, and further analysis of terrain and radio intercept can reduce the target area or identify an important target.
Threat forces employ a large number of RDF sets and communications intelligence (COMINT) analysts to exploit friendly use of the electromagnetic spectrum. The enemy's goal is to locate and destroy as many command, fire support, special weapons, and intelligence sites as possible during the first critical phase of the battle. He will continue to locate and destroy whenever possible; and when he locates sites that he cannot or does not want to destroy, these become his prime jamming targets. Jamming is an effective way to disrupt control of the battle. All it takes is a transmitter, tuned to your frequency, with the same type of modulation and with enough power output to override the signal at your receiver.
Jammers operate against receivers--not transmitters. There is equipment available which, when used with the AN/VRC-12 series radio sets, can counter the jamming threat of the enemy. See appendix E for more details.
There are many types of jamming signals that may be used against you. Some will be very difficult to detect and in some cases impossible. For this reason, an operator must always be alert to the probability of jamming and react accordingly when the radio has been silent for an inordinate amount of time. Others are quite clearly jamming signals. The more commonly used types are--
Random Noise Random Pulse
Stepped Tones Wobbler
Random Keyed Modulated CW Tone
Spark Recorded Sounds
Imitative Electronic Deception.
In addition to interception, RDF, and jamming, the enemy may use a compatible radio and a language expert to enter a friendly radio net. This is called imitative electronic deception (IED). The enemy IED experts are good and they are believable. If you permit them into your net, they will create confusion and destruction for your unit. They will have you shelling your own units, walking into ambushes, deploying to the wrong position, and making yourself a target.
6-4. Signal Security and ECCM
Now that you know what the enemy is going to try to do to you and how he plans to do it, you must also realize the effectiveness of his EW depends a lot on how effective we let it be. There is a saying that "You are your own worst enemy." In EW that is true because the less you do to protect yourself, the more the enemy can hurt you. This emphasizes the fact that a close relationship exists between ECCM and signal security (SIGSEC). Both of these defensive arts are based on the same principle. When the enemy does not have access to our essential elements of friendly information (EEFI), the less effective he becomes. The major impact of SIGSEC is to ensure that all friendly use of the electromagnetic spectrum is not exploitable by the enemy. ECCM are those actions taken to protect and ensure continued effective use of communications, surveillance, and acquisition devices from enemy ECM. SIGSEC techniques are designed to increase the security of our transmissions. ECCM techniques ensure you some degree of protection from enemy ECM and ESM measures. Both SIGSEC and ECCM should be preplanned and based on the enemy's ESM, ECM, and destructive capabilities.
SIGSEC techniques cover the areas of emission security (also called emission control or EMCON), transmission security, crypto security, and physical security. Each of these areas will protect you from one or more of the enemy's threats of interception, radio direction finding, jamming, imitative electronic deception, and destruction.
The first line of defense against enemy EW action, and the key to successful defense is control of electromagnetic emission. Radios and other emitters should be turned on only when required for the successful accomplishment of the mission. Enemy intercept analysts look for patterns which can be turned into usable information for the enemy commander. Making short transmissions, masking antenna locations, using directional antennas, and using the lowest possible power output are some of the actions that will protect you and your unit from enemy EW. Emission control can be total. All radios remain silent while the unit makes its tactical maneuver. It can be selective with some nets as directed nets and some nets as free nets. It can aid in deceiving the enemy, with some units on radio silence while others operate normally. EMCON should always be used and planned for. It is the first line of defense for a radio operator against interception and RDF. If the enemy does not know you are out there (because he can't pick up your radio transmissions), he cannot target you for destruction or for jamming.
The second line of defense is transmission security (TRANSEC). TRANSEC has to do with what and how information is transmitted. A message transmitted in the clear is the enemy's greatest source of information. After the enemy has intercepted your radio transmission, his language specialists will extract all possible intelligence from it. He hopes to learn EEFI. These critical items of information that must be protected can be remembered by the key words SELDOM UP. Each letter indicates a class of information as listed below.
Strength _____________Number of personnel, size of unit
Equipment ____________Type, quantity, condition
Logistics ____________Procedure for resupply, depots
Disposition __________Where, what positions, map coordinates
Organization _________How, what, chain of command, forces structure
Movement/Morale ______Where, how, when/good-bad
Units ________________Type, designation
Personalities ________Who, where
Using TRANSEC is absolutely essential for the radio operator. When the radio must be used, keep transmission time to an absolute minimum (20 seconds absolute maximum: 15 seconds maximum preferred); preplan your messages to avoid compromising any essential element of information; and, if you must send EEFI items, use brevity lists, if possible, and also encrypt the message. These measures decrease your transmission, help protect you from RDF, and deny the enemy valuable information. Included under transmission security are the authentication procedures that must be followed to protect you from the enemy's imitative electronic deception. (See FM 24-35 for instructions on authentication procedures.) Every radio operator must be aware of the dangers of and guard against IED.
Crypto security deals with codes, key lists, and communications security devices. This is the third line of defense for the radio operator. If you use a security device on your radio, the enemy will not get anything for his language specialists to work on. However, do not get a false sense of security. The need for emission control and transmission security still exists--probably more so; because, if the enemy can't get information, he might attempt to destroy or jam your station. Also, it is very important for all radio operators to use only authorized codes and to realize that using homemade codes is dangerous. Homemade codes offer no protection at all. Their use is not authorized and is a serious violation of security. This also includes trying to "talk around" a classified or sensitive piece of information. The enemy intelligence personnel are not fools, and trying something like "talking around" critical information does more harm than good. If critical information must be transmitted, it should be encrypted or sent by secure means. In a special situation where it is not possible to send by a secure means or to encrypt a message that must be sent, the possibility of what we will lose against what the enemy could gain must be weighed. Other factors, such as how fast the enemy could react to the information and what delaying the message for encryption could mean must also be considered.
Physical security is the fourth line of defense for the radio operator. Physical security means using common sense measures to protect your radio and related material, such as CEOIs and key lists, from unauthorized use and abuse. A radio is an important item of equipment; only well-trained and fully briefed personnel should use it.
ECCM techniques may be divided into two categories: preventive and remedial. Preventive ECCM are those procedures that can be used to avoid enemy ECM attempts. Remedial ECCM apply to jamming only; there are no remedial measures once you have been intercepted, detected, or deceived. ECCM procedures are covered in FM 24-33.
6-5. Capture Effect and Jamming Techniques
An inherent characteristic in FM communications is that a given station transmitting a signal will capture those receivers on the same frequency and in range for the receiver to detect the signal. This is the basis for netted communications for VHF FM radios. This FM capture effect is undesirable when receivers in a net are "captured" by a transmitter not in that net. This could be friendly interference or enemy interference. Friendly interference is usually unintentional whereas enemy interference is usually intentional.
We are mostly aware of obvious interference (jamming) by an enemy, such as stepped tones (bagpipes), random-keyed Morse Code, pulses, and recorded sounds. The purpose of this type of jamming is to block out reception of friendly transmitted signals and to cause a nuisance to the receiving operator. An operator usually can detect when the enemy is using this type of jamming against him.
This type of jamming is not obvious at all. With subtle jamming, no sound is heard from the receiver. The radio does not receive incoming friendly signals, yet everything seems normal to the operator. Subtle jamming takes advantage of design features of the AN/PRC-77 and AN/VRC-12 series radios. In order for an AN/PRC-77 to receive a signal in the "SQUELCH ON" mode (function switch in SQUELCH position) or an AN/VRC-12 series radio to receive a signal in the "NEW SQUELCH ON" mode, a 150-Hz tone must be on the received carrier signal. This 150-Hz tone is used to deactivate the squelch circuitry and allow the operator to hear the incoming message. In addition to this squelch feature, the AN/PRC-77 and AN/VRC-12 series radio receivers will lock onto the strongest carrier signal received and eliminate the reception of all other signals. For example, suppose you are operating an AN/PRC-77 in the "SQUELCH" mode or an AN/VRC-12 series radio in the "NEW SQUELCH ON" mode, and a strong, unmodulated (no 150-Hz tone) jamming signal appears on your frequency. If the jamming signal is stronger than any other signal on your frequency, the receiver will lock onto the jamming signal and will block out all others. Since there is no 150-hertz tone to deactivate the squelch, the receiver appears not to be receiving a signal. Neither is the call-light activated. In effect, the Threat jammers can block out the ability of these radios to receive a friendly transmission without the operator being aware that this is happening. This is called "SQUELCH CAPTURE" and is a subtle-jamming technique. Again, let us emphasize that with subtle jamming, everything appears normal to the operator. It should be noted that the operator should readily be able to detect the fact that he is being jammed in all other squelch positions.
6-6. Operator Actions
Radio operators must be able to determine whether or not their radios are being jammed. As was mentioned in the previous paragraph, this is not always an easy task. Threat jammers may employ obvious or subtle jamming techniques. These techniques may consist of powerful unmodulated or noise-modulated carrier signals transmitted to the operator's receiver. Unmodulated jamming signals are characterized by a lack of noise. Noise-modulated jamming signals are characterized by obvious interference noises. If radio operators suspect that their radios are the targets of Threat jamming, the following procedures will help them to make this determination.
The operator turns the function control from the SQUELCH to the ON position.
If no noise is present, this may indicate that the radio is being jammed by an unmodulated jamming signal. The operator should temporarily disconnect the antenna. If normal static noise returns with the antenna disconnected, there is a high probability that the radio is being jammed.
If a greater than normal level of noise or an obviously modulated signal is present, this may indicate that the radio is being jammed by a modulated jamming signal. The operator should temporarily disconnect the antenna. If normal static noise returns with the antenna disconnected, there is a high probability that the radio is being jammed.
If tests indicate the probability of jamming being present, the operator should follow local SOP to reestablish communications and also to initiate a MIJI report (see para 6-7) informing higher headquarters of the jamming.
AN/VRC-12 Series Radio
The operator turns the squelch control from the NEW SQUELCH ON to the NEW SQUELCH OFF position.
If no noise is present, this may indicate that the radio is being jammed by an unmodulated jamming signal. The operator should temporarily disconnect the antenna. If normal static noise returns there is a high probability that the radio is being jammed.
If a greater than normal level of noise or an obviously modulated signal is present, this may indicate that the radio is being jammed by a modulated jamming signal. The operator should temporarily disconnect the antenna. If normal static noise then returns there is a high probability that the radio is being jammed.
If tests indicate the probability of jamming being present, the operator should follow local SOP to reestablish communications and also to initiate a MIJI report (see para 6-7) informing higher headquarters of the jamming.
6-7. Meaconing, Intrusion, Jamming, and Interference (MIJI) Report
It must be reemphasized that, any time you suspect or know you are being jammed, or you know or suspect that the enemy is intruding on the net, you must report the incident immediately by secure means to higher headquarters. Such information is vital for the protection and defense of our radio communications.
Purpose of Field MIJI Reports.
Field MIJI reports serve two distinct purposes. First, initial MIJI reports facilitate battlefield evaluations of the enemy's actions or intentions and provide data for tactical countermeasures as appropriate. Second, complete and accurate follow-up reports ensure MIJI incidents are documented and evaluated on a national level, thus providing data for a continuing study of foreign electronic warfare capabilities and activities. To meet these diverse needs, field MIJI reports will be of two types.
- MIJI 1--An abbreviated initial report containing only those items of information necessary to inform headquarters of the incident and enable them to initiate evaluatory/retaliatory actions as appropriate.
- MIJI 2--A complete follow-up report containing all details of the incident which will be forwarded to the Joint Electronic Warfare Center (JEWC), San Antonio, Texas 78243 (message address is JEWC SAN ANTONIO TX//OPM//).
Transmission of MIJI Reports.
MIJI reports may be transmitted over nonsecure electronic means when secure communications are not available, however, the textual content of the MIJI report will be secured by an off-line (manual) system. Reports will be prepared in the format outlined below. Brevity numbers pertinent to specific line item information are provided for some items. These brevity numbers must be encoded in the numeral cipher/authentication system prior to transmission.
The MIJI 1 Report.
This report is forwarded through the chain of command to the unit operations center by the operator who is experiencing the MIJI incident. A separate report is submitted for each MIJI incident.
Item 1 - Type report. When being transmitted over nonsecure communications means, the numerals 022 are encrypted as Item 1 of the MIJI 1 report. When being transmitted over secure communications means, the term MIJI 1 is used as Item 1 of the MIJI 1 report.
Item 2 - Type MIJI incident. When being transmitted over nonsecure communications means, the appropriate numeral preceding one of the items below is encrypted as Item 2 of the MIJI 1 report. When being transmitted over secure communications means, the appropriate term below is used as Item 2 of the MIJI 1 report.
Item 3 - Type of equipment affected. When being transmitted over nonsecure communications means, the appropriate numeral preceding one of the terms below is encrypted as Item 3 of the MIJI 1 report. When being transmitted over secure communications means, the appropriate term below is used as Item 3 of the MIJI 1 report.
3 Navigational aid
Item 4 - Frequency or channel affected. When being transmitted over nonsecure communications means, the frequency or channel affected by the MIJI incident is encrypted as Item 4 of the MIJI 1 report. When being transmitted over secure communications means, the frequency or channel affected by the MIJI incident is Item 4 of the MIJI 1 report.
Item 5 - Victim designation and call sign of affected station operator. The complete call sign of the affected station operator is Item 5 of the MIJI 1 report over both secure and nonsecure communications means.
Item 6 - Coordinates of the affected station. When being transmitted over nonsecure communications means, the complete grid coordinates of the affected station are encrypted as Item 6 of the MIJI 1 report. When being transmitted over secure communications means, the complete grid coordinates of the affected station are Item 6 of the MIJI 1 report.
The MIJI 2 Report.
This is a complete report containing all details of the MIJI incident. Due to the number of items which require encryption when the report is transmitted over a nonsecure circuit, it is recommended that the report be delivered by messenger whenever possible. Either the operations officer, intelligence officer, or the electronic warfare officer is responsible for ensuring that a complete message report of the incident is submitted to the Joint Electronic Warfare Center (JEWC) within 24 hours of the incident.
Refer to FM 24-33 for a more detailed treatment of MIJI 1 reports and to AR 105-3 for details on MIJI 2 reports.
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