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Military

Chapter 2

FM Radio Operations

2-1. General

a. Single-channel FM voice radios are the primary communications means used in almost all tactical Army units below brigade level. FM radios give the tactical commander quick, reliable, and flexible communications needed to control the battle. The AN/VRC-12 series used by active and reserve units is the only family of FM radios currently used by the Army. With the fielding of SINCGARS, the Army may use two families of FM radios. The signal personnel of both AC and RC must prepare to operate tactical FM radio nets containing both families of radios.

b. FM radios must take some of the additional burden when shortages of multichannel equipment force reliance on other means of communications. FM radios add a great deal of flexibility to our communications system. This section addresses techniques useful in providing essential command and control communications in the face of equipment shortages.

2-2. Frequency Planning

a. Figure 2-1 shows a comparison of frequency ranges between the AN/VRC-12 series and the SINCGARS radio sets. In addition to the extended range of SINCGARS, channel-spacing problems must be anticipated when interfacing AN/VRC-12 and SINCGARS. The channel spacing for AN/VRC-12 is 50 kHz. The channel spacing for SINCGARS is 25 kHz. When interfacing, frequencies must end in 00 or 50.

b. Obviously, all radios for a particular net must be capable of operating on the same frequency. Net frequencies must be assigned with primary consideration given to the old radio's frequency tuning capabilities. This applies also to channel spacing. The AN/VRC-12 series has a channel every 50 kHz and the SINCGARS equipment every 25 kHz. Nets involving both series radios must consider these differences and plan for proper channel separation.

2-3. Planning Range

a. Older radios have less range than the newer radios, so the maximum planning range must not exceed that of the older radios. Antennas on vehicles are vertically polarized; therefore, polarization usually presents no problem. The distance problem may be eased by the careful placement of retransmission stations in the unit's area of operations. Retransmission is effective but must be carefully controlled and properly employed using electronic warfare considerations.

b. The range of the FM radio sets can be extended by the proper location and orientation of the antenna system in regard to the vehicle and terrain. Additional distance can be obtained using elevated ground plane antennas such as the RC-292 or OE-254. Field expedient antennas may also be used not only to increase range but also to provide more directivity while reducing interference and detection. The SINCGARS may use the RC-292 in the single-channel mode. The OE-254 may be used in single-channel and frequency-hopping modes. Field expedient antennas may be used with SINCGARS in single-channel mode.

c. The planning range can be further extended with retransmission operations.

(1) Retransmission, or retrans for short, offers the commander a valuable alternative when multichannel equipment is in short supply or absent. As with NRI, retransmission is often not used to maximum advantage because of lack of knowledge or lack of confidence in its effectiveness. A shortage of multichannel equipment requires better planning and use of all other communications assets; retransmission is no exception.

(2) A primary application of retransmission is the extension of a particular communications link such as an FM command net or a fire direction net. Another application might be a logistical link from brigade trains to the DISCOM area in the absence of multichannel. Traffic on this link would be for urgent requests for resupply of critical items only such as ammunition or POL, or for a contact team for the maintenance of critical items. Routine traffic should be sent by other means.

(3) Retransmission should also be used to support anticipated operations or planned moves of important elements. For example, if a brigade CP is moving to a certain location at 1600 hours, a retransmission station could provide a communications link back to the DTOC. Retransmission may also allow the brigade CP to locate in a position that provides better physical security while still maintaining its essential FM radio communications.

(4) Retransmission sites must be carefully chosen to maximize retransmission distance while at the same time minimizing enemy interception. Several alternate sites should be chosen for each retransmission facility to allow for periodic displacement.

d. HF radios, such as the AN/GRC-106 or the AN/PRC-104, are used for long-range communications.

2-4. FM Radio Security

a. The AN/PRC-25 is not capable of operations using secure equipment. The AN/PRC-77 and AN/VRC-12 series radios can be secured with VINSON. SINCGARS can also be secured with VINSON. Planners should attempt to exchange equipment if certain nets must be on-line secured, and leave other nets to use low-level encryption and authentication procedures. Regardless of which security system is used, all nets must use proper radio procedures.

b. Chapter 7 discusses COMSEC. It covers the use of proper procedures and encryption systems in voice and record communications. Part of our security effort must be directed at operations that prevent the enemy from locating our emitters or analyzing our traffic. This is essential to survival on the modern battlefield. Every operator and user of signal systems should read and practice the techniques described in FM 24-33 and ACP 125(D). All communicators should practice daily the basic ECCM techniques described below.

(1) Use the lowest power possible for the required communications when power settings are adjustable. This is especially important the closer the transmitter is to the FLOT.

(2) Reduce on-the-air communications time. Both the quantity and length of transmissions must be kept to a minimum to deny the enemy the opportunity to detect and exploit friendly communications. Minimal transmissions should be coupled with frequent moves for greater security against enemy direction finding efforts.

(3) Change call signs and frequencies and use the proper authentication and COMSEC practices as specified in the unit SOI.

(4) Train all radio operators to practice sound radiotelephone procedures and use them in all training and operations.

(5) Emplace antennas and all noncommunications emitters properly. Terrain masking is an invaluable technique for denying the enemy knowledge of your location and unit.

(6) Use digital communications terminals such as the AN/PSC-2 or TACFIRE digital message device when possible to take advantage of burst and error corrected transmission.

c. Many RC units are equipped with the AN/VRC-47 radio, enabling them (doctrinally) to operate in two separate radio nets at the same time. When the AN/VRC-47 is equipped with a KY-38 FM security device (or similar secure equipment), the RT-524 receiver-transmitter continues to function. But the R-442 auxiliary receiver will be inoperative since the KY-38 accommodates only the RT-524. The result is one AN/VRC-47 that was formerly engaged in a two-net function is now capable of only single-net operations. This shortfall must be considered in the planning. Similar problems occur when installing the KY-38 on the AN/VRC-44 and AN/VRC-48. If using the VINSON KY-57 security device, both the R-442 and RT-524 will operate in the secure mode.

2-5. Squelch Capabilities

a. The AN/VRC-12 series radios have the ON and OFF positions in both new squelch and old squelch. The old squelch was used with the AN/GRC-3 through -8 series radios which are no longer in the inventory. The AN/PRC-77, AN/PRC-25, and SINCGARS can be operated in new squelch or without squelch. The AN/VRC-12 series radios should be used in new squelch on or new squelch off mode only.

b. Squelch is particularly important when netting (interoperability) with allied forces whose tactical FM radios operate in the old squelch and have limited frequency range. Close coordination is required when netting with allied forces.

2-6. Net Radio Interface

a. NRI is a highly effective method for bridging the commander's two primary means of command and control: tactical radio and telephone networks. It is normally used only by commanders and key staff members, but in a "come-as-you-are" war more people may need to use NRI. This is because shortages of multi-channel equipment force the commander to find alternate communications routes, and NRI is one of the most flexible.

b. Commanders have not made full use of their NRI capabilities in the past because-

  • Some commanders and communications personnel do not know enough about NRI.
  • Some units lack technical expertise, resulting in lack of confidence by the commander in the NRI system.
  • Some commanders do not trust vital communications traffic to NRI systems because of lack of confidence in NRI.
  • NRI, at this point in time, is not capable of end-to-end encryption.

c. NRI extends communications distance because it connects the tactical radio into the division/corps wire system. Some NRI stations can also be used as retransmission stations, not simultaneously, but alternately in either mode. For example, the commander in his vehicle calls the NRI station to place a call back to the DTOC. The NRI operator attempts to call through, but the circuits are busy. As a result of thorough training and by knowing the SOI retransmission frequency, the operator switches over to the retransmission frequency and puts the call through to the NRI station serving the DTOC.

d. NRI stations, like other FM radio installations, must be moved periodically to various alternate sites, both to adequately serve the headquarters or elements it supports and to enhance security and survival. The move may be necessary to support a new tactical CP or to support fast-moving operations. These moves are supported by--

  • Planning acceptable communications sites.
  • Considering the mission and security requirements.
  • Planning and installing wire/cable to tie in to the division or corps wire/cable system.
  • Establishing a "jump" facility that moves into position to support operators before shutting down and moving to another facility.

Pre-positioning of wire/cable system terminations is absolutely necessary on the fast-moving battlefield.

e. NRI systems must connect to switchboards to have access to the telephone network. The switchboards can be manual or automatic boards and either can process NRI calls--but not without prior identification of the NRI circuits and adequate training of operators (both switchboard and radio). Switchboard operators must know telephone traffic diagrams well to react adequately and quickly. SOPs must identify those individuals who are authorized to use NRI circuits. All users of NRI systems must use low-level security procedures since NRI is not secure. NRI frequencies are found in the unit SOI. NRI procedures for both radio and telephone are found in the supplemental instructions in the unit SOI. All NRI users must use proper procedures when communicating.

f. The AN/GSA-7, AN/GRA-39, AN/GRA-6, and C-6709/G are associated items of NRI operation. More complete explanations are covered in FM 24-18.

(1) Radio set control AN/GSA-7 provides an interface between a radio set and a switchboard which can be located for planning purposes up to 16 km (10 miles) from the radio set. There are four methods of providing NRI (Figure 2-2), depending on the number of AN/GSA-7s in the system. The four methods are described in FM 24-18. These variations provide for both attended and unattended operation of the AN/GSA-7.

(2) Radio set control group AN/GRA-39 can provide remote control of receiver-transmitters up to approximately 3.2 km (2 miles). It can also provide NRI among SB-22, SB-86, SB-3082, and SB-3614 switchboards and receiver-transmitters such as the AN/PRC-25, AN/PRC-77, and AN/VRC-12 series radios. NRI operation with the AN/GRA-39 can be remoted up to 1.6 km (1 mile) between the switchboard and the receiver-transmitter. Specific procedures for remote operation and NRI operation are described in the operator's technical manuals for the radios and for the AN/GRA-39.

(3) Radio set control AN/GRA-6 can provide remote radio control of a radio set up to 3.2 km (2 miles). It can also be used to provide NRI between an SB-22 switchboard and the radio set.

(4) Radio set control C-6709/G provides the capability for manned integration between 4-wire tone signaling telephone communications systems and push-to-talk radio systems. The C-6709/G is compatible with both current and future wideband transmission requirements. The 300 Hz to 70 kHz baseband allows the unit to accommodate a wide variety of interfaces for data communications and other needs. Radio keying can be accomplished with manual control by the NRI operator, by DTMF procedures, and/or by automatic voice actuation. It contains an H-250 handset, an H-325 headset microphone, and connecting cables with a basic unit. The C-6709/G provides facilities for controlling transmitter/receiver circuits of a variety of tactical radios in a 4-wire switched system comprised of AN/TTC-38s, AN/TTC-39s, SB-3614(V), CNCEs, and radios with COMSEC, such as PARKHILL and VINSON.

2-7. FM Radio Operations Example

a. You are the S3 of the 52d Division (Mechanized) Signal Battalion. The DTAC is presently in the vicinity of the 1st Brigade. The G3 has notified you that the tactical CP is moving to the vicinity of the 3d Brigade and will be on site in four hours. No multichannel equipment is available to support the tactical CP or the 3d Brigade Headquarters. The commanding general's M577 has two FM radio sets, but only one RATT set is available for support at the new site.

b. How can you provide for minimum essential support for the tactical CP at the displacement location within four hours?

c. Obviously you cannot provide the tactical CP with the full doctrinal communications system at the displacement location. First, examine how the two FM sets could be employed. One set will be operating in the division command net. The other FM set must be used for an NRI circuit back to the extension node serving the 3d Brigade. In this situation, at least two priority circuits must be engineered from the extension node to the DTOC. These circuits would go to the G2/G3 and FSE in the DTOC. This configuration allows the CG to use his priority NRI to establish calls to the appropriate element in the DTOC. As a minimum-

  • Extension node NRI stations supporting the 3d Brigade must be reassigned and must be prepared to support the CG's high priority calls.
  • Dedicated circuits must be engineered from the extension node switchboard through the division communications system to the DTOC.
  • NRI and switchboard operators must be well briefed for this type of system.
  • DTOC personnel must be prepared to share the two priority lines from the extension node.

d. A retransmission station (if available) is needed to support the NRI system back to the extension node when actions to accomplish the above begin. Also, field wire lines must be installed (terrain and tactical situation permitting) to provide additional circuits to the new tactical CP location from the extension node. These wire circuits need a way that permits the NRI system to leave the air. This reduces the electronic signature of the tactical CP and improves its survivability. Also, the one RATT set at the tactical CP must be used for both command and operations traffic.

2-8. Technical Characteristics

FM operations are the backbone of combat communications. To plan communications networks effectively, the planner must know the technical characteristics of the radio equipment. Table 2-1 compares the important characteristics of the receiver-transmitter units that are the chief components of both the AN/VRC-16 series and the SINCGARS FM radios. Especially important are frequency, channel spacing, squelch, and secure equipment capability.

2-9. Typical Configurations

Current typical configurations of FM radio equipment are listed in Table 2-2.



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