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FM 24-18: Tactical Single-Channel Radio Communications Techniques

CHAPTER 8
SPECIAL OPERATIONS AND INTEROPERABILITY TECHNIQUES

Section I.
Retransmission and Remote Control Operations

8-1. Retransmission Operation for Radio Sets AN/VRC-45 and AN/VRC-49

Radio retransmission is an arrangement of two radios connected together to provide automatic retransmission of signals between two other radios that are too far apart to communicate directly with each other.

Special considerations.

Radio Sets AN/VRC-45 and AN/VRC-49 are radio configurations provided with Radio Set Control C-2299/VRC or RETRANS-Control C-10374/VRC for secure/nonsecure retransmission.

Frequency selection for retransmission.

To identify the radios involved in the retransmission operation, the retransmission site radios are identified as retransmission radios 2 and 3, and the distant radios as terminal radios 1 and 4. Radios 1 and 2 operate on one frequency; radios 3 and 4 on another frequency.

Frequencies selected for the receiver-transmitter units at the retransmission site must be such that the transmitting power of one radio will not interfere with the signals being received on the frequency of the other receiver-transmitter unit.

The frequencies selected will be at least 10 MHz apart.

Refer to your SOI and to TM 11-5820-401-10-1 to aid in selecting operating frequencies that will not cause interference problems.

Retransmission operation (nonsecure).

After the retransmission site radios are set up for retransmission, communications proceed automatically on a push-to-talk basis between the terminal radios.

Set the C-2299/VRC RETRANS switch to OFF.

When audio accessories are connected to the C-2299/VRC, set the C-2299/VRC RAD TRANS switch to position 1 to communicate through the receiver-transmitter connected to J701, and to position 2 to communicate through the other receiver-transmitter.

When the C-2299/VRC is connected to the radio-intercom system, the audio accessories connected to the control boxes must be used for radio communications. Set the MONITOR switch (on the AM-1780) to position A to communicate through "A" radio with terminal radio number 1, and to position 2 to communicate through "C" radio with terminal radio number 4.

Turn on and tune each receiver-transmitter to its assigned frequency. Establish a satisfactory squelch mode operation with each terminal radio before proceeding to use the radios for retransmission. Squelch is discussed in paragraph 8-8.

Notify each terminal radio station to communicate directly with the other station; then set the C-2299/VRC RETRANS switch to ON. When the C-2299/VRC is used in the radio-intercom system, also set the AM-1780/VRC INSTALLATION SWITCH to RETRANS.

Communications between the terminal radios progress automatically. While communications are in progress, the CALL indicator of the receiving radio lights and the blower motor of the other radio should be heard operating.

When the audio accessory is connected to the C-2299/VRC, the retransmission can be monitored by setting it to position 1 during the time retransmission is in progress from terminal radio number 1 to terminal radio number 4, and to position 2 when retransmission is in progress in the opposite direction.

When the C-2299/VRC is in the radio-intercom system, monitor the retransmission using the ALL position of the control box MONITOR switches.

The setting of the VOLUME controls of the receiver-transmitter units has no effect on the signal between the retransmission site radios.

While the C-2299/VRC RETRANS switch is set to RETRANS and the AM-1780/VRC INSTALLATION switch to RETRANS, the retransmission site radios cannot be keyed for transmission.

When retransmission is completed, or to establish contact with the terminal radios, set the C-2299/VRC RETRANS switch to OFF. In the radio-intercom system, also set the AM-1780/VRC INSTALLATION switch to OTHER.

Retransmission operation (secure).

After the retransmission site radios are set up for retransmission, communications proceed automatically on a push-to-talk basis between the terminal radios.

Turn on and tune each receiver-transmitter to its assigned frequency. Set squelch to NEW ON.

Turn on and load (key) each TSEC/KY-57 with appropriate variables if secure communications are desired at the RETRANS site.

NOTE: In tracked vehicles, ensure AM-1780/VRC installation switch is set to OTHER.

Establish secure communications with distant terminals (Radio 1 to Radio 2 and Radio 3 to Radio 4).

Notify one terminal to communicate directly with the other; immediately set C-10374/VRC RETRANS switch to RETRANS.

Communications between the terminal radios progress automatically. While communications are in progress, the call indicator of the receiving radio lights and the blower motor of the other radio should be heard operating.

To establish contact with either terminal radio, set C-10374/VRC RETRANS switch to OFF and call using the appropriate radio set (2 to 1 or 3 to 4).

8-2. Remote Operation of the AN/VRC-12 Series Radios

Remote control of the AN/VRC-12 series radios can be provided by Radio Set Control Group AN/GRA-39. The AN/GRA-39 may be used to provide remote control of radio transmission and reception by the receiver transmitter units up to approximately 3.2 kilometers (2 miles). This equipment may also be used to provide net radio interface between the SB-22/PT switchboard and the receiver-transmitter units. Installation and operating instructions for the AN/GRA-39 are provided in TM 11-5820-477-12.

Connections.

Connect the AN/GRA-39 to the receiver-transmitter using Control Cable C-2329/GRA-39. Connect the field wires between the AN/GRA-39 control boxes.

Operation.

Tune the receiver-transmitter to the assigned frequency. Use the squelch or nonsquelch mode, as desired. Use push-to-talk operation to control the receiver-transmitter from the AN/GRA-39.

Refer to TM 11-5820-477-12 for procedures to use the AN/GRA-39 for remote control of the receiver-transmitter.

8-3. Remote Operation of Radio Teletypewriter Sets Equipped with Radio Set AN/GRC-106

The Control Group AN/GRA-6 provides a means for controlling and operating the AN/GRC-106 from a remote site. In addition, the AN/GRA-6 permits control of the AN/GRC-106 through a continuous circuit, and for two-way telephone communications and ringing between the remote and local control operators. The local control portion consists of Local Control Unit C-434/GRC and the remote portion consists of Remote Control Unit C-433/GRC. In addition to these basic components, the AN/GRA-6 includes Handset H-33/PT with a connector plug and a push-to-talk switch and a CW-189/GR carrying or storage container. Connections between the local and remote control positions are made by a pair of field wires connected to the DC entrance box. Both units are battery operated and are in individual weatherproof cases. All operating controls and indicators are on the front panels. The C-434/GRC is shelf-mounted at the cab end of the shelter and is semi-permanently wired into the shelter. The C-433/GRC and associated accessory items are stored in the shelter. Refer to TM 11-5820-489-10 for a more detailed description of the AN/GRA-6.

8-4. Remote Operation of Radio Set AN/GRC-26D

The Remote Switching Control C-1474/GRC controls the operation of the radio set from a remote site up to 16 kilometers (10 miles). The C-1474/GRC consists of binding posts and jacks in a metal housing. On the right panel are four sets of binding posts: ONE WAY, REMOTE TEL, REC DX, and LOCAL TEL. The XMTR ON switch is on the front panel, and six telephone jacks are on the left panel. The TTY SWITCH coaxial connector is below the telephone jacks. A bracket at the top of the unit is for mounting the unit.

The modified C-1474/GRC has three jacks and two connectors on the right panel. The jacks are labeled LOC TEL, REC DX, and ONE WAY OR SEND DX. The connectors are labeled REMOTE and TTY SWITCH. The XMTR ON switch is on the front panel. A bracket at the top of the unit is for mounting the unit.

8-5. Remote Operation of Radio Teletypewriter Set AN/GRC-46

The AN/GRC-46 set may be operated from a remote site by using Radio Set Control Group OA-1754/GRC and Radio Set Control AN/GRA-6. Maximum distance for remote control of the set is 3.2 kilometers (2 miles), and remote CW or FSK operation is possible.

CW mode.

Plug the RE-479/GRC (connector J101) into the T-195/GRC-19 AUDIO receptacle.

Remove FUSE 15 amp 24 volt fuse cap from the T-195/GRC-19. Substitute the modified fuse cap provided with the RE-479/GRC.

Insert connector P101 of the WS-16/U into the jack of the modified fuse cap.

Mount the SA-708/GRC at the remote site as convenient. Use the angle bracket as required.

Connect Telegraph Key KY-116/U to receptacle J1 on the SA-708/GRC with a cable assembly such as CX-1852/U.

Connect an appropriate two-conductor line (not exceeding 3.2 kilometers (2 miles)) between the LINE binding posts of the SA-708/GRC and the T-195/GRC-19.

At the local site, place the SERVICE SELECTOR switch of the T-195/GRC-19 at CW. At the remote site, set the FSK ON switch on the SA-708/GRC in the down (off) position. The set is now ready for operation.

FSK mode.

Remove the jumper wire connecting terminals E2 and E5 (TTY LINE) on the J-668/GR interconnecting box. Reconnect the jumper wire between terminals E3 and E4.

Insert one end of a suitable two-conductor line such as WD-1/TT (not to exceed 3.2 kilometers (2 miles)) into J-668/GR terminals E2 and E5. At the remote site, connect the other end of the line to a teletypewriter set. Use the binding posts prescribed for two-wire operation.

Connect one end of another two-conductor line (not to exceed 3.2 kilometers (2 miles)) to the J-668/GR terminals E4 and E5. At the remote site, connect the other end of the line to the LINE binding posts of the SA-708/GRC.

Install Radio Set Control AN/GRA-6 between the local and remote sites, as required. This provides a voice and FSK monitoring capability.

The RE-479/GRC is not used for remote FSK operation.

The SERVICE SELECTOR switch on the T-195/GRC-19 must be at VOICE/FSK. The SEND-RECEIVE-MARK HOLD switch on the teletypewriter shelf must be at RECEIVE. At the remote site, the FSK ON switch on the SA-708/GRC must be in the up (on) position during teletypewriter transmission and in the down (off) position for reception.

Section II.
Secure Operations

8-6. Securing the AN/GRC-142 and -122 Series with the KW-7.

For information on secure operations with equipment, see TM 5815-334-10.

8-7. Securing the AN/GRC-26D and the AN/GRC-46 With the KW-7 (Figs 8-1, 8-2, 8-3, 8-4)

Identifying dummy boxes.

Two dummy boxes are provided in the AN/GRC-26D. The upper dummy box (J-2597/GRC) is for the OWR DX-SEND TTY Loop and the lower one (J-2598/GRC) is for the DX-RECEIVE PONY Loop.

NOTE: Only one dummy box (J-2498/GRC) is provided in the AN/GRC-46. It is used for OW DX-SEND TTY operations.

Removing dummy boxes (J-2597/GRC) or (J-2498/GRC).

Disconnect W7, CX-9998/U cable from E1, E3 and E5 terminals.

Remove jumper wire from E2 and E4 terminals.

Remove cables from J4, J7, J3 and J8.

Loosen the captive screws on both sides of the dummy box and remove the dummy box.

Installing the TSEC/KW-7 (for OWR DX-SEND OPERATION).

Position the TSEC/KW-7 in mount provided and perform the following actions on the equipment rear panel.

Connect jumper wire between E2 and E4 terminals.

Connect W7, CX-9998/U cable to terminals E1 (black wire), E3 (green wire), and E5 (white wire); connect the other end of the W7 wire to the J11 terminal on the Control Radio Set C-1123/GRC (AN/GRC-26D only).

Connect W7, CX-9998/U cable to terminals E1 (black wire), E3 (green wire), and E5 (white wire); connect the other end of the W7 wire to the SYNC input terminal on the Interconnecting Box J-2491/GR (AN/GRC-46 only).

Connect W5 wire to the J4 connector (LOOP IN-2).

Connect W4 wire to the J7 connector (LOOP OUT-1).

Connect W6 wire to the J3 connector (LOOP IN-1).

Connect W3 wire to the J8 connector (LOOP OUT-2).

Connect W2 AC POWER Cable (part of KW-7) to the J1 AC power connector, and the opposite end to the AC wall socket.

Adjust the BIAS ADJUST knob on the Control Radio Set C-1123/GRC for a reading of 60 MA prior to typing on the teletypewriter set (AN/GRC-26D only).

Adjust the BIAS and LINE ADJUST knobs on the Interconnecting Box J-668/GR for a reading of 60 MA prior to typing on the teletypewriter set (AN/GRC-46 only).

Removing dummy box (J-2598/GRC, AN/GRC-26D only).

Disconnect W7a, CX-9998/U cable from E1, E2 and E3 terminals.

Remove W4 wire from the J8 connector.

Loosen the captive screws on both sides of the dummy box and remove dummy box.

Installing the TSEC/KW-7 (for DX-RECEIVE OPERATION ONLY).

Position the TSEC/KW-7 in the mount provided and perform the following actions on the equipment rear panel.

Connect W7a, CX-9998/U cable to terminals E1 (black wire), E2 (white wire), and E3 (green wire); connect the other end of the W7a wire to the J16 terminal on the Control, Radio Set C-1123/GRC.

Connect W4 wire from the DX Receive TT98 to the J8 connector (LOOP OUT-2).

Connect shorting plugs (dummy plugs) to the J7, J3 and J4 connectors.

Connect W2 AC power cable (part of KW-7) to the J1 AC power connector, and the opposite end to the AC wall socket.

Figure 8-1. Wiring diagram for nonsecure operations, AN/GRC-26D.

Figure 8-2. Wiring diagram for secure operations, AN/GRC-26D.

Figure 8-3. Wiring diagram for nonsecure operations, AN/GRC-46.

Figure 8-4. Wiring diagram for secure operations, AN/GRC-46.

Section III.
Equipment Compatibility and Netting Procedures

8-8. FM Equipment Compatibility

Single-channel FM voice radio is the primary communications means used in practically all Army units. FM radio gives the tactical commander the quick, reliable, flexible communications support necessary to control the battle. The main family of FM radios used by the Army today includes the AN/VRC-12 series and the AN/PRC-77. With the fielding of the single-channel ground and airborne radio subsystem (SINCGARS), the Army will be using two families of FM radios. Also making its debut into the inventory is the extended frequency range, Multimode Field Radio Set AN/PRC-70. It is capable of operating FM, AM, or SSB over a frequency range of 2 to 76 MHz. It is further described in Appendix C. The communications personnel of both Active Army and Reserve Components must be prepared to plan for and operate tactical FM radio nets containing all three families of radios. Today's Army finds Active and Reserve Components training together and operating together. The old and new generations of FM radios do interoperate, and all it takes is proper planning and training to make the nets work. The following paragraphs give some planning considerations for operating nets with both families of radios.

Frequency planning.

Figure 8-5 shows a comparison between AN/VRC-12 series and SINCGARS-V radio sets. In addition to the extended range of SINCGARS-V, channel spacing problems must be anticipated when interfacing AN/VRC-12 and SINCGARS-V. The channel spacing for AN/VRC-12 is 50 kilohertz. The channel spacing for SINCGARS-V is 25 kilohertz. When interfacing is necessary, frequencies must end in 00 or 50.

Squelch.

The AN/VRC-12 series radios have the squelch capability of old squelch, new squelch or no squelch. The old squelch was for use with the AN/GRC-3 thru 8 series radios. The AN/PRC-77, AN/PRC-25 and SINCGARS-V radios have the capability of new squelch or no squelch. The AN/VRC-12 series radios should be used in new squelch or no squelch only.

Security.

The AN/PRC-25 has no capability for operations using secure equipment. The AN/PRC-77 and AN/VRC-12 series radios can be secured with Vinson or Nestor devices. SINCGARS-V can be secured with Vinson. Vinson and Nestor devices are not compatible, therefore, no interface is possible. 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 type of security system is used, all nets MUST USE proper radio procedures.

Figure 8-5. Frequency comparison chart.

Compatibility chart.

The following chart (table 8-1) compares the important characteristics of the receiver-transmitter units which are the chief components of both old and new series of FM radios. Of particular importance are frequency, channel spacing, squelch, and secure equipment capability.

Table 8-1. FM compatibility chart

8-9. AM Radio Operations

Equipment shortages or differences may cause serious problems for you. These problems can cause you to have to use one or two radio sets to pass all the traffic normally passed by a half dozen or more sets. It will help for you to remember that AM radios can pass traffic in either of three modes: voice, RATT, or CW. The following subparagraphs will help you to solve the problems just mentioned.

Design considerations for reduced assets

Alternate the radio between nets. Select the most important net to monitor and operate in. Enter the other nets only when necessary to pass traffic. Enter the other nets at preplanned times or notify the other stations by telephone or FM radio at unscheduled times.

Operate one AM radio in several nets using an established time schedule. The schedule should change every day and be randomly generated to preclude the enemy from analyzing your traffic pattern.

Preplan all messages by using brevity lists and codes as much as possible to shorten the amount of time actually spent on the air.

Use off-line teletypewriters of the telecommunications center to prepare teletypewriter tapes prior to submitting traffic to the RATT operator for transmission. This reduces the burden on the RATT operator and saves transmission time.

Use full-duplex (FDX) operation on equipment which has FDX capability. Much more traffic can be passed over FDX circuits than over half-duplex circuits, thereby reducing time required for passing traffic.

Transmit low priority traffic over alternate means, such as messenger or multichannel radio, if they are available.

Use one radio, if possible, for several individuals, staff sections, or units.

Establish a wire link with a distant station using the existing teletype-writer and secure device along with a Telegraph Terminal TH-5/TG or TH-22/TG when the radio or modem of a RATT system is defective. Speech-plus can also be provided using this technique by using Telegraph-Telephone Terminal AN/TCC-14 or AN/TCC-29.

Equipment considerations.

When different types of AM radios must work together in the same net, here are some suggestions which should help ensure communications.

SOI must include proper frequency assignments compatible to each type of radio equipment. All nets using two or more different AM radios are restricted to certain frequency ranges or modes of operation. (The various technical characteristics of all the AM radios currently in the Army inventory will be covered later in this chapter.) Frequency and mode assignments must be coordinated prior to joint operations when units with different AM radios may be involved together.

Radio planning ranges must be considered when planning nets. Certain AM radios have much more power than others, so planning ranges must be based on the least powerful radio's capabilities. Related to the distance factor is the type and polarization of antennas. Antennas must be properly polarized and correctly oriented. For extended ranges, a half-wave doublet antenna, such as the AN/GRA-50, should be used whenever time and terrain permit.

NOTE 1: When planning RATT operations with units with whom you do not normally operate, it is important that compatible speeds be selected. This is especially critical for units equipped with older generation teletypewriter sets, such as the TT-4, TT-98 and TT-76 teletypewriter-reperforator. These sets operate at speeds of 60, 66, 75, or 100 words per minute (WPM) depending on the worm gear installed in the set. The worm gear must be changed to be compatible with units operating at unlike speeds. Units who have mission support to North Atlantic Treaty Organization (NATO) and United States Army, Europe (USAREUR) units must ensure that 66 WPM gears are on hand and installed prior to deployment into these theaters since 66 WPM is the NATO/USAREUR standard.

FM 24-18 NOTE 2: When operating the AN/GRC-106, AN/GRC-122/142, and AN/VSC-2/3 with a whip antenna, the RF power output can be degraded on frequencies below 12 MHz. Additionally, prolonged keying of the radio set could cause damage to the amplifier. This is caused by a high SWR existing on low frequencies when the whip antenna is used. When operationally feasible, a doublet or other matched antenna should be used on low frequencies.

Various civilian radios with AM, CW, and SSB capabilities can be used provided prior approval is obtained. Under no circumstances will citizen's band procedures be used. When using civilian radio equipment, proper military procedures will be used.

Security considerations.

Some of the Army AM radios may be secured in the voice mode by using the KY-65. Most AM radios can be secured for RATT operation by using the TSEC/KW-7 or TSEC/KG-84 security devices. Some of the older radios have not been modified to accept the KW-7 at this time but can be altered to accept it as required. If time and situation permit, ensure that all your radios have been modified to accept security devices. When operating a nonsecured radio in voice or CW modes, it is essential to use codes or off-line encryption methods.

8-10. Technical Considerations

Old and new equipment.

Most active components use the newer families of SSB radios, whereas the Reserve Components have a combination of the older AM and the newer SSB equipment. The new equipment can work with the older equipment, but it takes just a little extra care to make it work correctly. The planner needs to know the technical characteristics of all the radio sets involved in order to plan the communications network properly. Table 8-2 is a comparison of technical characteristics of the AM radios in the Army inventory. (See the following Tables 8-3, 8-4, and 8-5.)

FM 24-18 Table 8-2. AM radio technical characteristics.

FM 24-18 Table 8-3. Technical capabilities of the AN/PRC-104

FM 24-18 Table 8-4. Technical capabilities of the AN/GRC-193.

FM 24-18 Table 8-5. Technical capabilities of the AN/GRC-213. Netting old and new equipment.

The most important technical characteristics to consider when netting two different radios are the type of tuning and type of emission.

As noted in Table 8-2., the older radios have continuous tuning whereas the newer radios have detent tuning. The difference between these two is that detent-tuned radios can tune only to certain frequencies and cannot tune to the in-between frequencies to which the continuous-tuned radios can tune. The NCS should have a new series radio set to which all radios can tune. If the NCS does not have a new series radio set, the operator should direct a station with a new series radio to provide the signal to which all others tune. Check Table 8-6 for compatible frequency ranges.

FM 24-18 Table 8-6. AM radio frequency ranges.

Types of emission must match. In the CW mode, the type of emission for both old and new series radio sets is the same. In the FSK mode, the type of emission is the same. Only the way the carrier shifts is different, and this problem can be overcome. (See paragraph 12.) The type of emission for voice, however, is different. The old series equipment uses double sideband (DSB) while the new series equipment uses SSB and compatible AM. Only the compatible AM mode of the new series radio can be used with the older equipment. SSB cannot be used to communicate with the older series equipment.

Another item of importance that must be taken into consideration when operating old and new series radio sets together is the difference in frequency ranges. The old series radio sets have a transmitting frequency range between 1.5 to 20 MHz, and a receiving frequency range between 0.5 to 32 MHz. The new series radio sets have a frequency range for transmitting and receiving from 2.0 to 29.999 MHz. So when operating between the old and the new series radios, the operating frequency must be within 2.0 to 20 MHz.

The most commonly used old series RATT sets are the AN/GRC-46 and the AN/GRC-26D. They are used by Reserve Components. The most commonly used new series RATT sets are the AN/GRC-142 and the AN/GRC-122 used by both Reserve Components and the active Army.

Characteristics of both old and new series equipment are listed in Table 8-7. (See Tables 8-3, 8-4, and 8-5.)

Table 8-7. Characteristics of both old and new generation AM radio sets.

8-11. Modes of Operations


The AN/GRC-46 and the AN/GRC-26D operate in the AM (DSB), CW, and FSK (850 Hz shift) modes.

The AN/GRC-142/122 operates in the SSB (using upper sideband), AM voice, narrow frequency shift keying (NSK) (85 Hz shift), FSK (850 Hz shift), and CW modes of operation.

8-12. Procedures for Interoperability

After establishing a compatible SOI net frequency, the following rules apply for netting old and new series radio sets for different modes of operations.

Voice operations (Table 8-8).

New series radio sets AN/GRC-142/122.

  • Tune for normal voice operation.
  • Change the SERVICE SELECTOR switch on the RT-662 or RT-834 from SSB/NSK to the AM position.
  • Conduct normal voice operation.

Old series radio sets AN/GRC-46 and AN/GRC-26D.

FM 24-18

  • Tune equipment as usual for voice operation.
  • While receiving voice signal from a station in the net using a new series radio set, rotate the CONTINUOUS TUNING dial for clearest voice reception.
  • Realign transmitter to receiver.
  • Conduct normal voice operation.

Table 8-8. Voice tuning procedures. CW operation (Table 8-9).

New series radio sets AN/GRC-142/122.

  • Tune radio set for normal operation.
  • Change the SERVICE SELECTOR switch on the RT-662 or RT-834 to the CW position.

NOTE: In the CW mode, the transmitted RF signal is 2 kHz higher than the frequency indicated on the RT's (Receiver/Transmitter) MC and KC controls.

  • Lower the operating frequency by 2 kHz on the RT-834/662. Key radio set with CW keying device and adjust beat frequency oscillator (BFO) control left or right for comfortable listening tone.
  • Conduct normal CW operation.

Old series radio sets AN/GRC-46, AN/GRC-26D.

  • Tune radio sets for normal CW operations.
  • While receiving a CW signal from a station in the net using a new series radio set, rotate the CONTINUOUS TUNING dial on the receiver until a clear CW signal is heard.
  • Realign transmitter to receiver.
  • Conduct normal CW operations.

FM 24-18 Table 8-9. CW tuning procedures. Radio teletypewriter operation.

NOTE: Other than netting with the continuous tuning dial for voice and CW operations, the operator of the new series radio set was the only one that had to make changes in the normal tuning procedures of his radio set. For RATT operations, both the operator of the old series and the new series radio sets must make changes from the normal tuning procedures. The primary reason is the position of the mark and space signals in relation to the carriers of the two types of equipment. Old series radio sets transmit the mark signal above the carrier and the space signal below the carrier. The new series radio sets transmit the mark signal below the carrier and the space signal above the carrier.

New series RATT sets AN/GRC-142/122.

  • Tune RATT equipment as usual for normal RATT mode of operation.
  • Change the service selector switch on the RT-662 or RT-834 to the FSK position.
  • Change the REC switch on the modem (MD-522) from normal to reverse.
  • Change the mode selector switch on the MD-522 to 850 Hz.
  • When receiving a TTY signal, adjust the BFO on the MD-522 for reverse scope alignment. If necessary, adjust the Frequency Vernier to assist BFO scope alignment when tuning the receiver to the receive signal.
  • Conduct normal RATT operation.

NOTE: When receiving a TTY signal from like equipment (new series radio to new series radio), the REC switch must go back to NORMAL in order to receive.

Old series radio teletypewriter set AN/GRC-46.

  • Tune RATT equipment as usual for normal RATT mode of operation (Table 8-10).
  • Change the service switch on the Converter CV-278 from normal to reverse.

FM 24-18

  • Adjust the Receiver R-392 to the tuning signal of the AN/GRC-142/122, VSC-2, or VSC-3. Adjust until a mark 40 signal to the right of 0 is received on the (CV-278) converter.
  • Realign transmitter to receiver.
  • Conduct normal RATT operation.

Table 8-10. RATT tuning procedures.

NOTE: When operating secure RATT and you are not receiving, the teletypewriters may run open. To stop them from running open, return the SERVICE switch on the Converter CV-278 to the MARK HOLD position. On the CV-116 set the MARK HOLD switch back to the MARK HOLD position.

Old series radio teletypewriter set AN/GRC-26D.

  • Tune RATT equipment as usual for normal RATT mode of operation (Table 8-11).
  • Change the MARK HOLD switch on the converter (CV-116) from XTAL (left) (NORM) position to the XTAL (right) (REV) position.
  • Adjust the Receiver R-390 to the tuning signal of the AN/GRC-142/122, VSC-2, or VSC-3. Adjust until a mark 50 signal to the right of 0 is received.
  • Realign transmitter to receiver.
  • Conduct normal RATT operation.

FM 24-18 Table 8-11. RATT tuning procedures.

NOTE 1: When receiving a TTY signal from like equipment (old series radio to old series radio), the service switch on CV-278 or the MARK HOLD switch on CV-116 must go back to the normal operating position in order to receive.

NOTE 2: When operating secure RATT and you are not receiving, the teletypewriters may run open. To stop them from running open, return the SERVICE switch on the Converter CV-278 to the MARK HOLD position. On the CV-116 set the MARK HOLD switch back to the MARK HOLD position. 8-26




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