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Table of Contents

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LESSON DESCRIPTION:

In this lesson you will learn the principles of radio teletypewriter operations and will become familiar with the components and characteristics of various radio teletypewriter sets.

TERMINAL LEARNING OBJECTIVE:

While voice communications is essential to the rapid dissemination of vital information in a tactical situation, you may also have the need to send hardcopy (paper) messages from time to time in your duties as a signal officer. It 'is important, then, that you have some means of transmitting this hardcopy message from one point to another. The equipment you will use for this is radio teletypewriter equipment.

1. Theory of RATT Operation.

RATT systems operate on a system of electronic pulses. Each RATT unit consists of a keyboard, transmitter, receiver, and printer combination. The keyboard converts the keystrokes entered by the operator into electronic pulses. Each character transmitted is actually made up of a start pulse, five intelligence pulses, and a stop pulse. All the pulses are 22 milliseconds in duration except for the stop pulse which is 31 milliseconds. The intelligence pulses are a combination of spacing impulses and marking impulses. Depending on the combination of these two types, the receiver-printer will decode the signal and print the corresponding character. Figure 4-1 shows the combinations of intelligence pulses for each character in the RATT character set. In most RATT sets, the marking and spacing impulses are transmitted on slightly different frequencies. This is called frequency-shift keying. FSK has an advantage in that it does not require any carrier modulation. Thus, RATT equipment can transmit messages more clearly in marginal conditions than voice radio. Figure 4-1 also shows the perforated tape that some RATT equipment uses. The perforated tape allows you to pre-record a message on the tape and then transmit it by running the tape through the RATT transmitter. Likewise, the receiver can print out on perforated tape if necessary. The perforated tape acts like a backup to the regular printer. Most RATT equipment also has voice communications capability built in so you have another backup for ensuring that messages are received.

Figure 4-1. Teletypewriter code character set and standard start-stop, five-unit code chart

2. RATT Equipment.

The following paragraphs present an overview of some of the more popular RATT equipment currently in use. As with the section on voice equipment, some of the RATT equipment may no longer be in use in regular units but may be used by some reserve units. Again, this course is not designed to teach you every aspect of each piece of equipment, and you are encouraged to read the technical manuals referenced for the various equipment presented. However, review of the technical manuals is not required for successful completion of this subcourse. Table 4-1 is a comparison of operating frequencies for each of the different pieces of equipment discussed. When netting old and new equipment, you must take into consideration the operating frequencies of the equipment with which you are trying to communicate.

Table 4-1. Comparison of operating frequencies

1. Teletypewriter Set AN/PGC-1. TM 11-5815-206-12 details this equipment. The PGC-1 is the most basic RATT system currently in use. It is a small transportable unit consisting of a keyboard and a printing device. The PGC-1 is used in fixed and transportable configurations.

2. Teletypewriter Set AN/FGC-20. TM 11-5815-200-12 details this equipment. The FGC-20 is similar to the PGC-1 with the exception of being larger and more durable. This unit is found primarily in fixed sites where portability is not a requirement.

3. Radio Teletypewriter Sets AN/GRC-46, AN/VSC-1, and AN/VRC-29. TM 11-5815-204-10 details this equipment. The GRC-46, VSC-1, and VRC-29 are older RATT sets that are used in mobile communications applications. They all use the AN/GRC-19 radio set as the basic transmitter-receiver, and they have the same operating frequencies and modes as that unit. The Army is replacing these units with the AN/GRC-106 based RATT sets.

4. Radio Set AN/GRC-26D.TM 11-5820-256-10 details this equipment. The GRC-26D is a high-power RATT station designed for use in fixed, semifixed, or mobile applications. The set consists of a transmitter, two receivers, and three teletypewriters. The GRC-26D operates in the same frequency bands and operational modes as the GRC-46 family. In its mobile configuration, the GRC-26D is shelter-mounted in a 2½ ton vehicle. The GRC-26D is generally considered obsolete and has been replaced in regular units by the GRC-106 based AN/GRC-122.

5. Radio Teletypewriter Sets AN/GRC-122, AN/GRC-142, AN/VSC-2, and AN/VSC-3. TM 11-5815-334-12, TM 11-5815-331-14, and TM 11-5815-332-14 details this equipment. These RATT sets are replacing the older AN/GRC-19 based systems described previously. They are based on the more modern and versatile AN/GRC-106 transmitter-receiver, and they have the same frequency and mode characteristics as that equipment. The following paragraphs briefly discuss the differences in each of these pieces of equipment.

3. RATT Network Communications.

As with voice radios, RATT stations are set up in networks according to the station's function within the command. The SOI and unit command structure will designate how these nets are set up. The procedures for communicating on a RATT net are very similar to those for communicating on a voice net. Proper training of operators and adherence to procedures are, if anything, more crucial in RATT communications. PROSIGNs must be used whenever possible to minimize the size and length of messages. Table 3-3 in lesson 3 lists most of the common PROSIGNs. Table 4-2 lists some additional PROSIGNs that are unique to RATT communications.

Table 4-2. Additional PROSIGNs

4. RATT Authentication and Net Procedures.

You should use authentication procedures for RATT communications just as you would for voice. The same rules of challenge and reply and transmission authentication apply. The one difference in authentication procedures between the two systems is that, in RATT communications, the reply to a challenge is sent twice. The following paragraphs illustrate opening and closing a RATT net using challenge and reply authentication.

The NCS calls the net, tells them to tune their radios, and asks how each station copies his transmission. His transmission is: "(Net call sign) DE (NCS call sign) ZRC2 INT ZBK K." The voice equivalent would be: "(Net call sign) this is (NCS call sign) net now, how do you copy me, over." You can see that the use of PROSIGNs significantly shortens the message and reduces the possibility of typographical errors.

The first substation in alphabetical order calls the NCS and reports that he reads the NCS clear or garbled. He then asks the NCS how he copies the substation's transmission. His transmission is: "(NCS call sign) DE (station call sign) ZBK1 INT ZBK K."

The NCS answers that he reads the substation clear or garbled and tells the substation to standby. His transmission is: "(Station call sign) DE (NCS call sign) ZBK1 (or ZBK2 if garbled) ZUJ AR." He ends his transmission with the PROSIGN for "Out" to indicate that he does not want the substation to reply.

The second substation in alphabetical order repeats the above procedure, and so on for the third and any additional substations on the net. When all stations have established contact with the NCS, the NCS will open the net using authentication. His transmission is: "(Net call sign) DE (NCS call sign) INT ZNB A J K." The NCS in this case has chosen A and J as his first authentication challenge.

The first substation in alphabetical order replies to the NCS's challenge and issues a counter challenge to the NCS. His transmission would be: "(NCS call sign) DE (Station call sign) ZNB F F INT ZNB P L K." Notice that the first substation transmitted the reply F twice in keeping with RATT procedures.

The remainder of the authentication process is like the same procedure in voice net operations. The NCS replies and issues a new challenge. The second substation replies to the challenge and issues another challenge that is replied to by the third substation and so on. When all substations have authenticated, the NCS will issue instructions on how the net will operate and pass message traffic.

When the net is ready to close down, the NCS will issue the close down order. His transmission will be: "(Net call sign) DE (NCS call sign) ZKJ1 K." As in voice operations, the first substation will challenge the NCS and the NCS will reply, validating the order to close the net. Each substation will then call the NCS in alphabetical order and acknowledge the order to close the net. When the last substation has acknowledged the close down order, all stations may secure their RATT stations.

5. Summary.

In this lesson you have learned about the operation of RATT systems. You have also learned about the main RATT systems currently in use for tactical radio communications and about the basic communications procedures associated with RATT communications. In the next lesson you will learn about some of the most modern tactical radio equipment currently in use.

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