Electronics Material Officer

Electronics Material Officer Course






















This lesson topic presents information about the Navy Tactical Data System. As EMO, you will be responsible for the maintenance of NTDS equipment. A familiarity with system purpose and operation, system equipment, training, and common problems will help you manage NTDS maintenance effectively.

The LEARNING OBJECTIVES of this LESSON TOPIC are as follows:

4.9 Describe Tactical Data System (TDS) equipment as related to:

a. Safety

b. Physical characteristics

c. Purpose

d. Limitations

e. Maintenance

f. Installation

g. Units

h. Modes of operation

i. Data transmission links

j. Other electronic subsystems

k. Technical documentation

l. Material condition


The student should review the "LIST OF STUDY RESOURCES" and read the Lesson Topic LEARNING OBJECTIVES before beginning the lesson topic.







To learn the material in this LESSON TOPIC, you will use the following study resources:

Written Lesson Topic presentations in the Module Booklet:

1. Lesson Topic Summary

2. Narrative Form of Lesson Topic

3. Lesson Topic Progress Check

Additional Materials:

1. Assignment Sheet

2. Answer Booklet


1. Shipboard Electronics Material Officer, NAVEDTRA 12969

2. Understanding Link 11, Logicon, Inc., Tactical and Training Systems, San Diego, CA, 1990.

3. OPNAVINST C3120.39(B)







This lesson topic will introduce you to the Navy Tactical Data System and will include a system overview, system operation/function, system equipment, transmission anomalies and their resolution, training, and net management. The lesson narrative is organized as follows:

Combat Systems Computer Equipment

A. Introduction to the Naval Tactical Data System

B. Introduction to Link 11

C. Link 11 Equipment

D. Link 11 Operation

E. Link 11 Anomalies and Transmission Problems/Resolution

F. Net Management










The NTDS is a computer-centered control system that coordinates the collection of data from ship's sensors (radar, sonar, navigation systems, electronic warfare, and fire control) and from external sources via communications links. NTDS allows commanders in the fleet to rapidly communicate tactical information between ships. Computers transfer information at extremely high speeds, information that was previously handled manually or by voice communications. Figure 4.3-1 shows a typical NTDS input/output configuration.



















Figure 4.3-1 NTDS Input/Output Configuration

The NTDS is designed around the unit computer concept. This concept is based on having standard computers operate together for greater capacity. NTDS computers receive data from various sensing devices that are in continuous contact with the outside environment. This data is received via transmission links. Received data is processed and correlated by NTDS computers

to evaluate each event as it happens, i.e., in real time. This data is viewed by operators as

symbols that represent friendly, hostile, and unknown air, surface, and subsurface contacts on NTDS displays. Figure 4.3-2 is a simplified diagram of an NTDS system equipment.
























Figure 4.3-2 NTDS Simplified Diagram



NTDS data transmission links are shown in Figure 4.3-3. These links provide operational commanders with accurate, high-speed tactical communications. Each link provides a capability to transfer data rapidly to ships, aircraft, and shore facilities without the delays imposed by human intervention. Link 4A, 11, and 16 are transmit/receive systems. Link 14 is receive only.

Link 11

Link 11 provides high-speed computer-to-computer transfer of tactical environment information, command orders, and participating unit status to all other tactical data systems with a nominal range of 300 miles. Tactical information transferred consists of surface, subsurface, air, and ES track information on friendly, hostile, and unknown identity tracks.


Link 14

Link 14 provides a means of transmitting track information, identity, engagement status, drop track reports, and gridlock information to units not capable of participating in the Link 11 net. This is basically a manual link relying on voice and manual TTY communications.

Link 4A

Link 4A enables the operational program to take control of the autopilot in a suitably equipped aircraft to control landing and takeoff, to pursue, or to follow collision intercept geometry. It can

control a flight to a strike area and return it to base without pilot action. The pilot has the option of:

l Going fully automatic

l Using the visual display to aid in interpreting the intercept controller's dialogue

l Totally ignoring the Link 4A transmission





















Figure 4.3-3 NTDS Communications Links


Link 16

Link 16 is essentially the same as Link 11. It is an automatic, high-speed data communications link that provides the same information as Link 11 however, Link 16 uses a merged message format, allowing interservice and NATO link operations, thus expanding our overall link capabilities. Link 16 can have either of the following titles, depending on usage: TADIL J (Tactical Data Information Link J), the NATO term for Link 16 or JTIDS (Joint Tactical Information Distribution System), the interservice term for Link 16. JTIDS is a high capacity, digital system that combines integrated communications and relative navigation and identification capabilities. The system permits secure, jam resistant data, and voice transfer in real time among the dispersed elements of the military services. The JTIDS architecture provides broadcast and point-to-point data communications for weapons systems, sensors, and command centers. The remainder of this lesson will address NTDS Link 11.


Link 11, also referred to as Tactical Digital Information Link (TADIL) A, uses a communications net formed by a group of participating units (PU). Digital information is exchanged between airborne, land-based, and shipboard tactical data systems using a standard message format. Link 11 communications are conducted in either the HF or UHF bands, or through limited range intercept satellite communications (LRI SATCOM). When operating in the HF band, Link 11 provides omnidirectional coverage up to 300 nautical miles (nm) from the transmitting site. When operating in the UHF band, the link provides omnidirectional coverage to approximately 25 nm ship-to-ship, or 150 nm ship-to-air. LRI SATCOM is used to effectively extend the UHF line of sight range for battle group tactical link operations. This link is known as the Tactical Relay Information Link (TACRIL). TACRIL uses an AN/ARQ-49 pod attachment to an aircraft operating with the task force and provides one channel for Link 11 operation and two channels for voice communications. Using UHF provides better communications security and using TACRIL reduces satellite channel congestion.


There are many different Link 11 equipment configurations. A representative configuration is shown in Figure 4.3-4 consists of a computer system, an encryption device, a data terminal set (DTS), an HF or UHF radio, a coupler, and an antenna.



























Figure 4.3-4 Link 11 Equipment Configuration

The Tactical Data System (TDS) receives data from sensors and collects this information in a database. Before this database can be shared with other TDS computers, it is converted into a message format. Commands and administrative information are also formatted into messages. The formatted data is encrypted by the TSEC/KG-40 and converted to an audio signal by the Data Terminal Set (DTS). An HF or UHF transmitter modulates an RF carrier with the audio signal and broadcasts it to other units participating in the link. When the transmitted signal is received, it is demodulated and sent to the DTS. The DTS converts the signal back into digital data. This data is decrypted by the TSEC/KG-40. The decrypted data, once again in the form of the message that originated at the transmitting unit, is sent to the TDS computer for processing.




Tactical Data System computers include the CP-642A/B, and currently, the AN/UYK-7 and AN/UYK-43. Table 4.3-1 shows typical TDS computer installations by ship class.


Table 4-3.1 TDS Computer Installations









Physically, these computers may appear quite different, but their Link 11 functions are identical. In addition to maintaining the tactical database, TDS computers manage displays, perform interim updates of track locations, respond to operator entries and inquiries, and control all peripheral input and output (I/O). All NTDS software must pass rigorous testing by the Navy Center for Tactical System Interoperability (NCTSI) in order to be certified.



The TSEC/KG-40 is the most commonly used encryption device. It provides COMSEC for 24 channels of tactical data that flow through the system. Each channel is encrypted to prevent interception. When the keylist is properly loaded into the TSEC/KG-40, COMSEC operation is fully automatic and requires no further operator intervention. Keylists are identified for the day and operations area. Keylist loading is cross-checked by two personnel to ensure proper operation.


The DTS is the heart of the Link 11 system (see Figure 4.3-5). In addition to encoding data into audio tones, it generates and recognizes protocol signals that control the operation of the net.

Several models of the DTS are in use. Table 4.3-2 provides a snapshot of different DTS models, the years they were first introduced, and installations by class. AEW and ASW aircraft use variations of the AN/USQ-76.






















Figure 4.3-5 Data Terminal Set


Table 4.3-2 Data Terminal Set Installations




AN/USQ-36 1964-66 CG, CGN, CV, CVN, FF, LCC

AN/USQ-59 1971-72 LHA

AN/USQ-63 1973-74 DD, LHA


AN/USQ-74 1981-87 CG, CGN, CV, DDG, FFG,



AN/USQ-92 1984 MULTOTS Detachments 1, 2

AN/USQ-83 (APE)1986-89 Tech Eval FF, AGF, AEGIS

MX512-P 1990 MULTS, USCG


* Receive only Link 11 (ROLE)

The DTS normally operates in half duplex mode, during which it can either send or receive data, but cannot do both simultaneously. The single exception is during system test, when it operates in a full duplex mode and can send and receive data simultaneously. The DTS functions as a modulator/demodulator (MODEM); i.e., it modulates and demodulates internally generated audio tones with tactical data. The composite signal is sent to the HF or UHF radio for transmission.

Additional functions performed by the DTS include error detection and correction (EDAC), Link protocol control, and NTDS interface control.

Error Detection and Correction (EDAC)

The DTS uses an EDAC parity check to identify and correct errors in receive data. The DTS requests and accepts tactical data in the form of a 24-bit data word from the TDS computer. To this data it adds an EDAC. This 6-bit code is also referred to as Hamming bits. The value of these bits is based on a parity check of specific combinations of the original 24 bits. The EDAC, or Hamming, bits allow error checking to be performed on received data. If a single bit error occurs, it can be located and corrected. A selection on the DTS determines whether a detected error is to be labeled or corrected.


Audio Signal Generation

The newly formed 30-bit word is used to phase modulate fifteen internally generated audio tones. The phase modulated audio tones, together with a Doppler correction tone, are then combined into a composite audio signal that is applied to either the HF or the UHF radio equipment for transmission.


Link Protocol Control

The DTS generates and recognizes protocol data that controls the type and number of link transmissions. This protocol data includes codes indicating the start of a transmission, the end of a transmission, and the number of the next unit to transmit.


NTDS Interface Control

NTDS interface is under the control of the DTS. The DTS signals when it has input data and when it wants output data. The operation of this interface is controlled by an external interrupt from the DTS, with a code designating the purpose of the interrupt. The DTS-to-NTDS interrupt codes include reset, end of receive, prepare to transmit, prepare to receive, and prepare to broadcast codes to control NTDS interface. When the DTS recognizes an output requirement, such as when it receives its own PU number, it generates a Prepare to Transmit interrupt. The NTDS then provides the requested channel activity. When the DTS recognizes an input requirement, such as a start code, it generates a Prepare to Receive interrupt. The NTDS sends and receives digital information only after being notified to do so by the DTS. The path from the DTS to the NTDS computer runs through the TSEC/KG-40 for encryption and decryption.


Link 11 transmitters and receivers provide point-to-point connectivity between widely separated units in the net. Both HF and UHF radios are used in Link 11. HF is used to establish a net when the range between units in the net is from 25 to 300 nm. For ranges of less than 25 nm, HF Limited Range Intercept (LRI) can be used. UHF is used when the range between units is less than 25 nm.

General Link 11 Radio Requirements

Link 11 radios must meet requirements that are different from radios designed for voice only operation. Because of the speed at which the link operates, link radio equipment must be able to keep up with the repetitive cycles of transmission and reception. On most ships, the only communications equipment that can be used for link operations is the link-capable equipment wired to NTDS patch panels. Other equipment should not be used for link operations. Additionally, transceivers dedicated for link operations should not be used for other purposes. A snapshot of HF and UHF Link 11 radio equipment is provided in Figure 4.3-3.


Table 4.3-3 Link 11 Radio Equipment Installations







T-1322/R-1903 42 DD, CG




AN/URC-109 1 LHD

AN/URT-23C/R-1051G 73 FFG, SSN

AN/URT-23D/R-1051H 10 CG, DDG



AN/URC-93 61 CGN, CG, CVN, CV, DG,


U AN/WSC-3 119 FF, DD, DDG, FFG, BB,







HF Radios

HF radios used for link operations use the link audio signal to amplitude modulate a carrier frequency. These radios, e.g., AN/SRC-23s, have several modes of operation, including Single Sideband Suppressed Carrier (SSBSC), Double Sideband Suppressed Carrier (DSBSC),

Independent Sideband Suppressed Carrier (ISBSC), Amplitude Modulation (AM), and Continuous Wave (CW). For Link 11 operations, the ISBSC mode is used. In this mode, the Link 11 audio signals generated by the DTS are applied to both the upper and lower sidebands for transmission. The RF carrier frequency is not transmitted because it conveys no intelligence. To extend the effective signal range, all signal power goes into the intelligence carrying sidebands. One advantage of using both sidebands with the same audio signal is that it affords a degree of diversity, so that signal fading and ambient noise can be overcome at the receiver. Whenever one sideband degrades, the other sideband does not degrade at the same time. The receiving unit can select the best data.


UHF Radios

UHF radios use frequency modulation (FM). In UHF radios, the Link 11 audio signal is used to frequency modulate a carrier frequency. This technique of modulation is more resistant to interference than amplitude modulation and UHF is less susceptible than HF to transmission anomalies. UHF is limited in range to LOS.


HF Antenna Couplers

Since link is a radio system, it must be able to tune the transmission line (cable) and the antenna to match the radio's output impedance to the antenna. This produces maximum output from the transmitter. Remember that some couplers provide the added advantage of allowing many transmitters to be fed into a single antenna. The resulting configuration is a multicoupler group. See Figure 4.3-13. Multicouplers require frequency management to prevent influence across adjacent channels. Typically, a 15% frequency separation between adjacent channels should be maintained. Failure to maintain sufficient frequency separation between adjacent channels will allow adjacent couplers to leak energy between units. Serious signal distortion, as well as costly equipment failure, could then occur.


UHF Couplers

In general, UHF antenna couplers are combined within the system group or are part of the UHF transceiver. Because they have smaller components and lower power requirements, UHF couplers typically perform quite well. Most UHF antenna couplers contain preset channels, a feature that permits faster tuning and remote selection of predesignated channels. On the surface, remote selection may appear to be a tactical advantage. The problem with users remotely selecting tactical channels is that they may create a frequency management nightmare. There must be a 15% frequency separation between adjacent predesignated channels; without it a tactical voice circuit might be heard on the Link 4 (nicknamed Dolly) circuit, or vice versa.



Antennas used for link communications are standard HF (AM spectrum) and UHF (FM spectrum) antennas, identical to those used for voice communications.

Frequency Standards

Link 11 DTSs, transmitters, and receivers are capable of operating with either an internally generated frequency standard or an external frequency standard. Using an external standard can help alleviate the problem of component frequency error. Typical frequency standards used are the AN/URQ-9, AN/URQ-10, and AN/URQ-23.


Patch Panels and Switchboards

Patch panels and switchboards are used in the Link 11 system to interconnect system equipment and components and allow technicians to isolate and test simple components. Patch panels and switchboards also allow redundancy to be designed into the system. Redundancy helps to ensure link capability even when a particular component (e.g., a radio) has failed or is undergoing preventive maintenance. Using patch panels and switchboards, we can meet both the connection requirements and provide maximum component flexibility by allowing operators to patch around inoperative equipment. Patching configurations are unique to each class of ship. A configuration may use both patch cables and manual switches, manual switches only, or a mix of computer-controlled input/output selectors.

A typical patching system is simple, but subject to failure. The 26 or 28 NTDS data channels require 52 to 56 individual wire conductors to connect the NTDS computer to the encryption unit and the encryption unit to the DTS. Double this number to satisfy both transmit and receive connection requirements. To accomplish the necessary switching between units, mechanical switches must have a section or wafer dedicated to each pair of conductors. Switches that have an open face construction have the greatest number of problems. Dust and dirt can damage the multiple-switch contact surfaces. The loss of a single bit of data along this pathway is very difficult to detect.




Transmissions are composed of preambles, a phase reference, control codes, a crypto frame, and message data.


The preamble, as previously discussed, consists of a two-tone signal composed of the 605 Hz tone and the 2915 Hz tone. The 2915 Hz tone is shifted in phase by 180 at the end of each frame. A transmission always begins with five frames of preamble.


Phase Reference Frame

The data frame that immediately follows the preamble is called the phase reference frame. It provides a reference phase for each of the data tones of the following frame. The difference between the phase angle of a given tone in the reference frame and that of the same tone in the

subsequent frame defines the phase shift for that tone. There is only one phase reference frame in a transmission. Each subsequent data frame acts as the phase reference for the frame that immediately follows it.


Control Codes

The operation of the link is controlled by control codes. The fifteen data tones allow the encoding of thirty bits of information, two bits per tone, in each frame. The control code is a special two-frame sequence. There are no Hamming bits associated with control codes. These codes can be recognized with up to four bits in error on each frame. The Link 11 control codes are the start code, the picket stop code, the control stop code, and the PU address codes. There are 62 possible PU address codes.


Address Code

An address code immediately follows either the phase reference frame, in the case of an NCS call-up, or the control stop code, in the case of an NCS report. The address code specifies which PU is to transmit next. Recognition of an address code identical to a ship's own address causes the DTS to issue a Prepare-to-Transmit interrupt to the NTDS computer.


Start Code

The start code immediately follows the phase reference frame. It specifies that a data report is about to begin. Recognizing the start code causes the DTS to issue a Prepare-to-Receive interrupt to the NTDS computer. If a start code is not received at the NCS's DTS within fifteen frames of the call-up, the NCS will poll the unit a second time. Since there are five preambles, a phase reference, and two frames of start code, eight of the fifteen frames are taken up with the initial building blocks of the transmission. To avoid having this response jammed by a second call-up, a picket DTS must begin its response within seven frames of receiving and recognizing its own address. In the real world, a picket DTS will usually respond within three frames of recognizing its own address.


Picket Stop Code

The picket stop code marks the end of a PU's data report. Recognizing a picket stop causes a DTS to issue an End-of-Receive interrupt to the NTDS computer. It takes both frames for the stop code to be recognized. The first frame of the stop code will be passed to the NTDS computer as data before the End-of-Receive interrupt is issued.


Control Stop Code

The control stop code, which is transmitted only by the NCS, marks the end of the NCS's own report. It is followed immediately by the address code of the next unit to transmit. Recognition of a control stop also causes a DTS to issue an End-of-Receive interrupt. Again, both frames of

the control stop are required for the stop code to be recognized and the first frame of the control stop is passed to the NTDS computer as data.


Message Data Frames

Between the start code and the stop code is the message data that originates from an NTDS computer. For each message data frame, the computer supplies 24 bits of information plus two bits representing the error code.


Crypto Frame

The first frame following the start code is actually generated within the KG-40. It is passed to the DTS while the first frame of message data is being encrypted. The first frame of message data is then passed to the DTS while the second frame is encrypted. In this way, the data is "pipelined" from the NTDS through the KG to the DTS.



The following five modes of operation are selected at the DTS: Net Synchronization, Net Test, Roll Call, Short Broadcast, Broadcast (or Long Broadcast).

Net Synchronization

The Net Sync transmission is a continuous series of preambles. Net Sync is initiated manually by the operator and continues until stopped manually by the operator. Operationally, it is often used as a first step in verifying RF connectivity between units.


Net Test

The Net Test transmission consists of a 21-frame repeating test pattern. This test pattern is a subset of the address codes. The transmission begins with preamble frames and a phase reference frame, and is then followed by the test pattern. Net Test mode is a test of connectivity between units. It is also a useful signal for setting the DTS audio input and output levels. The Net Test signal should be input to the DTS at 0 dBm. Net Test also checks the DTS's PU address receive circuits.


Roll Call

Roll Call is the normal mode of operation for Link 11. In roll call, one unit is designated as the Net Control Station (NCS). The remaining units serve as picket stations, or participating units (PU). The NCS's DTS controls the sequence in which the other PUs are polled. Each PU transmits its data when it is called. During the remainder of the time, a PU is receiving reports from the other members of the net. If a PU does not answer its call, the NCS will automatically poll a second time. If there is still no response, the NCS polls the next unit in the sequence.

When each polling sequence, or net cycle, is complete, the NCS reports its own information. In this way, tactical data is exchanged among the net members. The operation of the DTS, once initiated, is automatic. The types of transmissions that occur during roll call are the NCS call-up (interrogation), the picket reply, and the NCS report (interrogation with message).



The Short Broadcast is a single data transmission to all members of the net by a station that may be acting as either picket or NCS. It is initiated manually by the operator at the DTS. The Broadcast, or Long Broadcast, net mode consists of a continuous series of short broadcasts, separated by two frames of dead time. It is initiated manually by the operator at a station acting as either picket or NCS. It continues until the operator stops it manually.


Radio Silence

Radio Silence is the absence of any transmission. A PU in radio silence will receive data from other members of the net, but will not respond even if it is polled.



Transmission anomalies are defined as transmissions which, although occurring during operational conditions, do not match any of the Link 11 transmission structures presented under Modes of Operation. These anomalies are often, but not necessarily, symptomatic of equipment problems.


Empty Report

The report is empty or contains no data. The start code is immediately followed by the stop code. This can occur both in picket replies and in NCS reports.


Crypto Spike

The report contains only one frame of data. The transmission consists of a start code, a single frame of data, called a crypto frame, and a stop code. This can occur both in picket replies and in NCS reports.


NCS Report Call-Up

Units that use computer-enabled addressing must obtain the address of the next unit to poll from the NTDS computer. These units have a nonstandard call-up structure for the first time a PU is polled. Their first call-up resembles an NCS report that has only one frame. The address for the second call, however, is not an anomaly. It is automatically supplied from within the DTS and

looks like a normal call-up. Operationally, this idiosyncrasy has no effect except to increase the time it takes to complete one net cycle.


Too-Short Report

A minimum number of administrative messages are required in every transmission. If the frame count of a report, including the phase reference frame and control codes, is fewer than fourteen,

the NTDS software is not operating according to these requirements.



Sometimes a PU answers its call-up, but the NCS either does not hear or does not recognize the response. This situation can occur when the NCS has poor receive capability. It may also occur when the unit is very distant or has a weak signal. Sometimes it occurs when the NCS is trying to process a signal in diversity mode and the unit is transmitting only on upper sideband. It may

also occur for reasons that are still undetermined.



Atmospheric Interference

After the signal has been transmitted, it is subject to distorting effects such as skywaves, multipath interference, co-channel interference, and carrier frequency instability or drift. Some of these problems can be prevented; others are beyond your control. Understanding how the environment affects the transmitted signal will help you to recognize the problems you can do something about.

Intermodulation Distortion

The mixing of signals and their harmonics to produce new output frequencies in called intermodulation distortion. Link 11 uses 16 audio tones to modulate RF transmitters. This is a relatively broadband signal that covers a wide area of the spectrum when transmitted. These audio tones can mix with other modulation byproducts that occupy an even wider portion of the spectrum. This condition must be avoided. Ensure that technicians follow PMS procedures and other Link 11 maintenance publications (such as newsletters and bulletins). To avoid problems caused by intermodulation distortion and energy cross coupling, transmitter output power must be kept at the minimum required for connectivity.


EMI and Radio Frequency Interference

EMI and radio frequency interference (RFI) can have devastating effects on all shipboard communication circuits. Occasionally HF Link 11 signals can completely obliterate circuits operating in the UHF band. In such extreme cases, isolating the problem may be more difficult because the normal Link 11 "ping-pong" may not be heard in the upper frequency bands. Note that when Link 11 transmitter RF output power reaches a point that causes cross-coupling of

energy to other circuits. This causes a unique Link 11 "ping-pong" sound on the 1MC, Site TV channels, and other shipboard circuits. The preferred way to isolate an EMI/RFI problem is usually by selectively quieting the active data and voice circuits. After the problem emitter has been located, an operator should adjust its frequency, power output, or assigned antenna. Simply selecting another antenna for transmission may be a solution; changing frequencies may also be a solution. In either case, many hours of frustrating work may be required to isolate the problem emitter. Again, power levels of Link 11 emitters must be kept to minimum acceptable levels.


Co-Channel Interference

Failure to maintain sufficient frequency separation between adjacent channels will allow adjacent couplers to leak energy between units. This is called co-channel interference. For example, the AN/URA-38 antenna coupler, used in automatic mode with the AN/URT-23 transmitter, may actually retune itself to a transmission frequency on an adjacent channel whose separation is less than 15%. Serious signal distortion, as well as costly equipment failure, may then occur.


Carrier Frequency Error

Link 11 digital data is modulated onto individual audio tones using the quadrature phase shift keying technique of encoding described earlier. Carrier frequency error occurs when the transmitter's internal or external frequency standard is inaccurate. This causes the sidebands to be displaced by the amount of error. On demodulation, this frequency error will be perceived as a phase shift of the Link 11 tones. A good rule of thumb for estimating the amount of phase error is to assume that a 5 phase shift of the audio tones will result for every 1 Hz of carrier frequency error. For example, suppose a tone is intended to be shifted in phase by 45 to produce a certain data bit value. If the transmitter has only 1 Hertz of frequency error, then all data tones will have a 5 phase shift bias factor. At the distant station the 45 phase-shifted data tone will actually be shifted 50, or the intended 45 plus the 5 introduced by the carrier frequency error. As a greater and greater error in the carrier frequency is introduced, the phase shift of the data tones also increases. Eventually the phase difference angle is in the wrong quadrant, and the receiving station will decode an incorrect value for the data bits.

The DTS's Doppler correction function compensates for differences in relative velocity between platforms. It can compensate for frequency errors of up to 75 Hz. If the Doppler correction function has been disabled, however, good data cannot be decoded from a frequency-shifted signal. Carrier frequency error causes more degradation of the Link 11 signal than Doppler shift. A relative platform velocity of 58 knots will result in a Doppler shift error of only 100 parts per billion. This amounts to only a 1 phase error at 2 MHz and a 3 phase error at 6 MHz. To combat frequency errors, each ship is equipped with an extremely accurate frequency standard. Using this external, central frequency standard for all Link 11 systems will alleviate the problem of component frequency error. Link 11 transmitters, receivers, and DTSs should be connected to a calibrated frequency standard, such as the AN/URQ-10/23.



In the past, Link 11 was repaired by trial and error. This approach is ill-advised, considering the

complexity of the Link 11 system. The reasons that an action corrects a problem may not be understood. An action that coincides with another action may appear to be corrective, when in fact it has no impact. This trial and error approach evolves into a list of myths, or potentially corrective actions, that operators try repeatedly when problems arise, without understanding the real problem. Several recurring Link 11 problems that cause considerable net disruption have been identified. Symptoms on the LMS-11 and NTDS, possible underlying causes, and suggested corrective actions are summarized in "Understanding Link 11", Logicon, Inc., Tactical and Training Systems, San Diego, CA, 1990.

For any net, the number of potential sources of problems increases as the number of units increases. This means that for a net with a large number of PUs, you should assume there may be more than one problem. Each problem, however, needs to be addressed independently, and only one corrective action should be taken at a given time. To do otherwise may prevent you from identifying the actual cause of a problem. Certain human errors that result in loss of data are generally the easiest to correct. PU address entry errors, for example, are prevalent, have a profound effect on net performance, and are simple to correct. These are easily identified on the Link 11 Monitor System (LMS-11) displays, but can be checked manually without an LMS-11. Similarly, a radio switched to voice instead of data can cause tone attenuation and loss of data. Once the problem has been identified, the switch position is easily corrected. Complex Link 11 problems can be correctly diagnosed by isolating them to a particular PU (or PUs) in the net, and then to a location or subsystem on the PU. OPNAVINST C3120.39(B) supports this method of troubleshooting and should be consulted for more detailed information. The major sources of problems are inadequate planning, incorrect initialization, equipment malfunctions, and environmental factors.

In determining the severity of a problem, both the effect on other net participants and the effect on an individual unit must be considered. It is important to note that Link 11 troubleshooting requires at least a semi-operational net. Otherwise, your technicians are simply troubleshooting ownship's system for problems. A successful link depends on proper planning, proper initialization, proper equipment function, and favorable environmental conditions. The goal of Link 11 troubleshooting is to ensure that as many as possible of these conditions are met.

Inadequate Planning

Link operations are preceded by a planning phase at the BG level during which participants are identified, PU numbers and track blocks are assigned, frequencies are designated, and the capabilities and limitations of the participants are considered. When link operations are planned, procedures in the Link 11 SOP and battle group directives should be followed. There must be adequate frequency separation, the proper crypto must be designated, and all specifiable parameters must be communicated to all units. If planning is inadequate, at best, the link will be more difficult to manage; and, and at worst, it will not function at all.


Incorrect Initialization

Initialization procedures are very important. There are many opportunities for human error. Ensure that all pre-checks are performed and the initial setup is correct. This includes operator entries such as data link reference point, ownship position, PU number, and track block, as well as checking switch positions and settings for the crypto, DTS, and radio. No amount of

preplanning can compensate for sloppy initialization procedures.


Equipment Malfunctions

The most important step in troubleshooting after recognizing that there is a problem is to isolate the problem to a specific unit and then to the malfunctioning component. In most cases, the malfunctioning unit will be out of the net temporarily while fixing or reconfiguring its equipment. Initialization errors can sometimes masquerade as equipment malfunctions. For example, failure to enter ownship address correctly at the DTS may appear to be a transmit problem.


Environmental Factors

Do not rule out environmental factors. They fall into two groups, problems that are self-induced (EMI/RFI interference) and natural phenomena (thunderstorms, diurnal effect, and solar cycle). You should consider environmental problems as the source of a problem if, after you are sure that link operations have been well planned, initialization procedures have been properly followed, and no malfunctioning equipment can be found, the problem still persists. See Lesson Topic 3.2 for a description of these factors.



The quickest way to recognize that a problem exists is observing the operation of the net. Several monitoring techniques are available. An experienced operator can detect a problem by listening to the audio signal. Link monitor parameters calculated by the NTDS computer can be examined and information from the operator of the Battle Group Link 11 Monitor System, AN/TSQ-162(V) can be requested.

Audio Signal

Most DTSs have a head phone jack that allows monitoring of the audio signal received at the DTS. Audio can also be routed to a speaker. Operators and technicians should be able to recognize the ping-pong sound of the Link 11 call-up and response sequence.


DTS Indicators

Data Terminal Sets are equipped with indicators that provide information useful in determining the quality of Link 11 signal flow through the system. The specific indicators and their names may be different from one model of DTS to another. Interpreting these indicators properly provides information about the data flow in the system. At a minimum, each DTS monitors the following functions and indicates their condition:

l Transmit - The Transmit indicator lights up every time the DTS transmits. The interpretation of the Transmit indicator depends on whether the unit is operating as a picket or as NCS. In the NCS mode, the Transmit indicator lights up every time a PU is polled.

The only action required to start this indicator blinking is pressing the Transmit Initiate switch. In the picket mode the Transmit indicator indicates that the radio receiver is properly patched and is providing at least a minimum quality signal to the DTS, that the NCS is polling ownship's address, and that the DTS is processing and recognizing this address. This indicator does not provide any information about data flow between the DTS and the NTDS computer, nor does it provide any information about signal flow between the DTS and the radio, nor about the quality of the transmitted signal.

l Receive - The Receive indicator lights up every time the DTS recognizes a start code. It turns off when the subsequent stop code is recognized, indicating that the radio and DTS can provide this minimum quality of signal for the duration of an entire message.

l Net Busy - The Net Busy indicator lights up whenever the DTS receives audio input from the radio. Because this includes noise or static as well as the Link 11 signal, the only thing you can infer from this indicator is that the DTS is patched to a radio receiver.

l Transmit Data Error - The Transmit Data Error indicator lights up whenever a bit error is detected by the EDAC function of the DTS on the message data portion of the transmitted signal. This signal is monitored as a sidetone on the receive line. If no sidetone is returned to the DTS, for example, this indicator will light up every time the unit transmits.

A Transmit Data Error indicates a problem with ownship's signal pathway or equipment. The source of the problem may be on either the transmit side or the receive side of the system.

l Receive Data Error - The Receive Data Error indicator lights up when a bit error is detected by the EDAC function of the DTS on the message data portion of the signal received from another unit. This indicates a problem somewhere in the communications pathway between the transmitting unit's DTS and the receiving unit's DTS. Assuming that the audio signal generated by the transmitting unit was initially error free, the problem could be in the cabling to the transmitter, the transmitter itself, the transmitting antenna, the medium, the receiving antenna, the receiver, the cabling to the receiving DTS, or any combination of these locations.

l Code Error - The Code Error indicator lights when bit errors are detected in the control codes (start, stop, and address codes) of either a transmitted signal or a received signal. A lighted indicator means that a code was successfully processed even though it contained bit errors. There is no indicator light, however, for the more serious problem of a signal so poor that a code could not be recognized at all.

l Sync Complete - The Sync Complete indicator lights to indicate that the DTS is successfully receiving, recognizing, and synchronizing on the Net Sync signal (in Net Sync mode). Operation during roll call varies from model to model of DTS.


Net Cycle Time

Net Cycle Time (NCT) is a parameter calculated by the NTDS computer program of each unit as part of the link monitoring capability. It measures the average time between reporting opportunities, as measured by that unit. By comparing the NCTs measured by each PU, it can be

determined if one unit is being polled more often than other units, or if one unit is consistently missing call-ups.


Reception Quality

Reception Quality, or RQ, is a numeric value measuring the ability of every ship to receive every other ship. It is calculated by the NTDS system aboard every ship during normal roll call operations. It provides a means of diagnosing transmission and reception problems during the operation of the link. Every unit's data report contains a sequence number. Every NTDS follows this sequence for each PU and then assigns a grade. This grade depends on the answer to these questions: (1) Have any transmissions been missed? and (2) Did any data contain bit errors? If all PU transmissions have been received, and if the percentage of frames with bit errors is low, the highest grade, an RQ of seven, is assigned to that PU. An RQ value less than seven indicates that responses from a PU are missing, and/or message data contains bit errors. Table 4.3-4 illustrates the approximate correspondence between RQ and percentage of error-free message frames. This illustration assumes that every transmission is received. It does not take into account that an error in one frame causes two frames of information to be lost.

The RQ value for a PU is updated each time a transmission is received. If no transmission is received (for example, if the PU suddenly goes radio silent or the crypto becomes alarmed), the previously held value of RQ will remain in effect. Because the old RQ value is held through the period of unreceived transmissions, an RQ value does not necessarily, by itself, provide an

accurate indication of the quality of the link. RQ values must be carefully interpreted.





7 None 93 - 100 %

6 None 79 - 92 %

5 None 65 - 78 %

4 None 50 - 64 %

3 None 36 - 49 %

2 None 22 - 35 %

1 None 7 - 21 %


Table 4.3-4 Reception Quality (RQ) Value


A useful tool for using RQ to evaluate a link is the RQ matrix. This matrix will indicate what PU is held and the RQ. In Table 4.3-5, PU 6 holds PU 13 with an RQ of 2; PU 17 with a RQ of 7; PU 43 with on RQ of 7; and PU 61 with an RQ of 7.



PU 06 13 17 43 61


06 X 2 7 7 7

13 7 X 7 7 7

17 7 2 X 7 7

43 7 2 7 X 7

61 5 2 3 5 X


Table 4.3-5 RQ Matrix

PU 6 is receiving PU 13 with a RQ of 2. In fact, all units are receiving PU 13 poorly. But PU 13 is receiving all stations well. This may indicate that PU 13 has a poor transmitting system. Any equipment along the transmission data path is suspect: antenna, antenna coupler, radio, or DTS. On the other hand, PU 13's low RQ may indicate that the NCS is not receiving PU 13 and is polling on top of PU 13's responses, affecting data for all units. PU 61 is receiving all units poorly. All units are receiving PU 61's data well. This could indicate that PU 61 has a poor receiver.

Table 4.3-6 and Figure 4.3-6 illustrates how RQ values can be used to troubleshoot and manage the net. In this example, the pathway between PUs 30 and 43 is bad in both directions, which may indicate a problem with the RF medium range or extreme range. A change of frequency may help in this case. Shifting NCS to a geographically more central unit may also be a solution. Note that PU 56 appears to have difficulty in receiving all units, indicating a poor receiver.


Table 4.3-6 RQ Values


PU 30 43 56 62


30 X 2 6 7

43 3 X 7 7

56 2 4 X 3

62 6 7 7 X


























Figure 4.3-6 Evaluating Communication Pathways with RQ Matrix


AN/TSQ-162(V) Monitor System

The AN/TSQ-162(V), or Link 11 Monitor System (LMS-11), is a diagnostic tool designed to troubleshoot the analog portion of the Link 11 system, measuring both the performance of the net as a whole and the signal characteristics of each PU. There is at least one LMS-11 in every battle group. The LMS-11 provides Link 11 system operators with the ability to measure and analyze the quality of link communications in real time. By decoding a received Link 11 signal in much the same way a DTS does, the LMS-11 analyzes individual characteristics of the composite audio signal with respect to the power and phase of the frequency spectrum. It then calculates measurements of the overall signal power, signal-to-noise ratio (SNR), and Doppler shift. In addition, the LMS-11 measures mean and standard deviation of detected phase errors, counts the total number of frames transmitted and the number with bit errors, and calculates the bit error rate (BER). Bit errors can be caused by noise, simultaneous transmissions, uncorrected frequency errors, or tone attenuation. By viewing real-time displays and summaries of numerical data, the operator can use results from analysis and measurements performed by the LMS-11 to quickly detect and diagnose problems affecting the quality of communication in the overall net.

At the net level, operators can view the polling sequence as it occurs and observe which PUs are being polled and which PUs are responding. They can determine immediately whether a response contains message data or is empty. Operators can observe the net cycle time and the percentage of data being exchanged. With a single keystroke, an operator can change the focal point of the data displayed from the net level to the PU level. At the PU level, operators can identify individual units experiencing problems and can often isolate the cause(s) to one or more pieces of equipment. If necessary, the operator can also step through a PU's transmission and view the power and phase difference angles for each tone, on each sideband, one frame at a time. At the frame level, operators can look at the decoded value, the number of bit errors, and the SNR, as well as evaluating the individual tones. They can identify the effects of the NCS polling a second time on top of a PU response. A missed stop code can be confirmed.

Human error in setting up and operating Link 11 equipment, deviations from standard net protocol, and even some hardware anomalies can be identified quickly and without guesswork with the LMS-11.


Test Equipment

Once a problem has been recognized, an variety of test equipment is available for troubleshooting it to the component level. Power meters, spectrum analyzers, and oscilloscopes can be used to check out radios and patch panels. Computers rely on diagnostic programs. Some components of the Link 11 system have a Built-In-Test Evaluation (BITE) function that verifies their performance. Whenever that component is suspected of failing, its BITE can be executed to test it. The AN/USQ-74 DTS, for example, has five levels of maintenance tests that can isolate component failures. The AN/USQ-36 has a self-check function, which does not, however, check the I/O function. Technicians must be familiar with these tests so that they know precisely what is checked and what is not.

In addition to equipment self-tests, several system tests are available. One test is the Programmed Operational and Functional Analysis (POFA), a shipboard program to identify the number of link errors. Others include the Link 11 Audio Signal Simulator (LASS) and the "Quicklook" provided by the Multiple Units Link 11 Test and Operational Training System (MULTOTS) at Navy Center for Tactical System Interoperablilty (NCTSI) Detachments.


Programmed Operational and Functional Analysis (POFA)

The POFA test requires that a special diagnostic program be loaded into the NTDS computer. The test can execute in either the single-station mode or the multistation mode. A special switch setting on the TSEC/KG-40 is required for performing POFA tests. The single-station POFA is basically a loop-back test that circulates known data words from the NTDS computer, through the DTS, and back again to the NTDS computer. Single-station POFA can be run with or without the radio. It does not, however, check the operation of the entire system under dynamic conditions. See Figure 4.3-7.

Performing a multistation POFA involves a multiple number of ships. It tests more equipment than the single-station POFA. Known data words are generated from the NTDS computer on one ship, and are sent through the DTS and up and out through the radio to one or more other ships. See Figure 4.3-8 for multistation POFA data flow. Each receiving ship compares this data with a pattern known to its NTDS computer, counts the words in error, and transmits the count back to the original ship. Multistation POFA more closely approximates dynamic testing than any other diagnostic. Ideally, the test should be error free. Because the signal is transmitted on the air, however, several attempts may be required before an error free multistation POFA is achieved.






















Figure 4.3-7 Single Station POFA




























Figure 4.3-8 Multistation POFA


Link 11 Audio Signal Simulator (LASS)

The LASS is a portable unit that generates calibrated Link 11 audio signals. The parameters of these signals are specified in preprogrammed cartridges inserted into the LASS to simulate Link 11 transmissions. These cartridges allow the user to select individual audio parameters representative of a Link 11 transmitted signal for testing. The LASS can test output amplitude, phase shifts, bit error handling, Doppler shifts, signal-to-noise ratios, side band select and diversity processing.

LASS parameters which can be varied include the number of preamble frames, the signal-to-noise ratio, Doppler shift, control code bit errors, tone amplitude errors, and phase errors. Independent signals can be generated for each sideband. This known, calibrated signal can be inserted at any point in the audio pathway (e.g., as in Figure 4.3-9) and the results viewed on the NTDS display, or with the LMS-11. The LASS signal can be used to measure line losses, verify patch signal problems, or test the receive capability of the DTS. Since LASS also provides a radio keyline, the signal can be transmitted to another unit and used to test that unit's entire receive pathway, from the antenna to the NTDS computer.

























Figure 4.3-9 LASS Signal Insertion


MULTOTS Quicklook

MULTOTS Quicklook (Multiple Units Link 11 Test and Operational Training System) is a brief test of a unit's Link 11 system. It includes LMS-11 testing of the unit's hardware functions and selected MULTOTS testing of NTDS software functions as targeted by the unit. After the Quicklook is completed, a debriefing is provided, usually over the radio coordination circuit. A Services Summary Report is mailed to the unit as a follow-up to this debriefing. Quicklook services, technical assistance, and training are available from NCTSI (Navy Center for Tactical System Interoperablilty Detachments) at the following locations:

l DET 1 San Diego, CA

l DET 2 Norfolk, VA

l DET 3 Long Beach, CA

l DET 4 Signonella, Italy

l DET 6 Mayport, FL/Charleston, SC



Training in equipment maintenance, repair, and alignment techniques is provided by the Link 11 Technical Enhancement Training (TET) Program. This training is performed by the TET team aboard ship, using the ship's equipment. The TET team also evaluates how receptive ship's

technical personnel are to the training, and documents the calibration, usability, and availability of shipboard test equipment and technical documentation. Other resources for training include, but are not limited to, Combat Systems Training Group (CSTG), Fleet Technical Support Centers (FTSC), and Fleet Training Units. There are also two waterfront seminars available: one for operator/technicians and one for net managers. Each seminar lasts one day.



A successful team is the product of sound management policy and practices. In Link 11, that manager is called the Net Coordinator. In areas of Link 11 net management, the Net Coordinator is responsible to the Force Track Coordinator. Net management is the activity of planning, monitoring, and adjusting assignments, functions, parameters, and participation within the net. The goal of net management is to provide the connectivity necessary for implementing battle tactics. Net management, therefore, is a coordinated activity that starts before a net is activated and continues until after the net is terminated. This section concentrates on establishing and maintaining good link communications.


The NCS is the central controller for the Link 11 net. No rank or authority is associated with this function. Communication with the NCS is of primary importance. If a unit fails to recognize its own address, it will never transmit. If the NCS fails to recognize the unit's start code, it will jam the unit's response with a second call-up. The degree of communication among units is called connectivity. Perfect connectivity is where all units are exchanging tactical data accurately and completely. Connectivity can degrade as a result of equipment performance, RF propagation characteristics, and range. Selecting the unit to act as NCS is one of the most important decisions to be made in managing the net. Two principal features should determine the assignment of the NCS: equipment and location. The NCS should have the best operating Link 11 system and should be in the optimum location to remain in communication with all other units.



The manager of the net must know the material readiness of all Link 11 equipment units in the net before designating the NCS. An NCS assigned to a unit with a degraded system, such as a radio receiver with low sensitivity, could degrade the performance of the entire net. For example, suppose the net manager designates the "weak unit" as NCS. The "weak" NCS will then immediately begin missing valid PU responses to calls. The weak NCS will poll a PU a second time, fifteen frames later, while it is still on the air with the initial response. By reinterrogating the PU while it is still responding to the initial call, NCS both continues to miss the PU's response and prevents the other PUs in the net from hearing the response. This causes the entire net to be jammed. To a large extent then, NTDS front-end performance (antenna, coupler, transceiver, and DTS) ultimately dictates overall net performance.

The NCTSI detachments can replicate a battle group link scenario in interactive mode. In addition to NTDS program checkout, they can perform special operational readiness tests on the NTDS transmit and receive capabilities. Testing all possible combinations of equipment, not just

the transmitter and receiver known to be the best performers, is recommended. Subsequent test debriefings will give the net manager a feel for assets and liabilities. The FTSCs can assist ship's force in resolving problems that are beyond the scope of routine maintenance.



The NCS should be located in a position that allows it to receive each unit in the net by direct RF communication. The HF surface and air ranges are about 300 miles. A surface range for UHF is about 25 miles. For surface-to-air the UHF range can be extended to 150 miles. The use of an AEW platform with a UHF relay capability (Auto Cat) can be used to extend the UHF surface range.



The following factors affect the selection and usability of frequencies for Link 11:

l Limited number of available Link 11 designated frequencies

l 2-6 MHz band congestion

l Day/night radio propagation characteristics

l Solar flare and sunspot activity

l Self-inflicted radio frequency interference and its effect on other shipboard tactical circuits

l Aircraft limitations at frequencies below 4 MHZ


Frequencies with specific types of emissions characteristics are allocated to specific users. Ideally, no other user would ever use a frequency assigned to your battle group. In reality, radio frequencies are often infringed on by unauthorized users, especially in areas outside of the continental U.S. For that reason, a Net Coordinator may opt to stay on a certain HF frequency for weeks on end to ensure its continued availability.

Because the 2-6 MHz band offers the advantage of maximum groundwave coverage, there is often severe frequency congestion in this band. The best approach when seeking a clear HF frequency is to have the Communications Department personnel monitor available frequencies and make recommendations. These personnel will have the Communications Plan and will know what frequencies are available for Link 11 use. Additionally, the communications personnel will know what frequencies are currently in use and will be able to ensure correct separation of active frequencies. The Net Coordinator should require all track supervisors to monitor the link actively by listening to it. Monotonous as this is, link performance is directly related to frequency quality. A seasoned operator listening to the audio, can recognize frequency degradation almost immediately.

Frequency selection can also be dictated by the time of day. High frequencies during the day, lower frequencies at night. The ionosphere tends to disperse and move higher at night. A lower frequency will give you a better "bounce" at night due to the diminished ionosphere. Ship dispersement will also affect your selection of frequency. If all ships are within 25 miles of NCS, a 2-6 MHz frequency at noon may work just fine, or a shift to UHF may even be advisable.



One measure of Net Cycle Time (NCT) is the time required for NCS to complete a polling sequence of all PUs. Another measure of NCT is the average time between PU reporting opportunities. This is the PU NCT. It is calculated and reported by each PU in the net. The value measured by one PU may be different from that measured by other PUs, as well as being different from the NCT of the entire net. A PU's calculation of NCT will agree with that calculated by other units in cases where each unit is addressed only once during the cycle. If a unit is placed in the polling sequence twice per cycle, the calculation of net cycle time will be approximately half that of the value reported by other units, and significantly lower than the actual time required to poll all units. Factors affecting NCT include:

l The number of PUs polled

l The number of PUs that reply to the initial call

l The number of PUs that do not reply to either call

l The amount of data that each PU is transmitting


To allow timely responses to orders and keep display information accurate, each PU must transmit as often as possible. The frequency of a PU's transmission is determined by the NCT. Reducing the NCT allows more frequent transmission opportunities for each PU in the net.

Only the portion of NCT affected by the number of PU addresses polled and amount of data reported by each unit can be reduced. The remainder of the NCT is consumed in overhead, such as preambles, phase reference frames, and control codes. Because they administer net functions, these cannot be altered. NCT can be minimized by ensuring that all PUs respond to their first call-up. If necessary, any PUs that do not respond to their first call can be removed from polling. Thereafter, NCT can be reduced only by reducing the number of PUs called by NCS, or by limiting the quantity of data exchanged. A PU need not be called to receive net data.



The goal of net management is to provide the connectivity necessary for implementing battle tactics. The net manager must translate these tactics into operational requirements for the net.

The net manager controls which units transmit in a net, the frequency of the net access for each unit, the degree of efficiency required, and the amount of reporting reserve that should be maintained. The importance of one unit's data must be weighed against the inefficiency and extended net cycle time caused by its intermittent response pattern. The net manager can choose

to poll a unit that is currently in radio silence, knowing that a .6 second price is paid for providing the opportunity for a response. Net managers perform a quick trade-off analysis of the various actions they can take in a given situation versus their consequences and benefits.