SUMMARY—THE LINK-11 SYSTEM

Figure 2-19 shows the four possible phase shifts. A sixteenth tone, the 605-Hz Doppler correction tone, is added to the tone package. The Doppler tone is not phase modulated and is used to correct for Doppler shifts caused by the relative motion between the transmitting station and the receiving station. The 16 tones are combined into a composite audio signal and sent to the radio set. The radio set transmits the composite tone package on the carrier frequency in independent sideband form. During receive operations, the tone package is received from the radio set. The 30 bits of data are extracted from the tone package by determining the phase shift of each data tone with respect to the previous frame. The 30 bits, which contain 24-data bits and six-EDAC bits, are examined for errors. The six-EDAC bits allow for the detection of errors and provide enough redundancy to allow for correcting a single bit error. The operator can select whether or not the DTS attempts to correct detected errors, as explained earlier in this chapter. In the error detect (label) mode, a detected error is identified and labeled before it is sent to the CDS computer. In the error correction (correct) mode, the DTS attempts to correct a detected error, labels the error, and sends the data word to the CDS computer. The DTS is capable of receiving and processing both the upper sideband and the lower sideband when using a HF radio, depending on the position of the sideband select switch. When you are using a UHF radio, only the upper sideband is received and processed. If the sideband select switch is in the USB or the LSB position, only the designated sideband is processed. In the diversity (DIV) mode, the 30-bit word is generated by adding the relative phase angles of the USB and the LSB. Because of propagation anomalies, noise, and interference, the AUTO mode can be used to select the sideband (USB, LSB or DIV) that yields the most correct data automatically. In the AUTO mode the DTS processes a word from each sideband and the diversity combination. The decoded words are examined for errors in the following order or priority: DIV, USB, and LSB. A search of the three words is made to find a data word with no error. If one is found, it is selected for input to the CDS computer. If none is found, the RCV DATA ERR indicator is lit and the diversity combination data word is sent to the CDS computer. Radio Set Interface The DTS generates the following outputs to the radio set: upper sideband composite audio, lower sideband composite audio, and key line. It receives upper sideband composite audio and/or lower sideband composite audio. UHF radio sets use only the upper sideband signal and the key-line signal. The key-line signal controls the transmit and receive state of the radio set. The key line is set to transmit Link-11 data. When the key-line is cleared, the radio set returns to the receive mode. SUMMARY—THE LINK-11 SYSTEM This chapter has introduced you to the Link-11, or TADIL A, system. The following information summarizes important points you should have learned. LINK-11 SYSTEM— Link-11 is used to transmit REAL-TIME tactical information between CDS- equipped ships and similarly equipped ships, aircraft, and shore stations. The typical shipboard configuration of Link-11 hardware consists of the following: CDS computer Shipboard Gridlock System Cryptographic security device Data terminal set Communications switchboard HF or UHF radio set 2-19