135-HZ BEARING INFORMATION

Figure 2-5.—TACAN radiation pattern: A. cardioid pattern; B. Ampltitude-modulated pulse pairs. The aircraft TACAN equipment obtains bearing information by comparing the 15-Hz modulated signal with a 15-Hz reference burst signal it receives from the ground facility. The phase relationship between the 15-Hz modulated signal and the 15-Hz reference burst signal depends on the location of the aircraft in the cardioid pattern. The 15-Hz reference burst signals are transmitted when the maximum signal of the cardioid pattern aims due East. This group of 12 pulse pairs is commonly referred to as the North or main reference burst. You can see the relationship between the reference pulses and the cardioid pattern by comparing view A and view B of figure 2-5. 135-HZ BEARING INFORMATION Errors arising from imperfections in the phase measuring circuits and radio propagation effects are known as site error. These errors are significantly reduced by the addition of 32 outer parasitic elements added to the electronically scanned antenna. (See the section on the OE-273(V)/URN antenna group). Electronically switching these elements modifies the antenna cardioid pattern. Though the cardioid pattern is still predominant, it is altered by superimposed ripples. The aircraft now receives the 15-Hz signal with a 135-Hz ripple amplitude modulated on the distance data pulses (figure 2-6). To furnish a suitable reference for measuring the phase of the 135-Hz component of the envelope wave, the transponder is designed to transmit a coded 135- Hz reference burst similar to that explained for the 15- Hz reference. The 135-Hz reference group is commonly referred to as the auxiliary or aux reference b u r s t . The composite TACAN signal is composed of 2700 interrogation replies and noise pulse pairs-per-second, plus 180 North burst pulse pairs-per-second, 720 auxiliary burst pulse pairs-per- second, for a total of 3600 pulse pairs-per-second, or 7200 pulses-per-second. TACAN SIGNAL PRIORITIES Priorities have been established for transmission of the various types of TACAN signals. These priorities are as 1. 2. 3. 4. follows: Reference bursts (North and auxiliary) Identification group Replies to interrogations Squitter Therefore, the identification group, replies, or squitter will be momentarily interrupted for the transmission of either the main or auxiliary reference group. The transmission of replies or squitter will be interrupted every 37.5 seconds during the transmission of an identification code dot or dash. CHARACTERISTICS OF RADIO BEACON SIGNALS Depending on what channel (X or Y) the TACAN is on, the number of pulses-per-second and the pulse 2-4