Once again the resulting signal is referred to as

This preview shows 3 out of 5 pages.

Once again, the resulting signal is referred to as single sideband (SSB), but in this case we are using only the upper sideband (USB). Here again, the overall bandwidth has been reduced to around 3.5 kHz. Note that aircraft HF communication requires the use of the upper sideband (USB). DSB AM may also be available but is now very rarely used due to the superior performance offered by SSB.
Image of page 3

Subscribe to view the full document.

76 Aircraft communications and navigation systems Character Frequency A 312.6 Hz B 346.7 Hz C 384.6 Hz D 426.6 Hz E 473.2 Hz F 524.8 Hz G 582.1 Hz H 645.7 Hz J 716.1 Hz K 794.3 Hz L 881.0 Hz M 977.2 Hz P 1083.9 Hz Q 1202.3 Hz R 1333.5 Hz S 1479.1 Hz 5.3 SELCAL Selective calling (SELCAL) reduces the burden on the flight crew by alerting them to the need to respond to incoming messages. SELCAL is available at HF and VHF but the system is more used on HF. This is partly due to the intermittent nature of voice communications on long oceanic routes and partly due to the fact that squelch systems are more difficult to operate when using SSB because there is no transmitted carrier to indicate that a signal is present on the channel. The aircraft SELCAL system is defined in Annex 10 to the Convention on International Civil Aviation (ICAO), Volume 1, 4th edition of 1985 (amended 1987). The system involves the transmission of a short burst of audio tones. Each transmitted code comprises two consecutive tone pulses, with each pulse containing two simultaneously transmitted tones. The pulses are of 1 second duration separated by an interval of about 0.2 seconds. To ensure proper operation of the SELCAL decoder, the frequency of the transmitted tones must be held to an accuracy of better than ± 0.15%. SELCAL codes are uniquely allocated to particular aircraft by Air Traffic Control (ATC). As an example, a typical transmitted SELCAL code might consist of a 1 second burst of 312.6 Hz and 977.2 Hz followed by a pause of about 0.2 seconds and a further 1 second burst of tone comprising 346.7 Hz and 977.2 Hz. Table 5.1 indicates that the corresponding transmitted SELCAL code is ‘AM-BM’ and only the aircraft with this code would then be alerted to the need to respond to an incoming message. The RF signal transmitted by the ground radio station should contain (within 3 dB) equal amounts of the two modulating tones and the combination of tones should result in a modulation envelope having a nominal modulation percentage as high as possible (and in no case less than 60%). The transmitted tones are made up from combinations of the tones listed in Table 5.1. Note that the tones have been chosen so that they are not harmonically related (thus avoiding possible confusion within the SELCAL decoder when harmonics of the original tone frequencies might be present in the demodulated waveform). Table 5.1 SELCAL tone frequencies 5.4 HF data link ARINC’s global high frequency data link (HFDL) coverage provides a highly cost-effective data link capability for carriers on remote oceanic routes, as well as the polar routes at high latitudes where SATCOM coverage is unavailable. HFDL is lower in cost than SATCOM and many carriers are using HFDL instead of satellite services, or as a backup system. HFDL is still the only data link
Image of page 4
Image of page 5
You've reached the end of this preview.
  • Spring '10
  • MrLEE
  • Frequency, ........., Single-sideband modulation, HF radio

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern