Technical informations - Systems

ANNEX 10 Selcal

ANNEX 10 Aeronautical Regulations 4.8 SELCAL system RED A..S.
Questo sistema è utilizzato come supporto alle comunicazioni aeronautiche in HF. Per inviare il SELCAL viene impiegato il sistema H2B (es. USB a portante completa). I toni trasmessi hanno una tolleranza dello 0,15% cioè 2,25 Hz al centro del tono. La frequenza della portante della stazione di terra, ha una tolleranza di 10 Hz. I filtri impiegati in questo modulo di decodifica hanno una larghezza di banda di 3 Hz. La demodulazione di questi segnali nel modo SSB, non consente di avere la precisione richiesta per una corretta misurazione dei toni. Questi segnali devono essere demodulati o in modo AM, o in AM sincrona (in modo SSB) ma che abbia una velocità tale da consentire la sincronizzazione entro i 200 msc.

ANNEX 10 Aeronautical Regulations 4.8 SELCAL system RED A..S.

This module complies with ANNEX 10 of the Aeronautical Regulations 4.8 SELCAL system RED with all letters currently defined as A to S fully decoded.
When a SELCAL is transmitted, Aeronautical Ground stations use the H2B emission. ( i.e. USB with full carrier). According to the specification, the two pairs of tones transmitted have a quoted maximum decode tolerance of 0.15 % (i.e. 2.25 Hz at the top tone). Aeronautical Ground station carrier frequencies are supposed to have a frequency tolerance of 10 Hz maximum. Therefore, to comply fully with ANNEX10, the filters used in this decoding module have a bandwidth of 3 Hz.
Because of the above tight tolerances in ANNEX10, the demodulation of these signals in SSB mode does not give the accuracy in frequency needed for correct assessment of the tones. These signals must be demodulated either in the AM mode or using a carrier synchronisation facility in the SSB mode fast enough to synchronize within 200 msec.
This system will detect all tones-sequences complying with the above mentioned regulations and a SELCAL repeated immediately may be detected incorrectly. For example the sequence transmitted "ABCD ABCD" may be occasionally decoded as:

"ABCD CDAB ABCD"

Fortunately, because of the time-stamp added to each decoded sequence, it will be possible to identify the occasional occurrence of this situation.