Looking at the spectrum of the receivable signals around 7 MHz in the UKR skyes, as well as numerous and very frequent STANAG-4538 and L3Harris WHARQ waveforms, it may happen that we observe transmissions in frequency hopping mode (FHSS, or Frequency Hopping Spread-Spectrum) as shown in Figure 1.
Frequency hopping (also known as ECCM, Electronic counter-countermeasures) is the most commonly used Transmission Security (TRANSEC) technique. The frequency hopping capability provides advanced anti jam protection for communications. In HOP radio mode, the transmitter frequency changes so rapidly that it is difficult to intercept or jam the signal. For additional security, hopping data and digital voice data can be encrypted.
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| Fig.1 - FHSS transmission |
Me and my friend ANgazu from radiofrecuencias.es had the chance the analyze these signals and share the results. We observed transmissions which use 26 or 27 channels and occupy a bandwidth of 81 KHz, since each channel is 3 KHz wide (2700 + 300 Hz separation). Hopping rate is 8.88 sps with an hop time of ~112.5 ms (say 102 ms ON, 10.5 ms OFF).
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| Fig. 2 - FHSS single channel frequency occupation |
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| Fig. 3 - FHSS timing |
Like a single-channel serial tone waveform, the modulation used is 2400 Bd PSK8 for both voice and data (Fig. 4).
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| Fig. 4 - FHSS modulation |
This waveform is fielded in AN/PRC-150(C) radios by L3Harris. Wideband hopping covers a frequency band that is bounded by a lower and upper frequency specified in multiples of 100 Hz, frequency exclusion bands may also be programmed. AN/PRC-150 narrow band hopping uses frequencies within a defined bandwidth of the center frequency (Fc) as in the Table below: notice the reported 81 KHz bandwidth in case of 3.5 MHz <= Fc < 9.995 MHz.
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| Table 3.16 - L3Harris AN/PRC-150 operation manual |
An important aspect of hopping is synchronization, ie all radios in a net shall use the same frequency at the same time intervall: that alignment may be accomplished with the use of GPS, but is some in cases (very very rare) it uses the manual 3x4 sync sequences as shown in Figure 5.
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| Fig. 5 - 3x4 sync sequences |
If our guess is correct, we can assume a large employ of L3Harris equipmente in that (war) theatre.
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