Since any days I'm following transmissions on 14693.0 KHz/USB consisting of exchanges of messages between 2 nodes, as the different fading patterns in Figure 1 suggest.
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| Fig. 1 |
Messages are sent utilizing OFDM modulation occupyng a 2400 Hz bandwidth and consisting of 28 tones with a frequency spacing of ~86 Hz, each speech is modulated utilizing PSK2 at the symbol rate of 62.6 Bd (Fig. 2). The same results are obtained/verified by analyzing a single channel as shown in Fig. 3 (lowest tone).
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| Fig. 2 - OFDM parameters |
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| Fig. 3 - single speech (yhe lowest) analysis |
The parameters resulting from the analysis are very akin to those of the Skysweep Technologies proprietary "SkyOFDM" waveforms household (Table I). Quoting the SkySweeper mention Manual #82.2 General Description: "SkyOFDM is simply a state of art advanced velocity modem based on the OFDM and turbo coding technologies. It offers respective baud rates (300 -9600 bps) and 2 different interleaving options (short and long). besides there are 2 bandwidth options: 2.0 (OFDM-22) and 2.4 kHz (OFDM-28)".
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| Table I |
Note the different number, position and duration of the header tones compared to the values of the "original" waveform: this is most likely an improved version of the erstwhile SkyOFDM waveforms (Figure 4).
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| Fig. 4 |
The signal has an ACF value of about 957.8 ms which identifies a super-frame composed of 11 frames, the second with an ACF value of about 79.8 ms (Figure 5).
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| Fig. 5 - ACF values |
Direction uncovering tests utilizing TDoA algorithm (Figure 6) indicate an area north of Helsinki as the site of the transmitter (or rather, the radiating antenna): this makes sense because, acccording to any DXers, SkyOFDM waveforms were/are utilized by Finnish MFA and SkySweep Technologies was a Finnish advanced tech company. By the way, although there are inactive many references in the web to SkySweep, their authoritative website is no longer online since SkySweeper software was discontinued on June 1st 2009.
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| Fig. 6 - Direction Finding |
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