Some days ago my friend KarapuZ pointed my attention on a FSK transmission running on 8582.0 KHz/USB and that, at a first glance, appeared a bit uncommon. Once analyzed and decoded it was identified as PWZ-33 ERMRJ (Estação Rádio da Marinha no Rio de Janeiro) belonging to Brazilian Navy and operating in Pactor-FEC at 100Bd/200: just another proof of the "Occam's razor" (simpler theories are preferable to more complex ones).
Given the time we spent on signal analysis and the differences between Pactor-FEC modes, maybe is worth to publish a short post about it.
Given the time we spent on signal analysis and the differences between Pactor-FEC modes, maybe is worth to publish a short post about it.
Pactor-FEC is a synchronous simplex system based on Pactor and used for broadcast transmissions, ie it has no acknowledge return channel and the receiving stations perform error correction. The Pactor-FEC modem uses a FSK 200Hz shift waveform and operates adaptively so the baud rate can be either 100 or 200 Baud: during daylight time the speed of 200 Baud may be successfully used, while in night time, due to the propagation distortions, the speed may necessitate a reduction to 100 Baud.
The speed influences the period lenght of Pactor-FEC and due to the positive/negative coding, the BEE software is a bit confused and computes periods lengths as the double of the real ones and shows seemingly equal period lengths in both the cases (Figure 1):
200Bd speed: frame length 194 bits (period: 388 = 194+194)
100Bd speed: frame length 97 bits (period: 194 = 97+97)
200Bd speed: frame length 194 bits (period: 388 = 194+194)
100Bd speed: frame length 97 bits (period: 194 = 97+97)
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| Fig. 1 |
Indeed, altough Pactor-FEC frames consist of the same fields (header, data, status and 16 bit CRC calculated over the entire frame except the header) their lengths differ. As per above: at speed of 100 Baud the data field is 64 bits (8 bytes), while at 200 Baud the data field increases to 160 bits (20 bytes) as shown in Figs. 2,3.
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| Fig. 2 - Pactor-FEC 100Bd/200 frames |
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| Fig. 3 - Pactor-FEC 200Bd/200 frames |
To increase reliability data are transmitted twice (in positive/negative), as shown by a decoding of a short fragment in Fig. 4
In contrast to Pactor, all data blocks are in consecutive order with no or little space between them: indeed, Pactor 200Bd has 250-bit length frames (Fig. 5).
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| Fig. 4 |
In contrast to Pactor, all data blocks are in consecutive order with no or little space between them: indeed, Pactor 200Bd has 250-bit length frames (Fig. 5).
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| Fig. 5 - Pactor 200Bd Vs Pactor-FEC 200Bd |
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