1 September 2017

PWZ-33 Bazilian Navy and Pactor-FEC frame lengths


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.

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) 

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. 

Fig. 2 - Pactor-FEC 100Bd/200 frames
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

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).

Fig. 5 - Pactor 200Bd Vs Pactor-FEC 200Bd