# “56k modem” found in VHF band

While scanning the VHF spectrum with a SDR USB dongle, I found something that sounded like an old 56k modem (at around 160MHz, FM modulation, Europe region).

After some research, I found out that is relatively common to broadcast digital messages and warnings with sound tones (ACARS)... so I decided to try to demodulate the signal.

With the help of minimodem, and after some trial and error baud rate estimation (1200 baud, ASCII, 8N1), I finally got some text out of those recordings:

CAL/I0M039D042#EXAN:09158700403:250/END
CAL/I0M042D039#OKEY:151/END

CAL/I0M055D053#EXAN:15171500380:244/END
CAL/I0M041D039#EXAN:10139400412:237/END
CAL/I0M039D041#OKEY:150/END


Anyone knows something about these messages?

Wheater forecast? Airport broadcast? Thank you.

UPDATE:

I collected the signal for 24h, in order to determine if there was any kind of correlation with the weather (like temperature vs hour etc.). Afterward, I plotted the number following the EXAN keyword in the received string (here in bold: CAL/I0M055D053#EXAN:15171500380:244/END).

Here 4 of the resulting graphs (x-axis represent the sample number, not time). Judge by yourself:

As you can see, every signal seems to behave differently from the others, without any apparent correlation with wheater (pressure, temperature...).

I would exclude any boat tracking system because I live in a mountain valley, at least 80km from the sea. Moreover, the source does not seem to move and the packets signatures are recurrent day after day (at least for the moment).

PS: Thank you all for the interest, please feel free to move the question if inappropriate for "Electrical Engineering".

• might fit better on ham.stackexchange.com . Maybe you'd want to move this question there? – Marcus Müller Apr 22 '17 at 17:50
• @MarcusMüller this is outside a ham band. There might be some applicable knowledge, but it's not strictly a ham radio topic, especially as the question is about the meaning of the data - the radio encoding already being solved. – Chris Stratton Apr 22 '17 at 18:08
• Then it might be a better fit for reverse engineering se? Not really an electronics design question – PlasmaHH Apr 22 '17 at 18:12
• I would start looking here: sigidwiki.com for a pager protocol – PlasmaHH Apr 22 '17 at 18:19
• This question may end up being closed or migrated, but it is very interesting. – mkeith Apr 22 '17 at 18:26

160MHz lies in the Marine VHF Radio Band.

I guess the signal you received is for the marine vessel tracking system called AIS (Automatic identification system). In section Technical Specification of the Wikipedia Article you find that it uses 2 channels at 161.975MHz and 162.025MHz and carries a 1200baud digital signal which fits to the details you mentioned.

EDIT:
The signal you received is probably a DSC (Digital Selective Calling) transmission used in Marine VHF Communication (It is used to selectively call one particular maritime radio station).

See ITU-R M.493-12 Specifcation:

1.3.2 Frequency modulation with a pre-emphasis of 6 dB/octave (phase modulation) with frequency-shift of the modulating sub-carrier for use on VHF channels:

• frequency-shift between 1 300 and 2 100 Hz; the sub-carrier being at 1 700 Hz
• the frequency tolerance of the 1 300 and 2 100 Hz tones is ± 10 Hz; the modulation rate is 1 200 Bd;
• the index of modulation is 2.0 ± 10%.
• The details at your link do not seem to match those given in the question or even in your post, so while it is likely for a similar sort of system, it would not appear to be AIS. – Chris Stratton Apr 27 '17 at 20:25
• @Chris Stratton: What do you mean? 161.875MHz or 162.025MHz is "around 160MHz". AIS uses GMSK which is a special kind of FM. 1200 baud is exactly 1200 baud. So which details "do not" match??? – Curd Apr 27 '17 at 20:42
• Pretty much everything but the carrier frequencies mismatches. Perhaps you should re-read your link, as it doesn't say what you are claiming it does. – Chris Stratton Apr 27 '17 at 20:43
• Yes, it's probably not AIS, but DSC (also used with marine radio communication). See my edit. – Curd Apr 27 '17 at 20:52

We at work have wall clock that is synchronizing via radio band (with radio I mean that witch you can listen via FM or AM). I do not know exactly witch band is transmitting clock, but every hour they are synchronized. Clock is from Germany. Some time if they not getting good signal, then they shows wrong time :). I try to get more info about that clock.

• These systems do indeed exist, but usually use WWVB style national infrastructure broadcasts on HF, or else satellite navigation constellations like GPS (determining time is a required step in determining location). The signals would not look like this, making this a very improbably guess not a meaningful answer. – Chris Stratton Apr 27 '17 at 20:01
• My wristwatch has this feature. It works at a frequency about 70 kHz. – Ayhan Apr 27 '17 at 21:11
• That'd be DCF77, transmitted on 77kHz. The signal does not resemble a modem signal in any way, shape, or form. It can only be decoded by programs intended specifically for DCF77. If you did decode it, the data would not look at all like what the OP posted. – JRE Apr 28 '17 at 4:56