Thursday, June 14, 2012

The SDR Chronicles (#3: Discoveries)

I'm really starting to get the hang of the Excalibur Pro.

One very useful feature is the waterfall palette adjustment.  This is a bit different than the ones you've seen.  The two adjustments are for "bottom" and "gravity."  The first sets the low end of the color scale, while the second defines the place where the colors change the fastest.  These interact, and take some fussing, but they are useful when you need to pop out weak signals, or if you just want a brighter display.

While investigating signals around 4 MHz, I ran across some modes I hadn't heard before. According to Mike Chace-Ortiz, our digital editor and authority on such things, these are Tadiran radios.  He suspects Mexican military, and I do too.  That particular agency can really go through modes.

Given the disappearance of all their ALE nets with the cute call signs, a new Mexican military radio network is something of a major discovery here.  For various reasons, the confidence is high that these are indeed the people making all their latest weird noises.   The clear voice sounds like the way they train their operators, and also some stations are still using the older "Hooter," an analog voice scrambler made by Harris.

It appears that the new (at least for me) equipment has a number of selectable waveforms, best tuned in USB.  Nearly everything starts with a short, 1000 hertz beep, making tuning simple.  After this, one hears clear voice or several obviously encrypted waveforms.

These latter have the payload signal on the low end, but also include various lengths of high-pitched data signalling in 2-4 narrow audio bands centering around 2800 Hz. One is obviously a kind of vocoder-ish voice scrambling, while another is just the high band by itself. A third is a high-band "turkey call" beginning with the 1-kHz beep, and apparently some kind of crypto key/sync information, followed by a PSK waveform that detects as MIL-STD-110A. Being encrypted, it of course always errors out.

The SDR waterfall allowed rapid identification of these signals, and the recording feature made close analysis of the frequency range possible.  So far, all this activity is between 4.5 and 5 MHz, nearly always in 5-kHz channels.

Frequencies identified so far are:

4595 4617.5 4625 4820 4865 4880 4885 4890 4895 4900 4910 4915 4920 4930.  The 4617.5 does not fit this convention, but indeed that was the right frequency.

The speed with which all this data was gathered is a major indication of the usefulness of the SDR in the utility DX pursuit.  I'm sold.