Tuesday, November 3, 2015

Intercepting pager messages

Today I heard data bursts on 155.170 MHz and thought they were P25 or MotoTRBO. When they didn't produce any output in DSD+, I went on sigidwiki, chose VHF, and went through NFM signals until I heard one that sounded similar. It turned out to be a pager signal. I downloaded a program called PDW. It can be downloaded here.

I was then able to decode what turned out to be hospital messages. This form of interception is legal, according to what I read on this RTLSDR Blog post. In the post, they wrote, "In most countries it is perfectly legal to receive these messages, as they are plain text unencrypted, but it is illegal to act on the information received. Please respect your local laws."

I can't legally go into detail, but I will say I was able to read some interesting stuff. By law, you must not act on what you hear or divulge it to someone who wasn't present.

929.612 MHz has far more activity than VHF, at least in my area.

I recommend TV rabbit ears in a window with the adjustable parts vertically oriented. This forms a sort of vertical dipole. I've tried pager interception with a rooftop Yagi connected through a TV RF amp and it doesn't seem as good. It could be the RF amp corrupting any signal over 800 MHz, attenuation in the old cable wiring, or maybe TV Yagis just weren't meant for this frequency. Of course, the 150 MHz paging comes in significantly better with the Yagi, but that's understandable because it's near a VHF TV band, something the Yagi is good at, and the RF amp is designed for this frequency.

Extra tip: although HDSDR beats SDR# hands-down when it comes to CPU usage, SDR# seems to have a better FM demodulator, which means higher decoded message accuracy.

Here's where things get mysterious...

While monitoring 155.170 MHz, I heard a voice signal more than once. It seems people sometimes talk between pager bursts. I've embedded a recording of one such instance.

(Update 11/09/2015) I think this is an example of a voice page. Here is a good link to some info on this. Apparently pagers can receive voice messages.

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