I did my first radio fun today.
The Automatic Packet Reporting System (APRS) is a way to share local information of immediate value in real time. This information is often the position of its users, but can also include short messages and announcements, telemetry data like weather reports and storm forecasts.
You don’t need the internet to use it, so it doesn’t rely on there being a good mobile phone signal wherever you happen to be. Instead, it uses established packet radio protocols on various amateur bands. A handheld transceiver can receive information packets for a computer to decode and display, and also send encoded packets out for others to receive on the same shared frequency. A network of digital repeaters re-transmit the message, transporting it further than the handheld transceiver can reach. Internet-connected receivers, called IGates, also collect the data and relay it into the APRS Internet System to appear on sites like aprs.fi.
The encoding and decoding of the packets can be done by an ordinary smartphone, previously the job of a hardware terminal node controller (TNC). Apps like PocketPacket on iOS or APRSdroid on Android implement a TNC, or audio modem, in software. Transceivers like the Baofeng UV-5R can be bought for under £20. The phone and radio can connected via a simple cable, which can either be made at home or bought online. The final thing you need, of course, is an amateur radio licence, at least if you want to transmit.
I gave PocketPacket a go on a visit to Reality Checkpoint and Mill Road Cemetery. It worked pretty well, though I had to fiddle around with the settings a bit because it looked like for a little while I was just transmitting noise. In the end I used the maximal setting of 10 for VOX (voice operated transmission), the radio's audio gain at about a third of the maximum, and my phone on the second-to-lowest headphone setting.
So yes, despite aeroplane mode being enabled on my phone and therefore no link to the internet being available, I picked up other stations transmitting packets about themselves and sent a few test packets of my own. Hopefully I'll get the opportunity to test it again soon, ideally somewhere a bit more wild and remote.