As part of my interest in emergency communications, I’ve found myself rather interested in APRS, and now operate a few bits of infrastructure in San Francisco’s downtown region, which due to it’s dense nature is pretty hostile to radio in general, and low-power APRS especially.
A receive only iGate powered by a RTL-SDR stick, Raspberry Pi 3, and a 2-meter “bazooka” antenna mounted in the window of my apartment. Most of the time this is done using my packet “Go Kit”, but if that unit is in use elsewhere a second RaspberryPi can take over the duties.
A second receive only iGate, located in San Francisco’s Financial District at Market and California on the 23rd floor. Like TNDRLN, it’s RTL-SDR based, but uses a much smaller antenna and Linux virtual machine.