During the August 6, 2021, club meeting Jeff Liebermann AE6KS gave a technical talk on ADS-B and AIS. These are two tracking systems that receive the locations of aircraft and maritime vessels, respectively. Jeff posted a presentation page with notes and with links to the various topics and web sites that he covered.
ADS-B
Aircraft use 1.09-GHz and 978-MHz radio signals to broadcast position information to other aircraft and to ground stations. Aircraft use the position information for proximity alerts, and ground stations contribute received data to aircraft-tracking web sites like FlightRadar24 and FlightAware, usually in exchange for premium web-site features.
To receive aircraft position data, Jeff uses several inexpensive USB-based receivers, each using a tiny, cable-connected, omnidirectional antennas about four inches long. The receivers’ USB ends are inserted into Raspberry Pi single-board computers, which in turn send data to the aircraft-tracking web sites.
Jeff has had good luck with the RTL-SDR.COM receiver. If you want to assemble your own ADS-B receiver, you’ll also need a Raspberry Pi 3 or better, a power supply, an SD card for the Raspberry Pi’s storage, and an antenna. He recommends getting an inline bandpass filter, although that means that your receiver will be restricted to just one of the two frequencies. As far as cabling, at 1 GHz, cable loss can be significant. Jeff has found that RG-6U coax (75Ω CATV) works well. Be aware that using that cable with its normal F connectors requires several adapters.
Jeff notes that the typical, plastic Raspberry Pi enclosures provide inadequate cooling, and so you also should look for an enclosure that can add a cooling fan.
AIS
Maritime vessels transmit digital position information over the maritime VHF band. Coverage over the ocean seems to be unusually long range, until one learns that the vessel-mounted radios act as relays and send not only their own position data but any position data that they receive from other vessels.
The encoding used seems to be similar to 9600-baud packet radio. Although it might be possible to modify amateur packet-radio equipment to decode the position data, Jeff recommends finding a USB-based receiver for $25 to $30 instead. You’ll end up needing a RaspberryPi for an AIS receiver, too, unless you opt for much more expensive a stand-alone unit.