I run a Raspberry pi Zero W device which feeds data to the FlightRadar24 web site.
The receiver comprises a dedicated antenna covering the SSR frequency range (1,030 to 1,090 MHz), a high quality multipole cavity RF filter, a direct conversion digital radio receiver front-end (which you may recognise as a modified USB Digital TV receiver stick) and a Raspberry pi Zero W computer (RPi).
The processing of the signals is dealt with entirely within the RPi which runs a cut-down version of Linux and a special processing package provided by the FR24 service. In addition, the RPi runs BOINC grid computing software to ensure maximum use of resources.
The antenna is currently located indoors in the roof space above our garage. This will be upgraded to an outdoor mast on the side of the garage once planned refurbishments works on the garage have been completed during early 2023.
The most important part of the received, perhaps surprisingly, is the multipole cavity filter. With relatively low insertion loss and very high out of band rejection the filter renders the receiving chain more or less immune to interference from other sources. This improves the signal-to-noise of the wanted in-band signals received by the DVB-T stick. The DVB-T stick receiver has zero front-end filtering and although the device that I use has been upgraded with a higher quality local oscillator and better front-end components, the device will still have a relatively poor intermodulation characteristic. As a conseqnence, additoin of any front-end filtering is bound to reduce the level of intermodulation products caused by more or less any received signals in the DVB-T stick's reception range, notionally 50MHz to 2.2GHz!
Addition of the front-end filter more than quadrupled the average range of the receiver system overnight!
For maximum availability the ADSB receiver system is powered from a UPS. The limiting factor on the availability is the long-term stability of the FR24 software. At the moment, anecdotally, the device needs to be re-booted about once every 2-4 weeks. The underlying Raspbian linux OS will stay up for much longer. I have had one of these devices running for nearly one year without a reboot. It wasn't doing any work, though.
