SDR and receiver capabilities

RTL-SDR dongles

A receiver uses up to four RTL-SDR dongles, each with a job:

Job	Listens to	Sample rate	Default gain	Antenna
EOT	the end-of-train channel (457.9375 MHz), decoded	2.4 MS/s	40.2 dB	UHF (the shorter one)
VHF	every voice channel in the plan, plus the reference station	2.88 MS/s	20.7 dB	VHF (the longer one)
HOT	the head-of-train channel (452.9375 MHz)	2.4 MS/s	40.2 dB	UHF
ITC	the 220 MHz PTC network: channel 102 and the local channels of the bases it hears (ITC)	1.024 MS/s	40.2 dB	220 MHz (a whip about 33 cm long)

Each dongle sees a window about 2.2 MHz (at 2.4 MS/s) or 2.65 MHz (at 2.88 MS/s) wide, and hears every plan channel in it at once. The receiver centres the window to fit the channels with the most room, avoiding the centre spike every RTL-SDR has. A channel outside the window is listed as “uncovered”.
Jobs are automatic: free dongles take jobs in a role order, by default EOT, ITC, VHF, HOT (VHF covers both road calls and defect detectors, which share the channels). The receiver remembers which dongle (by serial) has which job. When a dongle is gone for more than a minute (thrown away, or swapped), its job goes to the next free dongle, so a replacement takes over within a minute without any setting.
The order can change: move jobs up or down, or switch one off (it then takes no dongle). The website can set the order for a receiver; so can the receiver’s console, the Mac app, or sdr.role_order in receiver.json.
Or a job for each dongle: the receiver’s page on RailSpotting lists its dongles (by an anonymous ID) and lets the owner pick EOT, ITC, VHF, HOT or off for each. The console, the Mac app and sdr.assign in receiver.json do the same by serial; a job chosen on the website wins.
Health checks: clipping (gain too high), a stalled dongle (reopened after 10 s), and a “stuck” dongle sending random data (unplug it for 10 s) are all detected and shown.

The reference station and radio checks

Each plan names a reference station: a NOAA weather radio transmitter that is always on. The VHF dongle measures it continuously; every 30 minutes the receiver reports a radio check (it passes at 10 dB or more above the noise), and warns if the signal stays weak. The status page graphs it over days, which shows quickly when an antenna or cable goes bad.

The Uniden SR30C scanner (optional)

A scanner connected with its USB cable (the CP2104 programming cable) adds a fourth “ear”:

	SDR dongle	Scanner
Channels at once	every channel in its window	one at a time, stepping through its memory
Detects	by signal level above the noise	by squelch (and level, when parked)
Audio, transcripts, EOT decoding	yes	no: only when and how strong

The scanner’s job is automatic too: it takes whatever no dongle covers. EOT first, then HOT if a dongle has EOT, otherwise it scans the voice channels. In the scan job, a HOT burst or a strong call on a road channel makes it park on EOT for 30 minutes. The website can set its job instead (scan, EOT, HOT or off). The receiver programs the scanner’s channel memory with the plan.

A receiver at a glance

With the default role order (EOT, ITC, VHF, HOT):

Kit	Covers
1 dongle	EOT (decoded)
2 dongles	EOT + ITC
3 dongles	EOT + ITC + all voice channels, detectors, radio checks
4 dongles	EOT + ITC + voice + HOT
+ scanner	fills what the dongles don’t: EOT, HOT or the voice channels

Without a 220 MHz antenna, switch ITC off in the role order (or move VHF ahead of it), so a second dongle takes the voice channels.

The receiver needs little computer: a 4-core Celeron with 2 GB RAM runs two dongles with speech recognition. Speech recognition is the heaviest part; turn transcription off on very small machines.