Field Guide · technology

Also known as: VITA 49, VRT, VITA Radio Transport, VITA-49

VITA 49, the VITA Radio Transport (VRT) standard, defines a packet format for carrying digitized IQ data together with synchronized metadata — precise timestamps, centre frequency, sample rate, bandwidth, and reference levels — so a digitizer and its downstream processing can be separate boxes on a network and still agree on exactly what each sample means.1 It is the interoperability layer of professional and defence SDR, standardising the “digital IF” that vendors previously each defined privately.

Signal Data Packetheader · timestamp · IQ Signal Data Packetheader · timestamp · IQ · · · Context Packetfreq · rate · bandwidth · ref level linked by Stream ID
A VITA 49 stream interleaves timestamped signal-data packets with context packets describing the RF parameters, tied together by a stream identifier.

How it works

A VRT stream is a sequence of two kinds of packet, sharing a Stream Identifier so a receiver knows which data belongs with which description:

  • Signal Data packets carry the payload — the IQ samples themselves, in a declared sample format (integer or float, a chosen bit width) — prefixed by a header and an optional high-precision timestamp. The timestamp pairs an integer-seconds field with a fractional field (often picosecond-resolution sample counts), so every packet is pinned to an absolute moment. This is what lets multiple digitizers be sample-aligned for phased arrays, direction finding, or beamforming.
  • Context packets carry the metadata that describes those samples: RF centre frequency, IF, sample rate, bandwidth, gain, reference level, and GPS/geolocation fields. They are sent when parameters change (and periodically as a keep-alive), so a late-joining receiver can recover the full state without a side channel.

VITA 49.2 extended the standard with a richer set of context fields and command packets for control, turning VRT from a one-way transport into a fuller interface. Because the format is self-describing in-band — every parameter needed to interpret the IQ rides in the same stream — VRT scales to many-channel, multi-receiver systems where a filename convention or a static config would not.

Relevance to SDR

VITA 49 dominates the professional, test-and-measurement, and defence SDR world: Ettus/NI USRP radios can emit VRT, signal-analysis platforms and the GNU Radio gr-vrt work speak it, and large sensor networks use it precisely because the timestamped context model keeps geographically separated digitizers coherent. It is the heavyweight counterpart to the hobbyist network IQ streaming protocols like rtl_tcp and spyserver: where those ship a bare byte stream and rely on out-of-band tuning commands, VRT bakes the metadata into every packet.

GopherTrunk does not implement VITA 49 — it is a consumer-SDR trunking decoder whose network inputs are the lightweight raw-IQ servers, and whose offline path reads plain IQ files with the parameters given on the command line. VRT is worth knowing here as the reference point for how a rigorous streaming format solves the same problem GopherTrunk solves informally: the timestamp and context fields VITA 49 standardises are exactly the sample rate, centre frequency, and timing that a bare capture forces you to track by hand. If a future GopherTrunk needed to ingest from professional receiver infrastructure, VRT is the format it would have to learn.

Sources

  1. VITA Radio Transport — Wikipedia, on the VRT/VITA 49 packet model, signal-data vs context packets, timestamping, and its role as the digital-IF interoperability standard. ↩

See also