Field Guide · protocol

Also known as: LRPT, Meteor LRPT, Meteor-M

Meteor-M LRPT (Low Rate Picture Transmission) is the digital weather-image downlink carried by Russia’s Meteor-M polar-orbiting satellites near 137 MHz. It is the modern replacement for analog NOAA APT: instead of an amplitude raster, LRPT sends compressed image data as a QPSK bitstream protected by a convolutional code that the receiver undoes with a Viterbi decoder.1 The payoff is sharper, calibrated imagery from the same simple 137 MHz ground station.

QPSKdemod Viterbidecode CCSDSpackets image 137 MHz QPSK downlink → soft bits → error-corrected packets → picture
LRPT recovers soft QPSK symbols, Viterbi-decodes the convolutional FEC, and reassembles CCSDS packets into a weather image.

Overview

LRPT flies on the Meteor-M “N2” series operated by Roscosmos and Roshydromet, the first of which reached orbit in 2014. Like APT it broadcasts continuously on a 137 MHz VHF carrier that any modest ground station can hear, but everything above the antenna is digital: the image is delivered as CCSDS space-packets carrying compressed pixels, so a good pass yields a clean, geometrically corrected picture rather than the noise-streaked grey scale of an analog signal.1

Technical characteristics

Property Value
Band ~137 MHz VHF
Modulation QPSK (some satellites/modes OQPSK)
Symbol/bit rate ~72 kbps (72k or 80k symbol variants)
Error correction Convolutional r=1/2 + Viterbi, with outer coding/interleaving
Framing CCSDS virtual channels / space packets
Content Multiple MSU-MR imager channels, JPEG-like compression

The forward error correction is what lets LRPT work at the same weak-signal margins as APT: soft QPSK decisions feed a Viterbi decoder that corrects the bit errors a fading 137 MHz pass inevitably produces, and the CCSDS layer packages the corrected bits into the imager channels.

History

Meteor-M LRPT arrived as the digital counterpart to the aging analog fleet, giving amateurs and forecasters a higher-quality 137 MHz image source. Early Meteor-M satellites suffered outages and a debris strike, but the series has been sustained, and open-source demodulators made LRPT a staple of the SDR weather-satellite hobby alongside its western sibling, the geostationary GOES HRIT.2

Deployment

LRPT is a receive-only broadcast with no user uplink. The ground station is essentially the same as for APT — a right-hand-circularly-polarised QFH or turnstile antenna, a 137 MHz SDR, and pass-prediction software — with the demod and Viterbi decode done in software.

Decoding it with GopherTrunk

GopherTrunk does not decode Meteor-M LRPT. It is a terrestrial land-mobile trunking scanner, whereas LRPT is a satellite image protocol with CCSDS framing and a QPSK/Viterbi chain that shares nothing with the land-mobile voice systems GopherTrunk implements. It is documented here as the digital successor to NOAA APT and a rewarding SDR-decoding project in its own right.

Sources

  1. Low-rate picture transmission — Wikipedia, for the LRPT QPSK downlink, convolutional coding, and CCSDS image format. ↩ ↩2

  2. Meteor (satellite) — Wikipedia, for the Meteor-M satellite series that carries LRPT. ↩

See also