PhosphorTEX - Real-Time NAVTEX Decoder

PhosphorTEX decodes NAVTEX (Navigational Telex) broadcasts in real time using your iPhone or iPad microphone. Hold it near a shortwave receiver tuned to 518 kHz, 490 kHz, or 4209.5 kHz and watch gale warnings, nav warnings, and maritime safety information appear on an authentic phosphor CRT display.

No internet connection required. No external hardware. No account. Just your iPhone or iPad and a maritime receiver.

FEATURES

Real-time NAVTEX decoding via microphone — CCIR-476 SITOR-B, 100 Bd, 170 Hz shift
Automatic character alignment and automatic polarity detection, so maritime receivers and SDR tuning both work with no toggles
Automatic frequency control that finds and tracks the 170 Hz tone pair anywhere from 800 to 2500 Hz
ZCZC…NNNN message framer with B1/B2/B3B4 header parsing
CARDS view: message cards coloured by subject (SAR/piracy, nav warning, met warning, ice report …)
TAPE view: raw teleprinter stream, character-by-character, with errors shown rather than guessed
Live FFT spectrum display and scrolling waterfall (SDR-style colour palette)
FEC health panel: each character measured against the second copy NAVTEX sends — DX valid %, RX fallback %, both invalid %, alternation lock
NAVAREA station schedule tab: 43 coast stations, 24-slot grid, current-slot highlight, UTC clock
Per-subject mute list (D-category SAR/piracy messages can never be muted)
Alert colour for SAR/piracy and serial-00 vital messages
Full message log with copy, share, and export of every message at once
Amber and green phosphor CRT themes with scanlines, vignette, and glow effects
Native on iPad in every orientation, with larger type and the cards, spectrum, waterfall and station schedule across the full width
No accounts, no tracking, no network access

DSP ENGINE

Per-sample I/Q quadrature demodulation with biquad low-pass filtering and asymmetric envelope tracking. Synchronous 7-bit SITOR-B decoding with DPLL clock recovery, two polarity lanes and a shift-until-valid character window checked against the CCIR-476 4:3 constant-weight code. 512-point FFT for frequency analysis and AFC. All signal processing runs entirely on-device.

REQUIREMENTS

iPhone or iPad with a microphone
iOS or iPadOS 26.4 or later

DECODER ACCURACY

PhosphorTEX is verified against real over-the-air 518 kHz WebSDR recordings of live maritime-safety traffic (Netherlands Coastguard and NCC-Hamburg nav warnings). Detailed test results and decoded output samples are available in the decoder test report.

WHERE TO FIND NAVTEX

518 kHz — International (English). The primary NAVTEX band.
490 kHz — National language broadcasts (Dutch, French, Portuguese, etc.)
4209.5 kHz — HF, tropical / long-range coverage.

A shortwave receiver (or SDR / WebSDR audio played through a speaker) tuned in USB with carrier positioned so the mark tone is around 1615 Hz is all you need. NAVTEX stations broadcast in assigned 10-minute slots within a 4-hour cycle — check the SCHEDULE tab for current and upcoming slots.

SUPPORT

Need help? Have a question, bug report, or feature request?

Ask a question — get help with using PhosphorTEX
Report a bug — let us know if something isn't working
Request a feature — suggest improvements or new functionality