ChannelSPURIOUS EMISSIONS
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Episode 8 · Weather Fax · Series 01: The Listening Station

How does a ship get a weather map with no internet?

A frame from the draft of episode 8, Weather FaxComing soon
Release
Length
3:03
On the dial
518 kHz, 4 to 22 MHz
Signal
NAVTEX

What it covers

A ship in the middle of the ocean still gets a weather map and storm warnings by radio: weather fax and NAVTEX. I explain how a picture travels as a sliding tone, how coast stations share one frequency for text warnings, and what my own station in the Puget Sound has really received so far, including what did not work.

The written guide: set this up yourself

Chapters

  1. 0:00Intro
  2. 0:17A picture as sound
  3. 0:46Lining it up
  4. 1:13Where it comes from
  5. 1:37Taking turns
  6. 2:00One message
  7. 2:20What I have received

Frames from the draft

Weather Fax: Intro
01 Intro
Weather Fax: A picture as sound
02 A picture as sound
Weather Fax: Lining it up
03 Lining it up
Weather Fax: Where it comes from
04 Where it comes from
Weather Fax: Taking turns
05 Taking turns
Weather Fax: One message
06 One message
Weather Fax: What I have received
07 What I have received

Gear in this episode

RX-888 Mk IIthe receiver that hears all of shortwave at once (mine is from OpenSourceSDRLab)
MLA-30+ loop antennathe small active loop antenna for shortwave
HP EliteDesk small form factor PCthe shortwave computer I call Raphael (mine was rescued from e-waste)
CyberPower CP1500PFCLCD UPS (1500 VA, sine wave)the battery backup that keeps almost all of my station running through a power cut

Software and references

ka9q-radiogithub.comthe program that cuts one channel for each broadcast out of the receiver's stream (open source, by Phil Karn; episode 5)
fldigigithub.coma free program that decodes weather fax and NAVTEX. My own decoders are small programs I wrote for my station; fldigi does the same job
Point Reyes weather fax: the charts and frequenciesweather.govthe US National Weather Service page for the broadcast I receive
Pacific weather fax scheduleocean.weather.govthe published schedule: which chart is sent at which time
Ocean Prediction Centerocean.weather.govthe office that makes the charts
NAVTEX (US Coast Guard)navcen.uscg.govwhat NAVTEX is and which US coast stations send it

What is simplified, and what is invented

  • The weather chart in this video is real: my station received it by radio. It is a 48-hour surface forecast for the North Pacific from the US National Weather Service (public domain).
  • The tone trace is computed from the pixels of that chart, smoothed. The blank margin of each broadcast line is left out.
  • The scan head moves at real speed only where the label says so. At real speed the picture shows every fourth line.
  • The split chart in "Lining it up" is a demonstration made with the real chart. The band heights there are not to scale; the time bar is.
  • The NAVTEX message is an example that I wrote in the real layout. I do not show received warnings, because they can name a vessel or a person. "Cape Example" is not a real place.
  • The amateur picture is a drawn stand-in. I do not show received pictures, because they carry call signs and faces.
  • The ring of time slots shows only the coast stations of the north-east Pacific, and the hand turns far faster than real time.
  • Map coastline: Natural Earth (public domain). Signal paths are drawn straight; real shortwave signals bounce off the ionosphere.
  • The intro drawing is an illustration and its warning text is an example. Many ships have satellite internet today; these radio broadcasts work without it.
  • All counts are one reading of my station, rounded.