H. P. Friedrichs (AC7ZL) Homepage
Exploring the Electromagnetic Spectrum: HF Weather Radio
Anyone who has ever walked down a sidewalk or crossed a parking lot on foot can probably remember a time when the glint of a lost quarter, lying on the ground, caught their attention. One might consider that a gift from Lady Luck. I'm more inclined to think of it as her reward for paying attention.
Right now we're at the bottom of the current sun cycle, and it's no secret that propagation conditions often leave much to be desired. While some people are discouraged when calling "CQ" yields no results, I like to take advantage of the general-coverage receiver built into my ICOM 718 to snoop around and see whatís going on elsewhere in the spectrum. Not long ago, while exploring the region below the 40-meter band, I noticed the "glint" of something interesting, and I was well-rewarded for my effort in going after it..
My favorite operating modes are CW and PSK-31. Regardless of what mode I operate, my ham shack computer is usually on, and the PSK waterfall generated by my Digipan software rolls slowly down the screen. One evening, I had abandoned calling CQ on 40 meters, and decided to slowly scan my way downward through the spectrum. When I reached a spot near 6.495 Mhz, I noticed a familiar "deedle-deedle" sound. "Hmmm," I thought, "this sounds like radio teletype." I glanced at the waterfall and noticed two tracks dribbling down the screen. It looked like a teletype signature, too, albeit a wide one. What was it saying? .
Figure 1: Screen capture of waterfall showing weather teletype signal
Digipan is a powerful and fully-functional piece of software to use for PSK-31 communications. It is, in fact, my favorite. On the other hand another package, MixW, offers the ability to operate in numerous other digital modes including PSK, FSK, MFSK, Hellschreiber, Throb, SSTV, Fax, and many more. I booted MixW, selected RTTY as the operating mode, and began tinkering with the mode parameters in a trial-and-error fashion.
Long story shortóThe radio was tuned to 6.495 Mhz, USB. The software was set for an 850 Hz spread, 75 Baud. I checked option boxes for "Inverted," "AFC" and "Unshift on Space." What emerged in the received-text window was something that looked like the following:
CYZX 270400Z 11005KT 3SM -RA BR FEW008 OVC#29 06/05 A2995 RMK SF2SC6/ WVXYLGQL0# 13011KT BUQQOP ZAMD CYZX 262349=+ 270024 1608KT 6SM -RA BR SCT006 OVC010 TEMPO 0016 11/2SM RE BR OVC006 BECMG 0607 13012KT BECMG 0911 20025G35KT FM1600Z 24020G30KT 6SM -SHRA BR BKN015 FM1900Z 25015G25KT P6SM SCT015 BKN025 TEMPO 1921 BKN015 RMK NXT FCST BY 06Z
Figure 2: Sample Weather Teletype Text
Help From An Expert
The text I received didnít exactly make sense, but certain letter clusters suggested that there was deeper meaning hidden there. I guessed that a "Z" following digits might mean Zulu time. "BECMG" looked like an abbreviation for "becoming," "CLR" seemed to say "clear" and "KT" surely meant "knots." I had already decided that this was some kind of weather bulletin when the line, "CANADIAN FORCES METOC CENTRE, HALIFAX, NOVA SCOTIA, CANADA" rolled onto the screen. Bingo! But what of all the other gobbletygook?
As luck would have it, I have not one but two meteorologists in my family. My sister, in fact, teaches weather courses to pilots, and the ability to decode the gibberish I had captured is among the core material covered in her courses. I sent her a chunk of the text I had saved. Predictably, she was able to read it like the menu at a restaurant. I was grateful enough, yet, she did one better: She directed me to the Rosetta stone with which I could decode the information myself! The FMH-1, or Federal Meterological Handbook, is available for download on the Internet.
Chapter 12 of the FMH-1 goes into considerable detail about the way observational data and symbols are ordered, encoded, and embedded in reports. Letís look at a line from the sample text and see what the message is really telling us.
Four letter symbol groups represent the ICAO (International Civilian Aviation Organization) identifier, the place from which the report observations were probably made. There are places on the Web where these identifiers can be looked up and translated. "CYZX" decodes to Greenwood, Nova Scotia, Canada.
A six-digit numeric group followed by the letter "Z" represents the day, hour, and minute of the report. In the context of the sample data, "270400Z" means that the report was generated on the 27th of the month, at 04:00 UTC.
A symbol group followed by "KT" gives you wind information, both direction and speed. The sample dataís "11005KT" decodes to a wind direction of 110 degrees and a speed of 5 knots. There can be variations in this group. For example, "11005G20KT" decodes the same way, but also indicates that there are 20-knot gusts.
"-RA" decodes to "light rain," "BR" means visibility is obscured by mist, and the abbreviations "FEW" and "OVC" give us details about the appearance of an overcast sky.
Finally, the "06/05" gives us the temperature "06" degrees and a dew point "05", while the number following the symbol "A" represents the air pressure in tens, ones, tenths and hundredths of inches of mercury as measured by an aircraft altimeter.
An almost endless list of additional codes and symbols describe whether the report was human or machine generated, the nature and distribution of cloud formations, barometric pressure trends, detailed wind behavior, precipitation, vertical and surface visibility, lightning, and even the atmospheric effects of volcanic activity.
That said, one must be prepared for occasional alphabetic garbage and symbols that wonít translate. Remember that RTTY is not an error-correcting communications mode. If fading, natural noise, or manmade noise corrupts the signal, youíll likely end up with patches of meaningless text. Some of this kind of corruption is evident in the sample text Iíve provided.
Back to the radio: I had just succeeded in capturing the weather data and had only begun to puzzle over its meaning when the transmission suddenly ceased. I guessed, correctly, that such transmissions are periodic and that the report would likely repeat. However, when the transmission resumed, the tone and nature of the signal I was receiving had changed entirely. The noise in my headphones now had a peculiar "scritch-scritch-scritch" tone.
I was not familiar with that specific sound. However, I had done some recent experiments with narrow-bandwidth mechanical television equipment. Mechanical television dates back as far as the late 1800ís. Equipment of this type utilizes primitive electronics and spinning aperture wheels to encode images. The video signals produced lie in the audible range- 20 kHz or less. The signal I was hearing on the radio was reminiscent of the signal produced by my mechanical television equipment, so I decided that the mystery signal must likewise be an image of some sort.
MixW supports a receive-only Fax mode. I selected Fax and tinkered with the settings. When adjusted to 120 LPM and FM modulation, MixW was able to reconstruct the chatter I was hearing into some kind of weather chart.
As time has passed, Iíve explored other regions of the HF spectrum for similar sounds. Finding them was slow going unit I stumbled upon the Worldwide Marine Radiofacsimile Broadcast Schedule on the Internet. This document lists numerous weather broadcast stations, the world over, with frequencies and broadcast times. In fact, using this document, I was able to identify appropriate frequencies and broadcast times to allow me to monitor the progression of Hurricane Gustav.
Figure 3: Screen capture of weather fax image. Note hurricane Gustav and tropical storm Hanna
In monitoring other stations, I noticed something interesting. Some stations transmit their weather text via Fax instead of RTTY or other digital character modes. Why? I donít know, but I would pose this guess. A burst of RF noise, even a fraction of a second in length, will corrupt bits in one or more RTTY characters, altering the data they represent. In a Fax, the same noise is represented by speckles or spots in an image. Even in the case where heavy interference leads to the generation of severely corrupted images, the human eye and brain are such effective pattern analyzers that the data is likely to remain 100% readable.
Give It A Try!
To receive and decode this kind of data yourself, youíll need four things: A computer, some software, a receiver, and an interface to link the receiver to the computerís soundcard. If you are a ham, and youíve already worked other hams with digital modes like PSK-31, the odds are that you already have everything you need.
If not, or you donít have the equipment, take heart. Any up-to-date computer, and most older machines, are more than capable of running the required software. There are several free RTTY programs available on the Internet, and several more pieces of affordable commercial software.
Youíll probably get your best Fax images from commercial software dedicated to this purpose, though the examples Iíve seen seem pricey.
For either mode, MixW is my choice. MixW is a fine product and it can be downloaded and used for free while in Demo mode, though it really is worth purchasing. My beef with it is that it provides no good way to save captured fax images intact, and there is no apparent means to correct for timebase errors (hence the slant to some fax images). Hereís another MixW tip: once youíve purchased your copy of MixW, back up your software and donít misplace your software key. WA2VOS can generate a new one for you, but not until heís satisfied that youíve been properly chastised.
I suspect a wide variety of receivers are suitable for monitoring weather transmissions, so you donít need a ham radio, as such. Youíll want something reasonably stable that will receive the HF bands. In addition, the radio must have SSB capability, or at least a BFO. Before I got my ham license and purchased a transceiver, I used to monitor both CW and digital modes like PSK-31 with a Grundig Yachtboy.
The interface between the radio and computer can be as elaborate or as simple as youíd like. Right now, I use a Tigertronics Signalink SL1+ with my ICOM transceiver. However, when I was engaged in listen-only activities with the Yachtboy described above, my interface consisted of nothing more than an audio transformer and a couple of resistors soldered inside of an empty Altoids Tin. Plans for simple homebrew interfaces are all over the Internet. Do a Google search on "PSK-31 Interface."
If you are not a ham, or youíve never worked any digital modes before, some of the terminology Iíve used will seem puzzling. In this case I would recommend the purchase of one additional item: The HF Digital Handbook by Steve Ford, WB8IMY. Fordís handbook is an excellent general introduction to HF digital communications and it answers those questions most likely to be asked by enthusiasts who are new to this kind of endeavor.
If all else fails, some HF weather transmissions are simply voice reports. As long as you've got some kind of receiver, it is possible to listen in on this interesting broadcast material.
The following is a brief collection of interesting URLs relevant to the article above.
Document Revision 2, 11/01/08
Document Revision 3, 04/28/09