Monday, April 28, 2008

Digital Mode of the Week: SITOR

SITOR stands for Simplex Telex Over Radio or Simplex Teleprinting Over Radio. It uses the same type of frequency-shift keying (FSK) as RTTY. Mark and space are used. The shift is always 170 Hz, and speed is always 100 baud.

SITOR was developed in the 1960s for use in the radiotelex and maritime narrowband direct printing services, as an improvement on RTTY. It adds error checking, reducing garble over noisy and fading HF circuits. The trade-off is that timing is far more important than in RTTY. Much tighter technical standards are needed for acceptable communication.

SITOR has two modes, A and B. SITOR-A is a fully synchronous two-way mode for traffic handling. Two stations alternate half-second transmit intervals with half-seconds for listening, exchanging short bursts which give the system its distinctive chirp-chirp-chirp sound. You can't mistake this one.

Mode A uses an error checking protocol named ARQ, for Automatic Repeat reQuest. Messages are broken up into 3-character blocks. The station sending these is the Information Sending Station (ISS). The other station is the Information Receiving Station (IRS). The IRS replies with a signal that the received block passed an error check (ACK for acknowledgement), or that it did not (NAK for negative acknowledgement).

Bad blocks are resent as many times as needed, within reason. The effect is that SITOR-A handles degraded circuits not by garbling the message but by slowing down, to a maddening snail's pace if necessary.

To copy SITOR-A, you need to be tuned to the ISS. Its bursts are a little longer than those of the IRS. With some experience, you can tell the difference. It is tricky for the casual listener to properly phase with the ISS. When you do, most decoders will simply print repeated blocks over and over again.

That incredible screech you hear on maritime telex channels is the SITOR-A tuning marker sent by the coast station. It consists of pulses at the baud rate sent in several bursts a couple of seconds long, usually followed by the station callsign in Morse code keyed with the mark tone.

SITOR mode B is a continuous broadcast system using Forward Error Correction (FEC). It sounds like a sped-up, less chattery version of standard RTTY. Being a broadcast, the only station transmitting is the sender. Everyone else listens. Characters are sent in a stream, but with a built-in redundancy in which each character is sent again three characters later. Such combined repetition sequences are called interleave in the jargon.

The concepts of ARQ, FEC, and interleave come up again and again in different digital modes. Many systems use different interleaves. In this one, ABCDE [end of message] would be sent ABCADBEC D E [end of message]. Characters not received twice are dropped, with the idea being that a missing character is better than a wrong one. Therefore, SITOR-B does not slow down on degraded circuits. If everything works right (a big if), it just stops printing.

SITOR-B is easier to receive than A, but it is still necessary to achieve sync for the error check. The special characters ALPHA and BETA are provided for phasing. This is especially evident in the NAVTEX service, which sends these phasing pairs between each of its short messages.

SITOR also uses a different transmission alphabet, called CCIR 476. The scheme is called 4/7. Each character is 7 bits long, but as an additional error check, there are always a total of 4 ones and 3 zeroes in the character. While a larger number of bits would usually mean a larger character set, this requirement means that most bit patterns are in fact error characters, and the character set is actually smaller. Again, there are LTRS and FIGS cases, selected by the appropriate shift characters. Again, optional USOS (UnShift On Space) is usually provided to partially deal with missed shift-outs.

A ham radio version of SITOR is called AMTOR. The differences between the two are slight, and for us, AMTOR can be considered the same thing. The same software usually works for both. ARRL headquarters station W1AW in CT transmits daily bulletins in AMTOR mode B on the same frequencies as RTTY. AMTOR mode A imposes timing and transmit/receive switching demands that amateur gear is rarely designed for, and it is not widely used.

Saturday, April 26, 2008


As you might have noticed, there was no RTTY or FEC SITOR from KSM on Point Reyes, CA today. Your radio is fine. They didn't transmit it. This means the encrypted text was not sent, and those of us who were waiting with virtual cipher machines at the ready will just have to wait another week.

The broadcast had to be cancelled due to illness of the engineer who does the RTTY. They will try again next Saturday, May 3. Times and frequencies are still the same as the ones mentioned below.

Thursday, April 24, 2008

KSM Encryption/ Decryption Exercise 4/26 and 5/3

KSM, a licensed commercial station using vintage equipment and antennas at historic KPH on Pt. Reyes, CA, will be doing a very interesting broadcast in World War II M-209 encryption. It's at 1900 and 2100 UTC on April 26 (also International Marconi Day) and a week later on May 3. The key will be broadcast, and listeners are invited to attempt decryption of the message with readily available freeware.

M-209, for those of us who weren't born yet, is a US military mechanical code machine slightly resembling a 6-rotor, patch board-less version of the better known German ENIGMA.

Here's the full release from Richard Dillman of MRHS:

KSM will broadcast messages encrypted with the military M-209 crypto machine via RTTY on 26 April and 3 May.

The idea for the broadcast came up in conversations between myself and Dave Ross as a way for the MRHS to support the Military Radio Collectors Group meet taking place in San Luis Obispo, CA on 2-3 May. We thought it would be fun to give the attendees something to copy on their vintage military RTTY gear and then exercise their M-209 skills by decoding the message. But we thought other listeners may enjoy trying their hand at decoding the message as well, thus this announcement (see below for information about a M-209 emulator in case you don't have access to the genuine article).

MRHS transmitter engineer Steve Hawes, who manages our RTTY broadcasts, was keen for the idea and Dave provided the text so all the pieces are in place. Here are the details:

Dates: 26 April and 3 May

Times: Approximately 1200pdt and 1400pdt

Modes: RTTY and FEC. Baudot transmissions are at 170cps shift, 45 baud. FEC transmissions are at 170cps shift, 100 baud

Frequencies: 8433.0kc, 12631.0kc

Text: Dave's text will start with a plaintext preamble and will include the settings for the M-209 as well as the key. That will be followed by the encrypted text in five letter groups.

Additional information:


M-209 emulator for those who wish to participate but don't have a M-209 -


International Marconi Day is April 26

International Marconi Day is a yearly amateur operating event also of some interest to utility listeners. This year, it lasts from 0000 UTC on 26 April 2008 (afternoon of the 25th in the US) to 2359 UTC, 26 April 2008.

It is organized by an amateur radio club near where Marconi transmitted his historic transatlantic signals. The home page is at

K6KPH, the amateur radio station of the Maritime Radio Historical Society at the historic KPH "Power House" on Pt. Reyes, CA, will be active starting at noon Pacific time, 1900 UTC, on Saturday, the 26th. Frequencies are 3550, 7050, 14050 and 21050 kHz.

As always, QSL to:

Denice Stoops
PO Box 381
Bolinas, California 94926

Tuesday, April 22, 2008

Firedrake Jammer Revealed (CD Available)

Every so often, I run across a web page that just makes my jaw drop off. The latest Oh My God is this Satdirectory article on the source of the Firedrake (Firedragon) jamming signal used by China to cover Falun Gong's "Sound of Hope" broadcast from Taiwan.

One can make a case that this item pertains to world broadcasting, not utilities. However, the signal pops up on or near utility frequencies regularly, as it chases SOH around the bands. Lately, it's been heard daily in the fertile ute hunting ground just above 20 meters, on 14410 kHz around 2300 UTC.

It's actually rather good music, when conditions permit clear reception. It has some real nice drumming. It certainly beats all the other noise blasted into the HF bands by governments that can't handle freedom of opinion.

Well, here's what Satdirectory has to say on Firedrake:

Shortwave Radio Enthusiasts and Ham Radio operators have been watching China's Firedrake with interest. They believe that the primary Firedrake transmitter location is on Hainan Island off the coast of Southern China, however it is believed that there may be other transmitter sites also in use. It has also been noted that the Firedrake audio is a one hour loop with no announcements. This got us thinking at Satdirectory; how does the Firedrake programming get to the transmitter site? Is it delivered by a tape or CD on repeat, or is it like most other Chinese radio, delivered by a satellite link to the transmitter?

Well, a search with our 3 meter dish has found Firedrake! The audio is transmitted on Chinasat 6B within the China National Radio (CNR) satellite feed circuits. Many of the China National Radio feeds are in stereo, however one channel that is solely mono is CNR 8 - The Voice of the Minorities broadcast which features programs in the Kazakh, Korean, Mongolian, Tibetan and Uighur languages. The CNR 8 audio feed to the Chinese transmitter sites can be found on the left audio channel of a feed circuit labelled "Lzh8Rdjy". On the right audio channel of this feed is the audio for the Firedrake transmitters.

Following our discovery we tuned up a shortwave receiver to 17780 kHz which at the time also had the jammer running. The audio from the satellite feed and the shortwave radio were synchronised with no delay. This confirmed that the Firedrake shortwave transmitter site was also being fed by the same satellite feed, otherwise we would have expected a delay of a second or so due to the satellite uplink and downlink path delay when compared to the shortwave broadcast.

Satdirectory has actually made a CD with the full, 60-minute cycle. Since the copyright status is unknown, this one is available for non-commercial use only, from support(at) The only costs are to cover shipping and duplicating.

There's also a 4-minute sample of the high-fidelity audio right off the downlink, in Windows .wma format. Yes, there's some of the cool drumming.

Get it right here. But do go to the site and check out the whole story, with technical paramaters for those with a shot at Chinasat 6B, and some hilarious propaganda art.

Monday, April 21, 2008

Digital Mode of the Week: RTTY (Part 2: Teleprinting over radio)

Having discussed the origins of the 5-bit teleprinting ("Baudot") code, now we can talk about one of the oldest digital modes. This is RTTY, for radioteletype. It has undergone several incarnations, from wire teleprinting (like news wires), through various types of hardware-based radio modems, and finally to just another digital option in multimode computer sound card packages.

RTTY is sent over the radio by keying a transmitter between two tone frequencies corresponding to binary states. These are called mark and space. The process is called frequency-shift keying (FSK, F1B emission).

Direct FSK is still used in some dedicated RTTY equipment, but audio frequency-shift keying (AFSK) is far more common today. It is done by generating the proper modulation of an audio tone, which is then sent to the audio input of a standard single-sideband voice radio. (Note that power is lowered due to RTTY having a continuous duty cycle.)

Either method produces the same signal in the receiver. To the ear, RTTY sounds like a continuous warbling tone with rather clicky bit transitions, and often kind of a busy, chattery pulsation to it.

The difference between mark and space frequencies is the shift (in Hz), and the number of bit transitions sent in a second is the speed (in baud). Sometimes you also see speed in words per minute.

Common speeds are 45 (actually 45.45), 50, and 75. Common shifts are 170, 450, and 850. Characters are sent asynchronously, as they are generated. The five data bits are preceded by a single start bit and then followed by 1, 1.5, or two stop bits. (In practice, a long stop bit just sounds like a brief pause on the tone, making RTTY sound even more chattery.)

RTTY's standard tone center (halfway between mark and space) varies a bit. For the most part, it's around 2210 Hz. RTTY used to require careful tuning to center the tones on their respective filters, but today's decoders are more forgiving. Usually, one simply clicks on or between the two peaks on a display, or the software jumps to the loudest signals. The tone center is sometimes relevant for frequency logging, however.

RTTY dial vs listed frequencies can get pretty ambiguous. One can easily be 2 or more kHz off, and not even be sure which way. If you hear nothing on the listed frequency, tune around.

The third important parameter is signal polarity, which can be "normal" or "reverse." This refers to whether mark or space is the lower of the two tones, preferably when tuned in LSB. Unknown signals require some trial and error to find the speed and polarity which print readable text. Shift is usually more evident.

RTTY has no error check, meaning that any problems with the signal will create gibberish, or no print at all. Missed characters on fades are just something we live with.

The good news is that RTTY may be primitive, but it's still heard all over the HF bands. The bad news is that most of this is encrypted, often in a secure military/ government mode called KG-84. There is absolutely no way to get meaningful copy from it. A few navies, particularly the French, still run RTTY test loops and an occasional real message in the clear. Remaining weather stations in Germany and Canada have regular RTTY schedules.

RTTY in the military can also be referred to as RATT.

Old time RTTY stations stored messages on long reels of perforated paper tape, a character at a time. A procedure named "tape relay" existed for the storage and forwarding of traffic. Tapes were punched by perforating devices attached to Teletype machines, for transmission later. Received traffic could be punched directly onto this tape by a reperforator, and relayed by sending the tape to a reader. Much of RTTY's operating nomenclature such as "brag tapes," "test slips," and "loops," is a holdover from this era.

1950s US Navy tape position

The American Radio Relay League (ARRL) headquarters station W1AW sends long RTTY bulletins daily, on time/frequency schedules listed all over the Internet. Shorter transmissions, by hams shooting the breeze or in operating contests, can be heard up or down maybe 10-15 kHz from 14080.

Thursday, April 17, 2008

Acarsd Pre-Release 1.7 Now Available for Testing

We finally got to see the new version 1.7 of acarsd, the free ACARS decoding and logging program that also does HFDL via data transfer from PC-HFDL. Beta versions of this new version have been hard to come by, after some users apparently misunderstood the meaning of the term "beta," as in "help us find the bugs."

This download is a "Release Candidate," something more than a beta but still not the official stable release. You can get it here. Right now the newest versions are Public 1.70 Release Candidate 3 30.03.2008 for Windows, and Public 1.70 Release Candidate 2 29.01.2008 for Linux.

I grabbed RC3 and installed it. The "Quick Install," a DOS program, was hard for me to understand, so I did the full install. There is now a setup screen in acarsd that runs the first time, and lets you set a lot of options that used to be deeply buried in the self-documenting acarsd.ini file. The graphic user interface now has its own .ini file, which I haven't looked at yet.

Several nice new features are apparent. The parsing of messages is a little better. What I was really interested in, however, was the expanded use of the ICAO24 airplane address to help identify aircraft making HF position reports. Basically, the idea is that instead of just logging all these planes as .NO-REG, the program uses this hex ID to look up the registration. This is a big improvement for HF users.

Since this is a pre-release, and since the acarsd documentation has never been especially detailed anyway, it took some digging in the .ini file before I found the option that would enable this search. Once I did that, the ICAO lookup worked as advertised.

So far I've had no major problems with version 1.7 RC. In fact, the only issue of substance I can think of is that so far I've been unable to change UP from Bahamasair to United Parcel Service. This issue is caused by the fact that UPS used to use a different IATA prefix.

It took a while to test all this out, since band conditions have been absolutely putrid. When you can't hear San Francisco on 6 or 8 MHz in Los Angeles, you know it's bad.

Sunday, April 13, 2008

Digital Mode of the Week: RTTY (Part 1: ITA2 Telegraphic Alphabet)

We begin a new Utility World series of basic descriptions of digital modes you hear on the air.

First up is one of the oldest modes, but it is still used widely. It's usually called RTTY, for Radioteletype. In the military, you'll also see references to RATT, also Radioteletype.

RTTY's earliest precursor is the Baudot telegraphy code developed in the 1870s by two people working for Jean-Maurice-Émile Baudot, a French engineer. (Yes, that's where we get the transmission speed unit "baud.") This was a 5-bit code, that became known as International Telegraph Alphabet #1 (ITA1, no longer used).

The Baudot code was improved by Donald Murray and others in the early 20th century, leading to an originally Western Union wireline standard called ITA2. This is still the one used for basic English-language Baudot teleprinting. It is still a 5-bit, 32-state, 58-character code, with longer pauses marking start and stop of characters. The name "Baudot" for ITA2 is not technically correct, but in practice the two names are interchangeable. Several ITA alphabets with higher numbers exist for use in other languages, and of course we don't even have time to talk about "third shift" modes for such non-Latin character set languages as Russian.

ITA2 has no lower case. The cases are LTRS (Letters; all upper case) and FIGS (Figures; numbers and punctuation). The case is changed by transmission of control characters corresponding to each. Since the normal mode is letters case, they can also be regarded as shift in (to figures) and shift out. A missed shift character leads to gibberish, and most RTTY systems have the option USOS (Unshift On Space) as a partial (and only partial) solution.

ITA2 bit states are based on timing, and they do not correspond to the base-2 places used in binary numerical notation, which was not used in mechanical teleprinting. Since a bit is technically a "binary digit," we should probably be calling them something else, but we won't. However, note how many of the ITA2 control characters carried over, with bit changes, into later binary computer codes such as American Standard Code for Information Interchange (ASCII).

The 5 bits were typically stored by punching holes in paper tape run through a perforator machine from reels resembling 16-mm movie film. These were read by tape keyers, which controlled a "current loop" connected to a "terminal unit" that interfaced with mechanical teleprinting machines. Many of these were made by the Teletype Corporation, and so "Teletype" is actually an old business trade name, though it has become somewhat generic.

Messages could be relayed by "reperforating" at the receive site. Note that the character pair RY is sent with alternating 01010 and 10101, testing all possible bit states in the code. Even in this electronic age, the test "slip" RYRYRY..., sometimes accompanied by THE QUICK BROWN FOX... is still common.

Here is the ITA2 code (click for bigger image):

Tuesday, April 08, 2008

New Cycle 24 Spot Emerges

After a couple of days with no visible sunspots whatsoever, a tiny Cycle 24 spot has just emerged.

Currently, there is something of a controversy over the precise nature of Cycle 24. While the mainstream prediction is for a fairly energetic cycle, there is an alternate theory. Its proponents argue that the length of Cycle 23 (longest ever recorded) is evidence that Cycle 24 will be late and weak, and in fact will begin a long term decline in solar activity leading to a sort of mini Maunder Minimum (the period in the seventeenth and eighteenth centuries of no sunspots at all, and a mini ice age in Europe). This, of course, would pretty much spell the end of consistent F-region skip propagation above 21 MHz.

Note that recent fading on HF was the result of a coronal hole, not sunspots or solar flares.

We shall see what we shall see.

HF-GCS Goes Crazy

For two days now, activity on the US Air Force High Frequency Global Communications System has been far busier than normal. Right now (0110 UTC) 11175.0 kHz USB is going crazy.

The activity resembles the Nightwatch net (a TACAMO airborne CP and supporting units), but there are way too many players. Emergency Action Messages (EAMs) are going out far more frequently than usual.

At 0110, "Aircraft 113" is working "Aircraft 115" for a radio check.

At 0122 FOUL LINE is passing multiple EAMs, and at 0124 he is "standing by for traffic."

Most other players are also using joint tactical callwords, though there is also a unit with a CHARLIE WHISKEY prefix (US Navy) simultaneously patching to Duty Office. Yes, I have QRM here, though they all seem able to hear each other where they are.

Highly experienced military monitor Jeff Haverlah heard one of the "for" format EAMs, sent to something like six units at once. According to Jeff, this hasn't happened in years.

Best guess is an exercise, and a big one. Anyone who knows what's up can e-mail this column at the usual drops.