Digital Mode Programs That Work #1: DSCdecoder

DSC stands for Digital Selective Calling. It's an automated mode used with the Global Maritime Distress and Safety System (GMDSS). DSCdecoder is a small, sleek little utility which does exactly what its name promises. It decodes DSC calls, on all three bands (MF, HF, and VHF). These are output to the screen with the MMSI (Maritime Mobile Service Identity) of the calling and called station, plus the message type and any optionally transmitted data such as the reply frequency and mode. You can have the program search over the Internet for the MMSI and substitute it with the station/vessel name, if found.

DSCdecoder is good to just leave going and see what accumulates. DSC is not a busy mode, but it's a fun one to DX. This program does a really nice job of grabbing the real weak stuff.

There's more, though. This program also decodes Navtex (Navigational Telex) and Differential Global Positioning System (DGPS). It does a similarly bang-up job on these. It's the best one of several ham-level programs that I use.

Navtex is a Sitor-B service on three primary frequencies. These are 518 kHz (main freq, nearly always in English), 490 kHz (specific nations and languages, not used in the US), and the more lightly used HF frequency of 4209.5 kHz. The Sitor-B waveform is very close to DSC's (on HF), though other parameters are quite different. Therefore, the decoder performs well on it too. Those who like to tweak things, and who can tune accurately, can set up the advanced decoder. The improvement is slight, but definite.

Navtex stations take turns broadcasting schedules of short, highly formatted, maritime safety and weather messages, from different coastal locations. These rotations typically repeat several times a day, usually every 4 hours. Professional GMDSS equipment keeps a list of headers and typically only prints new messages. Most large vessels will have a Navtex printer somewhere on the bridge.

Navtex is useful for weather forecasts, and it's also fun to DX. Navtex stations are designed for specific local coverage areas, but at night they can propagate amazingly well for this type of station.

The extra dividend is that any Sitor-B decoder that can handle Navtex will work on most other transmissions in this mode. These are all over HF, coming from coast guards and meteo services worldwide.

DGPS is an interesting mode, using minimum-shift keying (MSK). It takes some tuning around in its low band to find strong stations. Fortunately, these are being implemented pretty much worldwide.

Stations use two different baud rates, identifiable from their bandwidth. DGPS decoders for our use turn its packetized data streams into lines of rather cryptic text data. I mostly use this for DXing, and I have checked off an amazing number of DGPS stations, considering that it is designed as anything but a DX mode.

Not many PC programs do DGPS. (Mac users are lucky. They have MultiMode, an excellent program.) Two that do are this one and the no longer supported SkySweeper. I've run them side by side, and most of the time DSCdecoder has successfully recovered more packets.

DSCDecoder is available from www.coaa.uk. The first 21 days are free, so you can try it out, get over a bit of a learning curve, and see if it's for you. After that, it's 25 Euros (a little over 32 bucks at today's exchange).

Armed Forces Day SECDEF Message This Saturday

From Army MARS:

SECRETARY OF DEFENSE MESSAGE TEST VIA DIGITAL MODES.
The Secretary of Defense message will be transmitted via digital modes including RTTY, PACTOR, AMTOR, PSK-31, MFSK and MT63 from the stations listed below, including frequencies, mode, and date/time in Zulu (UTC). All frequencies are listed for center of intelligence. Offset as appropriate for your TNC. (Note: Not all stations may necessarily operate on all the frequencies listed, depending on propagation and available equipment.)

Army Stations

STATION: AAZ (HQ Army MARS Gateway, Fort Huachuca, Arizona)

6988.0 kHz
RTTY 11 May 0110Z
PACTOR FEC 11 May 0130Z
MT63 11 May 0220Z
PSK-31 11 May 0250Z
14402.0 kHz
RTTY 11 May 0110Z
PACTOR FEC 11 May 0130Z
MT63 11 May 0220Z
PSK-31 11 May 0250Z

STATION: WAR (Pentagon ARC/MARS Station, Arlington, Virginia)

6988.0 kHz
RTTY 10 May 1700Z
RTTY 10 May 2300Z
MT63 10 May 1715Z
MT63 10 May 2315Z
14440.0 kHz
PACTOR FEC 10 May 1730Z
PACTOR FEC 10 May 2330Z
Olivia 10 May 1745Z
Olivia 10 May 2345Z
Stations copying the Secretary of Defense message transmitted from AAZ/WAR should send their entries to Armed Forces Day Celebration, Commander NETCOM/9th ASC, Attn: NETC-OPE-MA (MARS) (31), Fort Huachuca, AZ 85613-5000.

Air Force Stations

STATION: AIR-2 (Scott Air Force Base)

7831.1 kHz
RTTY 10 May/1930Z
PACTOR 10 May/2000Z
MT63 10 May/2030Z
MFSK 10 May/2100Z
14877.1 kHz
RTTY 10 May/2130Z
PACTOR 10 May/2200Z
MT63 10 May/2230Z
MFSK 10 May/2300Z

Navy/Marine Corps Stations

STATION: NAV (HQ NAVMARCORMARS Radio Station, WILLIAMSBURG, VA)

7346.5 kHz
RTTY 75 baud 10 May/2340Z
AMTOR FEC 11 May/0010Z
MT63 11 May/0040Z
14480.0 kHz
RTTY 75 baud 10 May/2340Z
AMTOR FEC 11 May/0010Z
MT63 11 May/0040Z

STATION: NAV3 (NAVMARCORMARS Radio Station, CORPUS CHRISTI, TX)

7393.0 kHz
RTTY 10 May/2340Z
AMTOR FEC 11 May/0010Z
MT63 11 May/0040Z
13975.5 kHz
RTTY 10 May/2340Z
AMTOR FEC 11 May/0010Z
MT63 11 May/0040Z

STATION: NAV4 (NAVMARCORMARS Radio Station, GREAT LAKES, IL)

7375.0 kHz
RTTY 11 May/0240Z
AMTOR FEC 11 May/0310Z
MT63 11 May/0340Z
14468.5 kHz
RTTY 11 May/0240Z
AMTOR FEC 11 May/0310Z
MT63 11 May/0340Z

STATION: NBL (NAVMARCORMARS Radio Station, GROTON, CT)

7370.0 kHz
RTTY 10 May/2340Z
PACTOR FEC 11 May/0010Z
AMTOR FEC 11 May/0040Z
14393.0 kHz
RTTY 10 May/2340Z
PACTOR FEC 11 May/0010Z
AMTOR FEC 11 May/0040Z

STATION: NPL (NAVMARCORMARS Radio Station, SAN DIEGO, CA)

7350.0 kHz
RTTY 11 May/0240Z
PACTOR FEC 11 May/0310Z
AMTOR FEC 11 May/0340Z
14465.0 kHz
RTTY 11 May/0410Z
PACTOR FEC 11 May/0440Z
AMTOR FEC 11 May/0510Z

STATION: NUW (NAVMARCORMARS Radio Station, NAS WHIDBEY
ISLAND, WA)

7380.0 kHz
RTTY 11 May/0240Z
PACTOR FEC 11 May/0310Z
AMTOR FEC 11 May/0340Z
13530.0 kHz RTTY 11 May/0410Z
PACTOR FEC 11 May/0440Z
AMTOR FEC 11 May/0510Z

SUBMISSION OF SECRETARY OF DEFENSE TEST MESSAGE ENTRIES.

Transcripts of the RTTY, PACTOR, AMTOR, PSK-31, MFSK and MT63 receiving test should be submitted "as received". No attempt should be made to correct possible transmission errors. Provide time, frequency and call sign of the military station copied, including name, call sign, and address (including ZIP code) of individual submitting the entry. Ensure this information is placed on the paper containing the test message. Each year a large number of acceptable entries are received with insufficient information, or necessary information was not attached to the transcriptions and was separated, thereby precluding issuance of a certificate. Entries must be sent to the appropriate military address as follows:

a. Stations copying Secretary of Defense message transmitted from AAZ send entries to:
Armed Forces Day Celebration
Commander NETCOM/9th ASC
Armed Forces Day Celebration
Attn: NETC-OPE-MA (MARS) (31)
Fort Huachuca, AZ 85613-5000
b. Stations copying Secretary of Defense message transmitted from NAV, NAV-3, NAV-
4, NBL, NPL or NUW send entries to:
Armed Forces Day Celebration
Chief, Navy-Marine Corps MARS
Cheatham Annex Bldg 117
108 Sanda Ave
Williamsburg, VA 23185-5830
c. Stations copying Secretary of Defense message transmitted from AIR-2 send entries
to:
Armed Forces Day Celebration
AFCA / Chief, AF MARS
203W Losey St
Scott AFB, IL 62225

Details of the amateur crossband transmitting test are here.

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.

No KSM RTTY Today

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.

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:

MRCG - http://syzen.com/milradio/

M-209 emulator for those who wish to participate but don't have a M-209 -
http://users.telenet.be/d.rijmenants/en/m209sim.htm

MRHS - http://www.radiomarine.org

Press Release: US Coast Guard Continues HF Weather Broadcasts

Press Release
DATE: February 07, 2008 14:39:41 EST
FOR IMMEDIATE RELEASE
Office of Public Affairs
U.S. Coast Guard

Contact: (202) 475-3555


US Coast Guard Continues HF Weather Broadcasts

WASHINGTON - Last April the Coast Guard asked for public comment on the need to continue broadcasting high frequency (HF) high seas weather forecasts for single sideband voice, facsimile charts and text messages over radiotelex (e.g. HF NAVTEX). The Coast Guard required public comment because the infrastructure necessary to provide these services had exceeded its life expectancy and significant costs were involved to continue these services.

After reviewing and analyzing the substantial public response that overwhelmingly urged the continuation of these services, the Coast Guard's "business case study" concluded that it was necessary to continue HF weather broadcasts. The business case study, "An Impact Assessment of Discontinuing USCG High-Frequency Radio Broadcasts of NWS Marine Weather Forecasts" is posted at:
http://www.navcen.uscg.gov/marcomms/high_frequency/HF-WX_notice.htm

The study concluded:

"The responding public collectively perceives that the USCG HF broadcasts are essential to their safety. There is no viable alternative to the USCG HF broadcasts because present alternatives are perceived by the public to be out of financial reach. Also, marine weather forecasts available through these alternative sources may not guarantee the same level of accuracy, timeliness, and/or sufficiency as provided by the USCG HF broadcasts."

While the Coast Guard does not have funds necessary to replace all of its HF transmitters, funds are available to replace the 20 transmitters used for weather broadcasts.

###

The U.S. Coast Guard is a military, maritime, multi-mission service within the
Department of Homeland Security dedicated to protecting the safety and security of America.

------------

The full report is here. It is very comprehensive and informative, and worth the time to get and read it.

------------

Here's more from USCG Navcen:


The Coast Guard's HF infrastructure

The Coast Guard's HF infrastructure consisting of 123 10KW transmitters are no longer supportable. Repair parts are increasingly difficult to find, more expensive, and take can months to obtain. Funds should be available to replace many, but not all of these transmitters. Consequently all but the most essential HF services are or will be terminated. However, due to responsive received from the public and the conclusions of the business case report, the Coast Guard has decided to continue HF broadcasts of high seas weather forecasts and warnings without interruption. Transmitters used for this purpose will be included among those recapitalized.

The Coast Guard uses 20 high power transmitters to broadcast HF weather facsimile, voice and text (SITOR) high seas weather forecasts to mariners. Three additional Navy transmitters are used to broadcast weather information from Guam. The cost to replace one of these transmitters is ~$200K with installation ($4M total for those used for weather broadcasts).

Reliable, high power transmitters are needed to ensure mariners can reliably receive weather information anywhere within the National Weather Service’s area of responsibility.

[Presumably the services to be affected by the phase-out of the other transmitters include those on this list that are not required by GMDSS or other treaties. More when it happens. -Hugh]

USCG Seeking Input on HF Services

Here we go again. The frequently threatened HF services from the US Coast Guard are once again up for review. Users are requested to let the CG know how they use the service, and presumably if there is insufficient interest the services will be discontinued.

Reason given is that the equipment has reached the end of its service life, and new stuff costs money.

The following notice was recently published in that US Government catch all called the Federal Register:

Notice [USCG– 2007– 27656] in Federal Register / Vol. 72, No. 80 / Thursday, April 26, 2007

"High Frequency (HF) Radio Broadcasts of Marine Weather Forecasts and Warnings" (Summary below) for complete text, see: http://www.nws.noaa.gov/om/marine/waisgate.pdf

This is a necessary step in the Coast Guard's justification of
recapitalizing their HF equipment for continued operations.

Details of USCG weather broadcasts via HF radiofacsimile, voice and SITOR may be found at: http://www.nws.noaa.gov/om/marine/home.htm#dissemination

As a service to mariners, please inform potentially affected users of this Notice. It is very important that mariners reply via the Docket Management system or postal mail as called for in the Notice, or their input will not be taken into account.

Federal Register: April 26, 2007 (Volume 72, Number 80)] [Notices] [Page 20863-20865] From the Federal Register Online via GPO Access [wais.access.gpo.gov] [DOCID:fr26ap07-72]

And here's the accompanying letter from the Coast Guard:

DEPARTMENT OF HOMELAND SECURITY

Coast Guard

[USCG-2007-27656]


High Frequency (HF) Radio Broadcasts of Marine Weather Forecasts and Warnings

AGENCY: Coast Guard, DHS.

ACTION: Notice; request for public comments.

-----------------------------------------------------------------------

SUMMARY: The Coast Guard is soliciting public comment on the need to continue providing high frequency (HF) radio broadcasts of weather forecasts and warnings. Public comment is necessary in order to assess the demand for the HF radio broadcasts of weather forecasts in each of three forms: (1) Radiofacsimile; (2) voice; and, (3) Simplex Teletype Over Radio (SITOR), also known as Narrow Band Direct Printing (NBDP). The infrastructure necessary to provide these services has exceeded its life expectancy; the equipment is no longer manufactured, repairs are difficult to accomplish, and spare parts generally are not available. Because of the very significant costs involved to continue these specific HF radio services, the Coast Guard requires information on the extent to which these services are used by the public and what alternative services are being used or are available to obtain weather forecasts and warnings.

DATES: Comments and related material must reach the Docket Management Facility on or before August 24, 2007.
for complete text see a copy of the Notice at:
http://www.nws.noaa.gov/om/marine/waisgate.pdf

As we have seen before, the fate of USCG HF broadcasts is now in the hands of the users. The fax/weather/MSI you save will be your own.