First, the alert contains a warning to mariners that their "...SAFETY [is] ENDANGERED WHEN VHF RADIO DISTRESS ALERTS BY DIGITAL SELECTIVE CALLING (DSC) LACK LOCATION AND IDENTIFICATION INFORMATION."
Then the alert delivers what I find to be a surprise:
As the Coast Guard’s new marine radio network Rescue 21 becomes operational throughout the
U.S., rescue centers can now receive instant distress alerts from commonly used DSC-capable VHF
marine radios. However, approximately 90% of VHF DSC distress alerts received by the Coast
Guard do not contain position information, and approximately 60% do not contain a registered
identity. The Coast Guard cannot effectively respond to a DSC distress alert sent from such a radio.
This means that search and rescue efforts may normally be suspended when:
• no communications with the distressed vessel can be established,
• no further information or means of contacting the vessel can be obtained from other sources, and
• no position information is known.
I guess mariners are now on notice: if you send a distress alert with a DSC radio that lacks location and identity information, the USCG may suspend its search for you if the three conditions set forth above are met. What comes as a surprise is the notion that no position information could be obtained unless you sent it in your DSC distress alert message. The Rescue 21 radio network provides excellent direction-finding capability, and it should be able to provide a reasonable position fix for the origin of a distress alert message sent by DSC, even though the transmission duration is rather short. I believe the radio direction finder facilities of Rescue 21 can get a radio bearing on transmissions that are very short duration, measured in milliseconds. [UPDATE: the problem with Rescue 21 is the direction finding receiver is monitoring the voice distress channel, 156.800-MHz or Channel 16. It is not monitoring the DSC channel, 156.525-MHz or Channel 70.]
The USCG Navigation Center website, on the webpage related to special notices and alerts for digital selective calling (DSC) also mentions Marine Safety Alert 04-11.
I guess I have been overlooking this notice for four years. Now that I have become aware of it, I think it is proper to spread the word on this alert. The crux of the alert is to inform boaters that you really ought to get your digital selective calling (DSC) radio connected to a position finding device (like a GNSS receiver or a chart plotter with a receiver built-in) and to properly register and use an maritime mobile service identity (MMSI). Otherwise, the USCG might not really go to the max to find you in an emergency.
The alert comes from the office of the assistant commandant for marine safety, security, and stewardship. I don't know if that office sets official policy for rescue operations, but their warning is a bit ominous. I also find it curious that the alert is said to have been "[d]eveloped by the Spectrum Management and Telecommunications Policy Division (CG-652), United States Coast Guard Headquarters, Washington, DC." Again, I wonder if those guys are in charge of rescue policy.
In any case, the smart money as a recreational boater is to get yourself a DSC radio, get a proper MMSI, and interface the radio to a position finding device. Don't be part of that 90-percent of boaters sending DSC distress alert calls with no position information and for who the USCG may suspend its search.