DSC Distress Alert Message Test
http://continuouswave.com/whaler/refere ... sTest.html
I recently revisited the testing of the older RTCM SC-101 rated radio to observe its behavior when RECEIVING position information from other DSC radios. The results were again very surprising. I will present them here:
RTCM SC-101 RADIO RECEIVING DISTRESS ALERT
A distress alert was sent from a CLASS-D radio and received on an RTCM SC-101 radio. The display of the receiving radio failed to indicate position information. The radio's display only presented a message that a distress alert had been received. A careful reading of the receiving radio instruction manual did not provide any method to find the position information that was transmitted. The instruction manual explicitly failed to mention any notion of the the position of the vessel originating the distress alert. Here is an except from the manual:
- Receiving DSC calls
--Receiving a distress call
While monitoring Ch 70 and a distress
call is received:
- --Emergency alarm sounds for 2 minutes.
--Push any switch to stop the alarm.
--“DSC” appears and “RCV Distress” scrolls in the display,
then Ch 16 is automatically selected.
--Continue monitoring Channel 16 as a coast station may require
RTCM SC-101 RADIO RECEIVING POSITION REPORT
A position report was sent from a CLASS-D radio and received on an RTCM SC-101 radio. The display of the receiving radio only indicated the position information to one minute of resolution. This outcome is actually most surprising in that the radio displayed the position of the other vessel on a non-emergency position report, when it had failed to display the position of the vessel when sent in a distress alert.
RTCM SC-101 RADIO NMEA-0183
A further problem with the DSC radio under test that conformed only to RTCM SC-101 recommendations was a complete lack of any NMEA-0183 data output from the radio. This means the radio cannot be interfaced in any way to an electronic chart plotter display. More modern radios qualified to DSC CLASS-D typically provide a NMEA-0183 (or a NMEA-2000) communication port. When those radios receive DSC calls they typically produce a corresponding NMEA protocol output; the data can be connected to and displayed on a electronic chart plotter. In the case of a distress alert message, the ability to be able to visualize the position of the vessel in distress on an electronic chart in relation to your own vessel's position provides extremely useful information. You can immediately know the distance and bearing to another vessel from the navigation computer in the chart plotter, and that information is critical if being able to offer assistance in a distress situation.
RTCM SC-101 RADIO RECEIVING SUMMARY
The two behaviors of an RTCM SC-101 radio when interoperating with a CLASS-D DSC radio reveal further shortcomings of the RTCM SC-101 radio. The omission of the position information received from a DISTRESS ALERT call is the most serious deficiency in the radio. The loss of the position information sent by the vessel in distress is unforgivable. It is hard to understand how the receiving radio could not provide this information, particularly when the radio was able to provide position information received in a routine, non-distress, position report from another vessel.
The lack of any NMEA communication port on the RTCM SC-101 radio further degrades its usefulness in providing information about the position of other vessels received via DSC position reports.
The testing has reveal further and very serious deficiencies in the behavior of RTCM SC-101 radios. The decision of the FCC to ban the manufacture, import, sale, or installation of these older radios is justified by the poor behavior of these radios, particularly in reception of DISTRESS ALERT calls and the failure to display position information.
Although the Coast Guard has clarified that continued use of DSC radios with only RTCM SC-101 radios already installed is permitted, the risk to safety (of your own boat and to other boats) seems significant. Considering that a modern, new VHF Marine Band radio qualified to CLASS-D DSC features can be purchased for less than $150, the cost to eliminate the risk to safety posed by continued use of older DSC radios is very modest compared to the risk of failed rescue that those radios create.
The RTCM SC-101 radio tested was an ICOM IC M-402. The CLASS-D radio used to originate the test transmissions was a Standard-Horizon GX1500.
The information on my recent tested has been added to the initial article in the series on DSC DISTRESS ALERT testing.