Taroona
23-10-2009, 06:21 AM
I was looking to buy a new VHF with DSc and found this article. It appears that there is a bit of dumping of radio's going on that only have SC-101 capability for sending DSC distress signals. So if you are going to buy one make sure it is built to Class D requirements as a minimum
Les
VHF DSC compatibilityby Andrew Boon*
Further to the article Coastal Marine Communications by Jonathan Neeves (Afloat Apr’09) and other correspondence, the Coast Radio Hobart technical team’s experience might add a bit more confusion to the capabilities of VHF DSC.
Before installing a DSC monitoring capability, Coast Radio Hobart tested various models of VHF DSC radios. Our original intention was to establish a facility for vessels to automatically report their positions, either ad hoc or in response to a poll from a shore station, in addition to listening for DSC distress calls.
The idea was to install a fixed VHF receiver at one of our base station sites and feed the received signal into a computer running a programme called DSCdecoder.
This software has been used extensively in Europe and America for some years, but we didn’t get to first base with it. There is no local VHF DSC traffic so we set up two Standard Horizon GX1256S radios and made test calls. DSCdecoder could not decode any of the calls. The reason was that the baud rate of the radios was 0.6% slower than the nominal rate; this took a couple of weeks to find, with much cooperation from the programme’s author.
At the same time, we did a search of the internet for any reference to incompatibilities between various models of DSC radio. There is not much written about this subject (only one US Coastguard reference found), so we might have concluded that all is well, but subsequent testing dispelled that idea.
There are two standards relevant to recreational vessels (ie not requiring full GMDSS compliance): Class D and SC-101.
The Class D is an international specification (which has been altered slightly over the years) and SC-101 is a US standard which is now obsolete. All new DSC radios produced should meet the Class D standard.
A major difference between Class D and SC-101 is the lack of a dedicated DSC receiver in the SC-101 units; if you are listening to a transmission on another channel, you will not receive a DSC call.
We set up five radios on the bench and made DSC calls between them. The radios were: Standard Horizon GX1256S (two units), Icom IC-M422, GME GX600D and a Uniden Oceanus. Some of the results are listed below.
1. When a ‘Position Send’ (or ‘Position Report’) was sent to the Icom M422 from any of the other radios, the M422 displays the received latitude and longitude on a scrolling display, but it does not display the sending vessel’s ID (MMSI) or the time of the report.
2. When a Standard Horizon GX1256S or the Uniden Oceanus sends a ‘Position Request’ to the M422, the Icom sends the information (it was set for auto-acknowledge) and all information (MMSI, time, Lat/Lon) is displayed on the GX1256S and the Uniden. If the GPS is disconnected from the Icom, the response to a Position Request has the time of the last valid GPS position attached.
3. When the GME GX600D sends a ‘Position Request’ to the M422, the M422 replies (separately monitored) but the GME displays ‘No Reply’. The GME sends Position Request calls as ‘Safety’ category, the other radios send them as ‘Routine’ category. This affects the alert tone of the Icom (louder for Safety than for Routine).
4. When the Standard Horizon GX1256S sends a ‘Position Request’ to the GME GX600D, there is no response from the GME. (Note that the latest Class D specification does not require the capability for polling and position reporting calls.)
5. When the Standard Horizon sends a Position Request to the Uniden, the Uniden responds and all information is displayed. If the GPS is disconnected from the Uniden, it responds and the Standard Horizon displays ‘NO POSITION DATA’.
6. When any radio sends a Group Call, all other radios respond appropriately. (The GME did not respond with the group MMSI we initially used 015652500. Make sure your Group MMSI starts with 0503!)
7. When the Standard Horizon, GME or Uniden radios make an Individual Call to a shore station (the second Standard Horizon with MMSI 00503135), the shore station radio responds appropriately (alert tone, all radios switch to voice channel).
8. When the Icom M422 sends an Individual Call to the shore station, the shore station replies with an ‘Unable To Comply’ response and does not sound an alert tone. The Icom is an SC-101 radio but it complies with the Class D requirement that it not specify a voice channel when making an individual call to a shore station – the shore station is supposed to select the channel.
9. When the GME sends an Individual Call to the Icom, the time taken to send a manual acknowledgement from the Icom is too long for the GME to wait and it times out. The Icom cannot be set to auto-acknowledge Individual Calls.
Of the radios tested, only the GME was rated as meeting the DSC Class D standard. The Uniden claimed to meet Class D requirements but did not have a dedicated Ch70 DSC receiver (you can check this by opening the mute while on a voice channel and sending a DSC call to the radio – if it fails to respond to the DSC call, it does not have a dedicated Ch70 DSC receiver and is not Class D). The other radios were built to the SC-101 standard.
These tests are by no means conclusive, but based on our experience to date, it is no wonder VHF DSC is so little used. Any shore system we set up would have to cater for both Class D and SC-101 units, especially as the prices of the SC-101 units are now being discounted to move them off the shelves.
There’s a lot of them around. The long term future of a DSC-based position reporting system doesn’t look good, as any new radios may not have that feature. Maybe AIS will help!
Back to Jonathan Neeves’s article, in which he makes the statement: “So for those vessel owners with DSC enabled VHF radios – do not rely on your DSC emergency call to prompt your rescue – nobody is listening!” This has been disputed, as many VMR stations as well as ship stations are monitoring channel 70 for at least some of the time.
However, unless the VMRs have DSC equipment meeting Class A specifications (full GMDSS compliance for big ships), they (and ship stations other than Class A) are not able to acknowledge your DSC distress call.
If you have an SC-101 radio, there is a good chance that your radio will stay on Ch70 waiting for an acknowledgement, while the stations that received your call are voice-calling you on Ch16.
Try and remember that when it all hits the fan and you’re thinking that everyone is ignoring you!
There is value in using the DSC distress button – receiving stations will see your position displayed – but the procedure must be to manually switch to Ch16 after you have pressed the DSC distress button.
Coast Radio Hobart will monitor channel 70 for DSC traffic. We might hear you if you selectively call us, depending on what type of radio you have. We will hear you if you call us on channel 16.
* Andrew Boon is a member of Coast Radio Hobart technical team. aboon@bigpond.com
Les
VHF DSC compatibilityby Andrew Boon*
Further to the article Coastal Marine Communications by Jonathan Neeves (Afloat Apr’09) and other correspondence, the Coast Radio Hobart technical team’s experience might add a bit more confusion to the capabilities of VHF DSC.
Before installing a DSC monitoring capability, Coast Radio Hobart tested various models of VHF DSC radios. Our original intention was to establish a facility for vessels to automatically report their positions, either ad hoc or in response to a poll from a shore station, in addition to listening for DSC distress calls.
The idea was to install a fixed VHF receiver at one of our base station sites and feed the received signal into a computer running a programme called DSCdecoder.
This software has been used extensively in Europe and America for some years, but we didn’t get to first base with it. There is no local VHF DSC traffic so we set up two Standard Horizon GX1256S radios and made test calls. DSCdecoder could not decode any of the calls. The reason was that the baud rate of the radios was 0.6% slower than the nominal rate; this took a couple of weeks to find, with much cooperation from the programme’s author.
At the same time, we did a search of the internet for any reference to incompatibilities between various models of DSC radio. There is not much written about this subject (only one US Coastguard reference found), so we might have concluded that all is well, but subsequent testing dispelled that idea.
There are two standards relevant to recreational vessels (ie not requiring full GMDSS compliance): Class D and SC-101.
The Class D is an international specification (which has been altered slightly over the years) and SC-101 is a US standard which is now obsolete. All new DSC radios produced should meet the Class D standard.
A major difference between Class D and SC-101 is the lack of a dedicated DSC receiver in the SC-101 units; if you are listening to a transmission on another channel, you will not receive a DSC call.
We set up five radios on the bench and made DSC calls between them. The radios were: Standard Horizon GX1256S (two units), Icom IC-M422, GME GX600D and a Uniden Oceanus. Some of the results are listed below.
1. When a ‘Position Send’ (or ‘Position Report’) was sent to the Icom M422 from any of the other radios, the M422 displays the received latitude and longitude on a scrolling display, but it does not display the sending vessel’s ID (MMSI) or the time of the report.
2. When a Standard Horizon GX1256S or the Uniden Oceanus sends a ‘Position Request’ to the M422, the Icom sends the information (it was set for auto-acknowledge) and all information (MMSI, time, Lat/Lon) is displayed on the GX1256S and the Uniden. If the GPS is disconnected from the Icom, the response to a Position Request has the time of the last valid GPS position attached.
3. When the GME GX600D sends a ‘Position Request’ to the M422, the M422 replies (separately monitored) but the GME displays ‘No Reply’. The GME sends Position Request calls as ‘Safety’ category, the other radios send them as ‘Routine’ category. This affects the alert tone of the Icom (louder for Safety than for Routine).
4. When the Standard Horizon GX1256S sends a ‘Position Request’ to the GME GX600D, there is no response from the GME. (Note that the latest Class D specification does not require the capability for polling and position reporting calls.)
5. When the Standard Horizon sends a Position Request to the Uniden, the Uniden responds and all information is displayed. If the GPS is disconnected from the Uniden, it responds and the Standard Horizon displays ‘NO POSITION DATA’.
6. When any radio sends a Group Call, all other radios respond appropriately. (The GME did not respond with the group MMSI we initially used 015652500. Make sure your Group MMSI starts with 0503!)
7. When the Standard Horizon, GME or Uniden radios make an Individual Call to a shore station (the second Standard Horizon with MMSI 00503135), the shore station radio responds appropriately (alert tone, all radios switch to voice channel).
8. When the Icom M422 sends an Individual Call to the shore station, the shore station replies with an ‘Unable To Comply’ response and does not sound an alert tone. The Icom is an SC-101 radio but it complies with the Class D requirement that it not specify a voice channel when making an individual call to a shore station – the shore station is supposed to select the channel.
9. When the GME sends an Individual Call to the Icom, the time taken to send a manual acknowledgement from the Icom is too long for the GME to wait and it times out. The Icom cannot be set to auto-acknowledge Individual Calls.
Of the radios tested, only the GME was rated as meeting the DSC Class D standard. The Uniden claimed to meet Class D requirements but did not have a dedicated Ch70 DSC receiver (you can check this by opening the mute while on a voice channel and sending a DSC call to the radio – if it fails to respond to the DSC call, it does not have a dedicated Ch70 DSC receiver and is not Class D). The other radios were built to the SC-101 standard.
These tests are by no means conclusive, but based on our experience to date, it is no wonder VHF DSC is so little used. Any shore system we set up would have to cater for both Class D and SC-101 units, especially as the prices of the SC-101 units are now being discounted to move them off the shelves.
There’s a lot of them around. The long term future of a DSC-based position reporting system doesn’t look good, as any new radios may not have that feature. Maybe AIS will help!
Back to Jonathan Neeves’s article, in which he makes the statement: “So for those vessel owners with DSC enabled VHF radios – do not rely on your DSC emergency call to prompt your rescue – nobody is listening!” This has been disputed, as many VMR stations as well as ship stations are monitoring channel 70 for at least some of the time.
However, unless the VMRs have DSC equipment meeting Class A specifications (full GMDSS compliance for big ships), they (and ship stations other than Class A) are not able to acknowledge your DSC distress call.
If you have an SC-101 radio, there is a good chance that your radio will stay on Ch70 waiting for an acknowledgement, while the stations that received your call are voice-calling you on Ch16.
Try and remember that when it all hits the fan and you’re thinking that everyone is ignoring you!
There is value in using the DSC distress button – receiving stations will see your position displayed – but the procedure must be to manually switch to Ch16 after you have pressed the DSC distress button.
Coast Radio Hobart will monitor channel 70 for DSC traffic. We might hear you if you selectively call us, depending on what type of radio you have. We will hear you if you call us on channel 16.
* Andrew Boon is a member of Coast Radio Hobart technical team. aboon@bigpond.com