DSC Class-D v. Class-H

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
Posts: 11862
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

DSC Class-D v. Class-H

Postby jimh » Sat Apr 27, 2019 4:38 pm

The DSC Class-H radio is a handheld radio that has a minimal set of features to be easy to operate by people who might not be qualified as GMDSS radio operators. Previously some manufacturers, notably Standard-Horizon with their HX870 handheld, made handheld radios that were qualified as Class-D radios, that is, they met the minimum feature set required for DSC Class-D, even though that standard was not specifically intended for handheld radios.

Many fixed-mount radios qualified to DSC Class-D have included a feature in the category of VHF, ROUTINE, INDIVIDUAL calls known as POLLING. POLLING is a type of call in which one vessel sends a DSC request to a second vessel to have the second vessel's DSC radio reply with the position of the second vessel. Although this now is thought by many boaters to be a feature of all Class-D VHF Marine Band radios, that feature is not required. A careful study of the ITU-R M.493 recommendation, "Digital selective-calling system for use in the maritime mobile service", will show this to be true. For a second opinion, the USCG NavCen website page about DSC Classes also shows the same information. Only Class-A DSC radios are required to support polling.

Standard-Horizon was perhaps the first manufacturer of VHF Marine Band radios to introduce the POLING feature. Many boaters have come to expect that feature in a Class-D radio, even though POLLING is not a required feature.

When Standard-Horizon came out with their HX870 handheld radio, it was able to met all DSC Class-D requirements, and the radio also included some POLLING features. More recently, Standard-Horizon introduced a model HX890 handheld radio qualified to Class-H for DSC. The HX890 radio provide fewer polling features than the HX870 predecessor. Some have inferred that the change in POLLING features in these two DSC radios was attributed to the difference in Class-H and Class-D certification. But inasmuch as neither Class-D or Class-H required POLLING, that explanation for the difference in the two handhelds seems wrong. Apparently, any difference in POLLING features is due to the manufacturer's design of the radio and its features.

From what I can tell from anecdotal reports from users of both radios--I do not own either of them--the older HX870 radio can transmit a POLLING request, while the HX890 cannot. Both radios can respond to or ACKNOWLEDGE POLLING requests. This means a handheld HX890 cannot initiate polling requests from other ships, but it can respond to a POLLING request if another ships requests one. Or at least that is my inference from reading various reports about the two radios.

If anyone reading this owns a HX890, perhaps they can comment about what support is provided by those radios in regard to POLLING another vessel with a VHF, ROUTINE, INDIVIDUAL DSC call.