D-PRS is the act of converting Icom GPS information generated by Icom D-STAR
radios in GPS mode into TNC2 fomatted APRS strings. It is important to note that
D-PRS is not a protocol but a conversion specification. D-STAR digital voice
mode (DV mode) is a 4800 bps digital data stream with no error detection or
correction. Each transmission consists of a RF header followed by a 4800 bps
stream of bits. 3600 bps of this stream is dedicated to the AMBE encoded voice.
The remaining 1200 bps is used for synchronization and user-defined
capabilities. Icom made use of this user-defined space to pass radio messages
(simple display messages) and to pass serial data.
There is about 900 bps available for the serial data. Icom saw an opportunity to
incorporate GPS information into this serial data. The intent was to maintain
the primary purpose of DV mode, sending voice, while providing a potentially
valuable adjunct data stream, GPS information. I designed D-PRS to take this GPS
information and make it presentable to the APRS world. D-PRS removes the need
for designing special-purpose mapping software just for the unique Icom GPS data
stream. Note that Icom's use of the data portion of the DV signal for GPS is an
Icom implementation, not a D-STAR specification. The D-STAR specification
designates the data "sub-channel" but does not dictate what is done with it.
Because there is no error detection or correction on the data sub-channel, D-PRS
mandates a specific message format for radios running in GPS mode. This message
can be calculated on the D-PRS Calculator page. Icom introduced a new GPS mode
called GPS-A mode which is shorter in format and includes a CRC. I recommend
using GPS-A mode on GPS-A capable radios as a CRC is more reliable than an XOR
checksum and the likelihood of an error disrupting a single GPS-A line is less
than the likelihood of interrupting 3 lines in GPS mode. A full description of
D-PRS was delivered to the DCC conference in 2007 and can be found in the
D-PRS white paper.
Radios in GPS-A mode do not need to set their GPS message per the D-PRS
Calculator page. Instead, they should set their UNPROTO to:
- IC-2820 - API282,DSTAR*
- IC-92A - API92A,DSTAR*
Radios should beacon no more often than every 5 minutes. If you are active
on voice, turn beaconing off since everytime you transmit, GPS information is
transmitted if GPS or GPS-A mode is on.
Only the RMC and GGA GPS strings are used in GPS mode. Set your radio to only
use those two GPS strings.
If you are within range a repeater, set your URCALL to CQCQCQ, RPT1 to the
repeater call, and RPT2 to the gateway call (repeater call with "G" in the
eighth position). This will allow your data stream to reach the gateway but
prevent it from going any further. Most D-STAR repeaters now have D-PRS
implemented on the gateway. You do not have to be registered with the gateway
for this to work.
D-PRS Interface is a Windows GUI application that lets you run a mapping program
locally and watch what is happening on your local frequency. More information on
D-PRS Interface and its non-Windows equivalent can be found elsewhere on this
web site.