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[aprssig] APRS over HF PSK-63 using a new program

Chris Moulding chrism at crosscountrywireless.net
Tue Jan 19 17:44:22 UTC 2010


Hi,

I'm developing a version of my APRS program APRS Messenger to add APRS 
over HF PSK-63 to improve performance over the existing 300 bd packet HF 
system.

I know that a similar system came out some years ago as an add-on to 
UI-View but it didn't have error checking or receiver AFC.

I've got a version nearly ready for on air testing but before I release 
anything I would like to check a few points with the group.

I've tried to follow the existing AX.25 format as far as possible. For 
the on-air PSK-63 data I've added start and stop flags using the "~" 
character to the usual APRS message and beacon formats. I've added a 
short preamble of 5 dots "....." before the start flag to allow the 
receive station AFC to capture the incoming signal.

For the frame checksum I'm using the standard CRC-16 Modbus checksum to 
provide a four character checksum. This is different to the usual two 
character APRS frame checksum but it is a known standard providing a 
stronger error check with lots of code available on the internet for 
others writing similar programs.

I've added a receive only Igate to the program to port received packets 
stripped of the start / stop flags and frame check to the APRS-IS. The 
igated packets will be identical to packets received from existing VHF 
APRS clients. There will be no facility in the program to send internet 
packets to RF and no facility to digipeat. I'm trying to avoid the 
problems we see on HF APRS in Europe at present.

The program has a sound card PSK-63 modem built in with receive AGC and 
AFC so that signals over a ± 100 Hz range can be decoded. A receive 
squelch feature is added so that the transmitter will not key until the 
channel is seen to be relatively clear.

My main question is about the use of the CRC-16 Modbus checksum. Would 
this cause any problem? I'm willing to share the source code for the 
checksum with other developers and the program will be freeware so that 
all radio amateurs will be able to download it and use it after it's had 
some testing.

I would appreciate any comments.

73,

Chris, G4HYG

-- 
Chris Moulding
Cross Country Wireless (2009) Ltd
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax: +44(0)1204 410626
Mobile:  +44(0)7752 391908
Website: http://www.crosscountrywireless.net
Company number 6780346 registered in England and Wales  




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