[aprssig] Automatic omni DF query

Scott Miller scott at opentrac.org
Mon Feb 9 13:44:08 CST 2009

My new T2-301 trackers are frequency-agile and can measure incoming 
signal strength, so it occurred to me that it'd be useful to define an 
APRS query that'd cause all capable stations to QSY their receive side 
to the specified frequency for a second, take a reading, and then come 
back and report what they heard in omni-DF format.

Before anyone asks, these particular units won't work for 121.5 - I've 
only been able to get them to receive as low as 127 MHz.

Based on some experimentation with a service monitor, it looks like the 
usable RSSI range is roughly -128 dBm to -68 dBm.  If I'm reading my 
conversion chart right, for a 50-ohm system that's a little more than 
the standard S0-S9 range.  Seems kind of low at the top end, but I'll 
have to do some field testing and see what I get.

What I'm thinking of is something like this:

?DF146.520?<optional footprint>

The response should be a DFSshgd omni-DF extension followed by the 
frequency.  I'd prefer the frequency to be a standard part of that 
response, so clients could be made smart enough to separate out multiple 
DF plots.

Comments or suggestions?


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