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Meant to send this to the group...
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How long have you been listening to these stations? YAAC implements a feature called "vicinity plotting", where, for stations that are transmitting, but haven't yet transmitted a _valid_ position report, Mic-E report, or status message with Maidenhead position, YAAC guesses where the station might be based on the location of the first digipeater or I-gate (if their position is already known) to forward the station's packets. If the original station is sending _invalid_ position reports, they don't count for accurate location, as YAAC can't decode them.

As to why aprs.fi can properly locate those stations, that could be for either of two reasons:

1. aprs.fi has more capability to decode more invalid formats than YAAC does (assuming the stations are sending invalid-format position reports).

2. aprs.fi has a long-term database of history, and may be using the recorded last decodable position for those callsign-SSID combinations.

Note that YAAC can also use vicinity plotting in reverse to guesstimate the location of non-position-beaconing ("stealth") digipeaters that only meet their beaconing requirements by embedding their callsigns in the digipeat path (TRACE mode).