[aprssig] "location changes too fast > 500 km/h"

[Heikki Hannikainen]

On Thu, 27 Aug 2009, Keith VE7GDH wrote:

> to Hessu OH7LZB

You can also email me directly if you don't wish to bother the whole 
mailing list :)

> It seems that WA5DKW-1 somehow beaconed a position that placed him in
> Taiwan. It was suggested that the GPS was manufactured in Taiwan and
> had this as its last known position until it figured out where it was.
> Subsequent beacons contained the correct position, but looking at
> aprs.fi, the bogus Taiwanese location was shown on the map, while the
> raw data showed the correct location, but always tagged with
> [Location changes too fast (>500 km/h)].

> The problem of course is caused by them beaconing a bogus position in
> the first place, but for stations that are using aprs.fi to view their
> position, is there a way around this?

Currently, not really, other than me implementing a different kind of "bad 
GPS fix packet" detector. The current detector works fine for many people, 
who have GPSes which suddenly jumps hundreds of kilometers away from the 
correct location after providing a bunch of correct positions. It works 
really badly for people who either (1) have GPSes which, after power-on, 
gives a bad position fix and claims it's good, or (2) have trackers which 
ignore the "this position fix isn't any good" information provided by the 
GPS and transmit it anyway. I hadn't noticed these cases exist yet when I 
was designing the current detector.

There has been talk about a better algorithm which would collect, for 
example, the 5 most recent positions and discard the most recent one if it 
isn't consistent with the previous ones. It could allow huge speeds, too, 
if that's consistent with past behaviour. On the other hand, I would like 
it to filter out the cases where there are 3 stations using the 
same callsign at different fixed locations.

The current algorithm is described in more detail at:

http://oh7lzb.blogspot.com/2008/03/on-duplicate-and-delayed-packets.html

I've tried to mark articles like this under the "how-stuff-works" label:

http://oh7lzb.blogspot.com/search/label/how-stuff-works

   - Hessu