[aprssig] indoor APRS

Jim Lux jimlux at earthlink.net
Tue May 31 13:54:43 EDT 2005

Lots of interesting posts yesterday about location finding indoors.

As it happens, this is an area of great interest in the wireless comm 
business world, and there's also quite a lot of research interest (check 
out the work of Deborah Estrin and her students at UCLA, for 
instance).  Just about all the techniques described in the various emails 
have been done in one form or another, so you'd want to avoid "reinventing 
the wheel".

A couple generic comments:

1) What's the goal here?  Is it to push your location out to others, or is 
it to let your station know where it is?   The usual APRS model is that the 
station autonomously figures out where it is (either by keyboard entry, or 
from GPS, or any other technique that appeals to you), and then it 
broadcasts that position to all and sundry.  In general, you don't actually 
care what your position is (e.g. there are a number of "displayless" 
trackers around), just that it gets somewhere else.

So, if in a particular location there's some magic box that can figure out 
where you are, and push that information out, that would meet the need.

The appeal of the latter approach is that if you do it right, the roving 
nodes don't need ANY modification.  They could beacon their RF signal 
(indicating they're not getting any GPS fixes), and the magic box figures 
out where they're coming from, and deals with it accordingly.  No adding 
hardware to the rovers, no icky IR pods, etc.

2) If the rovers DO need to know their position, AND, they need to know it 
autonomously, then that's a generic localization problem, and one that has 
been studied copiously in the robotics world.  Google for "Where Am I?" by 
Borenstein at UofMich.  It's a several hundred page report that describes 
just about every method known to mankind. 
(http://www-personal.engin.umich.edu/~johannb/ will get you started).

In the APRS world, if you had the "magic locator box", then it could send 
the positions back to the rovers in a properly formatted message. I don't 
know if one has been defined in the APRS spec, but there should be one, 
because this scheme is at least as common in the robotics world as self 
localization.  Semantically, the message would be:  "From: MASTERBOX To: 
W6RMK-15 Payload: Your position is xxxx."  Whether W6RMK-15 reformats and 
repeats it is another story.  Or, whether the message should really be 
"From: MASTERBOX To:BIGWIDEWORLD Payload: W6RMK-15's current position is 
estimated to be xxxxx."  and let the rest of the world sort out who's got 
the best fix on W6RMK-15 (assuming that W6RMK-15 is also sending out it's 
own guess of the location).

3) As to mechanizations...

Assuming you want to use the 144.39 MHz transmission.   With a remarkably 
small number of receiving stations, just using power alone can get you 
fairly accurate positioning. (especially if all you want to know is which 
room you're in)  Sure, there's multipath, etc., but averaging over a 
reasonable time span (you're not driving at 60 mi/hr, after all) should 
work well.  You can also use some direction of arrival information.  I 
wouldn't use pseudo doppler... use a classical pair of figure 8 patterns 
(Adcock) or 3 antennas with coherent receivers, and do the processing on a 
PC at baseband.

If you've got multiple stations, you can use time of arrival (use the first 
arrival to edit out multipath), which is essentially a fancy phase 
measurement. You need stable oscillators at each station, but that's not 
all that tough these days, and you can transmit a low level continuous cal 
tone as well.

The best strategies use a combination of signal strength, direction of 
arrival, and multiple stations.  There's a lot of work in the acoustic area 
for this kind of thing (if you want to do high quality speech recognition, 
being able to figure out where the speaker(s) is (are) helps a whole lot).

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