[aprssig] indoor tracking.

Wes Johnston aprs at kd4rdb.com
Mon May 30 17:35:10 EDT 2005

Was thinking while I was travelling today and had an idea.  Follow me on

I gave a lot of thought to repeating GPS signals indoors.  This was
disussed last week, and the solution would involve getting a coax thru
the roof at dayton for example.  Even then, everyone would read the same
position as the out side antenna.  So, why not program a fake GPS
transmitter that would transmit a NMEA string that contained that
position?  Do it in such a way that any aprs tracker in proximity would

What if we were to build a IR transmitter that had a real time clock in
it, and each second transmitted a NMEA sentance with a preprogrammed
location and the correct UTC time?  Such a unit could be placed in the
entrances to the halls in dayton and programmed to the location of the
entryway.  By maintaining the approximate correct time, any tiny tracks
programmed for slotted beacons would still operate properly.

With the addition of a simple 40khz (or are they 38khz?) IR detector
assembly, any tracker such as a kpc3, a tiny track, a pocket track, an
opentracker can read serial data over an IR link.  Radio shack used to
sell such things for $4.  If the IR receiver module were wired open
collector to the serial data input of a tiny trak, and the normal GPS
was wired thru 10k resistor to the same serial input pin, when IR
signals were available, the IR serial signal would over power the GPS
serial signal.  Of course when you were outdoors, there would be no IR
transmission, so the IR module would be open collector and the GPS
serial signal would get thru. 

Now, you mount the pseudo IRGPS units to the door way just above the
header.  One on each side beaming a signal downward at 45 degrees.  As
you walk towards a doorway, your tracker "sees" the transmitter on your
side of the door and that transmitter is transmitting a GPRMC position
that is approximated to be, say 1/3 of the way across that room.  As you
walk thru the doorway, your tracker now sees the IRGPS on the opposide
side of the header which is transmitting a position that is about 1/3 of
the way across that room.  As long as the tiny tracks remember their
last known good position, they will remember the position they saw from
the IRGPS when it's time for them to TX.... even if you aren't near an
IR unit at that time.  It won't matter that your regular GPS is sending
bad data all the time.... when the IR signal is present it will override
the GPS data, and when the tiny track does start reading the bad GPS
reciever data, it will indicate no lock, and tiny track should continue
to remember the last good position.

This idea goes far beyond the application of dayton... imagine placing
these IRGPS units at your EOC.  People moving about the building would
be able to be tracked within the building and seamlessly walk outside
and continue to be tracked by a normal GPS unit.

More information about the aprssig mailing list