Brooke Clarke wrote:
> Your #1 is not Differential GPS, but what's commonly called "Poor Man's
> DGPS". ...

Nice writeup on various GPS and other positioning schemes, thanks.

One thing I haven't heard anyone mention is a scheme I thought of 20 years
ago before there was GPS.  I do some trail maintenence and other outdoor
things, and I wanted to somehow record my position as I walked so that the
path could be plotted on a map after I got home.

I was thinking of using existing AM radio stations as beacons at fixed
locations.  This scheme requires a base unit which can support any number
of mobile units.

Plant the base unit at a fixed known location.  Calibrate by taking mobile
unit readings for a minute or so at two known directions and distances
from the base unit (50 meters west, 50 meters south).  Then you can walk
around wherever you want to.  When you get back home, download the data
from the base and the mobile unit, and the computer can reconstruct your
track.

The way it works is that each unit locks onto the carrier of at least
three, preferably 5-10 AM radio stations.  At regular intevals, like every
100mS, each unit stores the current count of carrier cycles for each
station.  The counter only needs to be big enough to not wrap within the
sample period.

The sequence of counts therefore saves the phase relationship between the
carriers over time.  Since only whole carrier cycles are counted, each
individual measurement is +- 1/2 carrier cycle.  However, with 10
measurements per second and considering the walking speed of a human, the
error can be greatly reduced by filtering nearby samples.  For greater
accuracy, sample more often or move more slowly.

The purpose of the base station is to track the phases of all carriers at
a known fixed location over time.  Frequency shifts of the transmitters
are therefore recorded and can be cancelled out later.  The original
calibration step in effect allows for triangulating the location of each
transmitter.

This scheme was interesting to think about, but didn't seem useful anymore
once GPS became available.  I never did get around to building real
hardware.  Oh well.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu