Clever idea. Most AM radio stations should have decent short-term phase stability. From my experience in the industry, it's quite common to see the frequency to stay within 0.5 Hz of the assigned frequency for days or even weeks at a time and when the frequency moved, it was a very slow drift, like 0.1 Hz/day. Some of the technical problems are interesting. You have to strip off the sidebands to measure the carrier, so perhaps a limiter as found in an FM receiver might be used. Or, PLL to the carrier with a long time constant and measure the PLL. Of course, you would have a sky wave problem to worry about as well, where the signal arrives via both ground wave and sky wave and the phase relationship between the two gets mixed up. That's more of a night problem, of course, as the sky wave in daytime is usually pretty well down compared with the ground wave. Say two hours after sunrise and an hour or so before sunset for minimum sky wave problems. If you were going to use this technique for long distance measurements, I can see quite a few other difficulties, but for distances of a few hundred yards, those wouldn't be a problem. (To take one, you are measuring from the phase center of the antenna. In a directional array, that changes with azimuth. But if you are a few miles from the station and only move a short distance, you can take the phase center as a constant location.) Another interesting ranging approach is used in analog cellular. The base station generates a 6KHZ tone (called a SAT or supervisory audio tone) that is repeated back to the base station by the mobile. The base station measures the phase difference between the outgoing and returned 6 KHz SAT and calculates the distance based on the phase difference. It's a rough distance measurement of course. (The mobile has to meet a certain spec on delay, which is factored into the measurement.) If you can measure to +/- 1 degree, you get a total (out and back) measurement error of 140 meters. Jack -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Olin Lathrop Sent: Wednesday, November 12, 2003 6:42 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE:] GPS vs LPS 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 -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu