I thought we settled this one -- but I guess some of us just don't get it.


>> We located urban buses, in some cases down to less than 50cm with GPS.

Having been in the GPS satellite business for some years now - I know that is just plain CRAP !
Unless, of course, you left the GPS receiver stationery for at least 48 hours.


>> We found outside tolerances, in most cases, of no more than 3 meters.

Again, only with a stationery object. But since most parallel tracks are somewhat closer than 3 meters, this is still not good enough to safely tell which track the train is on, as the position could snap to the wrong (route) track. Even if you lock it on to one track, how can you be sure you locked it on the correct track. So still, with or without GPS, unless you use some sort of track based way to physically direct movement authorities solely to the correct train based upon its being on the correct track, you are still just guessing - with public safety at risk here. While a GPS system cannot guarantee that, track based control systems are already capable of such exclusive signaling - it is naturally inherent to their physics.


>> Writing a proper GIS data base of the route, which is closed, and doing a combination of verification, from inertial, wayside, and GPS sensors should correct for even slightly larger variations. The use of the route data base and inertial verification is one of the way to avoid total dependence on GPS signals.

Matching GPS measurements with those "routes" already is an absolutely integral part of using the GPS to locate a train's linear position on the track anyway. To claim it somehow avoids total dependence on the GPS is therefore circular logic. In fact, it was the snapping of an erroneous GPS reading to such a "route", the wrong route, that led to those stupid accidents that OPB referred to.

Inertial sensing of location (or "Dead Reaconning" in any form), as any naval or aviation navigator knows, requires periodic correction; correction from an external data source. So, is it your idea to periodically correct the inertial system with the GPS and then correct the GPS with the inertial data? Again, this is circular logic. Any corrections must come from an external fixed track based appliance.


>> This allows for the loss of satellites due to buildings or tunnels for short intervals.

The most common immediate cause of re-producible GPS errors are in fact, ghost reflections of the GPS signal from large buildings and other large metal objects. Moreover, the satellite cannot even be received under a freeway, let alone in a tunnel. Once it momentarily looses the signal, the GPS must start all over to synchronize and re-locate itself. In a moving vehicle, this can take quite some time; during which, all sorts of not quite accurate data is all that is available.

A friend, who is an electrical engineer, a member of the IEEE, and serves on the A.R.E.M.A. CBTC committees, has told me flatly, that all attempts so far to use GPS like radio ranging technology in tunnels and subways have so far failed - flat on their faces. The problem being that the tunnel walls themselves become a "waveguide" with an infinitely variable and unpredictable radio propagation delay; such unpredictable variability being the result of the infinitely unpredictable mass of the trains themselves. Since the whole GPS technology is based upon a presumed constant radio propagation rate, any unpredictable variability makes it entirely unusable.

>> One of the things we wondered, when we found several USGS markers and used these for verification of these very close tolerances was: how actually accurate were the survey markers? These of course were done by manual survey crew fifty to hundred years ago? Before we even knew about the earth's bulge?

Of course already - if the GPS seemed wrong in your experiments - it just has to be the USGS markers were what were wrong instead. And we still don't know a great deal about GPS signal refraction at the ionosphere. And until we do, we will not be able to compensate for the errors that phenomenon introduces into a GPS receiver.


On a final note: GPS was never designed or intended for safety critical use. And I'm sure your application was not safety critical either; therefore, rendering these anomalies to be mere curiosities. But signal engineers, such as OPB, and my friend at AREMA, must take them all very seriously if the railroads are to use GPS at all.

I hope that Mr. Poleburner's report that MetroLink is preparing to install a conventional track based form of PTS is accurate, and that our no-nothing senators stays out of it. Otherwise, I might never return to riding MetroLink.