Such system already exists, is called IMES, uses L1 band and is supported by standard Ublox 8 firmware. http://gpsworld.com/wirelessindoor-positioningopening-up-indoors-11603/ Leo > From: Stewart Cobb <stewart.cobb at gmail.com> > Peter Reilley suggests a backup to GPS using terrestrial transmitters. This > idea has been around since the early days of GPS....