Electronic device precision location via local broadcast signals

#This invention relates to precision location of electronic devices. More specifically, the invention relates to a method and apparatus for precision location of electronic devices, particularly indoor, via local broadcast signals.

#Present techniques for locating electronic devices (e.g., cellular phone, PDA or computer, etc.) including indoor locations (such as shopping malls and office buildings) require either: 1) satellite (global positioning signals “GPS”) signals; or 2) GPS and assistance via cellular signals to penetrate building structures, when required; or 3) triangulation using the cellular system. The cell sites are designed to have limited coverage, so finding useful signals from more than two cell sites is unlikely when indoors.

#Presently, two major approaches to GPS precision location dominate. The first, a mostly outdoor, satellite vehicle-based Global Positioning System that receives a feeble code division multiple access “CDMA-like” signal from several satellites in which a receiver (using complex search routines and hardware) determines its position via the delay calculated using the received GPS signal phase, the GPS almanac and ephermis. This procedure takes several minutes in weak signal environments. The second is a system that extends the above system through use of additional information supplied via a cellular wireless network.

#For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following detailed description taken in conjunction with the accompanying drawings in which:

#FIG. 1 illustrates a location system based on triangulation of terrestrial broadcast synchronized signals (cell phone includes lookup table for transmitter locations);

#FIG. 2 illustrates a location system based on triangulation of terrestrial broadcast un-synchronized signals where a local monitoring unit detects and or measures timing error among the transmitters and provides results to the cell phone for its measurement correction;

#GPS based positioning is not the only possible option for location tracking within buildings. The Rosum Corporation advised the FCC's Wireless Bureau on Jun. 29, 2001, that it is developing location technology based on timing of digital television (DTV) signals from distant cities. In an article on Rosum's website (a copy of which is submitted herewith), Rosum claims that at the receiving end, DTV signals are several orders of magnitude stronger than their GPS counterparts, thus, increasing their availability in indoors environments.

#Unfortunately, there is a flaw in Rosum's argument: their system uses the DTV signals from adjacent cities to effect triangulation with sufficient angular spread. These DTV signals are weak and distorted. The DTV sync signal (similar to a cellular pilot signal) is broadcast at over twice the power as the information to insure that the MPEG-2 encoded signal does not loose lock with the transmitter and hence, loose the picture. Rosum claims that the DTV signals are several orders of magnitude stronger than their GPS counterparts. This is true for local stations within a given city. But the CCIR propagation model (which they use) has a propagation loss of r−5 beyond 15 km and r−9 to the effective horizon. If a 100 kW ERP transmitter (+70 dBm) has a 90 dB loss from a distance city, the effective received power would be −20 dBm. Compared with GPS (−130 dBm) this is a quite a strong signal. The problem is the dispersion of the DTV signal through the atmosphere must be corrected in Rosum's proposed system. GPS uses two frequencies (L1 and L2) to correct for near-line-of-sight propagation dispersion through the atmosphere. Rosum uses a local receiver (similar to the European GSM E-OTD LMU) to correct for propagation errors from distant cities.

#Rosum uses DTV signals from asynchronous transmitters in distant cities for two reasons: 1) The digital transmitters do not need to be synchronous since acquisition (changing channels) requires a short time; and 2) The DTV broadcasters have located (and will continue to locate in the future) their antennas on the same towers currently housing antennas for present analog television transmissions.