Location tracking devices are well known and operate to provide a signal, indicating their present location, to a receiver at a remote location. Location tracking devices can be used, for example, to locate: vehicles, for fleet management, loss prevention, etc.; cargo, for logistics and loss prevention; people, for security and manpower management, etc.
Location tracking devices typically comprise a small device which includes location determination circuitry and radio transceiver circuitry to report the location determined by the location determination circuitry to a receiver at a remote location. Early location tracking devices generally employed radio direction finding (RDF) or similar technologies as the location determination circuitry while, more recently, most location tracking devices employ signals from a satellite-based positioning system, such signals from the Global Positioning System (GPS).
Location tracking systems employing satellite-based positioning are typically much preferred over systems employing other location determination technologies due to the accuracy which can be obtained with GPS and/or other satellite-based systems and due to the fact that, unlike RDF-based systems, the GPS infrastructure (the constellation of GPS satellites and related ground systems) is maintained and operated at no direct cost to entities which use it.
While GPS and other satellite-based systems offer many advantages over RDF-based or other location technologies, they do suffer from disadvantages in that, due to the high radio frequencies and relatively low power levels at which the signals are transmitted, reception of signals from the satellites at a receiver is easily compromised by obstructions such as geographic features, buildings, trees, etc.
To determine a location, a GPS receiver must have current almanac data and ephemeris data for at least three appropriate (depending upon their location relative to the GPS receiver) satellites and the receiver must have an initial estimate of its location.
Each GPS satellite broadcasts the almanac data for the entire GPS system and each GPS satellite broadcasts its own ephemeris data. One of the disadvantages of the GPS system is that almanac data and ephemeris data is transmitted from the GPS satellites at a relatively low data rate and thus a GPS receiver needs to receive the GPS satellite signals, uninterrupted, for a relatively long time to obtain the required almanac data and ephemeris data.
While GPS receivers store their last received almanac data and ephemeris data, and while almanac data is valid for relatively long periods of time, ephemeris data is valid for much shorter time periods. A GPS receiver which experiences intermittent or poor reception of signals from one or more GPS satellites may not be able to obtain the required ephemeris data, even though the GPS receiver does have sufficient signal reception to receive the other GPS satellite data required for location determination purposes. As location tracking devices are often operated in environments, such as cityscapes and urban areas, wherein GPS signal reception will be intermittent, this can result in poor performance of the location tracking system.
To address the weakness of GPS-based location tracking devices, a variety of enhancements generally referred to as Assisted GPS (AGPS) have been developed. In AGPS systems, ephemeris and/or almanac data is received at an infrastructure station, in the geographic area in which the location system is operating, which is expected to have good satellite signal reception. This received ephemeris and/or almanac data is then transmitted, via another non-satellite radio communication system (typically a cellular network data communication systems, such as GPRS), to the location tracking devices served by the infrastructure station which require up to date ephemeris data.
Thus, in an AGPS system, the necessary ephemeris and almanac data can be provided to the AGPS-based location tracking devices from the infrastructure station via a faster and/or more robust communication channel, increasing the probability that the AGPS-based location tracking device can make a valid location determination.
However, both GPS and AGPS-based location tracking devices still suffer from a problem in that, in addition to valid ephemeris and almanac data and reception of GPS satellite signals, a GPS receiver must also have an initial location position estimate to determine an actual location. The more accurate the initial location position estimate employed by the GPS receiver, the more quickly the actual location can be determined. Conversely, an inaccurate initial location position estimate can significantly increase the time required to determine an actual location and, in cases wherein intermittent reception of the GPS satellite signals is experienced, may prevent a location determination from being achieved.
While GPS and AGPS receivers typically store their last determined location and employ that as the initial location position estimate, it is not uncommon that a receiver has been moved a significant distance since it last determined a location and thus this initial location position estimate can be a very poor one.