Using Cellular Telephone Networks for GPS Anywhere
The first two articles to appear in the innovation column in GPS World more than 15 years ago were entitled "GPS: A Multipurpose System" and "The Limitations of GPS." In the first article, GPS was trumpeted as a revolutionary positioning and navigation technique that could be used in all sorts of unexpected ways. This premise has stood the test of time with new uses for GPS still being discovered. The second article reminded readers that GPS may not be a panacea for all of our positioning needs — that there are some situations in which GPS fails us. In particular, it was noted that GPS signals are "blocked" by buildings, making indoor use of GPS impossible. But what was impossible 15 years ago is possible today.
New designs have greatly improved the sensitivity of GPS receivers so they can make code-phase measurements even on the severely attenuated signals inside buildings. And if the signal is too weak for the receiver to extract the satellite navigation message itself, the necessary data can be sent to the receiver using a cellular telephone network, which also can supply timing information to help the GPS receiver acquire signals more quickly. In this month's column, we will investigate how this so-called "assisted GPS" works and why we can now say that GPS works (virtually) anywhere. — R.B.L.
New designs have greatly improved the sensitivity of GPS receivers so they can make code-phase measurements even on the severely attenuated signals inside buildings. And if the signal is too weak for the receiver to extract the satellite navigation message itself, the necessary data can be sent to the receiver using a cellular telephone network, which also can supply timing information to help the GPS receiver acquire signals more quickly. In this month's column, we will investigate how this so-called "assisted GPS" works and why we can now say that GPS works (virtually) anywhere. — R.B.L.
The integration of GPS into cellular telephones enables a potentially vast array of new applications ranging from consumer gimmicks through efficiency multipliers for enterprises to lifesaving safety and security applications. In the United States, the Enhanced-911 regulations have been and remain a major catalyst for this deployment. In Europe, the commercial potential of location-based services (LBS) is driving it. Regardless of the drivers, the technology convergence is happening with an ever-increasing momentum.
These new applications and the cell-phone environment itself, however, pose significant challenges for the GPS community. They demand GPS solutions that can be implemented in tiny spaces at extremely low cost in extremely high volumes, operate reliably in a much broader range of environments than was hitherto considered possible, acquire signals in seconds under extreme conditions and, for some applications, do so without the aid of stored orbital data.
These problems and the availability of cellular communications itself spawned the concept of assisted GPS (AGPS) in which the network assists the GPS receiver to perform its various functions. This article reports on AGPS developments leveraging cellular telephone networks to help acquire and deliver accurate GPS fixes from anywhere, anytime.
System Considerations
In developing a user system for AGPS, several factors must be considered, including the type of assistance to be provided by the network, the type of cell-phone network and the corresponding AGPS standards, and environmental factors such as radio frequency (RF) compatibility between the GPS module and the host platform.
Benefits of Assistance.
There are many types of assistance that can be provided by the network to the GPS receiver. The receiver could be a fully functional receiver capable of selecting satellites, acquiring signals, achieving time synchronization, extracting data, performing measurements, and computing its own navigation solution. Nevertheless, its acquisition speed can be enhanced through the provision of assistance.
Furthermore, it can avoid spending time to extract all of the required data from the satellite signals if most of this is supplied by the network.
Benefits of Assistance.
There are many types of assistance that can be provided by the network to the GPS receiver. The receiver could be a fully functional receiver capable of selecting satellites, acquiring signals, achieving time synchronization, extracting data, performing measurements, and computing its own navigation solution. Nevertheless, its acquisition speed can be enhanced through the provision of assistance.
Furthermore, it can avoid spending time to extract all of the required data from the satellite signals if most of this is supplied by the network.
Signal Levels.
More importantly, the range of signal levels at which the receiver can operate can be greatly increased if the receiver is relieved of the requirement to extract the 50 bits-per-second navigation data stream that is modulated onto the signals. This data cannot be extracted in a timely manner (or at all in many cases) if the received signal power is below about –172 dBW (–142 dBm) but code-phase measurements can still be made for much weaker signals than this. Since the navigation data is not location specific, it can be supplied by a remote receiver that has a clear line of sight to the same satellites.
The data supplied by the network in this way can include ephemeris coefficients, almanac coefficients, satellite health data, satellite clock error coefficients, atmospheric error coefficients, and so on. Additionally, excerpts from the data sequence can be supplied to facilitate coherent integration over periods much longer than a navigation data bit interval.
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