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New approaches in GPS based location systems
Handset Architecture Figure 1 is a typical “handset view” of a network assisted GPS system. The conventional tracking loops are replaced by snapshot memory and fast convolution processing.  Figure 1 Block Diagram of a typical DSP based GPS processing system At the request of either an external application, or the handset user, the server sends information on satellites in view at the handset’s approximate location, including Doppler predictions. After a snapshot of GPS satellite RF data has been stored in the handset memory, the DSP processes the data and returns the pseudorange measurements to the server, along with other statistical information. This snapshot approach allows the handset to gather GPS data when it is not transmitting, thus eliminating potential self-interference. Each of the messages between the handset and the location server is small (50-100 bytes). This represents a significant reduction in required communications bandwidth when compared to delivering differential corrections, almanac, ephemeris and/or satellite trajectory data to the handset. Figure 1 Block Diagram of a typical DSP based GPS processing system In the system described in Figure 1, received data is down-converted to a suitably low (~ 2MHz) intermediate frequency, digitized and stored in a buffer memory. This data is then operated upon using a programmable DSP IC. Unlike continuously tracking hardware correlator-based receivers, this snapshot processing technique is not subject to the fluctuating levels and changing nature of the signal environment. This server-aided GPS approach has been pioneered by Snaptrack (The company is now a part of Qualcomm) and improved upon conventional GPS performance by sharing processing and database functions between the mobile GPS receiver/processor (client) and a remote infrastructure (the server and reference network). The result is a highly sensitive, cost-effective, low-power, GPS receiving system that provides first fixes in a few seconds from a cold start, even when conventional GPS is unworkable or unreliable. Summary Both network and standalone GPS location technologies have inherent weaknesses, resulting in reduced accuracy, decreased availability and higher implementation costs. Assisted GPS utilizes the complimentary nature of both approaches to overcome situational weaknesses experienced by either network or GPS approaches working alone. The benefits of Assisted GPS approach include maximum availability, increased sensitivity, higher accuracy, lower complexity and a rapid time-to-first-fix. Apart from United States, large-scale adoption of this approach is beginning to happen in Japan where conventional GPS fails to perform in urban canyons like Tokyo. Attempts are being made to ensure that this technology is supported on a multitude of wireless networks deployed worldwide such as GSM, CDMA, TDMA, 3G etc. Successful compliance of the E-911 mandate by the end of 2004 will give more tooth to these advancements especially in United States. User experience and economics of incorporating this approach in a client/server environment will eventually drive this technology.
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