6.6 Performance Enhancement of GNSS Mobile Terminal Antennas in Difficult Environments

It is apparent from the discussion in the previous section that the multipath environment degrades the performance of GPS mobile terminal antennas. The time taken for the receiver to lock the GPS satellite (time to first fix (TTFF)) can reach up to 10 minutes, which is unacceptable for emergency calls. Provision of a reliable navigation service demands mitigation of these degrading effects. The performance of the terminal antennas in a cluttered environment can be enhanced by employing different techniques including assisted GPS, GPS signal reradiation, beamforming and diversity antennas.

6.6.1 Assisted GPS

Assisted GPS, also termed A-GPS, is a system where external sources including an assistance server with greater computing power and reference network provide the GNSS receiver with aiding information. Precise information of the GPS satellite orbit, clock data, initial position and time estimate is provided by the assistance server that gets help from the reference network [33]. The assistance server uses a wireless link to communicate with the GPS receiver. Provision of this data reduces the workload of the GPS receiver so it can operate more quickly and efficiently with assistance from the network. The TTFF can be decreased by a factor of 10, increasing the receiver's sensitivity [34]. This system is employed in mobile telecommunication n....