Global Positioning System Reference
In-Depth Information
CHAPTERTWELVE
GPS Receiver-Related Subjects
12.1 INTRODUCTION
In this chapter some subjects related to GPS receivers will be discussed. For
the most part, the almanac data used can be found on the Internet. When the
approximate time and the user location are known, the almanac data can be used
to estimate the number of visible satellites and their approximate Doppler fre-
quencies. This information can narrow down the satellite acquisition procedure.
A different means of acquisition will be presented that can be applied to
periodic as well as nonperiodic signals, such as the P code. This method also
consists of performing circular correlation in the frequency domain.
Other subjects include sampling frequency correction through the wide area
augmentation system (WAAS) signal, strong and weak signal conditions in acqui-
sition, simulation of GPS signals, impact on acquisition of filter bandwidth
in front of ADC, real time operation of a software receiver, passive altime-
ter, the number of ADC bit required under jamming conditions, and calculating
the satellite position through almanac data. An aided GPS system will be only
briefly discussed.
12.2 INFORMATION FROM ALMANAC DATA
(
1
)
From the acquisition method discussed in Section 10.10, it is evident that finding
the initial C/A code and the carrier frequency is a time-consuming process. It is
possible to obtain the almanac data (referred to as the YUMA data) from the inter-
net (
http://www.navcen.uscg.gov/ftp/GPS/almanacs/yuma/
). These data are usu-
ally updated about once a week. The satellite constellation can be obtained from
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