Global Positioning System Reference
In-Depth Information
performed by hardware. From the navigation data phase transition the subframes
and navigation data can be obtained. Ephemeris data and pseudoranges can be
obtained from the navigation data. The ephemeris data are used to obtain the
satellite positions. Finally, the user position can be calculated for the satellite
positions and the pseudoranges. Both the hardware used to collect digitized data
and the software used to find the user position will be discussed in this topic.
1.4 APPROACHES OF PRESENTATION
There are two possible approaches to writing this topic. One is a straightforward
way to follow the signal flow shown in Figure 1.1. In this approach the topic
would start with the signal structure of the GPS system and the methods to process
the signal to obtain the necessary the information. This information would be
used to calculate the positions of the satellites and the pseudoranges. By using
the positions of the satellites and the pseudoranges the user position can be found.
In this approach, the flow of discussion would be smooth, from one subject to
another. However, the disadvantage of this approach is that readers might not
have a clear idea why these steps are needed. They could understand the concept
of the GPS operation only after reading the entire topic.
The other approach is to start with the basic concept of the GPS from a
system designers' point of view. This approach would start with the basic concept
of finding the user position from the satellite positions. The description of the
satellite constellation would be presented. The detailed information of the satellite
orbit is contained in the GPS data. In order to obtain these data, the GPS signal
must be tracked. The C/A code of the GPS signals would then be presented.
Each satellite has an unique C/A code. A receiver can perform acquisition on the
C/A code to find the signal. Once the C/A code of a certain satellite is found,
the signal can be tracked. The tracking program can produce the navigation
data. From these data, the position of the satellite can be found. The relative
pseudorange can be obtained by comparing the time a certain data point arrived
at the receiver. The user position can be calculated from the satellite positions
and pseudoranges of several satellites.
This topic takes this second approach to present the material because it should
give a clearer idea of the GPS function from the very beginning. The final chapter
describes the overall functions of the GPS receiver and can be considered as
taking the first approach for digitizing the signal, performing acquisition and
tracking, extracting the navigation data, and calculating the user position.
1.5 SOFTWARE APPROACH
This topic uses the concept of software radio to present the subject. The software
radio idea is to use an analog-to-digital converter (ADC) to change the input sig-
nal into digital data at the earliest possible stage in the receiver. In other words,
Search WWH ::




Custom Search