GPS Software Receivers - Fundamentals of Global Positioning System Receivers: A Software Approach - page 186

CHAPTERNINE

GPS Software Receivers

9.1 INTRODUCTION

This chapter can be considered as a summary of all the previous chapters. As
mentioned in Chapter 1, this topic does not follow the signal path of a GPS
receiver but rather follows the design concept of the GPS. Therefore, the previ-
ous chapters are presented in the following order. The user position can be found
from the known positions of at least four satellites and the distances to them. In
Chapters 2 to 4 the satellite constellation and the earth-centered, earth-fixed coor-
dinate system are introduced and the equations to calculate the user position are
presented. The structure of the GPS signal and the acquisition and tracking of the
signal are discussed in Chapters 5, 7, and 8, respectively. Although this approach
should give a better understanding of the overall concepts associated with GPS,
the discussion may not flow smoothly from a receiver design perspective.
In this chapter the GPS receiver is discussed following the actual signal flow
through the receiver. The input signal will be digitized first followed by acquisi-
tion and tracking. Once the tracking is achieved, the output will be converted into
navigation data through subframe matching and parity checking. From the sub-
frames the ephemeris data such as the week number can be found. The position
of the satellite can be determined from the ephemeris data. The pseudoranges
between the receiver and the satellites can also be determined. Once all the nec-
essary information is obtained, satellite positions and the user position can be
calculated. Finally, the user position is put in the desired coordinate system. The
presentation in this chapter follows this order, which is shown in Figure 1.1.
The block adjustment of synchronized signal (BASS) is used for the tracking
program. The conventional approach will provide very similar results.