GPS软件接收机原理与MATLAB实现

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"GPS软件接收机基础 - 通过MATLAB平台进行研究" 全球定位系统（GPS）软件接收机是一种先进的技术，它将原本由硬件执行的GPS信号处理任务转移到了软件层面。这种技术允许更高的灵活性、可扩展性和成本效益，因为软件可以更容易地更新和调整以适应不同的需求和性能优化。在MATLAB平台上研究GPS软件接收机，可以利用MATLAB的强大的数学计算能力以及其丰富的信号处理工具箱。MATLAB是一个广泛用于科学研究和工程计算的交互式环境，特别适合于算法开发、数据分析、建模和仿真。通过MATLAB，开发者可以快速原型设计和测试GPS接收机的算法，如码相位跟踪、载波频率锁定、信号解调和定位计算等。 "基础全球定位系统接收机"通常涵盖以下几个关键知识点： 1. **GPS信号结构**：GPS信号由两种主要类型组成，即L1和L2载波，分别携带CA码（粗捕获码）和P码（精确码）。理解这些码的生成和解码对于软件接收机的设计至关重要。 2. **信号捕获**：在软件接收机中，首先要进行的是对GPS卫星信号的捕获，这包括搜索可能存在的信号频率和码相位。 3. **码相位跟踪**：一旦捕获到信号，就需要持续跟踪CA码或P码的相位，以保持与卫星信号的同步。 4. **载波频率恢复**：软件接收机需要准确地估计并跟踪载波频率，这通常通过锁相环（PLL）或数字下变频（DDC）实现。 5. **数据解调与解码**：GPS信号包含导航电文，需要经过解调和解码才能获取，这包括对二进制数据流的处理和校验。 6. **定位计算**：最后，软件接收机根据接收到的多个卫星信号进行多边形定位算法，如伪距法或载波相位差分定位，以计算出接收机的三维位置。 7. **MATLAB工具应用**：在MATLAB中，可以使用信号处理工具箱进行滤波、谱分析、同步和解码操作；用通信工具箱来构建通信系统模型；用控制工具箱实现 PLL 设计；而地理信息系统（GIS）工具箱则有助于显示和分析地理位置数据。 8. **性能评估**：软件接收机的性能指标包括定位精度、时间同步误差、跟踪锁定稳定性等，可以通过仿真和实际测试进行评估。通过上述内容，我们可以了解到，基于MATLAB的GPS软件接收机研究涉及了信号处理理论、计算机编程以及GPS系统原理等多个领域的知识，为理解和开发高效、灵活的GPS接收机提供了坚实的基础。

Preface to the First Edition

The purpose of this book is to present detailed fundamental information on a

global positioning system (GPS) receiver. Although GPS receivers are popularly

used in every-day life, their operation principles cannot be easily found in one

book. Most other types of receivers process the input signals to obtain the nec-

essary information easily, such as in amplitude modulation (AM) and frequency

modulation (FM) radios. In a GPS receiver the signal is processed to obtain the

required information, which in turn is used to calculate the user position. There-

fore, at least two areas of discipline, receiver technology and navigation scheme,

are employed in a GPS receiver. This book covers both areas.

In the case of GPS signals, there are two sets of information: the civilian code,

referred to as the coarse/acquisition (C/A) code, and the classiﬁed military code,

referred to as the P(Y) code. This book concentrates only on the civilian C/A

code. This is the information used by commercial GPS receivers to obtain the

user position.

The material in this book is presented from the software receiver viewpoint

for two reasons. First, it is likely that narrow band receivers, such as the GPS

receiver, will be implemented in software in the future. Second, a software

receiver approach may explain the operation better. A few key computer pro-

grams can be used to further illustrate some points.

This book is written for engineers and scientists who intend to study and

understand the detailed operation principles of GPS receivers. The book is at the

senior or graduate school level. A few computer programs written in Matlab are

listed at the end of several chapters to help the reader understand some of the

ideas presented.

I would like to acknowledge the following persons. My sincere appreciation

to three engineers: Dr. D. M. Akos from Stanford University, M. Stockmaster

from Rockwell Collins, and J. Schamus from Veridian. They worked with me

at the Air Force Research Laboratory, Wright Patterson Air Force Base on the

1.5 SOFTWARE APPROACH 3

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 book.

1.4 APPROACHES OF PRESENTATION

There are two possible approaches to writing this book. One is a straightforward

way to follow the signal ﬂow shown in Figure 1.1. In this approach the book

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 book.

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 book 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 book 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,

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