基于软件接收机的GPS L2C信号采集与数据处理-USB迷|专注于互联网分享

2024年3月12日发(作者：滑晟)

基于软件接收机的GPS L2C信号采集与数据处理

祝雪芬;杨阳;沈飞;杨冬瑞;王立辉;陈熙源

【摘要】L2C信号是由现代化GPS IIR-M卫星播发的新一代民用信号.基于软件接

收机的信号采集与数据处理的研究旨在提高信号在低信噪比环境下的捕获稳定性和

跟踪精度.L2C信号结构描述了其民用中码和民用长码的时分复用特征.信号采集部

分包括宽频天线和接收机前端的设计,工作频段为L1-L2频段,前端采用下变频和频

谱搬移来避免频谱混叠.数据处理中采用降频处理的方法能有效增强信号捕获的稳

定性,并利用圆周相关算法获得CM码的初始相位.码和载波跟踪数据处理包含计算

原理的介绍和跟踪环结构设计.最后,通过低信噪比环境下的实验来验证L2C信号采

集与数据处理方法的有效性.

【期刊名称】《中国惯性技术学报》

【年(卷),期】2014(022)006

【总页数】7页(P770-776)

【关键词】GPS L2C;信号采集;数据处理;软件接收机

【作者】祝雪芬;杨阳;沈飞;杨冬瑞;王立辉;陈熙源

【作者单位】东南大学仪器科学与工程学院,南京210096;东南大学微惯性仪表与

先进导航技术教育部重点实验室,南京210096;东南大学仪器科学与工程学院,南京

210096;东南大学微惯性仪表与先进导航技术教育部重点实验室,南京210096;东

南大学仪器科学与工程学院,南京210096;东南大学仪器科学与工程学院,南京

210096;东南大学微惯性仪表与先进导航技术教育部重点实验室,南京210096;东

南大学仪器科学与工程学院,南京210096;东南大学微惯性仪表与先进导航技术教

育部重点实验室,南京210096;东南大学仪器科学与工程学院,南京210096;东南大

学微惯性仪表与先进导航技术教育部重点实验室,南京210096

【正文语种】中文

【中图分类】TN967.1

Aiming to enhance applied range of GPS signal in weak signal

environment, the U.S. government started a program of a new GPS

modernization initiative, which had a remarkable improvement. A new civil

signal, L2C, is added to the original L2 signal. Compared with L1C/A, L2C

has lower data demodulation and carrier tracking threshold due to the

adoption of FEC (forward error correction) and TDM (time division

multiplex). Therefore, L2C signal is more appropriate for indoor

environment and wooded areas. Currently, L2C is transmitted by fourteen

modernized GPS IIR-M satellites. Overall, L2C signal can be fully available

in future years [1].

With the development of software radio technique, the user’s terminal

equipments, GPS software receivers, have been improved rapidly. GPS

software receiver introduces "software radio" concept into the receiver

designs, with the advantages of flexibility, easy to upgrade. GPS software

receiver can reduce the hardware testing costs and risks. Unlike

conventional GPS receivers, a GPS software receiver is mainly realized by

software except for the front-end part. New algorithms can be easily

developed without changing the design of the hardware [2].

Satellite navigation signals in indoor environment exist serious thermal

noise interference, signal fading and degradation, as well as multi-path

error and the positioning accuracy degradation. Common GPS receivers

are difficult to acquire or track the signals in low signal-to-noise ratio

environment. It is necessary to develop signal collecting and data

processing method of GPS L2C based on software receiver in order to

receive weak signals in the low signal-to-noise ratio environment.

The major work of this study include signal collecting and data processing

of GPS L2C based on software receiver platform. The rest of the paper is

organized as follows. It begins with a brief description about the structure

of GPS L2C signal. Section 2 introduces the GPS L2C signal collecting,

including the broadband antenna and the design of front-end. Section 3

introduces the GPS L2C data processing which mainly focuses on

acquisition and tracking. In section 4, simulation results and experiments in

low signal-to-noise ratio environment are designed to prove the

effectiveness of signal receiving and processing. Finally, some conclusions

are given.

The L2C PRN code consists of two code sets—the civil moderate (CM) and

civil long (CL) PRNs, and both are at the rate of 511.5kcps. These two

codes are multiplexed on a chip-by-chip basis to form a final code with

1.023Mcps. CM is the shorter period code with length 10 230 chips (20 ms),

and it is modulated by data messages with a symbol period of 50 Hz—

corresponding exactly to the CM code word length. The CL code is

unmodulated by data and has a length of 767 250chips (1.5 s). The CM and

CL codes are so highly synchronized that each CL code period contains

exactly 75CM code periods, as illustrated in Fig.1. The L2C signal is

transmitted by modulating a carrier at 1227.60 MHz with the composite

PRN-plus-data signal described above [3-4].

According to [5-8], digital intermediate frequency (IF) signal yk of single

satellite can be shown in equation (1).

Where A is called the amplitude of L2C signal; fI is called the frequency of

IF signal; fd means Doppler frequency shift of input signal; φ0 is called

initial carrier phase; CM(t) means CM code which is a rectangular pulse

with 20 ms period; CL(t) means CL code, and 1.5 s is its period; t0 and tk

mean starting time and current time of L2C signal; D(t) is called navigation

data value which is a rectangular pulse sequence with 20 ms pulse width T;

v(t) is the additive white Gaussian noise. The integral time of CM code is

restricted to 20 ms period. However, the integral time of CL code can be

much longer than it.

The basic idea of the GPS software receiver is that putting A/D converter

as close to the antenna as possible, and the broadband antenna is applied

to the entire RF(Radio Frequency) band to digitize analog signals.

Fig.2 is the scheme of GPS software receiver[9], as shown, the antenna

receives signals transmitted from the satellites, the RF front-end amplifies

the input signal to the appropriate magnitude and converts RF to the

appropriate IF. ADC is used to digitize the input signal. Antenna, RF front-

end and ADC is the hardware parts of GPS software receiver. After the

signal is quantized, the software processing is activated. Acquisition is to

find a rough estimation of the code phase and Doppler frequency shift of

visible satellites. Tracking obtains a more accurate carrier phase and

detects the phase change of the navigation data. Thereby the sub-frame

and the navigation data, the message and the pseudo-rang can be got and

finally the position, velocity and time can be calculated [2].

2.1 The antenna

The antenna is the first element of the receiving chain and is used to

induce a voltage from the incident radio waves. Although someone do not

consider the antenna as a front-end component, it is important to

underline its main features.

Fig.3 shows the antenna widely used in GNSS applications with GPS

receivers[10]. The antenna is the dual frequency, hemispherical and survey

grade. Thanks to its pattern, this antenna attenuates signals from low

elevations and provides good multipath rejection performance. Such an

antenna is active, therefore it incorporates a Low Noise Amplifier (LNA)

within its case. This antenna is widely known in the GNSS community and

it is often sited on the roof of navigation research labs.

2.2 RF front-end

The RF front-end converts one frequency of a radio signal to another, and

the architecture of RF front-end is shown in Fig.4. Converting input RF

signal to the analog IF signal by mixer, and then after ADC, the analog IF

signal finally becomes the digital IF signal. The advantage of this

architecture is that the required narrowband filter is relatively easy to

implement, and the amplifier circuit is relatively inexpensive. The

disadvantage is that the mixer and the crystal oscillator are relatively

expensive [11-13].

Generally, preamplifier consists of burned protection, filter and Low Noise

Amplifier (LNA). Its main function is to set the noise figure of the receiver

and suppress the out-of-band interference. Reference oscillator is to

provide the reference time and frequency and is a key component of the

receiver. Using the output of reference oscillator, frequency synthesizer

generates a local oscillator (Local Oscillator, LO) and a clock signal. One or

more of the LO and the input RF signal are mixed into the IF signal in the

Mixer.

The IF section of receiver is to filter the out-band noise and interference,

and make the amplitude level of the signal and noise raised to the signal

processing level. IF automatic gain control circuit section can be used to

control the operating voltage level, provide appropriate dynamic range

and suppress the pulse-type interference. After ADC the output of digital

IF signal is prepared for subsequent digital baseband signal processing

section.

2.3 Design of the front-end prototype

After the fundamental principle given in the previous sections, this section

focuses on the description of a real implementation of a front-end for a

software receiver, working in the L2 band. Fig.5 is a detailed block diagram

of the front-end prototype built with discrete components in a lab

experiment.

The receiving antenna in the scheme of Fig.5 is the GPS L1-L2 dual

frequency survey grade antenna, shown in Fig.3. The signal from the

antenna is further amplified by a high gain (i.e. 40 dB) LNA and filtered

using a passive band pass filter, with a -3 dB bandwidth of 40 MHz and

insertion losses within 2 dB.

The front-end frequency plan foresees a single down conversion to the

BB(Base Band) signal equal to 0.6 MHz, a LO provides a local carrier at

1227 MHz to the mixer. The LO is then used to generate other important

reference signals such as the sampling and the signal processing clocks.

The signal at BB is then filtered, amplified and finally converted to a digital

format by a 8-bit ADC. The gain introduced by the BB amplifiers is 43 dB.

At this point of the receiving chain the analog signal is converted at 0.6

MHz and is sampled with a rate of 10 MHz. As mentioned before, the

down sampling strategy is applied, the signal is further frequency down

converted and the spectrum of the digital signal is centered on 2.5 MHz,

avoiding aliasing [14].

Acquisition and tracking of signal are included in the part of data

processing. There circular correlation is utilized to implement the

acquisition algorithm and DLL tracking loop, the Costas carrier tracking

loop are foundations of the tracking algorithm.

3.1 Principle of Acquisition Algorithm

With the increasing demand for civilian GPS signal performance, the hot

issue goes to how to simplify calculation and shorten acquisition time

without the loss of acquisition performance. The acquisition algorithm

based on circular correlation is introduced in this paper to balance these

problems. Process of acquisition can be shown as next two steps .

Step 1. The IF signal sampler is used to get the L2C signal samples Ai (i=1,

2, 3,..., fst), where fs means the sampling frequency, and t means the

sampling time. The chip rate of the L2C signal rc is 1.023 MHz, where fs ≫

rc generally, so the samples can be folded to lower to the frequency rc.

Then the samples are divided into N parts, where N=t×rc, with M samples

each. After that, add samples together of each part to get N new samples

Ri (i=1, 2, …, N), which is shown in equation (2).

Step 2. Local code phase is produced by software and get N local code

phase samples Si (i=1, 2…N).

Then, the signal is processed by acquisition algorithm based on circular

correlation. After processing Ri and Si with FFT, complex conjugate is

performed on Si. The process of calculation is shown in equation (3).

Where Ci is the correlation of the input signal and the locally generated

signal. The magnitude of Ci can be written as:

After IFFT, the maximum absolute value can be compared with

predetermined threshold. If higher, the signals are successfully acquired.

The block diagram of the acquisition of GPS CM code signal by using

circular correlation[15] is shown in Fig.6.

3.2 Principle of tracking algorithm

CL code signal can obtain lower signal-to-noise ratio than CM code signal.

So the tracking algorithm uses CM code for demodulation and uses CL

code for tracking[16].

code for demodulatation and uses CL code for tracking[16]. Firstly, the

carrier in input signal is stripped by multiplying the local matched carrier.

Secondly, the CM code is stripped by local matched code. A simple frame

of signal demodulation is shown in Fig.7.

The integral time is limited due to 20ms’ period of the CM code.

Meanwhile, the demodulation and the tracking are individual by CM code

and CL code. Process of tracking can be expressed as next three steps.

Step 1. Data demodulation is involved in the CM code during integral time.

Discrete digital signal can be expressed as follows.

Where n is called the sampling number each 20 ms. Then both I (in-phase)

and Q (quadrature phase) baseband signals are separated from the digital

intermediate frequency signal. Accumulated results of CM signal are

mainly shown in equation (6) and equation (7) [5-8].

Where m is called the reference number of navigation message during 20

ms; The subscript CM means the CM code, and the subscript CL means the

CL code; I and Q mean in-phase and quadrature phase of the number m

coherent integrating range, respectively. ts means the difference of

received code phase and local code phase; ωL2 is called frequency of L2C

signal; ωdtk means carrier phase of receiver relative to IF signal. CL signal

can also get similar accumulated results.

Step 2. The code tracking loop is used to track the CM code phase, and the

frequency tracking loop is built to track the carrier[17]. When the phase-

locked loop tracks CM carrier phase, the phase error estimation is

extracted to control oscillator. A block diagram of code and carrier

tracking is shown in Fig.8.

Fig.8 shows the descriminator uses difference between advancing and

hysteretic energy to compensate initial signal until the local code phase

equals to input code phase. The feedback to adjust code phase and carrier

phase can get from three branches of ICM and QCM signal, then an effect

in code generator and carrier generator is got when local signal is different

from the received signal.

Step 3. Accumulation operation is used to the two signals and then gets IP

and QP values which are shown in equation (8) and equation (9) after

filtering the 2fI frequency signal.

Where N means the sampling number during the sampling period. The

ICM signal and the QCM signal constitute a compound signal whose phase

can be expressed as phase difference of local carrier and input signal

which can be shown as:

Equation (10) shows when the I signal has maximum, the output error is

smallest. Meanwhile, the Q signal gets the minimum. At this time, the

navigation message information is included in I signal.

In this paper, all the experiments and simulations were carried out in Key

Laboratory of Micro-Inertial Instrument and Advanced Navigation

Technology of Ministry of Education, School of Instrument Science and

Engineering, Southeast University. Receiver tracks 2 to 4 satellites with L2C

signal. Sampling time ranges from 30 s to 90 s. The sampling frequency is

set as 10 MHz.

Result shows that the signal-to-noise ratio of environment changes with

the time. When the signal data time is 1.2 s, the signal-to-noise ratio of the

No.15 satellite in inter- ferential environment is shown in Fig.9.

The designed front-end described in section 2 was used to validate the

algorithm of acquisition based on circular correlation. The circular

correlation acquisition algorithm was used on 20 ms data (one cycle of CM

code) to find the beginning point of the CM code and search the

frequency range of -4～4 kHz in 50 Hz each step.

The acquisition result of No.15 on 20 ms input data are shown in Fig.10.

The clear correlation peak proves the perfect detected result.

After acquiring, the beginning point of the CM code of No.15 satellite is

shown in Fig.11. It can be seen that the correlation peak appears at point

8811, so the beginning point of the CM code in 20 ms input data is at

point 8811.

Then the captured IF signal above is input to the tracking loop. Picture of

instant pertinent peak which is shown in Fig.12 expresses the changing

process of signal tracking. Fig.12 shows the adjustment of tracking loop

happens before 400 ms and it tends to be stable after 400 ms. Meanwhile,

the instant pertinent peak is approximate regardless of interferential

environment. So, the performance of tracking in poor environment proves

that L2C signal is suitable in weak environment.

This article mainly studies the front-end prototype of GPS L2C signal

collecting as well as the data processing based on software receiver. By

using the designed RF front-end and ADC, the signal collecting

architecture can get required digital IF signal whose narrowband filter is

relatively easy to implement. During data processing, by using the method

of frequency reduction processing, the stability of acquisition resulting

dada can be achieved. There initial phase of the CM code can be attained

after performing circular correlation. Signal tracking includes the signal

demodulation, the code tracking and the carrier tracking module. Finally,

the effectiveness of GPS L2C signal collecting and processing is proved in

low signalto-noise ratio environment. Although it is a preliminary test, the

method put forward in the paper offers foundation for the application of

GPS L2C signal in the new field.

【相关文献】

[1] Sun Liang. Study on acquisition and tracking algorithms of GPS L2C Signal and the

software implementation[D]. Tsinghua University, 2010.

孙亮. GPS L2C 信号的捕获跟踪算法及软件实现[D].清华大学, 2010.

[2] James Bao-Yen Tsui. Fundamentals of global positioning system receivers: A software

approach[M]. John Wiley, Sons, Inc, 2000: 133-165.

[3] Li Chengjun, Lu Mingquan, Feng Zhenming, et al. Study on GPS L2C acquisition

algorithm and performance analysis[J]. Journal of Electronics & Information Technology,

2010, 32(2): 296-300.

李成军，陆明泉，冯振明，等. GPS L2C 捕获算法研究及性能分析[J]. 电子与信息学报，2010，

32(2)：296-300.

[4] Kaplan E D, Hegarty C. Understanding GPS: Principles and applications[M]. Second

Edition. Artech house, 2006.

[5] Yang C. Joint acquisition of CM and CL codes for GPS L2 civil (L2C)

signals[C]//Proceedings of the 61st Annual Meeting of The Institute of Navigation.

Cambridge, MA, June 2005: 553-562.

[6] Muthuraman K, Klukas R, Lachapelle G. Performance evaluation of L2C data/pilot

combined carrier tracking[C] //Proceedings of ION-GNSS 21st International Technical

Meeting of the Satellite Division. Savannah, GA, 2008: 16-19.

[7] Razavi A, Gebre-Egziabher D, Akos D M. Carrier loop architectures for tracking weak

GPS signals[J]. Aerospace and Electronic Systems, 2008, 44(2): 697-710.

[8] Lim D W, Moon S W, Park C, et al. L1/L2CS GPS receiver implementation with fast

acquisition scheme[C]// Proceedings of Position, Location, and Navigation Symposium.

2006: 840-844.

[9] Chen Xiyuan, Zhang Kunpeng. Simulation and analysis of GPS and Galileo signals in

space[J]. Journal of Chinese Inertial Technology, 2007, 15(6): 653-657.

[10] Padros N, Ortigosa J I, Baker J. Comparative study of high-performance GPS receiving

antenna designs[J]. Antennas and Propagation, 1997, 45(4): 698-706.

[11] Akos D M. A software radio approach to global navigation satellite system receiver

design[D]. Ohio University, 1997.

[12] Spacek J, Puricer P. Front-end module for GNSS software receiver. multimedia signal

processing and communication[C]//Proceedings of 48th International Symposium

ELMAR-2006 focused on Multimedia Signal Processing and Communications. 2006: 211-

214.

[13] Adane Y, Bavaro M, Dumville M, et al. Low cost multi constellation front end for GNSS

software defined receivers[C]//Proceedings of ENC GNSS, 2009.

[14] Vaughan R G, Scott N L, White D R. The theory of bandpass sampling[J]. IEEE

Transactions on Signal Processing, 1991, 39(9): 1973-1984.

[15] Zhu Xuefen, Chen Xiyuan, Li Zigang. The architecture design of GPS software receiver

and implementation of its acquisition algorithm with fine frequency estimation [J]. Journal

of Southeast University (English Edition), 2008, 1(8): 38-41.

[16] Muthuraman K, Shanmugam S K, Lachapelle G, Evaluation of Data/Pilot tracking

algorithms for GPS L2C signals using software receiver[C]//Proceedings of the 20th

International Technical Meeting of the Satellite Division of The Institute of Navigation.

Fort Worth, USA, 2007: 2499-2509.

[17] Borre K, Akos D M, Bertelsen N, et al, A softwaredefined GPS and Galileo receiver: a

single-frequency approach[M]. Springer, 2007.