基于软件接收机的GPS L2C信号采集与数据处理

( 1. School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China; 2. Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology of Ministry of Education, Southeast University, Nanjing 210096, China)

( 1. School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China; 2. Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology of Ministry of Education, Southeast University, Nanjing 210096, China)

GPS L2C signal is a new civilian signal launched by the modernized GPS Block IIR-M satellite. The signal collecting and data processing based on software receiver is studied to improve the acquisition stability and tracking precision in low signal-to-noise ratio environment. The signal structure describes the time division multiplexing characteristic of civil moderate and civil long code. The part of signal collecting includes the broadband antenna and the design of front-end. The designed front-end of software receiver working in L1-L2 band applies frequency down conversion and moves the spectrum of the digital signal to avoid aliasing. In data processing, a frequency reduction processing method is put forward to increase the stability of acquisition. Circular correlation algorithm is applied to attain the initial phase of CM code. In the section of code and carrier tracking data processing, the computational principles are introduced and the structure of the tracking loop is designed. Finally, the experiments in low signal-to-noise ratio environment are designed and implemented, which prove the effectiveness of signal receiving and processing.

GPS L2C; signal collecting; data processing; software receiver

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.

1 The Structure of GPS L2C Signal

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].

Fig.1 L2C signal structure

According to [5-8], digital intermediate frequency (IF) signal ykof single satellite can be shown in equation (1).

Where A is called the amplitude of L2C signal; fIis called the frequency of IF signal; fdmeans Doppler frequency shift of input signal; φ0is 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; t0and tkmean 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.

2 GPS L2C Signal Collecting

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].

Fig.2 The scheme of GPS software receiver

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 GPS antenna

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].

Fig.4 The scheme of GPS front-end

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].

Fig.5 The block diagram of the front-end prototype

3 GPS L2C Data Processing

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 fsmeans the sampling frequency, and t means the sampling time. The chip rate of the L2C signal rc is 1.023 MHz, where fs≫ rcgenerally, 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 Riand Siwith FFT, complex conjugate is performed on Si. The process of calculation is shown in equation (3).

Fig.6 Flowchart of circular correlation acquisition algorithm

Where Ciis the correlation of the input signal and the locally generated signal. The magnitude of Cican 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.

Fig.7 Frame of signal demodulation of tracking

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. tsmeans the difference of received code phase and local code phase; ωL2is called frequency of L2C signal; ωdtkmeans 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 Block diagram of code and carrier tracking

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 ICMand QCMsignal, 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 IPand QPvalues which are shown in equation (8) and equation (9) after filtering the 2fIfrequency signal.

Where N means the sampling number during the sampling period. The ICMsignal and the QCMsignal 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.

4 Experiments Results

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.

Fig.10 The correlation peak of satellite 15

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.

Fig.11 Beginning of CM code of No.15 satellite

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.

Fig.12 The instant pertinent peaks of poor environments

5 Conclusions

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.

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1005-6734(2014)06-0770-07

10.13695/j.cnki.12-1222/o3.2014.06.013

基于软件接收机的GPS L2C信号采集与数据处理

祝雪芬1,2，杨 阳，沈 飞1，杨冬瑞1,2，王立辉1,2，陈熙源1,2
（1.东南大学 仪器科学与工程学院，南京 210096；2.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)

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。

GPS L2C；信号采集；数据处理；软件接收机

TN967.1

A

2014-07-21；

2014-11-17

国家自然科学基金项目资助（41104015，61203192）

祝雪芬（1983—），女，博士，副教授，从事卫星导航及组合导航。E-mail：zhuxuefen@seu.edu.cn

Signal collecting and data processing of GPS L2C based on software receiver

ZHU Xue-fen1,2, YANG Yang1,2, SHEN Fei1, YANG Dong-rui1,2, WANG Li-hui1,2, CHEN Xi-yuan1,2