Design and Implementation of a GNSS SignalCollection System Using Direct RF Sampling

Hongquan Liu, Yanhong KouSchool of Electronic and Information Engineering

Beihang UniversityBeijing, China

lhq@ee.buaa.edu.cn, buaakyh@126.com

Abstract-State-of-the-art commercial wideband ADCs (Analogto Digital Converter) already have the qualification for directRF (Radio Frequency) sampling, especially for bandpasssampling for L band GNSS (Global Navigation Satellite System)receivers. Compared with IF (Intermediate Frequency)sampling, this approach has outstanding features of simplestructure, high flexibility, as well as low distortion. This paperpresents the design and implementation of a GNSS signalcollection system using direct RF sampiing, where only theamplifiers and filters are employed between the antenna and theADC, and the collected data are transmitted to a computer viaGigabit Ethernet for storage and processing. As for the mostcrucial issues of direct RF sampling, the impacts of aliasingnoise and clock jitter are addressed, and an expression is givento calculate the requirements for aliasing noise. Then the designof FPGA (Field Programmable Gate Array) code and computersoftware is described. By means of a GNSS software receiver,the acquisition results and position solutions based on the signalscollected from the live satellites as well as a GNSS RF signalsimulator demonstrate that the system works effectively andprovides the flexibility for multi-constellation multi-signalstructure applications.

I. INTRODUCTION

With the modernization of GPS and GLONASS, as well asthe construction of Compass and Galileo, it's highlypromising to use signals from multi-constellation or multi­frequency for GNSS (Global Navigation Satellite System)applications. The positioning accuracy can be improved by theemployment of multi-frequency ionospheric error mitigationand multi-carrier ambiguity resolution technique. Multi­constellation signals bring more satellites in view, leading to alower PDOP (Position Dilution of Precision) and thus a betterpositioning accuracy. In an environment where signals aresusceptible to be blocked or deformed, multi-constellationprocessing can enhance the continuity, integrity andavailability ofnavigation services.

A GNSS signal collection system is a device for digitizingand collecting GNSS signals, as well as the hardwarefoundation for a GNSS software receiver. It plays an

This paper is supported by the national 863 program (2007AA12Z302).

irreplaceable role in the fields of navigation signal processing,information processing, and signal quality monitoring. It canalso be combined with a GNSS signal simulator to form aclosed-loop system for the test of GNSS signals. A traditionalGNSS signal collection system is usually based on thestructure of analog down-conversion, and signals are digitizedat IF (Intermediate Frequency). In view of SDR (Software­Defmed Radio), it is a compromise. A mixer usually generatesunwanted frequencies, which can contaminate the output [1].The mixer and the local oscillator also increase the complexityand the cost of the system. More importantly, different RFfront-ends are needed for collecting signals on different carrierfrequencies. Thus, the IF approach results in reducedflexibility and increased inconsistency of signals.

Direct RF sampling is more suitable for collecting multi­system and multi-frequency signals. Compared with the IFsampling, it can obtain a less distorted digital signal due to thereduced number of analog components. In addition, thisapproach provides a high flexibility, since the analog down­conversion is no longer required. When collecting signals ofdifferent frequencies, the only issue to pay attention to ischanging the filters and adjusting the sample rate (ifnecessary). Therefore, the consistency of the signals can beenhanced.

There are two different methods of direct RF sampling:lowpass sampling and bandpass sampling [2]. Lowpasssampling approach can achieve higher degree of flexibilityand yield simpler hardware architecture, because the bandpassfilters used to select frequency bands are not required. But thedigitized signal will have a very high data rate, which are hardto deal with. Bandpass sampling has some limits on the signalbandwidth and the sample rate, which reduces the flexibility.But it will significantly reduce the difficulty of the systemimplementation. A detailed comparison between direct RFbandpass sampling and analog down-conversion sampling canbe seen in [3]. D. M. Akos et al designed the prototypes ofsingle-frequency and multi-frequency GPS&GNSS receiverswith direct RF sampling [4---6]. Ville Syrjala et al analyzed thequantization and jitter effects in direct RF sampling systems

978-1-4244-4669-8/09/$25.00 ©2009 IEEE 105

The thermal noise at room temperature of T = 2900K isabout -174d.Bm/Hz. We can calculate the ideal RF front-endoutput power through the equation

two filters in the link, so the total insertion loss (Floss) is 5dB.The bandwidth of the filters is 30MHz, and the total stopbandrejection (Rs) of two filters is 35dB at 25MHz offset from thecenter frequency.

ITEM VALUE

Noise Figure ofLNA in Antenna (NFl 2dBGain ofLNA in Antenna 29dB

LNAGain 35dBAMP Gain 33dBLine Loss -25dB

[7]. This paper investigates the requirements of sample rate,aliasing noise, and sampling clock jitter for direct RFsampling of GNSS signals, and presents a GNSS signalcollection system using direct RF sampling. Analysis andvalidation about the data acquired has been done at the end ofthe paper using the GNSS software receiver and the GNSSsignal simulator developed by our laboratory.

II. DESIGN OF THE SIGNAL COLLECTION SYSTEM

As shown in Fig. 1 and Fig. 2, the hardware of the signalcollection system is composed of a RF front-end, an ADC(Analog to Digital Converter) module, and a Xilinx ML505FPGA Evaluation Platform. In addition, a computer is neededfor data storage and processing. After being received,amplified and filtered by the antenna and RF front-end, theanalog signal is digitized by the ADC module. Then the digitalsignal is preprocessed and transmitted to the computer by theML505 board.

TABLE I. RF FRONT-END LINK BUDGET

Where N is the noise floor expressed in dBm/Hz, B is the filterbandwidth . Gtolal is the total gain of the RF front-endexcluding the impact of filters. So the output level of the RFfront-end is -30.2dBm. This level of power can reachapproximately 4bits at the ADC's output. The AGC canincrease dynamic range of the front-end, maintain thequantization level, and suppress the pulse interference [9].

B. Impact ofAliasing Noise

The SNR (Signal to Noise Ratio) referenced to 1 Hz isoften used to represent the strength/quality of GNSS signals.This parameter, which is expressed as ClNo, is easily affectedby aliasing noise. The wideband noise in the input signal thatenters the ADC's input bandwidth will be aliased within theNyquist frequency. This will lead to an accumulation of noiseenergy, potentially degrading the ClNo. ADS5463 has ananalog input bandwidth of 2.3GHz, and we have selected asample rate of 100MSPS using the ladder gram method asproposed in [6]. So the noise will alias 46 times. Assumingthat the noises entering the ADC's bandwidth are amplifiedidentically, the total power of the out-of-band thermal noisewithin the analog input bandwidth is

N = N + G - F + 10 10g(1 + 10 (1OIog n-R,+ F,o" l/lO ) (4)o total loss

Where n is the number of the noise-aliasing, b is the Nyquistfrequency. The power ofnoise within the filter bandwidth is

The PSD (power spectral destiny) of the noise within theNyquist frequency can be figured out by the equation derivedfrom (2) and (3)

Figure I. Photo of the signal collection Hardware.

Figure 2. Block diagram ofthe signal collection hardware.

A. Link budget ofthe RF chain

GNSS signals arriving at the antennas near the ground areburied in the thermal noise. Taking the GPS L1 C/A signal foran example, the lowest received power is only about ­128.5d.Bm according to IS-GPS-200D interface specificationfile. The receiver RF front-end has to amplify the power of thenoise to meet the ADC's need [1]. The ADC we selected isTI's ADS 5463, which is a 12-bit, 500-MSPS ADC with areference voltage of 2AV. So we can figure out that its LSB(Least Significant Bit) is 0.586mV [8]. In terms of signalpower, it is -54.6dBm for a matched impendence of 500hm.The output power of the RF front-end can be calculatedaccording to TABLE I.

The total noise figure of the system is approximately equalto the noise figure of the LNA in the active antenna. There are

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P OUI = N + 10 log B + G IOlal - Floss

~hennal = N + Glotal - R , + 10 log n +10 log b

P jiller = N + Glotal - Floss + 10 log B

(1)

(2)

(3)

From (4) we can get the expression ofC/No

D. Design ofthe FPGA Code

The ML505 connects the ADC module and the computer.The on-board FPGA, a Xilinx Virtex5 XC5VLX50T,performs digital signal pre-processing, data buffering, as wellas Gigabit Ethernet interface. A block diagram of the FPGAdesign is shown in Fig. 3.

The output of ADS5463 is a LVDS (Low VoltageDifferential Signaling) signal, which should be translated intosingle-end signal before pre-processing. For a long-time datacollection, 4-bit digital signal will take a lot of disk space, and4-bit quantization is not necessary in most applications. The

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•••11

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Figure 4. Positioning result oflive GPS LI CIA signals.

The data were collected on July 30 2009, on the roof of thesixth floor of the NEW MAIN BUILDING in BUAA

E. Design ofthe Software

WinPcap is an industry-standard tool for link-layernetwork access in Windows environments: it allowsapplications to capture and transmit network packetsbypassing the protocol stack. We use functions of WinPcap toreceive data from the ML505. For high speed harddisk writing,a Memory-Mapped File technique is used. After recognizingthe packets by source address, the program will first check thepackets count information to get to know whether there areany data lost, and then strips off the source and destinationaddress. The data will be stored on the harddisk in a formatcompatible with the GNSS software receiver.

TOW 188941...eas tillle 188917 .000009

latitude longitude HAE39 :58 :42 . 87 116 :20:39.48 86.5126071

Spe e d Heading TIC _dt0 .974931 165.497235 9.999999

Figure 3. Block diagram ofFPGA code.

re-quantization module can re-quantize the signal to 2-bit or 1­bit, which will save a half/three quarters of the disk space. TheFIFO module ensures that the data are not lost when theEthernet is congested. The transmission control state machinewill append frame structure and packets count information tothe data packets . The Gigabit Ethernet MAC logic and theclock distribution logic are constructed by hard cores in FPGA.All the clocks used are originated from a common smallTCXO (Temperature Compensated Crystal Oscillators).

III. VALIDATION OF THE DESIGN

A. Data Collectedfrom Live Satellites

Fig. 4 shows the positioning result of the data collectedfrom live GPS satellites .

(6)

(5)

[2]1 /2

SNR = -20 log (21ifta)2+ (1;2& )

Where P, is the received power of signal. So we can see thatthe impact of aliasing noise on C/No is insignificant. But aprecondition is that the signal has been properly filtered. TheC/No considering the impact of aliasing noise is about 43.3dB­Hz, the degradation of C/No due to aliasing noise is onlyO.3dB in this case. In fact, the rejection of filters will be morethan 35dB in the band far away from the center frequency, sothe impact of aliasing noise will be even less. Because n isdetermined by the sample rate, the C/No will be affected bythe sample rate. A higher sample rate can result in a betterC/No, but a harder implementation.

C. Impact ofQuantization and Jitter

C/No could be changed by the impact of quantizationeffects and sampling clock jitter. l-bit quantization willdegrades SNR by 1.96dB, and 2-bit will degrades SNR by0.55dB [10]. Reference [7] gives a set of values of thesampling clock jitter requirements in direct RF samplingapplications. When the out-of-band interference exceeds thethermal noise by no more than IOdB, the jitter requirementsare easy to meet. The effects of jitter can be negligible whenthe analog input frequency is low enough, but the effects ofquantization are independent of the analog input frequency. Sowe can get the sampling clock jitter by comparing twomeasurements as follows [II]

Where f is the analog input frequency, ta is the total jitter, ande is the composite RMS DNL (Differential Non-Linearity) inLSBs, including thermal noise. The first measurement in thispaper is sampling a IMHz sine wave at 100MSPS. We can geta SNR of 63.4dBFS through a 5 average , 128K FFT. The e isl.77LSBs according to (6). Next, a I575.42MHz sine wave issampled at 100MSPS. The SNR degrades to 26.8dBFS . Thetotal jitter ta is 4.62ps. The ADC's aperture jitter is 0.15psaccording to the component's datasheet, so the jitter ofsampling clock is 4.618ps. This value completely meets therequirement in [7] in the case when the out-of-bandinterference exceeds the thermal noise by no more than 10dB.

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(Beihang University) . The antenna was shaded by buildingson the south and north side. That is why the ClNo parametersof some satellites are apparently lower than the others. Thecoordinate of the point on the Google Earth is latitude39°58'42.25 ", longitude 116°20'39.68", and altitude 55m. Theheight of the building is about 30m.

Fig. 5 is the acquisition result of the PRN 32 from the dataabove. A high ClNo of the collected signal causes a highcorrelation peak, and the noise floor is quite flat.

Gigabit Ethernet port can be used as the data storage andprocessing terminal. GPS satellite signals have been collectedby the system, and the digital data have been successfullyprocessed by the GNSS software receiver. Collection ofsignals of different structures and frequencies has also beenvalidated by the same receiver and the GNSS signal simulator.

lOW 386Pleas t iPie 299 .896499

latitude longitude HAE38:59 :59 . 92 11 6 : 8: 9 .11 188 .1381685

Spe e d Heading TI C_dt8 .887246 -112 .168891 9 .829898

Correlation Waveform of PRN32

Figure 6. Positioning result of the BOC(l ,I) signal at 1575.42MHz.

n e a s t illle 652.254743l a t itude longitude HAE38'59 '60 .00 115 '59'60 .00 100 .2151763

Speed Heading TIC_dt0.189123 32 .383461 0.020000

REFERENCES

[I] James BAO-YEN TSUl , Fundamentals of Global Positioning SystemReceivers: A Software Approach -2nd EDTION, WILEY, USA,pp.109-II0, 2005 .

[2] Rodney G. Vaughan, Neil L. Scott , D. Rod White , The Theory ofBandpass Sampling, IEEE Transactions on Signal Processing, vol. 39,No .9, pp.l973-1984, September 1991.

[31 M. L. Psiaki , D. M. Akos, and J. Thor , A comparison of "direct RFsampling" and "down-convert & sampling" GNSS receiverarchitectures, in Proc . Institute of Navigations GPS/GNSS meeting,Portland, USA, 2003, September 9-12 , 2003 .

[4] D. M. Akos, and 1. B. Y. Tsui, Des ign and implementation of a directdigitization GPS receiver front end, IEEE Tran sactions on MicrowaveTheory and Techniques, Vol. 44, No . 12, pp. 2334-2339, December1996.

[5] Jonas Thor , D. M. Akos , A Direct RF Sampling Multifrequency GPSReceiver, IEEE Position Locat ion and Nav igation Sympos ium, PalmSprings, CA, April 15-18, 2002 .

[6] D. M. Akos , A. Ene , and 1. Thor , A Prototyping platform forMultifrequency GNSS receivers, in Proc . Institute of NavigationGPS/GNSS Meeting, Portland, USA , 2003 .

[7] Ville Syrjala, Mikko Valkama and Markku Renfors, DesignConsiderations for Direct RF Sampling Receiver in GNSS Environment,in Proc . The 5th Workshop on Position ing, Navigation andCommunication , 2009.

[81 Nicholas Gray, ABCs of ADCs Analog-to-Digital Converter Basics,National Semiconductor, pp. 4-8 , June 2006.

[91 Asghar Tabatabaer Balaer, Andrew G. Dempster, Dennis Akos,Quantization Degradation of GNSS Signal Quality in the Presence ofCW RFI, Proceedings ofIEEE ISSSTA 2008, pp. 42-47 , 2008 .

[10] Spilker, J. 1. Jr., Digital Communication by Satell ite, pp. 550-555 ,Prentice Hall , Englewood Cliffs , NJ, 1995.

[II] Brad Brannon, Allen Barlow, Aperture Uncertainty and ADC SystemPerformance, Analog Devices Inc, Application Note AN-50I , 2006 .

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18 11 3 51 31 - 8 85 . 2 228 1 5 9 47 .111 32 8 8 8 8.9 8 8 8 8 8.8GDOp .. 2.621 HDOP - 1.145 UDOP - 1.994 IDOP- 1.259

Figure 7. Positioning result of the BPSK signal at 1561.098MHz.

For a direct RF sampling system, aliasing noise andsampling clock jitter should be taken into consideration, butactually, the requirements are not so hard to meet. A highsample rate and properly chosen filter can maintain the noiseat an acceptable level, and an inexpensive TCXO can meet thejitter requirement.

300

Figure 5. Acquisition result of the PRN 32 satelli te.

6x 10

500g~­4000

3000200 0

Rei Doppler (Hz) 0 0

IV. SUMMARY AND CONCLUSION

A GNSS signal collection system using direct RFsampling that can support GNSS signals of any frequencies isdesigned and implemented in this paper. Gigabit Ethernet isused for data transmitting , so any computer with a common

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10 11 4 156 56 3401.6 16 28 2 0 5 2 0 48. 2 111 12 4 73 54 1117.3 16282 0 52 0 49.0 1GDOp · 2 .986 HDOP· 1.397 UDOP· 2 .185 TDOP · 1.480

B. Data Collectedfrom the GNSS Signal Simulator

BOC (Binary Offset Carrier) and BPSK (Binary PhaseShift Keying) Signals of different carriers are generated by theGNSS signal simulator. Two carriers are located at 1575.42MHz and 1561.098 MHz respectively , and the output powerof the signals is -lOOdBm. The simulator and the signalcollection system are connected by a coaxial cable. Thecoordinate set by the simulator is at latitude 39°, longitude116°, and altitude 100m. The positioning results for the twosignals are shown in Fig. 6 and Fig. 7 respectively.

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