Development of a Prototyping Platform for Software ...

Development of a Prototyping Platform for Software GPS Receiver Falin Wu, Nobuaki Kubo, Akio Yasuda and Harumasa Hojo [email protected]

Laboratory of Communication Engineering Tokyo University of Marine Science and Technology

JIN-Spring, May 13, 2004 – p.1/21

Content Software GPS/GNSS Receiver GPS C/A Code Acquisition C/A Code Properties and Generation C/A Code Acquisition Conventional approach FFT approach GPS Signal Tracking Code Tracking Carrier Tracking Conclusion

JIN-Spring, May 13, 2004 – p.2/21

Background New Satellite Navigation Signals Modernized GPS (USA); Galileo (EU); Japanese Quasi-Zenith Satellite System (QZSS). New Positioning Applications E911 call service for wireless phones (US); E110 (Japan, Newspaper Asahi, Nov.6, 2003); Hardware size and power consumption of GPS receiver. Software GPS/GNSS receiver: the signal acquisition and tracking are implemented by software instead of hardware chip. JIN-Spring, May 13, 2004 – p.3/21

Software GPS/GNSS Receiver Four Classes PC based genuine software GPS/GNSS receiver; Digital signal processors (DSP) based software GPS/GNSS receiver; Field programmable gate array (FPGA) based software GPS/GNSS receiver; Simulation tools. Two Groups Radio Frequency (RF) sampling; Intermediate Frequency (IF) sampling. A prototype PC based and IF sampling software GPS receiver has been developed. JIN-Spring, May 13, 2004 – p.4/21

General Structure GNSS antenna

Frontend

IF signal

ADC

Hardware GNSS IF signal simulator

IF signal

IF data

Hard disk

Preprocessing: down sampling, amplitude and bias control

Tracking

Acquisition

Data decoding, pseudorange measurements Application processing (PVT solution) Software JIN-Spring, May 13, 2004 – p.5/21

Hardware GPS SIGNAL TAP (Accord Software and Systems Private Company)

Frequency Plan 15.42 MHz

135.42 MHz

1575.42 MHz LNA

Filter

Filter

ADC AMP

1440.0 MHz

120 MHz

Sampling Frequency: 2 MHz - 20 MHz. (Step: 1 KHz). JIN-Spring, May 13, 2004 – p.6/21

Content Software GPS Receiver GPS C/A Code Acquisition C/A Code Properties and Generation C/A Code Acquisition Conventional approach FFT approach GPS Signal Tracking Code Tracking Carrier Tracking Conclusion

JIN-Spring, May 13, 2004 – p.7/21

GPS C/A Code Properties C/A and P(Y) signals on L1 frequency SL1 = AP P (t) D (t) cos (2πf1 t + φ) +AC C (t) D (t) sin (2πf1 t + φ)

The C/A code is a bi-phase modulated signal with a chip rate of 1.023 MHz. C/A of sat #6 sampled at 5042500 Hz for 1 ms

60

50

Amplitude in dB

40

30

20

10

0

−10 −2500

−2000

−1500

−1000

−500 0 500 Frequency in KHz

1000

1500

2000

2500

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GPS C/A Code Properties (cont.) High Autocorrelation Peak: 1023 (a) Autocorrelation of satellite 19

1500 1000 500 0 −500

0

100

200

300

400

500

600

700

800

900

1000

800

900

1000

(b) Cross correlation of satellites 19 and 31

100 50 0 −50 −100

0

100

200

300

400

500

600

700

Low Cross-correlation Peaks: 63, -1 and -65 JIN-Spring, May 13, 2004 – p.9/21

GPS C/A Code Generation The local C/A codes and carrier replica signal are pre-generated, stored in memory, and used repetitively. G1 : 1 + x3 + x10 G1 generator

+ 1 2 3 4 5 6 7 8 9 10 Shift register

1.023 MHz clock

Reset all one

+

+

C/A code

Positions of these feedback determine the satellite ID

1 2 3 4 5 6 7 8 9 10

+ G2 generator

G2 : 1 + x2 + x3 + x6 + x8 + x9 + x10 JIN-Spring, May 13, 2004 – p.10/21

Acquisition (Conventional App.) Non coherent correlator in time domain I Digital IF

( j +1)NL −1

∑

2

( )

n = jNL

x [n ]

K −1

cos [Ωn ]

Q

CA [n + m ]

∑ ( j +1)NL −1

∑

R 2 [m ]

j =0

2

( )

n = jNL sin [Ωn ]

Correlation power #2 "  (j+1)N L−1 K−1 P P R2 [m] = x [n] · CA [n] · cos [Ωn] + j=0

"

(j+1)N P L−1 n=jN L

n=jN L

x [n] · CA [n] · sin [Ωn]

#2



JIN-Spring, May 13, 2004 – p.11/21

Acquisition (FFT Approach) Conventional approach can also be performed in circular convolution. L−1 P R [m] = x [n] · CA [((n + m))L ] n=0

Non coherent correlator in frequency domain I Digital IF

x [n ]

cos [Ωn ]

FFT

IFFT

2

K −1

∑

R 2 [m ]

j =0

Q sin [Ωn ]

FFT CA [n ]

R [m] = x [n] ⊗ CA [−n] = | {z } Circular convolution

F −1

F (x [n]) · F (CA [n])

∗


JIN-Spring, May 13, 2004 – p.12/21

Acquisition Results (FFT App.)

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Content Software GPS Receiver GPS C/A Code Acquisition C/A Code Properties and Generation C/A Code Acquisition Conventional approach FFT approach GPS Signal Tracking Code Tracking Carrier Tracking Conclusion

JIN-Spring, May 13, 2004 – p.14/21

GPS Signal Tracking CODE LOOP MA

sqrt

early

C/A

e/d select

Σ

late

carrier frequency

MA

sqrt

ADC prompt cw

output

lpf osc

lpf

arctan

90 lpf CARRIER LOOP

JIN-Spring, May 13, 2004 – p.15/21

Code Tracking Delay Lock Loop (DLL) CORRELATION

Prompt Prompt

Prompt Late

Early

Late

Early

-5 -3

0 (a)

3 5

Early

Late -5 -3

0

-5 -3

3 5

(b)

The discriminator output signal, ε =

0 (c)

3 5

yE yL

if ε = 1, the prompt code is perfectly aligned with the C/A code in the input signal; if ε > 1.5, the local codes should be shifted to the right; if ε < 0.8, the local codes should be shifted to the left. JIN-Spring, May 13, 2004 – p.16/21

Code Tracking (cont.) Correlation power of early, prompt and late channel. Correlation power of early, prompt and late channel for satellite 4

1400

Prompt Early Late

1200

Correlation power

1000

800

600

400

200

0

100

200

300

400

500 Time (ms)

600

700

800

900

1000

Code tracking loop keeps the prompt signal at maximum correlation. JIN-Spring, May 13, 2004 – p.17/21

Carrier Tracking Carrier Tracking Loop prompt IF

cw

output

lpf osc

carrier frequency

lpf

90

arctan

lpf CARRIER LOOP

θcorr = remainder (θold + θ, 2π)

Carrier Loop Filter A Second-order Filter: F (z) =

(C1 +C2 )−C1 ∗z −1 1−z −1

JIN-Spring, May 13, 2004 – p.18/21

Carrier Tracking (cont.) Outputs from carrier tracking loop Tracking: Correlation values for satellite 4 (C/N≈ 49)

1500

1000

500

0

−500

−1000

−1500

0

100

200

300

400

500 Time (ms)

600

700

800

900

1000

Navigation data phase transition ⇒ Navigation data ⇒ Ephemeris data and pseudorange ⇒ PVT JIN-Spring, May 13, 2004 – p.19/21

Summary A prototype PC based and IF sampling software GPS receiver; Two acquisition methods: Conventional approach and FFT approach have been introduced; The code and carrier tracking methods have been investigated. Future works Build a new hardware (antenna, front-end, and ADC) to sample the civil signals of modernized GPS, Galileo, and QZSS; Develop a genuine software GNSS receiver to acquire and track the signals of modernized GPS, Galileo, and QZSS. JIN-Spring, May 13, 2004 – p.20/21

Thank you for your attention! [email protected] Laboratory of Communication Engineering, Tokyo University of Marine Science and Technology JIN-Spring, May 13, 2004 – p.21/21