In this paper we investigate GPS signal acquisition and tracking using software GPS receiver. The software GPS receiver consists of a RF front-end, an ADC, and a software GPS program that runs on PC. The RF front-end down-converts the signal from RF to IF, and the ADC samples the IF signal. All the other processing including signal acquisition, tracking, data decoding, and solving position are all implemented in software using signal processing techniques. The local C/A code and carrier replica signals are pre-generated, stored in memory, and used repetitively during signal acquisition and tracking. The concepts of signal processing using software GPS receiver can be applied to the next generation Global Navigation Satellite System and Space Based Augmentation System receivers design

Wu, F

Yasuda, A

Zhang, K

In this paper we investigate GPS signal acquisition and tracking using software GPS receiver. The software GPS receiver consists of a RF front-end, an ADC, and a soft-ware GPS program that runs on PC. The RF front-end down-converts the signal from RF to IF, and the ADC samples the IF signal. All the other processing includ-ing signal acquisition, tracking, data decoding, and solv-ing position are all implemented in software using signal processing techniques. The local C/A code and carrier replica signals are pre-generated, stored in memory, and used repetitively during signal acquisition and tracking. The concepts of signal processing using software GPS re-ceiver can be applied to the next generation Global Nav-igation Satellite System and Space Based Augmentatio

Falin Wu

Kefei Zhang

Akio Yasuda

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Akio, “GPS signal acquisition and tracking using software GPS receiver

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GPS SIGNAL ACQUISITION AND TRACKING USING SOFTWARE GPS RECEIVER
Falin Wu∗ and Kefei Zhang
The RMIT University
Melbourne, Australia
∗falin.wu@rmit.edu.au
Akio Yasuda
Tokyo University of Marine
Science and Technology
Tokyo, Japan
ABSTRACT
In this paper we investigate GPS signal acquisition and
tracking using software GPS receiver. The software GPS
receiver consists of a RF front-end, an ADC, and a soft-
ware GPS program that runs on PC. The RF front-end
down-converts the signal from RF to IF, and the ADC
samples the IF signal. All the other processing includ-
ing signal acquisition, tracking, data decoding, and solv-
ing position are all implemented in software using signal
processing techniques. The local C/A code and carrier
replica signals are pre-generated, stored in memory, and
used repetitively during signal acquisition and tracking.
The concepts of signal processing using software GPS re-
ceiver can be applied to the next generation Global Nav-
igation Satellite System and Space Based Augmentation
System receivers design.
1. INTRODUCTION
Themodernization of USGlobal Positioning System (GPS),
the revival of Russian GLONASS and the advent of Euro-
pean Galileo System will bring the next generation Global
Navigation Satellite System (GNSS). To augment GPS for
aviation users the US, Europe and Japan have deployed
the Wide Area Augmentation System (WAAS), the Euro-
pean Geostationary Navigation Overlay System (EGNOS)
and MTSAT Satellite Augmentation System (MSAS), re-
spectively. Each of these Space Based Augmentation Sys-
tem (SBAS) services is free and interoperable, providing
coverage for most of North America, most of Europe and
Japan, respectively. At the same time Japan launches a
new regional SBAS plan, Japanese Quasi-Zenith Satellite
System (QZSS), a constellation of three to seven satellite
will broadcast GPS signals from orbits optimized for East
Asia regions. Although GNSS and SBAS will provide
users more navigation satellite signals, it is a challenge
for users to design the next generation GNSS receivers.
Currently GPS receivers consist of a Radio Frequency
(RF) front-end, an Applications Specific Integrated Cir-
cuit (AISC) for signal processing, and a CPU core for
higher level functions. Firmware can be loaded into the
CPU to change the performance parameters of the receiver.
However, design flexibility is very constrained by an AISC
that is hardwired with predefined tracking channels, corre-
lator and control loop characteristics. So a software GNSS
receiver is the best solution for next generation GNSS re-
ceiver design. Software GNSS receivers are those that
implement signal acquisition and tracking process not in
hardware, but in software [1, 2]. Each of GNSS, although
different, operates in the required underlying signal process-
ing. As such, the software GPS receiver architecture will
be used through this paper as an example but the concepts
can be applied to the design of any software GNSS re-
ceivers.
In this paper GPS signal acquisition and tracking us-
ing software GPS receiver are investigated. A prototype
software GPS receiver is developed. The receiver is based
on personal computer (PC) and samples GPS signal at in-
termediate frequency (IF). First, the structure of software
GPS receiver and data collection hardware is presented.
Second, the GPS Coarse/Acquisition (C/A) code acquisi-
tion is described. Then, the GPS signal tracking is dis-
cussed. Finally, some useful conclusions are given.
2. SOFTWARE GPS RECEIVER
Software GPS receiver uses an analog-to-digital converter
(ADC) to change the input GPS signal into digital data at
the earliest possible stage in the receiver. In other words,
the input signal is digitized as close to the antenna as pos-
sible [2]. Fig. 1 illustrates a general structure of soft-
ware GPS receiver [3, 4]. The signals transmitted from
the GPS satellites are received from an antenna. Through
a RF front-end, the input signal is amplified to a proper
amplitude and the frequency is converted to a desired fre-
quency. An ADC is used to digitize the input signal. The
antenna, RF front-end, and ADC are the hardware used in
the software GPS receiver. After the signal is digitized, a
software is used to process it. Acquisition means to find
the signal of a certain satellite. The tracking program is
used to find the phase transition of the navigation data.
From the navigation data phase transition, the sub-frames
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 posi-
tions. Finally, the user position, velocity, and time can
be calculated [2]. In this paper the GPS C/A code signals
are down-converted from 1575.42 MHz to 15.42 MHz and
sampled at 20 MHz using a GPS SIGNAL TAP of Accord
Software and Systems Private Company [5]. Therefore,
the center of the digitized signal is at 4.78 MHz. The sig-
nal samples can be processed in real-time and/or stored on
0-7803-9243-4/05/$20.00 ©2005 IEEE 843
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GNSS
antenna
RF
Front-end ADC
Hard disk
GNSS IF signal
simulator
Preprocessing: down sampling, amplitude and bias control
Tracking Acquisition
Data decoding, pseudorange measurements
Application processing (e.g. PVT solution)
Hardware
Software
IF signal
IF dataIF signal
Fig. 1. Structure of a general GPS receiver
the hard disk for post processing.
3. GPS C/A CODE ACQUISITION
The local C/A code and carrier replica signals are pre-
generated, stored in memory, and used repetitively during
signal acquisition and tracking.
3.1. GPS C/A Code Properties and Generation
The signal structure of GPS may be modified in the future.
However, at present, GPS satellites transmit two primary
L-band signals, termed L1 and L2, having carrier frequen-
cies of 1575.42 MHz and 1227.6 MHz, respectively. The
L1 frequency contains the C/A and P(Y) signals, while the
L2 frequency contains only the P(Y) signal. The C/A and
P(Y) signals in L1 frequency are in quadrant phase of each
other and they can be written as [2]
SL1 = APP (t)D (t) cos (2pif1t+ φ)
+ACC (t)D (t) sin (2pif1t+ φ) , (1)
where SL1 is the signal at L1 frequency, AP is the ampli-
tude of the P code, P (t) = ±1 represents the phase of the
P code, D(t) = ±1 represents the data code, f1 is the L1
frequency, φ is the initial phase, AC is the amplitude of
the C/A code, C(t) = ±1 represents the phase of the C/A
code.
The C/A code is a bi-phase modulated signal with a
chip rate of 1.023 MHz. Therefore, the null-to-null band-
width of the main lobe of the spectrum is 2.046 MHz.
Each chip is about 977.5 ns (1/1.023 MHz) long. The
transmitting bandwidth of the GPS satellite in the L1 fre-
quency is approximately 20 MHz to accommodate the P
code signal; therefore, the C/A code transmitted contains
the main lobe and several side lobes. The total code pe-
riod contains 1,023 chips. With a chip rate of 1.023 MHz,
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
+ +
+
+
1.023 MHz
clock
Reset
all one
C/A code
Shift register
G1 generator
G2 generator
Positions of these feedback
determine the satellite ID
Fig. 2. C/A code generator
1,023 chips last 1 ms; therefore, the C/A code is 1 ms
long. The code repeats itself every millisecond.
In order to find the beginning of a C/A code in the
received signal only a very limited data record is needed
such as 1 ms. If there is no Doppler effect on the received
signal, then one millisecond of data contains all the 1,023
chips. A guaranteed way to receive the first three sub-
frames is to take 30 seconds (or one page) of data. Thus,
one can take a minimum of 30 seconds of data and calcu-
late the user position [2].
The GPS C/A signals belong to the family of Pseudo-
Random Noise (PRN) codes known as the Gold codes [2].
Different C/A codes are used for different satellites. The
signals are generated from the product of two 1,023-bit
PRN sequences G1 and G2. Fig. 2 shows the C/A code
generator. One of the most important properties of the
C/A codes is their correlation. High autocorrelation peak
and low cross-correlation peaks can provide a wide dy-
namic range for signal acquisition. The maximum of the
autocorrelation peak is 1023, which equals the C/A code
length. The rest of the autocorrelation has three values 63,
-1, and -65. The cross-correlation also has three values
63, -1, and -65 [2].
3.2. GPS C/A Code Acquisition
The acquisition algorithm is used to determine whether a
certain satellite is in the input signal, and if so determine
its code phase and carrier frequency. The fast Fourier
transform (FFT) approach will be used to acquire GPS
C/A code.
The FFT approach performs circular convolution in
the frequency domain. A noncoherent correlator in fre-
quency domain can be adapted to the acquisition of GPS
signals as shown in Fig. 3 [6]. The discrete Fourier trans-
form (DFT) and its inverse is used to calculate the corre-
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I
Q
Digital IF
FFT
  
FFT
IFFT
1
0
K
j
−
=
∑[ ]x n [ ]cos nΩ
[ ]sin nΩ [ ]CA n
[ ]2R m2
Fig. 3. Non coherent correlator in frequency domain
Fig. 4. Acquisition result using FFT approach (Satellite
SV4)
lation value, R [6],
R [m] = x [n]⊗ CA [−n]︸ ︷︷ ︸
Circular convolution
= F−1
(
F (x [n]) · F (CA [n+m])∗) , (2)
where F , F−1 and ∗ denote FFT, inverse FFT and the
complex conjugate, respectively. n represents the nth sam-
ple andm represents the number of samples that the repli-
cated C/A code is phase shifted. First the input digitized
IF signal, x[n], is mixed to base band. After the carrier
removal, the in-phase (I) and quadrature-phase (Q) com-
ponents are used as the real and imaginary inputs when
calculating the DFT. The result is multiplied by the com-
plex conjugate of DFT of the C/A code, CA[n+m]. The
circular convolution is obtained by taking the magnitude
of the inverse DFT. The fast Fourier transform algorithm
is used to implement the DFT and inverse DFT, hence the
acquisition method may be called FFT approach.
Fig. 4 illustrates the result of acquisition using FFT ap-
proach. Because the Doppler frequency range that needs
to be searched is ±10 KHz, and the center of the signals
is at 4.78 MHz, see Sec. 2. It is important to determine
the frequency steps needed to cover range from 4.57 MHz
to 4.59 MHz. After the circular correlation, the highest
component occurs at 4.578 MHz and the beginning of the
C/A code is at 7547.
4. GPS SIGNAL TRACKING
Once a signal is acquired, the signal must be tracked in
order to obtain the navigation data. The tracking pro-
gram uses two parameters obtained from the acquisition
?
MA sqrt
MA sqrt
e/d select
lpf
lpf
osc lpf
ADC
90
C/A
carrier
frequency
prompt
cw output
arctan
early
late
CODE LOOP
CARRIER LOOP
Fig. 5. Code and carrier tracking loops
process: the beginning of the C/A code period and the car-
rier frequency of the input signal to start the processing.
Two loops are needed to track one GPS signal. One loop
is often referred to as the code loop, which tracks the C/A
code. The other one is the phase (or frequency) locked
loop, which tracks the carrier frequency of the down-converted
input signal. These two loops must be coupled together as
shown in Fig. 5 [2].
The code loop uses three locally generated C/A codes
to track the C/A code of the input signal. Three locally
generated codes are usually used: a prompt, an early and
a late code. The prompt code is applied to the digitized
input signal and strips the C/A code from the input signal.
The output will be a continuous wave (cw) signal with
phase transition caused only by the navigation data, which
is applied to the input of the carrier loop.
The output from the carrier loop is a cw with the car-
rier frequency of the input signal. This signal is used to
strip the carrier from the input digitized signal. The output
is a signal with only a C/A code and no carrier frequency,
which is applied to the input of the code loop.
4.1. Code Tracking
The code loop is known as a delay lock loop (DLL) [7].
The prompt code is intended to match the C/A code in
the input signal. The correlation outputs from three local
codes can be used to accurately determine the beginning
of the C/A code in the input signal. This information is
used to adjust the initial phase of the locally generated
prompt code to better match the code phase of the input
signal. The discriminator algorithm is to give a calculated
number on the code phase error. The discriminator output
signal, ε, is calculated by
ε =
y
E
y
L
, (3)
where y
E
and y
L
are the correlation powers of the early
code and the late code, respectively. If ε = 1 , the prompt
code is perfectly aligned with the C/A code in the input
signal; if ε > 1.1, the local codes should be shifted to the
right; if ε < 0.9, the local codes should be shifted to the
left [6].
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4.2. Carrier Tracking
The phase locked loop is used to track the input IF sig-
nal, which is obtained from the acquisition program. The
locally generated reference IF signal for the tracking pro-
gram should be closer to the input IF signal.
A second-order loop can track the rate of change in
Doppler and a third-order loop can be used to track con-
stant acceleration [2]. For a software GPS receiver, the
phase and frequency of the reference signal are adjusted
every ms or 10 ms. This operation may put the IF of the
locally generated signal very close to the IF of the input
signal, but may not be exactly equal to it. In this paper a
second order PLL is used as the filter to track the input IF
signal. The filter function is
F (z) =
(C1 + C2)− C1z−1
1− z−1 , (4)
where C1 and C2 are coefficients,
C1 =
1
k0k1
8ςωnts
4 + 4ςωnts + (ωnts)
2 , (5)
C2 =
1
k0k1
4 (ωnts)
2
4 + 4ςωnts + (ωnts)
2 , (6)
where ωn is a natural frequency,
ωn =
2β
ζ + 14ζ
, (7)
ζ is the damping ratio, β is the bandwidth, ts is the sam-
pling interval. The amplifier k0 represents the gain of the
phase comparator and the low-pass filter limits the noise
in the loop, and k1 is the gain of the voltage-controlled
oscillator [2].
The results of GPS signal tracking using the tracking
method proposed in this paper are illustrated in Fig. 6,
which plots the amplitude of the output signal. The data
from the satellite is taken from the prompt I-component
before it is fed into the arctangent discriminator, see Fig. 5.
Each data point is obtained from five milliseconds of dig-
itized data. The amplitude changes with time; this is the
transit effect of the tracking loop. Finally, the amplitude
reaches a steady state.
5. CONCLUSION
In this paper we have investigated GPS signal acquisition
and tracking using software GPS receiver. The FFT acqui-
sition approach and the code and carrier tracking methods
have been studied. A prototype software GPS receiver has
been developed. The receiver is based on a PC and sam-
ples GPS signals at IF. The concepts of signal processing
using software GPS receiver can be applied to any soft-
ware GNSS receiver design. We are developing a genuine
software GNSS receiver to acquire and track the signals
of modernized GPS, Galileo, and QZSS.
0 100 200 300 400 500 600 700 800 900 1000
−1500
−1000
−500
0
500
1000
1500
Tracking: Correlation Values for SV 4 (C/N ≈ 49)
Time (ms)
Fig. 6. Tracking results from conventional tracking phase-
locked loop (Satellite SV4)
6. REFERENCES
[1] Dennis M Akos, A Software Radio Approach to
Global Navigation Satellite System Receiver Design,
Ph.D. thesis, Ohio University, Athens, Ohio, USA,
August 1997.
[2] James Bao-Yen Tsui, Fundamentals of Global Posi-
tioning System Receivers - A Software Approach, A
John Wiley & Sons, Inc. Publication, 2 edition, 2005.
[3] Thomas Pany, Sung Wook Moon, Markus Irsigler,
Bernd Eissfeller, and Karl Fu¨rlinger, “Performance
assessment of an under sampling SWC receiver for
simulated high-bandwidth GPS/Galileo signals and
real signals,” in Proceedings of 16th International
Technical Meeting of the Satellite Division of The In-
stitute of Navigation, Portland, Oregon, USA, Sep-
tember 9-12 2003, pp. 103–116.
[4] Falin Wu, Nobuaki Kubo, Akio Yasuda, and Haru-
masa Hojo, “Development of a prototyping platform
for software GPS receiver,” The Journal of Japan In-
stitute of Navigation, vol. 111, pp. 193–200, Septem-
ber 2004, ISSN 0388-7405.
[5] Accord, GPS Signal Tap User’s Guide, Accord Soft-
ware & System Private Limited, Bangalore, India,
September 2003.
[6] Fredrik Johansson, Rahman Mollaei, Jonas Thor, and
Jo¨rgen Uusitalo, “GPS satellite signal acquisition and
tracking,” Undergraduate projects 1998, Division of
Signal Processing, Lulea˚ University of Technology, S-
971 87 Lulea˚, Sweden, August 21 1998.
[7] Pratap Misra and Per Enge, GLOBAL POSITIONING
SYSTEM, Signals, Measurements, and Performance,
Ganga-Jamuna Press, 2001.
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GPS signal acquisition and tracking using software GPS receiver

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GPS signal acquisition and tracking using software GPS receiver

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