Journal of Global Positioning Systems (2004)
Vol. 3, No. 1-2: 25-31
An Assisted GPS Acquisition Method using L2 Civil Signal in Weak
Signal Environment
Deuk Jae Cho
Department of Electronics, Chungnam National University, Korea
e-mail: [e-mail]; Tel: [phone]; Fax: [phone]
Chansik Park
School of Electrical and Computer Engineering, Chungbuk National University, Korea
e-mail: [e-mail]; Tel: [phone]; Fax: [phone]
Sang Jeong Lee
Division of Electrical and Computer Engineering, Chungnam National University, Korea
e-mail: [e-mail]; Tel: [phone]; Fax: [phone]
Received: 15 Nov 2004 / Accepted: 3 Feb 2005
Abstract. Recently, there has been increasing demands
on the positioning capability in weak signal environment
such as inside building and urban area. The present
assisted GPS technology uses GPS L1 signals only.
Meanwhile, according to the GPS modernization plan,
Block IIR-M GPS satellite will be first launched in 2005,
transmitting the civil code in L2 frequency as well as in
L1 frequency with the updated signal structure. Since the
L2 civil code has a worst-case cross correlation
performance of 45 dB (over 251 times better than 21 dB
cross correlation performance of the L1 C/A code), it will
be much more effective in weak signal environment. This
paper proposes an assisted GPS acquisition method using
L2 civil signals. It will show that the acquisition success
rate of the proposed assisted GPS acquisition method is
better than that of the existing assisted GPS method using
L1 signals in the same environment. The constellation of
the next generation GPS satellites is scheduled to launch
in 2005. Therefore, in order to design and test the assisted
GPS acquiring the L2 civil signal, it is necessary to
design a signal generator which can generate the L2 civil
signal. The signal generator will be designed using the
pseudo random noise (PRN) code generation method and
navigation message protocol defined in GPS ICD PIRN
200C-007B. Finally, through the simulations using the
designed signal transmitter, the success rate of the
proposed assisted GPS acquisition method will be
compared with that of the existing assisted GPS method
to show the performance improvements.
Key words: Acquisition, L2 Civil Signal, Weak Signal
1 Introduction
A signal processing of GPS receiver is composed of
signal acquisition, signal tracking and navigation in
accordance with function. Particularly, the performance
of the signal acquisition has influence on TTFF (Time to
First Fix) and RF sensitivity of GPS receiver. The RF
sensitivity of GPS receiver is defined as the minimum
power for acquiring the GPS signal. The GPS L1 C/A
signals and L2 civil signals (L2CS) in Block IIR-M
satellites are guaranteed minimum -128.5dBm and -
131.4dBm signal strength each into a 3dBi linearly
polarized user receiving antenna at worst normal
orientation when the satellite is above a 5-degree
elevation angle (ICD PIRN-200C-007B, 2002). It is
difficult for GPS receiver to acquire GPS signals in the
case of being obstacles in the line of sight since the GPS
signal strength is very low (Haddrell and Pratt, 2001).
From this viewpoint, it can be said that the RF sensitivity
of GPS receivers is the dominant factor that has influence
on the performance of GPS receivers. Since the L2 civil
code provides better protection (24dB) than C/A against
code cross correlation and continuous wave interference,
it will be much more effective in weak signal
environment.
This paper proposes an assisted GPS acquisition method
using L2 civil signals. In section 2, this paper summarizes
the structure and the property of L2 civil signal

26 Journal of Global Positioning Systems
comparing with those of L1 C/A code. In order to design
and test the assisted GPS acquiring the L2 civil signal not
existing yet, it is necessary to design L2CS generator. So
section 3 describes a software-based L2CS generator
designed in this paper. Section 4 proposes an acquisition
method for solving the problem of squaring loss in weak
signal environment since the long coherent integration
increases the number of frequency search cells and the
non-coherent integration of weak GPS signals induces the
squaring loss. In section 5, through the simulations using
the designed signal generator, it will show that the
acquisition success rate of the proposed assisted GPS
acquisition method is better than that of the existing
assisted GPS method using L1 signals in the same
environment.
2 L2 Civil Signal Structure
The new signal structure adds M (Military) codes and
enhances L2 civilian codes (Hartman et al, 2000). L2
civilian codes are composed of the L2 civil moderate
(CM) and L2 civil long (CL) codes as part of the L2
civilian enhancements. The spectrums of current and
proposed GPS signals are shown in Figure 1. And Table
1 shows the characteristics of L2CS and existing L1 C/A
code.
1227.60 MHz1575.42 MHz1176.45 MHz
L2 L1L5
P(Y)
P(Y)
C/A
C/A
P(Y)
P(Y)
C/A
C/A
P(Y)
P(Y)
P(Y)
P(Y)
L2CS
L2CS
P(Y)
P(Y)
C/A
C/A
P(Y)
P(Y)
L2CS
L2CS
M
MM
M
Present Signal
(Block II/IIA/IIR)
Next Generation
Of Capability
(Block IIR-M)
Civil Safety of Life
Applications
(Block IIF and beyond)
M MM
M
Fig. 1 Modernized GPS Signal Evolution
Tab. 1 Summary of Signal Characteristics
L1 C/A L2 CM L2 CL L2 CM/CL (TDM)
Code Type Gold Code Maximal Length
Code
Maximal Length
Code
Maximal Length
Code
Chip Rate
(Mchips/sec) 1.023 0.512 0.512 1.023
Code Length
(Chips) 1,023 10,230 767,250 1,534,500
Repeat Rate (msec) 1 20 1500 1500
Carrier Frequency
(MHz) 1575.42 1227.60 1227.60 1227.60
Bit Rate 50 bps 25 bps No message 50 sps
Tab. 2 Cross Correlation Protection
Carrier Frequency
(MHz)
Code Length
(chips)
Code Clock
(MHz) Phases Fully Available
Correlation
Protection
1,575.42 (L1 C/A) 1,023 1.023 Bi-Phase Now > 21 dB
1,227.60 (L2CS) 10,230 (CM)
767,250 (CL)1.023 Bi-Phase ~2013 > 45 dB
With the advent of the modernized GPS IIR-M satellites
there will be an immediate benefit to all civilian GPS
users including civil aviation. This is due to the
characteristics of the L2C code on the L2 frequency. The
L2C code signal is much more robust than the existing L1
C/A code and has much better cross correlation
properties. The minimum L2C code cross correlation
protection is 45 dB while 21 dB for the existing L1 C/A
code as summarized in Table 2 (Fontana et al., 2001).
This greater cross correlation protection is valuable in
many environments where a weak GPS signal may be
interfered with by another stronger GPS signal. It is
beneficial to emergency indoor positioning or to personal
navigation in wooded areas. Because of this great cross
correlation protection, the L2C code signal also has a
higher data recovery threshold and a better code tracking
performance. The superior cross correlation properties
also enable the GPS receivers to implement faster
acquisition strategies because it can reduce the number of
false alarms (Diggelen and Abraham, 2001).
The L2C code is composed of two multiplexed code.
Each of two codes is a disjoint repeating segment of a
maximal length code generated by a 27 bit shift register
with 15 taps defined by a coder initial state which in turn
determined by the satellite ID and code length. The
diagram of L2C code generator is shown in Figure 2
(ICD PIRN-200C-007B, 2002). The CM code signal is a
10,230 chip sequence repeating every 20ms. The CL code
signal is a 767,250 chip sequence repeating every 1.5
seconds.
LSB MSB
3+3+2+3+3+2+2+3+1+1+1+3
Output
3 3 2 3 3 2 2 3 1 11 3
Initial state of CL/CM
3 3 2 3 3 2 2 3 1 11 3
Compare
Last state of CL/CM
Fig. 2 L2C Code Generator
The L2C NAV (NAVigation) data can be either 50 bps
data or 25 bps data, which is coded with a rate 21 FEC
(Forward Error Correction) convolutional coder. The
coder state history is reset to zero at the beginning of each
data message. The resulting 50 sps (symbol per second)

Cho et al: An Assisted GPS Acquisition Method using L2 Civil Signal in Weak Signal Environment 27
L5 - Like CNAV
Message
25 bits/secRate ½ FEC
Legacy NAV
Message
25 Bits/sec
10,230 Chip
Code Generator
767,250 Chip
Code Generator
C/A Code
Generator
Legacy NAV
Message
50 bits/sec
Chip by Chip
Multiplexer
½
D1
D2 C1
C2
CM
Code
CL
Code
B1
B2
A1
A2
Transmitted
Signal
1.023 MHz
Clock
Fig. 3 L2C Signal Options in IIR-M Satellites
symbol stream is Modulo-2 added to the CM code. The
resultant CM, CL bit-trains are combined using a time-
division multiplex (TDM) method starting with the CM
code. The combined bit-trains are used to modulate the
L2 quadrature-phase carrier. The L2C NAV will have a
flexible message structure controllable by the Control
Segment. The structure of the navigation message for
L2C, CNAV, is basically same as that of the L5 signal. It
is more compact and more flexible than that of the
current NAV message. Instead of a fixed message format,
CNAV allows the Control Segment to specify the
sequence and timing of each message component
consisting of 300 bit subframe. Since the data rate of the
L2C signal is 25bps, each subframe requires 12 seconds
to be transmitted. The L2C signal options in IIR-M
satellites are shown in Figure 3. The signal options are
controlled by four switches whose preferred positions are
A1, B1, C1, D1.
3 L2 Civil Signal Generator Design and Analysis
The constellation of the next generation GPS satellites is
scheduled to launch in 2005. Therefore, in order to design
and test the assisted GPS acquiring the L2 civil signal, it
is necessary to design a signal generator which can
generate the L2 civil signal. The signal generator is
designed as shown in Figure 4.
CM Code
Generator
CL Code
Generator
TDM
∑
L2 Digitized IF
Carrier Generator
Noise
Generator
Rate 1/2
FEC
Data
Generation
+
XBand-pass
Filter AGC A/D
Fig. 4 Structure of GPS L2 Civil Signal Generator
Figure 4 shows the structure of the software-based L2C
signal generator. In this signal generator, the noise
generator has the zero-mean property as shown in Figure
5. And the output of the signal generator is shown in
Figure 6.
Fig. 5 The Output of Noise Generator
Fig. 6 The Output of the Signal Generator
4 A Proposed Assisted GPS Acquisition Method
4.1 The Squaring Loss
In general, in order to enhance RF sensitivity of GPS
receiver, it is necessary to increase the correlation
integration time over basic correlation time. Figure 7
shows a previously existing assisted GPS acquisition
method using both the coherent integration and the non-
coherent integration in weak signal environment. And
Equation (1) and Equation (2) show the coherent
integration and the non-coherent integration, respectively.

28 Journal of Global Positioning Systems
Lowpass Filter∑
=
e
M
k
k
e
Z
M1
1∑
=
e
N
k
k
e
Y
N1
1
ZY
(generated signal)
η
≥y
y
η
: detection threshold
yes
no
signal present
signal absent
(despreading mixer)
coherent/non-coherent
integration
)( k tx 0 T
r
ω
(received signal)
τ
ω
e
)
ˆ
exp()
ˆ
(2)( 0krkk tjTCtG ω −⋅=
Fig. 7 The Previously Existing Signal Acquisition Method in Assisted GPS
After low pass filtering, the output of the coherent
integration and the non-coherent integration are
∑
=
=e
M
k
k
e
Z
M
Y
1
1 (1)
() ()
∑
=
+= e
N
k
q
k
i
k
e
YY
N
y
1
22
1 (2)
where e M and e N are the number of the coherent
integration and the non-coherent integration, respectively,
k
Z is the output of the lowpass filter, Y and y are the
output of the coherent integration and the non-coherent
integration, respectively. Here Y is composed of the in-
phase component, i
k
Y and the quadrature-component,
q
k
Y.
The coherent integration is a technique integrating in-
phase correlation result e M times as given by Eq. (1).
Therefore, it is assumed that there is no sign inversion of
correlation by the navigation message bit transition or
assistance of sign inversion information during the
coherent integration. A relation of the number e
M of the
coherent integration and an allowable carrier frequency
error e
f is given by
()
ie
ie
Acq Tf
Tf
L
π
π
sin
log20 10
max = (3)
where peiTMT ⋅= is the coherent integration time, p T is
the period of integration, and Acq L is the acquisition loss.
Eq. (3) shows that the longer integration time requires the
less allowable carrier frequency error for the same signal
acquisition loss.
Since there is no navigation bit stream during the initial
acquisition time, the coherent integration technique is not
proper to be adopted in the signal acquisition process of
generic GPS receivers. For this reason, the generic GPS
receiver performs the coherent integration with
demodulating the navigation bit stream after the signal
acquisition. Here the purpose of the coherent integration
is to enhance the Signal-to-Noise (SNR) and to improve
the quality of measurements.
The non-coherent integration is a technique integrating
both the in-phase correlation result and the quadrature-
phase correlation result as shown in Eq. (2). Therefore it
is not necessary to know the navigation message bit
transition. That is, the non-coherent integration is not
influenced by sign inversion of the navigation message
bit during the integration, and an allowable carrier
frequency error is related not to the number of the non-
coherent integration but to integration time of correlation
values i
k
Y and q
k
Y. Therefore the non-coherent
integration technique is adopted to enhance RF sensitivity
of GPS receiver. But there is a disadvantage that the non-
coherent integration induces the squaring loss for weak
GPS signals. Particularly the squaring loss is the
dominant factor among the acquisition losses of assisted
GPS dealing with weak GPS signals.
The squaring loss is defined as the ratio of the SNR
before the non-coherent integration for the SNR after the
non-coherent integration.
nc
c
sq
L
α
α
= (4)
n
s
epcc
P
MTf
σ
α
2
2=
(
)
π
π
α
α
−
−







−
−
+Γ
=4
2
;1;
2
1
F1
2
1
2
2
11c
nc
where
(
)
⋅
Γ
is the gamma function,
(
)
⋅
11F is the
confluent hypergeometric function, c
α
is the SNR before
the non-coherent integration, and nc
α
is the SNR after
the non-coherent integration. Therefore, the squaring loss
is given by







−







−−





+Γ
−
=
π
α
πα
2
;1;
2
1
F1
2
1
2
4
2
11 c
c
sq
L (5)
And the squaring loss has properties as follows:
1,0 10 =< =
c
sqsq
c
LL
d
d
α
α
(6)

Cho et al: An Assisted GPS Acquisition Method using L2 Civil Signal in Weak Signal Environment 29
From Eq. (6), it is explained that Eq. (5) is a monotonic
decreasing function and the non-coherent integration
induces the squaring loss when the Signal-to-Noise Ratio
(SNR) is below 2/1 10 . As shown in Figure 8, if the SNR
before the non-coherent integration is below 2/1 10 , the
squaring loss exists. But if the SNR before the non-
coherent integration is above 2/1 10 , the squaring loss
does not exist.
11.2 1.4 1.6 1.822.2 2.4 2.6 2.8 3
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
αc (SNR before non-coherent integration) [ratio]
Lsq Squaring loss
Fig. 8 The Squaring Loss vs. the SNR before Non-coherent Integration
4.2 The Proposed Assisted GPS Acquisition Method
The signal acquisition method proposed in this paper is
shown in Figure 9. It integrates the inner products of
adjacent two pair of in-phase components and quadrature-
phase components sampled at different time instants as
given by Eq. (7) (Lee et al., 2003).
kkkkk QQIIY11 −−+= (7)







<−−
≥
=
∑
∑
=
=
p
p
N
k
kk
N
k
kk
YY
YY
y
1
1
0,
0,
where p
N is the number of the modified non-coherent
integration.
When only the noise exists, the modified non-coherent
integration method results in
(
)
(
)
(
)()
kqkqkikikn tntntntnty 11
)( −− += (8)
0))((
=
kn tyE (9)
2
))(var( σ =
kn ty (10)
The squaring loss does not occur because the inner
product of adjacent samples has the zero-mean property
as given by Eq. (9).
5 Performance Evaluation Test
To compare the signal acquisition performance of the
proposed assisted GPS acquisition method using L2CS
with that of the existing assisted GPS method using L1
signals in the same environment, this paper performed the
signal acquisition test using signal generator designed in
section 3.
First of all, this paper evaluates the GPS L1 signal
acquisition performance using the previously existing
acquisition method of Figure 7 and then evaluates the L1
and L2 civil signal acquisition performance using the
proposed acquisition method.
It is assumed that the existing acquisition method of
Figure 7 uses the navigation message bit information, and
performs the 50 times non-coherent integration after
∑
=
p
N
i
i
p
Y
N1
1
Y
η
≥
y
y
η
: detection threshold
yes no
signal presentsignal absent
Proposed Acquisition Scheme
∑
=
p
M
i
i
p
Z
M1
1
delay
ppTM
Y
~
∗
(
)
[
]
YSqr ~ Re Y
Lowpass Filter
Z
(generated signal)
(despreading mixer)
)( k tx 0 T
r
ω
(received signal)
τ
ω
e
)
ˆ
exp()
ˆ
(2)(0krkktjTCtG ω −⋅=
Fig. 9 The Proposed Signal Acquisition Method in Assisted GPS

30 Journal of Global Positioning Systems
coherent integration for 20 msec. The proposed method
does not use the navigation message bit information, and
performs the 200 times modified non-coherent integration
after the coherent integration for 5 msec.
Figure 10 and Figure 11 show the noise distribution of
the previously existing acquisition method and the
proposed acquisition method respectively. As noted
before, the proposed method has a nearly zero mean, but
the previously existing method has a non-zero mean. It
can be shown that the proposed method solves the
squaring loss problem as expected.
Fig. 10 Noise Distribution of the Existing Method
Fig. 11 Noise Distribution of the Proposed Method
Figure 12 shows the weak signal acquisition performance
with respect to the input signal levels. Here, the input
signal level is verified by comparing input level of the
real GPS L1 signal with output level of the software
based L1 signal generator through the post processing.
And the software based L2CS generator designed in this
paper adopts this experimental result.
The results of Figure 12 exclude the false alarm.
From Figure 12, it can be seen that the acquisition
success rate of the proposed assisted GPS acquisition
method is better than that of the existing assisted GPS
method using L1 signals in the same environment. And it
is evaluated that the assisted GPS using L2 civil signals is
much more available than the assisted GPS using L1 civil
signals in weak signal environment.
Fig. 12 The Signal Acquisition Success Rate
6. Conclusion
In this paper, the squaring loss of the previously existing
assisted GPS is derived and the acquisition method for
assisted GPS is proposed for solving the squaring loss
problem in weak signal environment. It is shown that the
proposed method solves the squaring loss problem by
making the mean of the noise distribution to the
integration output zero. Finally, the performance of the
proposed acquisition method is verified by signal
acquisition test using software based GPS signal
generator designed in this paper. It is concluded that the
proposed method for weak signal acquisition in assisted
GPS shows much enhanced performance in not only L1
civil signal but also L2 civil signal.
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