International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 1- June 2015
Code Generation for Different Navigation Systems
1
Sisira C S, 2R.Sindhu Rajendran
12
Department of Electronics and Communication Engineering,
Jawaharlal College of Engineering and Technology, Palakkad ,Kerala,India, Affiliated to University of Calicut.
2
Bangalore Institute of technology, Bangalore,Karnataka,India Affiliated to University of VTU
1
ABSTRACT
Today the use of GPS for both navigation and altitude
determination functions on LEO spacecraft has become
somewhat routine; however, it took years of research and
development efforts by a variety of organizations to get to this
point. In this paper we propose an analysis of the code
generation of two navigation systems, namely GPS and IRNSS
using the L1C/A, L2C and L5 Civil signals.
Keywords: GPS, IRNSS, C/A, CM and CL
I.
INTRODUCTION
US department of defense in the year 1973 started
Global Positioning System called GPS. In 1980‟s GPS
was made commercial for civil aviation purposes.
Finding the position of an object across the earth by
giving its coordinates is done by a satellite based
navigation system, GPS.
With a total of 32 satellites in GPS constellation having
24 working and 8 spare to replace any of these 24
satellites in case of malfunction or damage. In six
predefined orbits these 24 satellites revolves with 4
satellites in each orbit.
Indian Regional Navigation Satellite System (IRNSS) is
an indigenously developed independent satellite
navigation system. It is completely planned established
and controlled by Indian Space Research Organization
(ISRO).Restricted services for authorized users and
standard positing services for civilian users are the two
main services provided by IRNSS.L5 (1176.45MHz )
and S (2492.028) bands are used for the transmission of
IRNSS SPS services. The frequency in L5 band has
been selected in the allocated spectrum.
ORGANISATION OF THE PAPER
Session-I gives a brief introduction to the navigation
system, namely GPS and IRNSS, Session-II gives an
ISSN: 2231-5381
explanation of the GPS signal structure, The
Applications are explained in Session-III and the results
are explained in Session-IV. Session V gives the
conclusion.
II.
GPS SIGNAL STRUCTURE
L1 FREQUENCY
In order to help GPS receivers determine location and
synchronized time Global Positing System satellites
broadcast radio signals. Navigation messages and
ranging signals are included in GPS signals. Ranging
signals are used to measure the distance to t he satellite.
Ephemeris and almanac data are included in navigation
messages. Ephemeris is used to calculate position of
satellite in orbit. Information about the time and status of
satellite constellation is provided by almanac. Two
ranging codes are included in original GPS design:
Coarse/Acquisition code or C/A code, and Precision
code or P- code. C/A code is freely available to public.
Precision code is reserved for military applications.
A. Coarse/acquisition code
The SPS PRN ranging code is known as the
Coarse/Acquisition (C/A) code. Even though all
satellite transmits on same L –band frequency
appropriate code division multiplexing technique is
used to differentiate between the satellite. Transmission
at 1.023 megabits per second (Mbit/s) and repeating
every millisecond C/A code is a 1,023 bit long pseudo
noise (also pseudorandom binary sequence) (PN or
PRN code).
Only when they are exactly aligned these sequences
will match up or correlate. A unique PRN code is
transmitted by each satellite .This PRN code will not
correlate with any other satellites PRN code. Each PRN
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International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 1- June 2015
codes are highly orthogonal to one another which is a
III.
METHODOLOGY
form of Code Division Multiple Access. Thus multiple
satellites can be recognized by the receiver on same GENERATION OF L1C/A CODE
frequency.
B. Precision code
A 6.1871 × 1012 bits long(6,187,100,000,000 bits),
repeating once in a week and transmitting at 10.23
Mbit/s PRN code is each satellites P-code. Eliminating
any range ambiguity within solar system and increased
correlation gain are some characteristics of extreme
length P- code. Because of the length and complexity of
this code receiver cannot directly acquire and
synchronize the signals.
Composition of C/A code is studied to know about the
acquisition and tracking of C/A coded GPS signal. The
GPS C/A code is one kind of Pseudorandom noise
(PRN) codes also known as the Gold codes. A maximum
length pseudo random code is generated using two
maximum –length linear feedback shift register(LFSR)
having 10 stages ,G1 and G2,driven by a clock of
1.023MHz. 2n -1 length sequence is generated using a
shift register having n bits. A sequence of length 1023
bits are generated by both the shift registers G1 and G2
having 10 bits. Since all –zero sate is illegal both G1 and
G2 are initialized with ones. The feedback taps of the G1
and G2 LFSRs are defined by the generator polynomials.
L2C FREQUENCY
Modulation on L2 carrier and frequency of 1227.60
MHz a new signal was introduced. Augmentation of the
new signal is done by adding L2C code. Moderate
length codes (CM) and long length code (CL) are the
two multiplexed code signals in L2C signal. L2 CM
code is composed of 10230 chips with 20 milliseconds
long. L2 CL code has 767250 chips with a period of 1.5
seconds. Modulo2 addition of CM code with data is
done and the chips are time multiplexed with CL code
on chip by chip basis. A clock frequency of 511.5 KHz
is used for individual CM and CL code. Composite
clock frequency of 1.023 MHz is used for L2C code.
Both CM and CL code have aligned code boundaries.
CL period contain 75 CM periods. This time
multiplexed L2C sequence modulates the L2 (1227.6
MHz) and data is transmitted at 50 symbols per second
rate using an convolution encoder. CM period of 20
milliseconds is matched for each data symbol. Rather
than two signals in quadrature L2C signal is limited to
single bi-phase signal component. The reason behind
this is that it must share L2 frequency with new military
M-code and legacy P(Y) code. A data rate of 25bps can
be commanded to be coded in a rate ½ convolution
coder with the resulting 50 sps stream to be Modulo-2
added to the L2CM code.
G1(X) = 1+X3 +X10
G2(X) = 1+X2+X3+X6+X8+X9+X10
The output of the G2 LFSR for each C/A code is
delayed by the modulo-2 addition of two code phase
selection bits specific for each satellite. The C/A code is
generated by the modulo-2 addition of the output of the
G1 LFSR and the delayed output of the G2 LFSR.
Figure is a graphical presentation of how the C/A code
for a particular satellite is generated. This will work
since adding a phase-shifted version of a PRN sequence
to itself will shift the phase of the code, while not
changed the code. The positions of the code selection
bits determine the satellite identification. There are 32
sets of unique C/A codes for the 32 satellite numbers.
Another five are reserved for various applications such
as ground transmission.
Fig.1:C/A code generator
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International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 1- June 2015
The code phase assignment of the GPS satellites are Polynomial G1 and G2 are similar to the ones used by
given in the Table. The satellite vehicle identification GPS C/A signal. The G1 and G2 generators are realized
number is in column one. The second column contains by using 10 bits Maximum Length Feedback Shift
the PRN number of the satellites from 1 to 37. The code Registers (MLFSR). The initial state of G2 provides the
phase selections of the G2 generator of the satellites are chip delay. G1 and G2 are XOR„Ed for the generation of
listed in column three. The code delays measured in the final 1023 chip long PRN sequence. The time period
chips are presented in column four. The last column of the PRN sequence is 1 millisecond.
shows the first 10 bits of the C/A code generated for
each satellite in octal format.
GENERATION OF L2 CM AND CL CODE
CM
CM code which is 20 milliseconds in length and a
chipping rate of 511.5 Kbps. CM sequence is a linear
pattern which is short cycled every period of 10,230
chips by resetting with a specified initial state.
CL
The long length code (CL) signal which has a 767,250
chip sequence repeating every 1.5 seconds at a chipping
rate of 511.5 Kbps. Thus, the CL code is 75 times
longer than CM code. Both CM and CL codes are
generated from a single multiple return shift register
generator (MRSRG).
The L2C code generator polynomial is
1 + x3 + x4 + x5 + x6 + x9 + x11 + x13 + x16 + x19 + x21 +
x24 + x27
.
Fig .3: IRNSS L5 civil code generation
IV. RESULT
The code generation for two navigation system GPS
and IRNSS is carried out in VHDL and is dumped into
Vertex 5 FPGA using PPC440 which is used to
generate both GPS and IRNSS signals. The
Course/Acquisition code generation is different for
L1,L2 and L5 and the resultant Pseudorandom sequence
is obtained in VHDL.
V.
CONCLUSION
In this paper, we presented C/A code generation
algorithm of acquisition sub-systems using Xilinx ISE
13.2.Nineteenth satellite is taken into consideration in
developing C/A code generation to evaluate the
performance of our algorithm. In this paper, GPS
satellite C/A code generated a detailed analysis of the
principle, Carried out using VHDL simulation, a rough
simulation of the signal, VHDL simulation near real
satellite signal C/A code generation.
Fig .2: L2 CM/CL code generation
GENERATION OF L5 C/A CODE
For SPS code generation, the two polynomials G1 and
G2 are as defined below:
G1: X10 + X3 + 1 and G2: X10 +X9 + X8 + X6 +X3 + X2 +
1
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International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 1- June 2015
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