Research on modulation technique of high speed
Loran-C data channel
Qing Liang
Wei Xiong
Xi’an Institute of Posts and Telecommunications
Dept. of Electronics and Information Engineering
Xi’an, China
liangqing@xupt.edu.cn
Yan Li
The Air Force Engineering University
The Telecommunication Engineering Institute
Xi’an, China
xw5893@sina.com
liyankdy@163.com
Abstract—At the age of global positioning system (GPS), Loran-C
still has a lot of value. Loran-C can be useful in the new
navigation infrastructure both as a backup navigation and a
redundant source of GPS integrity information as provided by
WAAS. The time information, such as wide area augmentation
system (WAAS) message, can be added into the Loran-C by using
Loran-C data modulation. In this paper, several modulation
techniques are discussed in detail. To meet WAAS message
bandwidth requirements, on the base of Ninth Pulse
Communication, we present a new hybrid modulation technique
that can significantly increase the data rate of the Loran-C data
channel.
a system that can scale up in data rate thus giving legacy users
a transition period to upgrade to new receivers.
Keywords- Loran-C, WAAS, ninth pulse communication, pulse
position modulation, intrapulse frequency modulation
where A is the normalization constant related to the peak
amplitude, t is time in μs, and  d is the envelope-to-cycle
difference (ECD) in μs. Fig. 1 shows a single waveform of
Loran-C.
INTRODUCTION
Loran-C is a very widely used hyperbolic radio navigation
system which can provide an accurate position and time
service. Unlike the Global Positioning System, it is a terrestrial,
low frequency, high power system. These features make Loran
a good complement to providing backup to GPS. In our
country, we should not only continue to develop GPS, but also
to develop Loran-C [1].
Recent years we have made lots of efforts to modernize
Loran-C and increase the capabilities of Loran-C. Though not
originally designed with data broadcast capabilities, Loran-C
can broadcast satellite based augmentation system (SBAS),
such as the U.S. system, WAAS messages which provide
integrity and correction to GPS for aviation. The addition of the
message on Loran-C would allow the WAAS message to reach
users at high latitudes and in areas occluded from the
geostationary satellites. It is a key task for Loran-C to
broadcast 250 bps.
There will be three data modulation design ideas [2] for
the next generation. The first is the high speed Loran data
channel (HS-LDC). This design was created to support
broadcast of the WAAS message. The second LDC design is
ninth pulse communications (NPC). This design minimizes
interference on the navigation pulses by placing all the
modulation on one additional pulse. The result is a data rate of
roughly 50 bps or more. The third idea is termed scalable
Loran modulation (SLM). The goal of the design is to generate
SIGNAL MODEL
The Loran-C signal is formally defined by the USCG [4].
Loran-C transmitter transmits sequences of pulses of 100 kHz
RF energy. The envelope of a single pulse is defined as below:
0


E (t )   A(t   d ) 2 exp[2(t   d ) / 65]

undefined

for
t d
for  d  t  65   d (1)
for
t  65   d
60
40
normalised amplitude
I.
II.
20
0
-20
-40
-60
0
50
100
150
time（ us）
200
250
Figure 1. The waveform of Loran-C signal
The envelope E(t) modulates a 100 KHz carrier:
x0 (t )  E(t )sin(2 f0  Pc )
(2)
where Pc is 0 for a positive phase-code, and π for a negative
phase code, f 0 = 100 in KHz.
Loran-C transmissions from each station travel in a series
or group of pulses. For the master station, a series of nine
pulses are transmitted (eight spaced 1000μs apart, followed by
a ninth 2000μs later). The secondary stations transmit a series
of only eight pulses, each spaced 1000μs apart. The master
and each secondary in the chain transmit in a specified and
precisely timed sequence. First, the master station transmits.
Then, after an interval sufficient to allow the master signal to
propagate throughout the coverage area, the first secondary in
sequence transmits, and so forth. Each group’s pulses (nine for
the master and eight for secondary stations) repeat every group
repetition interval (GRI). The GRI is the length of time in
microseconds between the start of one transmission of master
in a Loran-C chain to the start of the next.
III.
LORAN-C DATA MODULATION
Loran-C can’t provide enough inside time information to
flag the reference pulse, and it has to be assisted another time
system to complete the procedure of timing. The time
information can be added into the Loran-C by using Loran-C
data modulation. The main modulations are discussed here.
A. Pulse Position Modulation (PPM）
Pulse position modulation is a system where the arrival
time of the Loran-C pulse is shifted to transmit data. The
Eurofix is an example of this technique. Only the last six pulses
are used and the code is balanced for every Loran-C
transmission. Fig. 2 shows PPM waveform using three-level
modulation (1μs advance, prompt, or 1μs delay).
Intrapulse frequency modulation is a system where the
difference phase and signal pattern by altering the frequency
to transmit date, which changing occur after the standard
tracking point. A gradual change in frequency will be result in
phase changing of up to 90 degree in 100μs. Fig. 3 shows a
single pulse configuration of standard Loran-C and intrapulse
frequency modulation.
IFM signal specification requires that 99% of the radiated
power in contained within 90-110 KHz. Usually, six pulses per
GRI are modulated with each pulse carrying four bits. With a
GRI of 5000(0.5s), this data rate is 20×6×4=480 bits. The
preference of IFM is better than PPM.
C. Ninth Pulse Communication (NPC)
Ninth pulse communication is a system where one pulse is
added at the following of the standard group pulse of eight
pulses for secondary station, nine for master station. This added
pulse is solely for data broadcast. Fig. 4 shows ten pulses
broadcasting for the master station.
Figure 4. Ninth pulse communication for the master station
NPC minimizes the number of pulses that need to be
modulated. It is important for integrity of Loran-C that the
eight navigation pulses are not modulated. Furthermore, since
the added pulse is not used for the navigation, it can be
changed in a more significant manner.
Figure 2. The waveform of PPM signal
B. Intrapulse Frequency Modulation (IFM)
IV.
A NEW MODULATION
The design of Loran-C data modulation must work with
the Loran-C transmitter, retain navigation, minimize
interference, and maintain spectrum and some legacy receiver
compatibility. On the basis of Ninth Pulse Communication, in
this paper, we present a new hybrid data modulation which can
achieve the data rates necessary for WAAS message
transmission.
The NPC only use one pulse after group of Loran-C
pulses for broadcast. We can use a combination of eight phase
shift and four envelope shift states to broadcast date message.
This idea can be express mathematically. n present the phase
shift, m present envelope shift.
State  50.625  m  1.25  n(  s)
Figure 3. The waveform of IFM signal
(3)
where, n=[0,1,…7], m=[0,1,2,3]. A separation of 50.625μs may
result in the desired Euclidean distance.
To achieve the maximum data rates, we can combine PPM
and IFM techniques, which can be called 9thIFM. The
additional pulse which has shifted the arrival alters the carry
frequency after the start of pulse 60μs. Here, we choice a four
level system which may have phase shift of -135, -45, +45,
+135 degree. Fig. 5 shows the waveform of 9thIFM.
It is assumed that the station broadcasts a minimum of 20
GRI per-second, the code rate of 9thIFM can be up to 2560bps.
V.
CONCLUSIONS
Loran-C data modulation is an important consideration in
the modulation of the system. We have detail discussion about
Loran-C data modulation, such as PPM, IFM, and NPC. To
achieve the maximum data rates, in this paper, we present a
new Loran-C data modulation-9thIFM. The 9thIFM technique
is not only meet the data rate requirements for WAAS type
message, but also have minimum degradation effect on the
navigation capabilities in PPM, IFM. In the future, we need do
the experiment to validate them.
REFERENCES
[1]
Figure 5. The waveform of 9thIFM
[2]
The additional pulse which has been modulated by
9thIFM can produce 32×4=128 states (see Table 1).
[3]
TABLE I.
State
Codes
Delay
(μs)
0
1
2
3
0
0
0
0
‥‥‥‥
8.75
8.75
8.75
8.75
50.625
50.625
50.625
50.625
‥‥‥‥
59．375
59．375
59．375
59．375
28
29
30
31
32
33
34
35
60
61
62
63
128 STATES OF 9THIFM
Phase
Bias
(degree)
-135
-45
45
135
State
Codes
64
65
66
67
-135
-45
45
135
-135
-45
45
135
92
93
94
95
96
97
98
99
-135
-45
45
135°
124
125
126
127
Delay
(μs)
101.25
101.25
101.25
101.25
‥‥‥‥
110
110
110
110
151.875
151.875
151.875
151.875
‥‥‥‥
160.625
160.625
160.625
160.625
[4]
Phase
Bias
(degree)
-135
-45
45
135
[5]
[6]
-135
-45
45
135
-135
-45
45
135
-135
-45
45
135°
[7]
[8]
[9]
John A. “Final report: vulnerability assessment of the transportation
infrastructure relying on the global positioning system”, U.S.
Department of Transportation.August29, 2001.
Sherman C. LO, Ben Jamin B. Peterson. “Loran data modulation:
extensions and examples”, IEEE Transactions on Aerospace and
Electronic Systems, vol. 43, no. 2, pp.628-644, 2007.
US Coast Guard. Loran-C user handbook. US Coast Guard,Washington,
DC COMDTPUB P16562.6, Nov. 1992.
Xiong Wei, Hu Yong-hui, Liang Qing. “Loran-C synchronous
interference suppression using improved adaptive algorithms”,
Proceedings of The 4th IEEE International Conference on Wireless
Communications, Networking and Mobile Computing, ChengDu, China
Sep. 2010.
James M. Boyer. “High speed Loran-C channel communications”.
Proceedings of 29th Annual Convention and Technical Symposium of
the International Loran Association, Washing，DC，2000
Sherman C.Lo ， Per Enge. “Data transmission using Loran-C”.
Proceedings of Department of Aeronautics and Astronautics, Stanford
University, pp.1324-1330,2002.
Dr.Benjamin Peterson，Perterson Integrated Geopositioning，LLC，
LT Anthony Hawes ， LTjg Kevin Shmihluk. “Loran data channel
communications using 9th pulse modulation Version 1.1”, USCG Loran
Support Unit, 2006.
Su JIAN-feng LIU Xue. “Study of Loran-C 9th pulse modulation
application”, In Time Transmit. Symposium on Frequency Control
technology, pp.37-39, 2008.
Liu Shu-jun. MATLAB 7.0 example and application of control system. China
Machine Press.2006（in Chinese）G. Eason, B. Noble, and I. N. Sneddon,
“On certain integrals of Lipschitz-Hankel type involving products of
Bessel functions,” Phil. Trans. Roy. Soc. London, vol. A247, pp. 529–
551, April 1955.