METHOD FOR IMPROVING THE TRACKING OF A DATA
TRANSMISSION SIGNAL OF A SATELLITE NAVIGATION SYSTEM
5 The field of the invention is that of satellite navigation systems
commonly called GNSS, with reference to the expression "Global
Navigation Satellite System."
In these systems, a satellite positioning receiver aboard a mobile
10 object provides position information in respect of the mobile object,
obtained by triangulation on the basis of the navigation signals emitted by
the satellites visible from the mobile object.
The receiver also receives data transmission signals which transmit
15 complementary data to the satellite positioning receiver, so as to improve
the services rendered to the users, for example, by improving the precision
or the integrity of the positioning of the mobile object.
Among the data transmission signals may be cited the augmentation
20 signals, arising from a GNSS augmentation system of the GBAS ("Ground-
Based Augmentation System") or SBAS ("Space-Based Augmentation
System") type, based on one or more ground stations, able to identify the
errors in the information provided by satellites and to dispatch, to the
satellite positioning receiver, augmentation signals transporting data
25 representative of individual corrections on the signals emitted by the
satellites. Satellite navigation systems comprising augmentation systems
make it possible to provide position information of better precision and
which is less impaired than that arising from satellite navigation systems
devoid of augmentation systems.
30
Current data transmission signals are dispatched with a
predetermined bitrate and a predetermined coding of the Viterbi type, which
limit their performance. Typically, for a signal transmitted over 1 second,
500 symbols are dispatched, coded over 2ms. The data are dispatched on
35 a data pathway (symbol-modulated carrier of the data to be transmitted by
the signal) but without any pilot pathway (non-message carrier pathway,
not modulated by symbols), thereby limiting the capabilities for tracking
these signals at the level of the receiver. Indeed, tracking is carried out
more easily on a pilot pathway than on a data pathway since the energy of
a pilot pathway can be accumulated in a coherent manner whereas on a
data pathway this accumulation must be done in an incoherent manner
thereby introducing an additional noise level.
5
One seeks to improve the quality of the services rendered by current
satellite navigation systems while retaining compatibility with current
services and developments, that is to say while using current receivers and
signals.
10
Currently, to improve the services rendered to users, an
improvement signal is dispatched to the receiver. However, the
improvement signal is added to the existing data transmission signal within
the limit of powers permitted by the regulations. The permitted power must
15 therefore be shared between the various signals.
Currently, the improvement signal comprises:
- either a data pathway, thereby making it possible to dispatch
20 additional data to the receiver so as to provide new services to the users,
but not making it possible to improve the tracking of the current data
transmission signal,
- or a pilot pathway and a data pathway, thereby reducing the
25 tracking and decoding performance for the new data pathway since the
power permitted by regulation is shared between the pilot pathway and the
data pathway.
The aim of the present invention is to propose a method for
30 improving the tracking of the data transmission signal, which is for example
an augmentation signal, of a satellite navigation system, which alleviates
the aforementioned drawbacks.
For this purpose, the subject of the invention is a method for
35 improving the tracking of a data transmission signal emitted to a receiver of
a satellite navigation system in addition to navigation signals which are
transmitted to it by the satellites, said method comprising:
- a step of transmission to the receiver, during a period called the
improvement period and by means of an improvement signal emitted in
coherence with said data transmission signal, of data or of other data
making it possible to predict said data, said data furthermore being
5 transmitted to the receiver by means of the data transmission signal during
a transmission period starting after the end of the improvement period,
- a step of reception, by the receiver, of said data or of said other
data which are transmitted by means of the improvement signal,
10
- a step of generating symbols serving to modulate the data
transmission signal during the transmission period, by means of said data
received or of said other data received,
15 - a step of tracking the data transmission signal while removing the
effect of the modulation of said data transmission signal by the symbols
generated.
Advantageously, the data transmission signal is an augmentation
20 signal.
According to a first characteristic, the data transmission signal is
transmitted with a first data coding of the Viterbi type.
25 Advantageously, the improvement signal is transmitted with a
second coding such that the threshold of demodulation of the improvement
signal by the receiver is lower than the threshold of demodulation of the
data transpission signal by the receiver.
30 Advantageously, the improvement signal is transmitted with a
second coding of the LDPC or Turbocode type.
Advantageously, the improvement signal is transmitted with
interleaving.
Advantageously, the duration of the improvement period is shorter
than the duration of the transmission period.
Advantageously, additional data are furthermore transmitted to the
receiver by means of the improvement signal.
The proposed solution makes it possible to improve the tracking of
5 data transmission signals such as, for example, augmentation signals,
used in the satellite navigation systems in addition to the navigation signals
emitted by the satellites, while preserving compatibility with existing
systems since the existing data transmission signals are utilized. The
proposed solution is usable with existing augmentation systems, for
10 example, the SBAS systems, which are deployed in the WAAS, MTSAT
and EGNOS systems.
The proposed solution does not make it necessary to add a pilot
pathway since the data transmission signal which is at the origin a data
15 pathway is no longer used as a data pathway but as a pilot pathway.
This makes it possible to allocate the whole of the permitted power
on the second data pathway on which the improvement signal is
transmitted and to obtain good performance in terms of quality of the
20 reception of the data on the improvement signal (high power). The
performance in terms of reception can be further increased by virtue of the
use of performance codings (for example LDPC) and of interleaving
schemes.
25 Moreover, given that the tracking is carried out on a pilot pathway,
rather than on a data pathway, the signal's tracking threshold, that is to say
the minimum power of the signal received by the receiver, is lowered so
that tracking is possible.
Furthermore, with the proposed solution, it is not necessary to
transmit at one and the same time a new pilot pathway and a new data
pathway in order to improve tracking and provide new services (by
transmitting additional data to the receiver). A single data pathway is
necessary. The energy necessary for the emission of the signals for
improvement and transmission of, for example, augmentation data is the
energy necessary for the emission of 2 transmission pathways and not for
3 transmission pathways.
The method according to the invention is furthermore an
autonomous method which is implemented solely on the basis of data, for
example of augmentation data, which are, themselves, provided by the
constructor of the data transmission system, for example, of the
5 augmentation system. The data are therefore rapidly at the disposal of the
emitter for dispatching the improvement signal to the receiver. This makes
it possible to ensure the dispatching of the data on the improvement
pathway with the speed suited to the rapid variation of the improvement
data, as compared with the navigation data dispatched by the satellites.
10 Typically, the augmentation data vary about every second, whereas the
GPS data vary every hour.
Other characteristics and advantages of the invention will become
apparent on reading the detailed description which follows, given by way of
15 nonlimiting example and with reference to the appended drawings in which:
- Figure 1 schematically represents the steps of the method
according to the invention,
20 - Figure 2 schematically represents an exemplary way of
transmitting data transmitted on a first and a second data pathway in the
method according to the invention.
From one figure to the other, the same elements are labeled by the
25 same references.
Figure 1 schematically represents the steps of the method according
to the invention corresponding to a method for improving a data
transmission signal transmitted, in addition to the navigation signals
30 dispatched by the visible satellites, to the receiver.
A data transmission signal can be in the form of an augmentation
signal which transmits, for example, augmentation data, which are
individual corrections on the navigation signals emitted to the receiver by
35 the visible GNSS satellites.
The method according to the invention will be described in respect of
an augmentation signal but it applies of course to any other data
transmission signal which could be transmitted to the receiver of the
satellite navigation system in addition to the navigation signals emitted by
the satellites.
The method according to the invention comprises a step 10 of
transmission, during successive improvement periods TAAi and by means
of an improvement signal emitted in coherence with the augmentation
signal, of data Nil which are furthermore transmitted by means of the
augmentation signal during successive transmission periods TAi starting
respectively after the end of the respective successive improvement
periods TAAi (that is to say of like index i).
Advantageously, "improvement signal emitted in coherence with the
augmentation signal" is understood to mean that the signals are coherent
at the level of the spreading codes, that is to say that the spreading codes
are transmitted in a synchronous manner, using for example a QPSK
modulation.
The augmentation signal is advantageously emitted by an
augmentation satellite. The improvement signal is likewise emitted by the
augmentation satellite. Stated otherwise, the improvement signal is emitted
by the same emitter as the augmentation signal. This characteristic makes
it possible to transform the first data pathway into a pilot pathway as we
shall see subsequently.
Moreover, this characteristic makes it possible to transmit, as we
shall see subsequently, the successive improvement data to the receiver
during improvement periods finishing before the respective successive data
transmission periods.
As a variant, instead of the data Ni it is possible to transmit to the
receiver, by means of the improvement signal, during the successive
improvement periods, other data Nai making it possible to predict the data
Ni.
Given that the example described deals with the case where the
data transmitted are augmentation data, the transmission periods TAi are
called augmentation periods hereinafter in the text.
The improvement signal being, in the example described, an
augmentation signal, the data Ni are augmentation data.
5 The method according to the invention can also comprise a step of
transmitting the data or other data during a single period.
The augmentation signal is dispatched on a data pathway, or
channel, VD1 and the improvement signal is dispatched on a second data
10 pathway, or channel, VD2 (or improvement pathway).
- The transmission step is represented in Figure 2 representing, over
time, the data which are transmitted on the first data pathway VD1 and on
the second data pathway VD2.
First N1, second N2 and third N3 augmentation data are transmitted
on the first data pathway VD1 during a first TAI, a second TA2 and
respectively a third TA3 consecutive augmentation period.
20 The first N1, second N2 and third N3 improvement data are
transmitted on the second data pathway VD2 during a first TAAI, a second
TAA2 and respectively a third TAA3 successive improvement period.
The first improvement period TAAI starts and finishes before the
25 start of the first augmentation period TAI. The second improvement period
TA2 starts and finishes before the start of the second augmentation period
TA2. The third improvement period TAA3 starts and finishes before the
start of the third augmentation period TA3.
..
30 Conventionally, as seen previously, the augmentation signal, or
more generally the data transmission signal such as is intended in the
method according to the invention, is transmitted without data coding or
with a coding using the former data transmission techniques such as Viterbi
coding without interleaving in the case of an augmentation signal. The
35 improvement signal is dispatched using the new data transmission
techniques such as interleaving, turbocode, etc.
The two data pathways aie modulated by symbols.
The bitrate in terms of symbols of the improvement signal is
advantageously greater than the bitrate in terms of symbols of the data
transmission signal, here the augmentation signal.
5
This makes it possible optionally to dispatch, to the receiver,
additional data Nsi by means of the improvement signal which are different
from the first data dispatched by means of the augmentation signal. This
characteristic makes it possible to ensure additional services by means of
10 the additional data. The quality of the existing services is thus improved
while offering new services which are not necessarily dedicated to the
aeronautical community.
In the representation of Figure 2, the bitrate of the symbols of the
15 improvement signal is such that the durations of the periods of transmission
of the improvement data by means of the improvement signal are shorter
than the durations of the periods of transmission of the augmentation data
by means of the augmentation signal.
20 This is represented in Figure 2. First Nsl, second Ns2 and third Ns3
additional data travel on the second data pathway, between the
consecutive transmissions of augmentation data, during a first Tsl, a
second Ts2 and a third Ts3 respective additional period. Stated otherwise,
the improvement signal transmits the respective data Ni, or the respective
25 other data Nai making it possible to predict the.respective data, as well as
respective additional data Nsi during the respective augmentation durations
TAi .
In the representation of Figure 2, the bitrate of the symbols of the
30 improvement signal is such that the duration of the period of transmission
of the improvement data Ni by means of the improvement signal and of the
additional data Nsi is equal to the duration of the period of transmission of
the augmentation data Ni by means of the augmentation signal.
35 The augmentation data Ni or the other data Nai allowing the receiver
to predict the augmentation data Ni which are transmitted by means of the
improvement signal are thereafter received 20 by the receiver.
The augmentation data Ni of index i which are dispatched by means
of the improvement signal are completely received before the dispatching
of the augmentation data of like index which are transmitted to the receiver
by means of the augmentation signal. Stated otherwise, the receiver
5 receives an item of data transmitted by means of the improvement signal
before the dispatching of an item of data of like index, transmitted by
means of the augmentation signal.
The advance with which the receiver receives the data Ni by means
10 of the improvement signal is exploited so as to erase the modulations on
the channel existing, that is to say on the first data pathway.
More precisely, the receiver generates the successive symbols 30
serving to modulate the augmentation signal during the respective
15 successive augmentation periods TAi, by means of the data Ni or of the
other data Nai respectively received. This is the data received by the
receiver and which have been transmitted by means of the improvement
signal. The receiver performs processings identical to those carried out by
the system to transform the augmentation data into symbols.
Once the symbols have been obtained, the receiver can track 40 the
data transmission signal, which here is an augmentation signal, with good
precision since by removing the effect of the modulation of the
augmentation signal by the symbols that it has generated, it is possible to
25 gain about 6dB for the tracking of the phase of the signal, for example.
This tracking step 40 carried out by demodulating the augmentation
signal by means of the generated symbols amounts to transforming the first
data pathway VD1 into a pilot pathway on which no data is transmitted. The
30 demodulation of the augmentation signal by means of the symbols
generated is called "symbol wipe off' with reference to the terminology
"data wipe OW' when entailing demodulation by means of data.
It has been seen that, on the second data pathway VD2, either the
35 data Ni which are transmitted on the first data pathway VDI, or other data
Nai which make it possible to predict these data Ni, are transmitted with a
slight advance. This second possibility affords a gain in terms of bitrate of
the improvement signal since there is no necessity to transmit the data Ni
in their entirety. When this second solution is used, an additional step of
predicting the data Ni is carried out at the level of the receiver, prior to the
step of generating the symbols 30.
Advantageously, in the case where the data transmitted by the
improvement signal are coded by means of a second data coding, the
second data coding is more efficacious than the first coding in such a way
that the threshold of demodulation of the improvement signal, by the
receiver, is lower than the threshold of demodulation of the augmentation
signal.
The second coding is, for example, a coding of the Turbocode or
LDPC type, with reference to the expression "Low Parity Density Code".
Advantageously, the improvement signal is transmitted with interleaving.
On the second data pathway VD2, the modulation by the symbols is,
for example, a CSK modulation with reference to the expression "Code
Shift Keying".
By using, on the second pathway, an LDPC coding and a CSK
modulation using symbols of 4 ms and, on the first pathway, a Viterbi
coding and symbols of 4 ms, an improvement in performance in terms of
data reception of the order of 5.2 dB is obtained.

CLAIMS
1. A method for improving the tracking of a data transmission signal
5 emitted to a receiver of a satellite navigation system in addition to
navigation signals which are transmitted to it by the satellites, characterized
in that it comprises:
- a step (10) of transmission to the receiver, during a period called
10 the improvement period TAAi, and by means of an improvement signal
emitted in coherence with said data transmission signal, of data Ni or of
other data Nai making it possible to predict said data Nil said data Ni
furthermore being transmitted to the receiver by means of the data
transmission signal during a transmission period TAi starting after the end
15 of the improvement period TAAi,
- a step (20) of reception, by the receiver, of said data Ni, or of said
other data Nai, which are transmitted by means of the improvement signal,
20 - a step (30) of generating symbols serving to'modulate the data
transmission signal during said transmission period TAi, by means of said
data Ni received or of the other data Nai received by the receiver and which
have been transmitted by means of the improvement signal,
25 - a step (40) of tracking the data transmission signal while removing
the effect of the modulation of said data transmission signal by the symbols
generated.
2. The method as claimed in the preceding claim, in which the data
30 transmission signal is an augmentation signal.
3. The method as claimed in any one of the preceding claims, in
which the data transmission signal is transmitted with a first data coding of
the Viterbi type.
4. The method as claimed in any one of the preceding claims, in
which the improvement signal is transmitted with a second coding such that
the threshold of demodulation of the improvement signal by the receiver is
lower than the threshold of demodylation of the data transmission signal by
the receiver.
5. The method as claimed in any one of the preceding claims, in
5 which the improvement signal is transmitted with a second coding of the
LDPC or Turbocode type.
6. The method as claimed in any one of the preceding claims, in
which the improvement signal is transmitted with interleaving.
10
7. The method as claimed in any one of the preceding claims, in
which the duration of the improvement period is shorter than the duration of
the data transmission period.
15 8. he method as claimed in the preceding claim, in which additional
data are furthermore transmitted to the receiver by means of the
improvement signal.
9. The method as claimed in any one of the preceding claims, in
20 which the improvement signal and the data transmission signal are emitted
- in coherence by the same emitter.
Dated this 18.06.2013
[NEHA SRIVASTAVA]
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]