Positioning system using pseudoJites operating in assisted mode.
The present invention concerns a system enabling an object
equipped with an appropriate receiver to determine its position within a
5 constrained area.
The positioning system of the present invention is more
particularly based on the use of pseudolites.
As is known in the art, pseudolites (pseudo-satellites) are devices
1 o operating in accordance with the same principles as satellites belonging to
constellations of satellites used in satellite navigation systems, known as
Global Navigation Satellite Systems (GNSS), such as the Global Positioning
System (GPS) or the Galilee system. Unlike satellites, pseudolites are
deployed on the ground. They may typically be distributed in a building and
15 are generally distributed in so-called constrained areas.
The general principle of pseudolite positioning systems is based
on the fact that said pseudolites transmit positioning signals the format of
which is identical or similar to that of messages transmitted by satellites of a
satellite navigation system. For good compatibility with existing systems,
20 notably the receivers, each pseudolite is generally allocated an identifier
corresponding to that of a satellite. In the context of constellations of
satellites, these identifiers are called spreading codes, as the person skilled
in the art knows. The range of the signals transmitted by pseudolites is
variable; it depends on their power and on their use. Objects equipped with
25 appropriate receivers can acquire these positioning signals. As in a classic
satellite navigation system, calculation of pseudodistances. between said
object and the pseudolites from which it has acquired t~e positioning signals,
followed by a triangulation calculation, enable the position of the object to be
determined. The principle of positioning by triangulation is known in itself: it is
30 a question of determining the position of a receiver as being at the
intersection of spheres with centers at the transmitters and the distance
between receiver and transmitters as radius. The calculations may be
effected onboard the object itself or remotely by a server.
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Pseudolite positioning systems are generally deployed in so-called
"constrained" areas. These are typically buildings in which positioning signals
transmitted by satellites in Earth orbit cannot be acquired because of the
masking effect of walls, ceilings, etc. They may be areas not covered by the
5 satellite navigation system concerned. Generally speaking, a constrained
area will be defined as an area in which positioning signals transmitted by
satellites cannot be acquired correctly. In contrast, the expression "open
area" may be used in respect of areas in which positioning signals
transmitted by satellites may be acquired by an appropriate receiver.
10 Moreover satellites from which a receiver can theoretically receive positioning
signals, because of the suitable relative position between said satellites and
said receiver, are referred to as "visible" to the receiver, while the other
satellites of the constellation are referred to as "non-visible". These terms
"visible" and "non-visible" may be used in the case of pseudolites.
15 The definitions given above of the terms "constrained area", "open
area", "visible satellite" and "non-visible satellite" are valid throughout the
remainder of the description and the claims.
It is also known in the art that satellite positioning systems are
20 often complemented by an assistance system. This technology is well known
to the person skilled in the art as Assisted-GNSS. These assistance systems
are generally based on a server called the assistance server the role of which
is to send information to the receiver concerning the constellation of
satellites, such as the position of the visible satellites, and other assistance
25 facilitating processing of the positioning signals. A pseudolite positioning
system may also have an assistance server of this kind. In so-called
"assisted" mode, this assistance server generally calculates the position of
the receiver based on calculations of pseudodistances that the latter supplies
it with. This mode of operation is also well known to the person skilled in the
30 art as MS-Assisted (Mobile Station Assisted) or UE-Assisted (User
Equipment Assisted) Mode.
The invention is used in such an assisted mode of operation.
The prior art discloses various technologies. Firstly, by way of
35 alternatives to pseudolite positioning, there are known WiFi positioning
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techniques, but this solution is acceptable only in static environments, and
necessitates the deployment of dedicated equipment for location in
constrained areas and relatively tedious calibration phases. Still in
connection with alternatives, there are also used positioning techniques
5 founded on the GSM or UMTS standards, for example, enabling a mobile
telephone to be located in the areas covered; however, the accuracy
achieved is of the order of a few tens of meters, which is not satisfactory.
Finally, known pseudolite positioning systems have a number of
drawbacks. In particular, they make no provision for moving from a
10 constrained area to an open area and vice versa in a continuous and
autonomous manner. Moreover, they generally do not provide for a cold start
without a knowledge of the initial position of the receiver. The known systems
generally involve the use of receivers specifically designed to operate in
constrained areas and to acquire positioning signals transmitted by
15 pseudolites.
In other cases they necessitate intervention on the receiver for the
latter to begin to acquire signals transmitted by pseudolites, which are
identified by specific spreading codes, potentially unknown to the receivers
the vocation whereof is to use satellites and that consequently know only the
20 spreading codes of said satellites. In any event, the modes of operation in
constrained areas and in open areas are generally not compatible, in the
sense that they may not be active simultaneously.
Moreover, an important constraint to be taken into consideration
lies in the fact that the spreading codes of satellites belonging to
25 constellations of satellites are reserved for said satellites. It is not possible to
use other codes without having to design dedicated receivers, because the
receivers commercially available are designed to acquire positioning signals
coming from the satellites.
30 An object of the invention is to alleviate these drawbacks by
proposing a pseudolite positioning system adapted to operate with standard
receivers able to acquire transparently, from the point of view of the receiver,
positioning signals transmitted by pseudolites as if they were positioning
signals transmitted by satellites belonging to a satellite constellation of a
35 satellite navigation system.
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The general principle of the invention therefore consists in
deceiving the receiver so that when it acquires positioning signals transmitted
by pseudolites provided in the constrained area in which it is located, said
receiver has the impression of acquiring normally signals transmitted by
5 visible satellites. This is made possible by the system defined by claim 1.
Accordingly, the invention consists in a system for positioning an
object provided with a receiver, which includes a set of pseudolites
transmitting positioning signals and distributed in a constrained area and
assistance means capable of communicating with said receiver and
1 o calculating the position of the object, and a server capable of dynamically
configuring the set of pseudolites, said pseudolites each further having a
spreading code corresponding to that of a satellite belonging to a satellite
constellation in a satellite navigation system, the dynamic configuration of the
pseudolites by the server being such that the spreading codes of said
15 pseudolites correspond to spreading codes of satellites of the satellite
constellation not visible to the receiver of the object, wherein assistance
means communicate to said receiver a list of spreading codes meant to be
those of the satellites visible to said receiver but actually corresponding to the
spreading codes of the set of pseudolites so as to deceive the receiver that
20 can acquire the positioning signals transmitted by the pseudolites and
communicate with the assistance means so that the latter calculates the
position of the object.
In one embodiment of the invention the position of each pseudolite
of the set of pseudolites in the constrained area being known to the
25 assistance means, the position of the object is calculated by triangulation
from the known positions of the pseudolites and pseudodistance
measurements carried out by the receiver.
In another embodiment the receiver may have access to the
ephemerides of the satellite constellation, the assistance means also
30 communicating to said receiver an erroneous current time corresponding to
the current time shifted by an offset so that the receiver consulting the
ephemerides determines that the spreading codes of the pseudolites
correspond to spreading codes of satellites of the satellite constellation
deemed to be visible to said receiver.
35
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The pseudolites advantageously transmit at the current time
positioning signals identical to the positioning signals that the satellites of the
satellite constellation would transmit of which they borrow the spreading code
at the erroneous current time.
5 The assistance means advantageously also communicate to said
receiver a non-integrity flag for the set of spreading codes of satellites of the
satellite constellation not allocated to pseudolites.
The server advantageously implements the assistance means by
means of appropriate programming.
1 o Other features and advantages of the invention will become
15
apparent in the light of the following description given with reference to the
appended drawings in which:
• figure 1 is a diagram of one example of the disposition
around the Earth of satellites of a satellite navigation
system;
• figure 2 is a diagrammatic representation of the operating
principle of a satellite navigation system in assisted mode.
The figure illustrates the definition given above, and well known to
the person skilled in the art, of the terms "visible satellite" and "non-visible
20 satellite". As figure 1 shows, at a given time, from the point A situated on the
surface of the Earth, only the satellites SV are visible, whereas the satellites
SN are not visible. Because of this, only positioning signals transmitted by
the visible satellites SV could be acquired by a receiver placed at the point A.
On the other hand, as mentioned above, the pseudolite positioning
25 system of the invention uses an assistance server controlling all of the
pseudolites equipping the constrained area concerned, in which the object to
be positioned is found. This assistance server is notably programmed to
perform the function of dynamic allocation of a spreading code to each of the
pseudolites of the constrained area.
30 Figure 2 represents the general principle of a so-called "assisted"
mode of operation of a satellite navigation system (Assisted-GNSS, see
above). In the figure 2 example, the assistance serverS collects information
transmitted by the satellites of the satellite constellation SAT belonging to a
satellite navigation system and, optionally, information transmitted by other
35 satellites, for example geostationary satellites G belonging to a data
•
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collection system. The assistance serverS exploits this information to be in a
position to supply a receiver R on demand with assistance data such as:
• location information,
• time references, such as the "GPS time",
• navigation information, such as the ephemerides and the
corrections applicable to the clocks of the satellites SAT,
• up to date corrections of the models relating to the
ionosphere,
• correction information for effecting differential navigation,
• information on the integrity of the positioning signals
received as a function of the transmitter satellite,
• the almanachs of the satellite constellation SAT,
• the list of the identifiers, generally the spreading codes, of
the visible satellites of the satellite constellation SAT as a
function of the area in which the receiver R is located,
• the ephemerides of the satellite constellation SAT extended
for a period of several days,
• etc.
20 The principle of the invention consists in deceiving the' receiver R
of the object to be positioned by appropriate programming of the assistance
serverS.
Firstly, the assistance server S is programmed to assign
dynamically to the pseudolites spreading codes which are those of the
25 satellites not visible from the constrained area concerned. This is possible
because the assistance server S knows the position of all of the satellites of
the satellite constellation SAT concerned and the corresponding
ephemerides. This point is crucial to avoiding any risk of interference, on the
one hand vis-a-vis users outside the constrained area, and on the other hand
30 vis-a-vis real satellites in areas in which positioning signals transmitted by
satellites may be received even in a constrained area: for example, inside a
building, there may be areas located near windows. It might not be possible
to distinguish positioning signals transmitted by pseudolites from those
transmitted by satellites.
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10
15
20
25
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According to the invention, the assistance server S supplies to the
receiver R assistance data modified with a view to deceiving said receiver R.
This modified data supplied by the assistance server comprises:
• A list of the spreading codes of the pseudolites visible from
the constrained area in which the receiver R is located,
corresponding to spreading codes of non-visible satellites
SN.
• Preferably a time reference deemed to correspond to the
current time, called the Time Of Week, but equal to the
current time offset so that, if the receiver R has stored in its
internal memory the almanachs of the constellations of
satellites with the object of determining autonomously the
list of visible satellites, said receiver detects no
inconsistency between the list of visible satellites -
corresponding in reality to the list of visible pseudolites
supplied by the assistance server S - and the list of visible
satellites that it would be able to determine for itself. The
offset applied must consequently be chosen appropriately,
so that the spreading codes of the pseudolites correspond
to spreading codes of satellites deemed to be visible at the
modified current time, equal to the current time shifted by
said offset.
• Optionally, a non-integrity flag for satellites deemed not to
be visible at the modified current time but in practice liable
to be visible at the real current time. A non-integrity flag is
interpreted by a receiver R as signifying that the signals
transmitted by the satellites to which the non-integrity flag
relates are erroneous and must not be acted upon. Such a
non-integrity flag concerning all of the satellites the
spreading code whereof has not been assigned to a
pseudolite may thus be broadcast to the receiver R. In this
way, the receiver R will not seek to acquire these satellites
and will therefore save energy; moreover, there will be no·
risk of conflict in the case of calculating positions from
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positioning signals coming both from pseudolites and from
satellites SV.
Moreover, in the assisted mode used in the context of the
5 invention, the pseudodistances are preferably calculated and the position of
the receiver is preferably determined by the assistance server or any
appropriate computer. Because of this, the receiver R does not need to
manipulate the pseudolite position information.
The assistance server S broadcasting modified assistance data
10 may be constituted of separate means comprising assistance means
calculating the modified assistance data to be broadcast and a server for
broadcasting said modified assistance data.
The main advantage of the invention is to propose a solution for
15 pseudolite positioning in constrained areas that is transparent for most
standard receivers able to acquire positioning signals in the context of a
global satellite navigation system. No hardware modification is required and
no particular software needs to be developed to implement the proposed
solution, which is based on programming and on an original configuration of
20 an assistance server enabling deception of said re·ceivers.

CLAIMS
1. A system for positioning an object provided witli a receiver (R), which
includes a set of pseudolites transmitting positioning signals and
distributed in a constrained area and assistance means (S) capable of
5 communicating with said receiver (R) and calculating the position of
the object, and a server (S) capable of dynamically configuring the set
of pseudolites, said pseudolites each further having a spreading code
corresponding to that of a satellite belonging to a satellite
constellation in a satellite navigation system, the dynamic
10 configuration of the pseudolites by the server being such that the
spreading codes of said pseudolites correspond to spreading codes
of satellites of the satellite constellation not visible (SN) to the receiver
(R) of the object, characterized in that assistance means (S)
communicate to said receiver (R) a list of spreading codes meant to
15 be those of the satellites visible to said receiver (R) but actually
corresponding to the spreading codes of the set of pseudolites so as
to deceive the receiver (R) that can acquire the positioning signals
transmitted by the pseudolites and communicate with the assistance
means (S) so that the latter calculates the position of the object.
20
2. The system as claimed in claim 1, the position of each pseudolite of
the set of pseudolites in the constrained area being known to the
assistance means (S), characterized in that the position of the object
is calculated by triangulation from the known positions of the
25 pseudolites and pseudodistance measurements carried out by the
receiver (R).
3. The system as claimed in any one of the preceding claims, wherein
the receiver (R) may have access to the ephemerides of the satellite
30 constellation, characterized in that the assistance means (S) also
communicate to said receiver (R) an erroneous current time
corresponding to the current time shifted by an offset so that the
receiver (R) consulting the ephemerides determines that the
spreading codes of the pseudolites correspond to spreading codes of
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satellites of the satellite constellation deemed to be visible to said
receiver (R).
4. The system as claimed in claim 3, characterized in that the .
*. 5 ^- pseudolites transmit at the current time positioning signals identical to
the positioning signals that the satellites of the satellite constellation
would transmit of which they borrow the spreading code at the
erroneous current time.
10 5. The system as claimed in any one of the preceding claims,
characterized in that the assistance means (S) also communicate to
said receiver (R) a non-integrity flag for the set of spreading codes of
satellites of the satellite constellation not allocated to pseudolites.
15 6. The system as claimed in any one of the preceding claims,
characterized in that the server (S) implements the assistance means
(S) by means of appropriate programming.