Determining an estimated location
Field of the Invention
The present invention relates to a receiver which is configured to receive signals
wirelessly from a plurality of pseudolites. More particularly, the present invention
relates to a receiver which is configured to determine an estimated location based
on which combination of two or more pseudolites it is currently within range of.
Background of the Invention
Global navigation satellite systems (GNSS) are well-known in the art, and enable the
location of a ground-based receiver to be accurately calculated. A GNSS receiver is
a receiver which calculates its location using signals received from a plurality of
satellites, based on triangulation. At present, the most widely used system is the
Global Positioning System (GPS), whilst other systems such as GLONASS, Galileo,
and Compass are at various stages of development. A GNSS comprises a plurality
of satellites (typically 20-30) which communicate with a receiver. Each satellite
includes a highly accurate atomic clock, and the clocks on all satellites within the
GNSS network are synchronised to one another. The satellites transmit radiofrequency
(RF) signals which include information which allows the receiver to
identify the location of the satellite when the signal was transmitted, as well as a
time at which the signal was transmitted. The receiver can therefore calculate the
distance travelled by the signal, since the velocity of the wave is known (i.e. the
speed of light). From this, the receiver calculates its position by triangulation, based
on signals from at least four satellites. That is, three satellites are required to
calculate the x, y and z coordinates of the receiver, whilst the fourth satellite is
required to calculate a timing offset between the internal clock of the receiver and
the internal clocks of the satellites.
However, one limitation of GNSS networks is that a line-of-sight is required
between the receiver and the satellite in order to receive the signals. This can make
it difficult or impossible for signals to be received in certain locations, for example
when the receiver is in a built-up area or is indoors. Systems have been developed
which allow a GNSS receiver to accurately calculate its position even when signals
from GNSS satellites are not available. These systems incorporate ground-based
transmitters, referred to as pseudohtes (a contraction of "pseudo-satellite"), which
transmit GNSS-like signals (i.e. signals formatted according to a GNSS message
structure) to the receiver. The pseudohtes include internal clocks which are
synchronised, and the location of each pseudolite is known to a high degree of
accuracy (the pseudolite locations are accurately measured during set-up of the
system, and this information is provided to the receiver). The receiver then
calculates its position using a GNSS-like method, i.e. by triangulation using signals
from at least four pseudohtes. Typically, in a pseudolite-based navigation system
the distance between a transmitter and receiver may be several metres or several
tens of metres. In order to accurately calculate the time-of-flight of a signal over
such distances, the internal clocks of the pseudohtes must be highly accurate, and
must typically be synchronised to within a nanosecond. As a result, conventional
pseudolite-based navigation systems are expensive, and require the pseudolite clocks
to be resynchronised periodically to ensure proper functioning of the system.
The present invention aims to address drawbacks inherent in known arrangements.
Summary of the Invention
According to the present invention, there is provided a receiver comprising an
antenna configured to receive signals wirelessly from a plurality of pseudohtes, each
one of the signals including information defining a pseudolite from which the signal
was transmitted, and means for determining an estimated location of the receiver
based on which combination of two or more pseudohtes the receiver is currently
within range of.
The receiver may identify the one or more pseudohtes based on the information
included in the received signals.
The receiver may be a global navigation satellite system GNSS receiver.
The receiver may be within range of a pseudolite when a signal received from the
pseudolite has a signal-to-noise ratio which is higher than a threshold value.
The receiver may be configured to determine the estimated location based on
known locations of the two or more pseudolites.
The receiver may be configured to determine the estimated location as a mean
location of the two or more pseudolites.
The receiver may be configured to measure the power of signals received from at
least three of the plurality of pseudolites, and to determine the estimated location
based on the measured signal powers and known transmission powers of the at least
three pseudolites.
The receiver may be configured to determine the estimated location independently
of a time taken for each one of the signals to reach the receiver from the two or
more pseudolites.
The receiver may be configured to determine the estimated location based only on
known locations of the two or more pseudolites, or the receiver may be configured
to determine the estimated location based only on known locations and known
transmission powers of the two or more pseudolites.
The receiver may be configured to receive signals formatted according to a global
navigation satellite system GNSS specification from a plurality of GNSS satellites,
and to receive the signals from the plurality of pseudolites according to the GNSS
specification.
The receiver may be configured to validate an authentication code included within a
signal received from one of the plurality of pseudolites.
If the authentication code is successfully validated, the receiver may be configured
to store information about the estimated location.
According to the present invention, there is also provided apparatus comprising a
plurality of pseudolites, and the receiver configured to receive signals wirelessly
from the plurality of pseudolites.
The plurality of pseudolites may be arranged so as to divide a space into a plurality
of regions, such that when the receiver is located within each one of the plurality of
regions it is within range of one or more pseudolites and out of range of other ones
of the plurality of pseudolites.
The receiver may be configured to determine which one of the plurality of regions it
is currently located in, by determining which combination of one or more
pseudolites it is currently within range of.
Each one of the plurality of pseudolites may be configured to transmit information
about its location and/or its transmission power to the receiver.
The plurality of pseudolites and the receiver may be configured for use in an indoor
environment where the receiver is not able to receive signals from a global
navigation satellite system GNSS satellite.
Each one of the plurality of pseudolites may include an internal clock, wherein the
internal clocks of the plurality of pseudolites are not synchronised with each other.
Brief Description of the Drawings
Embodiments of the invention will now be described, by way of example, with
reference to the accompanying drawings, in which:
Figure 1 illustrates a pseudolite navigation system, according to an embodiment of
the present invention;
Figure 2 illustrates the pseudolite navigation system of Fig. 1 in plan view;
Figure 3 illustrates a pseudolite navigation system comprising a plurality of nonoverlapping
cells, according to an embodiment of the present invention;
Figure 4 illustrates a plurality of estimated locations of a receiver as it moves
through the system of Fig. 3;
Figure 5 illustrates a pseudolite navigation system comprising three overlapping
transmission cells, according to an embodiment of the present invention;
Figure 6 illustrates a pseudolite navigation system comprising a plurality of
overlapping cells, according to an embodiment of the present invention;
Figure 7 illustrates a plurality of estimated locations of a receiver as it moves
through the system of Fig. 6;
Figure 8 illustrates a pseudolite message structure according to an embodiment of
the present invention;
Figure 9 illustrates a pseudolite-based information distribution system according to
an embodiment of the present invention;
Figure 10 illustrates a method of using authentication in a pseudolite navigation
system, according to an embodiment of the present invention; and
Figure 11 illustrates a pseudolite navigation system according to an embodiment of
the present invention.
Detailed Description
Referring now to Fig. 1, a pseudolite navigation system is illustrated according to an
embodiment of the present invention. As shown in Fig. 1, the system 100
comprises a first pseudolite transmitter 101 (hereinafter referred to simply as a
'pseudolite'), a second pseudolite 102, and a receiver 103. In the present
embodiment, the receiver 103 is a mobile telephone which includes a GNSS receiver
(e.g. a GPS receiver). However, in other embodiments of the present invention the
receiver may be any device which is capable of receiving GNSS signals.
As shown in Fig. 1, the first 101 and second 102 pseudolites each comprise an
antenna 104, 105 for transmitting signals to the receiver 103. However, unlike a
conventional pseudolite navigation system, in the present embodiment the receiver
does not calculate its location by calculating time-of-flight information for each
received signal. Instead, the transmission power of each one of the pseudolites 101,
102 is adjusted during setup of the system, such that each pseudolite defines a 'cell'.
Outside the cell, signals from that pseudolite are too weak to be detected by the
receiver 103. This will now be described in more detail with reference to Fig. 2.
As shown in Fig. 2, in the present embodiment, the antennae 104, 105 of the
pseudolites 101, 102 are arranged to radiate power isotropically (i.e. equally in all
directions). Therefore a transmission cell is established around each pseudolite, the
cell being spherical (circular when viewed in 2-D, as in Fig. 2) and having a radius r.
The radius r corresponds to a maximum effective range of the pseudolite, and as
shown in Fig. 2, both the first 101 and second 102 pseudolites are configured to
have the same maximum effective range r.
In more detail, for a constant transmission power the measured power of a received
signal will decrease as the distance between the receiver and transmitter increases.
The power loss may also be affected by various factors such as obstructions
between the transmitter and receiver, multipath effects, and RF noise. In the
present embodiment, the power loss can be approximated by an inverse square law
relationship. That is, for a distance D from the transmitter, the received power PR
can be estimated as:
where A is a constant, and P is the transmitted power. Preferably, the value use
for the transmitted power may be the EIRP ("effective isotropic radiated power")
value, which takes into account factors such as the antenna and other gains at the
transmitter. Although in the present embodiment an inverse square relationship i
used, in other embodiments, the power loss as a function of distance may be
modelled using a different proportionality.
Furthermore, in any environment there may be a certain level of background RF
energy. A receiver can only resolve a signal against this background noise when the
signal-to-noise ratio (SNR) of the received signal is higher than a threshold value.
The maximum effective range of a pseudolite may therefore be defined as the
maximum distance at which the SNR of the received signal is equal to or above this
threshold. Beyond this range, the receiver 103 is no longer able to distinguish the
transmitted signal from the background RF noise, and hence is not able to receive
transmissions from that particular pseudolite. Here, a receiver is said to be able to
"receive" a signal from a pseudolite if it is able to distinguish the signal against the
background noise in order to decode any information carried in the signal.
As described above, the cells 201, 202 surrounding the first 101 and second 102
pseudohtes are spherical in shape as the antennae 104, 105 are configured to radiate
energy isotropically. However, in other embodiments, shaped antennae may be
used to establish cells which are anisotropic in shape, i.e. non-spherical. Similarly,
in the present embodiment the cells 201, 202 are arranged to not overlap. That is,
the sum of the maximum ranges of each cell (2rin the present embodiment) is set
to be less than or equal to a separation distance between the antennae 104, 105 of
the two pseudohtes 101, 102. However, in other embodiments a plurality of
pseudohtes may be configured to create a plurality of overlapping cells, as will be
described later.
Continuing with reference to Fig. 2, the receiver 103 comprises means for
estimating its current position, for example a control module such as a processor, by
determining which of the pseudohtes it is currently able to receive transmissions
from. That is, the receiver 103 is configured to determine whether or not it is
currently located in one of the cells 201, 202 based on whether or not it is currently
able to receive signals from either one of the pseudohtes 101, 102. The signals
transmitted from the pseudohtes may include information identifying a pseudolite
which transmitted the signal, for example a pseudorandom number PRN code
which uniquely identifies a particular pseudolite amongst a plurality of pseudohtes.
To determine which pseudohtes the receiver is currently within range of, the
receiver may analyse signals received during a predetermined time period, the
predetermined time period being a period that is sufficiently short that the receiver's
location is substantially constant. Here, the receiver analyses a signal to identify the
pseudolite which transmitted the signal, for example by extracting a PRN code from
the signal.
In Fig. 2, by way of example, three separate receivers 203, 204, 205 are illustrated in
different locations. Each one of the receivers 203, 204, 205 may be substantially
similar to the receiver 103 of Fig. 1. The first receiver 203 is currently located in
the first cell 201 (i.e. the cell created by the first pseudolite 101), and can receive
signals from the first pseudolite 101 since it is within a maximum effective range of
the first pseudolite 101. At the same time, the first receiver 203 is outside of the
second cell 202 (i.e. the cell created by the second pseudolite 102), and is unable to
receive signals from the second pseudolite 102 since it is beyond a maximum
effective range of the second pseudolite 102. In contrast, the second receiver 204 is
located inside the second cell 201 and outside the first cell 202, and hence can
receive signals from the second pseudolite 102 but not from the first pseudolite 101.
The third receiver 205 is located outside both cells 201, 202, and hence cannot
receive signals from either the first 101 or second 102 pseudolite.
Each receiver 203, 204, 205 is provided with information about the positions of the
first 101 and second 102 pseudolites. For example, positional information about
each pseudolite may be pre-programmed into the receiver, or may be included in
signals transmitted by the pseudolite and received by the receiver. In the present
embodiment the cells of different pseudolites do not overlap, and so in any given
location the receiver is only able to receive signals from one of the pseudolites 101,
102. The receiver is configured to estimate its current location as being the position
of the pseudolite from which it is currently able to receive signals. Therefore the
first receiver 203 calculates its current location as being the position of the first
pseudolite 101, and the second receiver 204 calculates its current location as being
the position of the second pseudolite 102. The third receiver 205 cannot calculate
its current location as it is unable to receive signals from either of the pseudolites
101, 102.
The accuracy with which the current location of a receiver can be determined is
dependent on the size of each cell. For example, if the maximum effective range of
each pseudolite 101, 102 is set to be 10 metres, the location of a receiver can be
determined to within a distance of 10 metres. The cell sizes may be configured
during setup of the pseudolite navigation system 100, and may be selected according
to the number of pseudolites in the system, the total area which is to be covered,
and the desired accuracy.
In the present embodiment, each receiver is arranged to display its current location
to a user. Therefore, the user can be informed of their current location to an
accuracy that is determined by the cell radius. When a receiver is out of range of all
of the pseudolites, it is arranged to display a message to a user stating that its
current location cannot be determined. In another embodiment, if the receiver was
previously within range of one of the pseudolites 101, 102 (i.e. was previously
located in the first cell 201 or the second cell 202), the receiver is arranged to
continue to display its location as being the location of the pseudolite for the cell in
which it was previously located, until the receiver enters a new cell.
Referring now to Fig. 3, a pseudolite navigation system comprising a plurality of
non-overlapping cells is illustrated, according to an embodiment of the present
invention. A receiver is arranged to estimate its current position using a method
similar to that described above in relation to Fig. 2, and as such a detailed
description will be omitted here in order to maintain brevity.
The system 300 comprises a plurality of pseudolites which are arranged to provide
coverage within an area 301. The area 301 may be an area in which GNSS signals
from a satellite cannot be received. For example, the area 301 may be an enclosed
space such as a railway terminal or airport, and the pseudolites may be configured
such that each cell has a radius of about 10 metres. The pseudolite navigation
system 300 allows a receiver 302 to continue to receive GNSS-like signals and
calculate its position as it moves within the area 301, even though the receiver 302
is unable to receive normal GNSS signals from a satellite. However, in other
embodiments, the area 301 may be an area in which GNSS signals from a satellite
are able to be received
More specifically, in the embodiment of Fig. 3, the pseudolite navigation system 300
comprises ten pseudolites arranged on a regular hexagonal grid, such that the cells
appear close-packed in plan view. Each one of the pseudolites is configured to
transmit at the same power so that each pseudolite 303 has the same maximum
effective range, r . As the cells in the present embodiment are arranged to touch
without overlapping, the range r of each pseudolite 303 is set to correspond to half
of the separation distance between two nearest-neighbour pseudolites. The
pseudolites may be configured to transmit signals according to any multiple-access
transmission scheme, including but not limited to code-division multiple access
CDMA and time-division multiple access TDMA. In this way, for any given signal,
the receiver can identify the specific pseudolite from which the signal originated.
As shown in Fig. 3, by adjusting the transmission power of a plurality of pseudolites
to create a plurality of discrete transmission cells, the location of a receiver can be
determined with a high level of accuracy. Furthermore, a cell-based navigation
method such as that shown in Fig. 3 enables the receiver to calculate its location
without having to calculate time-of-flight information for a signal received from a
pseudolite. Therefore, it is not necessary for internal clocks of the pseudolites to be
synchronised with one another, which can simplify the setup and operation of the
system 300 in comparison to a conventional pseudolite navigation system. Also, the
internal clocks do not have to be highly accurate (i.e. do not need to be accurate to
within a few nanoseconds), and so lower-cost pseudolites may be used in
comparison to a conventional pseudolite navigation system.
Referring now to Fig. 4, a plurality of estimated locations of a receiver as it moves
through the system of Fig. 3 is illustrated. Specifically, an actual path 304 taken by
the receiver 302 is illustrated in both Figs. 3 and 4, and is shown by a dotted line.
The receiver 302 is configured to continually update its estimated location as it
moves within the system 300 of Fig. 3, by periodically determining which of the
pseudolites it is currently within range of. As long as the receiver 302 remains
within range of a given pseudolite (i.e. remains within the transmission cell of that
pseudolite), its location is estimated as being the position of that particular
pseudolite. Once the receiver 302 moves out of range of the pseudolite (i.e. exits
the cell), if no signals are received from other pseudolites its location is no longer
displayed. Alternatively, if the receiver 302 is now able to receive signals from
another one of the pseudolites, its new location is estimated as being the position of
a pseudolite from which the new signals are received.
In this way, as the receiver 302 moves between cells, a plurality of estimated
locations 401 are displayed in sequence. This sequence of estimated locations 401
comprises a series of discrete locations, which in Fig. 4 are illustrated as a plurality
of circles connected by a solid arrow. The arrow indicates the order in which the
locations 401 are displayed as the receiver moves through the area 301 (cf. Fig. 3).
Each discrete location is displayed for as long as the receiver 302 remains within a
corresponding one of the cells. In the present example, the receiver 302 moves in a
straight line through a total of four transmission cells, and so the estimated path
401 comprises four discrete locations. As shown in Fig. 4, the sequence of
estimated locations 401 closely approximates the actual path 304. Therefore in the
present embodiment, the cell-based pseudolite navigation system allows a moving
receiver 302 to be tracked without having to accurately measure the time-of- flight
of received signals.
Referring now to Fig. 5, a pseudolite navigation system comprising three
overlapping transmission cells is illustrated, according to an embodiment of the
present invention. The system 500 comprises a first pseudolite 501 configured to
create a first transmission cell 511, a second pseudolite 502 configured to create a
second transmission cell 512, and a third pseudolite 503 configured to create a third
transmission cell 513. Additionally in Fig. 5, three receiver locations 504, 505, 506
are illustrated along with corresponding estimated locations 514, 515, 516. In the
present embodiment, each pseudolite 501, 502, 503 is configured to have a different
maximum effective range, but in other embodiments some or all of the pseudolites
may be configured to have the same maximum effective range.
In more detail, the first 511, second 512 and third 513 cells are configured to
overlap. That is, for any given pair of neighbouring pseudolites, the sum of the
ranges of the two pseudolites is configured to be greater than the separation
distance of the two pseudolites. In this way, a plurality of overlapping regions are
created in which a receiver is able to receive signals from different combinations of
two or more pseudolites at once (i.e. whilst remaining at the same location). As
shown in Fig. 5, different estimated locations are obtained for different
combinations of pseudolites. In Fig. 5, regions in which a receiver is able to receive
signals from two pseudolites are shown as lightly shaded regions, whilst a central
region in which a receiver is able to receive signals from all three pseudolites is
shown as a darkly shaded region.
As shown in Fig. 5, a first receiver location 504 is located inside the first cell 511,
and outside the second and third cells 512, 513. Therefore, at the first receiver
location 504, a receiver is only able to receive transmissions from the first
pseudolite 501, and an estimated location 514 of the receiver corresponds simply to
the position of the first pseudolite 501. This is similar to the situation in which
cells are non-overlapping (cf. Fig. 3).
However, the second receiver location 505 is located in a region of overlap between
the first and second cells 511, 512. At this location 505, a receiver is therefore
within range of both the first pseudolite 501 and the second pseudolite 502, and can
receive signals from both first and second pseudolites 501, 502. The receiver is out
of range of the third pseudolite 503 (i.e. outside of the third cell 513) and hence
cannot receive signals from the third pseudolite 503. Finally, the third receiver
location 506 is located at a region of overlap between all three cells 511, 512, 513.
Therefore at the third receiver location 506, a receiver is within range of all three
pseudolites 501, 502, 503 and is able to receive signals from all three pseudolites
501, 502, 503.
In the present embodiment, when a receiver is at a location where it is able to
receive signals from a plurality of pseudolites, the receiver is configured to estimate
its current location as being an average position of said plurality of pseudolites.
Therefore, the estimated location 515 of a receiver at the second receiver location
505 corresponds to a mean position of the first and second pseudolites 501, 502.
Similarly, the estimated location 516 of a receiver at the third receiver location 506
corresponds to a mean position of the first, second and third pseudolites 501, 502,
503. In Fig. 5, the actual receiver locations are shown as solid circles, and the
estimated receiver locations are shown as broken (i.e. dashed) circles.
By way of example, if a current location of the receiver lies in two overlapping cells
(i.e. the receiver is able to receive signals from two pseudolites), and if the locations
of the pseudolites and receiver are expressed in 2-dimensional Cartesian
coordinates, the estimated location of the receiver may be calculated as:
where (x , ) are the coordinates of the receiver's estimated location, (x are
the coordinates of a first pseudolite from which signals are received, and (x 2 , 2) are
the coordinates of a second pseudolite from which signals are received. More
generally, when a receiver is located in a total of n overlapping cells, the coordinates
( , ) of the estimated location of the receiver may be calculated as:
n
n
n
å >.
n
where x , ) are the coordinates of the pseudolite.
Referring now to Fig. 6, a pseudolite navigation system comprising a plurality of
overlapping cells is illustrated, according to an embodiment of the present
invention. Like the embodiment of Fig. 3, the embodiment of Fig. 6 comprises ten
pseudolites arranged on a hexagonal grid within an area 601. The system 600 allows
the location of a receiver 602 to be estimated with a high degree of accuracy. As
with the embodiment of Fig. 3, the present embodiment may be used in an area
where GNSS signals are present (e.g. an outdoors area) in order to provide
augmented functionality, or may be used in an area where GNSS signals are not
present (e.g. an indoors location, or built-up area).
However, in the present embodiment, each pseudolite 603 is configured to have a
maximum range r2 which is greater than half the separation distance between two
neighbouring pseudolites, such that the plurality of cells overlap. This creates a
plurality of overlapping regions, enabling the location of the receiver 602 to be
determined with greater accuracy than a system which comprises the same number
of pseudolites, but in which the cells do not overlap (cf. Fig. 3) . Additionally, by
having cells which overlap, the "dead space" (i.e. regions in which a receiver is out
of range of all of the pseudolites, and hence unable to estimate its location) can be
substantially or completely eliminated. In the present embodiment, the plurality of
pseudolites are configured to provide coverage across substantially the entire area
601.
Referring now to Fig. 7, a plurality of estimated locations of the receiver as it moves
through the system of Fig. 6 is illustrated. As in Fig. 4, the plurality of estimated
locations 701 are displayed in sequence as the receiver 602 moves between cells.
This sequence of estimated locations 701 comprises a series of discrete locations,
which in Fig. 7 are illustrated as a plurality of circles connected by a solid arrow.
The arrow indicates the order in which the locations 701 are displayed as the
receiver moves through the area 601 (cf. Fig. 6) . In comparison with the system
comprising non-overlapping cells (cf. Figs . 3 and 4) , the present embodiment
enables the location of the receiver to be tracked with an even greater degree of
accuracy.
In a further embodiment of the present invention, a receiver is configured to
measure the received power PR of a signal from a pseudolite. Each pseudolite is
also configured to include information about its own transmission power P in the
signals which are transmitted to receivers . As described above, the power loss may
be approximated by an inverse square law. The receiver can therefore calculate the
distance between the receiver and pseudolite as D = k(P / P ) . The constant of
proportionality k may not be known, but can be assumed to be the same for signals
received from all pseudolites in the systems. Therefore, in the present embodiment,
if the receiver is at a location where it is able to receive signals from at least three
pseudolites, it may still calculate its location (x , ) by solving the following
simultaneous equations to eliminate the constant k
Here, ( , ) are the coordinates of a first pseudolite and D is the distance between
the first pseudolite and the receiver. Similarly, (x- , ) are the coordinates of a
second pseudolite, D is the distance between the second pseudolite and the
receiver, ( k , k) are the coordinates of a third pseudolite and D is the distance
between the third pseudolite and the receiver.
In the embodiments described above, a receiver estimates its current location based
on the known locations of a plurality of pseudolites, and based on whether or not it
is currently able to receive signals from each one of the pseudolites. Preferably,
information about the location of a particular pseudolite is included in the signals
transmitted by that pseudolite, as will now be described with reference to Fig. 8.
This information can be readily included in a GNSS-like message structure, ensuring
compatibility with a standard GNSS receiver. However, in other embodiments,
information about the positions of all pseudolites in the system may be pre
programmed into the receiver, or may be sent to the receiver when the receiver first
enters the pseudolite navigation system.
Figure 8 illustrates a pseudolite message structure according to an embodiment of
the present invention. The message structure corresponds to a structure used by
the 'Galileo' GNSS for transmissions on the E5a channel. The message structure of
Fig. 8 may therefore be suitable for use in embodiments of the present invention
where it is desired to provide compatibility with a Galileo GNSS receiver. In other
embodiments, alternative message structures may be substituted according to the
particular GNSS with which compatibility is desired (e.g. GPS, GLONASS etc.).
As shown in Fig. 8, the message 800 comprises a frame synchronisation field 801
and a page field 802. The synchronisation field 801 includes a fixed synchronisation
pattern, which for the E5a channel in the Galileo system is defined as the binary
sequence 101101 110000. The page field 802 is used to transmit the specific
navigation data, and comprises a word field 811 of 238 bits, and a tail 812 of 6 bits.
The word field 8 11 is subdivided into a page type field 821 of 6 bits, a navigation
data field 822 of 208 bits, and a cyclic redundancy check CRC field 823 of 24 bits.
Different GNSS specifications (GPS, Galileo, Compass, GLONASS etc.) define
different message structures. However, all GNSS-like messages will include a
navigation data portion (cf. Fig. 8) which holds information which will be used by
the receiver when calculating its position. Embodiments of the present invention
may therefore be made compatible with any given GNSS (the 'host' GNSS), by
retaining the message structure of the host GNSS and substituting the pseudolite
navigation data in the appropriate data portion of the message. When the receiver
is able to receive signals from the GNSS satellites, for example when it is outside,
the receiver can calculate its location using signals from the satellites. When the
receiver moves to a location where signals cannot be received from the satellites, for
example when it is indoors, the receiver can continue to calculate its location using
signals received from the pseudolites.
In more detail, a typical GNSS defines at least three different message types, which
are identified by means of a code included in a 'page type' field (e.g. the 6-bit page
type field 821 of Fig. 8). The message types can be broadly defined as 'ephemeris',
'almanac', and 'time and clock correction parameters'. Ephemeris data indicates the
precise orbit of the satellite from which the signal is being sent, almanac data
indicates the approximate orbits and positions of all satellites within the GNSS
constellation, and time and clock correction parameters are used to adjust the Rx
clock and compute pseudoranges. Additionally, GNSS satellites may transmit
service parameters (e.g. indicators of satellite health), which may be allocated a
separate 'service parameters' message type or may be included within the other three
message types.
In an embodiment of the present invention, the message structure of the host
GNSS is adapted as follows, for use in a cell-based pseudolite navigation system.
Firstly, ephemeris data is replaced by data which indicates the precise location of
the pseudolite which is sending the signal (e.g. latitude, longitude, floor number on
which the pseudolite is located, etc.). Secondly, the almanac data is replaced by data
which defines the system configuration (e.g. which pseudolites are included in the
system, and approximately where they are located). Thirdly, the time and clock
correction parameters are not needed as no synchronisation is required, and hence
this data is replaced by any other data which is desired to be transmitted. Here, the
data may include data which is not related to navigation, i.e. data which is not used
in determining a current location of the receiver.
Referring now to Fig. 9, a pseudolite-based information distribution system
comprising a plurality of pseudolites and a receiver is illustrated, according to an
embodiment of the present invention. The system further comprises a control unit
903 and a plurality of data storage units 905, 906, 907, 908 coupled to the control
unit 903 via a communications link 904. Each one of the plurality of pseudolites
901 and the receiver 902 may be substantially similar to any of the pseudolites and
receivers described above. In particular, the receiver 902 is configured to receive
signals from the plurality of pseudolites 901, and may be configured to determine its
current location based on any of the methods described above (for example, any of
the methods illustrated in Figs. 2 to 7).
In the present embodiment, each one of the plurality of pseudolites 901 is
configured to communicate with the control unit 903. The plurality of pseudolites
901 and control unit 903 may, for example, communicate over a wired local area
network (LAN) connection or a wireless LAN (WLAN) connection, or via a
combination of wired and wireless connections.
Furthermore, the control unit 903 is configured to communicate with a plurality of
data storage units 905, 906, 907, 908 via a communications link 904, which in the
present embodiment is the internet. However, other arrangements are possible.
For example, in another embodiment, the data storage units are local data storage
units which communicate with the control unit over a local area network.
The control unit 903 can request information from the data storage units 905, 906,
907, 908, or the data storage units 905, 906, 907, 908 can send information to the
control unit 903 without it first being requested. The information includes data to
be sent to the receiver 902 by one or more of the plurality of pseudolites 901. The
information can also include positional information associated with the data, the
positional information defining a location or area within a space in which the system
is deployed. The control unit 903 processes the received data and positional
information, and sends it to the plurality of pseudolites 901. The pseudolites are
configured to include the information in the GNSS-like messages sent to the
receiver 902. For example, as described above with reference to Fig. 8, the
information can be included in a 'time and clock correction parameters' GNSS
message type, since the receiver 902 does not need to calculate the time-of-flight of
a signal and hence the time and clock correction parameters are not required.
As an example, a system such as the one shown in Fig. 9 may be located in an
indoor environment such as a railway terminal. In this case, the data received from
the data storage units 905, 906, 907, 908 may comprise, for example, timetable
information about train services. The control unit 903 determines which services
are arriving or departing in a predetermined time (e.g. in the next 15 minutes), and
determines which platform each service is departing from or arriving into.
Positional information associated with each platform may be programmed into the
control unit 903 during configuration of the system, or can be determined by the
control unit 903 based on information about a physical layout of the railway
terminal. In this example, the positional information defines an area close to a
platform. The control unit 903 then sends the timetable data and associated
positional information to the plurality of pseudolites 901, which in turn transmit the
timetable data and positional information to the receiver 902.
The receiver 902 receives the timetable data and positional information, and selects
timetable data to be displayed based on a current location of the receiver 902. That
is, the receiver 902 uses the positional information and its current location to
determine whether it is currently located near one of the platforms, and if so,
displays the timetable data relevant to that particular platform. In this way, as a user
holding the receiver 902 moves around the railway terminal, the user is presented
with timetable information specific to a particular platform as they approach that
platform.
The data to be sent to the receiver is not limited to timetable information. For
example, the data may include information about services available in particular
areas of the railway stations, such as ticket machines, payphones, help desks and so
on. Similarly, the information distribution system of Fig. 9 is not limited to use in
railway terminals. In general, the system may be used in any area to provide
location-dependent information to a user.
Referring now to Fig. 10, a method of using authentication in a pseudolite
navigation system is illustrated, according to an embodiment of the present
invention.
In the first step 1001, a receiver receives a message from a pseudolite. The message
may be formatted according to a GNSS specification. For example, the message
may have a structure similar to the one illustrated in Fig. 8, although alternative
message structures may also be used. The receiver and the pseudolite may be
substantially similar to any of the receivers and pseudolites illustrated in any
embodiment.
Next, in the second step 1002, the receiver processes the received message and
extracts an authorisation code contained within the message. The authorisation
code may be contained within a reserved portion of the message. Then, in the third
step 1003, the receiver attempts to validate the authorisation code to confirm
whether or not the message is genuine, i.e. is received from an authorised
pseudolite. In this step, the receiver may use a predetermined validation algorithm
to validate the authorisation code.
In the fourth step 1004, the receiver determines whether or not to proceed based on
the result of the validation procedure in the previous step 1003. Specifically, if the
authorisation code cannot be validated, the process ends and the receiver takes no
further action. In one embodiment, the receiver may display an error message to
alert a user that a message has been received from an authorised source.
If the authorisation code is successfully validated, it can be assumed that the
message has been received from an authorised pseudolite, and so the receiver
proceeds to determine its current location in the fifth step 1005. To calculate its
current location, the receiver may use any of the methods of any embodiment (i.e.
any of the methods described herein).
Finally, in the sixth step 1006, the receiver stores a record of its current location in
an internal storage unit of the receiver. For example, the receiver may include an
internal flash memory for recording information about a journey, by recording the
receiver's current location at predetermined time intervals. When recording a
current location, the receiver may also mark the recorded location as
"authenticated", to denote that the location was calculated based on signals received
from one or more authorised pseudolites.
Referring now to Fig. 11, a pseudolite navigation system is illustrated, according to
an embodiment of the present invention. The system 1100 comprises a plurality of
receivers 1101, 1102, 1103 each of which is located in a vehicle. Each one of the
receivers 1101, 1102, 1103 may, for example, comprise a receiver of a GNSS-based
in-car satellite navigation system, commonly referred to as a 'sat-nav' system.
Alternatively, each one of the receivers 1101, 1102, 1103 may be a standalone GNSS
receiver, i.e. a dedicated receiver for use with the system 1100 of Fig. 11.
The system 1100 further comprises a plurality of toll booths 1104 located on a toll
road, such as a motorway. A plurality of pseudolites 1105 are located in or near the
toll booths 1104, and are configured to define a plurality of cells 1106 in a manner
similar to any of the embodiments described herein. Toll roads are well-known, and
comprise sections of road where a charge (or 'toll') is levied on drivers for using the
road. Each receiver may be configured to calculate its current location using any of
the methods disclosed herein (e.g. as illustrated in Figs. 2 to 7). Furthermore, each
one of the receivers 1101, 1102, 1103 is configured to record information about its
current location, if signals received from the plurality of pseudolites are successfully
authenticated (cf. step 1006 of Fig. 10). When a user completes their journey, the
recorded information is uploaded to a server, and an account belonging to the user
can be automatically debited if it is recorded that the receiver has passed through
one of the toll booths 1104. The system 1100 therefore allows drivers to be
automatically charged for use of the road when they pass through one of the toll
booths 1104.
In the present embodiment, the system 1100 uses an authentication method such as
the one shown in Fig. 10, in order to ensure that a receiver only calculates its
location based on signals received from authorised pseudolites. For example, if
authentication is not used, it may be possible for a user to avoid being charged for
use of the toll road, by providing their own pseudolite configured to continuously
transmit a signal to the receiver that causes the receiver to incorrectly calculate its
current location.
In some embodiments, the sixth step 1006 of the method in Fig. 10 may be omitted.
For example, if the authorisation code is successfully validated, the receiver may be
configured to determine its current location and then enable or disable certain
functionality of the receiver depending on the current location. In one
embodiment, a pseudolite-based navigation system may be used to monitor a
location of the receiver for security purposes, and the receiver may activate an alarm
when it detects that it has entered or left a certain area. The receiver may activate
an alarm built in to the receiver, and/ or may send a transmission to activate a
remote alarm, based on the current location determined in the fifth step 1005.
By using an authentication method such as the one illustrated in Fig. 10, it can be
ensured that the current location of the receiver is calculated only based on
messages received from authorised pseudolites. This may be particularly
advantageous when separate navigation systems are deployed in close proximity to
one another, by preventing a receiver of one system inadvertently miscalculating its
location based on a message from a pseudolite from the other system. Also,
authentication can prevent deliberate attempts to subvert a system by setting up an
unauthorised pseudolite or pseudolites which are designed to fool a receiver into
calculating its current location incorrectly. This may be particularly important in
embodiments where the pseudolite navigation system is used for security purposes,
or is used to automatically debit a user's account when a receiver moves from one
location to the next (e.g. as in the embodiment of Fig. 11).
Whilst certain embodiments of the present invention have been described above, it
will be clear to the skilled person that many variations and modifications are
possible without departing from the scope of the invention as defined by the claims.
Any feature of any embodiment described may be used in combination with any
feature of a different embodiment.
For example, embodiments of the present invention have been described in relation
to 2-dimensional arrays of pseudolites. However, in other embodiments the
pseudolites may be arranged in 3 dimensions, i.e. different ones of the pseudolites
may be located at different heights. In such embodiments, the receiver can calculate
its location in 3 dimensions.
Similarly, embodiments of the present invention have been illustrated in which the
pseudolites are arranged in a regular array (e.g. Figs. 3 and 6). However, this need
not necessarily be the case. In other embodiments, the pseudolites may be arranged
irregularly, and may have different operating ranges (i.e. may transmit at different
powers). In an embodiment of the present invention, when a pseudolite navigation
system is installed, each pseudolite may be located in any convenient location, e.g. a
pseudolite may be located alongside existing signage in an environment such as a
railway terminal. In such cases, the distribution of pseudolites may be determined
by the existing architecture of a structure in which the system is installed, and as
such the pseudolites may be arranged irregularly.
Additionally, embodiments of the present invention have been described in which a
receiver calculates its location as being the mean location of those pseudolites from
which signals are received. However, in other embodiments, alternative methods of
determining a location in a particular region (e.g. in a region of overlap) may be
used. For instance, each region may be associated with a particular location, which
may be arbitrarily chosen during setup of the system. In one embodiment, the
particular location may correspond to a geometric centre of the region of overlap.
The receiver can uniquely identify which region it is in, based on the combination of
pseudolites from which it receives signals (i.e. based on which ones of the
pseudolites it is currently within range of). Having identified which region it is in,
the receiver can then retrieve the coordinates of the particular location associated
with that region, rather than actively calculating a location itself based on locations
of the pseudolites. Information identifying the plurality of regions and their
associated locations may, for example, be stored in a lookup table which is sent to
the receiver. An example of such a lookup table is shown below, relating to the
configuration shown in Fig. 5 and using Cartesian coordinates corresponding to a
set of arbitrarily defined axes (not shown in Fig. 5):
In this embodiment, the receiver does not calculate a position based on the
locations of the pseudolites, but instead retrieves a location from the lookup table
based only on information about which ones of the pseudolites are currently within
range. As such, it may not be necessary to provide the receiver with information
about the locations of any of the pseudolite, since the receiver does not need to use
this information to determine its current location.
Claims
1. A receiver (103) comprising:
an antenna configured to receive signals wirelessly from a plurality of
pseudolites (101, 102), each one of the signals including information defining a
pseudolite from which the signal was transmitted; and
means for determining an estimated location (213) of the receiver based on
which combination of two or more pseudolites the receiver is currently within ran
of.
2. The receiver of claim 1, wherein the receiver is configured to determine th<
estimated location based on known locations of the two or more pseudolites.
3. The receiver of claim 2, wherein the receiver is configured to determine th<
estimated location as a mean location of the two or more pseudolites.
4. The receiver of claim 2 or 3, wherein the receiver is configured to measure
the power of signals received from at least three of the plurality of pseudolites, an
to determine the estimated location based on the measured signal powers and
known transmission powers of the at least three pseudolites.
5. The receiver of any one of the preceding claims, wherein the receiver is
configured to determine the estimated location independently of a time taken for
each one of the signals to reach the receiver from the two or more pseudolites.
6. The receiver of claim 5, wherein the receiver is configured to determine th
estimated location based only on known locations of the two or more pseudolites,
or
wherein the receiver is configured to determine the estimated location base
only on known locations and known transmission powers of the two or more
pseudolites.
7. The receiver of any one of the preceding claims, wherein the receiver is
configured to receive signals formatted according to a global navigation satellite
system GNSS specification from a plurality of GNSS satellites, and
wherein the receiver is further configured to receive the signals from the
plurality of pseudolites according to the GNSS specification.
8. The receiver of any one of the preceding claims, wherein the receiver is
configured to validate an authentication code included within a signal received from
one of the plurality of pseudolites.
9. The receiver of claim 8, wherein if the authentication code is successfully
validated, the receiver is configured to store information about the estimated
location.
10. Apparatus (100) comprising:
a plurality of pseudolites (101, 102); and
the receiver (103) according to any one of the preceding claims, the receiver
being configured to receive signals wirelessly from the plurality of pseudolites.
11. The apparatus of claim 10, wherein the plurality of pseudolites are arranged
so as to divide a space into a plurality of regions, such that when the receiver is
located within each one of the plurality of regions it is within range of one or more
pseudolites and out of range of other ones of the plurality of pseudolites.
12. The apparatus of claim 11, wherein the receiver is configured to determine
which one of the plurality of regions it is currently located in, by determining which
combination of one or more pseudolites it is currently within range of.
13. The apparatus of claim 10, 1 1 or 12, wherein each one of the plurality of
pseudolites is configured to transmit information about its location and/or its
transmission power to the receiver.
14. The apparatus of any one of claims 10 to 13, wherein the plurality of
pseudolites and the receiver are configured for use in an indoor environment where
the receiver is not able to receive signals from a global navigation satellite system
GNSS satellite.