U.S. patent application number 11/504291 was filed with the patent office on 2009-01-01 for inter-network operation of multiple location networks.
Invention is credited to David Small.
Application Number | 20090002238 11/504291 |
Document ID | / |
Family ID | 3832456 |
Filed Date | 2009-01-01 |
United States Patent
Application |
20090002238 |
Kind Code |
A1 |
Small; David |
January 1, 2009 |
INTER-NETWORK OPERATION OF MULTIPLE LOCATION NETWORKS
Abstract
The present invention discloses a system and method for allowing
a position receiver to determine position solutions from
positioning signals from a plurality of autonomous positioning
networks, each of which is synchronized to a different timebase.
Each of the plurality of autonomous networks comprises one or more
positioning-unit device, which is a specialized transceiver capable
of receiving and interpreting reference positioning signals from a
reference transmitter, a position receiver, another
positioning-unit device or all. At least one positioning-unit
device within a first autonomous positioning network receives
positioning signals from at least one positioning-unit device from
a second adjacent autonomous positioning network and measures the
timebase difference between the second network and the first
network and subsequently calculates a timebase clock correction.
The at least one positioning-unit device within the first network
incorporates the timebase clock correction into its positioning
signals for transmission. Other positioning-unit devices within the
first network receive and replay the positioning signals
incorporating the timebase clock correction. A position receiver
situated near the boundary of two of the plurality of networks
receives positioning signals incorporating the timebase clock
corrections from any positioning-unit device within the first
network and also receives positioning signals from one or more
positioning-unit devices from the second network. The position
receiver applies the timebase clock correction to the positioning
signals received from one or more positioning-unit devices from the
second network. The position receiver then calculates its own
position using positioning signals from the first network and the
second network. Hence, although the positioning signals from the
positioning-unit devices from the different networks are
synchronized to different timebases, the position receiver is able
to use these signals for position solutions.
Inventors: |
Small; David; (Canberra,
AU) |
Correspondence
Address: |
David Small;c/- Victoria Gaw
2561 Verbena Drive, Hollywood
Los Angeles
CA
90068
US
|
Family ID: |
3832456 |
Appl. No.: |
11/504291 |
Filed: |
August 16, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10494024 |
Apr 29, 2004 |
|
|
|
PCT/AU02/01495 |
Nov 1, 2002 |
|
|
|
11504291 |
|
|
|
|
Current U.S.
Class: |
342/464 |
Current CPC
Class: |
G01S 5/0289 20130101;
G01S 19/23 20130101; G01S 5/021 20130101; G01S 5/0226 20130101;
G01S 19/256 20130101; G01S 19/10 20130101; G01S 5/14 20130101; G01S
19/43 20130101; G01S 5/145 20130101; G01S 5/0081 20130101; G01S
1/24 20130101 |
Class at
Publication: |
342/464 |
International
Class: |
G01S 3/02 20060101
G01S003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2001 |
AU |
PR8643 |
Claims
1. A method for allowing a position receiver to determine position
solutions from positioning signals from a plurality of autonomous
positioning networks within a positioning system, wherein: a) each
autonomous positioning network is synchronized to a different
timebase; b) each of said plurality of autonomous positioning
networks comprises one or more positioning-unit device; c) a
position receiver is situated near the boundary of two of said
plurality of autonomous positioning networks; said method
comprising the steps of: i) at least one positioning-unit device
within a first autonomous positioning network receives positioning
signals from at least one positioning-unit device from a second
autonomous positioning network and measures the timebase difference
between said second autonomous positioning network and said first
autonomous positioning network and subsequently calculates a
timebase clock correction; ii) said at least one positioning-unit
device within said first autonomous positioning network
incorporates said timebase clock correction into its positioning
signals for transmission; iii) other positioning-unit devices
within said first autonomous positioning network receive and relay
said positioning signals incorporating said timebase clock
correction; iv) said position receiver receives positioning signals
incorporating said timebase clock correction from any
positioning-unit device within said first autonomous positioning
network; v) said position receiver receives positioning signals
from said one or more positioning-unit devices from said second
autonomous positioning network; vi) said position receiver applies
said timebase clock correction to said positioning signals received
from said one or more positioning-unit device from said second
autonomous positioning network; and vii) said position receiver
calculates its own position using said positioning signals from
said first autonomous positioning network and said positioning
signals from said second autonomous positioning network; such that
positioning signals received from positioning-unit devices from
said first and second autonomous positioning networks that are
synchronized to different timebases can be used for position
solutions.
2. A method according to claim 1, wherein said position receiver
determines its own position, velocity and time (PVT) using a single
point solution.
3. A method according to claim 1, wherein said position receiver
first chooses a single timebase from one of said plurality of
autonomous positioning networks and then applies clock corrections
to the timebase of subsequent autonomous positioning networks
before calculating a position solution.
4. A method according to claim 1, wherein said one or more
positioning-unit devices further include network identification
information in their positioning signals.
5. A method according to claim 4, wherein each of said one or more
positioning-unit devices transmits its own network identification
information to all other positioning-unit devices and position
receivers in view.
6. A method according to claim 5, wherein said one or more
positioning-unit devices determine the origin of reference clock
data for each positioning-unit device in view from said network
identification information.
7. A method according to claim 5, wherein said position receiver
determines the origin of reference clock data for each
positioning-unit device in view from said network identification
information.
8. A method according to claim 5, wherein said one or more
positioning-unit devices determine which positioning-unit device
positioning signals are associated with each autonomous positioning
network from said network identification information.
9. A method according to claim 5, wherein said position receiver
determines which positioning-unit device positioning signals are
associated with each autonomous positioning network from said
network identification information.
10. A method according to claim 9, wherein said one or more
positioning-unit devices and said position receiver can determine
which positioning-unit device positioning signals require clock
corrections as a result of having determined which positioning-unit
device positioning signals are associated with each autonomous
positioning network.
11. A positioning system for allowing a position receiver to
determine position solutions from positioning signals from a
plurality of autonomous positioning networks, wherein each
autonomous positioning network is synchronized to a different
timebase, said positioning system comprising: a) a plurality of
autonomous positioning networks, each comprising one or more
positioning-unit devices, wherein: i) at least one positioning-unit
device within a first autonomous positioning network receives
positioning signals from at least one positioning-unit device from
a second autonomous positioning network and measures the timebase
difference between said second autonomous positioning network and
said first autonomous positioning network and subsequently
calculates a timebase clock correction; ii) said at least one
positioning-unit device within said first autonomous positioning
network incorporates said timebase clock correction into its
positioning signals for transmission; iii) other positioning-unit
devices within said first autonomous positioning network receive
and relay said positioning signals incorporating said timebase
clock correction; b) a position receiver situated near the boundary
of two of said plurality of autonomous positioning networks,
wherein said position receiver: i) receives positioning signals
incorporating said timebase clock correction from any
positioning-unit device within said first autonomous positioning
network; ii) receives positioning signals from said one or more
positioning-unit devices from said second autonomous positioning
network; iii) applies said timebase clock correction to said
positioning signals received from said one or more positioning-unit
device from said second autonomous positioning network; and iv)
calculates its own position using said positioning signals from
said first autonomous positioning network and said positioning
signals from said second autonomous positioning network; such that
positioning signals received from positioning-unit devices from
said first and second autonomous positioning networks that are
synchronized to different timebases can be used for position
solutions.
12. A positioning system according to claim 11, wherein said
position receiver determines its own position, velocity and time
(PVT) using a single point solution.
13. A positioning system according to claim 11, wherein said
position receiver first chooses a single timebase from one of said
plurality of autonomous positioning networks and then applies clock
corrections to the timebase of subsequent autonomous positioning
networks before calculating a position solution.
14. A positioning system according to claim 11, wherein said one or
more positioning-unit devices further include network
identification information in their positioning signals.
15. A positioning system according to claim 14, wherein each of
said one or more positioning-unit devices transmits its own network
identification information to all other positioning-unit devices
and position receivers in view.
16. A positioning system according to claim 15, wherein said one or
more positioning-unit devices determine the origin of reference
clock data for each positioning-unit device in view from said
network identification information.
17. A positioning system according to claim 15, wherein said
position receiver determines the origin of reference clock data for
each positioning-unit device in view from said network
identification information.
18. A positioning system according to claim 15, wherein said one or
more positioning-unit devices determine which positioning-unit
device positioning signals are associated with each autonomous
positioning network from said network identification
information.
19. A positioning system according to claim 15, wherein said
position receiver determines which positioning-unit device
positioning signals are associated with each autonomous positioning
network from said network identification information.
20. A positioning system according to claim 19, wherein said
position receiver can determine which positioning-unit device
positioning signals require clock corrections as a result of having
determined which positioning-unit device positioning signals are
associated with each autonomous positioning network.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to systems and
methods for generating precise position determinations for a mobile
apparatus. In particular, the present invention applies to precise
time-of-arrival position determination systems. The present
invention is not constrained by prior art requirements such as
physical connections between transmitter beacons, such as the need
for atomic time standards connected to each transmitter, or the
need for differential correction techniques.
BACKGROUND OF THE INVENTION
[0002] It is well understood in the art that precise
time-of-arrival position determination is dependant upon the
accuracy of the transmitter clocks used. In its most rudimentary
form, three transmitter beacons positioned at known locations and
connected to a common clock via three identical length cables will
suffice as the basis for a time-of-arrival positioning system.
However this rudimentary positioning system is highly impractical
to manufacture and install due to the requirement for precisely
timed cables distributing high frequency timing signals over
potentially large distances between beacons. Alternatively,
precision atomic time standards, which have very low drift rates,
may be installed at each transmitter beacon and monitored using a
reference receiver positioned at a known location and connected to
a reference timebase. In response to positioning signals received
from the transmitter beacons, clock corrections are sent from the
reference receiver via an RF data link to each beacon, for
subsequent retransmission to user equipment. Modern satellite
positioning technologies such as GPS employ this technique, wherein
cesium and rubidium time standards are installed in each GPS
satellite, with the GPS Ground Control Segment continually
monitoring all GPS satellites and up-linking clock corrections to
each satellite every twenty four hours. These corrections are then
rebroadcast via each satellite's navigation message to GPS user
equipment, so that positioning algorithms within the GPS user
equipment can account for satellite clock error. With at least four
GPS satellites in view, a three-dimensional position is
accomplished in GPS user equipment using a standard technique known
as a conventional code-based GPS position solution. This standard
technique is also generally termed "a single point position" by
those skilled in the art.
[0003] Conventional Code-Based GPS Position Solution (Single Point
Position)
[0004] In conventional code-based GPS, the latitude, longitude, and
altitude of any point close to the earth can be calculated from the
propagation times of the positioning signals from at least four GPS
satellites in view. A GPS receiver makes range computations based
on the correlation of internally generated pseudorandom code (PRN)
sequences with received pseudorandom code sequences from each GPS
satellite. The measured ranges are referred to as pseudoranges as
there is a time difference, or offset, between the clocks on the
satellites and the clock within the GPS receiver. It is necessary
to ensure that the receiver's clock is synchronized with the
satellite constellation's clock in order to accurately measure the
elapsed time between a satellite's pseudorandom code sequence
transmission and reception of that pseudorandom code sequence by a
GPS receiver. A navigation message is also transmitted from each
satellite, which includes time information, satellite orbital
information, and satellite clock correction terms. For
three-dimensional positioning a GPS receiver requires four
satellite signals to solve for the four unknowns of position (x, y,
z) and time (t). For two-dimensional (2-D) positioning, altitude is
fixed, and three satellite signals are required to solve for three
unknowns of position (x and y) and time (t). A conventional
code-based GPS position solution is able to provide a GPS receiver,
with at least four satellites in view, the capability to determine
an absolute three-dimensional (3-D) position with an accuracy of
approximately 10 to 20 meters.
[0005] This Conventional Code-based GPS position solution is an
autonomous solution, which can determine position, velocity, and
time (PVT) without differential correction data from reference
receivers. It has therefore become known as a "single point"
position solution in the art.
[0006] Conventional Code-Based Differential GPS (Relative
Positioning)
[0007] With an established accurate atomic timebase the GPS
constellation is only capable of providing a GPS receiver with an
absolute three-dimensional position accuracy of approximately 10 to
20 meters. This is due to the corruption of positioning signals
from six major error sources: (1) ionospheric delay, (2)
tropospheric delay, (3) ephemeris error, (4) satellite clock error,
(5) GPS receiver noise and, (6) multipath. Ionospheric delay is the
varying time delay experienced by electromagnetic waves when
passing through bands of ionized particles in the ionosphere.
Tropospheric delay is the time delay experienced by electromagnetic
waves when passing through moisture in the lower atmosphere.
Ephemeris error is the difference between the actual satellite
location and the position predicted by satellite orbital data.
Receiver noise is the noise generated by the internal electronics
of a GPS receiver. Multipath is the signal delay caused by
localized signal reflections in close proximity to a GPS receiver.
The majority of these error sources are spatially correlated over
relatively short distances (i.e. tens of kilometers). This means
that two different GPS receivers within this proximity to one
another will observe the same errors. It is therefore possible to
improve the spatially correlated error sources using a technique
known as "Differential Correction". A reference receiver placed at
a well-known location computes an assumed pseudorange for each
satellite signal it detects. It then measures the received
pseudoranges from the GPS satellites and subtracts the assumed
pseudoranges from the received pseudoranges, forming a differential
range correction for each satellite in view. The reference receiver
then sends these corrections as digital data to the GPS receiver
via an RF data link. The GPS receiver subsequently adds these
corrections to the pseudoranges it measures (for the same
satellites in view to the reference receiver) before calculating a
position solution. Errors common to both reference receiver and the
GPS receiver are completely removed by this procedure. Uncorrelated
error sources such as multipath and receiver noise remain in the
pseudoranges and subsequently degrade position accuracy. Position
accuracies in the order of several meters are achievable with
code-based differential GPS correction in low multipath
environments.
[0008] Conventional Carrier-Based Differential GPS (Relative
Positioning)
[0009] Conventional carrier-based differential GPS (CDGPS)
calculates the difference between the reference location and the
user location using the differences between the carrier phases of
the satellites measured at the reference receiver and the user
receiver. A CDGPS reference receiver, installed at a well-known
location, calculates simultaneous carrier phase measurements for
all satellites in view, and then broadcasts carrier phase data to
the user receiver via an RF data link. The user receiver also
calculates simultaneous phase measurements for all satellites in
view, and subsequently computes a phase difference to determine the
position of the user receiver with respect to the reference
receiver location. The carrier phase measurements are a running
cycle count based on the Doppler frequency shift present on the
carrier frequencies from the GPS satellites. Each epoch, this
running cycle count (the value from the previous epoch plus the
advance in phase during the present epoch) is available from the
receiver. More specifically, the advance in carrier phase during an
epoch is determined by integrating the carrier Doppler offset over
the interval of the epoch, hence the name Integrated Carrier Phase
(ICP).
[0010] The user receiver can measure the fractional phase plus an
arbitrary number of whole cycles of the carrier, but cannot
directly determine the exact number of whole cycles in the
pseudorange. This number, known as the "integer cycle ambiguity",
must be determined by other means. Traditional strategies for
resolving carrier phase integer ambiguities fall into three broad
classes: search methods, filtering methods, and geometrical
methods. These traditional methods do not yield instantaneous
integer cycle ambiguity resolution. A technique, known as
"wide-laning", has been developed to overcome the non-instantaneous
integer cycle ambiguity problem. Wide-laning multiplies and filters
two carrier frequencies (traditionally the GPS L1 and L2
frequencies) to form a beat frequency signal. This beat frequency
wavelength is significantly longer than the wavelengths of the two
individual carriers. Consequently, resolution of the integers can
be accomplished by using pseudorange observations to determine the
integer ambiguity of the wider "lanes" formed by the beat frequency
signal. These, in turn, greatly reduce the volume of integers that
must be searched to resolve the integer ambiguity.
[0011] The main constraints for CDGPS methods are firstly the
integrity and latency of the RF data link, and, secondly, the lack
of time determination at the user receiver. The data bandwidth of
the RF data link constrains differential data update rates, causing
data latency and degrading position accuracy. Poor reception of
differential data caused by physical obstruction and multipath
causes data corruption, which degrades position accuracy at best,
and results in total link failure and no position update at worst.
The second shortcoming of CDGPS is the lack of time determination.
A conventional single point position solution solves for the four
unknowns of position (x, y, z) and time (t). CDGPS uses a process
known as "double differences", which eliminates the receiver clock
terms for both the reference receiver and the user receiver.
Therefore, the user receiver can determine accurate position with
respect to the reference receiver position, but cannot determine
time. This is unimportant if the user is simply, and only,
interested in position. However, precise knowledge of an accurate
system timebase is very beneficial to many user applications
involving computer networks and telecommunication systems. The lack
of time determination is a major problem associated with CDGPS
prior art systems.
[0012] Pseudolite Augmentation
[0013] Another approach used to aid GPS position determination is
the use of ground-based augmentation systems such as pseudolites.
Pseudolites can be incorporated into Conventional Code and
Carrier-based Differential GPS systems without any additional
infrastructure requirements. They can be used as additional ranging
signals, and also as RF data links to send differential corrections
to user equipment. Alternatively, pseudolites can be synchronized
to the GPS timebase. A GPS receiver determines GPS time from a
conventional code-based GPS solution using at least four GPS
satellites and passes the determined time to a co-located
pseudolite transmitter. The accuracy of the GPS timebase is
constrained by GPS error sources including ionospheric and
tropospheric delay, satellite clock error, satellite position
error, receiver noise, and multipath. Time accuracies of
approximately 50 to 100 nanoseconds are achievable by using the GPS
timebase method, however this translates to position accuracies
only in the order of tens of meters. This accuracy is much too
coarse for precise navigation systems.
[0014] Carrier-Based Differential GPS Using an "Omni-Marker"
Pseudolite
[0015] U.S. Pat. No. 5,583,513 to Cohen, titled "System and Method
for Generating Precise Code-based and Carrier Phase Position
Determinations" describes a differential correction method whereby
a so called "omni-marker" pseudolite serves as a channel for
relaying information to a position receiver for making differential
ranging corrections (Column 6, lines 43 to 46). The omni-marker
pseudolite can be described as a metaphorical mirror, whereby GPS
satellite signals are "reflected" in-phase from the known
omni-marker pseudolite position to the position receiver. Thus, the
out-going carrier and PRN code components of each of the beacon
marker signals is exactly phase coherent with respect to their
incoming counterparts in the GPS signals (Column 6, lines 28 to
32). A position receiver situated in an over-flying aircraft
receives positioning signals from the GPS satellites and also
receives "reflected" GPS positioning signals from the omni-marker
pseudolite, and subsequently computes differential range
measurements.
[0016] Cohen's differential method eliminates the need for a
traditional digital data link, as required by conventional code and
carrier-based differential systems. However, an omni-marker
position receiver must still receive both GPS satellites and
omni-marker signals to compute a differential range measurement.
Receiving omni-marker signals alone will not allow a position
computation. Also, the omni-marker must generate and transmit
individual carrier and PRN components for each GPS satellite in
view, making the omni-marker complex and expensive. Currently, this
would require up to twelve individual transmissions from a single
omni-marker. Further, an omni-marker position receiver requires
double the receive channels of a conventional differential GPS
receiver, adding to the cost and complexity.
[0017] Differential Range Measurements Using "Ground Transceiver"
Pseudolites
[0018] U.S. Pat. No. 6,121,928 to Sheynblat, titled "Network of
Ground Transceivers" describes a differential correction method
whereby a network of so called "ground transmitter" and "ground
transceiver" pseudolites serve as channels for relaying information
to a position receiver for the differential determination of user
position (Column 5, lines 31 to 36). Sheynblat teaches the use of
differential correction to overcome master clock bias (Column 5,
lines 23 to 36) and line biases introduced by the ground
transceiver hardware (Column 5, lines 38 to 67 and Column 6, lines
1 to 23). Sheynblat's differential methodologies and embodiments
include: (i) a user receiver differencing ground transceiver
signals with a ground transmitter signal (Column 5, lines 31 to 36,
and claim 2), (ii) a user receiver differencing multiple master
ground transmitter signals with a ground transceiver (Column 6,
lines 25 to 67, Column 7, lines 1 to 33), and (iii) a user receiver
differencing ground transceiver signals, which incorporate signals
that have been differenced with a satellite signal (Column 1, lines
34 to 67, Column 8, lines 1 to 34). Sheynblat's patent teaches an
advance of differential methodologies but does not teach, show, or
suggest a highly desirable system that would produce single point
position solutions in a roving position receiver from a network of
ground transceivers.
[0019] Prior art systems will not allow time-of-arrival position
determination without requiring at least one of: (a) a physical
connection between transmitter beacons; (b) an atomic time standard
at each transmitter; (c) synchronization to a GPS timebase; or (d)
some form of differential correction. A system that can provide
extremely precise time-of-arrival positioning signals, without any
of these constraints, is highly desirable. The present invention
achieves this desirable goal by chronologically synchronizing a
system of transceivers (hereafter referred to as a Positioning-Unit
Devices), as described below.
OBJECT OF THE INVENTION
[0020] It is an object of the present invention to provide a
positioning system and method for making precise code and carrier
phase position determinations without the need for physical
interconnections between Positioning-Unit Devices.
[0021] It is yet a further object of the present invention to
provide a positioning system and method for making precise code and
carrier phase position determinations without the need of atomic
time standards.
[0022] It is yet a further object of the present invention to
provide a positioning system and method for making precise code and
carrier phase position determinations without the need for a Global
Navigation Satellite System timebase.
[0023] It is yet another object of the present invention to provide
a positioning system and method for making precise code and carrier
phase position determinations without the requirement of
differential correction techniques.
[0024] It is yet a further object of the present invention to
chronologically synchronize Positioning-Unit Devices to a system
timebase, the system timebase not necessarily being of absolute
accuracy.
[0025] It is yet a further object of the present invention to
propagate a reference timebase through geographically distributed
Positioning-Unit Devices.
[0026] It is yet a further object of the present invention to
provide a roving position receiver with chronologically-synchronous
code phase pseudoranges, such that single-point code phase position
solutions can be determined without the aid of differential
correction.
[0027] It is yet a further object of the present invention to
provide a roving position receiver with chronologically-synchronous
carrier phase pseudoranges, such that once integer cycle
ambiguities are resolved, a single-point carrier phase position
solution can be determined without the aid of differential
correction.
[0028] It is yet a further object of the present invention to
provide a roving position receiver with precise network
time-transfer information.
SUMMARY OF THE INVENTION
[0029] The present invention is directed at a positioning system
and method for allowing a position receiver to determine position
solutions from positioning signals from a plurality of autonomous
positioning networks, each of which is synchronized to a different
timebase. Each of the plurality of autonomous networks comprises
one or more positioning-unit device, which is a specialized
transceiver capable of receiving and interpreting reference
positioning signals from a reference transmitter, a position
receiver, another positioning-unit device or all.
[0030] For the purposes of clarity and succinctness, the
positioning system and method of the present invention will be
summarized by describing inter-networking of two autonomous
positioning networks. A person skilled in the art of
inter-networking positioning networks will be able to expand on the
description and apply the system and method of the present
invention to a plurality of positioning networks without the need
of ingenuity. Therefore, the example disclosed in the summary of
the invention is not intended to set or limit the scope of the
invention.
[0031] At least one positioning-unit device within a first
autonomous positioning network receives positioning signals from at
least one positioning-unit device from a second adjacent autonomous
positioning network and measures the timebase difference between
the second network and the first network and subsequently
calculates a timebase clock correction. The at least one
positioning-unit device within the first network incorporates the
timebase clock correction into its positioning signals for
transmission. Other positioning-unit devices within the first
network receive and replay the positioning signals incorporating
the timebase clock correction. A position receiver situated near
the boundary of two of the plurality of networks receives
positioning signals incorporating the timebase clock corrections
from any positioning-unit device within the first network and also
receives positioning signals from one or more positioning-unit
devices from the second network. The position receiver applies the
timebase clock correction to the positioning signals received from
one or more positioning-unit devices from the second network. The
position receiver then calculates its own position using
positioning signals from the first network and the second network.
Hence, although the positioning signals from the positioning-unit
devices from the different networks are synchronized to different
timebases, the position receiver is able to use these signals for
position solutions.
[0032] In a preferred embodiment of the invention, the position
receiver determines its own position, velocity and time (PVT) using
a single point solution.
[0033] In another preferred embodiment of the invention, the
position receiver first chooses a single timebase from one of the
autonomous positioning networks and then applies clock corrections
to the timebase of subsequent positioning networks before
calculating a position solution.
[0034] In yet another preferred embodiment of the invention, the
positioning-unit devices further include network identification
information in their positioning signals.
[0035] In another preferred embodiment of the invention, each
positioning-unit device transmits its own network identification
information to all other positioning-unit devices and position
receivers in view.
[0036] In yet another preferred embodiment of the invention, each
positioning-unit devices determines the origin of reference clock
data for each positioning-unit device in view from the network
identification information.
[0037] In another preferred embodiment of the invention, the
position receiver determines the origin of reference clock data for
each positioning-unit device in view from the network
identification information.
[0038] In yet another preferred embodiment of the invention, each
positioning-unit device determines which positioning-unit device
positioning signals are associated with each of the positioning
networks from the network identification information.
[0039] In another preferred embodiment of the invention, the
position receiver determines which positioning-unit device
positioning signals are associated with each of the positioning
networks from the network identification information.
[0040] In yet another embodiment of the invention, the
positioning-unit devices and the position receiver can determine
which positioning-unit device positioning signals require clock
corrections as a result of having determined which positioning-unit
device positioning signals are associated with each of the
positioning networks from the network identification
information.
[0041] The methods described above wherein Positioning-Unit Devices
chronologically synchronize to at least one reference transmitter
will hereinafter be referred to as "Time-Lock".
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a graphical representation of one embodiment of
Time-Lock according to the present invention, incorporating a
single reference transmitter broadcasting to a plurality of
Positioning-Unit Devices, and a roving position receiver
determining an autonomous single point position solution.
[0043] FIG. 2 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating a
single reference transmitter broadcasting to a single
Positioning-Unit Device.
[0044] FIG. 3 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating a
single reference transmitter broadcasting to a plurality of
Positioning-Unit Devices.
[0045] FIG. 4 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating a
reference transmitter broadcasting through an intermediary
Positioning-Unit Device.
[0046] FIG. 5 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating a
plurality of reference transmitters broadcasting to a single
Positioning-Unit Device.
[0047] FIG. 6 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating a
Wide Area Augmentation System (WAAS) reference transmitter
broadcasting to four Positioning-Unit Devices. The Positioning-Unit
Devices subsequently transmit their own unique chronologically
synchronized positioning signals to a roving position receiver
situated in a satellite-occluded environment.
[0048] FIG. 7 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating a
Positioning-Unit Device reference transmitter broadcasting to three
other Positioning-Unit Devices. The Positioning-Unit Devices
subsequently transmit their own unique chronologically synchronized
positioning signals to a roving position receiver.
[0049] FIG. 8 is a graphical representation of another embodiment
of Time-Lock according to the present invention, incorporating two
autonomous networks of Positioning-Unit Devices, and a roving
position receiver situated at the boundary of the two networks. The
boundary Positioning-Unit Devices subsequently transmits
inter-network corrections to the roving position receiver.
[0050] FIG. 9 is a block diagram of Positioning-Unit Device
hardware, according to the present invention.
OVERVIEW
[0051] A Positioning-Unit Device is a specialized transceiver,
which is positioned at a known location and receives at least one
reference positioning signal from at least one reference
transmitter. Preferably, the reference transmitter is another
Positioning-Unit Device, or a WAAS satellite. In response to the
received reference positioning signals, the Positioning-Unit Device
chronologically synchronizes an internally generated positioning
signal to the reference transmitter timebase and transmits its
unique positioning signal to all other position receivers in view.
The minimum requirement for the formation of an autonomous location
network is at least two Positioning-Unit Devices chronologically
synchronized to one reference transmitter. A roving position
receiver in view of the all the transmitted signals within this
autonomous network is capable of determining autonomous code and
carrier single point position solutions without the need for
differential correction. Furthermore, the reference transmitter
oscillator does not need the inherent stability of atomic time
standards as required by prior art systems, thereby allowing an
inexpensive crystal oscillator to be used as a reference timebase
for the entire location network.
[0052] Thus, as detailed below, a Positioning-Unit Device may serve
as a metaphorical "channel" for distributing chronologically
synchronized positioning signals to a roving position receiver.
This allows the roving position receiver to calculate both code and
carrier-based single point position determinations, without the
need for physical connections between Positioning-Unit Devices,
without requiring atomic time standards or GNSS timebases, and
without requiring differential correction.
[0053] System and Method
[0054] FIG. 1 shows one configuration for a Positioning System that
generates precise position determinations using code and
carrier-based single point position calculations. A plurality of
Positioning-Unit Devices 101-1 & 101-2 are positioned at known
locations with respect to a reference co-ordinate system and
respectively receive at least one reference positioning signal 102
broadcast by at least one reference transmitter 103, which is also
positioned at a known location with respect to a reference
co-ordinate system. In response to the received reference
positioning signal 102 the Positioning-Unit Devices 101-1 &
101-2 transmit one or more unique positioning signals 104-1 &
104-2, which are chronologically synchronized to the reference
transmitter 103. A roving position receiver 105, situated within
the network of devices 101-1, 101-2 & 103, receives reference
positioning signals 102 from the reference transmitter 103 and
unique positioning signals 104-1 & 104-2 from the
Positioning-Unit Devices 101-1 & 101-2, and autonomously
calculates both code and carrier-based single point position
determinations from the network of chronologically synchronized
positioning signals.
[0055] Time-Lock
[0056] Time-Locked Positioning-Unit Devices synchronize to a common
chronological timebase, which can be of arbitrary value and have
arbitrary variance. Therefore any simple and inexpensive clock
source, such as a crystal oscillator, will suffice as the reference
clock in a reference transmitter. In the preferred embodiment a
temperature compensated crystal oscillator (TCXO) or better is
used. A Positioning-Unit Device first acquires a reference
transmitter positioning signal, and calculates a so-called
time-of-flight offset from the known co-ordinates of the reference
transmitter and the known co-ordinates of the Positioning-Unit
Device. The time-of-flight offset takes into consideration the
propagation time delay experienced by the reference positioning
signal whilst traveling from the reference transmitter to the
Positioning-Unit Device. In free space, electromagnetic waves
travel approximately one meter every three nanoseconds. Next, the
Positioning-Unit Device applies the time-of-flight offset to an
internally generated positioning signal and aligns this positioning
signal to the incoming reference positioning signal, thus bringing
the internally generated positioning signal into chronological
alignment with the reference transmitter. Specifically, Time-Lock
is achieved when a Positioning-Unit Devices' internally generated
positioning signal has frequency coherence with an incoming
reference positioning signal, and chronological coherence with the
reference transmitter timebase.
[0057] A reference positioning signal is transmitted via a radio
frequency (RF) carrier from a reference transmitter. The reference
positioning signal can be generated from any valid time source,
which may include Positioning-Unit Devices, Wide Area Augmentation
System (WAAS) satellites, Global Navigation Satellite System (GNSS)
satellites, Pseudolites, or any combination of valid sources.
Referring now to FIG. 2, a Positioning-Unit Device 201 located at a
known distance from a reference transmitter 202 receives a
reference positioning signal 203 transmitted by the reference
transmitter 202. The reference positioning signal 203 has a carrier
component, a unique pseudo-random code component, and a data
component. The Positioning-Unit Device 201 incorporates a position
receiver 204 and a co-located transmitter 205. The position
receiver 204 is capable of receiving positioning signals from all
reference positioning signals in view 203, and also positioning
signals from its co-located transmitter 205. In response to the
received reference positioning signal 203, the Positioning-Unit
Device 201 transmits a so-called slave positioning signal 206 from
its transmitter 205, which is received by the Positioning-Unit
Device position receiver 204. The slave positioning signal 206 has
a carrier component, a unique pseudo-random code component, and a
data component. The Positioning-Unit Device position receiver 204
receives and simultaneously samples the reference positioning
signal 203 from the reference transmitter 202 and the slave
positioning signal 206 from the co-located transmitter 205. A
transmission time difference is then calculated between the
received reference positioning signal 203 and the received slave
positioning signal 206. The transmission time difference, as used
in the preferred embodiment, is determined by: [0058] (a) Comparing
the integrated carrier phase (ICP) measurements determined from the
carrier components of the reference positioning signal 203 and the
slave positioning signal 206 to determine a carrier frequency
difference. [0059] (b) Demodulating and comparing the navigation
data components from the reference positioning signal 203 and the
slave positioning signal 206 to determine a coarse transmission
time difference. [0060] (c) Comparing the pseudorange measurements
determined from the pseudo-random code components of the reference
positioning signal 203 and the slave positioning signal 206 to
determine a code pseudorange difference. [0061] (d) Comparing the
instantaneous carrier phase measurements determined from the
carrier components of the reference positioning signal 203 and the
slave positioning signal 206 to determine a carrier phase
difference.
[0062] For precise time synchronization of the slave positioning
signal 206 to the reference transmitter 202 timebase the signal
propagation delay between the reference transmitter antenna 207 and
the Positioning-Unit Device position receiver antenna 208 must be
accounted for. The known geometrical distance in meters 209 from
the reference transmitter antenna 207 to the Positioning-Unit
Device position receiver antenna 208 can be converted to a signal
time-of-flight by the formula: time-of-flight=distance/speed of
light. The Positioning-Unit Device 201 incorporates a steered
transmitter clock 210, which can be adjusted in frequency by the
Positioning-Unit Device CPU 211. The correction to the steered
transmitter clock 210 is determined by the Positioning-Unit Device
CPU 211 from the time difference between the reference positioning
signal 203 and the slave positioning signal 206 which is measured
by the Positioning-Unit Device receiver 204, and offset by the
reference positioning signal time-of-flight 209. This brings the
slave positioning signal 206 into chronological synchronization
with the reference transmitter 202 timebase.
[0063] The process of differencing the received reference
positioning signal 203 with the slave positioning signal 206
eliminates the Positioning-Unit Device position receiver clock
term, thereby allowing the Positioning-Unit Device 201 to follow
the reference transmitter 202 timebase without any clock bias
caused by the local Positioning-Unit Device oscillator 212.
Furthermore, differencing between two channels of the same position
receiver 204 eliminates any receiver line bias or group delay
caused by the position receiver electronics.
[0064] Control States of a Positioning-Unit Device
[0065] In the preferred embodiment, Positioning-Unit Devices
Time-Lock to reference transmitters using the following control
states:
[0066] State 0: Reset
[0067] Reset all hardware
[0068] State 1: Acquire Reference
[0069] The Positioning-Unit Device CPU 211 initiates a search for a
reference positioning signal 203 by the Positioning-Unit Device
position receiver 204.
[0070] State 2: Lock to Reference
[0071] The Positioning-Unit Device position receiver 204 acquires a
reference positioning signal 203 and reference transmitter 202
position and time is demodulated from its navigation data component
by the Positioning-Unit Device CPU 211.
[0072] State 3: Synchronize Slave
[0073] The Positioning-Unit Device CPU 211 waits to allow for
coarse time alignment with the reference positioning signal
navigation data component. An internal clock generator is then
initiated by the CPU 211.
[0074] State 4: Initialize Slave
[0075] The Positioning-Unit Device CPU 211 determines an
appropriate and unique PRN code sequence for this particular
Positioning-Unit Device 201 and assigns this PRN code sequence to
the Positioning-Unit Device transmitter 205. The current frequency
offset for the reference positioning signal 203 (relative to the
Positioning-Unit Device oscillator 212) is also assigned to the
Positioning-Unit Device steered transmitter clock 210 by the
Positioning-Unit Device CPU 211. This serves to initialize the
Positioning-Unit Device transmitter 205 to a frequency that is
approximately the same as the frequency of the reference
positioning signal 203. The Positioning-Unit Device CPU 211 also
assigns the determined PRN sequence to a free receiver channel in
the Positioning-Unit Device position receiver 204. The receiver
channel is initialized with the same frequency offset and
pseudorandom code phase value as the Positioning-Unit Device
transmitter 205, in order to aid acquisition of the slave
positioning signal 206 by the Positioning-Unit Device position
receiver 204. The Positioning-Unit Device then initiates a
transmission of the slave positioning signal 206.
[0076] State 5: Acquire Slave
[0077] The Positioning-Unit Device position receiver 204 initiates
a search for the slave positioning signal 206.
[0078] State 6: Lock to Slave
[0079] The Positioning-Unit Device position receiver 204 acquires
the slave positioning signal 206 and a coarse slave time is
demodulated from its navigation data component.
[0080] State 7: Reference/Slave Frequency Lock
[0081] The simultaneous integrated carrier phase (ICP) measurements
for the reference positioning signal 203 and slave positioning
signals 206 are initialized (zeroed) and differenced by the
Positioning-Unit Device position receiver 204. This differenced
value represents the frequency and phase difference between the
reference positioning signal 203 and the slave positioning signal
206. A control loop within the positioning unit device CPU 211,
continuously applies corrections to the Positioning-Unit Device
steered transmitter clock 210 to maintain a zero ICP difference
between the reference positioning signal 203 and the slave
positioning signal 206, thus maintaining Frequency Lock.
[0082] Alternatively the received reference positioning signal
frequency offset value, as measured by the Positioning-Unit Device
position receiver 204, can be fed directly to the Positioning-Unit
Device steered transmitter clock 210 to create a so called
"Frequency Tracking System" (FTS). The steered transmitter clock
210 simply emulates the frequency offset of the incoming reference
positioning signal 203, thus maintaining Frequency Lock. This
method requires the Positioning-Unit Device oscillator 212 to be
common between position receiver 204 and transmitter 205.
[0083] State 8: Reference/Slave Code-Lock
[0084] Once State 7 Reference/Slave Frequency Lock is achieved the
time difference between the reference positioning signal 203 and
the slave positioning signal 206 can be accurately measured and any
time bias eliminated. Reference/Slave Code-Lock is achieved when
the Positioning-Unit Device steered transmitter clock 210 is slewed
the requisite amount of time to bring the reference and slave
positioning signals into PRN code alignment. The time-of-flight
value 209 is used to offset the measured reference-slave time
difference to remove the effect of the reference signal propagation
delay, and the calculated time difference is then applied as a
clock correction to the Positioning-Unit Device steered transmitter
clock 210. The clock correction is achieved by engaging the
Frequency Tracking System (FTS), and applying an additional
frequency offset to the steered transmitter clock 210 for a
predetermined time period. This additional frequency offset allows
the slave positioning signal 206 to slew in time until it becomes
time coherent with the reference transmitter 202 timebase. Once
this Time Slew is completed the control loop is re-engaged.
Alternatively, Code-Lock can be achieved by slewing the
Positioning-Unit Device transmitter 205 PRN code generator the
requisite amount of code phase (chips) whilst maintaining Frequency
Lock.
[0085] Code-Lock is based on PRN code accuracy, which is inherently
noisy. In the preferred embodiment stationary Positioning-Unit
Devices filter PRN code noise to a sub carrier cycle level.
[0086] State 9: Reference/Slave Phase Lock
[0087] Once State 7 Reference/Slave Frequency Lock and State 8
Reference/Slave Code-Lock are achieved, two time errors still
remain that must be corrected: (1) a 180 degree phase ambiguity
and; (2) a time-of-flight phase offset. [0088] (1) Correcting a 180
degree phase ambiguity: Data is demodulated from a PRN code
positioning signal using a specialized Phase-Lock-Loop, well-known
in the art as a "Costas Loop". The Costas Loop technique inherently
incorporates a 180 degree phase ambiguity, and therefore can
acquire and track positioning signals with a half cycle ambiguity.
This half cycle ambiguity represents an approximate 200 picosecond
time offset at 2.4 GHz. The Costas Loop ambiguity can be resolved
by reference to a predetermined sequence of data bits, generally
referred to as a preamble, transmitted in the navigation data
component by transmitters within the location network. When the
Costas Loop ambiguity is resolved, an arbitrary fixed phase
difference becomes evident between the position receiver phase
registers of the Frequency-Locked reference and slave positioning
signals. This arbitrary phase offset is due to the arbitrary phase
of the slave positioning signal and is adjusted in the following
step (2) below. [0089] (2) Correcting Time-of-Flight Phase Offset:
A fractional-cycle time-of-flight phase offset is present due to
the reference positioning signal propagation delay between the
reference transmitter antenna 207 and the Positioning-Unit Device
antenna 208. The geometrical distance 209 between the reference
transmitter and the Positioning-Unit Device can be represented as a
number of whole carrier cycles (the integer component) 213, plus a
fractional carrier cycle (the fractional component) 214. The
time-of-flight phase offset is the fractional cycle amount 214
computed from the known geometrical distance between the reference
transmitter antenna 207 and the Positioning-Unit Device antenna
208. The integer component 213 is corrected in the State 8
Reference/Slave Code-Lock control state described above. The
fractional component 214 however, is too fine to be corrected in
the State 8 Reference/Slave Code-Lock state, and must therefore be
corrected as a carrier phase adjustment. The Frequency Tracking
System (FTS) is engaged and the Positioning-Unit Device steered
transmitter clock 210 is time slewed the requisite fractional-cycle
amount (from its currently measured arbitrary phase value
determined in step (1) above) to a newly determined time-of-flight
phase value. The Time-Lock-Loop (TLL) is then re-engaged. The
Positioning-Unit Device carrier phase slave positioning signal 206
emanating from the Positioning-Unit Device antenna 208 is now
chronologically synchronized with the reference transmitter 202
carrier phase positioning signal emanating from the reference
transmitter antenna 207.
[0090] State 10: Reference/Slave All Lock
[0091] Once all of the above states have been achieved, the CPU 211
declares Time-Lock and the Positioning-Unit Device 201 begins
transmission of its now fully synchronized unique positioning
signal 215. The Positioning-Unit Device unique positioning signal
215 is now chronologically synchronized to the reference
transmitter 202 timebase with an accuracy of picoseconds, a
capability that is substantially beyond the capacity of any prior
art.
[0092] Unique Positioning Signals
[0093] In the preferred embodiment each Positioning-Unit Device
transmits a unique positioning signal, which consists of a carrier
component, a pseudorandom code component, and a navigation data
component. The carrier component is a sinusoidal radio frequency
wave preferably transmitted in the 2.4 GHz ISM band, though the
method of the present invention is equally applicable to other
frequency bands. The pseudorandom number (PRN) code component is
modulated upon the carrier component, and consists of a unique code
sequence which can be distinguished amongst other pseudorandom code
sequences transmitted by other devices on the same carrier
frequency. This technique is known as Code Division Multiple Access
(CDMA), and is well-known in the art. The navigation data component
is proprietary information modulated upon the pseudorandom code
component, and provides a communications link to transfer
navigation information to Positioning-Unit Devices and roving
position receivers. Navigation information may include network
time, Positioning-Unit Device locations, metaphorical "reference
clock lineage" information, and other desired network data.
[0094] Time-Lock Configurations
[0095] Time-Lock may be implemented in many different
configurations. These configurations include: [0096] 1. A single
reference transmitter broadcasting to a single Positioning-Unit
Device. [0097] 2. A single reference transmitter broadcasting to a
plurality of Positioning-Unit Devices. [0098] 3. One or more
reference transmitters broadcasting through intermediary
Positioning-Unit Devices [0099] 4. A plurality of reference
transmitters broadcasting to one or more Positioning-Unit Devices.
[0100] 5. Point position time synchronization
[0101] A Single Reference Transmitter Broadcasting to a Single
Positioning-Unit Device.
[0102] A single reference transmitter can be used to broadcast a
reference positioning signal to a single Positioning-Unit Device.
FIG. 2 shows a Positioning-Unit Device 201 situated at a known
location, and a reference transmitter 202 also situated at a known
location. The Positioning-Unit Device 201 receives the reference
positioning signal 203 transmitted by the reference transmitter 202
and the slave positioning signal 206 transmitted by the
Positioning-Unit Device transmitter 205. In response to the
received reference positioning signal 203 the Positioning-Unit
Device 201 determines the reference positioning signal propagation
delay 209 and applies an appropriate transmitter clock correction
to chronologically synchronize the carrier component, unique PRN
code component, and data component of its internally generated
slave positioning signal 206 to the carrier component, PRN code
component, and data component of the reference transmitter
positioning signal 203. The Positioning-Unit Device subsequently
transmits a unique positioning signal 215, which is chronologically
synchronized to the reference transmitter 202 timebase.
[0103] Two positioning signals are not sufficient to determine a
position solution in a roving position receiver. However, if the
reference transmitter is a WAAS satellite the Time-Locked
Positioning-Unit Device signal will be synchronous with GPS time to
picosecond level, and therefore can be used by a position receiver
as an additional precise ranging source for a conventional
code-based GPS solution.
[0104] A Single Reference Transmitter Broadcasting to a Plurality
of Positioning-Unit Devices.
[0105] A single reference transmitter can be used to form a network
of Positioning-Unit Devices when a plurality of Positioning-Unit
Devices is in clear view of the reference transmitter.
[0106] FIG. 3 shows a plurality of Positioning-Unit Devices
situated at known locations 301-1 & 301-2, and a reference
transmitter 302 also situated at a known location. The
Positioning-Unit Devices 301-1 & 301-2 receive the reference
positioning signal 303 transmitted by the reference transmitter
302. In response to the received reference positioning signal 303
each Positioning-Unit Device 301-1 & 301-2 determines its
respective signal propagation delay 304-1 & 304-2 from the
reference transmitter 302 and applies an appropriate transmitter
clock correction to chronologically synchronize the carrier
component, unique PRN code component, and data component of their
internally generated positioning signals to the carrier component,
PRN code component, and data component of the reference transmitter
positioning signal 303. Each Positioning-Unit Devices subsequently
transmits unique positioning signals 305-1 & 305-2, which are
chronologically synchronized to the reference transmitter 302
timebase.
[0107] One or More Reference Transmitters Broadcasting Through
Intermediary Positioning-Unit Devices.
[0108] One or more time-synchronized reference transmitters can be
used to form a network of Positioning-Unit Devices, without all
Positioning-Unit Devices being in clear view of a reference
transmitter. In this configuration the timing signal is cascaded
via intermediary Positioning-Unit Devices. When an intermediary
Positioning-Unit Device declares Time-Lock, subsequent
Positioning-Unit Devices can use this intermediary Positioning-Unit
Device as their reference positioning signal.
[0109] FIG. 4 shows a reference transmitter 401 situated at a known
location, and a first Positioning-Unit Device 402 also situated at
a known location. The first Positioning-Unit Device 402 receives
the positioning signal 403 transmitted by the reference transmitter
401. In response to the received reference positioning signal 403
the first Positioning-Unit Device 402 determines the signal
propagation delay 404 from the reference transmitter 401 and
applies an appropriate clock correction to chronologically
synchronize the carrier component, unique PRN code component, and
data component of its internally generated positioning signal to
the carrier component, PRN code component, and data component of
the reference transmitter positioning signal 403. The first
Positioning-Unit Device 402 subsequently transmits a unique
positioning signal 405, which is chronologically synchronized to
the reference transmitter 401 timebase.
[0110] A Second Positioning-Unit Device 406 situated at a known
location, but not in view of the reference positioning signals 410
due to signal obstruction caused by building 409, subsequently
receives positioning signal 405 from the first Positioning-Unit
Device 402. In response to the received positioning signal 405 the
second Positioning-Unit Device 406 determines the signal
propagation delay 407 from the first Positioning-Unit Device 402
and applies an appropriate clock correction to chronologically
synchronize the carrier component, unique PRN code component, and
data component of its internally generated positioning signal to
the carrier component, PRN code component, and data component of
the first Positioning-Unit Device positioning signal 405. The
second Positioning-Unit Device 406 subsequently transmits a unique
positioning signal 408 incorporating a carrier component, PRN code
component, and data component. This unique positioning signal 408
is chronologically synchronized to the first Positioning-Unit
Device 402 timebase, which is also chronologically synchronized to
the reference transmitter 401 timebase.
[0111] A Plurality of Reference Transmitters Broadcasting to One or
More Positioning-Unit Devices.
[0112] A plurality of time-synchronized reference transmitters can
be used to broadcast reference positioning signals to one or more
Positioning-Unit Devices. In this configuration any reference
signal error sources, such as multipath and tropospheric delay, can
be averaged between reference transmitters to improve timebase
accuracy.
[0113] FIG. 5 shows a Positioning-Unit Device 501 situated at a
known location, and a plurality of reference transmitters 502-1
& 502-2 with common timebase, also situated at known locations.
The Positioning-Unit Device 501 receives the reference positioning
signal 503-1, 503-2 transmitted by the reference transmitters 502-1
& 502-2. In response to the received reference positioning
signal 503-1, 503-2 the Positioning-Unit Device 501 determines the
signal propagation delays 504-1 & 504-2 from each reference
transmitter 502-1 & 502-2 and applies an appropriate clock
correction to chronologically synchronize the carrier component,
unique PRN code component, and data component of its internally
generated positioning signal to the carrier components, PRN code
components, and data components of the two reference transmitter
positioning signals 503-1 & 503-2. The Positioning-Unit Device
501 subsequently transmits a unique positioning signal 505, which
is chronologically synchronized to the timebase of the reference
transmitters 502-1 & 502-2.
[0114] Point Position Time-Lock
[0115] A Positioning-Unit Device is also capable of synchronizing
to a network timebase without the geometrical distance (reference
positioning signal propagation delay) between reference
transmitters and Positioning-Unit Device being known. For this
embodiment of Time-Lock, at least four Time-Locked Positioning-Unit
Devices must be in view. The Positioning-Unit Device, requiring to
enter the network, self-surveys its three-dimensional position by
calculating a single point position, which incorporates the
Positioning-Unit Device position receiver clock offset. The
Positioning-Unit Device position receiver clock offset accurately
provides network time (relative to the local position receiver
clock), which the Positioning-Unit Device slave transmitter can use
as an accurate network timebase. In the preferred embodiment the
Positioning-Unit Device uses a single point carrier solution to
determine accurate network time to the picosecond level, a
capability that is substantially beyond the capacity of prior art
systems.
[0116] WAAS Reference
[0117] In the preferred embodiment a reference transmitter is a
Wide Area Augmentation System (WAAS) Satellite. WAAS satellites are
geostationary communications satellites, which transmit GPS
differential corrections to GPS receivers. WAAS satellites also
transmit a unique positioning signal on the GPS L1 carrier
frequency of 1575.42 MHz. This unique positioning signal is
accurately synchronized to GPS time, with corrections provided for
UTC. Therefore, a WAAS satellite makes an ideal reference
transmitter, which is synchronous to the world standard timebase of
UTC.
[0118] In the preferred embodiment a Positioning-Unit Device
position receiver incorporates means for receiving positioning
signals from other Positioning-Unit Devices in the 2.4 GHz ISM
band, and also positioning signals from WAAS and GNSS satellites in
the L band frequencies. A Positioning-Unit Device may use a WAAS
satellite as a reference transmitter and Time-Lock its 2.4 GHz
slave positioning signal to the 1575.42 MHz WAAS positioning
signal. Time-Lock between disparate carrier frequencies is
initiated by coherently down-converting the incoming WAAS and
Positioning-Unit Device carriers to a common baseband frequency in
the Positioning-Unit Device position receiver. Time-Lock is then
performed with the methods previously described. Coherent
down-conversion requires the local oscillators in the
Positioning-Unit Device position receiver to be driven from a
common oscillator. In the preferred embodiment the common
oscillator generates clock information for all components of a
Positioning-Unit Device, including the position receiver,
transmitter, and central processing unit. Line biases and group
delay are taken into consideration when computing inter-frequency
Time-Lock, due to the disparate receive paths of the WAAS and
Positioning-Unit Device carrier frequencies prior to
down-conversion.
[0119] Referring now to FIG. 6, Positioning-Unit Devices 601-1,
601-2, 601-3 & 601-4 are placed in known locations with clear
view of the sky, and preferably in elevated positions such as on
top of hills 602-1 & 602-2 and/or tall buildings 603-1 &
603-2. If required, a directional receive antenna 604-1, 604-2,
604-3 & 604-4 may also be incorporated with each
Positioning-Unit Device 601-1, 601-2, 601-3 & 601-4 and
directed toward a geostationary WAAS satellite 605 (though these
additional antennas are preferred but not essential for the
method). Deploying directional antennas on Positioning-Unit Devices
helps to mitigate multipath and improve received signal to noise
ratios of the WAAS signal, which in turn improves reference
timebase accuracy. Each Positioning-Unit Device 601-1, 601-2,
601-3, & 601-4 Time-Locks to the WAAS satellite signal 606,
thus creating a precision UTC synchronized network with picosecond
accuracy. A position receiver 607 held by a pedestrian 608 is
situated inside a building 609. The WAAS satellite signal 606
cannot penetrate the building 609 due to its low signal power.
However, Positioning-Unit Device signals 610-1, 610-2, 610-3, &
610-4 from the Positioning-Unit Devices 601-1, 601-2, 601-3, &
601-4 can penetrate the building 609 due to their close proximity.
The position receiver 607 is capable of receiving Positioning-Unit
Device positioning signals from all four Positioning-Unit Devices,
which allows precise single point position determination in
satellite occluded regions. In addition, once the position receiver
607 has calculated a position solution, UTC can be determined
accurately. The present invention therefore also provides precision
UTC time transfer in satellite occluded regions. Moreover, when the
Position receiver 607 exits the building 609, signals from any
Positioning-Unit Devices 601-1, 601-2, 601-3 & 601-4, WAAS
satellites 605, or GNSS satellites in view can be used to form an
overdetermined position solution, adding position integrity to the
pedestrians calculated position.
[0120] Intermediary WAAS Reference
[0121] Positioning-Unit Devices placed in clear view of the WAAS
satellite may also be used as intermediary reference signals in
another embodiment. Positioning-Unit Devices that are unable to
receive WAAS satellite signals may use intermediary "backbone"
Positioning-Unit Devices as their time reference source. Therefore,
UTC may be distributed throughout the network without all
Positioning-Unit Devices being in clear view of the reference WAAS
satellite.
[0122] Positioning-Unit Device Reference
[0123] In the event of a WAAS satellite not being available, it is
preferable that at least one Positioning-Unit Device provides the
timebase for a network of Positioning-Unit Devices. Referring now
to FIG. 7, a first Positioning-Unit Device 701 situated at a known
location is designated as the reference transmitter and creates a
system timebase from its internally generated clock 702. Two
subsequent Positioning-Unit Devices 703 & 704 situated at known
locations Time-Lock to the first Positioning-Unit Device reference
positioning signal 705. A fourth Positioning-Unit Device 706, which
is situated at a known location but out of range of the first
Positioning-Unit Device 701, Time-Locks to the second
Positioning-Unit Device unique positioning signal 707. Therefore
the system allows accurate cascaded time transfer through
intermediary Positioning-Unit Devices. Position receiver 708
receives time-synchronous positioning signals 709 being transmitted
by all Positioning-Unit Devices in view 701, 703, 704, & 706
and subsequently calculates a single point position solution.
Further, the time calculated at the position receiver 708 will be
chronologically-synchronous with the reference clock 702 of the
reference Positioning-Unit Device 701. The arbitrary time value of
the reference clock 702 within the Positioning-Unit Device 701 is
of no consequence if the user is only concerned with position
determination. If the user wishes time alignment with a global
timebase, then the reference clock 702 within the reference
Positioning-Unit Device 701 needs to be steered to UTC.
[0124] Positioning-Unit Device Reference Steered by GNSS
Timebase
[0125] In the event of a WAAS satellite signal not being available,
and alignment to a global timebase is necessary for the network, it
is preferable that a reference Positioning-Unit Device be steered
to UTC by a GNSS timebase. A GNSS timebase requires a position
receiver, positioned at a known location, to compute a time
solution using at least one GNSS satellite. Time accuracies in the
order of 50 nanoseconds are achievable using this technique.
Relative time accuracy between Positioning-Unit Devices, which are
Time-Locked to the reference Positioning-Unit Device, will remain
at the picosecond level.
[0126] Inter-Network Position Solutions
[0127] A plurality of reference transmitters may be used to create
a plurality of autonomous networks. An autonomous network has its
own unique timebase, which is generated by the reference
transmitter. Position receivers that are situated within a single
autonomous network can determine position, velocity, and time (PVT)
using a single point position solution. The position receiver's
time will be determined relative to the network timebase (i.e. the
reference transmitter clock) and is termed an intra-network
position solution. Position receivers that are located at the
boundary of two autonomous networks, and receiving positioning
signals from Positioning-Unit Devices from both networks, must
first distinguish between the two network timebases before
determining their position. This can be described as an
inter-network position solution, and requires a roving position
receiver to first chose a single timebase and apply clock
corrections to the second timebase before computing a single point
position solution.
[0128] In the preferred embodiment, Positioning-Unit Devices also
include network identification (Network I.D.) information in their
network data. Network I.D. maps the reference-time
interconnectivity of Positioning-Unit Devices, such that
Positioning-Unit Devices and position receivers can determine the
origin and metaphorical "lineage" of reference clock data for each
Positioning-Unit Device in view. This allows a Positioning-Unit
Device or position receiver located at the boundary of two
autonomous networks to determine which Positioning-Unit Devices are
associated with each network, and therefore which Positioning-Unit
Devices require clock correction within the roving position
receiver position calculations. Each Positioning-Unit Device
receives Network I.D. information from all other Positioning-Unit
Devices in view, and in response generates and transmits its own
Network I.D. information to all other Positioning-Unit Devices and
roving position receivers in view.
[0129] Referring now to FIG. 8, there is depicted two autonomous
networks of Positioning-Unit Devices 801 & 802.
Positioning-Unit devices 801-1, 801-2, and 801-3 are in view of one
another and communicate to each other via positioning signals
803-1, 803-2, and 803-3. Positioning-Unit devices 802-1, 802-2, and
802-3 are in view of one another and communicate to each other via
positioning signals 804-1, 804-2, and 804-3. A Positioning-Unit
Device situated near the boundary of the two networks 801-3
receives Positioning-Unit Device positioning signals 804-3 from an
adjacent-network Positioning-Unit Device 802-3 and measures the
timebase difference, or clock bias, of the adjacent network
timebase with respect to its own network 801 timebase. The
Positioning-Unit Device 801-3 transmits clock corrections for the
adjacent-network Positioning-Unit Devices 802-1, 802-2, & 802-3
in its network data, which is incorporated in its positioning
signal 803-3. Positioning signals from only one adjacent-network
Positioning-Unit Device 802-3 needs to be received by
Positioning-Unit Device 801-3 when forming a network correction
value, as all clocks in an autonomous network are time coherent.
Furthermore, only one Positioning-Unit Device 801-3 need measure an
adjacent network, as its transmitted network clock corrections
which are sent in the network data of its positioning signal 803-3,
are received and relayed to other Positioning-Unit Devices within
its own network 801, for subsequent transmission 803-1 & 803-2
to roving position receivers 805.
[0130] The transmitted correction value, transmitted in the network
data of the Positioning-Unit Device 801-3 positioning signal 803-3,
is received by a position receiver 805 that is roving between
networks 801 & 802. The roving position receiver applies the
received network clock corrections from Positioning-Unit Device
801-3 and subsequently calculates a single point position solution
using all Positioning-Unit Device positioning signals in view
803-1, 803-2, 803-3, and adjacent network Positioning-Unit Device
positioning signal 804-3. With a single point position solution
calculated the roving position receiver 805 clock will be time
coherent with the network 801 timebase that provided the clock
corrections. Furthermore, the adjacent network Positioning-Unit
Device 802-3 can also receive positioning signals 803-3 from the
first Positioning-Unit Device 801-3 and measure the timebase
difference of the first network 801 with respect to its own network
802 timebase. The adjacent-network Positioning-Unit Device 802-3
then transmits clock corrections for the its adjacent-network
Positioning-Unit Devices 801-1, 801-2, & 801-3 in its network
data within its positioning signal 804-3, thereby allowing roving
position receivers 805 to choose between timebases, if
required.
[0131] Multiple Frequency Time-Lock
[0132] In the preferred embodiment a plurality of positioning
signals are transmitted on a plurality of frequencies from each
Positioning-Unit Device. Position receivers subsequently interpret
the plurality of positioning signals to generate a so called
wide-lane for integer carrier cycle ambiguity resolution (AR). RF
carrier signals experience a time delay whilst passing through
transmitter and receiver electronics, known as "group delay". Group
delay can vary many nanoseconds, depending on frequency and ambient
temperature. Therefore, a plurality of carrier frequencies
generated from a common oscillator and transmitted through the same
transmit path will experience unequal time delays due to the
carrier frequency differences, and further experience varying time
delays caused by temperature change of transmitter electronics.
This causes transmitted positioning signals that are not phase
coherent. Non phase-coherent positioning signals will induce range
errors into the wide-lane ambiguity resolution (AR) process.
[0133] A Positioning-Unit Device can eliminate the non-coherent
phase problem from a reference transmitter by transmitting a
plurality of frequency-diverse positioning signals, which are
individually time-locked to their respective incoming reference
positioning signals. A Positioning-Unit Device incorporates a
plurality of steered transmitter clocks, capable of steering a
plurality of positioning signals, which are transmitted on a
plurality of carrier frequencies. The Positioning-Unit Device
position receiver tracks the plurality of frequency-diverse
reference positioning signals, and also tracks the plurality of
frequency-diverse slave positioning signals. The Positioning-Unit
Device Time-Locks each frequency-diverse reference positioning
signal to its respective frequency-diverse slave positioning
signal, such that each slave positioning signal is chronologically
synchronized with the reference transmitter. The Positioning-Unit
Device then transmits its plurality of frequency-diverse
positioning signals, which are time-coherent with the group delay
from the reference transmitter. With at least three time-locked
Positioning-Unit Devices in view, a position receiver determines
wide-lane integer ambiguity resolution (AR) from each
Positioning-Unit Device in view. The reference transmitter group
delay has created an AR range error, which is common amongst the
Time-Locked Positioning-Unit Devices. Therefore the same AR induced
range error is evident on each Positioning-Unit Device pseudorange.
The position receiver interprets this common pseudorange error as a
receiver clock bias and eliminates the error in the single point
position calculation.
[0134] Network Co-ordinate Frame
[0135] A prerequisite for Time-Lock is the knowledge of the
Positioning-Unit Device positions with respect to a reference
co-ordinate frame. Any valid co-ordinate frame may be used, but in
the preferred embodiment the Earth Centered Earth Fixed (ECEF)
co-ordinate frame is used, which is also the co-ordinate frame used
by GPS and WAAS. In the preferred embodiment, Positioning-Unit
Devices self-survey from GNSS, and/or WAAS, and/or other
Positioning-Unit Devices to determine an ECEF co-ordinate.
[0136] Transmission Frequency
[0137] In the preferred embodiment, Positioning-Unit Devices
transmit in the unlicensed Industrial Scientific Medical (ISM) band
of 2.4 GHz to 2.48 GHz. The 2.4 GHz ISM band allows the development
of Positioning-Unit Device networks without regulatory constraint,
and without interference to current navigation systems such as GPS.
The 2.4 GHz ISM band also allows 83.5 MHz bandwidth, which can be
used for increased chipping rates of direct sequence spread
spectrum positioning signals, or the use of multiple carriers for
widelane integer cycle ambiguity resolution.
[0138] Description of Positioning-Unit Device Hardware
[0139] In the preferred embodiment, a Positioning-Unit Device
incorporates a position receiver, a transmitter, a central
processing unit (CPU), and a common oscillator. The position
receiver incorporates a plurality of receive channels capable of
receiving a plurality of positioning signals, each comprising a
carrier component, a PRN code component, and a data component. The
transmitter incorporates at least one RF carrier generator, at
least one PRN code generator, and at least one steered clock. The
CPU comprises means for interpreting positioning signals received
by the position receiver, responsive means to control the
transmitter steered clock and means to generate navigation data.
The common oscillator provides a coherent local timebase for all
components of the Positioning-Unit Device.
[0140] Referring now to FIG. 9, there is depicted a
Positioning-Unit Device 901 incorporating a position receiver 902,
a transmitter 903, a Central Processing Unit (CPU) 904, and a
common oscillator 905. The position receiver 902 incorporates a
plurality of receive channels 906, and the transmitter 903
incorporates one or more of carrier generator 907, one or more of
code generator 908, and one or more of steered clock 909. The CPU
904 includes means for position receiver communication 910, means
for transmitter communication 911, and means for transmitter
steered clock communication 912.
[0141] Positioning-Unit Device Position Receiver
[0142] A Positioning-Unit Device position receiver comprises at
least one receive channel capable of receiving and demodulating at
least one reference positioning signal from a reference
transmitter, and at least one receive channel capable of receiving
and demodulating at least one co-located transmitter slave
positioning signal. Preferably, a Positioning-Unit Device position
receiver is capable of receiving a plurality of reference
positioning signals for increased accuracy and integrity. The
Positioning-Unit Device position receiver preferably should also be
capable of receiving positioning signals from other
Positioning-Unit Devices transmitting in the 2.4 GHz ISM band, and
positioning signals from WAAS and GNSS satellites transmitting in
the microwave L band frequencies. A Positioning-Unit Device
position receiver tracks, demodulates, and interprets positioning
signals utilizing the same methodologies used in conventional GPS
receiver design. GPS receiver processing and design are well-known
in the art and are not a subject described here.
[0143] Positioning-Unit Device Transmitter
[0144] A Positioning-Unit Device transmitter has many similarities
to a conventional GPS pseudolite, With one major and critical
improvement: a steered transmitter clock. In the preferred
embodiment the steered transmitter clock is generated in the
digital domain using Direct Digital Synthesis (DDS) techniques. DDS
technology produces a digitally generated oscillator, which can be
frequency controlled to millihertz accuracies, thus allowing the
transmitter clock to be precisely "slaved" to an incoming reference
signal. The transmitter also incorporates at least one radio
frequency (RF) carrier generator, and at least one pseudorandom
number (PRN) code generator. The RF carrier generator produces the
carrier component, which is a sinusoidal radio frequency wave,
preferably transmitted in the 2.4 GHz ISM band, and the PRN code
generator produces the code component, which comprises a unique
code sequence that can be distinguished amongst other pseudorandom
code sequences transmitted on the same carrier frequency. A
plurality of codes can be generated on a plurality of frequencies
to produce a so called "wide lane", which allows carrier integer
cycle ambiguity to be resolved in a roving position receiver. In
the preferred embodiment Positioning-Unit Device transmitters are
pulsed in a Time Division Multiple Access (TDMA) scheme, such that
high power CDMA positioning signals do not jam weaker CDMA
positioning signals transmitted on the same carrier frequency. This
phenomenon is known as the "near/far problem" and is also
well-known in the art.
[0145] Positioning-Unit Device Central Processing Unit
[0146] The Positioning-Unit Device CPU comprises: [0147] a) Means
to determine the current position of the Positioning-Unit Device.
[0148] Position determination can be achieved through either
self-survey or through manual initialization. Self-survey requires
the Positioning-Unit Device to be in view of at least four other
reference Positioning-Unit Devices to determine a three-dimensional
single point position solution, or alternatively, a
Positioning-Unit Device may be in view of at least three GNSS
satellites plus at least one reference Positioning-Unit Device. In
this embodiment the reference Positioning-Unit Device supplies both
code and carrier differential corrections for all GNSS satellites
in view to the Positioning-Unit Device. The Positioning-Unit Device
then calculates an accurate position relative to the reference
Positioning-Unit Device. [0149] Manual initialization is achieved
by placing the Positioning-Unit Device at a predetermined location
and manually entering the geographical coordinate values into
Positioning-Unit Device memory. In the preferred embodiment a first
Positioning-Unit Device is manually initialized using precisely
known coordinates, with subsequent Positioning-Unit Devices
self-surveying from GNSS satellites and the first Positioning-Unit
Device. [0150] b) Means to initiate a reference signal search by
the position receiver. [0151] All channels of the position receiver
are set to search for any reference positioning signal in view.
[0152] c) Means to acquire at least one reference positioning
signal and extract network time and network data from the
navigation data component. [0153] d) Means to determine the signal
propagation delay from the reference transmitter to the
Positioning-Unit Device. [0154] Reference transmitter position
coordinates are first extracted from the reference positioning
signal navigation data, and compared to the known Positioning-Unit
Device location. The computed geometrical distance between
reference transmitter and Positioning-Unit Device is converted into
a time-of-flight offset. [0155] e) Means to initialize the slave
transmitter code generator with an appropriate unique PRN code.
[0156] f) Means to generate and pass appropriate network time and
network data to the transmitter, which is transmitted as the
navigation data component in the slave positioning signal. [0157]
Navigation Data is modulated upon the transmitter-generated PRN
code, which is subsequently modulated upon the
transmitter-generated RF carrier. Navigation data includes
time-of-week information, Positioning-Unit Device location, and
other network data such as location and status of other
Positioning-Unit Devices and GNSS satellites. [0158] g) Means to
apply the calculated time-of-flight offset and initialize the slave
transmitter to approximate network time and frequency. [0159] h)
Means to initiate the position receiver to search for the slave
positioning signal. [0160] i) Means to acquire the slave
positioning signal and apply a control loop to obtain frequency
coherence between the reference and slave positioning signals.
[0161] The CPU measures the instantaneous integrated carrier phase
(ICP) difference of the reference and slave positioning signals and
applies a control loop, known as a "Time-Lock-Loop (TLL)". The
output of the TLL applies correction values to the steered
transmitter clock, in order to zero the ICP difference. [0162] j)
Means to extract the transmitted slave time from the slave
positioning signal navigation data component and determine the time
difference between the reference positioning signal and slave
positioning signal. [0163] k) Means to Time Slew the steered
transmitter clock the requisite amount to zero the time difference
between the reference positioning signal and the slave positioning
signal, such that the slave positioning signal is chronologically
aligned with the reference transmitter time. [0164] l) Means to
declare Time-Lock status.
[0165] Common Oscillator
[0166] The common oscillator provides a coherent local timebase for
all components of the Positioning-Unit Device. In particular, the
same oscillator is used to drive the position receiver, the CPU,
and the steered transmitter clock. A coherent local timebase allows
open-loop frequency tracking of the received reference positioning
signal using a so called Frequency Tracking System (FTS). With FTS
the received reference positioning signal frequency offset, as
measured by the Positioning-Unit Device position receiver, is fed
directly to the Positioning-Unit Device steered transmitter clock.
The steered transmitter clock simply emulates the frequency offset
value of the incoming reference positioning signal, thus
eliminating the common oscillator term and maintaining
Reference/Slave Frequency Lock between the reference and slave
positioning signals. FTS aids in the acquisition and time
adjustment of the slave positioning signal.
[0167] Description of the Mobile System
[0168] A roving position receiver preferably comprises a plurality
of receive channels that are capable of receiving and interpreting
positioning signals from Positioning-Unit Devices, which are
preferably transmitting in the 2.4 GHz ISM band. The roving
position receiver is also preferably capable of receiving and
interpreting positioning signals from GNSS and WAAS satellites
transmitting in the L band frequencies. The roving position
receiver is preferably capable of demodulating navigation data
incorporating network data from all positioning signals in view.
This allows determination of Positioning-Unit Device network time,
GNSS time, Positioning-Unit Device locations, satellite locations,
and other network and GNSS data. In the preferred embodiment
network time is derived from GNSS time via WAAS satellites, thereby
making network time and GNSS time time-coherent. A roving position
receiver also preferably incorporates means to make code-based
pseudorange measurements for each positioning signal in view, means
to make carrier phase measurements for each positioning signal in
view, and means to solve for position, velocity, and time (PVT)
using single point position determination. Single point position
determination can be accomplished by using a conventional GPS
position solution, which is generally a form of least squares
regression that is well known in the art.
[0169] The roving position receiver preferably incorporates means
to determine integer cycle ambiguity. In the preferred embodiment
integer cycle ambiguity is resolved using wide-lane techniques.
Once integer cycle ambiguity is resolved, a precise carrier phase
pseudorange is determined from the roving position receiver to the
Positioning-Unit Device. The carrier pseudorange comprises an
integer number of carrier cycles (the integer component) plus a
fractional carrier cycle amount (fractional component or phase
component), and is termed a pseudorange due to the unknown position
receiver clock bias. Time-Locked Positioning-Unit Devices exhibit
time coherency to tens of picoseconds, thereby allowing a single
point position solution to be formed from the precise carrier
pseudoranges without the need for differential correction.
[0170] A position receiver tracks, demodulates, and interprets
positioning signals generated by a network of Time-Locked
Positioning-Unit Devices utilizing the same methodologies used in
conventional GPS receiver design. GPS receiver processing and
design, as well as Wide-Lane Ambiguity Resolution, are well-known
in the art and are not subjects described here.
[0171] It will of course be realized that whilst the above has been
given by way of an illustrative example of this invention, all such
and other modifications and variations hereto, as would be apparent
to persons skilled in the art, are deemed to fall within the broad
scope and ambit of this invention as is herein set forth.
* * * * *