Method and apparatus for improving the resilience of a positioning network

Abstract

Methods and apparatus are provided for improving the resilience of a positioning unit device to failures of a reference device that controls the network time base via timing information in its signals. In some embodiments, a predetermined positioning unit device in the positioning unit device is configured to monitor a reference signal, evaluate the serviceability of that signal, and take control of the network if it determines that the signal is not serviceable. Continuity of network synchronization is thereby maintained despite the reference signal being non-serviceable. The predetermined positioning unit device is preferably configured to obtain information about the reference signal from at least one other positioning unit device for use in evaluating the reference signal serviceability. If the reference device recovers from a failure, then it can re-enter the network by synchronizing its signal to a signal from one of the positioning unit devices and can then negotiate with the predetermined positioning unit device to resume control of the network.

Description

Field of the Invention

[0001] The present invention relates to methods and apparatus for improving the resiliency of positioning networks, particularly positioning networks in which the sources of positioning signals synchronize their positioning signals with reference signals received from a designated reference device. However, it will be recognized that the present invention is not limited to this particular field of use.

[0002] Related Applications

[0003] This application claims the benefit of priority of Australian Provisional Patent Application No. 2019904566, filed on 3 December 2019, the content of which is incorporated herein by reference in its entirety. Background Art

[0004] Any discussion of the prior art throughout this specification should in no way be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0005] The PCT application number WO03 / 038469A1, the content of which is incorporated herein by reference, discloses methods and systems for generating an accurate position solution of a mobile device using positioning signals transmitted from a synchronized network of ground-based transceivers called positioning unit devices. Each positioning unit device uses a so-called time-locked loop (TLL) to measure and correct timing errors relative to the time base of a designated reference transmitter, thereby establishing and maintaining a network of positioning unit devices that transmit positioning signals synchronized in time to the reference transmitter time base. Once a given positioning unit device has been synchronized to the reference transmitter time base, it can relay the time base to other positioning unit devices that do not have a clear line of sight to the designated reference transmitter, thereby propagating the time base through an extended network of positioning unit devices.

[0006] The TLL method disclosed in WO 03 / 038469A1 requires the reference transmitter and the positioning unit devices to be in known and fixed positions relative to a reference coordinate system. The published patent application number WO 2016 / 011505A1 (the content of which is incorporated herein by reference) discloses an extension of the TLL method to handle the case where the reference transmitter and the positioning unit devices move relative to each other. Generally, the reference transmitter or the positioning unit devices self-monitor their position and velocity, for example using an inertial navigation system or a position receiver, and broadcast this information in their positioning signals. The velocity information enables the estimation of the Doppler shift imposed on the positioning signals by the relative motion, while the position information enables the estimation of the propagation delay of the positioning signals.

[0007] The TLL synchronization process includes frequency alignment and time alignment steps. Preferably, the frequency alignment step is performed first so that the signal of the positioning unit device is coherent in frequency with the signal of the designated reference device before the signals are aligned in time. Simply put, the frequency alignment step ensures that the internal clock of the positioning unit device "ticks" at the same rate as the reference transmitter clock. To prevent clock drift and loss of synchronization with the reference transmitter, it is generally necessary for the positioning unit device to actively maintain the frequency alignment between its positioning signal and the reference signal. However, if the designated reference transmitter stops transmitting the reference signal, for example, due to a power outage or hardware failure, then the positioning unit device will have to stop transmitting its positioning signal, otherwise they will lose synchronization, resulting in a failure of the positioning network because their clocks drift independently. In addition, if the reference transmitter fails when the network is starting up, then the positioning unit devices will not be able to synchronize themselves.

[0008] In the description herein and in the subsequent claims, the term "comprising" and its variants shall be construed in an inclusive sense, synonymous with the term "including" and its variants. For example, the expression "a device comprising A and B" should not be limited to a device consisting only of elements A and B. Similarly, the term "or" should be construed in an inclusive rather than an exclusive sense. For example, unless the context clearly requires otherwise, the expression "A or B" should be interpreted to mean A or B or both A and B.

[0009] Object of the Invention

[0010] The object of the present invention is to overcome or improve at least one drawback of the prior art, or to provide a useful alternative.

[0011] The object of the present invention is to provide, in a preferred form, a positioning network comprising a plurality of positioning unit devices synchronized with the time base of a reference device, having improved resilience to reference device failures. Summary of the Invention

[0012] According to a first aspect of the present invention, there is provided a positioning network comprising:

[0013] a reference device configured to generate and transmit a reference signal according to a time base for the reference device; and

[0014] a plurality of positioning unit devices, each of the positioning unit devices being configured to generate a unique positioning signal and synchronize the unique positioning signal with the reference signal in time,

[0015] wherein a predetermined positioning unit device among the positioning unit devices is configured to:

[0016] (i) monitor the reference signal;

[0017] (ii) Evaluate the serviceability of the reference signal; and

[0018] (iii) In the case where it determines that the reference signal is not serviceable, take control of the positioning network,

[0019] such that other positioning unit devices can synchronize their unique positioning signals with the unique positioning signal of a predetermined positioning unit device in time, thereby maintaining or establishing the synchronization of the network although the reference signal is not serviceable.

[0020] In some embodiments, the monitoring of the reference signal includes measuring the quality of the reference signal received at a predetermined positioning unit device. In a preferred embodiment, the predetermined positioning unit device is configured to obtain information about the reference signal from at least one other positioning unit device for evaluating the serviceability of the reference signal. This information may include one or more measurements of the quality of the reference signal, or an evaluation of the serviceability of the reference signal. This evaluation may be based on one or more measurements of the quality of the reference signal.

[0021] In a preferred embodiment, the predetermined positioning unit device is configured to obtain information about the reference signal from one or more other positioning unit devices that routinely receive the reference signal and routinely receive the unique positioning signal from the predetermined positioning unit device. In some embodiments, each of the other positioning unit devices is configured to determine that it routinely receives that signal if it has received the signal within at least a predetermined percentage of a previous predetermined time period. Preferably, each of the other positioning unit devices is configured to broadcast whether it routinely receives the reference signal or the unique positioning signal from the predetermined positioning unit device.

[0022] In some embodiments, the quality of the reference signal includes received signal power, signal-to-noise ratio, or signal continuity.

[0023] In some embodiments, the reference device is configured, at startup: to search for unique positioning signals from one or more of the positioning unit devices; and if it detects a selected unique positioning signal among the unique positioning signals within a first predetermined startup period, then synchronize its signal to that unique positioning signal. The reference device is preferably configured to synchronize its signal to the unique positioning signal of the predetermined positioning unit device if detected. The reference device and the predetermined positioning unit device may be configured to negotiate the return of control of the positioning network to the reference device after the reference device has synchronized its signal to one of the unique positioning signals.

[0024] In some embodiments, the reference device and the predetermined positioning unit device are configured to be able to receive a time base from an external source such that the time base can be transmitted to a plurality of positioning unit devices or to one or more position receivers.

[0025] According to a second aspect of the invention, a method is provided in a positioning network that includes a reference device and a plurality of positioning unit devices, the reference device being configured to generate and transmit a reference signal according to a time base for the reference device, each positioning unit device being configured to generate a unique positioning signal and synchronize the unique positioning signal in time with the reference signal, the method for maintaining or establishing synchronization of the positioning unit devices in the absence of a servicable reference signal, the method including the following steps of a predetermined positioning unit device among the positioning unit devices:

[0026] (i) Monitoring the reference signal;

[0027] (ii) Evaluating the servicability of the reference signal; and

[0028] (iii) Taking control of the positioning network in the case where it determines that the reference signal is not servicable,

[0029] such that other positioning unit devices can synchronize their unique positioning signals in time with the unique positioning signal of the predetermined positioning unit device, whereby the synchronization of the network is maintained or established although the reference signal is not servicable.

[0030] In some embodiments, the step of monitoring the reference signal includes measuring the quality of the reference signal received at the predetermined positioning unit device. In a preferred embodiment, the step of monitoring the reference signal includes obtaining information about the reference signal from at least one other positioning unit device for evaluating the servicability of the reference signal. The information may include one or more measurements of the quality of the reference signal, or an evaluation of the servicability of the reference signal. The evaluation may be based on one or more measurements of the quality of the reference signal.

[0031] In a preferred embodiment, the predetermined positioning unit device obtains information about the reference signal from one or more other positioning unit devices that routinely receive the reference signal and routinely receive the unique positioning signal from the predetermined positioning unit device. In some embodiments, if each of the other positioning unit devices has received the signal within at least a predetermined percentage of a previous predetermined time period, then it determines that it routinely receives that signal. Preferably, each of the other positioning unit devices broadcasts whether it routinely receives the reference signal or the unique positioning signal from the predetermined positioning unit device.

[0032] In some embodiments, the quality of the reference signal includes received signal power, signal-to-noise ratio, or signal continuity.

[0033] In some embodiments, at startup, the reference device: searches for unique positioning signals from one or more of the positioning unit devices; and if it detects a selected unique positioning signal among the unique positioning signals within a first predetermined startup period, then synchronizes its signal to that unique positioning signal. The reference device preferably synchronizes its signal to the signal if it detects the unique positioning signal of a predetermined positioning unit device. After the reference device has synchronized its signal to one of the unique positioning signals, the reference device and the predetermined positioning unit device can negotiate to return control of the positioning network to the reference device.

[0034] In some embodiments, the reference device and the predetermined positioning unit device are capable of receiving a time base from an external source such that the time base can be transmitted to multiple positioning unit devices or to one or more position receivers. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0036] Figure 1 illustrates a positioning unit device that synchronizes its positioning signal to the time base of a reference device;

[0037] Figure 2 illustrates a positioning network including a reference device and multiple synchronized positioning unit devices, where a roaming position receiver can determine a position solution;

[0038] Figure 3 illustrates a positioning network according to an embodiment of the present invention;

[0039] Figure 4 illustrates a positioning network according to an embodiment of the present invention, showing the classification of positioning unit devices;

[0040] Figure 5 is a flowchart depicting a method of operating a positioning network according to an embodiment of the present invention, in which a series of steps are performed by an alternative reference device to maintain the continuity of network synchronization;

[0041] Figure 6 is a flowchart depicting the steps performed by a reference device of a positioning network at startup according to an embodiment of the present invention; and

[0042] Figure 7 illustrates a positioning network according to an embodiment of the present invention. DETAILED DESCRIPTION

[0043] Overview of the TLL Synchronization Process

[0044] Reference will be made Figure 1 to briefly discuss the time-locked loop (TLL) process disclosed in the above WO 03 / 038469A1. A positioning unit device 2 in a fixed and known position receives a reference signal 4 transmitted by a reference device 6 in another fixed and known position relative to a reference coordinate system (such as an Earth-centered Earth-fixed (ECEF) coordinate system), and synchronizes itself to the time base determined by the internal clock 8 of the reference device. The positioning unit device 2 incorporates a receiver 10, a transmitter 12, a manipulated transmitter clock 14, a CPU 16, and an oscillator 18. Once the positioning unit device 2 receives the reference signal 4, it transmits a slave version of the positioning signal 20 from its transmitter 12, which is received by its receiver 10. Each of the reference signal 4 and the slave positioning signal 20 has a carrier component, a pseudo-random code component, and a data component. The receiver 10 simultaneously receives and samples the reference signal 4 and the slave positioning signal 20 and measures the integrated carrier phase (ICP) difference between the two signals. The CPU 16 zeros the ICP measurements of the reference and slave positioning signals 4, 20 within the receiver 10 and then engages a control loop that continuously applies corrections to the manipulated transmitter clock 14 to maintain the ICP difference at zero, such that the slave positioning signal 20 has frequency coherence with the reference signal 4.

[0045] Once frequency coherence is achieved, the time difference observed between the pseudo-random code and data components of the reference and slave positioning signals 4, 20 becomes constant. This time difference includes a propagation time delay calculated from the known geometric distance 22 between the antennas 21 of the slave positioning unit device and the antenna 23 of the reference device, as well as a time offset, also known as a time bias, between the manipulated transmitter clock 14 and the reference device clock 8. The time bias can then be corrected to time-align the reference and slave positioning signals 4, 20.

[0046] When the slave positioning signal 20 has frequency coherence with the reference signal 4 and is time-aligned with the time base of the reference device, it is fully synchronized with the time base of the reference device. The transmitter 12 then increases the strength of the slave positioning signal 20, which now becomes the positioning signal 24 unique to the positioning unit device 2. Recall that the reference and positioning signals 4, 24 each have a carrier component, a pseudo-random code component, and a data component, and these signals are unique to each device and thus distinguishable, typically by virtue of their pseudo-random (PRN) code or the information in their data components.

[0047] Reference Figure 2, a mobile device in the form of a mobile position receiver 26 located within a network including a reference device 6 and a time-synchronized positioning unit device 2 can receive a positioning signal 24 from the positioning unit device and may also receive a reference signal 4, and autonomously calculate a carrier- or code-based position solution. More generally, a roaming position receiver can utilize positioning signals from any positioning unit device within sight to calculate a position solution, typically requiring signals from three or more positioning unit devices. Further implementation details of time-synchronized positioning networks are described in the above-mentioned PCT publication WO 03 / 038469A1, for example, regarding the transmission and interpretation of CDMA positioning signals and details about device hardware.

[0048] A Synchronized Positioning Network with Improved Resilience

[0049] It will be recognized from the foregoing description of the TLL synchronization process that, in order for a given positioning unit device to remain time-base synchronized with a designated reference device, it must continue to receive the time base encoded in the reference signal, either directly (if it has a clear view of the reference device) or cascaded through one or more intermediate positioning unit devices. If the reference device stops transmitting the reference signal, or if the reference signal becomes unusable for some reason, then the positioning unit device clocks will begin to drift in an individual and unpredictable manner, resulting in the loss of network synchronization. If the positioning unit devices have relatively inexpensive clocks, such as temperature-controlled crystal oscillators (TCXOs), then this may occur within a few seconds, and if they have more stable clocks, then more slowly, but the ultimate result is that the accuracy of the position solutions calculated by the roaming position receiver will soon be threatened unless the network is shut down.

[0050] Figure 3 Depicted is a positioning network 28 with improved resilience according to an embodiment of the present invention. The network 28 includes a reference device 6 and a plurality of positioning unit devices 2, the reference device 6 being configured to generate and transmit a reference signal 4 according to a time base derived from an internal clock 8. Each positioning unit device 2 is configured to generate a unique positioning signal 24 and synchronize its unique positioning signal with the reference signal 4 in time, enabling a roaming position receiver 26 to determine its position using the received positioning signals 24 (which may include the reference signal 4). The network 28 is said to be under the control of the reference device 6 when the positioning unit devices 2 synchronize their unique positioning signals 24 with the reference signal 4 either directly or via one or more intermediate positioning unit devices.

[0051] To provide improved resiliency to network 28, one of the predetermined positioning unit devices 2 is designated as "alternate reference device" 30 and is configured to: monitor reference signal 4; evaluate the serviceability of the reference signal; and assume control of network 28 in the event it determines that reference signal 4 is not serviceable. Generally, this will be as a result of partial or complete failure of reference device 6 and may manifest as a complete loss of reference signal 4, or the reference signal being weak, noisy or intermittent. Reference device 6 may fail during normal post-synchronization operation of network 28, in which case the positioning signals 24 of positioning unit devices 2 (including positioning signal 24-B of alternate reference device 30) should already be synchronized to reference signal 4, or reference device 6 may fail during startup of network 28. Once alternate reference device 30 (i.e., the predetermined positioning unit device) assumes control of the network, the other positioning unit devices 2 will be able to synchronize their positioning signals 24 in time to the positioning signal 24-B transmitted by alternate reference device 30, or remain synchronized thereto. Continuity of network synchronization can thus be maintained despite reference signal 4 being or becoming not serviceable. In addition, network 28 will still be able to start up despite failure of reference device 6. However, during normal network operation, reference device 6 is the "active reference", and alternate reference device 30 synchronizes its unique positioning signal 24-B to reference signal 4.

[0052] In a preferred embodiment, reference device 6 and alternate reference device 30 are positioning unit devices designated as having a special status prior to network startup. Preferably, the reference and alternate reference devices include this status information in the data component of their signals 4, 24-B. Alternatively or additionally, prior to network startup, information regarding the identities of reference and alternate reference devices 6, 30 is provided to the other positioning unit devices 2. The selection of reference device 6 and alternate reference device 30 from among the plurality of positioning unit devices 2 is influenced by several factors. For example, if the positioning unit devices are equipped with directional antennas rather than omnidirectional antennas, then it will generally be advantageous to select a positioning unit device at or near the edge of network 28 as reference device 6, with clear visibility of a large number of the other positioning unit devices 2. Similar considerations apply to the selection of alternate reference device 30. It is preferred to select a positioning unit device 2 that can directly receive reference signal 4 as alternate reference device 30. Similarly, reference device 6 should be able to directly receive the unique positioning signal 24-B from alternate reference device 30, as Figure 3As shown. The positioning unit device 2 that is not designated as the reference device 6 or the alternative reference device 30 can be conveniently referred to as a "slave device". When the positioning signal 24-B of the alternative reference device 30 is synchronized to the reference signal 4 transmitted by the reference device 6, the alternative reference device 30 is said to be in the "slave mode", and similarly, when the signal 4 of the reference device 6 is synchronized to the signal 24-B transmitted by the alternative reference device 30, the reference device 6 is said to be in the "slave mode".

[0053] Figure 4 Depicts a positioning network 28, which includes a reference device 6, an alternative reference device 30, and a plurality of positioning unit devices or "slave devices" 2 that can be divided into multiple categories. In particular, the slave devices that routinely receive signals 4, 24-B from both the reference device 6 and the alternative reference device 30 are classified as "layer 1 slave devices" 2-1. In a small network, it can be the case that all slave devices are layer 1 slave devices. By reciprocity, it can be expected that both the reference and alternative reference devices 6, 30 will routinely receive signals 24 from the layer 1 slave devices 2-1. Slave devices that receive signals from either the reference device 6 or the alternative reference device 30, but not routinely from both, can be classified as "layer 2 slave devices" 2-2, while those that do not routinely receive signals from the reference device 6 or the alternative reference device 30 can be classified as "layer 3 slave devices" 2-3. The layer 3 slave devices receive the network time base indirectly derived from the signal 24 of the reference device 6 or the alternative reference device 30 (as the case may be) via other already synchronized slave devices. As explained below, it is preferred that the operating positioning network 28 always has at least one layer 1 slave device 2-1. More preferably, there will be several layer 1 slave devices 2-1 in the operating network 28.

[0054] In a preferred embodiment, the slave devices 2, the alternative reference device 30, and the reference device 6 of the operating network 28 communicate with each other routinely or continuously, for example, via the exchange of status data or intentions included in the data components of their signals. In other embodiments, they can communicate via other means such as a wired or wireless local area network. The status data broadcast by the reference device 6 via wireless or wired means can include, for example, data about its identity and whether it is currently an "active reference", and the same is true for the alternative reference device 30. In a preferred embodiment, the slave devices 2 routinely include in the data component of their signals 24 (i) information about their layer status and (ii) information about the status of the signals 4, 24-B received from the reference or alternative reference devices 6, 30. In other embodiments, the slave devices provide this information when polled by the reference device 6 or the alternative reference device 30. Information from a layer 1 slave device indicating that it is currently not receiving a serviceable reference signal 4 or that the reference signal is particularly weak, noisy, or intermittent can indicate, for example, a problem with the reference device 6.

[0055] For example, if the signals from the reference or alternative reference devices 6, 30 are temporarily blocked, then the layer state of a given slave device 2 can change during the operation of the network 28. In a dynamic network such as that described in WO 2016 / 011505A1, the layer state can also change as a slave device or a reference device moves. For example, if a layer 1 slave device moves to a location where it no longer routinely receives the reference signal 4 or does not receive the reference signal at all, then it will downgrade itself from the layer 1 state and change the information it provides accordingly.

[0056] In some embodiments, to determine its layer state, each slave device 2 is configured to determine that it routinely receives a signal if it has received the signal within at least a predetermined percentage of a previous predetermined time period. For example, in a dynamic network where it is expected that the layer state of a slave device can change, if a given slave device 2 has received the reference signal 4 within at least 75% of the time in the previous 10 minutes, then it can determine that it routinely receives the reference signal 4. On the other hand, for a static network, the requirement for routine signal reception can be met if the relevant signal has been received within at least 90% of the time since the network was started. That is, the previous predetermined time period can correspond to the length of time the network has been operating. Alternatively, the requirement can be met if the relevant signal has been received continuously within the previous 5 minutes. In some embodiments, the criterion for a slave device 2 to receive a signal is whether the slave device can "track" the signal, for example, by means known in the field of CDMA signal processing. In other embodiments, the criterion can be that the received signal power is higher than a predetermined threshold, such as a threshold in the range of -90 to -110 dBm. It will be appreciated that for determining the layer state of a slave device, the assessment of whether a given signal is routinely received incorporates a historical element, which is different from the question of whether that signal is currently being received or is currently serviceable. Generally, it is preferred that the criterion for assessing routine reception of a signal is configurable.

[0057] In some embodiments, during normal operation of the positioning network 28, the reference device 6 asserts that it is the "active reference" and the alternate reference device 30 is in "slave mode". For reliable or robust operation of the network 28, it is important that the alternate reference device 30 be able to exit slave mode and assume control of the network in the event of a failure of the reference device 6 or more generally when the reference signal 4 becomes or is rendered inoperable. Assuming there is at least one layer 1 slave device 2-1 in the network 28, there will generally be a hold period during which the network can maintain synchronization via the unique positioning signal 24-B of the alternate reference device 30 in place of the reference signal 4. While it is preferred for the stability of network synchronization that the alternate reference device 30 assume control of the network as soon as possible, the alternate reference device 30 will generally have time during the hold period to evaluate the serviceability of the reference signal 4 before asserting the "active reference" state and controlling the network 28.

[0058] Ideally, at any given time, only one of the reference device 6 and the alternate reference device 30 should assert the "active reference" state, but a defective reference device 6 can continue to assert this state despite its signal 4 being inoperable. Thus, the positioning unit devices 2 can be configured to ignore the reference signal 4 once the alternate reference device 30 has asserted the "active reference" state, or to ignore it if they evaluate the reference signal 4 as inoperable.

[0059] In some embodiments, the alternate reference device 30 acts unilaterally, exiting slave mode and controlling the network 28 after determining that the reference signal 4 is inoperable based on measurements of one or more qualities of the reference signal 4 received at the alternate reference device 30 such as received signal power, signal-to-noise ratio, or signal continuity. For example, if the received power of the reference signal 4 instantaneously drops below a predetermined threshold or continuously drops below a predetermined threshold for a predetermined hold period, then the alternate reference device 30 can determine that the reference signal 4 is inoperable and continue to control the network. With the alternate reference device 30 now asserting the "active reference state", other positioning unit devices 2 will seek to synchronize their signals 24 with the signal 24-B transmitted by the alternate reference device 30. The criteria for determining the serviceability of the reference signal 4 are preferably configurable and can be set according to system requirements. For example, the predetermined threshold for received signal power can be in the range of -90 to -110 dBm, while the predetermined hold period can be in the range of 0.5 to 18 seconds, depending on the typical stability of the positioning unit device clocks. However, this unilateral approach risks the possibility that the alternate reference device 30 erroneously asserts network control, for example, if temporary occlusion prevents the alternate reference device 30 from receiving the reference signal 4 from a perfectly operating reference device 6.

[0060] In a preferred embodiment, the alternative reference device 30 additionally or alternatively obtains information about the reference signal 4 from at least one of the other positioning unit devices 2 in the network 28 for further input into the assessment of the serviceability of the reference signal. Preferably, the alternative reference device 30 routinely receives information about the reference signal, e.g., via the data component of the signal 24 from one or more other positioning unit devices 2, but in other embodiments the alternative reference device 30 actively polls the other positioning unit devices 2. The reference signal information may include one or more measurements of the quality of the reference signal 4, such as received signal power, signal-to-noise ratio, or signal continuity. Alternatively, the reference signal information may include an assessment of the serviceability of the reference signal 4, e.g., based on one or more measurements of the quality of the reference signal.

[0061] In a preferred embodiment, the alternative reference device 30 utilizes "first-hand" reference signal information received from one or more layer 1 slave devices 2-1 (i.e., positioning unit devices that routinely receive signals 4, 24-B from both the reference device 6 and the alternative reference device 30). The information may alternatively be received "second-hand" via relay by layer 2 slave devices 2-2, but this will generally only be necessary when there are no layer 1 slave devices 2-1 in the network 28. In a preferred embodiment, the alternative reference device 30 requests confirmation information about the reference signal 4 from a predetermined number or percentage of layer 1 slave devices 2-1 before proceeding to control the network 28. For example, before asserting control of the network, it may request confirmation information from at least one or two, or at least 50% to 100% of the layer 1 slave devices 2-1. Generally, the duration of the predetermined holding period during which the alternative reference device 30 assesses the serviceability of the reference signal should be shorter than the duration of the period used by the slave device 2 to determine its layer status, so that the alternative reference device 30 can obtain the desired reference signal information before the layer 1 slave device 2-1 degrades its layer status.

[0062] We now consider the case where the reference device 6 starts up. In a preferred embodiment, the reference device 6 is configured to check at startup whether the network 28 is operational, that is, whether it can receive signals 24 from one or more of the slave devices 2 in the network, typically including a signal 24-B from an alternative reference device 30 asserting an "active reference" state. If the reference device 6 does not detect a network signal during a first predetermined startup period (which would be the case when the network is starting up), then it starts up normally and asserts control of the network as an active reference. However, if the reference device 6 does detect a network signal 24 at startup (which would be the case when it is recovering from a failure), then it will enter the network 28 in slave mode, thereby synchronizing its signal 4 to a selected one of the detected signals via a standard TLL process. Preferably, if the reference device 6 detects a signal 24-B transmitted by the alternative reference device 30 during a first predetermined startup period (which could be in the range of 60 to 360 seconds, for example), then the reference device 6 will synchronize its signal 4 to that signal.

[0063] In some embodiments, the reference device 6 remains in this state indefinitely, ready to assume control of the network if the substitute reference device 30 fails to provide a serviceable signal, using methods similar to those previously described for the substitute reference device 30. In essence, the reference device 6 and the substitute reference device 30 will have interchanged roles. In other embodiments, once the reference device 6 has synchronized its signal 4 to the network time base, i.e., to the selected signal 24 from the synchronized positioning unit device 2, it may seek to re-assert the "active reference" state in a negotiated handover from the substitute reference device 30. In a preferred embodiment, this is performed via the reference and substitute reference devices 6, 30 monitoring the appropriate data bits in the data components of each other's signals 24-B, 4 and responding accordingly. In one embodiment, the synchronized reference device 6 marks that it is ready to resume control of the network, and then resumes control immediately or at an agreed time after it receives approval from the substitute reference device 30. In other embodiments, there is an additional level of negotiation, in which the reference device 6 initially indicates readiness to resume control, and then subsequently indicates an intent to resume control at an agreed time. When the reference device 6 changes state to "active reference", the substitute reference device 30 switches off its "active reference" state and reverts to slave mode.

[0064] refer to Figure 5The flowchart describes a method for operating a positioning network 28 according to an embodiment of the present invention. The flowchart shows steps performed by an alternative reference device 30 in maintaining the continuity of network synchronization through the loss of a servable reference signal 4. Optional steps are indicated by dashed outlines. Starting from a state where the alternative reference device 30 considers the network to be operating normally, the network is under the control of a reference device 6, and the process begins at step 42, where the alternative reference device 30 routinely monitors the reference signal 4 before evaluating the servability of the reference signal at step 44. Optionally, as indicated by step 46, the alternative reference device 30 obtains information about the reference signal from one or more layer 1 slave devices 2-1 for additional input to the evaluation of the reference signal servability. At decision point 48, the alternative reference device determines whether the reference signal 4 is servable. If so, then the alternative reference device 30 returns to step 42. If not, then it controls the network at step 50, for example, by exiting the slave mode and asserting an "active reference" state in the data component of its signal 24-B. Optionally, at step 52, the alternative reference device 30 or another positioning unit device 2 indicates to the network supervisor that there is a problem with the reference device 6 for fault determination or repair.

[0065] In some embodiments, the process ends here, and the positioning network 28 remains synchronized under the control of the alternative reference device 30. In other embodiments, the alternative reference device 30 periodically or continuously searches for the reference signal 4 at step 54, and at decision point 56, it determines whether the reference device has synchronized its signal 4 to the network timebase. If not, then the alternative reference device 30 returns to step 54 to continue searching for the reference signal while maintaining the "active reference" state. If so, then at step 58, the alternative reference device 30 and the reference device 6 negotiate the handover of control, and then at step 60, the alternative reference device hands over the control of the network to the reference device. In this embodiment, the process ends with the positioning network 28 remaining synchronized under the control of the reference device 6.

[0066] Figure 6 is a flowchart depicting steps performed by a reference device 6 of a positioning network 28 at startup according to an embodiment of the present invention. At step 62, the reference device searches for signals 24 from one or more positioning unit devices 2 in the network during a first predetermined startup period, and then at decision point 64, it determines whether it is receiving such network signals, which may include the signal 24-B from the alternative reference device 30. If so, which may be the case when the reference device 6 comes back online after a hardware failure, then at step 66, it synchronizes its signal 4 to the selected network signals 24, 24-B, i.e., starts in slave mode. The process may end there, with the reference device remaining in slave mode, or the reference device may negotiate with the alternative reference device to control the network, as previously referenced Figure 5As explained. On the other hand, if the reference device 6 determines that there is no network signal, which will generally be the case when the network is starting up, then in step 68, it continues its normal startup process and assumes control of the network 28.

[0067] In a preferred embodiment, the alternative reference device 30 is configured to operate in a manner similar to the reference device 6 at startup, except that before concluding that the reference device is inoperable and thus asserting control of the network, the alternative reference device 30 searches for a network signal during a second predetermined startup period that is preferably longer than the first predetermined startup period used by the reference device 6. When the network 28 is located to start up, it is expected that the reference and alternative reference devices 6, 30 will start their startup processes at approximately the same time, such that the first predetermined startup period should end before the second predetermined startup period. However, this is not necessary because if the second startup period ends first, then the alternative reference device 30 will control the network and the reference device 6 will enter the network in a slave mode. The second predetermined startup period can be in the range of, for example, 1.5 to three times longer than the first predetermined startup period.

[0068] In some embodiments, if neither the reference device 6 nor the alternative reference device 30 can provide a serviceable signal, then the resilience of the synchronous positioning network is further improved by designating another positioning unit device 2 as a second alternative reference device before the network starts up, where appropriate protocols are defined for communication between the various members of the network. If needed, more backup reference devices can also be designated, for example, depending on the size of the network or the criticality of its uninterrupted operation.

[0069] In the previously described embodiments, the time base of the reference device 6 of the network 28 to be propagated to the positioning unit device 2 is generated internally by the reference device. Alternatively, the reference device can obtain the time base from an external source. Such embodiments are useful for extended networks that transmit a time base (such as Coordinated Universal Time (UTC)) to positioning unit devices, as well as to devices such as position receivers that are capable of receiving and processing signals from the network.

[0070] Figure 7Depicts a positioning network 70 according to an embodiment of the present invention, including a reference device 6 and a plurality of positioning unit devices 2, where a predetermined one of the positioning unit devices is designated as an alternative reference device 30. The reference device 6 and the alternative reference device 30 are each configured to be able to receive a time base from an external source 72 and align their respective signals 4, 24-B with that time base. It is noted that it is necessary for the alternative reference device 30 as well as the reference device 6 to be able to receive an external time base to ensure that the external time base can be propagated in the event of a failure of the reference device 6. In a preferred embodiment, the external source 72 is a source of UTC, such as a Global Navigation Satellite System (GNSS), such as GPS, or an atomic clock pointing to UTC. In other embodiments, the reference device 6 and the alternative reference device 30 are configured to be able to receive a time base that is different from and possibly independent of UTC.

[0071] In normal operation of the network 70, the reference device 6 receives a time base from the external source 72 and then generates and transmits a reference signal 4 according to that time base. The positioning unit devices 2 within the field of view of the reference device 6, preferably but not necessarily including the predetermined alternative reference device 30, receive the reference signal 4 and then generate their own unique signals 24, and synchronize their signals to the received reference signal 4 in time using the TLL process described above. The positioning unit devices that are not within the field of view of the reference device 6 can synchronize their signals to the reference signal 4 in time by receiving and synchronizing with the signals transmitted by the positioning unit devices that have already been synchronized with the reference signal 4. By propagating this time base over an arbitrarily large area or distance, the signals 24 transmitted by an arbitrarily large number of positioning unit devices can thus be aligned with the time base obtained from the external source 72. Therefore, in addition to being able to calculate a position solution, devices such as position receivers 26 that are capable of receiving and processing signals 4, 24, 24-B from the network can also receive this time base, which can advantageously be UTC.

[0072] In this normal operating mode, the predetermined alternative reference device 30 operates as a standard positioning unit device in the network 70, synchronizing its unique signal 24-B with the reference signal 4, thus effectively ignoring the external source 72. However, if the alternative reference device 30 unilaterally or in consideration of information regarding the reference signals from one or more of the other positioning unit devices 2 determines that the reference device 6 is not transmitting a serviceable reference signal 4, then it switches to the "active reference" mode and synchronizes its unique signal 24-B to the time base obtained from the external source 72. The other positioning unit devices 2 can then synchronize their signals 24 with the signal 24-B transmitted by the alternative reference device either by directly receiving this signal 24-B or through cascading, thereby enabling the external time base to be transmitted in the absence of a serviceable reference signal 4.

[0073] Except for the source of the time base to which the signals of the networked positioning unit devices are synchronized, refer to Figure 7 The operation of the network 70 described is similar to that of the network 28 referred to in Figure 3 and 4 described. Generally speaking, the same protocols will be applied, for example, to determine whether the alternative reference device 30 should control the network or hand over the control back to the reference device 6.

[0074] Although the invention has been described with reference to specific examples, those skilled in the art will recognize that the invention can be implemented in many other forms.

Claims

1. A positioning network, comprising: A reference device configured to generate and transmit a reference signal according to a time base for the reference device; And A plurality of positioning unit devices, each of the positioning unit devices being configured to generate a unique positioning signal and synchronize the unique positioning signal in time to the reference signal, Wherein a predetermined positioning unit device among the positioning unit devices is configured to: (i) Monitor the reference signal; (ii) Evaluate the serviceability of the reference signal; And (iii) Take control of the positioning network in the case where it determines that the reference signal is not serviceable, Such that other positioning unit devices can synchronize their unique positioning signals in time to the unique positioning signal of the predetermined positioning unit device, thereby maintaining or establishing the synchronization of the network although the reference signal is not serviceable.

2. The positioning network according to claim 1, wherein the monitoring of the reference signal comprises: Measure the quality of the reference signal received at the predetermined positioning unit device.

3. The positioning network according to claim 1, wherein the predetermined positioning unit device is configured to obtain information about the reference signal from at least one other positioning unit device among the positioning unit devices for evaluating the serviceability of the reference signal.

4. The positioning network according to claim 3, wherein the information comprises one or more measurements of the quality of the reference signal.

5. The positioning network according to claim 3, wherein the information comprises an evaluation of the serviceability of the reference signal.

6. The positioning network according to claim 5, wherein the evaluation is based on one or more measurements of the quality of the reference signal.

7. The positioning network according to any one of claims 3 to 6, wherein the predetermined positioning unit device is configured to obtain information about the reference signal from one or more other positioning unit devices, and the one or more other positioning unit devices routinely receive the reference signal and routinely receive unique positioning signals from the predetermined positioning unit device.

8. The positioning network according to claim 7, wherein each positioning unit device among the other positioning unit devices is configured to determine that it routinely receives the signal if it has received the signal within at least a predetermined percentage of a previous predetermined time period.

9. The positioning network according to claim 7, wherein each positioning unit device among the other positioning unit devices is configured to broadcast whether it routinely receives the reference signal or the unique positioning signal from the predetermined positioning unit device.

10. The positioning network according to claim 2, claim 4 or claim 6, wherein the quality of the reference signal comprises received signal power, signal-to-noise ratio or signal continuity.

11. The positioning network according to any one of claims 1 to 6, wherein the reference device is configured to, at startup: Search for unique positioning signals from one or more of the positioning unit devices in the positioning unit device; and If it detects a unique positioning signal selected from the unique positioning signals within a first predetermined startup period, then synchronize its signal to the unique positioning signal.

12. The positioning network according to claim 11, wherein the reference device is configured to synchronize its signal to the unique positioning signal if the unique positioning signal of the predetermined positioning unit device is detected.

13. The positioning network according to claim 11, wherein the reference device and the predetermined positioning unit device are configured to negotiate to return control of the positioning network to the reference device after the reference device has synchronized its signal to one of the unique positioning signals.

14. The positioning network according to any one of claims 1 to 6, wherein the reference device and the predetermined positioning unit device are configured to be able to receive the time base from an external source, such that the time base can be transmitted to the plurality of positioning unit devices or one or more position receivers.

15. A method for maintaining or establishing synchronization of a network in a positioning network including a reference device and a plurality of positioning unit devices in the absence of a servicable reference signal, the reference device being configured to generate and transmit a reference signal according to a time base for the reference device, each positioning unit device being configured to generate a unique positioning signal and synchronize the unique positioning signal in time to the reference signal, the method including the following steps for a predetermined positioning unit device among the positioning unit devices: (i)Monitor the reference signal; (ii)Evaluate the servicability of the reference signal; and (iii)Take control of the positioning network in the case where it determines that the reference signal is not servicable, such that other positioning unit devices can synchronize their unique positioning signals in time to the unique positioning signal of the predetermined positioning unit device, thereby maintaining or establishing synchronization of the network although the reference signal is not servicable.

16. The method according to claim 15, wherein the step of monitoring the reference signal includes: Measure the quality of the reference signal received at the predetermined positioning unit device.

17. The method according to claim 15, wherein the step of monitoring the reference signal includes: Obtain information about the reference signal from at least one other positioning unit device among the positioning unit devices for evaluating the serviceability of the reference signal.

18. The method according to claim 17, wherein the information includes one or more measurements of the quality of the reference signal.

19. The method according to claim 17, wherein the information includes an assessment of the servicability of the reference signal.

20. The method according to claim 19, wherein the assessment is based on one or more measurements of the quality of the reference signal.

21. The method according to any one of claims 17 to 20, wherein the predetermined positioning unit device obtains information about the reference signal from one or more other positioning unit devices, the one or more other positioning unit devices routinely receive the reference signal and routinely receive a unique positioning signal from the predetermined positioning unit device.

22. The method according to claim 21, wherein, Each of the other positioning unit devices determines that it routinely receives the signal if it has received the signal within at least a predetermined percentage of a previous predetermined time period.

23. The method according to claim 21, wherein each of the other positioning unit devices broadcasts whether it routinely receives the reference signal or the unique positioning signal from the predetermined positioning unit device.

24. The method according to claim 16, claim 18 or claim 20, wherein the quality of the reference signal includes received signal power, signal-to-noise ratio or signal continuity.

25. The method according to any one of claims 15 to 20, wherein the reference device, at startup: searches for a unique positioning signal from one or more of the positioning unit devices; and if it detects a unique positioning signal selected from the unique positioning signals within a first predetermined startup period, then synchronizes its signal to the unique positioning signal.

26. The method according to claim 25, wherein the reference device, if it detects the unique positioning signal of the predetermined positioning unit device, then synchronizes its signal to the unique positioning signal.

27. The method according to claim 25, wherein, After the reference device has synchronized its signal to one of the unique positioning signals, the reference device and the predetermined positioning unit device negotiate to return control of the positioning network to the reference device.

28. The method according to any one of claims 15 to 20, wherein the reference device and the predetermined positioning unit device are capable of receiving the time base from an external source such that the time base can be transmitted to the plurality of positioning unit devices or one or more position receivers.

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