Method and apparatus for improving the resilience of positioning networks

Abstract

A method and apparatus are provided for improving the resilience of a network of positioning-unit devices to failure of a reference device that controls the network time reference via timing information in its signal. In some embodiments, a given one of the positioning-unit devices is configured to monitor the reference signal, assess the availability of the signal, and take control of the network if it determines that the signal is unavailable. This maintains continuity of network synchronization despite the reference signal being unavailable. Preferably, a given positioning-unit device is configured to obtain information about the reference signal from at least one other positioning-unit device to assess its availability. When the reference device recovers from the failure, it can rejoin the network by synchronizing its signal to a signal from one of the positioning-unit devices and then negotiate with the given positioning-unit device to resume control of the network.

Description

[Technical Field]

[0001] The present invention relates to methods and apparatus for improving the resilience of positioning networks, and in particular to methods and apparatus for positioning networks in which a source of a positioning signal synchronizes its positioning signal to a reference signal received from a designated reference device, although it will be understood that the invention is not limited to this particular field of use.

[0002] [Related Applications] This application claims priority to Australian Provisional Patent Application No. 2019904566, filed December 3, 2019, the contents of which are incorporated herein by reference in their entirety. [Background technology]

[0003] The discussion of prior art throughout this specification should in no way be taken as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0004] Published PCT Application No. WO 03 / 038469 A1, the contents of which are incorporated herein by reference, discloses a method and system for generating accurate position solutions for mobile devices using positioning signals transmitted from a synchronous network of ground transceivers known as Positioning-Unit Devices. Each Positioning-Unit Device uses a so-called Time-Locked Loop (TLL) to measure and correct its timing error relative to a designated reference transmitter time base, thereby establishing and maintaining a network of Positioning-Unit Devices that transmit positioning signals chronologically synchronized to the reference transmitter time base. Once synchronized to the reference transmitter time base, a given Positioning-Unit Device can propagate the time base through an extended network of Positioning-Unit Devices by relaying this time base to other Positioning-Unit Devices that do not have clear visibility of the designated reference transmitter.

[0005] The TLL methodology disclosed in WO03 / 038469A1 requires that the reference transmitter and positioning-unit device reside at known, fixed positions relative to a reference frame. Published patent application WO2016 / 011505A1, the contents of which are incorporated herein by reference, discloses an extension of the TLL methodology to address situations in which the reference transmitter and positioning-unit device are moving relative to each other. Typically, the reference transmitter or positioning-unit device self-monitors its own position and velocity, for example using an inertial navigation system or position receiver, and broadcasts this information in a positioning signal. The velocity information allows for estimation of the Doppler shift imposed on the positioning signal by relative motion, and the position information allows for estimation of the propagation delay of the positioning signal.

[0006] This TLL synchronization process includes a frequency alignment step and a time alignment step, with the frequency alignment step preferably being performed first so that the positioning-unit device's signal is frequency coherent with that of the designated reference device before the signals are aligned in time. Simply put, frequency alignment ensures that the positioning-unit device's internal clock "ticks" at the same rate as the reference transmitter's clock. Generally, a positioning unit must actively maintain frequency alignment between its positioning signal and the reference signal to prevent clock drift and loss of synchronization with the reference transmitter. However, if the designated reference transmitter stops transmitting its reference signal, for example, due to a power outage or hardware failure, the positioning unit must stop transmitting its positioning signal, otherwise its clock will drift independently and lose synchronization, resulting in a failure of the positioning network. Furthermore, if a reference transmitter failure occurs during network startup, the positioning-unit device itself will also lose synchronization.

[0007] In this specification and the following claims, terms such as "comprising" and "comprises" should be interpreted in an inclusive sense, synonymous with terms such as "including" and "includes." For example, the phrase "an apparatus comprising A and B" should not be limited to an apparatus that includes only elements A and B. Similarly, the term "or" should be interpreted in an inclusive sense rather than an exclusive sense. For example, unless the context clearly requires otherwise, the phrase "A or B" should be interpreted to mean either A or B, or both A and B. Summary of the Invention [Problem to be solved by the invention]

[0008] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0009] It is an object of the present invention in a preferred form to provide a positioning network including a plurality of Positioning-Unit Devices synchronized to the time base of a Reference Device, and having improved resilience to failure of the Reference Device. [Means for solving the problem]

[0010] According to a first aspect of the present invention, a reference device configured to generate and transmit a reference signal according to its own time base; a plurality of positioning-unit devices, each configured to generate a unique positioning signal and time-sequentially synchronize said unique positioning signal with said reference signal; wherein a given one of the positioning-unit devices is (i) monitoring the reference signal; (ii) assessing the serviceability of the reference signal; (iii) controlling the positioning network when determining that the reference signal is not available; wherein other positioning-unit devices synchronize their own unique positioning signals to the unique positioning signal of the given positioning-unit device in a time-series manner, thereby maintaining or establishing synchronization of the network despite the reference signal being unavailable. A positioning network is provided.

[0011] In some embodiments, monitoring the reference signal includes measuring the quality of the reference signal received at a given positioning-unit device. In a preferred embodiment, a given positioning-unit device is configured to obtain information about the reference signal from at least one other of the positioning-unit devices for use in assessing the usability of the reference signal. This information may include one or more measures of the quality of the reference signal, or an assessment of the usability of the reference signal. The assessment may be based on the one or more measures of the quality of the reference signal.

[0012] In preferred embodiments, a given positioning-unit device is configured to periodically receive the reference signal and to periodically receive a unique positioning signal from the given positioning-unit device or obtain information about reference signals from one or more other positioning-unit devices. In some embodiments, each of the other positioning-unit devices is configured to determine that it is receiving the signal periodically if it has received the signal for at least a predetermined percentage of a previous predetermined period. Preferably, each of the other positioning-unit devices is configured to broadcast whether it is periodically receiving the reference signal or unique positioning signal from the given positioning-unit device.

[0013] In some embodiments, the quality of the reference signal includes received signal power, signal to noise ratio, or signal continuity.

[0014] In some embodiments, the datum is configured, upon start-up, to look for unique positioning signals from one or more of the positioning-unit devices and, if a unique positioning signal is detected within a first predetermined start-up period, to synchronize its signal to a selected one of the unique positioning signals. Preferably, the datum is configured to synchronize its signal to the unique positioning signal of a given positioning-unit device if that signal is detected. The datum and a given positioning-unit device may be configured to negotiate handing control of the positioning network back to the datum after the datum has synchronized its signal to one of the unique positioning signals.

[0015] In some embodiments, the reference device and a given Positioning-Unit Device are configured to receive a time reference from an external source so that the time reference can be forwarded to multiple Positioning-Unit Devices or one or more position receivers.

[0016] According to a second aspect of the present invention, in a positioning network comprising a reference device configured to generate and transmit a reference signal according to its own time base, and a plurality of Positioning-Unit Devices each configured to generate a unique positioning signal and time-sequentially synchronise its unique positioning signal to the reference signal, there is provided a method of maintaining or establishing synchronisation of the Positioning-Unit Devices in the absence of an available reference signal, the method comprising: (i) monitoring the reference signal; (ii) assessing the availability of said reference signal; (iii) controlling a positioning network when determining that the reference signal is not available; wherein other positioning-unit devices time-sequentially synchronize their own unique positioning signals to the unique positioning signal of a predetermined positioning-unit reference device, thereby maintaining or establishing synchronization of the network even when the reference signal is not available.

[0017] In some embodiments, monitoring the reference signal comprises measuring the quality of the reference signal received at the given positioning-unit device. In a preferred embodiment, monitoring the reference signal comprises obtaining information about the reference signal from at least one other of the positioning-unit devices for use in assessing the usability of the reference signal. This information may include one or more measures of the quality of the reference signal, or an assessment of the usability of the reference signal. The assessment may be based on the one or more measures of the quality of the reference signal.

[0018] In preferred embodiments, a given positioning-unit device periodically receives a reference signal and periodically receives a unique positioning signal from the given positioning-unit device or obtains information about reference signals from one or more other positioning-unit devices. In some embodiments, each of the other positioning-unit devices is determined to be receiving the signal periodically if it has received the signal for at least a predetermined percentage of a previous predetermined period of time. Preferably, each of the other positioning-unit devices broadcasts whether it is periodically receiving a reference signal or a unique positioning signal from the given positioning-unit device.

[0019] In some embodiments, the quality of the reference signal includes received signal power, signal to noise ratio, or signal continuity.

[0020] In some embodiments, upon start-up, the reference device searches for unique positioning signals from one or more of the positioning-unit devices and, if it detects a unique positioning signal within a first predetermined start-up period, synchronizes its signal to a selected one of the unique positioning signals. Preferably, the reference device synchronizes its signal to the unique positioning signal of a given positioning-unit device if that signal is detected. The reference device and a given positioning-unit device may 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.

[0021] In some embodiments, the reference device and a given Positioning-Unit Device may receive a time reference from an external source so that the time reference can be forwarded to multiple Positioning-Unit Devices or one or more position receivers.

[0022] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0023] [Figure 1]FIG. 1 shows a positioning-unit device synchronizing its own positioning signal with the time base of a reference device. [Figure 2] 1 illustrates a positioning network including a reference device and a number of synchronized Positioning-Unit Devices, where a roving position receiver can determine a position solution; [Figure 3] 1 illustrates a positioning network according to an embodiment of the present invention; [Figure 4] 1 illustrates a positioning network showing classifications of Positioning-Unit Devices according to an embodiment of the present invention; [Figure 5] 4 is a flowchart illustrating a method of operating a positioning network in which a series of steps are performed by an alternate reference unit in maintaining continuity of network synchronization, according to an embodiment of the present invention. [Figure 6] 4 is a flowchart illustrating steps performed by a reference device of a positioning network at start-up, in accordance with an embodiment of the present invention. [Figure 7] 1 illustrates a positioning network according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0024] Overview of the TLL synchronization process Referring to Figure 1, the time-locked loop (TLL) process disclosed in WO 03 / 038469 A1 will be briefly described. A positioning-unit device 2, which resides at a fixed, known location relative to a reference coordinate system such as the Earth Centered Earth Fixed (ECEF) coordinate system, receives a reference signal 4 transmitted by a reference device 6, which resides at another fixed, known location, and synchronizes itself to a time reference determined by the reference device's internal clock 8. The positioning-unit device 2 includes a receiver 10, a transmitter 12, a steered transmitter clock 14, a CPU 16, and an oscillator 18. Upon receiving the reference signal 4, the positioning-unit device 2 transmits a slave version of a positioning signal 20 from the transmitter 12, which is received by the receiver 10. The reference signal 4 and the slave positioning signal 20 each have a carrier component, a pseudorandom 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 sets the ICP measurements of the reference signal and the slave positioning signal 4, 20 to zero in the receiver 10, and then engages a control loop that continuously applies corrections to the guided transmitter clock 14 to maintain the ICP difference at zero so that the slave positioning signal 20 has frequency coherence with the reference signal 4.

[0025] Once frequency coherence is achieved, the time difference observed between the pseudorandom code and data components of the reference positioning signal 4 and the pseudorandom code and data components of the slave positioning signal 20 will be constant. This time difference includes the propagation time delay calculated from the known geometric distance 22 between the Positioning-Unit Device antenna 21 and the reference device antenna 23, and the time offset, also referred to as the time bias, between the guided transmitter clock 14 and the reference device clock 8. The time bias can then be corrected to bring the reference and slave positioning signals 4, 20 into time alignment.

[0026] The slave positioning signal 20 becomes fully synchronized to the reference device's time base when it has frequency coherence with the reference signal 4 and is chronologically aligned with the reference device's time base. The transmitter 12 then increases the strength of the slave positioning signal 20, at which point the slave positioning signal 20 becomes a positioning signal 24 specific to the positioning-unit device 2. Recall that the reference signal and positioning signal 4, 24 each have a carrier component, a pseudorandom code component, and a data component, and therefore these signals are specific to each device and are therefore typically distinguishable by their pseudorandom (PRN) code or the information within their data component.

[0027] 2, a mobile device in the form of a roving position receiver 26 located in a network including reference devices 6 and time-synchronized positioning-unit devices 2 can receive position signals 24, and possibly reference signals 4, from the positioning-unit devices and autonomously calculate a code or carrier-based position solution. More generally, a roving position receiver can calculate a position solution utilizing positioning signals from any positioning-unit device within view, typically requiring signals from three or more positioning-unit devices. Further implementation details of a time-synchronized positioning network, for example regarding the transmission and interpretation of CDMA positioning signals, and device hardware, are described in the above-mentioned PCT publication WO 03 / 038469 A1.

[0028] Synchronous positioning network with improved resilience - Patent Application 20070122999 From the above description of the TLL synchronization process, it will be understood that for a given positioning-unit device to remain synchronized to the time reference of a designated reference device, it must continue to receive the time reference encoded in a reference signal transmitted to it either directly, if the reference device is clearly visible, or via one or more intermediate positioning-unit devices. If the reference device stops transmitting its reference signal, or if the reference signal becomes otherwise unavailable for any reason, the positioning unit's clock will begin to drift independently and unpredictably, resulting in a loss of network synchronization. This may occur briefly if the positioning-unit device has a relatively inexpensive clock such as a temperature-controlled crystal oscillator (TCXO), or more slowly if the positioning-unit device has a more stable clock, but ultimately will quickly compromise the accuracy of the position solution calculated by the roving position receiver unless the network is shut down.

[0029] Figure 3 shows an improved resilient positioning network 28 according to an embodiment of the present invention. The network 28 includes a reference device 6 configured to generate and transmit a reference signal 4 according to a time reference derived from an internal clock 8, and a plurality of positioning-unit devices 2. Each positioning-unit device 2 is configured to generate a unique positioning signal 24 and chronologically synchronize its unique positioning signal 24 to the reference signal 4, thereby enabling a roving position receiver 26 to determine its position using the received positioning signals 24, possibly including the reference signal 4. When the positioning-unit devices 2 synchronize their unique positioning signals 24 to the reference signal 4, either directly or via one or more intermediate positioning-unit devices, the network 28 is said to be under the control of the reference device 6.

[0030] To provide improved resilience to the network 28, a given one of the positioning-unit devices 2 is designated as an "alternate reference device" 30 and is configured to monitor the reference signal 4, assess its availability, and assume control of the network 28 if it determines that the reference signal 4 is unavailable. This condition typically occurs as a result of a partial or complete failure of the reference device 6 and can manifest as a complete loss of the reference signal 4 or as a weak, noisy, or intermittent reference signal. The reference device 6 may fail during normal post-synchronization operation of the network 28, when the positioning signals 24 of the positioning-unit devices 2, including the positioning signals 24-B of the alternate reference device 30, should have already synchronized to the reference signal 4, or during start-up of the network 28. Once the alternate reference device 30, i.e., a given positioning-unit device, assumes control of the network, the other positioning-unit devices 2 can chronologically synchronize or maintain synchronization with the positioning signals 24-B transmitted by the alternate reference device 30. This allows continuity of network synchronization to be maintained despite reference signal 4 being unavailable or becoming unavailable, and network 28 can 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 own positioning signal 24-B to reference signal 4.

[0031] In a preferred embodiment, the reference device 6 and alternate reference device 30 are positioning-unit devices that have been assigned special status prior to network startup. The reference device and alternate reference device preferably include this status information in the data component of their signals 4, 24-B. Alternatively or additionally, other positioning-unit devices 2 are provided with information regarding the identities of the reference device and alternate reference devices 6, 30 prior to network startup. The selection of the reference device 6 and alternate reference device 30 from among multiple positioning-unit devices 2 can be influenced by several factors. For example, if the positioning-unit devices are equipped with directional rather than omnidirectional antennas, it is generally advantageous to select as the reference device 6 a positioning-unit device at or near the edge of the network 28 that has a clear view of many other positioning-unit devices 2. Similar considerations apply to the selection of the alternate reference device 30. It is preferable to select a positioning-unit device 2 as the alternate reference device 30 that can directly receive the reference signal 4. Similarly, the reference device 6 should also be able to directly receive its own positioning signal 24-B from the alternate reference device 30 as shown in FIG. 3. Positioning-unit devices 2 that are not designated as reference units 6 or alternate reference units 30 may be referred to as "slaves" for convenience. An alternate reference unit 30 is said to be in "slave mode" when its positioning signal 24-B is synchronized to the reference signal 4 transmitted by the reference unit 6, and similarly, a reference unit 6 is said to be in "slave mode" when its signal 4 is synchronized to the signal 24-B transmitted by the alternate reference unit 30.

[0032] FIG. 4 illustrates a positioning network 28 including a reference device 6, an alternate reference device 30, and multiple positioning-unit devices or "slaves" 2, which can be divided into several categories. In particular, a slave that periodically receives signals 4, 24-B from both the reference device 6 and the alternate reference device 30 is classified as a "tier 1 slave" 2-1. In a small network, all slaves may be tier 1 slaves. Due to interdependencies, both the reference device 6 and the alternate reference device 30 are expected to periodically receive signals 24 from the tier 1 slave 2-1. Slaves that periodically receive signals from either the reference device 6 or the alternate reference device 30, but not both, are classified as "tier 2 slaves" 2-2, while slaves that do not periodically receive signals from either the reference device 6 or the alternate reference device 30 can be classified as "tier 3 slaves" 2-3. In some cases, tier 3 slaves receive the network time reference indirectly, derived from the reference device 6 or the alternate reference device 30, via signals 24 from other slaves to which they are already synchronized. As will be explained below, an operational positioning network 28 preferably has at least one first tier slave 2-1 at all times, and more preferably there are multiple first tier slaves 2-1 in an operational network 28.

[0033] In a preferred embodiment, the reference 6, alternate reference 30, and slave 2 of the operational network 28 periodically or constantly communicate with each other via information contained in the data components of their signals, e.g., to exchange status data or intent. In other embodiments, they may communicate via other means, such as a wired or wireless local area network. Status data broadcast by the reference 6 via wireless or wired means may include, for example, data regarding the identity of the reference 6 and whether it is currently the "active reference," and similarly for the alternate reference 30. In a preferred embodiment, slave 2 periodically includes in the data components of its signal 24 information regarding (i) its own stratum status and (ii) the status of signals 4, 24-B received from the reference or alternate reference 6, 30. In other embodiments, the slave provides this information when polled by either the reference 6 or alternate reference 30. Information from a first stratum slave indicating that it is not currently receiving a usable reference signal 4, or that the reference signal is particularly weak, excessively noisy, or intermittent, may suggest, for example, that there is a problem with the reference 6.

[0034] The stratum status of a given slave 2 may change during operation of the network 28, for example, if the signal from the reference or alternate reference devices 6, 30 is temporarily blocked. In dynamic networks such as those described in WO 2016 / 011505 A1, stratum status may also change when a slave or reference device moves. For example, a stratum 1 slave may be demoted from stratum 1 status if it moves to a location where it no longer receives the reference signal 4 regularly or at all, and the information it provides may change accordingly.

[0035] In some embodiments, each slave 2 is configured to determine its stratum status as periodically receiving a signal if it has received the signal for at least a predetermined percentage of the previous predetermined period. For example, in a dynamic network where a slave's stratum status can be expected to change, a given slave 2 may be determined to be periodically receiving the signal if it has received the reference signal 4 for at least 75% of the previous 10 minutes. On the other hand, in a static network, the requirement for periodic signal reception may be met if the relevant signal has been received for at least 90% of the time since network startup. That is, the previous predetermined period may correspond to the length of time the network has been operational. Alternatively, the requirement may be met if the relevant signal has been received continuously for the previous 5 minutes. In one embodiment, the criterion for a slave 2 receiving a signal is whether the slave can "track" the signal, such as by means well known in the art of CDMA signal processing. In other embodiments, the criterion may be that the received signal power exceeds a predetermined threshold, such as a threshold in the range of -90 to -110 dBm. It will be appreciated that evaluating whether a given signal is being periodically received for purposes of determining a slave's stratum status incorporates a historical element distinct from whether the signal is currently being received or currently available. In general, it is preferred that the criteria for evaluating periodic receipt of a signal be configurable.

[0036] In some embodiments, the reference device 6 asserts itself as the “active reference” and the alternate reference device 30 is in “slave mode” during normal operation of the positioning network 28. 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 if the reference device 6 fails, or more generally, if the reference signal 4 is not or becomes unavailable. When at least one tier 1 slave 2-1 is present in the network 28, there is typically a holdover period during which the network can maintain synchronization via the alternate reference device 30’s unique positioning signal 24-B in place of the reference signal 4. While it is preferable for network synchronization stability that the alternate reference device 30 assume control of the network as soon as possible, the alternate reference device 30 typically has time to evaluate the availability of the reference signal 4 during the holdover period before asserting “active reference” status and assuming control of the network 28.

[0037] Ideally, only one of the reference unit 6 and alternate reference unit 30 should claim "active reference" status at any one time, but a problematic reference unit 6 may continue to claim this status even though its signal 4 is not available. Therefore, the positioning-unit device 2 can be configured to ignore the reference signal 4 once the alternate reference unit 30 has claimed "active reference" status, or to ignore the reference signal 4 once it has assessed that it is not available.

[0038] In some embodiments, the alternate reference device 30 takes unilateral action to exit slave mode and control the network 28 after determining that the reference signal 4 is unavailable based on one or more quality measures, such as received signal power, signal-to-noise ratio, or signal continuity, of the reference signal 4 received at the alternate reference device 30. For example, if the received power of the reference signal 4 drops below a predetermined threshold, either momentarily or continuously for a predetermined holdover period, the alternate reference device 30 may determine that the reference signal 4 is unavailable and begin controlling the network. If the alternate reference device 30 asserts an "active reference state" at this point, other positioning-unit devices 2 will attempt to synchronize their own signals 24 to the signal 24-B transmitted by the alternate reference device 30. The criteria for determining the availability of the reference signal 4 can preferably 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, and the predetermined holdover period can be in the range of 0.5 to 18 seconds, depending on the typical stability of positioning-unit device clocks. However, this unilateral approach carries the risk that the alternate reference device 30 will erroneously claim control of the network if, for example, a temporary blockage prevents the alternate reference device 30 from receiving the reference signal 4 from the fully operational reference device 6.

[0039] In a preferred embodiment, the alternate 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 its assessment of reference signal availability. Preferably, the alternate reference device 30 periodically receives information about the reference signal from one or more of the other positioning-unit devices 2, for example via a data component of the signal 24, although in other embodiments the alternate reference device 30 actively polls the other positioning-unit devices 2. The reference signal information may include one or more measures 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 availability of the reference signal 4 based on, for example, one or more measures of the quality of the reference signal.

[0040] In a preferred embodiment, the alternate reference device 30 utilizes reference signal information received "directly" from one or more first-tier slaves 2-1, i.e., positioning-unit devices that periodically receive signals 4, 24-B from both the reference device 6 and the alternate reference device 30. Alternatively, information relayed via second-tier slaves 2-2 can be received "indirectly," but this method is generally only necessary when no first-tier slaves 2-1 are present in the network 28. In a preferred embodiment, the alternate reference device 30 requires corroborative information regarding the reference signals 4 from a predetermined number or percentage of the first-tier slaves 2-1 before it can begin to control the network 28. For example, the alternate reference device 30 may require corroborative information from at least one or two, or at least 50% to 100% of the first-tier slaves 2-1, before claiming control of the network. In general, the duration of the predetermined holdover period during which the alternate reference device 30 evaluates the availability of the reference signal should be shorter than the duration of the period used by the slave 2 to determine its stratum status, so that the alternate reference device 30 can acquire the desired reference signal information before the first stratum slave 2-1 downgrades its stratum status.

[0041] Consider now the situation in which the datum 6 starts up. In a preferred embodiment, the datum 6 is configured, upon startup, to check whether the network 28 is operational, i.e., whether it can receive signals 24 from one or more of the slaves 2 in the network, including signal 24-B from the alternate datum 30, which is generally claiming "active reference" status. If the datum 6 does not detect any network signals during a first predetermined startup period, such as during network startup, it starts up normally and claims control of the network as the active reference. On the other hand, if the datum 6 detects network signals 24 during startup, such as during recovery from a failure, it joins the network 28 in slave mode and synchronizes its signal 4 to a selected one of the detected signals via a standard TLL process. If the datum 6 detects signal 24-B transmitted by the alternate datum 30 during the first predetermined startup period, which may range from 60 to 360 seconds, for example, it preferably synchronizes its signal 4 to this signal 24-B.

[0042] In some embodiments, the reference device 6 remains in this state indefinitely, and if the alternate reference device 30 fails to provide a usable signal, it prepares to assume control of the network using a methodology similar to that described above for the alternate reference device 30. Essentially, the reference device 6 and the alternate reference device 30 have swapped roles. In other embodiments, once the reference device 6 synchronizes its signal 4 to the network time reference, i.e., to the selected signal 24 from the synchronized positioning-unit device 2, it can attempt to reassert the "active reference" status in a negotiated handover from the alternate reference device 30. In preferred embodiments, this is accomplished via the reference device and alternate reference device 6, 30 monitoring the appropriate data bits in the data component of each other's signal 24-B, 4 and responding accordingly. In one embodiment, the synchronized reference device 6 flags itself as ready to resume control of the network and resumes control immediately or at an agreed-upon time point after receiving approval from the alternate reference device 30. In other embodiments, there is a further level of negotiation where datum 6 first indicates it is ready to resume control and then indicates its intent to resume control at an agreed upon time. Once datum 6 changes its state to "active reference," alternate datum 30 turns off its "active reference" state and returns to slave mode.

[0043] A method of operation of a positioning network 28 according to an embodiment of the present invention will be described with reference to the flowchart of FIG. 5 , which illustrates steps performed by the alternate reference device 30 in maintaining continuity of network synchronization through the loss of an available reference signal 4. The dashed outline indicates optional steps. The process begins with the alternate reference device 30 viewing the network under the control of the reference device 6 as operating normally, and the alternate reference device 30 periodically monitors the reference signal 4 in step 42, and then evaluates the reference signal's availability in step 44. Optionally, as indicated by step 46, the alternate reference device 30 obtains information about the reference signal from one or more first-tier slaves 2-1 for further input into the evaluation of the reference signal's availability. At decision point 48, the alternate reference device 30 determines whether the reference signal 4 is available. If it is available, the alternate reference device 30 returns to step 42. If it is not available, the alternate reference device 30 takes control of the network in step 50, e.g., by exiting slave mode and asserting an "active reference" status in the data component of its own signal 24-B. Optionally, in step 52, the alternate reference device 30 or another positioning-unit device 2 indicates a problem with the reference device 6 to a network monitor for fault determination or repair.

[0044] In some embodiments, the process ends here, with the positioning network 28 maintaining synchronization under the control of the alternate reference device 30. In other embodiments, the alternate reference device 30 periodically or continuously searches for the reference signal 4 in step 54, and determines at decision point 56 whether the reference device has synchronized its signal 4 to the network time reference. If not, the alternate reference device 30 returns to step 54 and continues to search for a reference signal while maintaining the "active reference" state. The alternate reference device 30 and reference device 6 negotiate a handover of control in step 58, after which the alternate reference device returns control of the network to the reference device in step 60. In this embodiment, the process ends with the positioning network 28 maintaining synchronization under the control of reference device 6.

[0045] FIG. 6 is a flowchart illustrating steps performed by a datum 6 of a positioning network 28 during start-up, in accordance with an embodiment of the present invention. In step 62, the datum searches for signals 24 in the network or from one or more positioning-unit devices 2 for a first predetermined start-up period, and then determines at decision point 64 whether it is receiving such network signals, including signal 24-B, possibly from an alternate datum 30. If the datum 6 is receiving network signals, such as would be the case if it were coming back online after a hardware failure, it synchronizes its own signal 4 to the selected network signal 24, 24-B, i.e., starts up in slave mode, in step 66. The process can end here, with the datum remaining in slave mode, or it can negotiate with an alternate datum to take control of the network, as described above with reference to FIG. 5. On the other hand, if the datum 6 determines that no network signals are present, as is typically the case during network start-up, it continues with the normal start-up procedure in step 68 and assumes control of the network 28.

[0046] In a preferred embodiment, the alternate reference device 30 is configured to behave similarly to the reference device 6 upon startup, except that it searches for a network signal within a second predetermined startup period, preferably longer than the first predetermined startup period used by the reference device 6, before concluding that the reference device is not operational and, accordingly, asserts control of the network. It is expected that, upon startup of the positioning network 28, the reference and alternate reference devices 6, 30 will begin their startup procedures at approximately the same time, and thus the first predetermined startup period should end before the second predetermined startup period. However, this is not required, as if the second startup period ends first, the alternate reference device 30 will take control of the network and the reference device 6 will join the network in slave mode. The second predetermined startup period can be, for example, 1.5 to 3 times longer than the first predetermined startup period.

[0047] In some embodiments, the resilience of a synchronized positioning network is further improved by designating another positioning-unit device 2 as a second alternate reference device prior to network start-up, defining an appropriate protocol for communication between the various members of the network, in the event that neither the reference device 6 nor the alternate reference device 30 can provide a usable signal. More fall-back reference devices may be designated in this manner, if required, depending, for example, on the size of the network or the importance of its continued operation.

[0048] In the embodiments described above, the time reference for the reference device 6 to be disseminated to the network 28 of positioning-unit devices 2 is generated internally by the reference device. Alternatively, the reference device may obtain the time reference from an external source. Such an embodiment is useful for transferring a time reference, such as Universal Coordinated Time (UTC), to an extended network of positioning-unit devices and devices such as position receivers that can receive and process signals from the network.

[0049] FIG. 7 illustrates 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, with a predetermined one of the positioning-unit devices designated as an alternate reference device 30. Each of the reference device 6 and the alternate reference device 30 is configured to receive a time reference from an external source 72 and align its respective signal 4, 24-B to this time reference. Note that to ensure the external time reference is disseminated in the event of a failure of the reference device 6, it is necessary for both the reference device 6 and the alternate reference device 30 to be able to receive the external time reference. 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 steered to UTC. In other embodiments, the reference device 6 and the alternate reference device 30 are configured to receive a time reference different from, or even independent of, UTC.

[0050] In normal operation of the network 70, the reference unit 6 receives a time reference from an external source 72 and generates and transmits a reference signal 4 in accordance with this time reference. Positioning-unit devices 2 within the field of view of the reference unit 6, preferably but not necessarily including a predetermined alternate reference unit 30, receive the reference signal 4, generate their own signal 24, and time-synchronize this signal with the received reference signal 4 using the TLL process described above. Positioning-unit devices outside the field of view of the reference unit 6 can time-synchronize their own signal to the reference signal 4 by receiving and synchronizing with a signal transmitted from a positioning-unit device already synchronized to the reference signal 4. Thus, the transmitted signals 24 from any number of positioning-unit devices can be aligned to the time reference obtained from the external source 72, and this time reference can be spread over any wide range or distance. Thus, a device such as a position receiver 26 that can receive and process signals 4, 24, 24-B from the network can receive this time reference, which is advantageously UTC, in addition to being able to calculate a position solution.

[0051] In this normal operating mode, a given alternate reference device 30 behaves as a standard positioning-unit device within the network 70, synchronizing its own signal 24-B to the reference signal 4 and effectively ignoring the external source 72. However, if the alternate reference device 30 determines that no reference device 6 is transmitting a usable reference signal 4, it will unilaterally, taking into account information about reference signals from other or one or more positioning-unit devices 2, switch to an "active reference" mode and synchronize its own signal 24-B to a time reference obtained from the external source 72. Other positioning-unit devices 2 then synchronize their own signal 24 to the signal 24-B, either by direct reception of the signal 24-B transmitted by the alternate reference device, or by cascading, which allows an external time reference to be forwarded in the absence of a usable reference signal 4.

[0052] The operation of network 70 described with reference to Figure 7 is similar to the operation of network 28 described with reference to Figures 3 and 4, except that the networked positioning-unit device signals are synchronized to a source of time reference. In general, the same protocols apply as to whether, for example, alternate reference device 30 should take control of the network or return control to reference device 6.

[0053] Although the present invention has been described with reference to specific examples, those skilled in the art will appreciate that the present invention can be embodied in many other forms.

Claims

1. 1. A positioning network with improved resilience for enabling a roving position receiver to determine its position using received positioning signals, comprising: a designated reference device configured to generate and transmit a reference signal according to its own time reference; a plurality of positioning-unit devices, each configured to generate a unique positioning signal and time-sequentially synchronize said unique positioning signal with said reference signal; Equipped with a predetermined one of the positioning-unit devices that was designated as an alternate reference device prior to start-up of the positioning network; (i) monitoring the reference signal; (ii) assessing the availability of said reference signal; (iii) controlling the positioning network when determining that the reference signal is not available; wherein other positioning-unit devices synchronize their own unique positioning signals to the unique positioning signal of the given positioning-unit device in a time-series manner, thereby maintaining or establishing synchronization of the positioning network despite the reference signal being unavailable. Positioning network.

2. said monitoring of said reference signal comprising measuring a quality of said reference signal received at said given Positioning-Unit Device; The positioning network of claim 1 .

3. the given positioning-unit device is configured to obtain information about the reference signal from at least one other of the positioning-unit devices for use in assessing the usability of the reference signal; A positioning network according to claim 1 or 2.

4. the information includes one or more measures of the quality of the reference signal; The positioning network of claim 3 .

5. the information includes an assessment of the availability of the reference signal. The positioning network of claim 3 .

6. the evaluation is based on one or more measures of the quality of the reference signal; The positioning network of claim 5.

7. the given positioning-unit device is configured to periodically receive the reference signal and to periodically receive the unique positioning signal from the given positioning-unit device or obtain information about the reference signal from one or more other positioning-unit devices; A positioning network according to any one of claims 3 to 6.

8. each of the other positioning-unit devices is configured to determine that it is receiving the signal periodically if it has received the signal for at least a predetermined proportion of a previous predetermined period of time; The positioning network of claim 7.

9. each of the other positioning-unit devices is configured to periodically broadcast whether it is receiving the reference signal or the unique positioning signal from the given positioning-unit device; A positioning network according to claim 7 or 8.

10. the quality of the reference signal includes received signal power, signal-to-noise ratio, or signal continuity; A positioning network according to claim 2, 4 or 6.

11. The designated reference device, upon start-up, searching for a unique positioning signal from one or more of said Positioning-Unit Devices; if the unique positioning signals are detected within a first predetermined start-up period, synchronizing the signal of the aircraft with a selected unique positioning signal from the unique positioning signals; 11. The positioning network according to claim 1, configured to:

12. the designated reference device is configured to synchronize its own signal to the specific positioning signal of the given positioning-unit device when the specific positioning signal of the given positioning-unit device is detected; The positioning network of claim 11.

13. the designated reference device and the given positioning-unit device are configured to negotiate returning control of the positioning network to the designated reference device after the designated reference device has synchronized its signal to one of the unique positioning signals; A positioning network according to claim 11 or 12.

14. the designated reference device and the given Positioning-Unit Device are configured to receive the time reference from an external source so that the time reference can be forwarded to the plurality of Positioning-Unit Devices and / or one or more position receivers; A positioning network according to any one of claims 1 to 13.

15. 1. A method of maintaining or establishing synchronization of a positioning network in the absence of an available reference signal, comprising: a designated reference device configured to generate and transmit a reference signal according to its own time base; and a plurality of Positioning-Unit Devices, each configured to generate a unique positioning signal and time-sequentially synchronize said unique positioning signal to said reference signal, said positioning network enabling a roving position receiver to determine its position using received positioning signals, the method comprising: a) determining whether a given one of said Positioning-Unit Devices, designated as an alternate reference device prior to start-up of said positioning network, (i) monitoring the reference signal; (ii) assessing the availability of said reference signal; (iii) controlling the positioning network when determining that the reference signal is not available; wherein other positioning-unit devices synchronize their own unique positioning signals to the unique positioning signal of the given positioning-unit device in a time-series manner, thereby maintaining or establishing synchronization of the positioning network despite the reference signal being unavailable. method.

16. the step of monitoring the reference signal comprises measuring a quality of the reference signal received at the given Positioning-Unit Device; 16. The method of claim 15.

17. and wherein said step of monitoring said reference signal comprises obtaining information about said reference signal from at least one other one of said positioning-unit devices for use in assessing said usability of said reference signal.

17. The method of claim 15 or 16.

18. the information includes one or more measures of the quality of the reference signal; 18. The method of claim 17.

19. the information includes an assessment of the availability of the reference signal.

18. The method of claim 17.

20. the evaluation is based on one or more measures of the quality of the reference signal; 20. The method of claim 19.

21. the given positioning-unit device periodically receives the reference signal and periodically receives the unique positioning signal from the given positioning-unit device or obtains information about the reference signal from one or more other positioning-unit devices; 21. The method of any one of claims 17 to 20.

22. each of the other positioning-unit devices determines that it is receiving the signal periodically if it has received the signal for at least a predetermined percentage of a previous predetermined period of time; 22. The method of claim 21.

23. each of the other positioning-unit devices periodically broadcasting whether it is receiving the reference signal or the unique positioning signal from the given positioning-unit device; 23. The method of claim 21 or 22.

24. the quality of the reference signal includes received signal power, signal-to-noise ratio, or signal continuity; 21. The method of claim 16, 18 or 20.

25. The designated reference device, upon start-up, searching for a unique positioning signal from one or more of said Positioning-Unit Devices; if the unique positioning signals are detected within a first predetermined start-up period, synchronizing the signal of the aircraft with a selected unique positioning signal from the unique positioning signals; 25. The method of any one of claims 15 to 24.

26. when the specific positioning signal of the predetermined positioning-unit device is detected, the designated reference device synchronizes its own signal with the specific positioning signal; 26. The method of claim 25.

27. the designated reference device and the given positioning-unit device negotiate to return control of the positioning network to the designated reference device after the designated reference device synchronizes its signal to one of the unique positioning signals; 27. The method of claim 25 or 26.

28. the designated reference device and the given Positioning-Unit Device are capable of receiving the time reference from an external source so as to be able to forward the time reference to the plurality of Positioning-Unit Devices and / or one or more position receivers; 28. The method of any one of claims 15 to 27.

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