Wideband high frequency based precision time transfer

Timing interrogation methods and communication devices utilizing such timing interrogation methods are disclosed. The timing interrogation method may include: sending a round-trip timing interrogation preamble from a first radio node to a second radio node via the ionosphere; receiving a round-trip timing response at the first radio node via the ionosphere from the second radio node; receiving a time of arrival record at the first radio node via the ionosphere from the second radio node, wherein the time of arrival record indicates the time of arrival of the round-trip timing interrogation preamble at the second radio node according to timing information maintained by the second radio node; calculating timing information based on the time of arrival record and a propagation time of the round-trip timing interrogation preamble and the round-trip timing response; and adjusting time information maintained by the first radio node according to the calculated timing information.

BACKGROUND

Satellite systems, such as the Global Positioning System (GPS), utilize satellites to provide location and precision timing information to GPS receivers. During conditions referred to as anti-access area-denial (A2AD) conditions, however, reception of satellite signals (and hence precision timing information) may be severely limited and/or completely denied. To help determine timing information when satellite access has been denied, alternative non-satellite-dependent systems such as star trackers and/or eLoran may be utilized.

It is noted, however, that existing non-satellite-dependent systems are unable to cover the entire earth and are therefore unable to support transfer of operationally relevant precision time to a location anywhere in the world.

SUMMARY

In one aspect, embodiments of the inventive concepts disclosed herein are directed to a method. The method may include: sending a round-trip timing interrogation preamble from a first radio node to a second radio node via the ionosphere; receiving a round-trip timing response at the first radio node via the ionosphere from the second radio node; receiving a time of arrival record at the first radio node via the ionosphere from the second radio node, wherein the time of arrival record indicates the time of arrival of the round-trip timing interrogation preamble at the second radio node according to timing information maintained by the second radio node; calculating timing information based on the time of arrival record and a propagation time of the round-trip timing interrogation preamble and the round-trip timing response; and adjusting time information maintained by the first radio node according to the calculated timing information.

In a further aspect, embodiments of the inventive concepts disclosed herein are directed to a method. The method may include: receiving a round-trip timing interrogation preamble sent from a first radio node to a second radio node via the ionosphere; recording a time of arrival record at the second radio node, wherein the time of arrival record indicates the time of arrival of the round-trip timing interrogation preamble at the second radio node according to timing information maintained by the second radio node; sending a round-trip timing response from the second radio node to the first radio node via the ionosphere; and sending the time of arrival record from the second radio node to the first radio node via the ionosphere.

In another aspect, embodiments of the inventive concepts disclosed herein are directed to a radio. The radio may include at least one transmitter-receiver and at least one processor in communication with the at least one transmitter-receiver. The at least one processor may be configured to initiate a timing interrogation process utilizing the at least one transmitter-receiver. The timing interrogation process may include: send a round-trip timing interrogation preamble from the radio to at least one other radio via the ionosphere; receive a round-trip timing response from the at least one other radio via the ionosphere; receive a time of arrival record from the at least one other radio via the ionosphere, wherein the time of arrival record indicates the time of arrival of the round-trip timing interrogation preamble at the at least one other radio according to timing information maintained by the at least one other radio; calculate timing information based on the time of arrival record and a propagation time of the round-trip timing interrogation preamble and the round-trip timing response; and adjust time information maintained by the radio according to the calculated timing information.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the inventive concepts disclosed and claimed herein. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the inventive concepts and together with the general description, serve to explain the principles and features of the inventive concepts disclosed herein.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of the inventive concepts disclosed herein, examples of which are illustrated in the accompanying drawings.

Embodiments in accordance with the inventive concepts disclosed herein are directed to timing interrogation methods and communication devices utilizing such timing interrogation methods. More specifically, a wideband high frequency (WBHF) waveform may be combined with timing interrogation methods in accordance with the inventive concepts disclosed herein to facilitate transfer of precise knowledge of time. As will be described in details below, the timing interrogation methods and communication devices utilizing such timing interrogation methods may be utilized to transfer precise knowledge of time from one device to another device located anywhere in the world without utilization of any satellite.

Referring toFIG. 1, an illustration depicting a timing interrogation method100carried out between two radio nodes102and104within a communication system is shown. More specifically, each node102and104within the communication system may maintain a clock that indicates a standard time (e.g., Coordinated Universal Time or the like) with a predetermined fixed time update period (e.g., every 1-second). If a node (e.g., node102) realizes that its knowledge of time has degraded, that node102may need to query some other nodes within the communication system to obtain their knowledge of time. Obtaining the knowledge of time from other nodes may help the node102determine how far its 1-second time marks have drifted, which may in turn help the node102make appropriate corrections.

To help the node102make this determination, the node102may first attempt to locate one or more nodes104that have (or have access to) the knowledge of time. It is contemplated that the node102may utilize various techniques to locate the one or more nodes104with the knowledge of time. For instance, standard HF waveform tools such as Automatic Link Establishment (e.g., 4th Generation ALE), Wideband HF Signals in Space (SIS), as well as other techniques may be utilized to locate the one or more nodes104without departing from the broad scope of the inventive concepts disclosed herein.

As shown inFIG. 1, suppose the node102has located a particular node104with the knowledge of time, a round-trip timing (RTT) interrogation technique may then be initiated to facilitate transfer of the knowledge of time from the node104to the node102. For instance, the node102may send an outgoing interrogation message to the node104in a step106, and upon receiving such an interrogation message, the node104may send a response back to the node102in a step110, allowing the knowledge of time to be transferred from the node104to the node102.

Also as shown inFIG. 1, both the outgoing interrogation message and the response may be sent upwards and may refract off of the ionosphere to support long distance communication. To overcome any potential effects of ionospheric scintillation, the timing interrogation method100in accordance with the inventive concepts disclosed herein may implement a short burst RTT interrogation technique. With short burst RTT interrogation, messages utilized to facilitate timing interrogation may be kept sufficiently short so that they fit inside a time frame where the scintillation effects can be considered negligible (or non-existent). It is contemplated that this time frame may be predetermined based on the coherence bandwidth of a scintillating HF channel. For example, if it is determined that the coherence bandwidth of a scintillating HF channel in the mid latitudes is 10 Hz, it means that the path of a particular refraction may remain constant for about 100 milliseconds, after which the refraction properties may change. Therefore, in this example, 100 milliseconds may be determined to be the time frame where the scintillation effects can be considered negligible.

To keep the messages short, the node102(may be referred to as the interrogator102for clarity) may transmit a short burst RTT interrogation preamble that contains no data in the step106. In certain implementations, the short burst RTT interrogation preamble may be coded using transmission security, or TRANSEC, for security purposes, and the header information may be omitted entirely, making the short burst RTT interrogation preamble a data-less and header-less TRANSEC sequence that serves the purpose of timing interrogation. It is contemplated that the structure of this short burst RTT interrogation preamble may be set according to the initial 32-bit synchronization preamble of the WBHF message standard. It is noted that 32 bits may be considered a reasonable length to insure a unique correlation with respect to noise, yet not too long to unnecessarily consume the transmission time. It is to be understood, however, that the length of the short burst RTT interrogation preamble may vary without departing from the broad scope of the inventive concepts disclosed herein.

When the node104(may be referred to as the responder104for clarity) receives the short burst RTT interrogation preamble, the responder104may immediately record the time of arrival (TOA) data in a step108based on the knowledge of time maintained by the responder104. However, instead of sending the TOA data to the interrogator102immediately, the responder104may be configured to withhold the TOA data and send the TOA data to the interrogator102at a later time to avoid competing against a RTT response that needs to be sent within the predetermined time frame (100 milliseconds in the example presented above). In certain implementations, the responder104may be configured to send the TOA data after the completion of the short burst RTT interrogation.

The RTT response refers to a message that the responder104is configured to send out in response to receiving the short burst RTT interrogation preamble. It is noted that the responder104may need some processing time to switch from a receive mode to a transmit mode before the responder104is ready to send out the RTT response. To minimize any potential processing time variations during this switch, a predetermined amount of turnaround time may be specified in certain implementations to ensure that the responder104has adequate time to handle the switch, and in case the responder104is able to complete the switch faster than the predetermined turnaround time, the responder104may be forced to wait till the end of the predetermined turnaround time to send out the RTT response. It is contemplated that this predetermined amount of turnaround time may be agreed upon by all nodes participating in the communication system (including the interrogator102and the responder104). In certain implementations, the predetermined amount of turnaround time may be 1 millisecond, but it may vary without departing from the broad scope of the inventive concepts disclosed herein.

The RTT response sent from the responder104may be delivered to the interrogator102in the step110. The interrogator102may also expect to receive the TOA from the responder104as a separate message (as previously mentioned). Based on the RTT response received, the interrogator102may determine the amount of time it took to complete the interrogation (commonly referred to as the propagation time), and by knowing both the propagation time and the TOA provided by the responder104, the interrogator102may calculate accurate timing information in a step112and adjust its own reference of time (e.g., the time information or the clock maintained by the interrogator102) accordingly in a step114.

It is noted that the purpose of keeping the RTT interrogation preamble and the RTT response short as described above is so that they can be transmitted within the time frame where the scintillation effects may be considered negligible. The following example may help illustrate the effectiveness of this approach. More specifically, suppose the interrogator102and the responder104both implement a relatively low data rate of 1800 bits/second, meaning that it would take the interrogator102about 17.8 milliseconds to complete transmission of a 32-bit short burst RTT interrogation preamble. Further suppose that the distance from the interrogator102to the ionosphere and from the ionosphere to the responder104are both5000nautical miles, meaning that it would take about 30.9 milliseconds of propagation time for the RTT interrogation preamble to reach the ionosphere (shown as step106A inFIG. 1) and another 30.9 milliseconds of propagation time for the RTT interrogation preamble to reach the responder104(shown as step106B inFIG. 1).

Similarly, from the perspective of the responder104, it would take the responder104about 17.8 milliseconds to complete the reception of the 32-bit short burst RTT interrogation preamble, and after a predetermined amount of turnaround time (e.g., 1 millisecond as previously mentioned), the responder104may send a RTT response back to the interrogator102. Continuing with the example above, it would take about 30.9 milliseconds of propagation time for the RTT response to reach the ionosphere (shown as step110A inFIG. 1) and another 30.9 milliseconds of propagation time for the RTT response to reach the interrogator102(shown as step1108inFIG. 1).

The following time table provides a summary of these process steps:

It is noted that the propagation time from the interrogator102up to the ionosphere and the propagation time from the ionosphere back down to the interrogator102(corresponding to steps106A and1108inFIG. 1) do not need to be counted in the coherence time calculations. In other words, the coherence time calculations may start when the RTT interrogation preamble reaches the ionosphere and end when the last bit of the RTT response refracts off of the ionosphere. This means that entries2and7in the time table presented above are not applicable to the total time consumption calculation. The true total time consumption illustrated in this example is 98.4 milliseconds, which is under the 100-millisecond target, indicating that the short burst RTT interrogation technique in accordance with the inventive concepts disclosed herein is able to support RTT interrogation for distances as far as 10,000 nautical miles apart without being significantly affected by the ionospheric scintillation.

It is to be understood that the specific references to data rates, data lengths, and distances in the examples above are merely exemplary and are not meant to be limiting. The examples are shown to illustrate that the message structure configured in accordance with the inventive concepts disclosed herein allows the interrogation preamble and response to be transferred within the coherence time of an HF channel, and it is contemplated that specific implementations may vary without departing from the broad scope of the inventive concepts disclosed herein. It is estimated that utilizing the timing interrogation method100in accordance with the inventive concepts disclosed herein may provide the interrogator102with accurate knowledge of time within less than ±100 microseconds of “truth” and can be delivered while maintaining a covert HF signal-to-noise ratio that is 6 dB below the noise floor. It is also noted that this accuracy is not degraded by distance, which may be appreciated in various operational conditions.

It is also to be understood that the specific references to HF and/or WBHF channels in the examples above are merely exemplary and are not meant to be limiting. It is contemplated that the timing messages (e.g., the RTT interrogation preambles and the RTT responses) may be sent using various types of channels and/or modulation techniques without departing from the broad scope of the inventive concepts disclosed herein.

FIG. 2is a simplified block diagram depicting an exemplary radio node200configured to support the timing interrogation method100presented above. As shown inFIG. 2, the node200may include one or more processors202, one or more non-transitory processor-readable memories204, and one or more transmitters and/or receivers206. The one or more non-transitory processor-readable memories204may be utilized to store processor-executable code. The one or more processors202may be implemented as dedicated processing units, application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or various other types of processors or processing units. When the processor-executable code stored in the one or more non-transitory processor-readable memories204is executed by the one or more processors202, the one or more processors202may carry out the various functions and method steps previously described, allowing the node200to function as either an interrogator or a responder, depending on the specific operating condition at the time of the execution.

It is to be understood that embodiments of the inventive concepts disclosed herein may be conveniently implemented in forms of a software, hardware or firmware package. Such a package may be a computer program product which employs a computer-readable storage medium including stored computer code which is used to program a computer to perform the disclosed function and process of the inventive concepts disclosed herein. The computer-readable medium may include, but is not limited to, any type of conventional floppy disk, optical disk, CD-ROM, magnetic disk, hard disk drive, magneto-optical disk, ROM, RAM, EPROM, EEPROM, magnetic or optical card, or any other suitable media for storing electronic instructions.

It is to be understood that the specific order or hierarchy of steps in the processes disclosed is an example of exemplary approaches. It is to be understood that the specific order or hierarchy of steps in the processes may be rearranged while remaining within the broad scope of the present disclosure. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented.

It is believed that the inventive concepts disclosed herein and many of their attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the broad scope of the inventive concepts or without sacrificing all of their material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.