Precision two-way time transfer over meteorburst communications channels

Patent metadata

Field	Value
Publication number	US-12015992-B2
Application number	US-202217835148-A
Country	US
Kind code	B2
Filing date	Jun 8, 2022
Priority date	Jul 7, 2021
Publication date	Jun 18, 2024
Grant date	Jun 18, 2024

Abstract

Systems and methods for clock synchronization are disclosed. A client synchronization system (CSS) may transmit RF pulses to a server CSS over micrometeorite ionization trail (MMIT) channels, and may receive RF pulses from the server CSS over MMIT channels, each received RF pulse following transmitted RF pulse. The client CSS may receive from the server CSS over MMIT channels measurement data including pulse arrival times at the server CSS. The measurement and transmission data may be correlated to identify RF-pulse pairs, each pairing a transmitted RF pulse received by the server CSS with a received RF pulse received from the server CSS, both over the same MMIT. TWTT analysis may be applied to timing data the pairs to compute time offsets between a client clock and a server clock. An analytical model of clock drift may be applied to the offsets to synchronize the client clock to the server clock.

First claim

What is claimed is:

1. A client clock synchronization system (CSS) configured for communications with a server CSS over micrometeorite ionization trail (MMIT) channels, the client CSS comprising:
an antenna;
a transceiver coupled with the antenna and configured for switching between transmit and receive modes; and
a computing system coupled with the transceiver and with a client clock, and configured for carrying out operations including:
via the antenna, with the transceiver in transmit mode, transmitting a sequence of radio-frequency (RF) pulses to the server CSS over a first multiplicity of MMIT channels, wherein transmission information indicating a respective local transmit identifier and respective transmission time of each transmitted RF pulse is maintained at the client CSS;
via the antenna, with the transceiver in receive mode, receiving a first plurality of RF pulses from the server CSS over a second multiplicity of MMIT channels, each received RF pulse consecutively following one of the transmitted RF pulses of the sequence, wherein reception information indicating a respective local receive identifier and arrival time of each received RF pulse is maintained at the client CSS;
via the antenna, with the transceiver in receive mode, receiving measurement data from the server CSS over a third multiplicity of MMIT channels, wherein the measurement data comprise, for each of a second plurality of RF pulses received by the server CSS, a respective remote receive identifier, a respective arrival time at the server CSS, and a respective signal-to-noise (SNR) measurement;
correlating the measurement data with the transmission information to identify respective transmitted RF pulses of the sequence with respective RF pulses of the second plurality received by the server CSS at the respective arrival times with the respective SNR measurements;
correlating the transmission times of the identified respective transmitted RF pulses with the reception information to determine a third plurality of respective RF-pulse pairs, wherein each respective RF-pulse pair comprises (i) a particular transmitted RF pulse that was received by the server CSS over a particular MMIT channel and (ii) a particular RF pulse that was received from the server CSS over the same particular MMIT channel and consecutively following transmission of the particular transmitted RF pulse;
applying two-way time transfer (TWTT) analysis to timing data of each RF-pulse pair to compute a set of time offsets of the client clock with respect to a server clock of the server CSS; and
applying an analytical model of clock drift to the set of time offsets to synchronize the client clock with the server clock.

2. The client CSS of claim 1 , further comprising a switched amplifier component coupled with the transceiver, and configured for switching the transceiver between transmit and receive modes at a switching rate within a range of [5 Hz, 150 Hz], while operating within a power range of [0.5 kW, 2.5 kW],
wherein transmitting with the transceiver comprises the switched amplifier component switching the transceiver to transmit mode,
wherein receiving with the transceiver comprises the switched amplifier component switching the transceiver to receive mode.

3. The client CSS of claim 1 , wherein the antenna comprises a directional antenna,
wherein transmitting the sequence of RF pulses to the server CSS over the first multiplicity of MMIT channels comprises transmitting the sequence of RF pulses at a target azimuthal angle substantially directed to the server CSS, and at a target elevation angle directed toward the Earth's ionosphere and substantially coincident with an incident angle that reflects toward the server CSS, and,
and wherein receiving the first plurality of RF pulses from the server CSS over the second multiplicity of MMIT channels comprises receiving the first plurality of RF pulses from the target azimuthal angle, and directed from the Earth's ionosphere substantially along the target elevation angle.

4. The client CSS of claim 1 , wherein the sequence of RF pulses is transmitted in a periodic sequence of pulse repetition intervals (PRIs), one transmitted RF pulse per PRI,
wherein each PRI comprises a transmit window followed by a receive window,
wherein each RF pulse of the sequence is transmitted during one of the transmit windows,
wherein each RF pulse of the first plurality is received during one of the receive windows,
wherein the respective local transmit identifier of a given transmitted RF pulse of the sequence is a sequence number of the PRI in which the given transmitted RF pulse was transmitted,
wherein the respective remote receive identifier of a given RF pulse of the second plurality is a sequence number tracked by the server CSS of the PRI in which the given RF pulse of the second plurality was received by the server CSS,
and wherein correlating the measurement data with the transmission information comprises matching the respective local transmit identifiers with the respective remote receive identifiers.

5. The client CSS of claim 4 , wherein the PRI is 25 milliseconds.

6. The client CSS of claim 1 , wherein receiving the first plurality of RF pulses comprises:
during each respective receive window that consecutively follows each transmitted RF pulse, applying a matched filter to data samples from a received signal sampled at a fixed sampling rate; and
applying a threshold to the matched filter output of each respective receive window to determine which receive windows yield a detected pulse,
and wherein the operations further include determining an arrival time of each received RF pulse by fitting an arrival-time curve to each detected pulse.

7. The client CSS of claim 6 , wherein the sampling rate is 25 MHz.

8. The client CSS of claim 6 , wherein fitting the arrival-time curve to each detected pulse comprises applying an interpolation to the data samples of each detected pulse to compute an effective time resolution higher than that of the sampling rate.

9. The client CSS of claim 1 , wherein the set of time offsets comprises sparse, intermittent time offset measurements with associated uncertainties derived from SNR measurements of both the first plurality of RF pulses measured by the client CSS and the second plurality of RF pulses measured by the server CSS,
wherein the analytical model of clock drift comprises a control algorithm with a state representation of time offset and frequency offset, and a sequential state estimator configured to compensate for the sparse, intermittent time offset measurements,
and wherein applying the model of clock drift to the set of time offsets to synchronize the client clock with the server clock comprises:
updating a covariance matrix of the analytical model for each respective time offset of the set;
computing an updated state based on the respective offset; and
applying control to adjust the client clock according to the updated state.

10. The client CSS of claim 1 , wherein applying the analytical model of clock drift to the set of time offsets to synchronize the client clock with the server clock comprises synchronizing the client clock to within 4 nanoseconds agreement with the server clock.

11. The client CSS of claim 1 , wherein transmission of RF pulses and reception of RF pulses are carried out using a common, first RF frequency,
and wherein reception of the measurement data is carried out using one of: time multiplexing with a second RF frequency that is the same as the first RF frequency, or frequency multiplexing with a second RF frequency that is different than the first RF frequency.

12. A server clock synch

Assignees

• Stanford Res Inst Int

Inventors

• Sparr Robert H
• Wodin Jesse
• Landrum Justin D
• Strus Joseph M
• Erkocevic Boskin
• Cooper Jacob M
• Watt David
• Arndt Joshua A

Classifications

• H04W56/0015 Primary

  one node acting as a reference for the others · CPC title

• H04B7/145

  Passive relay systems · CPC title

• H04B7/22

  Scatter propagation systems {, e.g. ionospheric, tropospheric or meteor scatter} · CPC title

• H04W56/005 Primary

  compensating for timing error by adjustment in the receiver · CPC title