Precision relative positioning and free space time transfer between moving platforms

Patent metadata

Field	Value
Publication number	US-2018317184-A1
Application number	US-201715582555-A
Country	US
Kind code	A1
Filing date	Apr 28, 2017
Priority date	Apr 28, 2017
Publication date	Nov 1, 2018
Grant date	—

Abstract

Aspects herein describe techniques for synchronizing clocks between two moving platforms (e.g., land vehicles, ships, aircraft, and the like) using optical signals generated from lasers. In one aspect, a method for mixing an intermediate signal with a local reference clock in a method of communicating between moving platforms includes mixing the intermediate signal with a sine representation of the local reference clock to generate a first offset error; mixing the intermediate signal with a cosine representation of the local reference clock to generate a second offset error; and combining the first and second offset errors to generate the fine phase offset.

First claim

What is claimed is:

1 . A method for mixing an intermediate signal with a local reference clock in a method of communicating between moving platforms, the method comprising:
transmitting a first wireless signal from a first moving platform to a second moving platform, the first wireless signal comprising modulated data (i) indicating a first timestamp for a first clock operating in the first moving platform at a first frequency and (ii) identifying a timing pulse in the first wireless signal indicating when the first clock is at the first timestamp; receiving, at the first moving platform, a second wireless signal transmitted by the second moving platform, the second wireless signal comprising modulated data (i) indicating a second timestamp for a second clock operating in the second moving platform at a second frequency different from the first frequency and (ii) identifying a timing pulse in the second wireless signal indicating when the second clock is at the second timestamp; determining a first phase offset between the first clock and the second clock by evaluating the second timestamp;
wherein determining the first phase offset between the first clock and the second clock comprises:
identifying a coarse phase offset and a fine phase offset, wherein a value of the fine phase offset is less than a clock cycle of the second clock;
wherein identifying the fine phase offset comprises:
recovering the second clock at the first moving platform based on the second wireless signal;
mixing the recovered clock with the first clock to identify an intermediate signal; and
mixing the intermediate signal with the local reference clock in the first moving platform to identify the fine phase offset, wherein the first clock is synthesized from the local reference clock; and

combining the coarse phase offset and the fine phase offset to yield the first phase offset

transmitting the first phase offset from the first moving platform to the second moving platform; receiving, at the first moving platform, a second phase offset transmitted by the second moving platform, wherein the second phase offset defines an offset between the first clock and the second clock relative to the second moving platform and is based on the first timestamp; and synchronizing the first clock and the second clock at the first moving platform based on the first and second phase offsets, wherein mixing the intermediate signal with the local reference clock comprises:
mixing the intermediate signal with a sine representation of the local reference clock to generate a first offset error;
mixing the intermediate signal with a cosine representation of the local reference clock to generate a second offset error; and
combining the first and second offset errors to generate the fine phase offset.

2 . A method for mixing an intermediate signal with a local reference clock in a method of communicating between moving platforms, the method comprising:
transmitting a first wireless signal from a first moving platform to a second moving platform, the first wireless signal comprising modulated data (i) indicating a first timestamp for a first clock operating in the first moving platform at a first frequency and (ii) identifying a timing pulse in the first wireless signal indicating when the first clock is at the first timestamp; receiving, at the first moving platform, a second wireless signal transmitted by the second moving platform, the second wireless signal comprising modulated data (i) indicating a second timestamp for a second clock operating in the second moving platform at a second frequency different from the first frequency and (ii) identifying a timing pulse in the second wireless signal indicating when the second clock is at the second timestamp; determining a first phase offset between the first clock and the second clock by evaluating the second timestamp;
wherein determining the first phase offset between the first clock and the second clock comprises:
identifying a coarse phase offset and a fine phase offset, wherein a value of the fine phase offset is less than a clock cycle of the second clock; and
combining the coarse phase offset and the fine phase offset to yield the first phase offset;

recovering the second clock at the first moving platform based on the second wireless signal; identifying a difference between an edge of the first clock and an edge of the second clock over a predefined window of time greater than or equal to a beat period of the local reference clock used to synthesize the first clock; identifying an adjustment for the fine phase offset based on when the difference between the edges of the first and second clocks is at a minimum; transmitting the first phase offset from the first moving platform to the second moving platform; receiving, at the first moving platform, a second phase offset transmitted by the second moving platform, wherein the second phase offset defines an offset between the first clock and the second clock relative to the second moving platform and is based on the first timestamp; and synchronizing the first clock and the second clock at the first moving platform based on the first and second phase offsets.

3 . The method of claim 2 , further comprising:
monitoring a flip-flop to determine when an edge of the first clock and an edge of the second clock cross; identifying samples of the second wireless signal captured at the first moving platform corresponding to when the edges of the first and second clocks cross; and identifying an adjustment for the coarse phase offset based on the identified samples.

4 . A method for mixing an intermediate signal with a local reference clock in a first moving platform, the first moving platform comprising:
a pulse encoder configured to transmit a first wireless signal from a first moving platform to a second moving platform, the first wireless signal comprising modulated data (i) indicating a first timestamp for a first clock operating in the first moving platform at a first frequency and (ii) identifying a timing pulse in the first wireless signal indicating when the first clock is at the first timestamp; a recovery module configured to receive a second wireless signal transmitted by the second moving platform, the second wireless signal comprising modulated data (i) indicating a second timestamp for a second clock operating in the second moving platform at a second frequency different from the first frequency and (ii) identifying a timing pulse in the second wireless signal indicating when the second clock is at the second timestamp; and control logic configured to determine a first phase offset between the first clock and the second clock by evaluating the second timestamp, wherein the pulse encoder is further configured to transmit the first phase offset from the first moving platform to the second moving platform and the recovery module is further configured to receive a second phase offset transmitted by the second moving platform, wherein the second phase offset defines an offset between the first clock and the second clock relative to the second moving platform and is based on the first timestamp,
wherein determining the first phase offset between the first clock and the second clock comprises:
identifying a coarse phase offset and a fine phase offset, wherein a value of the fine phase offset is less than a clock cycle of the second clock, and
wherein identifying the fine phase offset comprises:
recovering the second clock at the first moving platform based on the second wireless signal;
mixing the recovered clock with the first clock to identify an intermediate signal; and
mixing the intermediate signal with the local reference clock in the first moving platform to identify the fine phase offset, wherein the

first clock is synthes

Assignees

• Boeing Co

Inventors

• Bushnell Glenn S

Classifications

• H04L7/0075

  with photonic or optical means · CPC title

• H04W56/001 Primary

  Synchronization between nodes · CPC title

• H04L7/0016

  correction of synchronization errors · CPC title

• H04B10/00

  Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication · CPC title

• H04W56/0025 Primary

  synchronizing potentially movable access points · CPC title