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METHOD OF ACCURATE TIMEKEEPING THROUGH OPTICAL INTER-SATELLITE CROSS-LINK

IP.com Disclosure Number: IPCOM000009718D
Original Publication Date: 2000-Jan-01
Included in the Prior Art Database: 2002-Sep-12
Document File: 3 page(s) / 124K

Publishing Venue

Motorola

Related People

Dan Bishop: AUTHOR [+2]

Abstract

This paper describes a method of intersatellite timekeeping of a satellite constellation based on exchange of time via optical cross-links. Through proper selection of bandwidth of the optical link and the clock one can demonstrate constellation time variance as low as several nanoseconds.

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Technical MO-LA @ Developments

     METHOD OF ACCURATE TIMEKEEPING THROUGH OPTICAL INTER-SATELLITE CROSS-LINK

by Dan Bishop and John Herzberg

ABSTRACT

  This paper describes a method of intersatellite timekeeping of a satellite constellation based on exchange of time via optical cross-links. Through proper selection of bandwidth of the optical link and the clock one can demonstrate constellation time variance as low as several nanoseconds.

INTRODUCTION

  Accurate satellite constellation timekeeping is an important function in order to coordinate com- munications and maneuvers. One such example is a LEO (low earth orbit),TDMA/FDMA communica- tion constellation for which efficient use of the bandwidth and time slots will maximize user data rates. In such a system the user equipment makes tightly synchronized handoffs between satellites without disruption of traffic channel communica- tion. The current timekeeping approach relies on commercial receivers, which communicate with the
U.S. government's GPS constellation.

Satellites in a multi-satellite constellation would

currentIy coordinate time by fitting themselves with a GPS receiver. These receivers are capable of trian- gulating signals from several GPS satellites to obtain local position and time The current 3-sigma variation in time produced by such a GPS receiver and thus the constellation is approximately 160 ns.

CONSTELLATION CROSS-LINK TIMEKEEPING

  For systems using optical cross-links, one can demonstrate constellation time variance of the order of several nanoseconds with proper selection of the bandwidth of the optical link and clock capabilities available on each satellite,. For a 10 Gigabit/second

cross-links are used the system could keep time to
0.25 nsec. With a 100 Mbit/set system, timing could be 10 nsec, still at least an order of magnitude better than GPS.

CONCEPT OF OPERATION

  The satellite sequence of operation is shown in Figure 1. Precise time measurements can be accom- plished by having both satellites initiate a range measurement flag as near as possible to their main timing pulse. To work effectively, the counters on each satellite must be large enough to encompass the offset in both satellite clocks plus the full optical mund trip time between each satellite. By logging the departure and the receipt of their range signal in addition to logging the r...