Redundant AFS TKS with Seamless AFS Switch-over Victor S. Reinhardt & Charles B. Sheckells Hughes Space and Communications Company P. O. Box 92919, Los

Redundant AFS TKS with Seamless AFS Switch-over

Victor S. Reinhardt & Charles B. SheckellsHughes Space and Communications Company

P. O. Box 92919, Los Angeles, CA 90009

• Overview of Redundant Atomic Frequency Standard (AFS) Time Keeping System (TKS)

• Theory of TKS Operation & Experimental Results

• Advantages of TKS for GPS other Time Distribution Systems

Copyright 2005 Victor S. Reinhardt--Rights to copy material is granted so long as a source reference is listed on each page, section, or graphic utilized.

31st NASA-DOD Precise Time and Time Interval Planning Meeting (Dana Point, California), 1999.


Redundant AFS TKS

• 10.23 MHz Output Phase Locked to AFS Input• Utilizes Multiple 13.4 or 10 MHz (5MHz with x2) AFS’s• Phase & Frequency Adjustable (1ps, 0.02uHz Resolution) without

Disturbing AFS’s• Monitors Phase of Hot Spare AFS (1.3 ps Resolution)• Seamless Switchover (No Phase or Frequency Change) to Hot Spare

D/A

VCXO10.23 MHz

Event Clock& PLL Processor

Frequency& Other

Commands

HighIsola-tion

SelectSwitch Down-

Converter Monitors

VCXO vs AFSAFS1 - AFS2

13.4 & 10 MHzAFS’s

Down-Converter

T&C Frequency Synthesizer Units (FSU)

100

Hz

AF

S1-

4


What is Seamless AFS Switch-Over?

Non-SeamlessSwitchover

Time

AFS1On-Line

AFS2On-Line

f1

f2

Ph

as

e

Switch-Over

Hot Spare AFS

No change at Switch-Over

Time

AFS1On-Line

AFS2On-Line

Ph

as

e

Switch-Over

f1

f1f

SeamlessSwitchover


TKS Breadboard

• Non-Redundant Breadboard Built

• All Critical Functions Tested

FSU Breadboard

High Isolation Switch (92 dB Measured

Isolation)


Redundant AFS TKS

• 1 ps Phase Measurement System– Downconverters & Event Clock

• PLL Processor– Computes VCXO-AFS Phase Differences From Event Clock Epochs– Implements Digital PLL with Phase and Frequency Offsets – Monitors AFS1 - AFS2 Phase and Frequency – Implements Seamless Switch-over to Hot Spare

D/A

VCXO10.23 MHz

Event Clock& PLL Processor

Frequency& Other

Commands

HighIsola-tion

SelectSwitch Down-

Converter Monitors

VCXO vs AFSAFS1 - AFS2

13.4 & 10 MHzAFS’s

Down-Converter

T&C Frequency Synthesizer Units (FSU)

100

Hz

AF

S1-

4


• Down Converter Conserves Phase Difference Between AFS & VCXO: IF = VCXO- AFS

• Event Clock Measures Epochs tn of 100 Hz IF’s to:

• Processor Computes n = VCXO- AFS from tn:

1 ps Phase Measurement System

VCXO fo = 10.23 MHz

AFS fAFS = 13.4 MHz

180

KHz

76

3.53 KHz Cs

2.76 KHz Rb

N

N= 3800 CsN= 4858 Rb

fb 100 Hz

3.17

MHz

4

t =1/fo = 0.98 ns

xn = n/(2fo) = (n - tnfb)/fo x = t(fb/fo2) = 0.96 ps


Phase Measurement System Experimental Noise Floor

103 s102 s101 s100 s

10-11

10-12

10-13

10-14

10-15

MeasuredTKS Noise

Floor

Standard Cs

Rby


FSDU Control Processor

+

-

PhaseOffset

ComputePhase

+

FrequencyOffset

-

PhaseError

Limiter

Integrate

VCXOLook-Up

Table

& fEstim-ators

I/O &Control Logic

Commands

Precharge

2nd OrderLoop Filter

x 2n

98 nsEvent Clock

VCXO- AFS & AFS1- AFS2

To D/AFrom D/C

10.23 MHzfrom VCXO

Integratex n

2/s


Precharge Improves Transient Response

• Precharge Pre-loads Integrator at Each New Frequency Command• Generates More Ideal Stepped Frequency Response

Difference from Ideal Step (with Precharge)

0

1

2

3

4

0 0.5 1 1.5 2Time - seconds

Ph

ase

- n

s

No Precharge

Precharge

-0.2

-0.1

0

0.1

0.2

1.5 s EpochsP

has

e -

ns


Seamless Switchover

• Processor Monitors Primary AFS– Beat Failure– AFS Health Through AFS Telemetry & Command– Can Monitor Stability vs VCXO

• Processor Monitors Hot Spare– Measures Phase and Frequency Relative to Primary AFS– Can Operate VCXO on Average of AFS’s

• At Detected Primary Failure or Switchover Command– Switches PLL Phase Lock to Hot Spare– Inserts Phase and Frequency Offsets to Minimize Change to VXCO

Output


-0.5

0

0.5

1.5 s Epochs

Clo

ck

Re

ad

ing

- n

s(W

rap

pin

g S

ca

le)

Seamless Switchover Measured Results

SeamlessSwitchover

Non-SeamlessSwitchover

Transient< 50 ps

23 nsStep


Impact of Seamless Switchover TKS on GPS Availability

• For Current System When AFS Fails on GPS Satellite – Must Identify Failed Satellite– Must Turn-on and Monitor new AFS– Must Reset OCS Kalman Filters and Restablish Predictions– 1-2 Weeks of Spacecraft downtime

• In Satellite with Seamless Switchover TKS– Pre-failure Features

• Advance Check-out of Spare & Main AFSs• No SC Downtime for Clock Maintenance• Diagnose AFSs from SV On or Off Line

– Seamless Switchover to 2nd Unit• No Satellite Downtime During or After AFS Switchover• Switchover Does Not Glitch OCS Kalman Filters


Simulation of Impact of Seamless Switchover on 24 Hr Prediction

0

100

200

300

400

-3 0 3 6 9Hours from Switchover

Ph

as

e -

ns

Seamless Non-Seamless

0

20

40

60

80

-3 0 3 6 9


Conclusions

• Data Has Been Presented Showing the TKS

– Provides Redundant and Steerable AFS Output– Has Full Stability of Reference AFS’s at Sub-

Nanosecond Level– Provides On-board AFS Diagnostic Capability– Implements Seamless Switchover at Sub-

Nanosecond Level

• Data Also Presented Showing Seamless AFS Switchover Can Eliminate 1-2 Week Satellite Down Time After AFS Failure

Documents

Redundant AFS TKS with Seamless AFS Switch-over Victor S. Reinhardt & Charles B. Sheckells Hughes Space and Communications Company P. O. Box 92919, Los