Transcript Redundant Atomic Frequency Standard Time Keeping System

31st NASA-DOD Precise Time and Time Interval Planning Meeting (Dana Point,
California), 1999.
Redundant AFS TKS with
Seamless AFS Switch-over
Victor S. Reinhardt & Charles B. Sheckells
Hughes 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.
Redundant AFS TKS
13.4 &
10 MHz
AFS’s
VCXO
T&C
DownConverter
DownConverter
D/A
100 Hz
AFS1-4
High
Isolation
Select
Switch
Event Clock
& PLL Processor
Frequency Synthesizer Units (FSU)
10.23 MHz

Frequency
& Other
Commands
 Monitors
VCXO vs AFS
AFS1 - AFS2
• 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
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.
What is Seamless AFS
Switch-Over?
Hot Spare
AFS
f1
AFS1
On-Line
f2
f
Seamless
Switchover
Phase
Phase
Non-Seamless
Switchover
f1
No change at
Switch-Over
f1

AFS2
On-Line
Switch-Over
Time
AFS1
On-Line
AFS2
On-Line
Time
Switch-Over
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.
TKS Breadboard
• Non-Redundant
Breadboard Built
• All Critical Functions
Tested
FSU Breadboard
High Isolation Switch
(92 dB Measured
Isolation)
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.
Redundant AFS TKS
13.4 &
10 MHz
AFS’s
VCXO
T&C
DownConverter
DownConverter
D/A
100 Hz
AFS1-4
High
Isolation
Select
Switch
Event Clock
& PLL Processor
Frequency Synthesizer Units (FSU)
10.23 MHz

Frequency
& Other
Commands
 Monitors
VCXO vs AFS
AFS1 - AFS2
• 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
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.
1 ps Phase Measurement System
AFS fAFS = 13.4 MHz
4
3.17
MHz
N= 3800 Cs
N N= 4858 Rb
76
180
KHz
3.53 KHz Cs
2.76 KHz Rb
fb 
100 Hz
VCXO fo = 10.23 MHz
• Down Converter Conserves Phase Difference Between
AFS & VCXO: IF = VCXO- AFS
• Event Clock Measures Epochs tn of 100 Hz IF’s to:
dt =1/fo = 0.98 ns
• Processor Computes n = VCXO- AFS from tn:
xn = n/(2pfo) = (n - tnfb)/fo
dx = dt(fb/fo2) = 0.96 ps
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.
Phase Measurement System
Experimental Noise Floor
10-11
Standard Cs
10-12
Rb
y
10-13
Measured
TKS Noise
Floor
10-14
10-15
100 s
101 s
102 s
103 s
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.
FSDU Control Processor
From D/C
To D/A
10.23 MHz
from VCXO
98 ns
Event Clock
+
Compute
Phase

-
+

-
Phase
Error
Limiter
x 2n
2nd Order
Loop Filter 
Integrate
x n2/s
VCXO
Look
-Up
Table
Phase Integrate
Frequency
Offset
Precharge
Offset
& f
Commands
I/O &
EstimControl Logic
ators
VCXO- AFS & AFS1- AFS2
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.
Precharge Improves Transient
Response
No Precharge
3
Phase - ns
Phase - ns
4
Difference from Ideal Step
(with Precharge)
0.2
2
Precharge
1
0
0.1
0
-0.1
-0.2
0
0.5
1
1.5
Time - seconds
2
1.5 s Epochs
• Precharge Pre-loads Integrator at Each New
Frequency Command
• Generates More Ideal Stepped Frequency Response
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.
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
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.
Seamless Switchover
Measured Results
Clock Reading - ns
(Wrapping Scale)
0.5
Seamless
Switchover
Non-Seamless
Switchover
0
Transient
< 50 ps
23 ns
Step
-0.5
1.5 s Epochs
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.
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
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.
Simulation of Impact of Seamless
Switchover on 24 Hr Prediction
Phase - ns
400
80
60
300
40
200
20
0
100
-3
0
3
6
9
0
-3
0
3
6
Hours from Switchover
Seamless
Non-Seamless
9
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.
Conclusions
• Data Has Been Presented Showing the
TKS
– Provides Redundant and Steerable AFS Output
– Has Full Stability of Reference AFS’s at SubNanosecond Level
– Provides On-board AFS Diagnostic Capability
– Implements Seamless Switchover at SubNanosecond Level
• Data Also Presented Showing Seamless
AFS Switchover Can Eliminate 1-2 Week
Satellite Down Time After AFS Failure
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.