1. 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. 31st NASA-DOD Precise Time and Time Interval Planning Meeting (Dana Point, California), 1999.

2. 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 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 VCXO 10.23 MHz   Event Clock & PLL Processor Frequency & Other Commands High Isola- tion Select Switch Down- Converter  Monitors VCXO vs AFS AFS 1 - AFS 2 13.4 & 10 MHz AFS’s Down- Converter T&C Frequency Synthesizer Units (FSU) 100 Hz AFS1-4

3. 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? Non-Seamless Switchover Time AFS1 On-Line AFS2 On-Line  f1f1 f2f2 Switch-Over Hot Spare AFS No change at Switch-Over Time AFS1 On-Line AFS2 On-Line Switch-Over f1f1 f1f1 ff Seamless Switchover

4. 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)

5. 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 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 AFS 1 - AFS 2 Phase and Frequency –Implements Seamless Switch-over to Hot Spare D/A VCXO 10.23 MHz   Event Clock & PLL Processor Frequency & Other Commands High Isola- tion Select Switch Down- Converter  Monitors VCXO vs AFS AFS 1 - AFS 2 13.4 & 10 MHz AFS’s Down- Converter T&C Frequency Synthesizer Units (FSU) 100 Hz AFS1-4

6. 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. Down Converter Conserves Phase Difference Between AFS & VCXO:  IF =  VCXO -  AFS Event Clock Measures Epochs t n of 100 Hz IF’s to: Processor Computes  n =  VCXO -  AFS from t n : 1 ps Phase Measurement System VCXO f o = 10.23 MHz AFS f AFS = 13.4 MHz 180 KHz  76 3.53 KHz Cs 2.76 KHz Rb NN N= 3800 Cs N= 4858 Rb f b  100 Hz 3.17 MHz 44  t =1/f o = 0.98 ns  x n =  n /(2  f o ) = (n - t n f b )/f o  x =  t(f b /f o 2 ) = 0.96 ps

7. 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 3 s10 2 s10 1 s10 0 s 10 -11 10 -12 10 -13 10 -14 10 -15 Measured TKS Noise Floor Standard Cs Rb  y 

8. 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  + - Phase Offset Compute Phase  + Frequency Offset - Phase Error Limiter Integrate VCXO Look -Up Table  & f Estim- ators I/O & Control Logic Commands  Precharge 2nd Order Loop Filter x 2  n 98 ns Event Clock  VCXO -  AFS &  AFS1 -  AFS2 To D/A From D/C 10.23 MHz from VCXO Integrate x  n 2 /s

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. 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)

10. 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

11. 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 Seamless Switchover Non-Seamless Switchover Transient < 50 ps 23 ns Step

12. 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

13. 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

14. 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 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