The CHOMPTT Precision Time Transfer CubeSat Mission

Transcription

1 The CHOMPTT Precision Time Transfer CubeSat Mission John W. Conklin*, Paul Serra, Nathan Barnwell, Seth Nydam, Maria Carrascilla, Leopoldo Caro, Norman Fitz-Coy

2 Background and Motivation GPS constellation Common View Non-common View T2L2 mission [P. Guillemot et al 2006] John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 2/14

3 CHOMPTT: CubeSat Handling Of Multisystem Precision Time Transfer GPS A t gpsa + position A t gpsb + position B GPS B Ground Station Clock discrepancy SLR Facility John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 3/14

4 Concept of Operations John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 4/14

5 Satellite Overview Antenna, GPS, Radio (UHF/VHF) ADCS (active magnetic) EPS Batteries CDH (MSP430) OPTI MAC CSAC Instrument Boards Retroreflector, Light Collectors John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 5/14

6 Optical Precision Time-transfer Instrument (OPTI) Demo SLR Emulator Space Segment Laser, Pulse driver Beam Splitter t ground CSAC Event Timer t cubesat CSAC Event Timer APD t 0 ground APD t 2 ground APD t 1 cubesat John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 6/14

7 Characteristic Atomic Clocks (Microsemi) Chip Scale Atomic Clock (CSAC) Standard Cesium Rubidium Allan Deviation (time error) 6000 sec (20 nsec) Power 0.12 W 5 W Mass 35 g 85 g Miniature Atomic Clock (MAC) 6000 sec (6 nsec) Size (LxWxH) x x mm 51 x 51 x 18 mm John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 7/14

8 10 ps Event Timer Time-to-digital converter measures fine time Measurement based on propagation delay Autonomous temperature compensation using DLL Low power (132 mw) 10 ps single shot accuracy (12 ps measured) MSP430 microcontroller - course time TDC-GPX PD TDC Start TDC time (fine time) TDC Stop Clock True time Pulse counter (coarse time) Counter reading Ti MSP430 John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 8/14

9 Optics & Light Detection PLX retroreflector 25 mm diam, 50 FOV Space capable Avalanche photodetectors (2) Si (532 nm, 1064 nm): 500 ps rise InGaAs (1064 nm): 140 ps rise Light collection Light collected by optical fiber on nadir face 12 max incidence GRIN lens focuses light onto APD APD PLX Retroreflector APD electronics Fiber coupler / TEC John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 9/14

10 Timewalk Correction 0.5 V to 2.5 V Δt = 230 psec Apparent timing variations due to pulse amplitude variations Atmosphere, attitude, range, Solution: Time both rising and falling edges of pulse Threshold Time-to-digital converter Start Pulse Amplitude Time Time Stamp Signal Clock Stop 1 Stop 2 John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 10/14

11 Measured Performance Clock difference (2 CSACs) measured using OPTI breadboard Several glitches filtered in software 50 χ (nsec) Elapsed time (ksec) John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 11/14

12 Timing Error Budget Timing error, t (nsec) GPS Time (20 nsec) 10 nsec 1 nsec Predicted Timing Budget Breadboard One Orbit Averaging time τ (sec) John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 12/14

13 Laser Communication 2-Pulse Position Modulation (2 slots per pulse) Synchronization string provides phase, rate, and masks SLR delays Fine time required only for first timing pulse Timed laser pulse Synchronization string Timing data (20 bytes) Checksum (2 bytes) Repeated if low link quality TRUE/1 FALSE/0 Sync. error Comm. Loss or sync. error John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 13/14

14 Status and Future Prototype OPTI (silver) fabricated and tested Summer 2014 OPTI integrated into CHOMPTT satellite Fall 2014 Qualification testing at NASA KSC Vibration, Shock Next Generation Satellite Laser Ranging System (NASA) Thermal Vac (at UF) Selected for ELaNA launch in Developing SLR collaborations Starfire optical range at Kirtland AFB, NM NGSLR managed by Goddard, MD Starfire Optical Range (AFRL) John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 14/14

15 Backup slides John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 15/14

16 Future applications Concept of Disaggregated Navigation System: 1. Command station performs time transfer with reference satellite 2. Satellite with atomic clock synched with time standard 3. Navigation satellites synced to atomic clock using rf (no ionospheric effects) 4. Navigation receivers determine location and time from navigation satellites John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 16/14

17 Payload Overview John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 17/14

18 SLR Emulator Laser Collimator Focusing Lens APD Box t 2 ground De-focusing Lens Clock Event Timer Laser and Pulser Beam Splitter APD Box t 0 ground John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 18/14

19 Space Segment Event Timer Clock Focusing Lens APD Box Retroreflector John W. Conklin, 2014 Spring CubeSat Developers Workshop, Cal Poly 19/14