The CHOMPTT Precision Time Transfer CubeSat Mission John W. Conklin*, Paul Serra, Nathan Barnwell, Seth Nydam, Maria Carrascilla, Leopoldo Caro, Norman Fitz-Coy *[email protected] 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 CHOMPTT: CubeSat Handling Of Multisystem Precision Time Transfer GPSA GPSB tgpsA + positionA tgpsB + positionB Ground Station SLR Facility Clock discrepancy John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 3/14 Concept of Operations John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 4/14 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 Optical Precision Time-transfer Instrument (OPTI) Demo SLR Emulator Laser, Pulse driver Space Segment tground CSAC Beam Splitter Event Timer APD t0 ground tcubesat CSAC APD t2 ground John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly Event Timer APD t1cubesat 6/14 Atomic Clocks (Microsemi) Characteristic Chip Scale Atomic Clock (CSAC) Miniature Atomic Clock (MAC) Standard Cesium Rubidium Allan Deviation (time error) 3.3x10-12 @ 6000 sec (20 nsec) 9.5x10-13 @ 6000 sec (6 nsec) Power 0.12 W 5W Mass 35 g 85 g Size (LxWxH) 40.64 x 35.31 x 11.42 mm 51 x 51 x 18 mm John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 7/14 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 John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly Ti MSP430 8/14 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 APD PLX Retroreflector Light collection • Light collected by optical fiber on nadir face • • 12˚ max incidence GRIN lens focuses light onto APD APD electronics Fiber coupler / TEC John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 9/14 Timewalk Correction • Apparent timing variations due to pulse amplitude variations • • 0.5 V to 2.5 V → Δt = 230 psec Atmosphere, attitude, range, … Solution: Time both rising and falling edges of pulse Time-to-digital converter Threshold Start Pulse Amplitude Signal Time Clock Time Stamp John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly Stop 1 Stop 2 10/14 Measured Performance • Clock difference (2 CSACs) measured using OPTI breadboard • Several “glitches” filtered in software χ (nsec) 50 0 –50 –100 0 5 10 15 Elapsed time (ksec) 20 John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 25 5 11/14 Timing Error Budget Timing error, ∆t (nsec) 102 GPS Time (20 nsec) 101 10 nsec Predicted Timing Budget 1 nsec 100 10–1 100 Breadboard 101 One Orbit 102 103 Averaging time τ (sec) John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 104 12/14 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 Synchronization string Timing data (20 bytes) Checksum (2 bytes) Timed laser pulse 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 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 • Thermal Vac (at UF) • Selected for ELaNA launch in 2016-2017 • Developing SLR collaborations • Starfire optical range at Kirtland AFB, NM • NGSLR managed by Goddard, MD Next Generation Satellite Laser Ranging System (NASA) Starfire Optical Range (AFRL) John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 14/14 Backup slides … John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 15/14 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 Payload Overview John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 17/14 SLR Emulator Laser Collimator Focusing Lens APD Box t2ground De-focusing Lens Clock Event Timer Beam Splitter Laser and Pulser John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly APD Box t0ground 18/14 Space Segment Event Timer Clock Focusing Lens APD Box Retroreflector John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 19/14
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