The CHOMPTT Precision Time Transfer CubeSat Mission

Transcription

1 The CHOMPTT Precision Time Transfer CubeSat Mission Nathan Barnwell, Lucas Bassett-Audain, Paul Buchman, Maria Carrasquilla, Leopoldo Caro, David Keister, Olivia Formoso, Seth Nydam, Blake Richards, Paul Serra, John W. Conklin CubeSat Handling Of Multisystem Precision Time Transfer 1/18

2 Background and Motivation Application of precision time transfer to space: Satellite navigation systems ( x = c t) Beyond LEO Global time standards Test of general relativity Satellite encryption/authentication Optical time transfer More resilient to ionospheric effects than RF (~1/f 2 ) CNES T2L2 (2008), hosted payload on Jason-2 CHOMPTT objectives: <200 ps time transfer error (6 cm) <20 ns clock drift after 1 orbit (6 m) Real time clock update GPS Constellation Gravity Probe A (1976) Common View T2L2 mission [P. Guillemot et al 2006] Non-common View 2/18

3 Time Transfer t space t 1 space t 0 ground t ground 2 t ground 3/18

4 Optical Precision Time-transfer Instrument (OPTI) 4/18

5 OPTI Flight Configuration Interface Board Support Board Event Timers (2 ) CSAC MAC Light Collectors (2 ) 5 Retroreflector (not shown) 5/18

6 OPTI Engineering Model Bottom (Nadir facing) MAC CSAC Event Timers (2 ) Photodiodes (2 ) TEC controllers and reverse bias voltage Interface & power regulator (High altitude balloon configuration) Top (Zenith facing) 6/18

7 Clocks Characteristic Chip Scale Atomic Clock (CSAC) Miniature Atomic Clock (MAC) Standard Cesium Rubidium Allan Deviation (time error) 6000 s (20 ns) Power 0.12 W 5 W Mass 35 g 85 g 6000 s (6 ns) Size (LxWxH) x x mm 51 x 51 x 18 mm Clocks from Microsemi 7/18

8 Optics Retroreflector Fiber and ferrule APD GRIN lens Fiber and ferrule Band-pass filter 8/18

9 OPTI Time Transfer Demo SLR Emulator Space Segment Laser, Pulse driver Beam Splitter t ground CSAC Event Timer t space CSAC Event Timer APD t 0 ground APD t 2 ground APD t 1 space 9/18

10 (ns) Measured Performance Clock difference (2 CSACs) measured using OPTI breadboard Elapsed time (ks) 10/18

11 Timing error, t (ns) Timing Error Budget 10 2 GPS Time (20 nsec) nsec 1 nsec Predicted Timing Budget 10 1 Measured One Orbit Averaging time τ (s) 11/18

12 High Altitude Balloon Testing ~100,000 ft. for 6+ hours Successful OPTI operations in near-space environment Obtained system health data Successful power cycle test OPTI 12/18

13 OPTI View in Space 13/18

14 Satellite Overview 14/18

15 Concept of Operations 15/18

16 Satellite Laser Ranging Facility Townes Institute Science & Technology Experimentation Facility (TISEF) managed by UCF 50 cm satellite tracking telescope 1 km testing range TISEF (Kennedy Space Center) 16/18

17 Future Work Completion and testing of 2U bus Tailoring TISEF SLR facility for mission & OPTI testing ELaNA launch ~ /18

18 Sponsors and Collaborators 18/18

19 Backup Slides 19/18

20 Laser Communication 2-Pulse Position Modulation (2 slots per pulse) High precision measurement only on the first pulse Synchronization string provides phase and rate for communication, masks SLR Delay Timed laser pulse Synchronization string Timing data (20 bytes) Checksum (2 bytes) TRUE/1 FALSE/0 Sync. error Comm. loss or sync. error 20/18

21 Timewalk Correction Threshold Pulse Amplitude Time Time Stamp Signal Clock Time-to-digital converter Start Stop 1 Stop 2 21/18

22 Timewalk Correction Apparent timing variations due to pulse amplitude variations Atmosphere, attitude, range, Solution: Time both rising and falling edges of pulse 0.5 V to 2.5 V Δt = 230 psec 22/18

23 Photodetectors 2 avalanche photodiodes: InGaAs for 1064 nm, 150 ps rise time Si for 532 nm ps rise time Photodetector in linear mode Temperature regulated by Thermal-Electric Coolers Photodetectors are fiber-coupled Pulse sent back by a PLX retroreflector 25 mm diameter, 50 FOV Space Capable 23/18

24 Optics Band-pass filters Increase SNR Light collected by a multimode optical fiber on nadir face 12 max incidence 200 μm diameter GRIN Lens focuses light onto APD 24/18

25 10 ps Event Timers- Fine Time 2 independent channels Fine time on short intervals, course time on long duration Time-to-digital converter- measures fine time Integrated, off-the-shelf: Acam TDC GPX Measurement based on propagation delays Autonomous calibration using Delay Lock Loops Low power (<150 mw) 10 ps single shot accuracy (12 ps measured) TDC-GPX 25/18

26 10 ps Event Timers- Fine Time Counter-measures coarse time Ti MSP430 microcontroller used as counter MSP430 TDC and counter are synchronized on a chosen clock rising edge Within 7 μs TDC range PD TDC Start TDC time (fine time) TDC Stop Clock True time Pulse counter (coarse time) Counter reading 26/18

27 Mission Overview CHOMPTT will demonstrate technology for enhanced GPS and future disaggregated navigation systems CHOMPTT is a precision timing satellite equipped with atomic clocks synchronized with a ground clock, via laser pulses Optical frequencies reduce ionospheric time delay uncertainties relative to radio frequencies Robust against signal interference / jamming Payload with low size (1U), mass (1 kg), and power (7 W) Real-time clock phase & frequency corrections via modulated laser pulses 27/18

28 Objectives Primary Objective Demonstrate low cost, precision time transfer between an atomic clock on the ground and one on a CubeSat to 200 psec (short term) Secondary Objectives Achieve timing accuracy of 1 ns over 1 orbit (long term) Onboard real-time calculation of CubeSat clock discrepancy Compare CubeSat s clock to GPS time Utilize CubeSat to compare two spatially separated ground atomic clocks 28/18