View
217
Download
4
Category
Preview:
Citation preview
CubeSat Handling Of Multisystem Precision Time Transfer
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 1/18
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/f2)
• 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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
GPS Constellation Gravity Probe A (1976)
Common View Non-common View
T2L2 mission [P. Guillemot et al 2006]
2/18
Time Transfer
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
t0 ground
tground
t2ground
tspace
t1space
3/18
Optical Precision Time-transfer Instrument (OPTI)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 4/18
OPTI Flight Configuration
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
5
Interface Board
Support Board
Event Timers
(2×)
CSAC
MAC
Light Collectors
(2×)
Retroreflector
(not shown)
5/18
OPTI Engineering Model
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
TEC controllers andreverse bias voltage
Interface & power regulator
(High altitude balloon
configuration)
Bottom
(Nadir facing)Top
(Zenith facing)
Event Timers (2×)
CSAC
MAC
Photodiodes (2×)
6/18
Clocks
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Characteristic Chip Scale Atomic Clock (CSAC) Miniature Atomic Clock (MAC)
Standard Cesium Rubidium
Allan Deviation(time error)
3.3x10-12 @ 6000 s(20 ns)
9.5x10-13 @ 6000 s(6 ns)
Power 0.12 W 5 W
Mass 35 g 85 g
Size (LxWxH) 40.64 x 35.31 x 11.42 mm 51 x 51 x 18 mm
Clocks from Microsemi
7/18
Optics
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Retroreflector Fiber and ferrule
APD Fiber and ferrule Band-pass filter
8/18
GRIN lens
OPTI Time Transfer Demo
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
t space
SLR Emulator Space Segment
CSACEvent Timer
APD
t1space
APD
Laser,Pulse driver
Beam Splitter CSAC
t0 ground
Event Timer
t ground
APD
t2 ground
9/18
Measured Performance
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
50
0
–50
–100
(n
s)
0 5 10 15 20 525
Elapsed time (ks)
Clock difference (2 CSACs) measured using OPTI breadboard
10/18
Timing Error Budget
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
GPS Time (20 nsec)
10 nsec
1 nsec
Predicted Timing Budget
One OrbitMeasured
100 101 102 103 104
10–1
102
Averaging time τ (s)
100
101
Tim
ing
erro
r,
t(n
s)
11/18
High Altitude Balloon Testing
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
• ~100,000 ft. for 6+ hours
• Successful OPTI operations in near-space environment
• Obtained system health data
• Successful power cycle test OPTI
12/18
OPTI View in Space
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 13/18
Satellite Overview
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 14/18
Concept of Operations
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 15/18
Satellite Laser Ranging Facility
• Townes Institute Science & Technology Experimentation Facility (TISEF) managed by UCF
• 50 cm satellite tracking telescope
• 1 km testing range
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
TISEF (Kennedy Space Center)
16/18
Future Work
• Completion and testing of 2U bus
• Tailoring TISEF SLR facility for mission & OPTI testing
• ELaNA launch ~2017
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 17/18
Sponsors and Collaborators
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 18/18
Backup Slides
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 19/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Timed laser pulse
Synchronization string Timing data (20 bytes) Checksum (2 bytes)
FALSE/0 Comm. lossor sync. error
Sync. errorTRUE/1
20/18
Timewalk Correction
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Threshold
Time Stamp
Pulse Amplitude
Time
Time-to-digital converter
Start
Stop 1
Stop 2
ClockSignal
21/18
Timewalk Correction
• Apparent timing variations due topulse amplitude variations• Atmosphere,
attitude, range, …
• Solution: Time both rising andfalling edges of pulse
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
0.5 V to 2.5 V Δt = 230 psec
22/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 23/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 24/18
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)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
TDC-GPX
25/18
10 ps Event Timers- Fine Time
Counter-measures coarse time
• Ti MSP430 microcontroller used as counter
• TDC and counter are synchronized on a chosen clock rising edge
• Within 7 μs TDC range
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
MSP430
PD
Clock
TDC Start
TDC Stop
Counterreading
TDC time (fine time)
Pulse counter(coarse time)
True time
26/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 27/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 28/18
Recommended