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
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
GPS Constellation
Gravity Probe A (1976)
• 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
Common View
Non-common View
T2L2 mission [P. Guillemot et al 2006]
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
2/18
Time Transfer
tspace
t1 space
t0 ground
t2 ground
tground
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
3/18
Optical Precision Time-transfer
Instrument (OPTI)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
4/18
OPTI Flight Configuration
Interface Board
Support Board
Event Timers
(2×)
CSAC
MAC
Light Collectors
(2×)
5
Retroreflector
(not shown)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
5/18
OPTI Engineering Model
Bottom
(Nadir facing)
MAC
Top
(Zenith facing)
CSAC
Event Timers (2×)
Photodiodes (2×)
TEC controllers and
reverse bias voltage
Interface & power
regulator
(High altitude
balloon
configuration)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
6/18
Clocks
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
5W
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
7/18
Optics
Retroreflector
APD
GRIN lens
Fiber and ferrule
Fiber and ferrule
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Band-pass filter
8/18
OPTI Time Transfer Demo
SLR Emulator
Laser,
Pulse driver
Space Segment
t ground
CSAC
Beam Splitter
Event Timer
APD
t0 ground
APD
t2 ground
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
t space
CSAC
Event Timer
APD
t1space
9/18
Measured Performance
Clock difference (2 CSACs) measured using OPTI breadboard
50
 (ns)
0
–50
–100
0
5
10
15
20
255
Elapsed time (ks)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
10/18
Timing Error Budget
102
Timing error, t (ns)
GPS Time (20 nsec)
101
10 nsec
Predicted
Timing Budget
1 nsec
100
One Orbit
Measured
10–1
100
101
102
103
Averaging time τ (s)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
104
11/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
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
TISEF (Kennedy Space Center)
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
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
Synchronization string
Timing data (20 bytes)
Checksum (2 bytes)
Timed laser pulse
TRUE/1
FALSE/0
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Sync. error
Comm. loss
or sync. error
20/18
Timewalk Correction
Threshold
Time-to-digital
converter
Pulse
Amplitude
Start
Time
Time
Stamp
Signal
Clock
Stop 1
Stop 2
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
21/18
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
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
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
TDC-GPX
• 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
25/18
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
PD
TDC Start
TDC time (fine time)
• Within 7 μs TDC range
TDC Stop
Clock
True time
Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly
Pulse counter
(coarse time)
Counter
reading
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