CubeSat Handling Of Multisystem Precision Time

Transfer

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

The CHOMPTT Precision Time Transfer CubeSat Mission

Nathan Barnwell, Lucas Bassett-Audain, Maria Carrasquilla, Jonathan Chavez, Olivia Formoso, Asia Nelson, Anh Nguyen, Seth Nydam, Jessie Pease, Tyler Ritz, Steven Roberts, Paul Serra, Evan Waxman, John W. Conklin

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Background and Motivation

•  

GPS Constellation Gravity Probe A (1976)

Common View Non-common View

T2L2 mission [P. Guillemot et al 2006]

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Time Transfer

t0 ground

tground

t2 ground

tspace

t1 space

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Optical Precision Time-transfer Instrument (OPTI)

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1 2

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5

6

7

8

9

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

OPTI Flight Configuration

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Channel A Channel B

Supervisor

Retroreflector Array

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

OPTI Flight Configuration

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Channel A Channel BSupervisor

Retroreflector Array

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

OPTI Engineering Model

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Key Hardware

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Retroreflector

Avalanche Photodiode with Thermal Electric

Cooler

Cesium Atomic Clock

Event Timer

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

OPTI Time Transfer Demo

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t space

SLR Emulator Space Segment

CSAC

Event Timer

APDt1

spaceAPD

Laser, Pulse driver

Beam Splitter CSAC

t0 ground

Event Timer

t ground

APDt2

ground

Retroreflector

Measured timing error: 100 psec (3 cm) @ 1 sec 17 nsec (5 m) @ 6000 sec

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Measured Performance

50

0

–50

–100

χ (n

s)

0 5 10 15 20 525

Elapsed time (ks)

Clock difference (2 CSACs) measured using OPTI breadboard

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Timing Error Budget

GPS Time (20 nsec)

10 nsec

1 nsec

Predicted Timing Budget

One OrbitMeasured

100 101 102 103 10410–1

102

Averaging time τ (s)

100

101

Tim

ing

erro

r, Δ

t (ns

)

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

OPTI View in Space

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Satellite Overview

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Pumpkin Chassis

EDSN

Adapter Plate

OPTI

Nadir Plate

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Concept of Operations

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Satellite Laser Ranging Facility

• Townes Institute Science & Technology Experimentation Facility (TISTEF) managed by UCF

• 50 cm satellite tracking telescope • Optical Beacon on CHOMPTT

• 1 km testing range

TISEF (Kennedy Space Center)

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VAB ~13 km Range Target ~1 km

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Satellite Laser Ranging Facility

• Laser: Coherent Flare NX • 1030 nm • 500 µJ • 1 ns pulses • 2 kHz repetition rate

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Laser

Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Schedule

• Testing OPTI Engineering Model @ TISTEF • Summer 2016

• Integrate EDSN + OPTI • Fall 2016

• CubeSat delivery • March 2017

• ELaNA XIX launch • June 2017

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Sponsors and Collaborators

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Backup Slides

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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 pulseSynchronization string Timing data (20 bytes) Checksum (2 bytes)

FALSE/0 Comm. loss or sync. error

Sync. errorTRUE/1

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Timewalk Correction

Threshold

Time Stamp

Pulse Amplitude

Time

Time-to-digital converter

Start

Stop 1

Stop 2

ClockSignal

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Timewalk Correction

• Apparent timing variations due topulse amplitude variations

• Atmosphere, attitude, range, …

• Solution: Time both rising andfalling edges of pulse

0.5 V to 2.5 V→ Δt = 230 psec

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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

MSP430

PD

Clock

TDC Start

TDC Stop

Counter reading

TDC time (fine time)

Pulse counter(coarse time)

True time

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

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

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Nathan Barnwell, 2016 Spring CubeSat Developers’ Workshop, Cal Poly

Nadir face

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