CubeSat Handling Of Multisystem Precision Time...

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

1


Background and Motivation

•

GPS Constellation Gravity Probe A (1976)

Common View Non-common View

T2L2 mission [P. Guillemot et al 2006]

2


Time Transfer

t0 ground

tground

t2 ground

tspace

t1 space

3


Optical Precision Time-transfer Instrument (OPTI)

4

1 2

34

5

6

7

8

9

10


OPTI Flight Configuration

5

Channel A Channel B

Supervisor

Retroreflector Array


OPTI Flight Configuration

6

Channel A Channel BSupervisor

Retroreflector Array


OPTI Engineering Model

7


Key Hardware

8

Retroreflector

Avalanche Photodiode with Thermal Electric

Cooler

Cesium Atomic Clock

Event Timer


OPTI Time Transfer Demo

9

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


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

10


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

)

11


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


OPTI View in Space

13


Satellite Overview

14

Pumpkin Chassis

EDSN

Adapter Plate

OPTI

Nadir Plate


Concept of Operations

15


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)

16

VAB ~13 km Range Target ~1 km


Satellite Laser Ranging Facility

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

17

Laser


Schedule

• Testing OPTI Engineering Model @ TISTEF • Summer 2016

• Integrate EDSN + OPTI • Fall 2016

• CubeSat delivery • March 2017

• ELaNA XIX launch • June 2017

18


Sponsors and Collaborators

19


Backup Slides

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

FALSE/0 Comm. loss or sync. error

Sync. errorTRUE/1

21


Timewalk Correction

Threshold

Time Stamp

Pulse Amplitude

Time

Time-to-digital converter

Start

Stop 1

Stop 2

ClockSignal

22


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

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

24


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


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

26


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


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


Nadir face

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