The CHOMPTT Precision Time Transfer CubeSat Mission John W. Conklin*, Paul Serra, Nathan Barnwell, Seth Nydam, Maria Carrascilla, Leopoldo Caro, Norman Fitz-Coy

*jwconklin@ufl.edu

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 2/14

Background and Motivation •

Common View

GPS constellation

Non-common View T2L2 mission [P. Guillemot et al 2006]

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 3/14

CHOMPTT: CubeSat Handling Of Multisystem Precision Time Transfer

SLR Facility Ground

Station

tgpsA + positionA

tgpsB + positionB

GPSA GPSB

Clock discrepancy

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 4/14

Concept of Operations

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 5/14

Satellite Overview Antenna, GPS, Radio (UHF/VHF)

ADCS (active magnetic)

EPS

Batteries

CDH (MSP430)

MAC

Instrument Boards

CSAC

Retroreflector, Light Collectors

OPT

I

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 6/14

Optical Precision Time-transfer Instrument (OPTI) Demo

tcubesat

SLR Emulator Space Segment

CSAC Event Timer

APD

t1cubesat

APD

Laser, Pulse driver

Beam Splitter CSAC

t0 ground

Event Timer

tground

APD

t2 ground

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 7/14

Atomic Clocks (Microsemi) Characteristic Chip Scale Atomic Clock

(CSAC) Miniature Atomic Clock (MAC)

Standard Cesium Rubidium

Allan Deviation (time error)

3.3x10-12 @ 6000 sec (20 nsec)

9.5x10-13 @ 6000 sec (6 nsec)

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

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 8/14

10 ps Event Timer • Time-to-digital converter – measures fine time

• Measurement based on propagation delay

• Autonomous temperature compensation using DLL

• Low power (132 mW)

• 10 ps single shot accuracy (12 ps measured)

• MSP430 microcontroller - course time

Ti MSP430

TDC-GPX

PD

Clock

TDC Start

TDC Stop

Counter reading

TDC time (fine time)

Pulse counter (coarse time)

True time

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 9/14

Optics & Light Detection • PLX retroreflector

• 25 mm diam, 50˚ FOV

• Space capable

• Avalanche photodetectors (2)

• Si (532 nm, 1064 nm): 500 ps rise

• InGaAs (1064 nm): 140 ps rise

• Light collection

• Light collected by optical fiber on nadir face

• 12˚ max incidence

• GRIN lens focuses light onto APD

PLX Retroreflector

Fiber coupler / TEC

APD electronics

APD

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 10/14

Timewalk Correction • Apparent timing variations due to

pulse amplitude variations

• Atmosphere, attitude, range, …

• Solution: Time both rising and falling edges of pulse

Threshold

Time Stamp

Pulse Amplitude

Time

Time-to-digital converter

Start

Stop 1

Stop 2

Clock Signal

0.5 V to 2.5 V → Δt = 230 psec

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 11/14

Measured Performance • Clock difference (2 CSACs) measured using OPTI breadboard

• Several “glitches” filtered in software

50

0

–50

–100

χ (n

sec)

0 5 10

15 20 5 25 Elapsed time (ksec)

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 12/14

Timing Error Budget

GPS Time (20 nsec)

10 nsec

1 nsec

Predicted Timing Budget

One Orbit Breadboard

100 101 102 103 104 10–1

102

Averaging time τ (sec)

100

101

Tim

ing

erro

r, ∆t

(nse

c)

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 13/14

Laser Communication • 2-Pulse Position Modulation (2 slots per pulse)

• Synchronization string provides phase, rate, and masks SLR delays

• Fine time required only for first ‘timing’ pulse

Timed laser pulse

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

Repeated if low link quality

TRUE/1

FALSE/0 Sync. error

Comm. Loss or sync. error

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 14/14

Status and Future • Prototype OPTI (silver) fabricated and tested Summer 2014

• OPTI integrated into CHOMPTT satellite Fall 2014

• Qualification testing at NASA KSC

• Vibration, Shock

• Thermal Vac (at UF)

• Selected for ELaNA launch in 2016-2017

• Developing SLR collaborations

• Starfire optical range at Kirtland AFB, NM

• NGSLR managed by Goddard, MD

Next Generation Satellite Laser Ranging System (NASA)

Starfire Optical Range (AFRL)

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 15/14

Backup slides …

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 16/14

Future applications • Concept of Disaggregated

Navigation System:

1. Command station performs time transfer with reference satellite

2. Satellite with atomic clock synched with time standard

3. Navigation satellites synced to atomic clock using rf (no ionospheric effects)

4. Navigation receivers determine location and time from navigation satellites

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 17/14

Payload Overview

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 18/14

De-focusing Lens

APD Box

t2ground

Focusing Lens

APD Box t0

ground Beam Splitter

Laser Collimator

Event Timer Clock

Laser and Pulser

SLR Emulator

John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 19/14

Space Segment

Retroreflector

APD Box

Focusing Lens

Event Timer Clock