the chomptt precision time transfer cubesat...

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

*[email protected]

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

Background and Motivation• Precision time transfer to space important for:

• Satellite nav systems, e.g. GPS (∆𝑥 = 𝑐 ∆𝑡)

• International time standards

• Test of general relativity

• Satellite encryption/authentication

• Technique: exchange of short light pulses

• Optical frequencies less affected byionosphere relative to rf

• European T2L2 (2008) washosted payload

• CHOMPTT Objectives:

• <10 ns time transfer over 1 orbit

• Real time clock update Common View

GPS constellation

Non-common ViewT2L2 mission [P. Guillemot et al 2006]


CHOMPTT: CubeSat Handling Of MultisystemPrecision Time Transfer

SLRFacilityGround

Station

tgpsA + positionA

tgpsB + positionB

GPSAGPSB

𝑡'()*+,-

𝑡.()*+,-

𝑡/0+12345

𝑡()*+,-

𝑡0+12345

𝜒 = 𝑡/0+12345 −𝑡.()*+,- + 𝑡'()*+,-

2+ ∆𝑡

Clock discrepancy


Concept of Operations


Satellite OverviewAntenna, GPS, Radio (UHF/VHF)

ADCS (active magnetic)

EPS

Batteries

CDH (MSP430)

MAC

Instrument Boards

CSAC

Retroreflector, Light Collectors

OPT

I


Optical Precision Time-transfer Instrument (OPTI) Demo

tcubesat

SLR Emulator Space Segment

CSACEvent Timer

APD

t1cubesatAPD

Laser,Pulse driver

Beam SplitterCSAC

t0 ground

Event Timer

tground

APD

t2 ground


Atomic Clocks (Microsemi)Characteristic Chip Scale Atomic Clock

(CSAC) Miniature Atomic Clock (MAC)

Standard Cesium RubidiumAllan 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


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

Counterreading

TDC time (fine time)

Pulse counter(coarse time)

True time


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 opticalfiber on nadir face

• 12˚ max incidence

• GRIN lens focuseslight onto APD

PLX Retroreflector

Fiber coupler / TEC

APD electronics

APD


Timewalk Correction• Apparent timing variations due to

pulse amplitude variations

• Atmosphere, attitude, range, …

• Solution: Time both rising andfalling edges of pulse

Threshold

Time Stamp

Pulse Amplitude

Time

Time-to-digital converter

Start

Stop 1

Stop 2

ClockSignal

0.5 V to 2.5 V→ Δt = 230 psec


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 525Elapsed time (ksec)


Timing Error Budget

GPS Time (20 nsec)

10 nsec

1 nsec

Predicted Timing Budget

One OrbitBreadboard

100 101 102 103 10410–1

102

Averaging time τ (sec)

100

101

Tim

ing

erro

r, Δt(

nsec

)


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. Lossor sync. error


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)


Backup slides …


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


Payload Overview


De-‐focusing Lens

APD Box

t2ground

Focusing Lens

APD Boxt0ground

Beam Splitter

Laser Collimator

Event TimerClock

Laser and Pulser

SLR Emulator


Space Segment

𝒕𝟏𝒄𝒖𝒃𝒆𝒔𝒂𝒕

Retroreflector

APD Box

Focusing Lens

Event Timer Clock

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