CubeSat Handling Of Multisystem Precision Time Transfer

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

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• 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

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

GPS Constellation Gravity Probe A (1976)

Common View Non-common View

T2L2 mission [P. Guillemot et al 2006]

2/18

Time Transfer

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

t0 ground

tground

t2ground

tspace

t1space

3/18

Optical Precision Time-transfer Instrument (OPTI)

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly 4/18

OPTI Flight Configuration

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

5

Interface Board

Support Board

Event Timers

(2×)

CSAC

MAC

Light Collectors

(2×)

Retroreflector

(not shown)

5/18

OPTI Engineering Model

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

TEC controllers andreverse bias voltage

Interface & power regulator

(High altitude balloon

configuration)

Bottom

(Nadir facing)Top

(Zenith facing)

Event Timers (2×)

CSAC

MAC

Photodiodes (2×)

6/18

Clocks

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

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 5 W

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

7/18

Optics

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

Retroreflector Fiber and ferrule

APD Fiber and ferrule Band-pass filter

8/18

GRIN lens

OPTI Time Transfer Demo

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

t space

SLR Emulator Space Segment

CSACEvent Timer

APD

t1space

APD

Laser,Pulse driver

Beam Splitter CSAC

t0 ground

Event Timer

t ground

APD

t2 ground

9/18

Measured Performance

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

50

0

–50

–100

(n

s)

0 5 10 15 20 525

Elapsed time (ks)

Clock difference (2 CSACs) measured using OPTI breadboard

10/18

Timing Error Budget

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

GPS Time (20 nsec)

10 nsec

1 nsec

Predicted Timing Budget

One OrbitMeasured

100 101 102 103 104

10–1

102

Averaging time τ (s)

100

101

Tim

ing

erro

r,

t(n

s)

11/18

High Altitude Balloon Testing

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

• ~100,000 ft. for 6+ hours

• Successful OPTI operations in near-space environment

• Obtained system health data

• Successful power cycle test OPTI

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

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

TISEF (Kennedy Space Center)

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

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

Timed laser pulse

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

FALSE/0 Comm. lossor sync. error

Sync. errorTRUE/1

20/18

Timewalk Correction

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

Threshold

Time Stamp

Pulse Amplitude

Time

Time-to-digital converter

Start

Stop 1

Stop 2

ClockSignal

21/18

Timewalk Correction

• Apparent timing variations due topulse amplitude variations• Atmosphere,

attitude, range, …

• Solution: Time both rising andfalling edges of pulse

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

0.5 V to 2.5 V Δt = 230 psec

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• 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

TDC-GPX

25/18

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

Nathan Barnwell, 2015 Spring CubeSat Developers’ Workshop, Cal Poly

MSP430

PD

Clock

TDC Start

TDC Stop

Counterreading

TDC time (fine time)

Pulse counter(coarse time)

True time

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