The CHOMPTT Precision Time Transfer CubeSat...
Transcript of 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
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]
John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 3/14
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
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 OverviewAntenna, 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
CSACEvent Timer
APD
t1cubesatAPD
Laser,Pulse driver
Beam SplitterCSAC
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 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
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
Counterreading
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 opticalfiber on nadir face
• 12˚ max incidence
• GRIN lens focuseslight 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 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
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 525Elapsed 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 OrbitBreadboard
100 101 102 103 10410–1
102
Averaging time τ (sec)
100
101
Tim
ing
erro
r, Δt(
nsec
)
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. Lossor 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 Boxt0ground
Beam Splitter
Laser Collimator
Event TimerClock
Laser and Pulser
SLR Emulator
John W. Conklin, 2014 Spring CubeSat Developers’ Workshop, Cal Poly 19/14
Space Segment
𝒕𝟏𝒄𝒖𝒃𝒆𝒔𝒂𝒕
Retroreflector
APD Box
Focusing Lens
Event Timer Clock