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DSAC Technology Demonstration Mission Results and FutureTodd Ely (PI/PM)John Prestage (co-I), Robert Tjoelker (co-I), Eric Burt, Angela Dorsey, Daphna Enzer, Da Kuang, Dave Murphy, David Robison, Jill Seubert, and Rabi Wang
October 28, 2021
j p l . n a s a . g o vOctober 2021
The DSAC Mission
sftp
GA MOC/SOCColorado
ViaSat GS(Atlanta)
CMD & TLM
GPS Sat 1
GPS Sat 2
GPS Sat n
GA Orbital Test Bed (OTB)• 720 km altitude• 24° inclination
DSAC teamJPL
Launch June 25, 2019USAF STP-2(SpaceX Falcon Heavy)
• Collect GPS phase & range data• Collect DSAC telemetry• Validate clock instability < 2 ns @ one-day
• achieved < 0.3 ns @ one-day and more• Validate as a navigation instrument
• sufficient accuracy for onboard radio nav• Operate for at least one year
• Actually over two – mission completed September 18, 2021
j p l . n a s a . g o vOctober 2021
To have a space demo mission you need a launch …
USAF STP-2SpaceX Falcon Heavy
OTB one of 26 spacecraft
Launch June 25, 2019
j p l . n a s a . g o vOctober 2021
DSAC Payload on the Orbital Test Bed Spacecraft
GPS Receiver
Ultra-Stable Oscillator (USO)
General Atomics Electromagnetic SystemsGroup (GA-EMS)Orbital Test Bed (OTB)
DSAC Physics Unit
j p l . n a s a . g o vOctober 2021
DSAC has proven to be very reliable and robust
DSAC 2020-2021 Operations Overview
• Mission operations from 8/20/2019 to 9/18/2021 for a duration of 760.2
• Operations periods include:• USO on: 84.6%• Clock on: 78.2%• GPS data: 74.9%
• 10 long runs (67 days longest)• All run terminations due to S/C
safe modes• 2 known radiation-induced clock
faults – recovered• No known clock hardware faults
j p l . n a s a . g o vOctober 2021
DSAC Technology: How it Works
Key Reliability Features: - practical• No lasers • No cryogenics • No microwave cavity• Low light shift (QP), no light shift (MP)• Low consumables
Microwave Input
Input light system
Detection light system
Local Oscillator
Controller
Key Performance Features:106-107 199Hg+ trapped ions• No wall collisions, high Q microwave line• Buffer gas cooled to ~300K • Two trap system• Quadrupole (QP) trap – low Doppler sensitivity• Multi-pole (MP) trap – lower Doppler sensitivity
(Flight demo operated with only the QP trap)
State selection• Optical Pumping from 202Hg+ lamp• 1-2 UV photons per second scattered
High Clock Transition• 40,507,347,996.8 Hz – low magnetic sensitivity
Adapts to variety of Local Oscillators - flexible
j p l . n a s a . g o vOctober 2021
Nature Journal Publication of DSAC Results
Stability at one-day of 3e-15➡ significantly better than required 2e-14
Drift of 3.0e-16/day➡ establishes space clock record
Two subsequent long runs had similar long term stability
10-15
2
3
45678
10-14
2
3
45678
10-13
2
3
4
Alla
n D
evia
tion
100 101 102 103 104 105 106
Time (s)
NO DRIFT REMOVAL
Measurement system + DSAC
DSACOptimized DSAC
DSAC measured on the ground (MP mode)
DSAC requirement
j p l . n a s a . g o vOctober 2021
DSAC Flight Experience Overview: What we have learned to date• Improved temperature predictions vs. beta angle • USO performance in space nominal• Lamp function in space – better than on ground!• Degree of interaction with the SAA – USO and Clock (9)• Relativity corrections require J2 effects (10)
• Measure fundamental clock sensitivities in space:Ø Background pressure: Limits on gas evolution (11)Ø Magnetic: Characterization of external magnetic field variations (12)Ø Doppler Effects: Ion number sensitivity and method to reduce (13)Ø Overall temperature: measured sensitivity with caveats (14)
j p l . n a s a . g o vOctober 2021
-2.0 x10-3
-1.5
-1.0
-0.5
0.0
0.5
Clo
ck F
requ
ency
Erro
r
600005000040000time (s)
55 x103
50
45
40
35
30
PMT counts191002a Daily
average clock_frequency_errorcompared to dim_count(CFE consists of data from10/2, 10/3, 10/4, 10/6, and 10/9)
Clock control loop efficiently filters out SAA-induced PMT variations
South Atlantic Anomaly (SAA) Impact on Operations
PMT variations due to SAA passage
Clock control loop response to orbital environmental variations – USO responding to temperature changes
Clock control loop response to SAA transits – radiation is causing the USO to ‘pull’ in frequency and the clock is responding to control it out
Clock Control Loop:
PMT counts:
j p l . n a s a . g o vOctober 2021
GPS phase dynamically corrected for gravitational effects (red shift)
Characterizing Gravitational Effects
Must include higher order terms (J2) in the gravitational potential
10-15
10-14
10-13
10-12
Alla
n De
viatio
n
102 103 104 105 106
Averaging Time (s)
NO DRIFT REMOVAL
No corrections
Relativity corrections, but no GPSR temperature corrections
Measurement floor after relativity and GPSR temp correction
Estimated clock noise
j p l . n a s a . g o vOctober 2021
Limit on 𝝙f/f due to background gas evolution at < 4e-16/day
Characterizing Background Gas Evolution Effects
Residual frequency offsets when ion number stable and known temperature effect removed
-1.5 x10-13
-1.0
-0.5
0.0
0.5
1.0
1.5Fr
eque
ncy
Offs
et
800 x1036004002000Time (s)
j p l . n a s a . g o vOctober 2021
Strength of this technology - no magnetic perturbations observed
Characterizing External Magnetic Field Effects
Peak effect below clock sensitivity
15
10
5
0
-5
-10M
agne
tic F
ield
(uT)
2.0 x1061.51.00.50.0Time (s)
200110a MagnetometerMagnetic field as measuredby magnetometerC (weak clockdirection) where sensitivity is7e-14/G. Data from 12/15/19 to1/10/20.
250 mGpp (100x lab!!)
AD of external magnetic field-induced frequency variations
20 days
Optimal clock noise level
Clock + Measurement noise during
52-day run
j p l . n a s a . g o vOctober 2021
Ion number stability 𝝙f/f ~ 1e-15/day achieved after 1 day of warmup
Characterizing Doppler Effects
4 x10-13
3
2
1
0
Frac
tiona
l Fre
quen
cy
6005004003002001000Signal Size Measurement Number
200110 yt_ss_qptempcomparison: real freq (red)to frequency derived from-5.9e-17*D_ss - 7e-16*D_qpt
y(t) actual yf_ss_qpt
• Red: Measured y(t)• Blue: Predicted y(t) using only ion number variations (2nd-order Doppler effect)• Excellent agreement Ion number variations dominant effect• Temperature effect relatively small
Actual freq offsets (re
d trace)
Modeled freq response
using ONLY second order
Doppler shift (blue tra
ce)
j p l . n a s a . g o vOctober 2021DSAC-2 will remove stray magnetic effects to achieve typical 2e-15/C
Characterizing Temperature Effects• Measured sensitivity @ +/- 1e-14/C – varying slowly with time • Should be closer to 2e-15/C and not time dependent• Doesn’t limit performance, but higher than expected• Time variability indicated by a stray magnetic effect & may be reduced by smaller ion cloud• Long-term AD may improve with smaller cloud – one test below
• Small cloud yields signal with ~5400 counts vs ~20K for larger cloud (52 day result published in Nature)
• Final long run with small cloud still being analyzed• Temperature sensitivity time variability correlated with beta angle - beta angle causes thermal
gradients - can aggravate stray magnetic fields
Large signal run
Small signal run
j p l . n a s a . g o vOctober 2021
DSAC Performance Summary
• Demonstrated operational robustness and met 1-day stability requirementü 3e-15 (2e-14 required)
• Set stability records for space clocksü 3e-16/day drift
• Refined understanding of temperature sensitivityü < 1e-14/ºC and able to achieve 2e-15/ºC
• Improved life time estimates due to Hg, Neon, and background gasesü > 7 years
• DSAC lamp life: ü > 4 years (expected 3-5 years), no degradation in space
15
j p l . n a s a . g o vOctober 2021
What is Possible with a Deep Space Atomic Clock?
Lunar Gateway
Enhanced Autonomous Navigation
DSN Extension
Radio Science & Fundamental Physics
Lunar/MartianPositioning System
j p l . n a s a . g o vOctober 2021
Autonomous Nav Example – Mars Approach & Entry
• Onboard one-way uplink radiometric data using DSAC combined with optical improves on optical-only navigation by an order of magnitude
• Radio tracking reduced by 90% relative to traditional two-way, ground based tracking• Atmosphere entry knowledge ~ 150 m (order of magnitude improvement compared to
ground-based navigation)
17
Elapsed Time (hrs) Elapsed Time (hrs)
Onboard Gimbaled Optical Only Onboard CPF-Phase from 2 hr/day DSN & Onboard Gimbaled Optical
j p l . n a s a . g o vOctober 2021
DSAC-2 significantly reduces SWaP, increases operational life, and improves stability for improved navigation and radio science
DSAC-2 TDO Hosted on VERITAS
Free running USO
DSAC-1 in spaceDSAC-2 expected
10-16
10-15
10-14
10-13
10-12
10-11
Alla
n D
evia
tion
100 101 102 103 104 105Time (s)
courtesy D. Enzer (JPL 335E)
DSAC-2 DSAC-1Required CBE Actuals
Stability2E-13 at 1-s2E-14 at 1000 s3E-15 at 1-d
1e-13 at 1-s 1e-14 at 1000 s 1e-15 at 1-d
1.5E-13 at 1-s< 3E-14 at 1000 s~ 3E-15 at 1-d
Power 42 W 33 W 56 WMass 13 kg 10 kg 19 kgVolume 13 L 10 L 19 LLifetime 2 yr >5 yr 2–3 yrs
DSAC-1USO
PIU
+ +
DSAC-2 concept• includes USO & clock control• fits inside a GPS footprint and/or
commercial rack mount• the TDO will raise TRL to 9
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