Time-Of-Flight and Ranging Experiments on the Lunar Laser Communication Demonstration

Time-of-Flight and Ranging Experiments on the Lunar Laser Communication Demonstration

M. L. Stevens, R. R. Parenti, M. M. Willis, J. A. Greco, F. I. Khatri, B. S. Robinson, D. M. Boroson

Stanford PNT Symposium 12 November 2015

This work is sponsored by National Aeronautics and Space Administration under Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government. NASA Metric Tracking System

• RF satellite ranging performed using specialized 1-MHz waveforms applied to communication loop-back links • Precision ranging requires dedicated measurements performed over a period of several hours • Range accuracies of the order of 10 meters are achievable

White Sands S-Band Tracking Antenna Loop-Back Configuration

Stanford PNT Seminar 11Nov15 MLS- 2 Autonomous Navigation Concept

• NASA MSFC is developing a system architecture for solar- system wide navigation using embedded headers in comm links • LEO cubesat demo concept in development

NASA’s Multi-spacecraft Autonomous Positioning System

Anzalone, 29th AIAA/USU Conference on Small Satellites 2015

Stanford PNT Seminar 11Nov15 MLS- 3 TOF Enables Planetary Science

• Time-of-Flight (TOF) measurements are an enabler for: – Planetary science, gravity, internal structure of planets, moons Mollweide Projection of Lunar Gravity Anomalies

Far side Near side

GRAIL LOLA laser gravity altimeter anomalies

Lemoine, et al. GRAIL: Gravity Recovery and Interior Laboratory “High Degree GRAIL Gravity Models” LOLA: Lunar Orbiter Laser Altimeter Journal of Geophysical Research: Planets (2013)

Stanford PNT Seminar 11Nov15 MLS- 4 Europa Clipper Mission

• Primary mission: measure Europa gravity – Look for tidal changes indicative of a liquid ocean that might harbor life

Stanford PNT Seminar 11Nov15 MLS- 5 Outline

• LLCD Mission • TOF System Architecture • TOF Data

Stanford PNT Seminar 11Nov15 MLS- 6 LLCD and LADEE

Lunar Atmosphere and Dust Environment Explorer (LADEE) Science mission – 100 days • Orbit Moon • Measure fragile lunar atmosphere LLCD • Measure electrostatically transported dust grains • NASA’s first lasercom • High-rate dupex comm • cm-class real-time ranging using comm signals • Novel space and ground technologies • 30-day mission

Stanford PNT Seminar 11Nov15 MLS- 7 LLCD Space Terminal on LADEE

Modular design allowed for 0.5-W transmitter 4-inch telescope balanced placement in small LLCD Optical Fully-gimballed spacecraft. Units fiber- and Module Inertial cable-connected. stabilization

LLCD Controller Module LLCD Modem Module Space Terminal: mass ~ 30 kg; power ~ 90 W

Stanford PNT Seminar 11Nov15 MLS- 8 Primary LLCD Ground Terminal (LLGT) at White Sands

Ground Terminal Design • Single gimbal • Four 16-inch receive telescopes • Four 6-inch transmit telescopes • All fiber-coupled superconducting nanowire single-photon detectors • Air-conditioned globe for optics • Clamshell dome for weather protection

Transportable Design • Novel architecture allows transportability • Shipping container houses modem, computers, office • Transported to White 19-meter antennas in background Sands NASA site LLGT gimbal on pedestal is ~4-meters tall

Stanford PNT Seminar 11Nov15 MLS- 9 Major Accomplishments

• Longest laser communication link ~400,000 km • Highest data rates ever demonstrated to/from moon – 20 Mbps up, 622 Mbps down • Operation through the atmosphere under a wide range of conditions – Including thin clouds • Real-time reliable command and data delivery via Lasercom – Demonstrated RF-free operation – Entire spacecraft buffer downlinked in minutes – Loopback of multiple high-rate video streams and other file transfers

Lunar Lasercom LADEE Lunar Lasercom Spacecraft Ground Terminal Space Terminal (LLGT) (LLST) White Sands, NM 20 Mbps

622 Mbps NASA ARC

Stanford PNT Seminar 11Nov15 MLS- 10 … and Time-of-Flight

• Time-of-Flight (TOF) of signals using high-rate uplink and downlink communication system clocks • In addition to duplex communication, 2-way TOF requires: – Common time reference on forward and return links – Downlink phase-locked to received uplink in space terminal – High-stability time reference for measuring two-way time-of-flight

Lunar Lasercom LADEE Lunar Lasercom Spacecraft Ground Terminal Space Terminal (LLGT) (LLST) White Sands, NM 20 Mbps

622 Mbps NASA ARC

Stanford PNT Seminar 11Nov15 MLS- 11 LADEE / LLCD Mission Parameters

• LADEE orbital period ~ 2 hrs – Visible from earth for about half of orbit • Communication links available when LADEE is visible – Duplex phase-locked communications required for LLCD TOF • Lasercom intervals limited to ~20 2 hr minutes by power and temperature – 100 passes, 135 intervals of duplex comm (14.2 hours) • LADEE ephemeris (orbit parameters) measured using NASA’s Satellite Tracking Network in dedicated ranging sessions

Stanford PNT Seminar 11Nov15 MLS- 12 LLGT-LADEE Range and Doppler in Lunar Orbit

5 Example Pass x 10 May 01 2012 00:00

3743.74 2 2

Doppler (one-way) Km)

relative ± 6.7 ppm 3 carrier ± 1.3 GHz 3723.72 0 0 DL slot ± 33 kHz clock UL slot ± 2.1 kHz

Range (x 10 (x Range clock

Range Velocity Range (km/s)

LLGT-LLST (km) Range Range Velocity (Km/sec) Velocity Range

3703.7 -2-2 0 10 20 30 40 50 6060 Time (minutes) Lunar orbit varies by 40,000 km over month

Stanford PNT Seminar 11Nov15 MLS- 13 Outline

• LLCD Mission • TOF System Architecture • TOF Data

Stanford PNT Seminar 11Nov15 MLS- 14 Ranging Based on Communication Synchronization

16 slots per symbol, 200 ps

FAS Frame Alignment Sequence 16 PPM CW Codeword • Need perfect bit-alignment of symbols, codewords, frames, to have any communication • Slot timing errors typically reduced to where communication loss is < 0.1 dB – Usually only a few % of a slot time • Phase- and frequency-locking loops are designed as part of communication receivers – Designed to track through Doppler, fades, clock imperfections, delay variations, etc • Symbol, codeword, and frame synchronization often accomplished using embedded symbols as part of communication signaling

Stanford PNT Seminar 11Nov15 MLS- 15

Ranging Based on Communication Synchronization

Everything we need for TOF is already built into the communication hardware

16 slots per symbol, 200 ps

Stanford PNT Seminar 11Nov15 MLS- 16

Communication System Time Scales

• Uplink and Downlink clocks are phase locked and fractionally related – 1 uplink slot (3.2 ns) = 16 downlink slots (200 ps) = 1 downlink symbol – Phase difference measured, integrated phase yields change in distance • Synchronous UL / DL frame clocks compared at ground terminal

– Time delay measurement yields absolute distance offset

LLCD Designs Frequency Duration Distance Comm Downlink requires accuracy to Slot 4.977 GHz 200 ps 6 cm << 200 ps Symbol 311 MHz 3.2 ns 96 cm Codeword 81.9 kHz 12.2 us 3.7 km Coarse Phase Range TDM Frame 5.1 kHz 195.27 us 58.5 km Comparison ambiguity Uplink Slot 311 MHz 3.2 ns 96 cm Symbol 19.4 MHz 51.4 ns 15.4 m Codeword 2.5 kHz 390 us 117 km TDM Frame 160 Hz 6.25 ms 1873 km

Stanford PNT Seminar 11Nov15 MLS- 17 Space Terminal Clock Architecture

170 Hz BW during comm

• Downlink clock is phase locked to received uplink clock • Downlink frame is synchronized to uplink frame by command for absolute distance measurements – 39 measurement intervals synchronized by command – Automated synchronization possible in future missions

Single master clock locks downlink to uplink

Stanford PNT Seminar 11Nov15 MLS- 18 Ground Terminal Time-of-Flight Systems

Fine Resolution Source clock Frequency stability (63 µm, 20 kS/s)* 4 Expected < 8e-12 at 2.5 seconds

Time-of-Flight Coarse16 21Range MSB’s** (58.5 km, 160 S/s)

63 µm 50-100 Hz BW

58.5 km • Measured and archived all system performance metrics – 12.6 GB of fine and coarse resolution TOF data Stanford PNT Seminar 11Nov15 MLS- 19 Outline

• LLCD Mission • TOF System Architecture • TOF Data

Stanford PNT Seminar 11Nov15 MLS- 20 Processing Issues of TOF Phase Data

• Each sawtooth is one cycle (360º) of 311.04 MHz

2. Samples in rollover regions result in phase errors

[straight-line fit correction applied]

0 5 10 15 20 25 -50

1. Phase shift reversal at Doppler null 0 50

[simple linear4 mapping applied]

x 10

2.5 100 150

2 200

1.5

250 Raw Sample Raw 1 300

0.5 350 Phase Samples (ADC) Samples Phase 0 761 762 763 764 765 766 767 768 769 770 Time [s]

3. Slight non-linearity of detector results in residual beat-frequency noise in data [removed with filter]

Stanford PNT Seminar 11Nov15 MLS- 21 Relative Change in Distance

• Using only fine dataRelative Change in Distance (m) • Measured and ephemeris set to zero at start • Comparison of measured to ephemeris prediction at time light arrives at LADEE

Residual Noise After Removing

Polynomial Fit

90000 m 90000 3.8 cm rms

Stanford PNT Seminar 11Nov15 MLS- 22 Mission TOF Engineering Data

• Two-way time-of-flight residual noise measured – Standard deviation in 1 s blocks calculated – Averaged over all data – σ = 44.3 ps (1.3 cm) • Very close to expected • Much better than 200ps promised • Data archives, extraction and processing software sent to NASA

science and navigation teams )

s 150

p (

100

l 50

a 44.3 ps (1.3 cm)

i s

e 0 R 0 50 100 150

Measurement Interval

Stanford PNT Seminar 11Nov15 MLS- 23 Detector Non-Linearity

• Each sawtooth is one cycle (360º) of 311.04 MHz

Slight non-linearity of detector results in residual beat-frequency noise in data 2

Beat Frequency Artifact

4 Removed by 200 Hz Filter

x 10

2.5 1 Phase Sensor Noise 2

1.5 0 Raw Sample Raw 1

0.5 Phase Samples (ADC) Samples Phase 0 761 762 763 764 765 766 767 768 769 770 -1

Time [s] Differential Distance (cm) Distance Differential

-2

Residual beat-frequency noise removed Stanford PNT Seminar 11Nov15 MLS- 24 with post-processing filtering One-way Residual Gaussian Noise

Gaussian fit to filtered noise

Black: Measured Red: Gaussian fit

Standard deviation 0.93cm Noise BW ~20 Hz

LLCD TOF precision is 2 orders of magnitude finer than RF ranging systems currently in use

Stanford PNT Seminar 11Nov15 MLS- 25 Low-Frequency Variations

• Some measurements show Residual after removing ephemeris estimate low-frequency variations 14 No filtering

• Possible causes 12 )

– Measurement noise m c

( 10

– Platform movement c

n 8 a

• Roll, pitch, yaw t s

i 6 D

– Temperature or signal power

a i

– Real orbital disturbance t 4

r e

f 2

• Resolution pending further f

i D analysis 0

-2 0 200 400 600 800 1000 1200 Time (sec) Is this noise or real orbital disturbance?

Stanford PNT Seminar 11Nov15 MLS- 26 Low Frequency Variations

• Some measurements show low frequency variations Residual after removing ephemeris estimate 4 Linear term removed

• Possible causes 3 )

– Measurement noise m c

( 2

– Platform movement e c

n 1 a

• Roll, pitch, yaw t s

i 0 D

– Temperature or signal power

a i

– Real orbital disturbance t -1

r e

f -2

• Resolution pending further f

i D analysis -3

-4 0 200 400 600 800 1000 1200 Time (sec) Is this noise or real orbital disturbance?

Stanford PNT Seminar 11Nov15 MLS- 27 Low Frequency Variations

• Some measurements show low frequency variations Residual after removing ephemeris estimate 4 0.2 Hz Low-pass filtered

• Possible causes 3 )

– Measurement noise m c

( 2

– Platform movement e c

n 1 a

• Roll, pitch, yaw t s

i 0 D

– Temperature or signal power

a i

– Real orbital disturbance t -1

r e

f -2

• Resolution pending further f

i D analysis -3

-4 0 200 400 600 800 1000 1200 Time (sec) Is this noise or real orbital disturbance?

Stanford PNT Seminar 11Nov15 MLS- 28 Low Frequency Variations

• Some measurements show low frequency variations Residual after removing ephemeris estimate • Possible causes 0.2 Hz low-pass filtered – Measurement noise – Platform movement • Roll, pitch, yaw – Temperature or signal power – Real orbital disturbance • Resolution pending further analysis

Is this noise or real orbital disturbance?

Stanford PNT Seminar 11Nov15 MLS- 29 Summary

• LLCD included a measurement of time-of-flight using the high- speed clocks in the communication system • Mission completed 100 passes – 14.2 hours of duplex comm – 12.6 GB of TOF data – Standard deviation of residual noise in 2-way TOF = 44.3 ps (1.3 cm) • Preliminary ranging estimates show: – Centimeter precision of one-way relative distance – Gaussian residual noise with typical standard deviation of 0.93cm – Two orders of magnitude better than RF ranging systems in use • NASA science and navigation teams are performing fine analysis of ranging

Stanford PNT Seminar 11Nov15 MLS- 30 We believe that high-rate communication- signal-based time-of-flight systems could be highly useful in future navigation and science missions

Stanford PNT Seminar 11Nov15 MLS- 31 References

We believe that high-rate communication- signal-based time-of-flight systems could be highly useful in future navigation and science missions

• D.M. Boroson, B.S. Robinson, D.A. Burianek, D.V. Murphy, F.I. Khatri, J.M. Kovalik, Z. Sodnik, Overview and results of the Lunar laser communication demonstration. Proc. SPIE 8971 (2014) • B.S. Robinson, D.M. Boroson, D. Burianek, D. Murphy, F. Khatri, A. Biswas, Z. Sodnik, J. Burnside, J. Kansky, D. Cornwell, “The NASA Lunar Laser Communication Demonstration—Successful High-Rate Laser Communications to and from the Moon”; Space Ops (2014) • Willis, M.M.; Robinson, B.S.; Stevens, M.L.; Romkey, B.R.; Matthews, J.A.; Greco, J.A.; Grein, M.E.; Dauler, E.A.; Kerman, A.J.; Rosenberg, D.; Murphy, D.V.; Boroson, D.M., "Downlink synchronization for the lunar laser communications demonstration," in Space Optical Systems and Applications (ICSOS), 2011 International Conference on , vol., no., pp.83-87, 11-13 May 2011

Stanford PNT Seminar 11Nov15 MLS- 32