Long distance frequency comparison via satellite at the

Long distance frequency comparison via satellite at the 1 E-18 level 8 th International Symposium of Time and Space Sept 14 -16, 2016, St. Pertersburg, Russia Wolfgang SCHÄFER Time. Tech Gmb. H, Stuttgart, Germany www. timetech. de w olfgang. schaefer@timetech. de +49 -711 -678 -08 —

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 2 2 Do we need 1 E-18? What for? This talk concentrates on ground-to-ground tests, in support of • Re-definition of the Si unit 1 Second “Once measurement capability is available” • Development and verification of novel clocks • Fundamental Metrology • Frequency is most accurate reference for other Si units • Relativistic geodesy (1 cm ~ 1 E-18) • Secure communications

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 3 Geographical distribution of the laboratories that contribute to TAI and time transfer equipment as of April 2013 [BIPM] Online: http: //www. bipm. org/en/bipm-services/timescales/tai. html

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 4 4 Example: 920 km dark fiber link between PTB and MPQ, Germany Two antiparallel compensated fibers allow for common-clock performance characterisation and remote optical clock comparisons at the same time. Instability @ 1000 s: ~ 10 -18 Uncertainty: 10 -19 level [1] K. Predehl, A 920 km Optical Fiber Link for Frequency Metrology at the 19 th Decimal Place , Ph. D-thesis, LMU, 2 012. Today’ most accurate links: Ground-based optical fiber Regional Solution to the problem

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 5 Disadvantages of current TWSTFT links covering long distances • Mostly operated at low bandwidth for cost reasons (1 or 2. 5 MChip/s) • Commercial communication satellites, transponders must be paid for • No special equipment on board, „bent-pipe transponder“ • „ Simple“ ground station hardware Most important TWSTFT has about 2. . 3 orders lower performance compared to advanced clocks Is there growth potential? Support the re-definition of the second?

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 6 Historical Evolution of Clock Performance [2] Schiller, S. et al, The STE QUEST Mission, A Space Test of the Equivalence Principle in the quantum domain, Cosmic Vision M 3 Selection, Paris, 21. 1. 2014, p. 14 TWSTFT ACES MWL 1 E-18 ?

Sept 14 th , 2016 Long distance frequency comparison via satellite at the 1 E-18 level 7 Will satellite links meet the challenges? Review of Past and Current Satellite Links

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 8 8 How it began (TU-Berlin 1977, Univ. Stuttgart 1983) MITREX: “non-coherent Delay-lock loop [3] Hartl, P. et al, „High Accuracy Global time Transfer Via Geosnchronous Telecommunication Satellites with MITREX“, Zeitschrift für Flugwiss. Weltraumforsch. 7 (1983), Heft 5, p

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 9 9 TWSTFT Principle, „bent-pipe“ transponder using PRN-coded signals Clock A Tx Two-Way Link Rx TIC Clock BRx Tx TICGeo Satellite Station A Station B • Time Transfer configuration „(A-B)/2“ + Bi-directional links eliminate path-induced errors, incl Tropo and Iono + Potential of 1 E-17 / day frequency transfer uncertainty, > 20 MChip/s + Time-transfer: Ub ~ 1 ns (demonstrated) + Potential of 100 ps time transfer (extrapolated)

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 10 10 Link Calibration using Mobile Station Reproducibility: ~ 250 ps after 6 months, typical @ 1 MChip/s VNIIFTRI operates ist own mobile calibration station.

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 11 Space-based Systems “ Active Elements in Space”

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 12 12 First Active System on Satellite: PRARE on-board Meteor-3, ERS-2, Space-Based PN Ranging, Emphasis on Geodesy PRARE Precise RAnge- and Range-rate Equipment (1990), Uni Stuttgart, GFZ Ranging Ua 0. 9 cm @ 15 s normal points Velocity Ua 0. 015 mm/s @ 15 s normal points • Range measurements in X-band between sat and ground stations (CDMA in uplinks), 10 MChip/s PN-code, negative SNR • Additional S-band downlink for ionosphere compensation Courtesy F. Flechtner, GFZ

Sept 14 th , 2016 Long distance frequency comparison via satellite at the 1 E-18 level 1313 ACES MWL: Concept and Performance (PRARE heritage) • Noise Requirements: – 230 fs @ 300 s – 1. 2 ps @ 5000 s – 5. 5 ps @ 1 d – 10 ps @ 10 d • -Configuration • “ Ideal mirror“ESA/Airbus (prime), launch 2017 [4] Salomon, C. et al 2001 Cold atoms in space and atomic clocks: ACES Comptes Rendus de l’Académie des Sci-ences — Series IV – Physics Vol. 2 , Iss. 9 pp. 1313 —

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 14 14 ACES MWL Key Concepts: How to improve performance by 2 orders of magnitude? • Active element on board to achieve 1 E-17 • Ground Station with Radome (thermally stable) • Code- and Carrier phase measurements • 2 Bands , Ku and S-band, high ratio – Differential Code Phase – Differential Carrier Phase • Frequency transfer by Carrier Phase, ( Ku-band, ~ 60 ps ) • Time transfer by modulation (PRN, 100 MChip/s, 10 ns / Chip ) • Integer carrier cycle identification • Carrier phase continuity , pass by pass • Carrier exhibits lower Multi-Path • Differential Delay Calibration Tx vs Rx to 5 ns to meet -Configuration requirements (ISS shows high acceleration) Engineering Model Ground Terminal Radome removed

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 15 15 Under Development: HEO MWL (ACES MWL Heritage) Driven by proposed STE-QUEST Mission • Highly elliptical orbit (HEO) – Apogee height: 51000 km – Perigee height: 700 km • Ground-to-space and common-view ground to ground comparisons • Required accuracy for ground-to-ground comparison: 5 x 10 -19 • Flicker phase-noise floor: 92 fs • Proposed ground stations are at – Boulder (NIST) – Torino (IT) – Tokyo (NICT) • Ground demonstrator breadboard currently under development [2] Schiller, S. et al

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 16 16 Requirements for HEO MWL, suitable for STE-QUEST • Frequency Uncertainty (Ub) – Space-to-ground: 3 x 10 -17 – Ground-to-ground: 5 x 10 -19 @ 1 week Many similarities to ACES-MWL • Frequency stability: Frequency uncertainty of 1 x 10 -18 @ 1 day Frequency uncertainty of 3 x 10 -19 @ 1 week. Frequency uncertainty of 1 x 10 -17 @ 10000 s Target TDEV: 100 fs (red line)

Sept 14 th , 2016 Long distance frequency comparison via satellite at the 1 E-18 level 1717 17 Link Stability Simulations, using ASTOS TM soft ware (MDEV) • Simulation includes – White PN due to link budget – Flicker of phase noise – Orbit dynamics – Antenna phase pattern – Bi-directional link (Ka-band X-band combination) – Margin was left for • Temperature variations • Environment Simulated MDEV: < 1 E-18 @ 10000 s „ too good to be true“

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 18 18 Some TDEV measurements using ACES-MWL hardware, NO Doppler, Differential Receiver channels alone • Ground-Ground potential Stability: – Differential delay between channels – 1 st mixer is in common mode – TDEV: 4 fs @ 800 s on Carrier Phase (Req. 100 fs) 4 fs Differential Ground-Ground 60 fs End-end Ground-Space „ Common Clock“ elements

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 19 Candidate Novel Links

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 20 20 A Possible Path to the Future: Laser Communication Terminals (LCT), LEO-LEO and GEO-LEO demonstrated • Bi-directional by design : ideal for two-way time transfer & ranging, inter-satellite • 2 Gbit/s, C/No > 100 d. BHz • Positive signal-to-noise ratio with potential for – fs-level timing jitter – µm-level ranging performance • Space-ground links currently under investigation [5] Gregory, M. et al 2010 TESAT Laser Communication Terminal Performance Results on 5. 6 GBit Coherent Inter-satellite and Satellite to Ground Links ICSO 2010.

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 21 21 Expected performance, Continuous Laser Communication Links Communication electronics adapted to timing and ranging High bit-rate: 100 MBit/s. . 10 GBit/s, Eb/No ~6 d. B, GEO-LEO: 2 Gbit/s GEO-LEO fs-level timing @ 1 s GEO-LEO Ranging at µm level @ 1 s Suitable for GRACE-like applications 5 fs 1 µm

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 22 22 Alternative Optical Link: Pulsed Optical Links LEO / ACES Corner Cube Reflector (GFZ, Potsdam)ELT / SLR: Single photon detection (part of ACES) Ultimate Performance limited by pulse counter, repetition rate I. Prochazka, Univ. Prague, Czech Rep. ps-uncertainty not (yet) sufficient to achieve STE-QUEST specifications (Required: 100 fs) Excellent for time calibration Neubert, R. et al, 17 th Workshop on Laser Ranging, Bad Koetzting, Germany, (2011)MEO / GEO Open Reflector

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 23 23 Available µWave Technology, EHF band, 30… 300 GHz [6] F. Boesetal 2014 Ultrabroadband. MMICbasedwirelesslinkat 240 GHzenabledby 64 GS/s. DAC Proc. 39 th. IEEEIRMMWTHz, Tucson, AZ pp. 12. [7] Lewark, U. J. et al 2013 Link budget analysis for future E-band gigabit satellite communication links (71 -76 and 81 -84 Ghz) CEAS Space Journal pp 1 -6 [8] Recommendation ITU-R P. 676 -3 Attenuation by atmospheric gases. High speed terrestrial radio communication [6], [7] 43 d. Bi horn antenna, 240 GHz Atmospheric attenuation, [8] „ Radio Windows“

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 24 24 Comparison Optical vs. µWave Optical or µWave? Look outside…. Then answer for yourself.

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 25 Some µWave Link Examples

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 26 26 How to design a Link to 1 E-1 8 ? (1 -page recipe) • Establish one actively compensated link space-to-ground for each ground station • Stabilise each link individually to the required performance • Well-designed links show (1/ ) ADEV performance characteristics • Use both modulation (PRN coding) and Carrier Cycle Identification • Ensure pass-to-pass continuity, „count carrier cycles“ • Operate the links simultaneously: „ Ideal Mirror“ in space • Rely on „Common-Clock“ approach in Space, including not just the clock, but also including – Transmitter electronics (100% of noise is common to both links) – Receiver 1 st Local Oscillator (~90% of noise is common) • On Ground, use „brute-force“ stabilisation , incl. – Very good thermal stabilisation (better than 1 K) – Reduce Flicker phase noise by all means 00 PLL/2 1 NC B 0 b. DLL /2 NCBW T Ua Code (DLL): Ua Carrier (PLL): Links can be tailored to basically any requirement !

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 27 27 Link Budget Examples: Ground to GEO for 50, 70 and 200 GHz • Noise floor is at fs level Potential to reach 10 -19 frequency uncertainty in 1 day • Distance: 36000 km • Atmospheric loss estimated from ITU data for an elevation of 20° • A worldwide system requires 3 GEO satellites to cover the entire Earth • Objectives: – Moderate transmitter power: 3 W – Earth coverage: Space antenna beamwidth 17 ° Ground to GEO Frequency GHz 50 70 200 Modulation MChip/s 250 500 2000 Transmitter power W 3 3 3 Satellite antenna diameter cm 2. 6 1. 8 0. 6 Ground antenna diameter m 1. 2 3. 0 Receiver C/N 0 d. BHz 50 50 48 Code jitter @ 1 s ps 18 9 3 Carrier jitter @ 1 s fs

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 28 28 Alternative and preferred satellite location: Earth-Moon Lagrange Point MLP 1 • Good performance with low transmitter power possible (fs and µm level) • Carrier performance competitive with assumed optical link performance • All weather operation • Link distance: ~300000 km • 2, 5° antenna beamwidth needed for Earth coverage • Satellite is visible 6. . . 12 h each day (moon visibility) • Only 1 satellite !!! • Low satellite velocity, excellent for -Configuration Ground to Lagrange Point (Moon) Frequency GHz 50 70 200 Modulation MChip/s 500 2000 Transmitter power W 6 6 6 Satellite antenna diameter cm 18 13 4 Ground antenna diameter m 0. 73 0. 63 0. 73 Receiver C/N 0 d. BHz 49 47 49 Code jitter @ 1 s ps 10 12 2. 6 Carrier jitter @ 1 s fs

Sep 14 -16, 2016, Russia 8 th International Symposium Metrology of Time and Space 29 29 Summary and Conclusions • ACES-MWL hardware is an excellent starting point • Satellite links are still reality and needed in regions with low infrastructure. • Orbits: GEO or better Lagrange Point Earth-Moon (L 1, others) • µWave (EHF, > 30 GHz) preferred, All-Weather capability • 1 E-18 uncertainty ground-to-ground feasible within less than 1 day (relativistic geodesy 1 cm) • 1 E-18 at intercontinental distances , using today‘s technology • Growth potential (1 E-19) using higher bands ( to 200 GHz ) and larger RF bandwidths ( to 5 GHz ), through atmosphere • Bi-directional communication Laser-links very promising for inter-satellite links (above atmosphere) • Pulsed lasers for calibration (intermitted operation)