ACES payload and requirements
The ACES payload (shown on Figure 3.8) has a volume of about 1m3, a total mass of 230 kg and a power consumption of 450 W (11). It includes the hydrogen maser SHM, the cold cesium clock PHARAO, a GPS/GALILEO receiver, a two way time transfer in the S and Ku band MWL and a two way time transfer in the optical band (500nm) ELT. The FCDP measures the phase/frequency difference between PHARAO and SHM anddistributes the reference signal to the transfer links. XPLC is the payload computer.
PHARAO (Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbit) clock, which is being developed by the French space agency CNES, is the first Primary Fre- quency Standard (PFS) specially designed for operation in space. A PFS is a cesium atomic clock which realizes the unit of time, the second, with high accuracy. PFS op- eration is based on the measurements of the hyperfine transition of cold cesium atoms at the frequency 9,192,631,770 Hz. Its operation is similar to ground based atomic fountains but optimized for microgravity operation. Main components of PHARAO are the cesium tube, the laser source, the microwave source and the computer.
Signal merging and data handling
Telecommand, telemetry and data handling for all ACES instruments and short and long-term servo loop is ensured by the eXternal PayLoad Computer (XPLC), developed by ASTRIUM under ESA responsibility.
ACES will operate a phase locked loop which will stabilize the medium term stability of the PHARAO local oscillator on the clock signal generated by SHM which has a better medium term stability (Figure 3.11). Data corrections are sent every 250 ms. Short, medium and long term stability are defined on time intervals of 0.0001s to 1s, 1s to 1000s and 1000s to ∞, respectively. In order to compare the 100 MHz signals of SHM and PHARAO the Frequency Comparison and Distribution Package (FCDP) is used.
It was developed by ASTRIUM and TimeTech under ESA responsibility. FCDP is the central node of the ACES signal management also responsible for signal distribution to the time and transfer link.
Instability of the hydrogen maser for τ > 3000s will be measured and corrected by a second servo loop. XPLC receives the PHARAO measurements and sends the correction to SHM at a rate of several hundreds seconds. Resulting fluctuations of the ACES timescale based on the SHM signal are expected to be around 10 ps per day.
Time and frequency transfer: MWL and ELT
The implemented microwave link MWL (57), developed by ASTRIUM, Kayser-Threde and TimeTech, is one of the key elements of ACES surpassing the performances of the currently used transfer links (TWSTFT and GPS) by one to two orders of magnitude. It is a two-way system with two different frequencies that allows both space to ground and ground to ground clock comparisons. It has 4 channels allowing 4 simultaneous ground users. MWL has a high carrier upload and download frequencies (13.5 and 14.7 GHz, respectively). Another frequency in the downlink S-band (2.2 GHz) is used to correct the ionosphere time delay by measuring Total Electron Content (TEC). A code phase measurement removes the phase ambiguity between separated ground space clock comparisons.
International Atomic Time contribution
International Atomic Time (or TAI from Temps atomique international) is a time scale built by the Bureau International des Poids et Mesures (BIPM) and created as a weighted average of over 300 ground atomic clocks over the world (Figure 3.17). With a high accuracy of 10−16 and well defined gravitational potential, PHARAO has a possibility to give a significantly weighted contribution during mission duration. TAI relies on ground clock comparisons using GPS satellites and TWSTFT links with 1 day fluctuations on the order of 10−15, while ACES will allow worldwide clock comparison at an unprecedented level of 10−17, further improving on the TAI. With the development of optical clocks having better performances than their ce- sium counterparts a redefinition of the second is foreseen in the future. A global clock comparison system provided by ACES will help in this endeavor.
Optical bench layout
The optical bench (the optical assembly diagram is given on Figure 4.11) is dense and has optical component placed on both sides of the bench. The main components are:
• 2 852 nm Extended Cavity Diode Lasers (ECDL) delivering the 2 main frequencies with frequency stabilization units.
• 2 backup Extended Cavity Diode Lasers (ECDL).
• 2 optically injected slave diode lasers (SL) amplifying the laser power.
• 2 backup optically injected slave diode lasers (SL).
• 4 magnetically shielded caesium cells: 2 for frequency stabilization of the ECDLs.
• 6 photodiodes: to control the laser frequency through fluorescence and absorption in the cesium cells.
• 4 optical isolators (OI): to prevent optical feedback which would induce pertur- bations on the laser frequency.
Table of contents :
List of Figures
List of Tables
2 Atomic clocks
2.1 Principle and performances
2.1.1 Clock accuracy
2.1.2 Clock stability
2.1.3 Clock development
18.104.22.168 Selection of cesium and alternatives
2.2 Atomic fountain – primary frequency standard
2.2.1 Cesium atomic fountain operation
22.214.171.124 Atom cooling (Doppler and sub-Doppler cooling) and the launch
126.96.36.199 Ramsey interrogation
2.3 Perspective and other clocks
3 ACES mission
3.1 International Space Station
3.1.1 Orbit and environments
3.1.2 Instrument positions
3.1.3 Vehicle support
3.2 ACES payload and requirements
3.2.3 Signal merging and data handling
3.2.4 Operation modes
3.2.5 Time and frequency transfer: MWL and ELT
3.3 Scientific objectives
3.3.1 Fundamental physics
188.8.131.52 Gravitational red-shift
184.108.40.206 Drift of fine structure constant
220.127.116.11 Anisotropy of light
3.3.2 Geodesic application
3.3.3 International Atomic Time contribution
4.1 PHARAO architecture
4.2 PHARAO development
4.3 Microwave source
4.3.1 Microwave synthesis chain
4.4 Laser source
4.4.1 Optical bench layout
18.104.22.168 ECDL output
4.5 Electronic control system
4.6 Cesium tube and operation
4.6.1 Atom capture
4.6.2 Atom cooling
4.6.3 Preparation and selection
4.6.6 Magnetic shields
4.6.7 Experimental ground operation
22.214.171.124 Experimental setup
126.96.36.199 Initial starting, optimization an results of the clock .
5 PHARAO frequency stability
5.1 Sources of noise in PHARAO
5.1.1 Quantum projection
5.1.2 Detection system noise
5.1.3 Detection laser noise
5.1.4 Local oscillator noise – Dick effect
5.1.5 Micro-vibration effect
5.2 Experimental results and discussion
6 PHARAO frequency accuracy: preliminary evaluation on the FM
6.1 Second order Zeeman effect
6.1.1 Flight model shields characterization with a magnetic probe
188.8.131.52 Shield architecture
184.108.40.206 Shield characterization experimental setup
220.127.116.11 The external B3 shield
18.104.22.168 Individual B2 and B1 shields
22.214.171.124 Shield combinations B1+B2, B2+B3 and B1+B2+B3 .
126.96.36.199 Magnetic field homogeneity
188.8.131.52 Space qualification of the shield
6.1.2 Magnetic results of the flight model by using cold atoms
184.108.40.206 Axial and transverse field attenuation
220.127.116.11 Magnetic field evaluation
18.104.22.168 Conclusion to FM shield experiments
6.1.3 Active compensation
22.214.171.124 Model description
126.96.36.199 Postulate I
188.8.131.52 Postulate II
184.108.40.206 Postulate III
220.127.116.11 Active compensation system
18.104.22.168 Experimental setup
22.214.171.124 Axial external field pattern and results
126.96.36.199 Orbital external field testing
188.8.131.52 Axial results
184.108.40.206 Degradation of results for the total field and the track- ing procedure
220.127.116.11 Shield attenuation as a function of external field ampli- tude and demagnetization
6.1.4 Conclusion to Zeeman shift
6.2 Black body radiation
6.2.2 PHARAO thermal architecture, development and temperature uncertainty
18.104.22.168 STM experimental results and modelization
22.214.171.124 FM modelization
126.96.36.199 FM experimental results
188.8.131.52 Measurement uncertainties
6.2.3 Conclusion to black body
6.3 Cold collision
6.3.2 Collision frequency simulation
6.4 Doppler effect (DCP)
6.5 Conclusion to systematic effects
8 Appendix 1
8.1 Ramsey interrogation
8.1.1 Classical representation
8.1.2 Quantum physical interpretation of interference
8.1.3 Semiclassical representation
184.108.40.206 Single oscillatory field – Rabi magnetic resonance method225
220.127.116.11 Double oscillatory field – Ramsey magnetic resonance method
8.1.4 Fictitious spin representation
9 Appendix 2
10 Appendix 3
10.1 Sensitivity function
10.2 Clock frequency shift calculation
11 Abstract (in english)
12 R´esum´e (en francais)